Source: http://patents.com/us-8902408.html
Timestamp: 2019-07-22 10:27:36
Document Index: 266065806

Matched Legal Cases: ['Application No. 201010251189', 'Application No. 201010251189', 'Application No. 2010', 'Application No. 2010', 'Application No. 102010038955', 'Application No. 2014', 'Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 2010', 'Application No. 2010']

US Patent # 8,902,408. Laser tracker used with six degree-of-freedom probe having separable spherical retroreflector - Patents.com
United States Patent 8,902,408
Bridges December 2, 2014
A method for measuring three-dimensional coordinates of a probe center includes: providing a spherically mounted retroreflector; providing a probe assembly; providing an orientation sensor; providing a coordinate measurement device; placing the spherically mounted retroreflector on the probe head; directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector; measuring the first distance; measuring the first angle of rotation; measuring the second angle of rotation; measuring the three orientational degrees of freedom based at least in part on information provided by the orientation sensor; calculating the three-dimensional coordinates of the probe center based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom; and storing the three-dimensional coordinates of the probe center.
Bridges; Robert E. (Kennett Square, PA)
Bridges; Robert E.
Faro Technologies Inc. (Lake Mary, FL)
13/453,002
US 20130155386 A1 Jun 20, 2013
13407983 Feb 29, 2012 8467072
13370339 Feb 10, 2012
61592049 Jan 30, 2012
61448823 Mar 3, 2011
61442452 Feb 14, 2011
61475703 Apr 15, 2011
Current U.S. Class: 356/4.01; 342/118; 342/140; 342/147; 342/156; 342/5; 342/61; 342/73; 356/3.01; 356/5.01; 356/625; 356/9; 367/118; 367/13; 367/21; 367/27; 367/28; 367/64
Field of Search: ;356/3.01,4.01,5.01,9,625 ;342/5,61,73,118,140,147,156 ;367/13,21,27,28,64,118
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The present application claims the benefit of U.S. Provisional Patent Application No. 61/592,049, filed Jan. 30, 2012, the contents of which are hereby incorporated by reference. The present application also claims the benefit of U.S. patent application Ser. No. 13/407,983, filed Feb. 29, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/448,823, filed Mar. 3, 2011, the contents of which are hereby incorporated by reference. The U.S. patent application Ser. No. 13/407,983 also claims the benefit of U.S. patent application Ser. No. 13/370,339, filed Feb. 10, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/442,452, filed Feb. 14, 2011, the contents of which are hereby incorporated by reference. The U.S. patent application Ser. No. 13/407,983 also claims the benefit of U.S. Provisional Patent Application No. 61/475,703, filed 15 Apr. 2011, and U.S. Provisional Patent Application No. 61/592,049, filed Jan. 30, 2012, the contents of which are hereby incorporated by reference.
1. A method for measuring three-dimensional coordinates of a first point and a second point, the method comprising: providing a spherically mounted retroreflector, the spherically mounted retroreflector including a retroreflector mounted in a retroreflector body, the retroreflector body having a first spherical shape over a first portion of its outer surface, the first portion having a target center, the retroreflector configured to receive a first beam of light and to return a second beam of light, the second beam of light being a portion of the first beam of light, the second beam of light traveling in a direction substantially opposite the direction of the first beam of light; providing a probe assembly, the probe assembly including a probe stylus and a probe head, the probe stylus including a probe tip, the probe tip having a second spherical shape over a second portion of its surface, the second portion having a probe center, the probe head configured to receive the spherically mounted retroreflector and to permit rotation of the spherically mounted retroreflector about the target center while keeping the target center at a substantially constant position relative to the probe assembly; providing a coordinate measurement device, the coordinate measurement device including a first motor, a second motor, a first angle measuring device, a second angle measuring device, a distance meter, a position detector, a control system, an orientation sensor, and a processor, the first motor and the second motor configured together to direct the first beam of light to a first direction, the first direction determined by a first angle of rotation about a first axis and a second angle of rotation about a second axis, the first angle of rotation produced by the first motor and the second angle of rotation produced by the second motor, the first angle measuring device configured to measure the first angle of rotation and the second angle measuring device configured to measure the second angle of rotation, the distance meter configured to measure a first distance from the coordinate measurement device to the spherically mounted retroreflector based at least in part on a third portion of the second beam of light received by a first optical detector, the position detector configured to produce a first signal in response to a position of a fourth portion of the second beam of light on the position detector, the control system configured to send a second signal to the first motor and a third signal to the second motor, the second signal and the third signal based at least in part on the first signal, the control system configured to adjust the first direction of the first beam of light to the position in space of the spherically mounted retroreflector, the orientation sensor configured to measure three orientational degrees of freedom of the probe assembly, the processor configured to determine three-dimensional coordinates of the probe center, the three-dimensional coordinates based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom; placing the spherically mounted retroreflector on the probe head; moving the probe center to the first point; directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector; measuring the first distance; measuring the first angle of rotation; measuring the second angle of rotation; measuring the three orientational degrees of freedom based at least in part on information provided by the orientation sensor; determining a first set of three-dimensional coordinates of the probe center based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom; storing the first set of three-dimensional coordinates of the probe center; removing the spherically mounted retroreflector from the probe head; moving the target center to the second point after removing the spherically mounted retroreflector from the probe head; directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector with the target center in the second point; measuring a second distance with the target center in the second point; measuring a third angle of rotation with the target center in the second point; measuring a fourth angle of rotation with the target center in the second point; determining a second set of three-dimensional coordinates of the target center based at least in part on the second distance, the third angle of rotation, and the fourth angle of rotation; and storing the second set of three-dimensional coordinates of the target center.
