DE10159336A1 - Self-adjustment of sensor involves holding reference voltage constant or reducing or increasing it depending on relationship of output voltage to upper and lower thresholds - Google Patents

Abstract

The method involves holding a reference voltage constant if the output voltage is less than or equal to an upper threshold voltage and greater than or equal to a lower threshold voltage, increasing the reference voltage if the output voltage is greater than the upper threshold voltage or reducing the reference voltage if the output voltage is less than the lower threshold voltage. AN Independent claim is also included for the following: a self-adjusting sensor.

Description

Technical field

The invention relates to a method for self-adjustment of a sensor and a self-adjusting sensor, especially for capacitive level detection of a medium in a container and for distance monitoring.

State of the art

Capacitive sensors for non-contact are known and widely used Level monitoring of solid or liquid media through container walls and the query of various objects at a distance within the Detection range of a sensor field, that is the switching distance.

The principle of operation of such sensors is based on influencing the active one Field of a capacitive sensor by introducing the dielectric (Er) of the objects or media. By introducing these dielectrics, the Capacitance of a capacitor, which is determined by the capacitively active area of the sensor on the one hand and earth or the sensor parts lying at ground potential on the other hand is formed, enlarged. If a certain capacity value is exceeded, the Sensor off a switching signal; the sensor is "activated". This is e.g. B. the case when the level of a medium sufficiently covers the active area of the sensor or an object has fallen below a certain distance from the active area.

The response sensitivity (switching distance on an earthed metal plate) of a capacitive sensor must always be connected to the respective measurement or over monitoring task (application) can be adapted. This results from the fact that the Surrounding the active area always "preloaded" by other capacitive influences becomes. This is the case with the capacitive level measurement z. B. the capacity of Container wall through which measurements are taken or at distance query unwanted objects in the background that are also within the Er range of the sensor, but should not be included. E.g. at the level detection through a container wall already causes  Wall a substantial capacitive preload, which can be higher than that actual capacity of the medium to be recorded. It also determines the wall thickness also the distance to the medium. In addition, z. B. substances such as granules and oil low, water, however, high Er values or capacities.

The sensor must therefore depend on the wall thickness, wall material and what is to be detected Medium can be set individually in order to record the level precisely can. A sensor that was set too sensitive would already be due to the Wall of an empty vessel change to the actuated state while one is closed Insensitive sensor no switching signal even when the active area is fully covered triggers. Also when querying the distance of objects z. B. an immediate metal wall behind it a too sensitive sensor in the continuously operated Bring condition, since this is only the Would recognize metal wall. The sensor must therefore be precise Sensitivity that is just sufficient in this example A switching signal exceeds the total capacity of the object + metal wall trigger. The respective capacitive preload causes a kind undesired "offsets" or displacement of the "working point", which are caused by a corresponding sensor setting is compensated.

State-of-the-art capacitive sensors work with critically coupled sensors RC oscillators that respond to small changes in capacitance with oscillation or Decay (amplitude evaluation) or strong frequency influence (Frequency evaluation) react. There are also some externally controlled processes which deliver a capacitance-dependent DC voltage. The setting of the Sensitivity is usually achieved by changing the parameters of the capacitive process itself, usually the change of a gain factor in the RC Oscillator (starting threshold), or by changing the threshold of a Comparator, which when a certain adjusting DC voltage is reached triggers the switching signal.  

The following methods for setting the sensitivity are common and state of the art:

• - With the help of a small screwdriver, one in the sensor Potentiometer with X-gear spiral or 270 degree rotation angle to the desired one Sensitivity adjusted. This process is usually time consuming and requires some Practice with often critical attitudes.
• - With the help of a small button on the sensor, is opened in the running application Adjustment is carried out automatically. To do this, the application must be in be brought to a certain state, e.g. B. Vessel empty or full. Depending on the condition are then different keys in a specific order or a key for to press certain periods of time. LEDs also signal the adjustment status and may need to be observed and, depending on the display, further entries may be be performed. The user must study the operating instructions and to press the mostly water-protected button (s) under film or rubber have suitable tools on hand. State of the art are also sensors with Bus interfaces for PC connection. Here the adjustment is done via the PC keyboard. In in any case, this method also requires some time and effort. Such "Teachable" sensors work i. d. Usually with a microprocessor and A / D converter. The capacitance-dependent output voltage of the capacitive transducer digitized and the microprocessor uses this to generate a specific one Algorithm a more or less correct switching point. For this there exist e.g. B. also the published documents DE 195 07 094 A1 and DE 197 15 146 A1.

DE 197 15 146 A1 is a method with two separate, independent mutually variable thresholds E1 and E2 become known. This leads to one time-consuming learning process, which must record all the states of an application, e.g. B. Level full or empty, and cannot run independently without operation.

Technical task

The invention has for its object a self-adjusting sensor and a Specify method for self-adjustment of a sensor, being a fully auto  automatic adjustment of the sensor to a variety of applications without any Control element on the sensor and without user intervention.

Disclosure of the invention and its advantages over the prior art

The object is achieved according to the invention by a method for the automatic self-adjustment of a sensor, comprising a sensor with an output which outputs an output voltage and a first comparator to which the output voltage and a reference voltage are applied, the first comparator outputting a switching signal, if the output voltage is greater than the reference voltage and otherwise does not emit a switching signal, or vice versa, characterized in that the reference voltage is automatically tracked to the output voltage as follows:

• a) If the output voltage is both
  • is less than or equal to an upper limit voltage, which is given by the sum of the reference voltage plus a first voltage distance,
    as well as
  • is greater than or equal to a lower limit voltage, which is given by the difference of the reference voltage minus a second voltage distance,
    so the reference voltage is kept constant,
  • - If the output voltage is greater than the upper limit voltage, the reference voltage is raised, and
  • - If the output voltage is less than the lower limit voltage, the reference voltage is lowered.

The object is further achieved by a method for the automatic self-adjustment of a sensor, comprising a transducer with an output that outputs an output voltage, an analog-digital converter to which the output voltage is applied and to which the latter assigns a digital output value, and a EDP device into which the digital output value is read and which compares the output value with a reference value and emits a switching signal if the output value is greater than the reference value and otherwise does not emit a switching signal, or vice versa, characterized in that
that the reference value is automatically updated as follows:

• a) If the initial value is both
  • is less than or equal to an upper limit value, which is given by the sum of the reference value plus a first distance value,
  as well as
  • is greater than or equal to a lower limit value, which is given by the difference of the reference value minus a second distance value,
  so the reference value is kept constant,
• b) if the initial value is greater than the upper limit, the Reference value raised, and
• c) if the initial value is less than the lower limit, the Reference value lowered.

The object is further achieved by a self-adjusting sensor comprising a transducer with an output that outputs an output voltage and a first comparator with a signal input to which the output voltage is applied via a first line and a reference input to which one of one Voltage source output reference voltage is applied, wherein the first comparator outputs a switching signal if the output voltage is greater than the reference voltage, and otherwise does not give a switching signal, or vice versa, characterized in that the voltage source is a tracking circuit to which the output voltage is also applied and which is the reference voltage

• a) keeps constant if the output voltage is both
  • is less than or equal to an upper limit voltage, which is given by the sum of the reference voltage plus a first voltage distance,
    as well as
  • is greater than or equal to a lower limit voltage, which is given by the difference of the reference voltage minus a second voltage distance,
• b) or increases if the output voltage is greater than the upper one Limit voltage,
• c) or reduced if the output voltage is lower than the lower one Limit voltage.

The object is further achieved by a self-adjusting sensor comprising a transducer with an output that outputs an output voltage, an analog-to-digital converter to which the output voltage is applied and to which it assigns a digital output value, and an EDP device , into which the digital output value can be read and which is able to compare the output value with a reference value stored in the EDP device and emits a switching signal if the output value is greater than the reference value and otherwise does not emit a switching signal, or vice versa, characterized .
that the computer equipment the reference value

• a) holds constant if the initial value is both
  • is less than or equal to an upper limit value, which is given by the sum of the reference value plus a first distance value,
    as well as
  • is greater than or equal to a lower limit value, which is given by the difference of the reference value minus a second distance value,
• b) or increases if the initial value is greater than the upper limit,
• c) or reduced if the initial value is less than the lower limit.

According to a variant of the invention, the reference voltage is only then raised or lowered when the output voltage for a first predeterminable or the second predefinable response time has remained greater or smaller than that upper or lower limit voltage. In this case, the tracking circuit lifts the  Only then does the reference voltage rise or fall, if the output voltage for a first specifiable or second specifiable Response time has remained greater or smaller than the upper or lower Limit voltage. In this way, the tracking of the Reference voltage to higher and / or lower values and thus the Self-adjustment of the sensor if required with any time Delay, that is, any response time. So z. B. advantageously an undesirable response of the tracking z. B. short Disruptions in the output voltage can be prevented.

