GB1597275A

GB1597275A - Distance measuring system

Info

Publication number: GB1597275A
Application number: GB22619/78A
Authority: GB
Original assignee: MITEC MODERNE INDUSTRIETECHNIK GmbH
Current assignee: MITEC MODERNE INDUSTRIETECHNIK GmbH
Priority date: 1977-05-26
Filing date: 1978-05-25
Publication date: 1981-09-03
Also published as: DE2723835C2; NL8800031A; FR2392398A2; AU517107B2; AT374930B; SE7805662L; CH628995A5; DE2723835A1; NL7805787A; JPS53147570A; AU3654578A; FR2392398B2; JPS601590B2; SE439553B

Abstract

This device operates in accordance with the principle of measuring the transit time of a light pulse. The light beam (j1) emitted by a transmitting diode (14) fed by a laser pulse generator (10) passes to a transmitter lens (12). The greatest proportion of the light beam is directed to a target object. The light rays (j3) reflected from the target object pass to a receiving diode (23) of a measuring channel (I). A small proportion of the light beam (j1) emitted by the transmitting diode is dispersed at the transmitter lens and conducted as reference beam (j2) to a receiving diode (15) of a reference channel (II). Each of the receiving diodes is allocated one parallel resonant circuit (17; 16) each as load impedance. The emitted laser pulse triggers the reference and the measuring channel in optical manner. To obtain the time-significant signal edges, the first sinewave zero crossing is detected in the reference channel and the second sinewave zero transition of the sinusoidal oscillation started in the relevant parallel resonant circuit is detected in the measuring channel. The sensitivity of the device can be increased by the galvanic isolation between the transmitter and receiver. Using a parallel resonant circuit as load impedance has the further advantage that the optical filters previously required are no longer needed. <IMAGE>

Description

(54) DISTANCE MEASURING SYSTEM (71) We, MITEC MODERNE INDUS TRIETECHNIK Gmbh of Ottostr.61 D 8012 Ottobrunn, German Federal Republic a Company organised and existing under the laws of the German Federal Republic, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a distance measuring system based on the principle of measuring the transit time of a light pulse to and from a target, by means of two channels, of which one channel, the so-called reference channel, serves to obtain a start signal, while the other channel, the so-called measuring channel, serves to obtain a stop signal, and is an improvement in the system described and claimed in Patent No.

1540774.

In the system described in the above mentioned patent similar reference and measuring channels are employed. The present invention provides a distance measuring system, in which optical triggering of separate reference and measuring channels is effected and differences in the response times of the two channels are corrected.

According to the present invention, there is provided a distance measuring system as claimed in claim 4 of British Patent No.

1540774, in which each transmitted light pulse optically triggers the reference channel to provide an indication of the beginning of the transit time and each received reflected pulse optically triggers the measuring channel to provide an indication of the end of the transit time, and the distance calculated from the transit time of the transmitted light pulse to and from a target is corrected by subtracting therefrom a distance calculated from the time difference for operation of the reference and measuring channels determined by simultaneous electrical triggering of the reference and measuring channels, which is effected immediately after a measuring operation.

In the preferred embodiment the measuring channel includes a variable gain amplifier and the output signal from a comparator is used as a control signal to reduce the gain of this amplifier for at least the signal derived from the next received reflected pulse. For this purpose the variable gain amplifier in the measuring channel has means for reducing the gain of the amplifier in steps, which means operate to reduce the gain of the amplifier by one step each time the comparator produces an output signal.

This invention will now be described by way of example with reference to the drawings in which the three figures together make up a block schematic diagram of a system according to the invention.

A laser pulse J1 is emitted by a transmitting diode 14 towards a target. On passing through a transmitter lens 12 a small part Jz of the pulse is reflected as stray light, and passes via an optical fibre constituting a photo-conductive channel 11 to a receiving diode 15 of a reference channel II, where it triggers an oscillator circuit 16, the signal from which is passed via an impedance converter 18 and an amplifier 19 to a detection device 20 for detecting passage of the signal through zero, a signal SF1 being generated at the output of the detection device 20 on such detection.This signal is accurately timed, and indicates that the pulse J is just leaving the transmitter lens 12 for the target; this signal, therefore, is used to initiate the start 21a of a time measuring device 21, and at the same time an operating cycle control system 22 is triggered, as shown at 22a.

Somewhat later, in dependence on the transit time of the pulse to and from the target, a reflected pulse J3 reaches a receiving diode 23 of a measuring channel I via a receiving lens 13, and triggers an oscillator circuit 17, the signal from which is passed via an impedance converter 24 to an amplifier 25 which unlike the amplifier 19 in the reference channel II has a gain or amplification factor which can be reduced in steps u to a maximum of n. The amplified sinusoidal al oscillation is passed to a detection device 26 for detection of the second passage of the signal through zero.

Thus, the first passage through zero is detected in the reference channel II, while the second such passage is detected in the measuring channel I. This ensures that any drift in the channels I and II relative to each other will never cause the time-significant signal of the measuring channel I to occur before the time-significant signal SF1 in the reference channel II when the distance measured is zero. This makes it impossible for the system to fail in the course of a measuring operation.

The time-significant signal SFZ generated by the detection device 26 is passed to the stop input 21b of the time measuring device 21. Shortly afterwards the time measuring device 21 generates the signal "measuring terminated", and clocks the operating cycle control system 22 one step S1 further, thus initiating storage of the result of the time measuring operation in a store 27 for uncorrected measuring values.