2. The method of claim 1, wherein the step of placing the spherically mounted retroreflector on the probe head further includes placing the spherically mounted retroreflector into an indexed position.
3. The method of claim 2, wherein: the step of providing a spherically mounted retroreflector further includes providing the spherically mounted retroreflector with a collar, the collar including indexing features; the step of providing a probe assembly further includes providing the probe head with a mating structure, the mating structure having mating features; and the step of placing the spherically mounted retroreflector on the probe head further includes moving the spherically mounted retroreflector into the indexed position by registering the indexing features with the mating features.
4. The method of claim 1, wherein the step of placing the spherically mounted retroreflector on the probe head further includes rotating the spherically mounted retroreflector relative to the probe head into a desired orientation without translating the target center.
5. The method of claim 4, wherein: the step of providing a probe assembly further includes providing the probe head with a clamping device, the clamping device configured to hold the spherically mounted retroreflector in a fixed orientation; and the step of placing the spherically mounted retroreflector further includes clamping the spherically mounted retroreflector into the desired orientation.
6. The method of claim 4, further includes the step of determining probe compensation parameters, the probe compensation parameters including at least information indicative of an orientation of the spherically mounted retroreflector in relation to the probe assembly.
7. The method of claim 6, wherein the step of determining probe compensation parameters further includes steps of: providing a nest configured to accept the probe tip, the nest further configured to permit rotation of the probe tip about the probe center, the probe center held at a substantially fixed point in space; placing the probe tip into the nest; rotating the probe tip; measuring a collection of quantities, the collection of quantities including the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom, each of the measured values obtained for a plurality of different rotations of the probe tip; and calculating the probe compensation parameters based at least in part on the collection of quantities.
8. The method of claim 1, further including steps of: incorporating a pattern into the retroreflector; providing an optical system including a lens and a photosensitive array, the lens configured to form an image of at least a portion of the patterned retroreflector on the photosensitive array; converting the image into a digital data set; and calculating the three orientational degrees of freedom based at least in part on the digital data set.
9. The method of claim 8, wherein, in the step of providing a spherically mounted retroreflector, the retroreflector is a glass cube-corner retroreflector.
10. The method of claim 1, wherein, in the step of providing the probe assembly, the probe assembly operates without the use of electrical power.
11. The method of claim 1, wherein the probe assembly includes an actuator button.
12. The method of claim 1, wherein in the step of providing the probe assembly the probe stylus is removed and is replaced by a second probe stylus.
13. The method of claim 1 wherein the step of placing the spherically mounted retroreflector on the probe head includes magnetically coupling the spherically mounted retroreflector to the probe head.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker can use the position of the light on the position detector to adjust the rotation angles of the mechanical axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) an SMR that is moved over the surface of an object of interest. The gimbal mechanism used for a laser tracker may be used for a variety of other applications. As a simple example, the laser tracker may be used in a gimbal steering device having a visible pointer beam but no distance meter to steer a light beam to series of retroreflector targets and measure the angles of each of the targets.
Several laser trackers are available or have been proposed for measuring six, rather than the ordinary three, degrees of freedom. Exemplary six degree-of-freedom (six-DOF) systems are described by U.S. Pat. No. 7,800,758 ('758) to Bridges et al., the contents of which are herein incorporated by reference, and U.S. Published Patent Application No. 2010/0128259 to Bridges et al., the contents of which are herein incorporated by reference.
In the past, six-DOF probes and SMRs have been separate accessories that were relatively expensive. What is needed is an accessory that is relatively inexpensive and that combines the functionality of an SMR and a six-DOF probe.
