Abstract:
A method for locking onto and tracking a selected retroreflector target with a laser tracker includes steps of: actuating by the operator the handheld appliance and sending a wireless signal; responding to the wireless message by repetitively carrying out steps in the following loop and exiting the loop when an exit condition is met: reflecting part of the cone of light by the at least one retroreflector target and capturing an array image on the photosensitive array; determining which retroreflector target meets the retroreflector target criterion; determining whether the position detector is receiving the reflected beam; establishing that the exit condition is met when the position detector receives the reflected beam and the reflected beam comes from the selected retroreflector target; and steering the first light beam toward the selected retroreflector target.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Non-Provisional of U.S. Provisional Application Ser. No. 61/702,864, filed Sep. 19, 2012. This application is also a Continuation-In-Part of U.S. patent application Ser. No. 13/340,730, filed Dec. 30, 2011; U.S. patent application Ser. No. 13/340,730 is a Continuation-In-Part of U.S. patent application Ser. No. 13/090,889, filed on Apr. 20, 2011, which claims priority to U.S. Provisional Patent Application No. 61/326,294, filed Apr. 21, 2010, the entire contents of each which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The following background art may be regarded as useful for the understanding of the invention: U.S. Patent Publication No. 2012/120391 A1; CA Patent No. 2,811,444 A1; and, U.S. Patent Publication No. 2012/120415 A1. 
     The present invention relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point, where it is intercepted by a retroreflector target. The instrument finds the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer. The angles are measured with an angular transducer such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. An example of such a device is a laser tracker. 
     A coordinate measuring device closely related to the laser tracker is the total station. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering or retroreflective targets. Hereinafter, the term laser tracker is used in a broad sense to include total stations. 
     Ordinarily a laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The apex of the cube corner, which is the common point of intersection of the three mirrors, is located at the center of the sphere. It is common practice to place the spherical surface of the SMR in contact with an object under test and then move the SMR over the surface being measured. Because of this placement of the cube corner within the sphere, the perpendicular distance from the apex of the cube corner to the surface of the object under test remains constant despite rotation of the SMR. Consequently, the 3D coordinates of a surface can be found by having a tracker follow the 3D coordinates of an SMR moved over the surface. 
     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. The position of the light that hits the position detector is used by a tracker control system to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) the SMR. 
     Angular transducers such as angular encoders attached to the mechanical azimuth and zenith axes of the tracker may be used to determine the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements obtained by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR or other retroreflector target. 
     As mentioned previously, two types of distance meters may be found in laser trackers: interferometers and absolute distance meters (ADMs). In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement. Initially, absolute distance meters were only able to measure stationary targets and for this reason were always used together with an interferometer. However, some modern absolute distance meters can make rapid measurements, thereby eliminating the need for an interferometer. 
     In its tracking mode, the laser tracker will automatically follow movements of the SMR when the SMR is in the capture range of the tracker. If the laser beam is broken, tracking will stop. The beam may be broken by any of several means: (1) an obstruction between the instrument and SMR; (2) rapid movements of the SMR that are too fast for the instrument to follow; or (3) the direction of the SMR being turned beyond the acceptance angle of the SMR. Following a beam break, in some modes of operation the beam by default remains fixed at the point of the beam break or at the last commanded position. It may be necessary for an operator to visually search for the tracking beam and place the SMR in the beam in order to lock the instrument onto the SMR and continue tracking. In another mode of operation, the beam may be automatically directed back to the SMR through the use of a camera system, as discussed hereinbelow. 
     Some laser trackers include one or more cameras. A camera axis may be coaxial with the measurement beam or offset from the measurement beam by a fixed distance or angle. A camera may be used to provide a wide field of view to locate retroreflectors. A modulated light source placed near the camera optical axis may illuminate retroreflectors, thereby making them easier to identify. In this case, the retroreflectors flash in phase with the illumination, whereas background objects do not. One application for such a camera is to detect multiple retroreflectors in the field of view and measure each in an automated sequence. 
