Apparatus for measuring distances by measuring time of travel of a measuring light pulse

• US 4,521,107 A
• Filed: 09/26/1984
• Issued: 06/04/1985
• Est. Priority Date: 02/03/1981
• Status: Expired due to Fees

• Current Assignee: Bayerische Motoren Werke AG
• Original Assignee: Mitec-Moderne Industrietechnik GmbH
• Inventors: Mehnert, Walter, Chaborski, Hoiko
• Primary Examiner(s): BUCZINSKI, STEPHEN C
• Application Number:
  US06/654,807
• Time in Patent Office: 251 Days
• Field of Search: 356/5, 343/13, 343/7.3, 367/95, 367/97
• US Class Current: 356/5.06
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G01S 17/14 wherein a voltage or current pulse is initiated and terminated in accordance with the pulse transmission and echo reception respectively, e.g. using counters
• CPC Class Codes: G01S 7/4818 using optical fibres

Abstract

An apparatus measures the distance of a target by measuring the time of travel of a light pulse from the measuring apparatus to the target and back wherein it is important to achieve high accuracy in timing to correlate, for a distant target, the time of transmission with the start of measuring the time of travel and the time of receiving a reflected light pulse and the end of the time measurement. For a near target, the start of distance measurement is correlated with receipt of a reflected measuring light pulse and a stop signal is correlated with a delayed reference light pulse. For this purpose a reference light pulse is branched off from a measuring light pulse and passed by light guides to a single receiver unit. The reference light pulse is passed to the receiver unit either through a short or a long light path depending on whether the target is distant or near, respectively. Attenuators are provided in each path to insure that the start and end of a time measurement of target distance can be performed without interfering signals.

Claims

1. Distance measuring apparatus for measuring the time of travel of individual unmodulated measuring light pulses to a target and therefrom, by reflection, back to the apparatus which for this purpose comprises:

a light pulse transmitter for emitting on triggering thereof individual unmodulated light pulses of short duration,

a light beam splitter means for splitting individual light pulses from said transmitter into two parts, a first of which is directed toward an optical transmitting means and a second of which is directed into a reference light path means, said optical transmitting means transmitting said first light pulse part as a measuring light pulse toward said target,

an optical receiving means for receiving a reflected measuring light pulse from the target,

a receiving channel comprising an electro-optical receiving means for receiving from said optical receiving means said reflected measuring light pulse and from said light beam splitter means, via said reference light path means, said second part of each light pulse as a reference light pulse and transforming each received light pulse into an electrical signal, electrical circuit means in said receiving channel for processing each electrical signal to produce time-related distance measuring signals for starting and stopping of said measurement of time of travel of each measuring light pulse, said receiving channel, on responding to each received light pulse having a recovery time which must elapse before the next light pulse can be processed, said reference light path means including intermediate its ends, connected to said light beam splitter and said electro-optical receiving means, first and second functionally parallel reference light path branches of different length and selector means for selecting one of such first and second reference light path branches for supplying the reference light pulse via the selected branch to said electro-optical receiving means, said first reference light path branch comprising a short reference light path branch to transit time through which for the reference light pulse is relatively short and said second reference light path branch comprising a long reference light path branch the transit time through which for the reference light pulse is relatively long, control means for controlling operation of said selector means as a function of target distance, the selector means being controlled to supply reference light pulses to the electro-optical receiving means from the short reference light path branch for long distance targets and for supply of reference light pulses to the electro-optical receiving means from the long reference light path branch for short distance targets, said relatively long transit time for the reference light pulse being at least equal to two times said recovery time of said receiving channel, and a time measuring means connected to the receiving channel and supplied therefrom with said time-related distance measuring signals for measuring the time of travel of each measuring light pulse.

2. A distance measuring apparatus as claimed in claim 1, wherein said control means also provides for trigger pulses for triggering said transmitter and being operative to detect whether, on supplying a reference light pulse to said electro-optical receiving means via said short reference light path branch, a given measuring light pulse and the corresponding reference light pulse arrive at the electro-optical receiving means within a time shorter than or equal to said receiving channel recovery time, and, should this occur, to reject the measurement, to automatically repeat the measuring operation by again triggering the transmitter and to effect operation of said selector means to supply the resulting reference light pulse to said electro-optical receiving means via said long reference light path branch.

3. A distance measuring apparatus as claimed in claim 1, wherein said reference light path means includes in advance of said first and second light path branches an optical delay means the transit time through which for a reference light pulse is short compared to the transit time of that reference light pulse through said long reference light path branch but sufficiently long to ensure receipt of the reference light pulse by the electro-optical receiver means free of interference signals generated on light pulse transmissions by the transmitter.