A method for measuring three-dimensional coordinates of a probe center includes the steps of: providing a spherically mounted retroreflector, the spherically mounted retroreflector including a retroreflector mounted in a retroreflector body, the retroreflector body having a first spherical shape over a first portion of its outer surface, the first portion having a target center, the retroreflector configured to receive a first beam of light and to return a second beam of light, the second beam of light being a portion of the first beam of light, the second beam of light traveling in a direction substantially opposite the direction of the first beam of light. The method also includes: providing a probe assembly, the probe assembly including a probe stylus and a probe head, the probe stylus including the probe tip, the probe tip having a second spherical shape over a second portion of its surface, the second portion having a probe center, the probe head configured to receive the spherically mounted retroreflector and to permit rotation of the spherically mounted retroreflector about the target center while keeping the target center at a substantially constant position relative to the probe assembly. The method further includes: providing an orientation sensor, the orientation sensor configured to measure three orientational degrees of freedom of the probe assembly; providing a coordinate measurement device, the coordinate measurement device including a first motor, a second motor, a first angle measuring device, a second angle measuring device, a distance meter, a position detector, a control system, and a processor, the first motor and the second motor configured together to direct the first beam of light to a first direction, the first direction determined by a first angle of rotation about a first axis and a second angle of rotation about a second axis, the first angle of rotation produced by the first motor and the second angle of rotation produced by the second motor, the first angle measuring device configured to measure the first angle of rotation and the second angle measuring device configured to measure the second angle of rotation, the distance meter configured to measure a first distance from the coordinate measurement device to the spherically mounted retroreflector based at least in part on a third portion of the second beam of light received by a first optical detector, the position detector configured to produce a first signal in response to a position of a fourth portion of the second beam of light on the position detector, the control system configured to send a second signal to the first motor and a third signal to the second motor, the second signal and the third signal based at least in part on the first signal, the control system configured to adjust the first direction of the first beam of light to the position in space of the spherically mounted retroreflector, the processor configured to determine three-dimensional coordinates of the probe center, the three-dimensional coordinates based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom. The method also includes: placing the spherically mounted retroreflector on the probe head; directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector; measuring the first distance; measuring the first angle of rotation; measuring the second angle of rotation; measuring the three orientational degrees of freedom based at least in part on information provided by the orientation sensor; calculating the three-dimensional coordinates of the probe center based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom; and storing the three-dimensional coordinates of the probe center.
FIG. 11 is a block diagram of the computing and communication elements of a laser tracker in accordance according to an embodiment of the present invention;
FIGS. 14A and 14B are front views of a six-DOF SMR magnetically attached to a six-DOF probe base and a six-DOF SMR clamped onto a six-DOF probe base, respectively, according to an embodiment of the present invention;
FIG. 15A is a front view of a probe assembly and a six-DOF SMR placed on a compensation fixture, and FIG. 15B is a cross-sectional view of the compensation fixture according to an embodiment of the present invention;
FIGS. 16A-C are front views indicating the yaw, pitch, and roll movements of the six-DOF probe to obtain compensation parameters according to an embodiment of the present invention;
FIG. 17 is a front view of a probe assembly and a six-DOF SMR, the probe assembly and SMR having indexing and mating features according to an embodiment of the present invention;
FIG. 18 is a flow chart of a method for measuring three-dimensional coordinates according to an embodiment of the present invention;
FIG. 19 is a flow chart of a method that begins at reference marker A of FIG. 18, according to an embodiment of the present invention;
FIG. 20 is a flow chart of a method that begins at reference marker B of FIG. 19, according to an embodiment of the present invention;
FIG. 21 is a flow chart of a method that begins at reference marker A of FIG. 18, according to an embodiment of the present invention;
FIG. 22 is a flow chart of a method that begins at reference marker C of FIG. 21, according to an embodiment of the present invention; and
FIG. 23 is a flow chart of a method that begins at reference marker A of FIG. 18, according to an embodiment of the present invention.
Magnetic nests 17 may be included on the laser tracker for resetting the laser tracker to a "home" position for different sized SMRs--for example, 1.5, 7/8, and 1/2 inch SMRs. An on-tracker retroreflector 19 may be used to reset the tracker to a reference distance. In addition, an on-tracker mirror, not visible from the view of FIG. 1, may be used in combination with the on-tracker retroreflector to enable performance of a self-compensation, as described in U.S. Pat. No. 7,327,446, the contents of which are incorporated by reference.
FIG. 3 is a block diagram showing optical and electrical elements in a laser tracker embodiment. It shows elements of a laser tracker that emit two wavelengths of light--a first wavelength for an ADM and a second wavelength for a visible pointer and for tracking. The visible pointer enables the user to see the position of the laser beam spot emitted by the tracker. The two different wavelengths are combined using a free-space beam splitter. Electrooptic (EO) system 100 includes visible light source 110, isolator 115, optional first fiber launch 170, optional interferometer (IFM) 120, beam expander 140, first beam splitter 145, position detector assembly 150, second beam splitter 155, ADM 160, and second fiber launch 170.
Beam expander 140 may be set up using a variety of lens configurations, but two commonly used prior-art configurations are shown in FIGS. 4A, 4B. FIG. 4A shows a configuration 140A based on the use of a negative lens 141A and a positive lens 142A. A beam of collimated light 220A incident on the negative lens 141A emerges from the positive lens 142A as a larger beam of collimated light 230A. FIG. 4B shows a configuration 140B based on the use of two positive lenses 141B, 142B. A beam of collimated light 220B incident on a first positive lens 141B emerges from a second positive lens 142B as a larger beam of collimated light 230B. Of the light leaving the beam expander 140, a small amount reflects off the beam splitters 145, 155 on the way out of the tracker and is lost. That part of the light that passes through the beam splitter 155 is combined with light from the ADM 160 to form a composite beam of light 188 that leaves that laser tracker and travels to the retroreflector 90.