Likewise, according to another variant of the invention, the reference value is first then raised or lowered when the initial value for a first specifiable or the second predefinable response time has remained greater or smaller than that upper or lower limit. In this case, the IT facility lifts the Only then does the reference value increase or the EDP device lowers the reference value then when the initial value for a first predeterminable or second predeterminable Response time has remained longer or shorter than the upper or lower Limit.

The first and / or the second response time does not need to be constant in time to be. You can e.g. B. depending on the temporal behavior of the Output voltage or the output value and / or the reference voltage or reference value are changed, d. H. be variable.

The first and / or the second voltage spacing or the first and / or the second distance values can be constant over time. According to another variant the first and / or the second voltage spacing or the first and / or the second distance value changes over time. Here, the first and / or the second voltage spacing or the first and / or the second Distance value especially depending on the temporal behavior of the Output voltage or the output value and / or the reference voltage or the reference value can be changed. In a further refinement of this The first and / or the second voltage spacing or the first take a variant  and / or the second distance value as a function of time Average obtained average value of the amount of the slope Output voltage or the output value and / or the reference voltage or the reference value to or from. The voltage distances or distance values can z. B. for sensors with non-linear but constant distance Output voltage characteristic curve during operation individually with the amount of Output voltage or the output value can be adjusted.

The first and second voltage spacing can be the forward voltage of one or the other Be connected in series of diodes, which means that these voltage gaps can be provided in hardware in a very simple manner.

By connecting additional diodes, additional diodes, the Voltage gaps can be increased gradually. By bridging individual diodes with switches can further change the voltage spacing be gradually changeable. Of course, the voltage gaps also be continuously changeable.

The rate of change of the reference voltage or the reference value over time can be limited to a predeterminable maximum value, so that the reference voltage or the reference value z. B. sudden changes in the output voltage or Output values follow only at a limited speed. This can e.g. B. then be advantageous if the output voltage with high-frequency interference is superimposed. The rate of change of the reference voltage or the The reference value need not be constant, but can be temporal be changeable and z. B. from the difference between output and Depend on the reference voltage or output and reference value. According to one Variant of the invention starts the rate of change of the reference voltage over time or the reference value after the start of each tracking process with a certain value and then decreases asymptotically with time. The maximum can be changed over time, i.e. it can be variable over time.  

The tracking circuit can be a pure analog circuit without digital components be and z. B. a first and a second diode and a charging capacitor comprise, the two diodes antiparallel between the output of the up taker and the reference input of the comparator are switched and the Re remote input of the comparator is connected to ground via the charging capacitor.

The tracking circuit can also be a hybrid circuit or a digital one Circuit without a microprocessor. Furthermore, the tracking circuit can Analog / digital converter and a microprocessor, the Output voltage is applied to the analog / digital converter and this the Digitize output voltage and send it in digital form to the microprocessor is able to output, which the reference voltage depending on the Output voltage to calculate their temporal behavior and use a Microprocessor output is able to deliver.

In one embodiment of the invention, the tracking circuit comprises the following components:

• a) a second and a third comparator, each with a signal input, one each Reference input and one switching signal output each, the second and third Comparator then and only emits a switching signal via its output if there is a higher voltage at its signal input than at its Reference input,
• b) a series circuit comprising a third resistor, a third diode D3, one fourth diode and a fourth resistor, the third resistor with its connection facing away from the third diode to a voltage pole and the fourth resistor with its connection facing away from the fourth diode Ground is laid and the third and fourth diodes in the forward direction are switched so that a current from the voltage pole through this series circuit flows by mass,
• c) a second line which is between the third and the fourth diode of the Series connection branches and is connected to the output of the transducer, so that the output voltage is present between these diodes,  
• d) a third line, which is between the third resistor (R3) and the third Diode branches off from the series circuit and to the reference input of the third Comparator is created,
• e) a fourth line, which is between the fourth resistor and the fourth Diode branches from the series circuit and to the signal input of the second Comparator is created,
• f) a binary counter with a counter input, a direction input, outputs as well as with an R or R2R network connected to these outputs Resistors, the direction input with the switching signal output of the second comparator is connected and the binary counter has an internal counter reading incremented if a clock pulse arrives at its counter input and at its Direction input there is a high signal, and the internal counter reading decremented if a clock pulse arrives at its counter input and at its Direction input there is a low signal, and the binary counter via its outputs and the resistance network is a variable, the internal Meter reading is able to deliver proportional voltage to a fifth line, which to the signal input of the third comparator and to the Reference inputs of both the second and the first comparator are created,
• g) an AND gate with a first and a second AND input and one logical AND output, which is applied to the counter input of the binary counter,
• h) a clock generator which emits clock pulses via its clock output, wherein the clock output is connected to the first AND input of the AND gate, and
• i) an OR gate with a first and a second OR input and one logical OR output, which is connected to the second AND input of the AND gate is connected, the switching signal output of the third comparator to the first OR input and the switching signal output of the second comparator the second OR input is connected.

To increase the first or second or third or fourth diode the forward voltage at least a first or second or third or fourth Additional diode can be connected in series, whose forward direction with that of first or second or third or fourth diode matches. This allows a gradual increase in the upper or lower voltage gap.  

The tracking circuit can be an analog / digital converter and a Digital circuit include, the output voltage at the analog / digital Converter is applied and this digitize the output voltage and in is able to output in digital form to the digital circuit which the Reference voltage depending on the output voltage or its to calculate temporal behavior and to the reference input of the first Comparator is able to deliver.

The tracking circuit can flow through a first direct current seventh resistor and / or one through which a second direct current flows eighth resistance, the voltage drop across the seventh or eighth resistor serves as the first or second voltage gap.

According to the invention, the sensor is fully automatically adapted to a Many applications without any control element on the sensor and without on grab the user. The sensor according to the invention advantageously needs only to be installed and learns the correct setting on the Application that he "sees" after installation. When changing the application the sensor adjusts itself automatically: it "teaches" itself.

The automatic adjustment of the reference voltage is largely carried out Analogy to the drag pointer principle and z. B. using normal analog technology, simple digital technology or a microprocessor. On There is a significant advantage over conventional teach technology in that it is no longer required any sensor operation. Furthermore, the correct settings of the sensor be made with the invention with even greater precision than with the conventional teach technology. Depending on the implementation, the invention also allows one Adjustment of sensor sensitivity in "real time". This can be done in "dynamic Applications "with rapidly changing requirements can be of great advantage.

The tracking according to the invention with rigid "working windows", which according to the invention depending on the time course of the output voltage Inter alia, be moved in their entirety, i. H. the two voltage spacings dU1  and dU2 can, once they have been specified, remain constant.

Of course, however, if necessary, for. B. in the case of a strongly curved Distance-voltage characteristic curve of a sensor, individual adjustments of this Values e.g. B. depending on the output voltage Ua z. B. by a Microprocessor.

The tracking according to the invention is highly advantageous compared to the stand of the technique. A sensor according to the invention can be implemented with very little technical or implement software expenditure. Another major advantage of In particular, the invention is that an automatic reset of the Sensor always takes place when changing the application, d. H. an adjustment is in Real time possible. The learning process is as short as desired, only one state is sufficient an application, for example a full vessel, for teaching.

A sensor according to the invention can be used in particular for normal standards applications are used. The sensor can be a z. B. capacitive or a inductive or an acoustic sensor or a sensor for electromagnetic Radiation, e.g. B. optoelectronic sensor, or a mechanical touch sensor or a capacitive button. The main one is compared to conventional sensors Difference that the external adjustment of the sensor z. B. by setting a Potentiometer using a screwdriver or control button due to the Self-adjustment according to the invention advantageously omitted; this will in addition, the waterproof or airtight encapsulation of the Sensor much easier, since none of the sensors according to the invention Controls guided through the sensor housing or mechanically from the outside need to be accessible. There is also no need to lay cables for one electrical remote adjustment of the sensor. These advantages come e.g. B. then completely particularly advantageous to wear when the sensor is not on or not without Other or only accessible at hazardous locations, for example under water, in radioactive rooms (e.g. within Nuclear power plants), in rooms with a high toxic load or in potentially explosive atmospheres Rooms.  

Furthermore, a sensor according to the invention compensates for shifts in the working point, which, if necessary, during the application B. by dirt, dew formation, Moistening of the medium (e.g. outdoors when it rains) or changing the medium occur automatically, resulting in the maintenance required to operate the sensor significantly reduced. The invention enables "plug and play" in practically all Applications.

Another advantage of the invention is that a variety of conventional Sensors easily with a tracking circuit according to the invention retrofitted and then using the method according to the invention can be operated.

