GB2161932A - Acoustic distance sensing system - Google Patents

Abstract

In apparatus for sensing distances in air by timing delay from transmission of an ultrasound or sonic pulse to the return of its echo variable amplification is applied to the received echo signal. The amplification is determined by digital data held in a memory (38) addressed by a counter (40,41) which is incremented progressively after transmission of the pulse. The data at the addressed location determines amplification by a digital to analogue converter (34). The change with time follows an experimentally determined curve to compensate for the attenuation in air of the pulse and its echo so that weaker spurious echoes are discriminated against. <IMAGE>

Description

SPECIFICATION Distance sensing This invention relates to the sensing of distances in air, by means of ultrasound (or possibly audible sound) which is transmitted as a pulse from a distance sensing device and reflected back to it by an object at a distance from the device. The time from transmission of the pulse to receipt of its echo is measured, and provides an indication of the distance. Equipment for this purpose is already known, for example, the Sonarange device made by Freedom Engineering Limited, the apparatus described in U.K. patent specification No. 2 082 325A of Sonic Tape PLC, and that described in German OLS 2 755 556A of Dieter Haffer.

In general the device will be used to measure distances. One particular application of such devices is for building-surveyors and estate agents, measuring the rooms within a building. The invention is also applicable however to devices which sense whether the distance to a reflecting object is above or below a prescribed limit value, or between a pair of limits, without generating an actual measurement. Devices to assist the reversing of heavy vehicles by detecting obstructions within a specified range would fall within this category. The invention in also applicable to ultrasonic rangefinders for cameras, where the sensing device does not produce any measurement read out, but is coupled to operate the camera focus automatically.

With such apparatus it is desirable to exclude echoes from objects which are not on the centre axis of the transmitted pulse, sine these can give rise to false measurement values. For exclusion of such spurious echoes, it is known to apply variable amplification to the received echo. The amount of amplification is made to increase with time following the transmission of an ultrasonic pulse.

U.K. Application No. 2 082 325 suggests use of a voltage ramp generator to produce a linearly increasing signal which in turn controls amplification to increase linearly with time. In fact a variable amount of attenuation is made to decrease linearly with time: the effect is equivalent. Use of a logarithmic change with time is also suggested. Either way, the varying amplification is generated by analogue circuitry.

We have now appreciated that it is both possible and desirable to provide a better match of the amount of amplification to what is required. This is accomplished in this invention by utilising stored information.

According to this invention there is provided apparatus for sensing distances in air comprising means for transmitting a sonic or ultrasonic signal, means to receive an echo of the signal and convert it into an electrical signal, variable amplification/attenuation means to amplify and/or attenuate received signals by an amount dependent on the time elapsed from transmission of the pulse, and means to measure or otherwise sense the time elapsed between transmission of the said sonic or ultrasonic signal and return of the echo, characterised in that the variable amplification/attenuation means comprises means to store information associated with the amplification and/or attenuation required at a multiplicity of times following transmission and means to utilise the said stored information to apply the required amplification and/or attenuation to received signals.The apparatus may have means to display a digital distance measurement derived from the measured time. Normally the overall amplification of received signals will increase with the time elapsed since transmission.

The transmitted signal preferably consists of one, or a succession, of pulses of sound or ultrasound, each pulse containing a number of cycles of the transmitted frequency or frequencies. Such a pulse might also be termed a "burst".

By means of the invention the overall amplification of received signals can be made to increase progressively over a period of time following transmission, with the amount of amplification chosen so as to counteract fairly closely the attenuation with distance of the ultrasonic or sonic signal. The increase in amplification with time can be made to follow a theoretical curve. or a curve determined experimentally.

Theoretically it can be expected that the amplification should be increased in proportion to the square of the distance travelled, which is also is proportion to the square of the time elapsed.

We have found that a curve of required voltage amplification of the electrical signal against distance, measured with particular transmitting and receiving transducers, did approximately increase with the square of the distance.

The stored information could possibly be such as to provide a continuous variation in the amplification or attenuation. Preferably however the stored information is digital information associated with a multiplicity of time intervals.

The preferred arrangement uses a semi-conductor memory which may be a read-only memory.