The light from the fiber network 166 enters ADM electronics 164 through optical fibers 168, 169. An embodiment of prior art ADM electronics is shown in FIG. 7. Optical fiber 168 in FIG. 3 corresponds to optical fiber 3232 in FIG. 7, and optical fiber 169 in FIG. 3 corresponds to optical fiber 3230 in FIG. 7. Referring now to FIG. 7, ADM electronics 3300 includes a frequency reference 3302, a synthesizer 3304, a measure detector 3306, a reference detector 3308, a measure mixer 3310, a reference mixer 3312, conditioning electronics 3314, 3316, 3318, 3320, a divide-by-N prescaler 3324, and an analog-to-digital converter (ADC) 3322. The frequency reference, which might be an oven-controlled crystal oscillator (OCXO), for example, sends a reference frequency f.sub.REF, which might be 10 MHz, for example, to the synthesizer, which generates two electrical signals--one signal at a frequency f.sub.RF and two signals at frequency f.sub.a). The signal fRF goes to the light source 3102, which corresponds to the light source 162 in FIG. 3. The two signals at frequency f.sub.LO go to the measure mixer 3310 and the reference mixer 3312. The light from optical fibers 168, 169 in FIG. 3 appear on fibers 3232, 3230 in FIG. 7, respectively, and enter the reference and measure channels, respectively. Reference detector 3308 and measure detector 3306 convert the optical signals into electrical signals. These signals are conditioned by electrical components 3316, 3314, respectively, and are sent to mixers 3312, 3310, respectively. The mixers produce a frequency f.sub.if equal to the absolute value of f.sub.LO-f.sub.RF. The signal f.sub.RF may be a relatively high frequency, for example, 2 GHz, while the signal f.sub.IF may have a relatively low frequency, for example, 10 kHz.
The reference frequency f.sub.REF is sent to the prescaler 3324, which divides the frequency by an integer value. For example, a frequency of 10 MHz might be divided by 40 to obtain an output frequency of 250 kHz. In this example, the 10 kHz signals entering the ADC 3322 would be sampled at a rate of 250 kHz, thereby producing 25 samples per cycle. The signals from the ADC 3322 are sent to a data processor 3400, which might, for example, be one or more digital signal processor (DSP) units located in ADM electronics 164 of FIG. 3.
FIG. 6E shows a position detector assembly according to embodiments of the present invention that includes an optical conditioner 149E. Optical conditioner contains a lens 153 and may also contain optional wavelength filter 154. In addition, it includes at least one of a diffuser 156 and a spatial filter 157. As explained hereinabove, a popular type of retroreflector is the cube-corner retroreflector. One type of cube corner retroreflector is made of three mirrors, each joined at right angles to the other two mirrors. Lines of intersection at which these three mirrors are joined may have a finite thickness in which light is not perfectly reflected back to the tracker. The lines of finite thickness are diffracted as they propagate so that upon reaching the position detector they may not appear exactly the same as at the position detector. However, the diffracted light pattern will generally depart from perfect symmetry. As a result, the light that strikes the position detector 151 may have, for example, dips or rises in optical power (hot spots) in the vicinity of the diffracted lines. Because the uniformity of the light from the retroreflector may vary from retroreflector to retroreflector and also because the distribution of light on the position detector may vary as the retroreflector is rotated or tilted, it may be advantageous to include a diffuser 156 to improve the smoothness of the light that strikes the position detector 151. It might be argued that, because an ideal position detector should respond to a centroid and an ideal diffuser should spread a spot symmetrically, there should be no effect on the resulting position given by the position detector. However, in practice the diffuser is observed to improve performance of the position detector assembly, probably because the effects of nonlinearities (imperfections) in the position detector 151 and the lens 153. Cube corner retroreflectors made of glass may also produce non-uniform spots of light at the position detector 151. Variations in a spot of light at a position detector may be particularly prominent from light reflected from cube corners in six-DOF targets, as may be understood more clearly from commonly assigned U.S. patent application Ser. No. 13/370,339 ('339) filed Feb. 10, 2012, and Ser. No. 13/407,983 ('983), filed Feb. 29, 2012, the contents of which are incorporated by reference. In an embodiment, the diffuser 156 is a holographic diffuser. A holographic diffuser provides controlled, homogeneous light over a specified diffusing angle. In other embodiments, other types of diffusers such as ground glass or "opal" diffusers are used.
The purpose of the spatial filter 157 of the position detector assembly 150E is to block ghost beams that may be the result, for example, of unwanted reflections off optical surfaces, from striking the position detector 151. A spatial filter includes a plate 157 that has an aperture. By placing the spatial filter 157 a distance away from the lens equal approximately to the focal length of the lens, the returning light 243E passes through the spatial filter when it is near its narrowest--at the waist of the beam. Beams that are traveling at a different angle, for example, as a result of reflection of an optical element strike the spatial filter away from the aperture and are blocked from reaching the position detector 151. An example is shown in FIG. 6E, where an unwanted ghost beam 244E reflects off a surface of the beam splitter 145 and travels to spatial filter 157, where it is blocked. Without the spatial filter, the ghost beam 244E would have intercepted the position detector 151, thereby causing the position of the beam 243E on the position detector 151 to be incorrectly determined. Even a weak ghost beam may significantly change the position of the centroid on the position detector 151 if the ghost beam is located a relatively large distance from the main spot of light.