     Some laser trackers have the ability to measure with six degrees of freedom (DOF), which may include three coordinates, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. Several systems based on laser trackers are available or have been proposed for measuring six degrees of freedom. 
     As explained hereinabove, tracking of a retroreflector target stops when the beam is broken. In some cases, such a beam break is intentionally created by the operator—for example, to use to beam to provide a marker for the alignment of target stands or instruments. In other cases, a beam break is unintentional or unavoidable—for example, when an operator rotates the retroreflector target too much or passes the retroreflector behind an object in moving from one point to another. In cases where the beam break is unwanted, it is desirable to provide a way to conveniently steer the beam back onto the retroreflector target. 
     One method known in the art for conveniently steering a beam back onto a retroreflector target is to illuminate the retroreflector target with a cone of light, to view the illuminated retroreflector target with a locator camera placed in close proximity to the light source producing the cone of light, to evaluate the position of the retroreflector image on a photosensitive array contained within the locator camera, and to activate motors of the laser tracker to drive the beam of light from the tracker toward the retroreflector target. This action may be repeated if necessary to lock the light beam from the tracker onto the retroreflector target. The locking of the light beam on the retroreflector target may be recognized by the position detector receiving a relatively large amount of retroreflected light. 
     In one implementation of this method for steering a beam onto a retroreflector target, the locator camera system automatically finds and locks onto a nearby retroreflector target whenever the tracker loses the beam. However, this method is limited in some respects. In some cases, numerous retroreflector targets may be located within a measurement volume, and the operator may want to direct the tracker beam to a different target than the one automatically selected by the tracker following a beam break. In other cases, the operator may want the tracker beam to remain fixed in direction so that a stand or instrument may be aligned to it. 
     One way around this difficulty that is known in the art is to use gestures to control the behavior of a laser tracker. In one implementation using gestures, a retroreflector target is followed using one or more locator cameras and associated proximate light sources. In this implementation, the cameras may detect a particular gesture by evaluating the motion of an illuminated retroreflector target or evaluating a pattern in the power of light from the retroreflector targets. A potential disadvantage in the use of gestures is that the operator must remember the correspondence between tracker commands and gestures. 
     What is needed is a flexible and convenient method for acquiring retroreflector targets. In some cases, it is desirable to recapture a retroreflector target following a beam break. In other cases, it is desirable to direct a tracker beam, either broken or unbroken, to a different retroreflector target. 
     SUMMARY 
     A method for locking onto and tracking a selected retroreflector target with a laser tracker, the locking onto and tracking carried out under direction of an operator, the method including steps of: providing at least one retroreflector target; providing the laser tracker, the laser tracker having a structure, a first light source, a distance meter, a first angular transducer, a second angular transducer, a position detector, a camera, a second light source, and a processor, the structure rotatable about a first axis and a second axis; the first light source configured to produce a first light beam that cooperates with the distance meter, the first angular transducer configured to measure a first angle of rotation about the first axis, the second angular transducer configured to measure a second angle of rotation about the second axis, the position detector configured to receive a reflected beam, the reflected beam being the first light beam reflected by a retroreflector target, the camera including a lens system and a photosensitive array, the second light source configured to provide a cone of light, the first light beam and the cone of light being fixed in relation to the structure, the second light source configured to cooperate with the camera, the camera having a field of view, the processor configured to operate the laser tracker. The method also includes the steps of: providing a transceiver coupled to the laser tracker or coupled to a computer in communication with the laser tracker, the transceiver including a receiver and optionally a transmitter; providing a handheld appliance configured to wirelessly communicate with the transceiver; positioning the at least one retroreflector target within the field of view of the camera; actuating by the operator the handheld appliance and sending in response to the actuation a wireless