Addresses in the memory hold digital data which determines the amount of amplification/attenuation for a particular time interval. The addresses are read in turn and so the amount of amplification/attenuation varies as the addresses are read. The amplification/attenuation may be effected by a digital-to-analog converter which receives the echo signal(s) and the digital data read from the addresses.

Conveniently the data for each time interval is held in a single address, and an address counter is incremented (or decremented) for each time interval. However data for one time interval could possibly be held at two or more addresses read in succession. The address counter may increment or decrement through a range of addresses and stop at the final address reached, so that after the address counter reaches the final address, all subsequent echo signals are subjected to the same amplification/attenuation.

The apparatus may be arranged to repeat the transmission of pulses automatically, resetting an address counter each time a pulse is transmitted.

If the apparatus is arranged to transmit repeated pulses automatically, it may provide a display of a distance measurement derived from an echo of the most recent pulse transmitted, or it may display a figure which is an average from echoes of several pulses.

It is also known to reduce spurious echoes by transmitting the ultrasonic pulses through a horn or sound lens to make the transmitted beam directional. Echoes may likewise be received through a directional horn or sound lens. (The same transducer and horn may be used for both).

This expedient may also be used with this invention if desired, although the need to do so may be rather less.

Embodiments of this invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram schematically indicating the circuitry of a first form of distance measuring device; Figure 2 shows a modification; Figure 3 is a circuit diagram for the variable amplification/attenuation stage; and Figure 4 is a diagram corresponding to Fig. 1 for a distance measuring device which incorporates a microprocessor.

As shown by Fig. 1, the output of a crystal oscillator 10 running at somewhat above 100 KHz is fed to a decades counter 1 2 and to a dividing circuit 14. This divides the oscillator signal to produce a low frequency signal of around 5 Hz which provides the 200 m sec. interval at which ultrasonic pulses are transmitted.

The 5 Hz signal triggers a monostable 1 6 to produce a reset pulse (hereinafter designated RESET) which initiates the transmission of an ultrasonic pulse. As will be explained below RESET causes a "transmit enable" pulse (hereinafter designated TREN) to be generated after a short delay. This TREN pulse enables a 40 KHz oscillator 1 8 whose output signal is amplified at 20 and supplied to an output transducer 22. The 40 KHz signal is also fed to a counter 26 which counts the cycles until twenty-four cycles have been generated, whereupon the counter 26 inhibits the 40 KHz oscillator, terminating the transmitted pulse and holding the oscillator 18 inhibited until the next TREN pulse. The counter 26 should of course be reset between TREN pulses. Fig. 2 shows a possible modification.The TREN pulse triggers a monostable 1 7 to produce a 0.6 m sec. pulse which enables the 40 KHz oscillator for its duration. The counter 26 is omitted.

Any echo is received by transducer 30. The echo signal passes through a circuit 32 functioning as an analog switch. This circuit is inhibited by TREN, and a contains an RC network which extends the period of inhibition so that no echo can pass during a period from the start of transmission to slightly after the end of transmission. This eliminates spurious echoes by suppressing all echoes from within a minimum range of approx 45 cm.

From the analog switch circuit 32 the echo signal(s) pass into the variable amplifying circuitry of Fig. 3.

Fig. 3 shows circuitry which applies variable amplification to the echo signals received. The circuitry of Fig. 3 also controls the amount of this amplification and generates the TREN signal to start transmission of an ultrasonic pulse.

The echo signals(s) from analog switch 32 are input to a digital-to analog converter 34, and output on line 36 as a fraction of the input signal. That fraction is governed by eight bits of digital data supplied from a preprogrammed, programmable read-only memory 38. An eight bit address is applied to memory 38 from two four bit binary counters 40,41 connected so that the carry output of counter 40 is the input to counter 41.

After an ultrasonic pulse is transmitted the counter 40 is incremented regularly, so that the address in PROM 38 is incremented. The data stored in the PROM is such that the amplification of the echo signal(s) in the digital-to-analog converter increases with time elapsed since transmission of the ultrasonic pulse. The increase with time follows a curve experimentally determined with the same types of transducer 22,30. This curve is such that the amplification exactly compensates for the attenuation in air of ultrasound pulses and their echoes from the centre axis of the transmitted ultrasound pulse. Consequently after this variable amplification stage, all echoes from within a given range along the centre axis have approximately equal magnitude (enabling the device to discriminate against weaker spurious echoes).

