Miniature laser tracker

• US 6,618,132 B1
• Filed: 09/12/1997
• Issued: 09/09/2003
• Est. Priority Date: 09/12/1997
• Status: Expired due to Fees

• Current Assignee: Lawrence Livermore National Security LLC (Government of the United States of America)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Vann, Charles S.
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US08/928,242
• Time in Patent Office: 2,188 Days
• Field of Search: 356/141.1, 356/4.07, 356/141.4, 359/169, 359/529, 250/566, 250/568, 235/462, 235/462.03, 235/384, 235/494
• US Class Current: 356/141.1
• CPC Class Codes: G01S 17/42 Simultaneous measurement of distance and other co-ordinates indirect measurement G01S17/46
• CPC Class Codes: G01S 17/46 Indirect determination of position data
• CPC Class Codes: G01S 7/481 Constructional features, e.g. arrangements of optical elements
• CPC Class Codes: G01S 7/4817 relating to scanning
• CPC Class Codes: G01S 7/499 using polarisation effects

Abstract

This small, inexpensive, non-contact laser sensor can detect the location of a retroreflective target in a relatively large volume and up to six degrees of position. The tracker'"'"'s laser beam is formed into a plane of light which is swept across the space of interest. When the beam illuminates the retroreflector, some of the light returns to the tracker. The intensity, angle, and time of the return beam is measured to calculate the three dimensional location of the target. With three retroreflectors on the target, the locations of three points on the target are measured, enabling the calculation of all six degrees of target position. Until now, devices for three-dimensional tracking of objects in a large volume have been heavy, large, and very expensive. Because of the simplicity and unique characteristics of this tracker, it is capable of three-dimensional tracking of one to several objects in a large volume, yet it is compact, light-weight, and relatively inexpensive. Alternatively, a tracker produces a diverging laser beam which is directed towards a fixed position, and senses when a retroreflective target enters the fixed field of view. An optically bar coded target can be read by the tracker to provide information about the target. The target can be formed of a ball lens with a bar code on one end. As the target moves through the field, the ball lens causes the laser beam to scan across the bar code.

Claims

1. A laser tracker for detecting a retroreflective target on an object, comprising:

a laser which produces an output beam;

a linear polarizer tilted at an angle with respect to the laser beam which passes a beam having a first linear polarization;

a quarter wave plate positioned after the polarizer which passes the beam and converts the beam to a circular polarization;

a reflector affixed to a target;

a scanning optical system for directing the circularly polarized beam into a field of view wherein the beam will illuminate said retroreflective target which is present in the field of view, thereby producing a reflected beam which is directed back to the quarter wave plate which converts the beam to a beam having a second linear polarization orthogonal to the first linear polarization, and then to the linear polarizer which reflects the beam;

a photodetector positioned to receive the reflected beam which is also focused onto the detector by the optical system, and producing a detector signal;

a signal processing system connected to the detector to obtain information about the target from the detector signal, wherein said scanning optical system comprises;

a first cylindrical lens between the laser and polarizer to diverge the beam in one dimension to produce an expanding planar beam;

a scanning mirror assembly after the quarter wave plate to scan the beam over a scanning plane; and

a second cylindrical lens before the photodetector to focus the reflected beam onto the photodetector.

2. The laser tracker of claim 1, further comprising:

proximity sensor for detecting the start of each scanning cycle.

3. The laser tracker of claim 2, wherein the photodetector is a position sensitive detector (PSD) which generates two output signals whose total is proportional to the intensity of the focused reflected beam incident on the PSD and whose difference is related to the position of the focused reflected beam on the PSD.

4. The laser tracker of claim 3, wherein the signal processing system comprises:

a signal processor for processing analog output signals from the proximity sensor and the PSD;

a digitizer connected to the signal processor to digitize analog signals from the signal processor; and

a computer connected to the digitizer for processing the digitized signals from the digitizer.