As explained hereinabove, the position detector performs two important functions--enabling tracking and correcting measurements to account for the movement of the retroreflector. The position sensor within the position detector may be any type of device capable of measuring a position. For example, the position sensor might be a position sensitive detector or a photosensitive array. The position sensitive detector might be lateral effect detector or a quadrant detector, for example. The photosensitive array might be a CMOS or CCD array, for example.
In an embodiment, the fiber network 166 of FIG. 3 is fiber network 420C of FIG. 8C. Here the optical fibers 184, 186, 168, 169 of FIG. 3 correspond to optical fibers 447, 455, 423, 424 of FIG. 8C. The fiber network 420C includes a first fiber coupler 445 and a second fiber coupler 451. The first fiber coupler 445 is a 2.times.2 coupler having two input ports and two output ports. Couplers of this type are usually made by placing two fiber cores in close proximity and then drawing the fibers while heated. In this way, evanescent coupling between the fibers can split off a desired fraction of the light to the adjacent fiber. The second fiber coupler 451 is of the type called a circulator. It has three ports, each having the capability of transmitting or receiving light, but only in the designated direction. For example, the light on optical fiber 448 enters port 453 and is transported toward port 454 as indicated by the arrow. At port 454, light may be transmitted to optical fiber 455. Similarly, light traveling on port 455 may enter port 454 and travel in the direction of the arrow to port 456, where some light may be transmitted to the optical fiber 424. If only three ports are needed, then the circulator 451 may suffer less losses of optical power than the 2.times.2 coupler. On the other hand, a circulator 451 may be more expensive than a 2.times.2 coupler, and it may experience polarization mode dispersion, which can be problematic in some situations.
FIGS. 9 and 10 show exploded and cross sectional views, respectively, of a prior art laser tracker 2100, which is depicted in FIGS. 2 and 3 of U.S. Published Patent Application No. 2010/0128259 to Bridges et al., incorporated by reference. Azimuth assembly 2110 includes post housing 2112, azimuth encoder assembly 2120, lower and upper azimuth bearings 2114A, 2114B, azimuth motor assembly 2125, azimuth slip ring assembly 2130, and azimuth circuit boards 2135.
FIG. 11 is a block diagram depicting a dimensional measurement electronics processing system 1500 that includes a laser tracker electronics processing system 1510, processing systems of peripheral elements 1582, 1584, 1586, computer 1590, and other networked components 1600, represented here as a cloud. Exemplary laser tracker electronics processing system 1510 includes a master processor 1520, payload functions electronics 1530, azimuth encoder electronics 1540, zenith encoder electronics 1550, display and user interface (UI) electronics 1560, removable storage hardware 1565, radio frequency identification (RFID) electronics, and an antenna 1572. The payload functions electronics 1530 includes a number of subfunctions including the six-DOF electronics 1531, the camera electronics 1532, the ADM electronics 1533, the position detector (PSD) electronics 1534, and the level electronics 1535. Most of the subfunctions have at least one processor unit, which might be a digital signal processor (DSP) or field programmable gate array (FPGA), for example. The electronics units 1530, 1540, and 1550 are separated as shown because of their location within the laser tracker. In an embodiment, the payload functions 1530 are located in the payload 2170 of FIGS. 9, 10, while the azimuth encoder electronics 1540 is located in the azimuth assembly 2110 and the zenith encoder electronics 1550 is located in the zenith assembly 2140.
The azimuth encoder electronics 1540 and zenith encoder electronics 1550 are separated from one another and from the payload electronics 1530 by the slip rings 2130, 2160 shown in FIGS. 9, 10. This is why the bus lines 1610, 1611, and 1612 are depicted as separate bus line in FIG. 11.
The laser tracker electronics processing system 1510 may communicate with an external computer 1590, or it may provide computation, display, and UI functions within the laser tracker. The laser tracker communicates with computer 1590 over communications link 1606, which might be, for example, an Ethernet line or a wireless connection. The laser tracker may also communicate with other elements 1600, represented by the cloud, over communications link 1602, which might include one or more electrical cables, such as Ethernet cables, and one or more wireless connections. An example of an element 1600 is another three dimensional test instrument--for example, an articulated arm CMM, which may be relocated by the laser tracker. A communication link 1604 between the computer 1590 and the elements 1600 may be wired (e.g., Ethernet) or wireless. An operator sitting on a remote computer 1590 may make a connection to the Internet, represented by the cloud 1600, over an Ethernet or wireless line, which in turn connects to the master processor 1520 over an Ethernet or wireless line. In this way, a user may control the action of a remote laser tracker.