4. A distance measuring apparatus as claimed in claim 3, wherein said optical delay means, said long and short reference light path branches and said reference light path means are all constituted of light guide fibres.

5. A distance measuring apparatus as claimed in claim 4, wherein the light guide fibres for said optical delay means and for said long reference light path branch are in coil form.

6. A distance measuring apparatus as claimed in claim 4, wherein the first and second light path branches together with said reference light path, leading thereto from said light beam splitter means, form a Y-shaped divider, the bottom leg of the Y receiving each reference light pulse from said beam splitting means, one of the upper legs of the Y directing a part of each reference light pulse into one end of the long reference light path branch and the other of the upper legs of the Y directing a part of each reference light pulse into one end of the short reference light path branch.

7. A distance measuring apparatus as claimed in claim 6, wherein the opposite ends of the long and short reference light path branches converge forming with said reference light path means, leading to the electro-optical receiving means, an inverted Y shaped divider, one of the legs of which receives the reference light pulse from the long reference light path branch and the other leg of which receives the reference light pulse from the short reference light path branch, the common leg of the Y directing each reference light pulse to said electro-optical receiving means.

8. A distance measuring apparatus as claimed in claim 7, wherein said selector means comprises two optical attenuation means for attenuating the reference light pulse from substantially no attenuation to substantially total attenuation of said attenuation means, the one of said two attenuation means being arranged in the short reference light path branch and the other one of said two attenuation means being arranged in the long reference light path branch and the two attenuation means being oppositely controlled.

9. A distance measuring apparatus as claimed in claim 6, wherein said selector means comprises an optical change-over switch controlled by said control means to connect one of the long and short reference light path branches to said electro-optical receiving means.

10. A distance measuring apparatus as claimed in claim 9, wherein the selector means is positionable to pass to said electro-optical receiving means at least part of said reference light pulse in said long reference light path branch as well as at least part of said reference light pulse in said short reference light path branch for determining transit time difference of a reference light pulse through the long reference light path branch and through the short reference light path branch.

11. A distance measuring apparatus as claimed in claim 4, wherein said short reference light path branch includes attenuation means of fixed attenuation value for compensating for the attenuatiion experienced by the reference light pulses in passing through the long reference light path branch, due to the length of the light guide fibre constituting said branch.

12. A distance measuring apparatus as claimed in claim 1, wherein said reference light path means includes in advance of said first and second light path branches an optical attenuation unit which attenuates reference light pulses passing through said reference light path means to approximate the intensity of the corresponding measuring light pulse when received by the electro-optical receiving means.

13. A distance measuring apparatus as claimed in claim 9, wherein the optical change-over switch comprises three light guide fibres disposed in a common plane, two of said three fibres being fixed, the third being movable between said two fixed light guide fibres for alternate alignment with the ends thereof, the positions of the third fibre being defined by stops.

14. A distance measuring apparatus as claimed in claim 13, wherein a sleeve of ferromagnetic material is provided over the movable light guide fibre end, said optical change-over switch incorporating magnet means for moving, on reaction with the ferromagnetic sleeve, the movable light guide fibre between its aligned positions with the ends of said fixed fibres.

15. A distance measuring apparatus as claimed in claim 1, wherein said receiving channel includes a resonance means the input of which is connected to the output of said electro-optical receiving means, said resonance means producing an oscillation signal in response to each light pulse received by said electro-optical receiving means, and a first comparison circuit having a zero-transition comparator unit for producing a time-related distance measuring start or stop signal for the time measuring means at a given zero-transition of the signal of said resonance system.

16. A distance measuring apparatus as claimed in claim 15, wherein said receiving channel further comprises an amplifier connected between said resonance system and said first comparison circuit, said amplifier having an automatic voltage control circuit for maintaining its zero direct voltage output constant.

17. A distance measuring apparatus as claimed in claim 16, wherein said first comparison circuit includes a first comparator unit, producing a control signal when the output signal of the resonance system after being amplified by said amplifier, becomes greater than a first reference level, and a second comparator unit producing a control signal when the output signal of the resonance system, after being amplified by said amplifier becomes smaller than a second reference level, whereby said second reference level has a smaller absolute value than the first reference level, said first and second reference levels being different from said zero-level, said first comparison circuit further comprising a memory circuit for storing the control signal of said first comparator unit on occurrence thereof and enabling, through a logic circuit, said zero-transition comparator unit upon the occurrence of the control signal of the second comparator only if the latter control signal is produced after the control signal of the first comparison unit.

