Laser scanning system

• US 5,831,719 A
• Filed: 04/12/1996
• Issued: 11/03/1998
• 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:
  US08/631,165
• Time in Patent Office: 935 Days
• Field of Search: 356/4.01, 356/5.01-5.15, 356/141.1, 356/141.4, 356/376
• US Class Current: 356/5.13
• 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 laser scanner for measuring objects in a scene having a plurality of objects and a calibration target within a range less than a predetermined maximum object distance, comprising:

a waveform generator that generates a predetermined reference waveform;

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

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

optical means for processing said reflected intensity-modulated light, said optical means including an intensity-control 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 positioned to receive said processed intensity-modulated light from the optical means, said photodetector converting energy in said light into an amplitude-modulated range signal;

a first electrical circuit for converting the amplitude-modulated range signal into a converted waveform that has falling and rising edges that correspond to the zero crossings in said amplitude-modulated range signal;

a second electrical circuit for converting the reference waveform into a second converted waveform that has falling and rising edges that correspond to the zero crossings in said reference waveform;

a phase comparator coupled to receive said first and second converted waveforms and output a signal indicative of the phase difference therebetween; and

a processor means for calibrating measurements of distances to objects in the scene from distances to the calibration target.

2. The laser scanner of claim 1 wherein said intensity-control aperture comprises a circular shape.

3. The laser scanner of claim 1 wherein said waveform generator comprises means for forming a sinusoidal waveform, so that said reference waveform has a sinusoidal shape and said intensity-modulated laser beam has a sinusoidally-varying intensity.

4. The laser scanner of claim 1 wherein said first electrical circuit includes a circuit for converting the amplitude-modulated range signal into an approximately square waveform so that said first converted waveform comprises said square waveform.

5. The laser scanner of claim 4 wherein said second electrical circuit includes a circuit for converting the reference waveform into an approximately square waveform so that said second converted waveform comprises said square waveform.

6. The laser scanner of claim 1 wherein said optical scanning system includes a horizontal scanning mirror.

7. The laser scanner of claim 6 wherein said optical scanning system includes a vertical scanning mirror operated cooperatively with the horizontal scanning mirror to scan a predetermined area.

11. The laser scanner of claim 1 wherein:

said first electrical circuit includes a circuit for converting the amplitude-modulated range signal into an approximately square waveform so that said first converted waveform comprises said square waveform; and

said second electrical circuit includes a circuit for converting the reference waveform into an approximately square waveform so that said second converted waveform comprises said square waveform.

8. A method for measuring objects and a calibration target in a scene having a range less than a predetermined maximum object distance, comprising the steps of:

providing a laser scanner that includes

a waveform generator that generates a predetermined reference waveform,

a laser diode that supplies an intensity-modulated laser beam responsive to the reference waveform, and

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

situating said calibration target within said scene, said calibration target having a plurality of steps with predefined distances between them;

scanning the scene with said intensity-modulated laser beam to provide intensity-modulated reflected light from the objects in said scene;

processing said intensity-modulated light reflected from the objects in the scene, including transmitting a first percentage of light reflected from the maximum object distance and a second, smaller percentage of light reflected from closer objects;

applying said processed intensity-modulated light to a photodetector to provide an amplitude-modulated electrical range signal;

converting the range signal into a converted waveform that has falling and rising edges that correspond to the zero crossings in said amplitude-modulated range signal;

converting the reference waveform into a second converted waveform that has falling and rising edges that correspond to the zero crossings in said reference waveform;

detecting the phase difference between said first and second converted waveforms and providing a phase difference signal indicative of distances to the objects and the calibration target distances in the scene; and

processing the phase difference signal using information from the calibration target to calibrate measurements which arc used to define distances to the objects in the scene.

9. The laser scanner of claim 8 wherein the step of processing the intensity-modulated light is performed with an intensity-control aperture having a circular shape.

10. The laser scanner of claim 8 wherein said waveform generator comprises means for forming a sinusoidal waveform, so that said reference waveform has a sinusoidal shape and said intensity-modulated laser beam has a sinusoidally-varying intensity.

12. The laser scanner of claim 8 wherein said optical scanning system includes a horizontal scanning mirror and a vertical scanning mirror operated cooperatively with the horizontal scanning mirror to scan a predetermined area.

13. 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;

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.

14. The electrical processing circuit of claim 13 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.

15. The electrical processing circuit of claim 14 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.

16. A method of scanning a scene with a laser scanner, comprising the steps of:

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

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.