Laser range finding apparatus

• US 5,949,530 A
• Filed: 02/13/1997
• Issued: 09/07/1999
• Est. Priority Date: 02/27/1996
• Status: Expired due to Term

• Current Assignee: Sick AG (Sick)
• Original Assignee: Sick AG (Sick)
• Inventors: Wetteborn, Hainer
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US08/800,162
• Time in Patent Office: 936 Days
• Field of Search: 356/4.01, 356/5.01, 356/141.1, 180/169
• US Class Current: 356/5.01
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• 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 17/42 Simultaneous measurement of distance and other co-ordinates indirect measurement G01S17/46
• CPC Class Codes: G01S 2013/9315 Monitoring blind spots
• CPC Class Codes: G01S 2013/93274 on the side of the vehicles
• CPC Class Codes: G01S 7/4865 Time delay measurement, e.g. time-of-flight measurement, time of arrival measurement or determining the exact position of a peak peak detection in noise, signal conditioning G01S7/487
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

A laser range finding apparatus comprises a pulse laser (11), a light deflecting device, a photodetector arrangement (22) having an opto-electronic photodetector (23) and also an electronic control and evaluation system (10) in which a sensing signal representative for the range of an object is determined in accordance with the pulse transit time method from the time between the transmission and reception of a light pulse taking account of the speed of light. The electronic control and evaluation system (10) includes means for measuring the total electrical charge which has flowed via the photodetector (23) during the reception of a light pulse and/or for the measurement of the pulse width of the received light pulse. The compensation takes place as a result of the transit time measurement errors which arise as a result of the signal dynamics in dependence on the measured electrical charge or pulse width on the basis of respective correction values which were determined by the electronic control and evaluation system with respect to charges or pulse widths and pulse transit times, which were measured with respect to at least one reference object (86) having different reflectivities and arranged at a defined range from the light deflecting device (15).

Claims

1. Laser range finding apparatus comprising a pulsed laser (11), a light deflecting device (15), a photodetector arrangement (22) having an opto-electronic photodetector (23) and an electronic control and evaluation system (10), wherein the pulsed laser (11) is controlled to transmit light pulses (12) and the sequential light pulses (12) that are transmitted are deflected via the light deflecting device (15) at changing angles into a measurement region (13), wherein the light pulses (12'"'"') reflected back from an object (14) located in the measurement region (13) are received by the photodetector arrangement (22), wherein a sensing signal representative for the range of the object (14) from the light deflecting device (15) is found in the electronic control and evaluation system (10) in accordance with the pulse transit time method from the time between the transmission and the reception of a light pulse (12, 12'"'"') on taking account of the speed of light, and wherein a compensation is also effected for transit time measurement errors arising as a result of signal dynamics, characterized in that

the electronic control and evaluation system (10) includes means (C, 36, 37;

34,

     38) for measuring the total electrical charge which has flowed via the photodetector (23) during the reception of a light pulse (12'"'"') and/or for measuring the pulse width of the received light pulse (12'"'"'); and

in that the compensation of the transit time error which occurs as a result of the signal dynamics, takes place in dependence on the measured electrical charge and/or the pulse width on the basis of respective correction values which are determined by the electronic control and evaluation system (10) as a result of charges and/or pulse widths and pulse transit times which were measured with respect to at least one reference object (86) arranged at a defined distance from the light deflecting device (15) and having different reflectivities.

2. Apparatus in accordance with claim 1, characterized in that the reference object (86) is arranged outside of a defined, monitored angular scanning range (54) and is swept over by the transmitted pulsed light beam (21) deflected with continuously changing angles.

3. Apparatus in accordance with claim 1, characterized in that the reference object (86) has a surface (30) of changing reflectivity, and preferably steplessly changing reflectivity, which is struck by the transmitted pulsed light beam (21), and which extends along a circular arc concentric to the axis of rotation (17) of the rotatable light deflecting means (15), whereby the reflectivity changes along the circular arc.

4. Apparatus in accordance with claim 3, characterized in that the surface (30) is formed by a foil, undercoat or the like of high initial reflectivity, which is increasingly painted with black paint.

5. Apparatus in accordance with claim 1, characterized in that the reference object (86) is formed by a specific number of reference object elements which preferably lie at a mutual spacing on a circular arc concentric to the axis of rotation (17) of the rotatable light deflecting device (15);

and in that the surfaces of the different reference object elements, which are struck one after the other by the transmitted pulsed light beam (21), have different reflectivities.

