Laser scanning system

• US 5,940,170 A
• Filed: 08/19/1998
• Issued: 08/17/1999
• Est. Priority Date: 04/12/1996
• Status: Expired due to Fees

• Current Assignee: Holometrics Incorporated
• Original Assignee: Holometrics Incorporated
• Inventors: Berg, Johnny L., Gerhard, Francis H.
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US09/136,393
• Time in Patent Office: 363 Days
• Field of Search: 356/5.1-5.15, 356/141.1, 356/141.4, 356/376
• US Class Current: 356/5.1
• CPC Class Codes: G01B 11/00 Measuring arrangements characterised by the use of optical techniques
• CPC Class Codes: G01B 11/04 specially adapted for measuring length or width of objects while moving
• CPC Class Codes: G01B 11/24 for measuring contours or curvatures
• CPC Class Codes: G01S 17/36 with phase comparison between the received signal and the contemporaneously transmitted signal
• CPC Class Codes: G01S 17/42 Simultaneous measurement of distance and other co-ordinates indirect measurement G01S17/46
• CPC Class Codes: G01S 17/88 Lidar systems specially adapted for specific applications
• CPC Class Codes: G01S 17/89 for mapping or imaging
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating
• CPC Class Codes: G01S 7/4972 Alignment of sensor

Abstract

A laser scanner for measuring the spatial properties of objects in a scene within a range less than a predetermined maximum object distance. A laser diode supplies a laser beam that is intensity-modulated by a reference waveform from a waveform generator. An optical scanning system scans the scene with the intensity-modulated laser beam, and receives reflected intensity-modulated light and supplies it to an optical processing system that includes an aperture that transmits a first percentage of light reflected from the maximum object distance and a second, lesser percentage of light reflected from objects closer than the maximum object distance. A photodetector receives the processed light and converts the energy into an amplitude-modulated range signal. A mixer down-converts the frequency into an IF frequency, and a first circuit converts the range signal to the form of a square wave. A phase comparator receives the processed range signal and the reference waveform and outputs a highly accurate signal indicative of the phase difference therebetween. In some embodiments a calibration target may be situated within the scene having a plurality of steps with predefined distances to use as a calibration for measurements of the spatial relationships within the scene.

Claims

1. A transmitter and receiver for range scanning of a scene having a plurality of objects up to a predetermined maximum range distance o_max used in a phase-difference scanner-ranging system comprising:

a laser diode for emitting an intensity-modulated laser beam;

means for scanning the scene with the laser beam and receiving reflected light from the objects in said scene;

a collecting lens positioned to receive the reflected light from the scanning means;

an intensity-control substantially circular aperture with fixed opening size spaced apart from the collecting lens at a predetermined distance D1 or less, an image plane of the collecting lens being located at a distance D1 from the collecting lens for the objects located at a distance O₁ from the collecting lens that corresponds to the distance of an image of a reflection from said predetermined maximum range distance o_max, whereby the light reflected from the objects that are closer then o_max is increasingly blocked by the aperture to maintain nearly constant light intensity exiting the aperture; and

a photodetector positioned to receive the light transmitted through the intensity-control aperture;

whereby the photodetector receives a substantially constant reflected signal from the object even though intensity of the reflected laser beam at the collecting lens is varying due to factors that can include object reflectivity and distance variations between the object and the collecting lens, thus enabling accurate phase-angle measurements in the phase-difference scanner ranging system.

2. The laser scanner of claim 1 wherein said intensity-control aperture comprises a circular shape, wherein said light transmitted to the photodetector defines a spot on said photodetector, and further comprising means for maintaining a spot size and position on said photodetector that is approximately constant.

3. The laser scanner of claim 1 wherein the intensity-control aperture is positioned at said image plane distance i_o that corresponds to the distance of an image of a reflection from said predetermined maximum range distance.

4. The laser scanner of claim 1 wherein said optical scanning system includes a horizontal scanning mirror and a vertical scanning mirror orthogonally positioned with respect to said horizontal scanning mirror.