FIG. 13 shows an embodiment of a locator camera system 950 and an optoelectronic system 900 in which an orientation camera 910 is combined with the optoelectronic functionality of a 3D laser tracker to measure six degrees of freedom. The optoelectronic system 900 includes a visible light source 905, an isolator 910, an optional electrooptic modulator 410, ADM electronics 715, a fiber network 420, a fiber launch 170, a beam splitter 145, a position detector 150, a beam splitter 922, and an orientation camera 910. The light from the visible light source is emitted in optical fiber 980 and travels through isolator 910, which may have optical fibers coupled on the input and output ports. The light may travel through the electrooptic modulator 410 modulated by an electrical signal 716 from the ADM electronics 715. Alternatively, the ADM electronics 715 may send an electrical signal over cable 717 to modulate the visible light source 905. Some of the light entering the fiber network travels through the fiber length equalizer 423 and the optical fiber 422 to enter the reference channel of the ADM electronics 715. An electrical signal 469 may optionally be applied to the fiber network 420 to provide a switching signal to a fiber optic switch within the fiber network 420. A part of the light travels from the fiber network to the fiber launch 170, which sends the light on the optical fiber into free space as light beam 982. A small amount of the light reflects off the beamsplitter 145 and is lost. A portion of the light passes through the beam splitter 145, through the beam splitter 922, and travels out of the tracker to six degree-of-freedom (DOF) device 4000. The six-DOF device 4000 may be a probe, a scanner, a projector, a sensor, or other device.
FIG. 14A shows an embodiment of a six-DOF probe 4400. The probe 4400 includes a six-DOF SMR 4434 placed on a magnetic nest 4432 that includes a strong magnet 4433. The six-DOF probe 4400 includes a stylus 4410, a body 4420, and a head 4430. The stylus 4410 includes a probe tip 4414 and a probe shaft 4412. The head 4430 includes the magnetic nest 4432 configured for rotating the six-DOF SMR 4434 about the SMR center but without translating the SMR center. The six-DOF SMR 4434 may be used advantageously as a stand-alone target. In this way a six-DOF SMR 4434 and six-DOF probe 4400 are both obtained for almost the same price as the six-DOF SMR alone. In an embodiment, the six-DOF SMR includes lines of intersection among the reflecting surfaces, the lines of intersection visible to an orientation camera such as the camera 910. Aspects of a six-DOF SMR are discussed in more detail in patent applications '339 and '983. In general, the six-DOF laser tracker has the ability to measure six degrees of freedom of a probe having a retroreflector target. One way of obtaining such a six-DOF measurement, as described hereinabove, is to incorporate marks into a cube corner retroreflector and to view these marks with an orientation camera to determine the three orientational degrees of freedom of the six-DOF SMR or six-DOF probe.
Other methods of determining the three orientational degrees of freedom may be used. In the present application, the term orientation sensor shall be applied to all such devices capable of being used with a laser tracker to determine the three orientational degrees of freedom. A first example of an alternative method for determining the three orientational degrees of freedom is to place at least three points of light on a probe that contains a retroreflector. By observing the points of light, it is with some configurations possible to determine the three orientational degrees of freedom. A second example of an alternative method for determining three orientational degrees of freedom is to use a combination of two sensor methods. A first sensor method is to allow a small quantity of light striking a cube-corner retroreflector to pass through the retroreflector to strike a position detector, which might be a CMOS array or a position sensitive detector, for example. Such a method permits determine of the pitch and yaw angles of the retroreflector, which in an embodiment is attached to a probe assembly. A second sensor method is to use a mechanical pendulum coupled to an angular encoder to measure the orientation of the probe in relation to the gravity vector. This measurement gives an angle that is closely related to the roll angle of the probe. By combining the results of the first sensor and the second sensor, it is possible to obtain the three orientational degrees of freedom. Whatever method is used, the idea of incorporating a separable SMR from the measurement device may be applied. In all cases, the term orientation sensor represents the apparatus needed to measure the three orientational degrees of freedom.
FIG. 14B shows an embodiment of a six-DOF probe 4450. It is like the six-DOF probe 4400 except that it includes a constraint 4460. The constraint includes an element that comes in contact with the six-DOF SMR 4434--for example, a machined piece of metal, a plastic cover, or a strap. The constraint 4460 is brought into tight physical contact with the six-DOF SMR 4434 by means of a securing mechanism 4464. Examples of suitable securing mechanisms include hooking clamps and screw clamps. Two possible advantages of including a clamping mechanism are decreased chance of moving the six-DOF SMR, which might require repeating the compensation procedure described herein below, and decreased chance of bumping the SMR off of the probe and onto the floor. The magnet 4433 shown in FIG. 14B is optional.
An advantage of the probe embodiments shown in FIGS. 14A-B is that six-DOF probing capability can be added to six-DOF SMR capability for very low cost. Another advantage is that these embodiments require no source of electrical power since the six-DOF SMR may be used in a completely passive manner if desired.