The binary counters 40,41 are reset to zero by a signal CRST just after the trailing edge of-a RESET signal from the monstable 1 6, and they are then incremented until there is overflow from the higher order counter 41, as will be explained below. This signal is active low, and is designated CARRY.

A Schmitt trigger 42 with an RC network generates a clock signal (hereinafter MCK) of around 4 KHz. This is inverted at 44 to a signal MCK which is divided by flip-flop 46 to give a 2 KHz secondary clock signal SCK. MCK and MCK are continuous square waves. However, SCK does not run continuously, CARRY is inverted at 47 and fed to the clear input of flip-flop 46, thus inhibiting the generation of the SCK signal. Consequently SCK starts when the counters 40, 41 are reset.

The SCK signal is shaped by Schmitt trigger 48 and input to the binary counter 40. When the active low CARRY from 41 appears, it is fed to the second input of Schmitt trigger 48, stopping further incrementing of the counters 40,41. The counters remain in this condition until the next RESET signal from the monostable 1 6 causes CRST to effect a reset of the counters 40,41 and hence causes CARRY to disappear. ~~~~~~ Depending on the type of counter employed, the overflow signal CARRY may appear while the counters 40,41 register their highest count, or as that count disappears. In the latter case the counters 40,41 will stop on address zero as the count following the highest binary address 1111 1111 (hexadecimal address FF).The data held by PROM 38 at these addresses 00 and FF can then correspond to the final and penultimate points on the experimental amplification curve respectively and can be substantially equal.

The "chip-select" inputs of the digital-to-analog converter 34 and of the PROM 38 are held enabled. The inverted clock signal MCK is fed to an inverting "write enable" input WR of the digital-to-analog converter.

The generation of the counter reset (CRST) and TREN signals is as follows. The reset signal (RESET) from the monostable is fed to the D input of flip-flop 50 and to the clear input of flipflop 52 where it forces the counter reset line 53 low.

The MCK signal strobes the RESET signal into flip-flop 50 to send line 51 high. After the trailing edge of RESET the next leading edge of MCK strobes the high on line 51 into flip-flop 52, providing a high counter reset pulse (CRST) on line 53 for one cycle of MCK. This resets counters 40,41, and hence starts generation of SCK.

The first leading edge of SCK strobes the CRST signal into flip-flop 54, from which it is output as the TREN signal.

The Schmitt trigger 56 with resistor and capacitor connected to it forces the clear inputs of flip-flops 50 and 54 high for a short time when power is first supplied, so that the line 51 and the TREN lines are initially set into a low state, and spurious signals on them are avoided.

The following Table sets out the sequence of signal states and changes from one RESET pulse to the next, assuming that CARRY appears when the highest binary count disappears from the counters 40,41.

EVENT CONSEQUENCE/STATE OF OTHER SIGNALS ADDRESS (Hexade cimal) CARRY, SCK, RESET, TREN ,51 and CRST all low. 00 RESET # CAST held low MCK # 51t 00

51 stays high, with CARRY, SCK, TREN and CRST low until .

RESET; MCK # 51# CRST# CARRY# 00 MCK # MCK# SCK# TREN# CRST high 00 MCK # CRST# MCK # MCK# SCK# MCK # MCK # MCK# SCK# TREN# SCK increments address MCK # MCK + MCKf SCK # MCK # MCK # MCK; SCK increments address 02 above four lines repeat until address FF, whereafter

MCK # MCK ; MCKf SCK MCK # MCK # MCK# SCK# increments address 00 CARRY# SCK# System has now reverted to top line of table.

The following Table sets out the sequence of signal states and changes from one RESET pulse to the next, assuming that CARRY appears as SCK goes low during the highest binary count.

EVENT CONSEQUENCE/STATE OF OTHER SIGNALS ADDRESS (Hexade cimal) CARRY, SCK, RESET, TREN, 51 and CRST all low. FF RESET # CRST held low MCK # 51# FF

51 stay, high, with CARRY, SCK, TREN and CRST ].ow until ..

RESET; MCK # 51# CRST# CARRY# 00 MCK # MCK# SCK# TREN# CRST high 00 MCK # CRST# MCK # MCK# SCK# MCK # MCK # MCK# SCK# TREN# SCK increments address ol MCK # MCK # MCK* SCK MCK r MCK # MCK # SCKf increments address 02 above four lines repeat until address FE, whereafter ....