5. A transmitter and receiver for range scanning of a scene having a plurality of objects up to a predetermined maximum range distance o_max used in a phase-difference scanner-ranging system comprising:

a laser diode for emitting an intensity-modulated laser beam;

means for scanning the scene with the laser beam and receiving reflected light from the objects in said scene;

a collecting lens positioned to receive the reflected light from the scanning system;

an intensity-control substantially circular aperture with fixed opening size spaced apart from the collecting lens at a predetermined distance D1 equal to or greater than an image plane distance i_o that corresponds to the distance of an image of a reflection from said predetermined maximum range distance o_max ;

a collimating lens having a focal length f_c approximately positioned with its focal length at said intensity-control aperture;

a narrow band filter situated in the collimated beam, said narrow band filter comprising material for substantially transmitting essentially wavelengths emitted from the laser diode;

a focusing lens positioned to focus the collimated, narrow band beam; and

a photodetector coupled to receive the focused beam from the focusing lens,

whereby the photodetector receives a substantially constant reflected beam signal from the object even though intensity of the reflected beam at the collecting lens is varying due to factors that can include object reflectivity and distance variations between the object and the collecting lens, thus enabling accurate phase-angle measurements in the phase-difference scanner ranging system.

6. The laser scanner of claim 5 wherein said intensity-control aperture comprises a circular shape.

7. The laser scanner of claim 5 wherein the intensity-control aperture is positioned at said image plane distance i_o that corresponds to the distance of an image of a reflection from said predetermined maximum range distance.

8. The laser scanner of claim 5 wherein said optical scanning system includes a horizontal scanning mirror and a vertical scanning mirror orthogonally positioned with respect to said horizontal scanning mirror.

9. A method of regulating reflected light from a scene having a plurality of objects up to a predetermined maximum range distance o_max for use in a phase-difference ranging system, the method comprising:

intensity-modulating a laser diode supply thereby outputting an intensity-modulated laser beam;

receiving reflected intensity-modulated light from the objects in the scene;

applying the reflected intensity-modulated light to a collecting lens;

directing the light from the collecting lens to an intensity-control substantially circular aperture with fixed opening size positioned at a distance D1 or less from the collecting lens for the objects located at a distance O₁ from the collecting lens corresponding to an image from a reflection at the predetermined maximum range distance o_max, whereby light reflected from the objects that are closer then o_max is increasingly blocked by the aperture to maintain nearly constant light intensity exiting the aperture; and

applying the light energy transmitted through the intensity-control aperture to a photodetector to convert the light energy into an electrical signal;

whereby the photodetector receives a substantially constant reflected beam signal from the object even though intensity of the reflected beam at the collecting lens is varying due to factors that can include object reflectivity and distance variations between the object and the collecting lens, thus enabling accurate measurements in the phase-difference ranging system.

10. The method of claim 9 further comprising the steps of:

applying the light energy from the intensity-control aperture to a collimating lens positioned with its focal length at said intensity-control aperture to provide a collimated beam;

filtering the collimated beam with a narrowband filter that substantially transmits the wavelength of the laser diode;

focusing the filtered beam on said photodetector.

11. The method of claim 9 further comprising the step of applying the beam from the collecting lens to said intensity-control aperture that comprises a circular aperture.

12. The method of claim 9 further comprising the step of applying the beam from the collecting lens to said intensity-control aperture positioned approximately at said image plane distance i_o that corresponds to the distance of an image of a reflection from said predetermined maximum range distance.

13. The method of claim 9 further comprising the step of scanning the scene with a horizontal scanning mirror and a vertical scanning mirror operated cooperatively with the horizontal scanning mirror.

14. The method of claim 9 further comprising the steps of:

providing a calibration target having a plurality of steps with predefined distances therebetween;

positioning the calibration target within the scene;

scanning the scene including the calibration target to supply calibration data and object data from the objects in the scene; and

processing the object data using the calibration data as a known distance reference.