A sensor according to the invention can be used for any application for which a conventional sensor can be used. B. for the following applications:

• - Level monitoring (all media the standard sensor can, water, oil, Granules etc.) with common wall thicknesses,
• - Distance monitoring with background suppression.

Brief description of the drawing, in which:

Fig. 1 is a linear distance-output voltage characteristic of a capacitive transducer,

Fig. 2 is a schematic diagram of a conventional capacitive sensor, which is to be adjusted by external setting of a potentiometer,

Fig. 3 is a schematic circuit diagram of another conventional capacitive sensor, a potentiometer which is adjusted by an external setting,

Fig. 4 is a block diagram of a sensor of the invention with a tracking circuit,

FIG. 5a is an example of a time course of the output voltage of a capacitive transducer and the associated reference voltage, wherein the tracking of the present invention is not activated,

Fig. 5b shows the temporal course of a sensor-switching signal, which is one of the voltage waveforms of Fig. 5a,

Fig. 5c, d, e examples of time curves of the output voltage of a capacitive transducer and the associated reference voltages, respectively, the tracking according to the invention is activated,

Fig. 6a, b, c mechanical analogies to the invention for tracking various operating states,

Fig. 7a, b for examples of the process of self-alignment of a fiction, modern sensor after installation into an application, and

Fig. 8, 9, 10 embodiments of the invention Nachführschaltungen.

Ways of Carrying Out the Invention

To further explain the invention, a conventional capacitive transducer with an adequately linear dependence of its output voltage Ua on the distance of the active surface of the sensor from a grounded metal plate, also called "target", is considered as the starting point with reference to FIG. 1; the sensor therefore has an approximately linear distance-output voltage characteristic, as shown in FIG. 1 using an example. This linear behavior can be achieved very easily, inter alia, with a conventional RC oscillator which is frequently used in the capacitive sensor technology with the prior art, with the appropriate dimensions and adapted rectification of the oscillator output voltage.

In a conventional sensor according to the prior art; FIG. 2 is now the output via an output A 1 output voltage Vout of the pickup 1 is compared in a comparator 31 with an adjustable reference voltage Uref. For this purpose, the output voltage Ua is connected to the signal input 31 A via a first line L1 and the reference voltage Uref is connected to the reference input 31 B of the comparator 31 via a seventh line L7. The reference voltage Uref is generally generated with a potentiometer 3 applied to a voltage Uo and adjusted by tapping, whereby the sensor sensitivity is adjusted, ie the sensor can be adjusted, which is shown in FIG. 2 using an example.

In other conventional sensors, the reference voltage is not at Comparator set - rather, it always remains at the same fixed value - but the sensor output voltage Ua can by changing the Gain factor of the oscillator or other parameters in the sensor set, d. H. be adjusted.

Exceeds in Fig. 2, at the signal input 31 of A adjacent sensor output voltage UA, the predetermined from the potentiometer 3, present at the reference input 31 B reference voltage Uref, so on ( "tips") of the comparator 31 in the "positive" state, it means generates a switching signal and outputs this via a sixth line L6, which corresponds to the switching state "on". Otherwise, the comparator 31 is in the "negative" state, ie it does not emit a switching signal, which corresponds to the switching state "Off". With the aid of a first and a second resistor R1, R2 ( FIG. 3), a small hysteresis is generally also generated, so that the comparator 31 always tilts clearly and stably into one of the “positive” or “negative” states. Since this hysteresis is usually chosen to be very small, it is neglected in the following considerations, whereby the following condition applies:
R2 <<< R1.

In contrast, according to the invention, the reference voltage Uref is set automatically. A self-adjusting sensor according to the invention comprises a fully automatic tracking circuit, through which a reference voltage Uref correctly adjusting the sensor is electronically derived from the output voltage Ua. The potentiometer 3 is no longer required. Fig. 4 shows an embodiment of the invention according to a block diagram of a sensor with a tracking circuit 10, which below with reference to FIGS. 8 and 9 by means of embodiments 10 A, 10 B thereof will be explained in more detail. The output voltage Ua is tapped from line L1 and fed to the tracking circuit 10 . The tracking circuit 10 functions as voltage source generating the reference voltage Uref and outputs 31 B to the reference input.

The mode of operation of a preferred variant of a tracking of the reference voltage Uref according to the invention is further explained below with reference to FIGS. 5, 6 and 7.

To further explain the invention, reference is now made to FIG. 5a. The upper limit voltage UGo is given by the reference voltage Uref plus a first voltage distance dU1, which is also referred to below as the "upper work window". The lower limit voltage UGu is given by the reference voltage Uref minus a second voltage gap dU2, which is also referred to below as the "lower working window". An "upper work window" and a "lower work window" are thus generated. The width of these “work windows” is given by the first and second voltage spacing dU1, dU2.

Each of these two working windows is therefore due to two different voltages defined, namely by Uref and UGo or by Uref and UGu. The Reference voltage Uref is always higher than UGu and less than UGo. In a preferred variants of the invention are the amount of dU1 and the amount dU2 always constant.

In Fig. 5a an example of a time characteristic of the sensor output voltage Vout is also shown. In the example shown in FIG. 5a, this exceeds the reference voltage Uref (sensor "actuated") after a certain time (time t1) with the result that the comparator 31 ( FIG. 4) changes to the "positive" switching state and a switching signal S gives ( Fig. 5b). After a further period of time (time t2) in the sensor output voltage Ua shown in FIG. 5a again falls below the reference voltage Uref (sensor "not actuated") with the result that the comparator 31 ( FIG. 4) changes to the "negative""Switching state changes and no more switching signal is emitted ( Fig. 5b).

In Fig. 5a the condition that the output voltage Vout is both less than the upper limit voltage Ugo as well as greater than the lower limit voltage UGU satisfied. According to the invention, the reference voltage Uref is therefore kept constant in FIG. 5a . The upper and lower limit voltages UGo, UGu thus also remain constant, so that the position of the two working windows in FIG. 5a remains unchanged.

Fig. 5c is another example of time characteristics of the output voltage Vout, the reference voltage Uref and the two limit voltages Ugo, UGU. The output voltage is zero volts up to a time t3 and then rises to a certain plateau value Uao, which is reached at a time t5. This time course can e.g. B. correspond to a switching-on process of a level sensor taking place at the time t = 0, the output voltage Ua starting to rise from the time t3, for. B. because the level of a container to be monitored by the sensor has grown so much that it begins to noticeably influence the output voltage Ua.

Correspondingly, the reference voltage Uref also initially has the value zero volts. The upper and the lower limit voltage UGo and UGu are according to the invention given by the sum of the reference voltage Uref plus the first voltage distance dU1 or by the difference of the reference voltage Uref minus a second voltage distance (dU2), which means that the reference voltage Uref , the upper limit voltage UGo and the lower limit voltage UGu always run parallel to each other. The upper and lower limit voltages UGo and UGu therefore initially run horizontally at a distance dU1 and dU2 above and below the zero volt line in FIG. 5c.

According to the invention, the reference voltage does not change until time t4, since the condition is fulfilled that the output voltage Ua is both less than or equal to the upper limit voltage UGo and greater than or equal to the lower limit voltage UGu. At time t4, however, the output voltage Ua exceeds the upper limit voltage UGo. According to the invention, the reference voltage Uref is now raised. the upper and lower limit voltages UGo, UGu also rise in parallel. The position of the “work window” defined above accordingly moves upwards from time t4 in FIG. 5c, ie the reference voltage Uref and the “work window” are tracked from the output voltage Ua from time t4.

At time t5, the output voltage Ua in the example of FIG. 5c reaches the plateau value Uao and does not continue to increase, for example because the maximum fill level of the container to be monitored has been reached, but the output voltage Ua is still greater than the upper limit voltage UGo, which is why Reference voltage Uref and thus also the two limit voltages UGo, UGu continue to increase according to the invention.

At time t6, the upper limit voltage UGo has also reached the plateau value Uao, ie the condition that the output voltage Ua is both less than or equal to the upper limit voltage UGo and greater than or equal to the lower limit voltage UGu is fulfilled. According to the invention, the reference voltage Uref and with it also the upper and lower limit voltages UGo, UGu remain constant. The reference voltage Uref and thus the sensor have adjusted themselves according to the invention. The upper limit voltage UGo now coincides with the output voltage Ua; In Fig. 5c, these two voltages are shown slightly offset from one another only for reasons of clarity.

According to the variant of the invention explained with reference to FIG. 5c, the reference voltage increases more slowly in the time interval t4 to t6 than the output voltage Ua in the time interval t3 to t5. Such behavior of the reference voltage Uref can be advantageous for various reasons, e.g. B. when the output voltage Ua is superimposed by external interference pulses ("spikes").