MCK MCK # MCK# SCK# MCK t MCK # MCKt SCKf # increments address FF MCK MCK ; MCK SCK # CARRY; System has now reverted to top line of table.

From the digital-to-analog converter 34 the echo signal(s) are fed to further stages of amplification 60, a tuned circuit 62 to filter out any spurious received signals of the wrong frequency, and then a detector stage 64. This generates an output signal if any echo signal which has passed through the preceding stages is above a predetermined minimum amplitude, and accordingly is recognised as a genuine echo from a target object approximately on the centre axis of the transmitted pulse.

When an ultrasound pulse is transmitted, the loading edge of the TREN signal starts the counter 12 for the crystal clock signals. An output signal from the detector stage 64 stops the counter 12 so that the count recorded by the counter 12 is directly proportional to the distance from the device to the target object.

The next RESET signal strobes the count into a buffer and display driver 66 which displays it on a seven segment display 68. The next CRST signal resets the counter 12. Hence the display 68 shows the latest distance measurement made. As is known, the frequency of the oscillator 10 may be chosen so that the count from counter 12 which is displayed on the display 68 is a direct measurement in metres, or alternatively in feet, from the device to the target. As is also conventional the frequency of oscillator 10 may be adjusted automatically to compensate for variations in temperature, and hence variations of the speed of sound in air.

Fig. 4 shows an arrangement using a microprocessor. Many sections are the same and are indicated by the same reference numerals. In particular the circuitry of Fig. 3 is utilised without change.

The same transducer 22 is used for both transmission and reception. This known expedient could also be employed in the arrangement of Fig. 1, and is shown in U.K. 2082325A. The input to the analogue switch circuit is connected across a pair of diodes in parallel opposition, which are in series with the transducer 22.

The output of a crystal oscillator 70 is fed to a binary counter 72 whose output is connected to a parallel input port of the microprocessor 74 (or to a parallel input/output chip which is in turn connected to the microprocessor, if the microprocessor does not incorporate a parallel input port).

RESET is generated by the microprocessor under the command of its operating program. This causes the circuitry of Fig. 3 to run exactly as described above, and TREN enables the 40 KHz oscillator 18 to generate an ultrasonic pulse as described, and it also starts the binary counter 72. When a returning echo causes an output pulse from the detector stage 64, this output pulse immediately stops the counter 72. It also generates an interrupt to the microprocessor 74.

When the microprocessor services this interrupt it reads the (stopped) count from the counter 72 and from it calculates the distance from the measuring device to the target object which reflected the echo. The distance may be displayed on a seven segment display driven by the microprocessor through a display driver stage, or used by it for further calculation. Possibly the apparatus could be arranged to display an average of the distances measured by means of several transmitted pulses.

The device preferably includes means enabling the microprocessor to read in a digital measurement of temperature, so that it can allow for the ambient temperature in calculating the distance to the target object.

When servicing the interrupt the microprocessor also generates a signal to reset the counter 72. If no echo is detected so that the counter 72 overflows, the resulting carry signal from it may be used as an interrupt for the microprocessor. If this is done RESET signals might not be generated at regular intervals, but follow servicing of an interrupt.

The details of an operating program for the microprocessor will depend on the particular microprocessor employed, but functions which it must provide are indicated above, viz to generate the RESET pulses, service the interrupts by reading and resetting the counter 72, and manipulate the count so read in.

The crystal oscillator 70 preferably runs at 1 MHz. However, a lower frequency might be employed, for example if it proved impractical to count such a high frequency.

It is by no means essential, but it is possible that the pulses from crystal oscillator 70 (especially if a 1 MHz signal) could provide the clock signal for the microprocessor.

Various modifications are possible. One possibility would be for the counter 1 2 or 72 to be preset each time an ultrasonic pulse was transmitted. The preset count would correspond to a predetermined distance.

The clock signals from the crystal oscillator 10 or 70 would decrement the counter, and the device would be arranged to give an alarm signal if an echo was received before the counter had decremented to zero. In such an arrangement the exact time for return of an echo is not recorded, but it is sensed as being above or below a prescribed value.