6. Apparatus in accordance with claim 5, characterized in that the measured values which are received with respect to the reference object elements arranged at the same distance from the light deflecting device (15) are linked together by a polynomial and that intermediate values are determined via the polynomial.

7. Apparatus in accordance with claim 1, characterized in that the light deflecting device (15) sweeps over a 360°

deflection angle and preferably continuously rotates in one direction of rotation.

8. Apparatus in accordance with claim 1, characterized in that the means (C, 36, 37) for the measurement of the total electrical charge which has flowed via the photodetector (23) during the reception of one light pulse (12'"'"') includes a capacitor (C) connected in series to the photodetector (23) and also a buffer amplifier (36) connected after the capacitor.

9. Apparatus in accordance with claim 8, characterized in that the capacitor (C) is connected in series via a diode, preferably a Schottky diode (37), with the photodetector (23).

10. Apparatus in accordance with claim 1, characterized in that the photodetector (23) is connected via a preamplifier, preferably via a transimpedance amplifier (38), to one input of a comparator (34), which is, moreover, provided with a reference input (35) defining a detection threshold (79) and the output signal of which is used in particular for the measurement of the pulse width of the received light pulse (12'"'"').

11. Apparatus in accordance with claim 10, characterized in that the output signal of the comparator (34) is simultaneously used for the transit time measurement.

12. Apparatus in accordance with claim 1, characterized in that the reference object (86) is repeatedly sensed before and/or during the normal operation of the apparatus in order to obtain the measurement values required for the determination of the correction values.

13. Apparatus in accordance with claim 1, characterized in that measurement values obtained during a specific number of scans with respect to the reference object (86) are averaged to improve the accuracy in the electronic control and evaluation system (10).

14. Apparatus in accordance with claim 1, characterized in that the compensation in the electronic control and evaluation system (10) of the transit time measurement errors which arise as a result of the signal dynamics first takes place in dependence on the measured electrical charge and on the basis of corresponding derived correction values or correction functions, when the measured pulse width of the received light pulse (12'"'"') has exceeded a specific threshold value.

15. Apparatus in accordance with claim 14, characterized in that the threshold value is at least substantially the same as the value of two to ten times and in particular of two to five times the pulse width of the transmitted light pulse (12), or corresponds to a value at which the pre-amplifier is greatly overmodulated or saturated.

16. Apparatus in accordance with claim 1, characterized in that the electronic control and evaluation system (10) includes a time measurement unit for the determination of the transit times and pulse widths.

17. Apparatus in accordance with claim 1, characterized in that the electronic control and evaluation system (10) includes a microprocessor.

18. Apparatus in accordance with claim 1, characterized in that the opto-electronic photodetector is a photodiode, in particular an avalanche diode (23).

19. A laser range finding apparatus comprising a pulsed laser directing light pulses towards an object that is spaced from the apparatus;

a photodetector receiving light pulses reflected by the object;

an evaluation circuit generating distance signals reflecting a distance between the object and the apparatus from a pulse transit time between a transmission and reception of light pulses by the apparatus using the speed of light;

a deflector arranged between the laser and the object for angularly deflecting sequential light pulses to change their angular positions relative to the object;

a transmitter sending to the evaluation circuit angular position signals representative of the angular positions of the sequential light pulses;

the evaluation circuit being adapted to determine the distance between the object and the apparatus from the distance signal and the angular position signal;

a control circuit operatively connected with the evaluation circuit for measuring at least one of an electrical charge which has flowed through the photodetector during the reception of a light pulse and a pulse width of a received light pulse; and

a reference object located at a defined distance from the apparatus;

the evaluation circuit further generating needed correction values for correcting errors in the distance signals caused by signal dynamics by comparing the at least one of the electrical charge which has flowed through the photodetector and the pulse width with at least one of a corresponding electrical charge which has flowed through the photodetector and a pulse width resulting from a reference laser light pulse reflected by the object.

20. Apparatus according to claim 19 wherein different portions of the reference object have different reflectivities.

21. Apparatus according to claim 19 wherein the evaluation circuit generates the correction values also as a function of a pulse transit time of the reference laser light pulse.