A further variant of the invention will now be explained with reference to FIG. 5d, in which the output voltage shows the same course as in FIG. 5c. However, the reference voltage Uref and thus also the limit voltages UGo, UGu in FIG. 5d increase just as quickly in the time interval t4 to t6 as the output voltage Ua in the time interval t3 to t5. The times t5 (reaching the plateau voltage Uao by Ua) and t6 (reaching the plateau voltage Uao by UGo) therefore coincide in FIG. 5d. Such behavior of the reference voltage Uref can e.g. B. can be advantageous if the self-adjustment of the sensor is done very quickly or is to be realized by means of the lowest possible hardware outlay, without own delay elements

The reference voltage Uref thus became analogous to the output voltage Ua tracked to the drag pointer principle, d. H. the output voltage Ua has the Reference voltage Uref at a certain voltage distance, namely dU1, "pulled behind". The "work windows" were "dragged along". in this connection when "dragging" a certain, predeterminable distance dU1 between Ua and Uref complied with.

Uref is tracked by the distance dU1 through Ua, so that Ua = Uref + dU1 or Uref = Ua-dU1. From time t4 ( FIG. 5d), ua is larger by dU1 than the reference voltage Uref. The comparator 31 ( FIG. 4) generates a switching signal (high signal) because Ua is greater than Uref, ie the sensor is actuated from time t4 ( FIG. 5d).

The output voltage Ua now decrease z. B. due to a falling fill level after the time t6 ( Fig. 5c, 5d) from a time t7 again ( Fig. 5e). According to the invention, however, the reference voltage Uref and thus also the two limit voltages UGo, UGu remain constant as long as Ua does not drop below UGu; ie the two "work windows" keep their position. The reference voltage Uref, ie the working point of the sensor, is thus "fixed", ie the adjustment of the sensor which is automatically brought about according to the invention remains unchanged as long as Ua does not drop below UGu (or rise above UGo).

If Ua now drops below the "fixed" value of Uref (time t8 in FIG. 5e), the comparator 31 ( FIG. 4) no longer emits a switching signal or a low signal, ie the sensor switches off, which indicates The first voltage spacing dU1, which can be predetermined as desired, is in this example a measure of the sensitivity with which the sensor responds to a falling fill level.

Now the output voltage Ua drops even further and falls below the lower limit voltage UGu at a time t9 ( FIG. 5e). According to the invention, the reference voltage Uref is now reduced, FIG. 5e relating to such a variant of the invention in which the reference voltage Uref falls at the same speed as the output voltage Ua from time t9. The upper and lower limit voltages UGo, UGu also drop parallel to Uref. The position of the “work window” defined above accordingly moves downward from time t9 in FIG. 5e, ie the reference voltage Uref and the “work window” are tracked from the output voltage Ua from time t9 until Ua no longer drops (time t10 in Fig. 5e). According to the invention, the reference voltage Uref and with it also the upper and lower limit voltages UGo, UGu remain constant. The reference voltage Uref and thus the sensor have adjusted themselves according to the invention. The lower limit voltage UGu now coincides with the output voltage Ua; in Fig. 5e these two voltages are shown slightly offset from each other only for reasons of clarity.

Conversely, Ua increases again, e.g. B. because the level rises again, remain the reference voltage Uref and the position of the "work window" "fixed" and change only reappears when Ua exceeds the upper limit voltage UGo. Then the reference voltage and the "working window" will go up again "pulled along", i.e. H. tracked the output voltage Ua. Before that, Ua however, the "fixed" reference voltage Uref, and the sensor switches again on. The arbitrarily definable second voltage spacing dU2 is therefore in the present Example a measure of the sensitivity with which the sensor is sensitive to one increasing level.

Of course, in a complete analogy to FIG. 5c, a variation in which the reference voltage drops more slowly than the output voltage is also possible when tracking the reference voltage downward with reference to FIG. 5e. Furthermore, as shown in FIGS. 5a, c, d and e, the output voltage Ua need of course not be linear in sections; the course can also be curved. According to a variant of the invention, the rate of rise or fall of the reference voltage Uref is not constant, as shown in FIGS. 5a, c, d and e, but variable over time, e.g. B. The amount increases progressively with time. Of course, the time dependencies of the rise and fall speed of the reference voltage Uref need not be the same in opposite directions.

According to further variants of the invention, the Reference voltage Uref does not start immediately as soon as the output voltage Ua the upper limit voltage UGo exceeds or, the lower limit voltage UGu below. Rather, the tracking of the reference voltage Uref begins above according to one of these variants only when the output voltage Ua for a certain first predefinable response time period T1 has remained longer than that upper limit voltage UGo. According to another of these variants, the Tracking of the reference voltage Uref down only when the off output voltage Ua for a specific second predefinable response time period T2 has remained smaller than the lower limit voltage UGu. These variants that can of course also be combined with each other, z. B. then be advantageous if the self-adjustment of the sensor according to the invention is not by short, e.g. B. vibration-related disturbances of the output voltage Ua should be triggered.

Can be compared to the reference to FIG. 5d, 5e described tracking procedure with a mechanical pointer principle, which will now be explained in more detail with reference to Figs. 6a, 6b and 6c. On a main pointer 22 a driver claw 23 is rigidly attached transversely to this so that the main pointer 22 and the driver claw 23 form an asymmetrical cross 22 , 23 , the driver claw 23 forming the crossbar of the cross 22 , 23 and on one side by one Route X1, on the other side, overcomes a route X2 via the main pointer 22 . The main pointer 22 is pivotally mounted, like clock hands, about a center of rotation 24 and has a drive (not shown) which is able to pivot the main pointer 22 clockwise and counterclockwise about the center of rotation 24 . Due to the rigid connection to the main pointer 22 , the driving claw 23 follows all pivoting movements of the main pointer 22 around the center of rotation 24 .

A trailing pointer 21 is also pivotably arranged around the center of rotation 24 in a manner similar to a clock hand, the driver claw 23 crossing the trailing pointer 21 without being mechanically connected to it. The slave pointer 21 does not have its own drive.

The driver claw 23 has at both ends stops 23 A, 23 B for the pointer 21 , which limit the size of the relative displacement between the driver claw 23 and the pointer 21 and thus also the maximum angular distance between the main pointer 22 and the pointer 21 to a certain range; when the driver claw 23 moves beyond this clearance, the drag pointer 21 is pulled along by the driver claw 23 via one of the stops 23 A, 23 B, and thereby tracks the pivoting movements of the main pointer 22 about the center of rotation 24 at a certain distance. If the distances X1, X2, as in the present example of Figs. 6a-6c are not equal, but the cross 22, 23 of the main pointer is asymmetrical with respect to 22, the maximum angular distance between the main pointer 22 and drag link 21 in clockwise movements different from that when moving counterclockwise.

The maximum angular distance by which the main pointer 22 can be deflected in the clockwise or counterclockwise direction with respect to the drag pointer 21 is predetermined by the length of the distances X1 or X2 and by the distance of the crossing point between the driving claw 23 and the main pointer 22 from the center of rotation 24 .

The drag pointer 21 is initially in a certain rest position. Now the main pointer 22 by its drive z. B. in a clockwise direction with respect to the slave pointer 21 , the driver claw 23 strikes first with its stop 23 A on the slave pointer 21 . Now the main pointer 22 by its drive z. B. deflected even further clockwise relative to the slave pointer 21 , the latter is pulled clockwise by the main pointer 22 via the driver claw 23 into a new rest position, ie, is tracked ( FIG. 6a). The main pointer 22 now has a new asymmetrical margin with respect to the slave pointer 21 , which is as large on both sides as the previous margin, but is shifted clockwise with respect to this.

Now the main pointer 22 is pivoted counterclockwise ( Fig. 6b). Again, the slave pointer 21 of the movement of the main pointer 22 is tracked by the Mitnehmerkralle 23 as soon as the main pointer 22 its margin - this time in the counterclockwise direction - leaves; the slave pointer 21 is then again adjusted to a new rest position, etc. ( FIG. 6c). The drag pointer 21 only comes into action when the main pointer 22 leaves its scope.

This mechanical example is consistently transferred to sensor technology in the invention. The position of the main pointer 22 corresponds to the output voltage Ua, that of the drag pointer 21 to the reference voltage Uref. The margin of the main pointer 22 with respect to the drag pointer on both sides corresponds to the size of the first or second voltage spacing dU1, dU2 or the width of the "working window". The tracking of the trailing pointer 21 by the main pointer 22 thus corresponds to that of the reference voltage Uref by the output voltage Ua; In both cases, the tracking becomes active only when a predetermined scope between the tracking and tracking size has been exhausted. The switching point of the sensor is reached when Ua = Uref, which corresponds to the situation (not shown) in the mechanical analogy of FIG. 6, that the main and trailing pointers are overlapping.