15. An electrical processing circuit for a laser scanner that has a waveform generator for supplying a reference waveform at a reference frequency that modulates a laser diode to emit an intensity-modulated laser beam, said electrical processing circuit including a scanning system that scans a scene with the intensity-modulated laser beam and supplies reflected intensity-modulated light to a photodetector that converts the reflected intensity-modulated light into an electrical range signal, comprising:

a first mixer coupled to receive the range signal at the reference frequency and down-convert it to a lower, IF frequency;

an IF amplifier coupled to receive the IF frequency output of the first mixer;

a voltage clamping circuit coupled to receive the output from the IF amplifier and limit the voltage to a predetermined value;

a DC amplifier coupled to receive the output signal from the voltage clamping circuit;

a comparator coupled to receive the output signal from the DC amplifier and supply an approximately square first waveform signal whose zero crossings substantially correspond with the zero crossings of the input signal and whose amplitude is substantially independent of the amplitude of the input signal;

a second mixer coupled to receive the reference waveform at the reference frequency and down-convert it to the lower, IF frequency;

a phase detector coupled to compare the first square waveform signal with the down-converted reference waveform and provide an output indicative of the phase difference therebetween; and

a low signal circuit for detecting an electrical range signal having a value less than a predetermined value, and responsive thereto, for replacing said range signal with an alternate range signal having a predetermined minimum value.

16. The electrical processing circuit of claim 15, wherein the lower frequency is within a range of about 2 MHz to 4 MHz.

17. The electrical processing circuit of claim 15 further comprising a low gain, high frequency amplifier for amplifying the electrical range signal before providing it to the mixer.

18. The electrical processing circuit of claim 15, further comprising:

a second IF amplifier coupled to receive the IF frequency output of the second mixer;

a second voltage clamping circuit coupled to receive the output from the second IF amplifier and limit the voltage to a predetermined value;

a second DC amplifier coupled to receive the output signal from the second voltage clamping circuit; and

a second comparator coupled to receive the output signal from the second DC amplifier and supply an approximately square second waveform signal whose zero crossings substantially correspond with the zero crossings of the reference signal.

19. The electrical processing circuit of claim 18 further comprising a low signal circuit for detecting an electrical range signal having a value less than a predetermined value, and responsive thereto, for replacing said range signal with an alternate range signal having a predetermined minimum value.

20. The electrical processing circuit of claim 15 wherein said low signal circuit is coupled to the output of said DC amplifier, said low signal circuit including a circuit for sensing the signal from said DC amplifier and a circuit for supplying an alternate signal if said signal from the DC amplifier is below a predetermined value.

21. The electrical processing circuit of claim 15 wherein said voltage clamping circuit comprises a Schottky diode coupled to ground and further comprises an amplifier coupled to receive the input signal and supply it to the input of the Schottky diode.

22. A laser scanner comprising:

a waveform generator that generates a predetermined periodic reference signal;

a laser diode that supplies an intensity-modulated laser beam responsive to the reference signal;

an optical scanning system for scanning a scene with the intensity-modulated laser beam and receiving a reflected signal of intensity-modulated light from objects in the scene;

a photodetector positioned to detect the reflected signal and supply a range signal responsive to an amplitude thereof;

a phase detector coupled to compare the reference signal with the reflected signal and provide an output signal that includes;

a) a range signal corresponding to the phase-difference between the reference signal and the reflected signal and b) a ripple signal;

an analog-to-digital (A/D) converter coupled to receive the output signal of the phase detector, and to sample said output signal responsive to sampling command signals from a synchronization circuit; and

means for supplying the sampling command signals to the A/D converter such that the sampling signals are generated at a time interval following a zero crossing of the reference signal but before the crossing of the range signal, whereby such sampling of the range signal provides consistent phase-difference measurements.

23. The laser scanner of claim 22 wherein the means for supplying a sampling signal comprises a pulse synchronization circuit coupled to receive the reference signal, said pulse synchronization circuit comprising a frequency divider for reducing the frequency of the reference signal and a phase shift circuit for delaying the output of the frequency divider by a predetermined time interval, said pulse synchronization circuit coupled to provide said sampling signal to said A/D converter.