message to the transceiver; determining a retroreflector target criterion; responding to the wireless message by repetitively carrying out steps in a loop including the following steps (a)-(e) and exiting the loop when an exit condition is met: (a) reflecting part of the cone of light by the at least one retroreflector target and capturing an array image on the photosensitive array; (b) determining which retroreflector target meets the retroreflector target criterion, the determining based at least in part on the array image, the retroreflector target that meets the retroreflector target criterion referred to as the selected retroreflector target; (c) measuring a signal level with the position detector and determining, based on the signal level, whether the position detector is receiving the reflected beam; (d) establishing whether the exit condition is met, the exit condition being met if and only if the position detector receives the reflected beam and the reflected beam comes from the selected retroreflector target; (e) activating the first motor and the second motor to steer the first light beam toward the selected retroreflector target; activating the first motor and the second motor to steer the reflected beam so as to keep the reflected beam on the position detector; and measuring to the first retroreflector target a distance with the distance meter, a third angle with the first angular transducer, and a fourth angle with the second angular transducer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  is a perspective view of a prior art laser tracker; 
         FIG. 2  is a perspective view of a prior art laser tracker system; 
         FIG. 3  is a block diagram of electronics of a prior art laser tracker system; 
         FIG. 4  is a perspective view of a laser tracker used with a wireless appliance according to an embodiment of the present invention; 
         FIG. 5  is a perspective view of a laser tracker obtaining reflected light with locator cameras according to an embodiment of the present invention; 
         FIG. 6  is a perspective view of a laser tracker sending a light beam to a retroreflector according to an embodiment of the present invention; 
         FIG. 7  is a flow chart showing steps in a method for selecting, locking onto, and tracking a retroreflector target according to an embodiment of the present invention; 
         FIG. 8  is a perspective view of a laser tracker in which the beam is not locked onto any retroreflector within a field of multiple retroreflectors according to an embodiment of the present invention; 
         FIG. 9  is an exemplary user interface provided on a handheld appliance according to an embodiment of the present invention; 
         FIG. 10  is a perspective view of a laser tracker used with a wireless appliance containing a inertial measurement unit according to an embodiment of the present invention; 
         FIG. 11  is a perspective view of a laser tracker used with a wireless appliance containing a inertial measurement unit according to another embodiment of the present invention; and 
         FIG. 12  is a perspective view of a laser tracker used with a wireless appliance containing an inertial measurement unit according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A prior art laser tracker  10  is illustrated in  FIG. 1 . An exemplary gimbaled beam-steering mechanism  12  of laser tracker  10  comprises zenith carriage  14  mounted on azimuth base  16  and rotated about azimuth axis  20 . Payload  15  is mounted on zenith carriage  14  and rotated about zenith axis  18 . Zenith mechanical rotation axis  18  and azimuth mechanical rotation axis  20  intersect orthogonally, internally to tracker  10 , at gimbal point  22 , which is typically the origin for distance measurements. Laser beam  46  virtually passes through gimbal point  22  and is pointed orthogonal to zenith axis  18 . In other words, laser beam  46  is in a plane normal to zenith axis  18 . Laser beam  46  is pointed in the desired direction by motors within the tracker (not shown) that rotate payload  15  about zenith axis  18  and azimuth axis  20 . Zenith and azimuth angular encoders, internal to the tracker (not shown), are attached to zenith mechanical axis  18  and azimuth mechanical axis  20  and indicate, to high accuracy, the angles of rotation. Laser beam  46  travels to external retroreflector  26  such as the spherically mounted retroreflector (SMR) described above. By measuring the radial distance between gimbal point  22  and retroreflector  26  and the rotation angles about the zenith and azimuth axes  18 ,  20 , the position of retroreflector  26  is found within the spherical coordinate system of the tracker. 
     Laser beam  46  may comprise one or more laser wavelengths. For the sake of clarity and simplicity, a steering mechanism of the sort shown in  FIG. 1  is assumed in the following discussion. However, other types of steering mechanisms are possible. For example, it would be possible to reflect a laser beam off a mirror rotated about the azimuth and zenith axes. The techniques described here are applicable, regardless of the type of steering mechanism. 
