GB2218815A - Calibration of signal conditioning equipment - Google Patents

Abstract

The voltage applied to signal conditioning equipment such as an amplifier (4) is adjusted for calibration purposes by driving additional current through a resistor (6) that forms a permanent part of the input circuit. The input voltage to the amplifier is measured at access points (5c, 5d) and the current driven through the resistor (6) may be supplied by an arrangement 7-10, Figure 2. Trip and alarm settings may be tested in this way. The apparatus may be used with thermocouples as shown or with transducers in the form of variable resistances or potentiometers. <IMAGE>

Description

CALIBRATION OF SIGNAL CONDITIONING EQUIPMENT This invention relates to the calibration of signal conditioning equipment with analogue inputs. Such equipment may form a part of a system for the monitoring or control of an industrial process or distribution network.

For the purposes of the present specification, signal conditioning equipment is defined as that which accepts signals from transducers that are responsive to physical variables. Such signals are typically of the order of a few millivolts to a few volts. The aforesaid equipment will either filter, isolate or amplify these signals and may then transmit them to control or supervisory apparatus located nearby or at a distance. The transmitted signal may be analogue, multi-bit digital, or in the form of a simple digital signal that indicates a 'safe' or 'unsafe' condition. In all these situations there are parameters of the signal conditioning equipment such as the amplification characteristics - often known as the 'span' and 'zero' settings and the threshold of the safe condition - often known as the 'trip' or 'alarm' setting - whose values must be established with confidence.These parameters may need to be changed from time to time during the commissioning or subsequent modification of the associated plant. Such plant might comprise an industrial process such as that of an oil refinery or a distribution network such as that linking water catchment with consumers.

Hence there is a need for a convenient method for the adjustment and verification of such parameters. Since in many cases the associated plant must remain in continuous operation, the method should take the minimum time and cause the minimum physical or electrical disturbance to the instrument system.

According to the present invention there is provided a method for the calibration of signal conditioning equipment as hereinbefore defined that includes obtaining a chosen internal input test voltage by firstly modifying the pre-existing external input voltage, said modification being effected by connecting an adjustable current source across a resistance connected in series with the input of said signal conditioning equipment, secondly measuring the voltage at said internal input, and thirdly adjusting said current source so as to obtain said chosen internal input voltage.

According to another aspect of the present invention there is provided a calibration device including first a resistance with first access points connected at each of its terminals, said resistance being also connected in series with the input of signal conditioning equipment as thereinbefore defined, said access points being arranged to enable the injection of current so as to modify the voltage presented to the internal circuits of said equipment, and second access points at the input of said internal circuits arranged to enable the measurement of said modified voltage.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 shows a diagram of the input circuit of signal conditioning equipment adapted to include a calibration device according to the invention.

Figure 2 shows the circuit diagram of input calibration apparatus according to the invention.

Figure 3 shows a diagram of the output circuit of a signal conditioning amplifier adapted according to one aspect of the invention.

Referring to Figure 1, a thermocouple 1 is shown connected via a cold junction 2 to a signal conditioning equipment input terminal strip 3 at which the external input voltage is developed. Said terminal strip is connected internally to the input amplifier 4 via the resistor 6. To implement the present invention the resistor 6 (which is a permanent part of the equipment and which might also serve other circuit functions) is connected by access points 5a and 5b to an adjustable source of current such as that shown in Figure 2. Said current source uses a battery 7 and may have its polarity set by the reversing switch 8 and its magnitude set by resistors 9 and 10 so that the voltage across resistor 6 in cooperation with the voltage of the thermocouple 1 and its cold-junction 2 establish the desired input voltage to the amplifier 4.This desired voltage is measured by the millivoltmeter 11 in Figure 2, connected via access points 5c and 5d.

It will be seen that this method of establishing a desired amplifier input voltage has the following advantages: a) Provided that both the millivoltmeter 11 and the amplifier 4 have sufficiently high input resistance, the emf developed across the thermocouple is unchanged by the calibration process.

Therefore, any other signal conditioning equipment connected thereto will function without disturbance.

b) The normal operational circuit of 1, 2, 3, 4 and 6 is not interrupted during calibration. Therefore, there can be no doubt about its correct restoration after calibration.

c) The access connections 5c and 5d do not carry any significant current. Therefore, the resistance of these connections will not cause error.

If the signal conditioning equipment is a trip unit whose set threshold is to be verified, then the above described apparatus is used as follows. The source of current is adjusted by switch 8 and resistors 9 and 10 so that the threshold is just reached and the condition signal observed to change state, and the millivoltmeter 11 is then read and the equivalent temperature determined from the known characteristics of the thermocouple and the cold-junction temperature.

If the signal conditioning equipment is an amplifier whose output is to be transmitted in analogue form, then the addtional measurement facility shown in Figure 3 may be provided, wherein further access points 5e and 5f in the current-output circuit of output amplifier 12 allow the voltage developed across resistor 13 to be measured and the output current calculated.

During the use of the calibrator circuit of Figure 2, the injected current may be adjusted to obtain some desired millivolt value on the indicator 11. It is within the scope of this invention to replace switch 8 and resistors 9 and 10 by an automatic current adjusting circuit which will use error feedback to set the millivolt value as desired.

The millivolt readings may be translated into the equivalent physical values by scaling and linearising circuits associated with the indicator 11 so as to simplify the operation of the device.

It will be evident that the principles described may be applied in other situations. For example, electronic methods of cold-junction compensation are known wherein the thermocouple is connected directly to the terminal block of the defenectent-ensatin9 emf signal-conditioning equipment, and a temperaturejcomp is introduced into the amplifier circuit. In this case, access point 5c may be connected so as to include or exclude this emf as may be desired. If the trnnsducer is a potentiometer or a variable resistance then 3 wire or 4 wire input connections are known, and additional calibration access points may be provided to enable the measurement of the excitation voltage or current.

Claims (9)

1. A method for the calibration of signal conditioning equipment as hereinbefore defined that includes obtaining a chosen internal input test voltage by firstly modifying the pre-existing external input voltage, said modification being effected by connecting an adjustable current source across a resistance connected in series with the input of said signal conditioning equipment, secondly measuring the voltage at said internal input, and thirdly adjusting said current source so as to obtain said chosen internal input voltage.

2. A method according to claim 1 in which the output of said equipment is also measured.

3. A calibration device including first a resistance with first access points connected at each of its terminals, said resistance being also connected in series with the input of signal conditioning equipment as hereinbefore defined, said access points being arranged to enable the injection of current so as to modify the voltage presented to the internal circuits of said equipment, and second access points at the input of said internal circuits arranged to enable the measurement of said modified voltage.

4. A device according to claim 3 in which one of the first access points shares the same physical conductor as one of the second access points.

5. A device according to claims 3 or 4 in which additional access points are provided to facilitate the measurement of the output of said signal conditioning equipment.

6. A device according to any of the above claims which also includes means for the injection of said current and means for the measurement of said modified voltage.

7. A device according to claim 6 in which said current injection means are controlled automatically so as to establish a desired value of said modified voltage.

8. A device according to claims 6 or 7 in which scale conversion and linearisation means are provided in order that said modified voltage may be expressed to or by the user in physical units appropriate to the signal that is to be conditioned.

9. Apparatus according to Figure 1, 2 or 3 of the accompanying drawing.