A possible modification would be for the RESET signal of Fig. 1 or Fig. 3 to be of precise length and used directly to start a transmitted pulse, and fix its length. For this the leading edge of RESET would be used to enable the 40 KHz oscillator 18, and its training edge would disable the oscillator, terminating the ultrasonic pulse.

With such an arrangement RESET could be used instead of CRST to reset the counters 40,41 with the consequence that the counters would start to increment at the end of a transmitted ultrasound pulse instead of its beginning.

For special applications the data stored in PROM 38 might give amplification which did not increase continuously with time. For example the increase might follow a curve with a notch in it, to exclude a known unwanted echo.

The PROM 38 might be large enough to hold data for several amplification curves. In this event the user might be able to select which curve was followed. To accomplish this signals to three out of eleven address lines could be provided by manually operable switches, or controlled by keyed-in commands to the microprocessor, while the remaining eight lines are addressed by the counters 40,41 as described above. As an example, the PROM could hold data for a standard curve used normally, a curve of similar shape but giving higher amplification for use with poorly reflecting targets, and a notched curve for a special application.

When the apparatus of this invention utilises a microprocessor, a possibility is to employ RAM as the semi-conductor memory which holds the digital data determining the amount of amplification/attenuation for a particular time interval. This RAM could be loaded with the required data by the microprocessor as an initial operation when the apparatus was switched on.

The microprocessor might then compute some or all of the data.

Claims (11)

1. Apparatus for sensing distances in air comprising means for transmitting a sonic or ultrasonic signal, means to receive an echo of the signal and convert it into an electrical signal, variable amplification/attenuation means to amplify and/or attenuate received signals by an amount dependent on the time elapsed from transmission of the pulse, and time sensing means to measure or otherwise sense the time elapsed between transmission of the said sonic or ultrasonic signal and return of the echo, characterised in that the variable amplification/attenuation means comprises means to store information associated with the amplification and/or attenuation required at a multiplicity of times following transmission and means to utilise the said stored information to apply the required amplification and/or attenuation to received signals.

2. Apparatus according to claim 1 wherein the stored information is such that the amplification of received signals increases, and/or attenuation thereof decreases, approximately in proportion to the square of the distance travelled by the sonic or ultrasonic signal.

3. Apparatus according to claim 1 or claim 2 wherein the stored information is digital information associated with a multiplicity of discrete time intervals.

4. Apparatus according to claim 3 wherein the information is stored in a memory chip with addresses in the memory holding digital data which determines the amount of amplification/attenuation for a particular time interval.

5. Apparatus according to claim 4 wherein the memory chip is a read-only memory.

6. Apparatus according to claim 4 or claim 5 wherein the amplification/attenuation is effected by a digital-to-analog converter which receives as inputs the echo signal(s) and the digital data read from the read-only memory.

7. Apparatus according to claim 4, claim 5 or claim 6 wherein the data for each time interval is held at a single address of the memory, the apparatus including an address counter which is incremented (or decremented) as each time interval passes.

8. Apparatus according to claim 5 comprising means generating clock pulses, a counter for said pulses, means causing said counter to commence counting said pulses when a said sonic or ultrasonic signal is transmitted.

said counter being connected to the address lines of the read only memory, so that the count registered by the counter determines the memory location addressed, the data lines of said memory being connected to digital inputs of a digital to analogue converter, and received echo signals also being input to said converter, whereby the amplification/attenuation applied to an echo signal is determined by the digital data at the memory location currently address by the counter.

9. Apparatus according to claim 7 or claim 8 arranged to repeat the transmission of the sonic or ultrasonic signal automatically, resetting the address counter for each transmission.

10. Apparatus according to any one of the preceding claims wherein said time sensing means comprises a counter which generates a numerical value proportional to time from transmission to return of an echo, the apparatus further comprising display means to display a digital distance measurement derived from said numerical value, and which is a measure of distance to the target returning the echo.

11. Apparatus according to any one of the preceding claims wherein said time sensing means comprises a counter which generates a numerical value proportional to time from transmission to return of an echo, the apparatus comprising means to average a plurality of said values obtained from repeated transmitted signals and echoes thereof.

1 2. Apparatus for sensing distances in air, substantially as any herein described with reference to the accompanying drawings.