The reason why it is uncorrected is that the time-significant signals SF1 and SFZ have different retardations which are inherent in the reference channel II and the measuring channel I, and which are mainly due to the different detection of passage of the signals through zero as mentioned above.

This is immediately followed by a signal "storage terminated" which via a clock input 22b of the operating cycle control 22, sets up the step S2. The two oscillator circuits 16 and 17 are then impulsed electrically at the same time, and time-significant signals SF1 and SF2, as described before, are again received. The time measuring device 21 is again started and stopped as in the previous optical triggering operation. The signal "measuring terminated", acts this time, however, via the clock input 22b of the operating cycle control system 22, to switch this to step S3, which results in the storage of the time measuring operation in a correction store 28. The store 28 thus contains a value indicating the difference in response times between the reference channel II and the measuring channel I.

All that is now required is to subtract the value stored in store 28 from the uncorrected measuring value stored in the store 27, and which was obtained at step S1 of the operating cycle control system 22, in order to obtain the true measuring value indicating the distance measured. For this purpose, the signal "storage terminated", from the store 28 in step S3 moves the operating cycle control system 22 one step further on to S4.

At step S4 the required difference is calculated and appears as the true measuring value at the output of a difference store 29. This true value can, if necessary, be indicated in a display device 30 and/or utilized for control purposes, etc.

Owing to the fact that simultaneous electrical triggering of the oscillator circuits 16 and 17 in the reference channel II and measuring channel I, this corresponding to measurment of a distance of "zero", follows immediately after the actual measuring operation, and by the subsequent calculation of the difference between the time measuring values, of which the first was obtained by optical triggering and the second by electric triggering, any difference, for example due to drift, in the two channels I and II, and in the time measuring device 21 can be eliminated.

As the functional operation continues, a signal "storage terminated" coming from the difference store 29 sets up step S5 via the clock input 22b of the operating cycle control system 22, this causing the scaling 21c of the time measuring device 21 to be calibrated. On termination of this process, the time measuring device 21 generates a signal "calibration terminated" and restores the operating cycle control system 22 to zero, and the cycle of operations described in the foregoing is repeated.

Simultaneously with zeroing of the operating cycle control system 22, a counter 31 which counts as far as "n" is zeroed. This counter 31 serves to alter the amplification factor of the amplifier 25 in the measuring channel I. In conjunction with a comparator 32, which is connected to the output of the measuring channel amplifier 25, this serves to prevent any over-driving of the measuring channel amplifier 25 in the event of targets with a considerable reflection power.

For this purpose the amplitude of the amplified signal in the measuring channel I is compared with a reference voltage UREF in the comparator 32. This reference voltage is selected to ensure that an amplitude of the signal which is of the same magnitude but reduced by the selected amplification factor will not over-drive the amplifier 25 and that measurement will be effected in the linear range of the amplifier.

If, however, the amplitude of the signal exceeds the value to which the reference voltage has been set, the comparator 32 moves the counter 31 on by one step, and at the same time the start described before, for the operating cycle control system 22 is cancelled, no measuring value storage being effected, although the previous measuring value is retained. On the arrival of the next pulse this operation is repeated, as is also the altering of the amplification factor in the measuring channel 25, until the signal amplitude received no longer exceeds, in the comparator 32, the reference voltage selected. Only after this control operation can the next measuring value, as explained in the foregoing, be obtained.

WHAT WE CLAIM IS: 1. A distance measuring system as claimed in claim 4 of British Patent No.

2. A distance measuring system as claimed in claim 1, in which the first sinusoidal passage through zero of the signal triggered by the transmitted light pulse in the reference channel is used to provide the said indication of the beginning of the transit time while the second sinusoidal passage through zero of the sinusoidal oscillation triggered in the measuring channel by the received pulse reflected from the target is used to provide the said indication of the end of the transit time.

3. A distance measuring system as claimed in claim 1 or claim 2, in which the measuring channel includes a comparator in which the amplitude of the signal in the channel derived from a received reflected pulse, is compared with a reference signal amplitude, and in the event that the amplitude of the signal in the channel exceeds the reference amplitude, no distance measurement is effected with that signal.

4. A distance measuring system as claimed in claim 3, in which the measuring channel includes a variable gain amplifier and the output signal from the said comparator is used as a control signal to reduce the gain of this amplifier for at least the signal derived from the next received reflected pulse.

5. A distance measuring system as claimed in claim 4 in which the variable gain amplifier in the measuring channel has means for reducing the gain of the amplifier in steps, which means operate to reduce the gain of the amplifier by one step each time the said comparator produces an output signal.

6. A distance measuring system as claimed in any preceding claim, in which stray light from a transmitting optical channel by way of which each light pulse is transmitted, is used to trigger the reference channel, while light frcm a receiving lens by way of which each reflected pulse is received, is used to trigger the measuring channel.

7. A distance measuring system, as claimed in claim 6, in which the reference channel is triggered by stray light from the optical channel via a photo-conductive fibre or photo-conductive channel.

8. A distance measuring system substantially as hereinbefore described with reference to the drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. pulse this operation is repeated, as is also the altering of the amplification factor in the measuring channel 25, until the signal amplitude received no longer exceeds, in the comparator 32, the reference voltage selected. Only after this control operation can the next measuring value, as explained in the foregoing, be obtained. WHAT WE CLAIM IS:

1. A distance measuring system as claimed in claim 4 of British Patent No.