Under the convention that a movement of the hands 21 and 22 in a clockwise direction of an increase in the voltage Uref and Ua and an opposite movement of the pointer corresponds to 21 and 22, a decrease of these voltages, the situation shown in Fig. 6 therefore corresponds to an actuated Sensor (Ua <Uref) and a shift (tracking) of the reference voltage Uref and the working window upwards. The situation in FIG. 6b corresponds to an unactuated sensor (Ua <Uref) and a "fixed" position of the reference voltage Uref and working window. The situation of Fig. 6c corresponds to an unactuated sensor (Ua <Uref) and a shift (tracking) of the reference voltage Uref and the work window down.

In the most common cases in the practice of sensor technology, the "working window" only when installing or removing the sensor in or out of the application postponed. The output voltage Ua then usually varies in the application itself only within the window widths dU1, dU2 by which the reference voltage Uref (= "Working point"). This is mostly due to the fact that the container wall at one Level application makes such a large contribution to the total capacity that also without medium, the output voltage Ua hardly ever drops to 0 volts, but only to a smaller value than the one who is fully covered by the Medium. The maxima and minima of the output voltage Ua are included most applications i. d. Usually only within the width of the respective "Working window". For each application, the two "work windows" automatically by tracking the reference voltage Uref according to the invention through the output voltage Ua to the correct value, the operating point is then automatically correct.

Some numerical values determined or advantageously set in practical operation of a capacitive level sensor are given below:

• 1. Ua = 0 V with sensor in the air (distance infinite),
  Ua = 7 V with sensor fully damped, maximum capacitance or distance to a grounded plate = 0
  first voltage distance: dU1 = 0.3 V
  second voltage gap: dU2 = 1.2 V
• 2. Level application of the same sensor with medium water and plexiglass container with 6 mm wall thickness: Ua = 6 V with fully covered sensor, Ua 4 , 4 V with empty container.
  Now the sensor is mounted on a full container. The operating point is shifted with Ua to Uref = 5.7 V (ie UGu = 4.5 V). The sensor switched on because Ua is above Uref. The fill level now slowly decreased. If the active sensor surface is covered by about half, Ua falls below the reference voltage Uref "fixed" at 5.7 V; the sensor switched off. When the vessel was empty, Ua now dropped to 4.4 V. The lower limit voltage UGu was 100 mV higher, namely UGu = 4.5 V. The working windows and Uref were therefore adjusted downwards by 100 mV. Uref was now automatically reduced to 5.6 V according to the invention. When the liquid level rose again, the sensor therefore switched on again at about 40% coverage. Due to the window shift, the switching point (working point) has only shifted slightly, the application is fully fulfilled.
• 3. Level application of the same sensor with medium plastic granulate beads in containers with 2 mm wall thickness made of plexiglass: Ua = 3.8 V when the container is full, Ua = 2.8 V when the container is empty.
  The sensor was mounted on a full vessel. The reference voltage was shifted with Ua to Uref = 3.5 V, the lower limit voltage to UGu = 2.3 V, the sensor switched on because Ua was 0.3 V above the automatically fixed 3.5 V reference voltage according to the invention , The level now dropped. If the active sensor area is covered by about half, the value falls below 3.5 V and the sensor therefore switches off. With a further decrease to the empty state, Ua reached the lowest value of 2.8 V. This value was therefore above the lower limit voltage UGu. There was therefore no tracking. When the level rose again, the sensor switched on again at Ua = 3.5 V and central coverage.

For a further general illustration of the automatic adjustment process (teach-in process) of a sensor according to the invention, FIGS . 7a, 7b show the associated time profiles of the voltages using the example of a fill level application. The sensor is initially in the air (time period t11-t12 in FIG. 7a, t18-t19 in FIG. 7b) and then (at time t12 in FIG. 7a, at time t19 in FIG. 7b) on a container mounted, which is to be monitored by the sensor for whether it is full or not. In particular, two extreme cases are possible and relevant in practice: The container to be monitored for its fill level can initially be full ( Fig. 7a) or empty ( Fig. 7b) after the sensor has been installed and commissioned.

In the example on which FIG. 7a is based, the sensor is mounted on an already full container at time t12. As a result, the output voltage Ua rises to the highest value for Ua occurring in the application, namely Ua _max . The reference voltage Uref and the limit voltages UGo, UGu are tracked according to the invention at time t12. According to the invention, the sensor is actuated correctly (switching signal high, container full) and is automatically correctly adjusted from time t12 without the need for an operation or external adjustment of the sensor. At time t13, the fill level and thus the output voltage Ua begin to drop. At time t14, Ua falls below the value of Uref: the sensor switches off correctly (switching signal low).

At time t15 the container is empty; Ua therefore reaches the minimum possible value in this application, namely Ua _min , which, however, in the present example is still greater than the lower limit voltage UGu, so that the reference voltage Uref and the limit voltages UGu, UGo are not tracked. Then the container is refilled in the present example (t15-tt17). At time t16, the output voltage Ua again reaches the value it had at time t14, namely Uref; the sensor switches on again.

In the example on which FIG. 7b is based, the sensor is mounted on an empty container at time t19. Due to the inherent capacity of the container wall and possibly other disturbing effects, the output voltage Ua does not remain at the value zero, but rather rises to the minimum value for Ua occurring in the application, namely to Ua _min . According to the invention, Uref and the limit voltages UGo, UGu are tracked, but this is not yet sufficient to finally adjust the sensor: Ua is larger than Uref between t19 and t21, so that the sensor trips even though the container is empty or not yet full is: First of all, there is a false trigger.

The filling of the container begins at time t20. Therefore, the fill level of the medium and thus Ua continue to increase; Uref and the "work window" are "dragged along", ie tracked according to the invention. At time t21 the container is full; Ua has reached the value Ua _max and does not rise any further, Uref and the "work windows" are "fixed", they "now sit correctly" and the sensor has adjusted itself permanently at this point at the latest. At time t21, the container begins to be emptied; Among other things, begins to fall. At time t22, Ua falls below the "fixed" value of Uref, the sensor switches off correctly (switching signal low). In the time interval t23 to t24, the container is empty, at t24 refilling begins at time t25 - the permanently "fixed" reference voltage Uref is exceeded - the sensor, which is still correctly adjusted, switches on again (switching signal high).

The voltage spacings dU1, dU2 are advantageously chosen such that the range given by dU1 and dU2 due to the fluctuation of the output voltage Ua between Ua _min and Ua _max (cf. mechanical drag pointer analogy in FIG. 6) is not exhausted: Uref remains for the entire further application of the sensor "fixed". The sensor must therefore, so to speak, go through the process once or "see" the medium once in order to adjust itself to it.

Fig. 8 is a schematic circuit diagram of Figure 4 shows a simple technical embodiment of the invention with an embodiment 10A of the tracking circuit 10 of Fig. With a first and a second diode D1, D2, and a charging capacitor C. The tracking circuit 10A includes, in this simple embodiment, no further blocks and acts as a voltage source, which generates the reference voltage Uref and outputs 31 B to the reference input. The two diodes D1, D2 are connected in anti-parallel between the output A 1 of the pickup 1 and the reference input of the comparator 31 B 31st The reference input 31 B of the comparator 31 is also connected to ground via the capacitor C.

The output voltage Vout may be within the pickup 1 through a signal path there in the same intermediate (not shown) non-inverting voltage follower, which has a voltage gain of 1 has buffered or low impedance at the output A 1 be provided. The output 1 A is connected to the signal input 31 A of the comparator 31 via the line L1 and the first resistor R1.

The forward voltages of the diodes D1, D2 produce the widths dU1, dU2 "Working window" of a total of approx. 1.2 V or 0.6 V each. The voltage Ua increases by more than 0.6 V via Uref at C, the first diode D1 becomes conductive and charges C. the value Ua-0.6 V = Uref. The forward voltage of the diode D1 thus corresponds to that Value of the upper voltage gap dU1 in the above. Presentation. The "work window" are now set. If Ua drops again, the diode D1 and the Voltage value Uref is held by the charging capacitor C1: Uref is "fixed".

If Ua falls below Uref, the sensor switches off (no switching signal or low signal on line 6 ). If Ua rises above Uref, the sensor switches on again. If Ua drops below the value UGu = Uref-0.6 V, the second diode D2 becomes conductive and the voltage at C follows Ua down until Ua remains: Uref is tracked according to the invention. The sensor is now not activated until Ua again exceeds Uref, so it has adjusted itself to a new application.