An embodiment of a six-DOF probe 4200 is shown in FIG. 15A. The probe includes a probe head 4240, a probe body 4220, and a probe stylus 4210. The six-DOF SMR 4234 is held in place by a constraint 4260. The constraint 4260 includes an element 4262 that comes in contact with the six-DOF SMR 4234. The element 4262 may be, for example, a machined piece of metal, a plastic cover, or a strap. The constraint 4260 is brought into tight physical contact with the six-DOF SMR 4234 by means of a securing mechanism 4264. Examples of suitable securing mechanisms include hooking clamps and screw clamps. The six-DOF SMR 4234 sits on a nest base 4332, which in an embodiment is magnetic. The probe body 4220 includes a housing 4224, optional actuator buttons 4226, 4227, and optional nest storage 4228. The probe housing 4224 is contoured to be held by a hand. The probe stylus 4210 includes a probe tip 4214, a probe shaft 4212, a probe connector 4216, and a probe clamp 4218. The probe clamp enables a variety of styluses having different lengths, angles, and shapes to be attached to the six-DOF probe.
In an embodiment, the six-DOF SMR 4480 of FIG. 17 includes indexing features 4471, and the probe head 4475 includes a mating structure 4472 that includes mating features 4473. The indexing features and the mating features are configured to fix the orientation of the six-DOF SMR within the nest 4432 without shifting the spherical center of the SMR relative to the six-DOF probe assembly. The use of indexing and mating features of FIG. 17 is that they enable the tracker to be immediately used based on probe compensation parameters obtained from the factory or from a compensation procedure carried out previously.
In another embodiment, the angle of the beam of light from the tracker to the probe body 4420 is changed by rotating the six-DOF SMR 4434, 4234 relative to the body. This may be done by rotating the six-DOF SMR 4434, 4234 on the nest 4432, 4232. To find the angle of rotation of the six-DOF SMR, a compensation procedure may be quickly carried out in which the probe is placed in a fixed nest 4250, as shown in FIG. 15A. The probes in FIGS. 14A and 14B may equally well be used with the compensation procedure described herein. In FIG. 15A, the probe tip 4214 is placed in the fixed nest 4250. The operator may place the fixed nest on any convenient surface and hold it in place with one hand while moving the six-DOF probe 4200 with the other hand, the movement made according to method described herein below.
The fixed nest may be any of several types. One type of fixed nest 4250, depicted in the cross-sectional view 4280 of FIG. 15B, includes a body 4255 onto which are embedded three small spheres (two of these are 4251A, 4251B) placed 120 degrees apart and positioned to support the probe tip 4214 or 4414. A small magnet 4249 may be placed in the fixed nest 4250 to provide a downward force on the probe tip 4214, 4414. The three small spheres hold the center of the spherical probe tip fixed in space but allow the six-DOF probe 4200, 4400, or 4450 to be rotated to any desired angle. Because the center of the probe tip remains fixed, the calculated value of the probe tip should remain constant regardless of the rotation angles of the six-DOF probes. This makes it possible to solve for the orientation of the six-DOF SMR with respect to the rest of the probe. Another name for a nest containing three small spheres spaced 120 degrees apart is "trihedral hollow."
Other types of nest configurations may be used. In an embodiment, small protrusions are used rather than small spheres. Such protrusions are used, for example, in the nests 17 of FIG. 1. In another embodiment, the fixed nest includes a conical seat. As the name suggests, a conical seat includes a cone, usually with regions of the cone provided for making contact with the probe tip. If designed correctly, nests of the sort shown in FIG. 17 or nests formed as conical seats can provide good performance. Usually, the trihedral hollow provides the best possible performance.
In its position in FIG. 14A, 14B, or 15A, the six-DOF SMR has no free degrees of translational freedom, but it has three degrees of orientational freedom. In an embodiment, the probe is rotated about three different axes to obtain the information needed to determine the values of the three orientational degrees of freedom. For convenience we can let the three angular degrees of freedom be the yaw angle, the pitch angle, and the roll angle. The yaw angle 4263 is the angle of rotation about the axis 4261 that passes through a reference direction of the six-DOF probe. In an embodiment, the reference axis passes through the center of the body 4220 and the center of the stylus 4210. However, the stylus need not be arranged in this way and may instead be placed at any desired angle. The pitch angle is the angle 4266 about the axis 4265, as shown in FIG. 16B. The roll angle is the angle 4270 about the point that passes through 4268 and is perpendicular to the axes 4261 and 4265. The roll angle in this case is not the roll angle of the six-DOF SMR but rather a system-level roll angle. However, the information obtained from performing the rotations of FIGS. 16A, 16B, and 16C is sufficient to determine the orientation of the six-DOF SMR relative to the rest of the probe assembly. In general, it is important to rotate to cover three angles such as the yaw, pitch, and roll angles to get enough information to determine the orientation of the six-DOF SMR on the probe assembly, and this in turn is needed to determine the coordinates of the probe tip as it is moved to points on the surface of a workpiece.
The probe assembly may be conveniently attached to the six-DOF probe body for storage while the six-DOF probe is used to perform measurements. FIG. 15A shows a fixed nest 4250B magnetically attached to a location 4228 on the probe body 4220. It would also be possible to attach the probe body to the probe with a tether or a mechanical snap-on. Of course, the fixed nest 4250 may simply be kept nearby and made available when needed.