18. A distance measuring apparatus as claimed in claim 17, wherein said first and said second comparator units define a first reference level and a second reference level, respectively, being adapted for comparison with a predetermined half-oscillation of each of said oscillation signal.

19. A distance measuring apparatus as claimed in claim 18, wherein said first comparison circuit further comprises a delay unit for delaying the enabling signal for the zero-transition comparator unit so that a later zero-transition, rather than the first zero-transition occurring after production of the control signal of the second comparator unit, is used for producing a time-related distance measuring signal for starting or stopping the time measurement.

20. A distance measuring apparatus as claimed in claim 19, wherein the delay of said delay unit is such that the fourth zero-transition after exciting the oscillation signal is used for producing a time-related distance measuring signal.

21. A distance measuring apparatus as claimed in claim 20, wherein the logic circuit is formed by a normally reset D-flip-flop, whose data input receives an enabling signal produced at the same time as the control signal of the first comparator unit, and whose clocking input receives a clocking pulse produced at the same time as the control signal of the second comparator unit and whose one output produces, in the set condition of the D-flip-flop, a signal enabling the zero-level comparator unit, which signal is delayed by said delay unit.

22. A distance measuring apparatus as claimed in claim 21, wherein the D-flip-flop is reset on the first reaction of the zero-transition comparator unit.

23. A distance measuring apparatus as claimed in claim 22, wherein the zero-transition comparator unit is latched by way of a gate circuit, by the output signal delayed by the delay unit, of the D-flip-flop as well as by a signal produced in the rest condition by one of the outputs of the zero-transition comparator unit, and wherein said signal is prevented by a gate circuit from latching the zero-transition comparator unit, when the D-flip-flop is set.

24. A distance measuring apparatus as claimed in claim 17, further comprising a first latching circuit preventing said first comparator unit from producing further control signals in response to further half-oscillations of each oscillation signal after having produced a first control signal, and a second latching circuit for preventing said second comparator unit from producing further control signals in response to further half-oscillations of each oscillation signal after having produced a first control signal, the two latching circuits being disabled by a reset signal produced a short time before a light pulse arrives at the receiving channel.

25. A distance measuring apparatus as claimed in claim 17, wherein the memory circuit is only ready for receiving a control signal produced by the first comparator unit when a time slot signal is produced, the time slot signal being produced a short time before expected receipt of a light pulse by the receiving channel and being matched in time to the greatest distance to be measured whereby the comparison of the amplitude of the oscillation signal with the two reference levels is only enabled shortly before receipt of said light pulse in the receiving channel, and immediately after production of said control signals of said first and said second comparator units, said comparison is again locked.

26. A distance measuring apparatus as claimed in claim 16, further comprising an attenuation unit in the light path between the optical receiving means and the electro-optical receiving means, means for controlling the sensitivity of said electro-optical receiving means, means for controlling the output power of said transmitter, and a second comparison circuit comprising the output signal of said resonance system, after being amplified by said amplifier, with a third reference level denoting the top limit of a linear range of said amplifier, which second comparison circuit produces, if the absolute value of said amplitude is greater than said third reference level, a control signal for discarding the time of travel measurement produced, for controlling said output power of said transmitter and/or the sensitivity of said electro-optical receiving means and/or for activating said above noted attenuation unit and for initiating a new distance measurement attempt.

27. A distance measuring apparatus as claimed in claim 26, wherein said electro-optical receiving means is a photodiode, the sensitivity of the electro-optical receiving means being changed by changing the bias voltage of said diode.

28. A distance measuring apparatus as claimed in claim 26, wherein said controllable attenuation unit is connected in the light path between the optical receiving means and a photodiode of the electro-optical receiving means and comprises two light guide fibre ends, arranged in a common plane, one of said two ends being fixed while the other is movable in said plane between two positions in one of which the end face of the movable light guide fibre end is in complete alignment with end face of the fixed light guide fibre and in the other position of which the end face of the movable light guide fibre is in only partial alignment with the end face of the fixed light guide fibre whereby only part of the light is transmitted to the electro-optical receiving means.

29. A distance measuring apparatus as claimed in claim 28, wherein a sleeve of ferromagnetic material is provided on movable light guide fibre, said controllable attenuation unit further comprising a magnet arrangement for moving the movable light guide fibre between its two positions.