The width of the "working window" can be adjusted by inserting further diode sections (Additional diodes) varied in series with the diodes D1 and D2 in 0.6 V steps become. The advantage of this embodiment is a small amount of circuitry. Using a comparator in C-Mos or FET technology Uref stopped for a few minutes. The circuit is therefore suitable in excellent for dynamic applications in which a constant, periodic change in the measured variable is ensured.

FIG. 9 shows a further embodiment 10B according to the invention of a tracking circuit which has a simple digital binary counter 44 and a second and a third comparator 32 , 33 . Each of these comparators 32, 33 has a signal input 32 A and 33 A, a reference input 32 B and 33 B and a switching signal output 32 C and 33 C and are on this then and only then a switching signal when the at its signal input 32 A and 33 A the voltage applied is greater than the voltage applied to its reference input 32 B and 33 B voltage.

A positive voltage pole 7 is connected to ground via a series circuit of a resistor R3, a third diode D3, a fourth diode D4 and a resistor R4, the two diodes D3 connected, D4 in the forward direction so that of the positive terminal 7 via this Series connection R3, D3, D4, R4 a current flows to ground. The voltage of the voltage pole 7 is at least 0.6 V higher than the maximum output voltage Ua _{max of} the sensor 1 .

The output voltage Vout may be within the pickup 1 through a signal path there in the same intermediate (not shown) non-inverting voltage follower, which has a voltage gain of 1 has buffered or low impedance at the output A 1 be provided. The output 1 is A, as in Fig. 4 and Fig. 8, via the line L1 and placed the resistor R1 to the signal input 31 A of the first comparator 31, which also includes the reference input 31 B and the switching signal output 31 C over which the first comparator, a switching signal on the line L6 outputs 31 when the voltage applied at the signal input 31 of a voltage is higher than the berthing at the reference input 31 B voltage Uref. The tracking circuit 10 B acts as a voltage source, which generates the reference voltage Uref and outputs it to the reference input 31 B.

Between the two diodes D3, D4, a second line L2 branches off which is connected to the line L1, and thus to the output A 1 of the pickup 1 so that between the diodes D3, D4, the output voltage Vout is present. The signal input 31 A is also, as already shown in Fig. 4 and Fig. 8, 31 connected back through resistor R2 to the output of 31 C of the comparator, wherein the ohmic value of R2 is very much greater than that of R1 (R2 <<< R1).

A third line L3 branches off between the resistor R3 and the diode D3, which line is connected to the reference input 33 B of the third comparator 33 . Furthermore, a fourth line L4 branches off between the resistor R4 and the diode D4, which line is connected to the signal input 32 A of the second comparator 32 . Due to the forward voltages of the diodes D3, D4, the line L3 is at a voltage of Ua + 0.6 volts, the line L4 is at a voltage of Ua-0.6 volts.

The forward voltage of the fourth diode D4 forms the upper voltage gap dU1, d. H. the width of the upper "working window" (0.6 V), and the forward voltage of the third Diode D3 forms the lower voltage gap dU2, d. H. the width of the bottom "Working window" (also 0.6 V). These values can be added by inserting more Diode sections (additional diodes) can be increased in series with diodes D3 and D4.

The binary counter 44 has a plurality of z. B. six outputs Q1 to Q6, all of which are separately connected to a resistor 45 on an R or R2R network 45 . Each of the resistors 45 is connected to a fifth line L5, which is connected both to the signal input 33 A of the comparator 33 and to the reference input 32 B of the comparator 32 and to the reference input 31 B of the comparator 31 .

The binary counter 44 is used in conjunction with the R or R2R network 45, which is used for simple D / A conversion, to generate the reference voltage Uref. Via its outputs Q1 to Q6 and the network 45 , the binary counter outputs a voltage on a line L5 which is proportional to an internal counter reading of the binary counter 44 and is therefore variable in stages. 6 bits are sufficient to subdivide this voltage range into 64 steps (the more the better). This voltage is used as the reference voltage Uref and is applied via the line L5 both to the signal input 33 A of the third comparator 33 and to the reference input 32 B of the second comparator 32 and to the reference input 31 B of the first comparator 31 .

The inventive tracking of the reference voltage Uref generated by the binary counter 44 is realized in the embodiment of FIG. 9 as follows.

A clock generator 41 outputs via its output 41 A clock pulses to the one, first AND input 42 A of an AND gate 42 , the logic AND output 42 C of which is applied to the count input 44 A of the binary counter 44 . The binary counter 44 can be switched over a direction input 44 B between up and down counting mode (ie between incrementing and decrementing the internal counter reading); it is in the up-count mode when a high signal is present at direction input 44 B, and otherwise in the down-count mode. With each clock or pulse at the counter input 44 A, the voltage Uref on the R network (line L5) then increases by 1/64 in the forward counting mode or decreases by 1/64 in the downward counting mode. The counter reading can only vary between the value zero (no voltage is generated) and the value 63 (the highest possible voltage is generated).

The switching signal output 32 C of the comparator 32 is connected to the direction input 44 B of the binary counter 44 and also to the one OR input 43 B of an OR gate 43 . The output 33 C of the comparator 33 is connected to the other OR input 43 A of the OR gate 43 .

The logical OR output 43 C of the OR gate 43 is connected to the other AND input 42 B of the AND gate 42 . The AND gate 42 outputs via its logical AND output 42 C the clock pulses of the clock generator 41 to the counter input 44 A of the binary counter 44 , provided that a switching signal is present at the other, second AND input 42 B of the AND gate.

Now the voltage (Ua-0.6 V) across resistor R4 (and thus via line L4 also at signal input 32 A of comparator 32 ) rises above the value of Uref (line L5), which means that Ua becomes greater than Uref + 0.6 V, ie Ua exceeds the upper limit voltage UGo = Uref + 0.6 V, the comparator 32 switches on: it outputs a switching signal via its output 32 C. This passes through the OR gate 43 to the second AND input 42 B of the AND gate 42 with the result that the clock pulses of the clock generator 41 reach the counter input 44 A of the binary counter. At the same time, however, as stated above, the output 32 C of the comparator 32 is connected to the direction input 44 B of the binary counter 44 , with the result that the binary counter 44 is put into the up-counting mode by the comparator 32 , ie the internal counter reading and thus Uref increase until the comparator 32 switches off. Uref is therefore tracked according to the invention by the increase in Ua at a distance dU1 = 0.6 V.

The reference voltage Uref and the two "work windows" were thus pushed up and Uref "fixed". Ua can now move within the range spanned by dU1, dU2 and, depending on the instantaneous size of the output voltage Ua, trigger a switching signal "sensor actuated" or not via the comparator 31 .

The third comparator 33 is switched off in this state (ie it does not emit a switching signal at its switching signal output 33 C), since the voltage on line L3, ie at the reference input 33 B of the comparator 33 (Ua + 0.6 V), is higher is as uref. As long as Ua does not leave the above-mentioned margin, both the second comparator 32 and the third comparator 33 are switched off; neither of them emits a switching signal. A high signal is therefore not present at any of the inputs 43 A, 43 B of the OR gate 43 , ie no high signal is also present at the second input 42 B of the AND gate 42 . The pulses of the clock generator 41 therefore do not reach the counting input 44 a of the binary counter, with the result that Uref remains unchanged: Uref remains "fixed".

If the output voltage Ua now drops by more than 0.6 V below the reference voltage Uref, ie Ua leaves the "scope", the voltage applied to reference input 33 B of the third comparator 33 falls below the value (= Ua + 0.6 V) the previously "fixed" reference voltage Uref. The comparator 33 outputs via its output 33 C a switching signal via the OR gate 43 to the second AND input 42 B of the AND gate 42 , whereby the clock of the clock generator 41 is enabled on the count input 44 A. No switching signal is present at direction input 44 B since second comparator 32 is switched off. The binary counter 44 thus counts down, the internal counter reading decrements, the reference voltage Uref drops until it falls below the value at the reference input 33 B; the third comparator 33 then switches off, the counter reading remains, Uref and the “work windows” are shifted downward and the reference voltage Uref is “fixed” to a new value.

A particular advantage of the embodiment of FIG. 9 is its ability to store the last generated working point, ie the "fixed" value of Uref, as long as the counter 44 does not change its count as long as Ua remains in the "scope". If the supply voltage of the meter is buffered, for example using a C-Mos module with the lowest possible current consumption and storage capacitor, the last value of Uref or meter reading is retained even after the supply voltage has been removed. The last meter reading can also be saved in an EE-Prom and thus be available again after a complete shutdown of the system. The arrangement can be realized with inexpensive components. No microcontroller and no software is required.