     In exemplary laser tracker  10 , cameras  52  and light sources  54  are located on payload  15 . Light sources  54  illuminate one or more retroreflector targets  26 . Light sources  54  may be LEDs electrically driven to repetitively emit pulsed light. Each camera  52  includes a photosensitive array and a lens placed in front of the photosensitive array. The photosensitive array may be a CMOS or CCD array. The lens may have a relatively wide field of view, say thirty or forty degrees. The purpose of the lens is to form an image on the photosensitive array of objects within the field of view of the lens. Each light source  54  is placed near camera  52  so that light from light source  54  is reflected off each retroreflector target  26  onto camera  52 . In this way, retroreflector images are readily distinguished from the background on the photosensitive array as their image spots are brighter than background objects and are pulsed. There may be two cameras  52  and two light sources  54  placed about the line of laser beam  46 . By using two cameras in this way, the principle of triangulation can be used to find the three-dimensional coordinates of any SMR within the field of view of the camera. In addition, the three-dimensional coordinates of the SMR can be monitored as the SMR is moved from point to point. 
     Other arrangements of one or more cameras and light sources are possible. For example, a light source and camera can be coaxial or nearly coaxial with the laser beams emitted by the tracker. In this case, it may be necessary to use optical filtering or similar methods to avoid saturating the photosensitive array of the camera with the laser beam from the tracker. Another possible arrangement is to use a single camera located on the payload or base of the tracker. 
     As shown in  FIG. 2 , auxiliary unit  70  is usually a part of prior art laser tracker  10 . The purpose of auxiliary unit  70  is to supply electrical power to the laser tracker body and in some cases to also supply computing and clocking capability to the system. It is possible to eliminate auxiliary unit  70  altogether by moving the functionality of auxiliary unit  70  into the tracker body. In most cases, auxiliary unit  70  is attached to general purpose computer  80 . Application software loaded onto general purpose computer  80  may provide application capabilities such as reverse engineering. It is also possible to eliminate general purpose computer  80  by building its computing capability directly into laser tracker  10 . In this case, a user interface, possibly providing keyboard and mouse functionality is built into laser tracker  10 . The connection between auxiliary unit  70  and computer  80  may be wireless or through a cable of electrical wires. Computer  80  may be connected to a network, and auxiliary unit  70  may also be connected to a network. Plural instruments, for example, multiple measurement instruments or actuators, may be connected together, either through computer  80  or auxiliary unit  70 . 
     The laser tracker  10  may be rotated on its side, rotated upside down, or placed in an arbitrary orientation. In these situations, the terms azimuth axis and zenith axis have the same direction relative to the laser tracker as the directions shown in  FIG. 1  regardless of the orientation of the laser tracker  10 . 
     In another embodiment, the payload  15  is replaced by a mirror that rotates about the azimuth axis  20  and the zenith axis  18 . A laser beam is directed upward and strikes the mirror, from which it launches toward a retroreflector  26 . 
       FIG. 3  is a block diagram depicting prior art dimensional measurement electronics processing system  1500  that includes a laser tracker electronics processing system  1510 , 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  1570 , and an antenna  1572 . The payload functions electronics  1530  may include 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 a payload, while the azimuth encoder electronics is located in the azimuth assembly and the zenith encoder electronics  1550  is located in the zenith assembly. 
     Many types of peripheral devices are possible, but here three such devices are shown: a temperature sensor  1582 , a six-DOF probe  1584 , and a personal digital assistant,  1586 , which might be a smart phone or a remote control, for example. The laser tracker may communicate with peripheral devices in a variety of means, including wireless communication over the antenna  1572 , by means of a vision system such as a camera, and by means of distance and angular readings of the laser tracker to a cooperative target such as the six-DOF probe  1584 . 