5. The laser tracker of claim 4, wherein the scanning plane is orthogonal to the beam plane and the computer includes:

a range calculator for calculating target range from the ratio of reflected beam intensity measured as the total PSD output signal to laser output beam intensity;

an in-plane angle calculator for calculating the angle of the target in the scanning plane from the difference in arrival time of the reflected beam at the PSD and the start of a scanning cycle from the proximity sensor;

an out-of-plane angle calculator for calculating the angle of the target out of the scanning plane from the difference between the two PSD output signals.

6. The laser tracker of claim 1,

wherein the signal processing system comprises:

a digitizer connected to the photodetector to digitize the analog signals from the photodetector; and

a computer connected to the digitizer for processing the digitized signals from the digitizer,

wherein the reflective target is an optically bar coded target and the computer includes software for processing bar coded information from the target.

7. The laser tracker of claim 1, wherein the target is an optically bar coded retroreflective target.

8. The laser tracker of claim 1, further comprising a bandpass filter which passes the laser beam positioned before the photodetector.

9. The laser tracker of claim 1, wherein the photodetector is selected from the group consisting of a position sensitive detector, an array of photodiode elements and a camera.

10. A laser tracker for detecting a retroreflective target on an object comprising:

a laser which produces an output beam;

a first cylindrical lens after the laser to diverge the beam in one dimension to produce an expanding planar beam;

a linear polarizer after the first cylindrical lens and tilted at an angle with respect to the laser beam which passes a beam having a first linear polarization;

a quarter wave plate positioned after the polarizer which passes the beam and converts the beam to a circular polarization;

a scanning system for scanning the circularly polarized expanding planar beam in a scanning plane across a field of view wherein the beam will illuminate a retroreflective target which is present in the field of view, thereby producing a reflected beam which is directed back to the quarter wave plate which converts the beam to a beam having a second linear polarization orthogonal to the first linear polarization, and then to the linear polarizer which reflects the beam to a reflected path;

a proximity sensor for detecting the start of each scanning cycle;

a bandpass filter which passes the laser beam positioned in the reflected path;

a second cylindrical lens after the filter to compensate for the beam divergence produced by the first cylindrical lens to produce a focused reflected beam;

a position sensitive detector (PSD) positioned to receive the focused reflected beam and which generates two output signals whose total is proportional to the intensity of the focused reflected beam incident on the PSD and whose difference is related to the position of the focused reflected beam on the PSD;

a signal processing system connected to the PSD and proximity sensor to obtain information about the target from the detector signal.

11. The laser tracker of claim 10, wherein the scanning plane is orthogonal to the beam plane and the signal processing system includes a computer which calculates:

target range from the ratio of reflected beam intensity measured as the total PSD output signal to laser output beam intensity;

the angle of the target in the scanning plane from the difference in arrival time of the reflected beam at the PSD and the start of a scanning cycle from the proximity sensor;

the angle of the target out of the scanning plane from the difference between the two PSD output signals.

12. The laser tracker of claim 10, wherein the laser is a diode laser.

13. A laser tracker for detecting a retroreflective target on an object, comprising:

a laser which produces an output beam;

a linear polarizer tilted at an angle with respect to the laser beam which passes a beam having a first linear polarization;

a quarter wave plate positioned after the polarizer which passes the beam and converts the beam to a circular polarization;

a bar coded retroreflecting target;

a diverging lens after the quarter wave plate to produce a diverging beam which is directed into a fixed field of view wherein the beam will illuminate said retroreflective target which is present in the field of view, thereby producing a reflected beam which is directed back through the diverging lens to converge the beam to the quarter wave plate which converts the beam to a beam having a second linear polarization orthogonal to the first linear polarization, and then to the linear polarizer which reflects the beam into a reflected path;

a bandpass filter which passes the laser beam positioned in the reflected path;

a photodetector positioned to receive the reflected beam and producing a detector signal;

a signal processing system connected to the detector to obtain information about the target from the detector signal, wherein the target is a ball lens having a bar code on a reflective surface thereof.

14. The laser tracker-target combination of claim 13, wherein the target is a corner cube having a bar code on a reflective surface thereof.