FIG. 10 illustrates an embodiment of the invention with an analog-digital converter 50 and an EDP device 51 , which in particular can be a microprocessor. The output A 1 of the pickup 1 is to write the output voltage Ua which is applied to the analog-to-digital converter 50th This digitizes the output voltage Ua and assigns it a digital output value Wa, which is read into the EDP device 51 via a bus 55 . In software, this compares the output value Wa with a reference value Wref stored in the EDP device 51 in a memory 54 and emits a switching signal via an output line 52 if the output value Wa is greater than the reference value Wref. Furthermore, an upper distance value dW1 and a lower distance value dW2 are stored in the memory 54 .

The EDP device 51 is set up so that it the reference value Wref

• a) holds constant if the initial value Wa both
  • is less than or equal to an upper limit value WGo, which is given by the sum of the reference value Wref plus the first distance value dW1,
    as well as
  • is greater than or equal to a lower limit value WGu, which is given by the difference between the reference value Wref minus the second distance value dW2,
  • - or increases if the initial value Wa is greater than the upper limit value WGo,
  • - or lowered if the output value Wa is less than the lower limit value WGu.

According to a further variant of the invention, the tracking circuit comprises one Microprocessor and an A / D converter. The voltage Ua of the transducer is digitized here with the analog / digital converter. The invention Tracking is in the microprocessor z. B. by calculating the reference voltage Uref from Ua-dU1 = Uref and the lower limit voltage UGu reproduced. Also the shift and comparator function is compared by the microprocessor carried out by Uref and Ua with hysteresis. The microprocessor reads constantly Cycles the digitized values of Ua and delivers the switching signal directly "Sensor activated" (eg high signal) or "Sensor not activated" (eg low signal).

In this embodiment, all degrees of freedom are given. The widths dU1, dU2 of the "work window" can be specified as desired. Furthermore, a linear characteristic curve of the transducer according to FIG. 1 is not absolutely necessary, but a steady increase or decrease of Ua z. B. with the level in a container or the distance to an object to be detected. The microprocessor then adjusts the width of the "work window" depending on the Ua to the respective slope of the characteristic curve. Furthermore, an adaptation to any output voltage range Ua _min to Ua _{max of} any sensors is possible. The last operating point Uref is saved when the supply voltage is interrupted, for. B. automatically in an EE-Prom. When using a capacitive transducer 1 with a linear characteristic, the software effort is particularly low. With a stroke of Ua from 0 (sensor in the air) to 7 V (distance to the metal plate = 0) z. B. for dU1 of a value of 0.3 V, for dU2 a value of 0.6 V to 1.2 V has proven to be particularly favorable for a large number of applications.

Industrial applicability

The method according to the invention is suitable in capacitive sensors and. a. For Level applications of liquid and solid media, automatic readjustment in the case of adhering media, for example when querying glues and varnishes which increases the layer thickness on the container wall with the filling at which Double sheet control for paper polling, capacitive buttons, distance polling with background suppression, label query, for example with a Slot sensor, and the like.

The leading figure is Fig. 8.

List of reference numbers

1

pickup

1

A exit from

1

1

B active sensor surface

3

potentiometer

7

voltage pole

10

.

10

A,

10

B tracking circuits

21

Peak values

22

main pointer

23

Mitnehmerkralle

23

A,

23

B stops of

23

24

turning center

31

.

32

.

33

first, second, third comparator

31

A,

32

A,

33

A signal inputs from

31

.

32

.

33

31

B

32

B

33

B Reference inputs from

31

.

32

.

33

31

C.,

32

C.,

33

C switching signal outputs from

31

.

32

.

33

41

clock generator

42

AND gate

42

A,

42

B

42

C first, second AND input of

42

, AND output from

42

43

OR gate

43

A,

43

B

43

C first, second OR input of

43

, OR output from

43

44

binary counter

44

A,

44

B Counter input, direction input from

44

45

Resistor network

50

Analog to digital converter

51

EDP device

52

Output management from

51

54

Storage

55

bus
D1-D4 diodes
L1-L7 first to seventh lines
Q1-Q6 outputs from

44

R1-R6 resistors

Claims (29)

1. A method for the automatic self-adjustment of a sensor, comprising a sensor ( 1 ) with an output ( 1 A) which outputs an output voltage (Ua), and a first comparator ( 31 ) to which the output voltage (Ua) and a reference voltage (Uref) are applied, the first comparator ( 31 ) emitting a switching signal if the output voltage (Ua) is greater than the reference voltage (Uref) and otherwise not emitting a switching signal, or vice versa, characterized in that
that the reference voltage (Uref) of the output voltage (Ua) is automatically updated as follows:

• a) If the output voltage (Ua) both
  is less than or equal to an upper limit voltage (UGo), which is given by the sum of the reference voltage (Uref) plus a first voltage distance (dU1),
  as well as
  is greater than or equal to a lower limit voltage (UGu), which is given by the difference of the reference voltage (Uref) minus a second voltage distance (dU2),
  so the reference voltage (Uref) is kept constant,
• b) if the output voltage (Ua) is greater than the upper limit voltage (UGo), the reference voltage (Uref) is raised, and
• c) if the output voltage (Ua) is lower than the lower limit voltage (UGu), the reference voltage (Uref) is reduced.

2. A method for the automatic self-adjustment of a sensor, comprising a sensor ( 1 ) with an output ( 1 A) which outputs an output voltage (Ua), an analog-to-digital converter ( 50 ) to which the output voltage (Ua) is applied which assigns a digital output value (Wa), and an EDP device ( 51 ) into which the digital output value (Wa) is read and which compares the output value (Wa) with a reference value (Wref) and emits a switching signal, if the output value (Wa) is greater than the reference value (Wref) and otherwise does not emit a switching signal, or vice versa, characterized in that
that the reference value (Wref) is automatically tracked to the initial value (Wa) as follows:

• a) If the initial value (Wa) is both less than or equal to an upper limit value (WGo), which is given by the sum of the reference value (Wref) plus a first distance value (dW1),
  as well as
  is greater than or equal to a lower limit value (WGu), which is given by the difference of the reference value (Wref) minus a second distance value (dW2),
  so the reference value (Wref) is kept constant,
• b) if the initial value (Wa) is greater than the upper limit value (WGo), the reference value (Wref) is increased, and
• c) if the output value (Wa) is less than the lower limit value (WGu), the reference value (Wref) is lowered.

3. The method according to claim 1 or 2, characterized in that the reference voltage (Uref) is only raised or lowered when the output voltage (Ua) for a first specifiable or second specifiable Response time (dT1, dT2) has remained longer or shorter than the upper or lower limit voltage (UGo, UGu), or the reference value (Wref) only then is raised or lowered when the initial value (Wa) for a first predefinable or second predefinable response time (dT1, dT2) greater or has remained smaller than the upper or lower limit (WGo, WGu). 4. The method according to claim 1 or 2, characterized in that the sensor is a capacitive or an inductive or an acoustic sensor or a sensor for electromagnetic radiation, e.g. B. optoelectronic sensor, or a mechanical push button or a capacitive push button.   5. The method according to claim 1 or 2, characterized in that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) are kept constant over time. 6. The method according to claim 1 or 2, characterized in that the first and / or the second voltage spacing (dU1, dU2)) or the first and / or the second distance value (dW1, dW2) can be changed over time 7. The method according to claim 5, characterized in that that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) depending on the time Behavior of the output voltage (Ua) or the output value (Wa) and / or the reference voltage (Uref) or the reference value (Wref) is changed. 8. The method according to claim 7, characterized in that that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) as a function of one time averaged average of the amount of the slope of the Output voltage (Ua) or the output value (Wa) and / or the Reference voltage (Uref) or the reference value (Wref) increased or be made smaller. 9. The method according to any one of claims 1 to 7, characterized in that the rate of change of the reference voltage (Uref) or the Reference value (Wref) is limited to a predeterminable maximum value. 10. The method according to claim 9, characterized in that the maximum value is changed over time. 11. The method according to any one of claims 1 to 10, characterized in that that the first and / or the second response time period (dT1, dT2) is dependent the time behavior of the output voltage (Ua) or the output value  (Wa) and / or the reference voltage (Uref) or the reference value (Wref) to be changed. 12. Self-adjusting sensor, comprising a sensor ( 1 ) with an output ( 1 A), which outputs an output voltage (Ua), and a first comparator ( 31 ) with a signal input ( 31 A), to which a first line (L1 ) the output voltage (Ua) is applied, and a reference input ( 31 B) to which a reference voltage (Uref) emitted by a voltage source ( 10 , 10 A, 10 B) is applied, the first comparator ( 31 ) emitting a switching signal , if the output voltage (Ua) is greater than the reference voltage (Uref) and otherwise does not emit a switching signal, or vice versa,
characterized in that the voltage source ( 10 , 10 A, 10 B) is a tracking circuit ( 10 , 10 A, 10 B) to which the output voltage (Ua) is also applied and which the reference voltage (Uref)

• a) holds constant if the output voltage (Ua) is both less than or equal to an upper limit voltage (UGo), which is given by the sum of the reference voltage (Uref) plus a first voltage distance (dU1),
  as well as
  is greater than or equal to a lower limit voltage (UGu), which is given by the difference of the reference voltage (Uref) minus a second voltage distance (dU2),
• b) or increases if the output voltage (Ua) is greater than the upper limit voltage (UGo),
• c) or reduced, if that of the output voltage (Ua) is less than the lower limit voltage (UGu).