FIG. 18 is a flow chart of a method 4900 for measuring three-dimensional coordinates of a probe center. A step 4905 is providing a spherically mounted retroreflector, the spherically mounted retroreflector including a retroreflector mounted in a retroreflector body, the retroreflector body having a first spherical shape over a first portion of its outer surface, the first portion having a target center, the retroreflector configured to receive a first beam of light and to return a second beam of light, the second beam of light being a portion of the first beam of light, the second beam of light traveling in a direction substantially opposite the direction of the first beam of light.
A step 4910 is providing a probe assembly, the probe assembly including a probe stylus and a probe head, the probe stylus including the probe tip, the probe tip having a second spherical shape over a second portion of its surface, the second portion having a probe center, the probe head configured to receive the spherically mounted retroreflector and to permit rotation of the spherically mounted retroreflector about the target center while keeping the target center at a substantially constant position relative to the probe assembly.
A step 4915 is providing an orientation sensor, the orientation sensor configured to measure three orientational degrees of freedom of the probe assembly.
A step 4920 is providing a coordinate measurement device, the coordinate measurement device including a first motor, a second motor, a first angle measuring device, a second angle measuring device, a distance meter, a position detector, a control system, and a processor, the first motor and the second motor configured together to direct the first beam of light to a first direction, the first direction determined by a first angle of rotation about a first axis and a second angle of rotation about a second axis, the first angle of rotation produced by the first motor and the second angle of rotation produced by the second motor, the first angle measuring device configured to measure the first angle of rotation and the second angle measuring device configured to measure the second angle of rotation, the distance meter configured to measure a first distance from the coordinate measurement device to the spherically mounted retroreflector based at least in part on a third portion of the second beam of light received by a first optical detector, the position detector configured to produce a first signal in response to a position of a fourth portion of the second beam of light on the position detector, the control system configured to send a second signal to the first motor and a third signal to the second motor, the second signal and the third signal based at least in part on the first signal, the control system configured to adjust the first direction of the first beam of light to the position in space of the spherically mounted retroreflector, the processor configured to determine three-dimensional coordinates of the probe center, the three-dimensional coordinates based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom.
The step 4925 is placing the spherically mounted retroreflector on the probe head.
The step 4930 is directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector.
The step 4935 is measuring the first distance.
The step 4940 is measuring the first angle of rotation.
The step 4945 is measuring the second angle of rotation.
The step 4950 is measuring the three orientational degrees of freedom based at least in part on information provided by the orientation sensor.
The step 4955 is calculating the three-dimensional coordinates of the probe center based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom.
The step 4960 is storing the three-dimensional coordinates of the probe center. The method 4900 concludes at a reference marker A.
FIG. 19 is a flow chart for a method 5000 that begins at reference marker A of FIG. 18 and further includes the step of determining probe compensation parameters, the probe compensation parameters including at least information indicative of an orientation of the spherically mounted retroreflector in relation to the probe assembly.
FIG. 20 is a flow chart for a method 5100 that begins at reference marker B of FIG. 19. The step 5105 is providing a nest configured to accept the probe tip, the nest further configured to permit rotation of the probe tip about the probe center, the probe center held at a substantially fixed point in space. The step 5110 is placing the probe tip into the nest. The step 5115 is rotating the probe tip. The step 5120 is measuring a collection of quantities, the collection of quantities including the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom, each of the measured values obtained for a plurality of different rotations of the probe tip. The step 5125 is calculating the probe compensation parameters based at least in part on the collection of quantities.
FIG. 21 is a flow chart for a method 5200 that begins at reference marker A of FIG. 18. The step 5205 is removing the spherically mounted retroreflector from the probe head. The step 5210 is directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector. The step 5215 is measuring the first distance. The step 5220 is measuring the first angle of rotation. The step 5225 is measuring the second angle of rotation. The step 5230 is determining the three-dimensional coordinates of the target center.
FIG. 22 is a flow chart for a method 5300 that begins at reference marker C of FIG. 21. The step 5305 is placing the spherically mounted retroreflector on the probe head. The step 5310 is directing the first beam of light from the coordinate measurement device to the spherically mounted retroreflector. The step 5315 is measuring the first distance. The step 5320 is measuring the first angle of rotation. The step 5325 is measuring the second angle of rotation. The step 5330 is measuring the three degrees of orientational freedom. The step 5335 is calculating the three-dimensional coordinates of the probe center based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the three orientational degrees of freedom.
FIG. 23 is a flow chart for a method 5400 that begins at reference marker A of FIG. 18. The step 5405 is incorporating a pattern into the retroreflector. The step 5410 is providing an optical system including a lens and a photosensitive array, the lens configured to form an image of at least a portion of the patterned retroreflector on the photosensitive array. The step 5415 is converting the image into a digital data set. The step 5420 is calculating the three orientational degrees of freedom based at least in part on the digital data set.
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