13. Self-adjusting sensor, comprising a transducer ( 1 ), with an output ( 1 A), which outputs an output voltage (Ua), an analog-to-digital converter ( 50 ), to which the output voltage (Ua) is applied and which of these assigns a digital output value (Wa), and an EDP device ( 51 ) into which the digital output value (Wa) can be read and which assigns the output value (Wa) with a reference value (Wref) stored in the EDP device ( 51 ) is able to compare and emits a switching signal if the output value (Wa) is greater than the reference value (Wref), and otherwise does not emit a switching signal, or vice versa, characterized in that
that the EDP device ( 51 ) the reference value (Wref)

• a) holds constant if the initial value (WUa) is both less than or equal to an upper limit value (WGo), which is given by the sum of the reference value (Wref) plus a first distance value (dW1),
  as well as
  is greater than or equal to a lower limit value (WGu), which is given by the difference of the reference value (Wref) minus a second distance value (dW2),
• b) or increases if the initial value (Wa) is greater than the upper limit value (WGo),
• c) or reduced if the initial value (Wa) is less than the lower limit value (WGu).

14. Sensor according to claim 12 or 13, characterized in that the tracking circuit ( 10 , 10 A, 10 B) only raises or lowers the reference voltage (Uref) when the output voltage (Ua) for a first or second predetermined Response time (dT1, dT2) has remained greater or smaller than the upper or lower limit voltage (UGo, UGu), or the EDP device ( 51 ) only increases or decreases the reference value (Wref) when the output value (Wa ) has remained larger or smaller than the upper or lower limit value (WGo, WGu) for a first predefinable or second predefinable response time period (dT1, dT2). 15. Sensor according to claim 12 or 13, characterized in that that the sensor is a capacitive or an inductive or an acoustic sensor or a sensor for electromagnetic, e.g. B. optoelectronic sensor, or a mechanical push button or a capacitive push button.   16. Sensor according to claim 12 or 13, characterized in that that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) is constant over time. 17. Sensor according to claim 12 or 13, characterized in that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) can be changed over time. 18. Sensor according to claim 17, characterized in that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) depending on the time Behavior of the output voltage (Ua) or the output value and / or the Reference voltage (Uref) or the reference value (Wref) are changeable. 19. Sensor according to claim 18, characterized in that the first and / or the second voltage spacing (dU1, dU2) or the first and / or the second distance value (dW1, dW2) as a function of one time averaged average value of the amount of the slope Output voltage (Ua) or the output value (Wa) and / or the Reference voltage (Uref) or the reference value (Wref) increase or lose weight. 20. Sensor according to claim 12 or 13, characterized in that that the first and second voltage spacing (dU1, dU2) the forward voltage of a Diode (D1, D4, D2, D3) or a series connection of diodes. 21. Sensor according to one of claims 12 to 20, characterized in that that the rate of change of the reference voltage (Uref) or the Reference value (Wref) is limited to a predeterminable maximum value. 22. Sensor according to claim 21, characterized in that the maximum value is variable over time.   23. Sensor according to one of claims 12 to 22, characterized in that that the first and / or the second response time period (dT1, dT2) is dependent the time behavior of the output voltage (Ua) or the output value (Wa) and / or the reference voltage (Uref) or the reference value (Wref) are changeable. 24. Sensor according to claim 12, characterized in that the tracking circuit ( 10 , 10 A) is an analog circuit ( 10 A) without digital components or a digital circuit without a microprocessor. 25. Sensor according to claim 24, characterized in that the tracking circuit ( 10 , 10 B) comprises a first and a second diode (D1, D2) and a charging capacitor (C), the two diodes (D1, D2) antiparallel between the Output ( 1 A) of the transducer ( 1 ) and the reference input ( 31 B) of the comparator ( 31 ) are switched and the reference input ( 31 B) of the comparator ( 31 ) is connected to ground via the charging capacitor (C). 26. Sensor according to claim 12, characterized in that the tracking circuit ( 10 A) comprises the following components:

• a) a second and a third comparator (32, 33) each having a signal input (32 A, 33 A), each having a reference input (32 B, 33 B) as well as one shift signal output (32 C, 33 C), wherein the second or third comparator ( 32 or 33 ) emits a switching signal via its output ( 32 C or 33 C) only when a higher voltage is present at its signal input ( 32 A or 33 A) than at its Reference input ( 32 B or 33 B),
• b) a series circuit (R3, D3, D4, R4) of a third resistor (R3), a third diode D3, a fourth diode (D4) and a fourth resistor (R4), the third resistor (R3) with its one of the third diode (D3) facing away from a voltage pole ( 7 ) and the fourth resistor (R4) with its terminal facing away from the fourth diode (D4) is connected to ground and the third and fourth diodes (D3, D4) in each case Forward direction are switched so that a current flows from the voltage pole ( 7 ) via this series circuit (R3, D3, D4, R4) to ground,
• c) a second line (L2), which branches off between the third and the fourth diode (D3, D4) of the series connection (R3, D3, D4, R4) and connected to the output (1 A) of the transducer (1) is , so that the output voltage (Ua) is present between these diodes (D3, D4),
• d) a third line (L3), which branches off between the third resistor (R3) and the third diode (D3) from the series circuit (R3, D3, D4, R4) and to the reference input ( 33 B) of the third comparator ( 33 ) is created,
• e) a fourth line (L4), which branches off between the fourth resistor (R4) and the fourth diode (D4) from the series circuit (R3, D3, D4, R4) and to the signal input ( 32 A) of the second comparator ( 32 ) is created,
• f) a binary counter (44) with a counting input (44 A), a directional input (44 B), outputs (Q1-Q6), and connected with a to these outputs R or R2R network with resistances 45, wherein the direction input (44 B) is connected to the switching signal output ( 32 C) of the second comparator ( 32 ) and the binary counter ( 44 ) increments an internal counter reading if a clock pulse arrives at its counter input ( 44 A) and on at its direction input ( 44 B) high signal is present, and decrements the internal counter reading, if appropriate, (44 a) arrives at its counting input a clock pulse and, at its towards the entrance (44 B) a low signal is applied, and the binary counter 44 via its outputs (Q1-Q6), and Resistor network ( 45 ) is able to deliver a voltage (Uref), which is variable in steps and proportional to the internal meter reading, to a fifth line (L5), which is connected to the signal input ( 33 A) of the third comparator ( 33 ) and to the reference current inputs ( 32 B, 31 B) of both the second and the first comparator ( 32 , 31 ) are created,
• g) an AND gate ( 42 ) with a first and a second AND input ( 42 A, 42 B) and a logical AND output ( 42 C), which is applied to the counter input ( 44 A) of the binary counter ( 44 ) is
• h) a clock generator ( 41 ) which emits clock pulses via its clock output ( 41 A), the clock output ( 41 A) being connected to the first AND input ( 42 A) of the AND gate ( 42 ), and
• i) an OR gate ( 43 ) with a first and a second OR input ( 43 A, 43 B) and a logic OR output ( 43 C) which is connected to the second AND input ( 42 B) of the AND Gate ( 42 ) is connected, the switching signal output ( 33 C) of the third comparator ( 33 ) to the first OR input ( 43 A) and the switching signal output ( 32 C) of the second comparator ( 32 ) to the second OR input ( 43 B) is connected.

27. Sensor according to claim 25 or 26, characterized in that that to the first or second or third or fourth diode (D1, D2, D3, D4) at least one first or second or third or fourth additional diode in series is switched, the forward direction of which of the first or second or third or fourth diode (D1, D2, D3, D4) matches. 28. Sensor according to one of claims 12 to 21, characterized in that the tracking circuit ( 10 ) comprises an analog / digital converter and a digital circuit, the output voltage (Ua) being applied to the analog / digital converter and this the output voltage digitize and output in digital form to the digital circuit, which is able to calculate the reference voltage (Uref) depending on the output voltage (Ua) or its temporal behavior and is able to deliver it to the reference input ( 31 B) of the first comparator ( 31 ) , 29. Sensor according to claim 12, characterized in that the tracking circuit ( 10 ) comprises a seventh resistor through which a first direct current flows and / or an eighth resistor through which a second direct current flows and that the voltage drop across the seventh or eighth resistor is the first or second voltage spacing (dU1, dU2) is used.