     In an embodiment, a separate communications bus goes from the master processor  1520  to each of the electronics units  1530 ,  1540 ,  1550 ,  1560 ,  1565 , and  1570 . Each communications line may have, for example, three serial lines that include the data line, clock line, and frame line. The frame line indicates whether or not the electronics unit should pay attention to the clock line. If it indicates that attention should be given, the electronics unit reads the current value of the data line at each clock signal. The clock signal may correspond, for example, to a rising edge of a clock pulse. In an embodiment, information is transmitted over the data line in the form of a packet. In an embodiment, each packet includes an address, a numeric value, a data message, and a checksum. The address indicates where, within the electronics unit, the data message is to be directed. The location may, for example, correspond to a processor subroutine within the electronics unit. The numeric value indicates the length of the data message. The data message contains data or instructions for the electronics unit to carry out. The checksum is a numeric value that is used to minimize the chance that errors are transmitted over the communications line. 
     In an embodiment, the master processor  1520  sends packets of information over bus  1610  to payload functions electronics  1530 , over bus  1611  to azimuth encoder electronics  1540 , over bus  1612  to zenith encoder electronics  1550 , over bus  1613  to display and UI electronics  1560 , over bus  1614  to removable storage hardware  1565 , and over bus  1616  to RFID and wireless electronics  1570 . 
     In an embodiment, master processor  1520  also sends a synch (synchronization) pulse over the synch bus  1630  to each of the electronics units at the same time. The synch pulse provides a way of synchronizing values collected by the measurement functions of the laser tracker. For example, the azimuth encoder electronics  1540  and the zenith electronics  1550  latch their encoder values as soon as the synch pulse is received. Similarly, the payload functions electronics  1530  latch the data collected by the electronics contained within the payload. The six-DOF, ADM, and position detector all latch data when the synch pulse is given. In most cases, the camera and inclinometer collect data at a slower rate than the synch pulse rate but may latch data at multiples of the synch pulse period. 
     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, and 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. 
     A method and apparatus in an embodiment of the present invention is shown in  FIGS. 4-7 . Laser tracker  10  sends out laser beam  46  as in  FIG. 1 , but in one instance the beam has been broken and is no longer aligned with the retroreflector target  26 . In another instance, the beam is still locked onto a retroreflector target, but the operator wants to lock onto a different retroreflector target. In either case, an operator actuates a handheld appliance  410  (step  705  in  FIG. 7 ), for example, by pressing a button or pushing a soft key appearing on a UI of the handheld appliance  410 . The actuation causes transceiver  415  to emit wireless signal  420 , which might be an RF, microwave, or infrared signal, for example, that is received by one or more transceivers  65 A,  65 B, and  65 C in the computer  80 , auxiliary unit  70 , and tracker  65 , respectively (step  708  in  FIG. 7 ). In this document, the term transceiver is used to denote any device that contains a transmitter, receiver, or both transmitter and receiver. The auxiliary unit  70  may contain the power supply and optionally the master processor  1520 , and hence may be considered part of the laser tracker  65 . 
     The reception of the wireless signal by the one or more transceivers  65 A,  65 B, and  65 C causes the software to obtain a retroreflector target criterion (step  710  in  FIG. 7 ). The retroreflector target criterion is a criterion that describes the characteristics of the target that the tracker is to lock onto. The criterion may, for example, be the retroreflector that is closest to the beam of light  46  from the laser tracker. Other examples of retroreflector target criteria are described hereinbelow. The criterion may be chosen by the operator by pressing a soft key on the UI of the handheld apparatus, for example. The criterion may instead be provided by the operator by choosing a default setting ahead of time, for example, in a Properties or Settings menu of software used with the laser tracker. 
     A further response to the wireless signal  420  is illustrated in  FIGS. 4-6 . The one or more transceivers  65 A,  65 B, and  65 C respond (step  715  in  FIG. 7 ) by executing software that causes a repetitive sequence of events (steps  720 - 740  in  FIG. 7 ) to be carried out. One or more illuminators  54  proximate to each locator camera  52  emit a cone of light (step  720  in  FIG. 7 ), which may be modulated in time, for example, by flashing the light on and off. The cone of light may cover a relatively wide angle—for example, 60 degrees. Any retroreflector within the cone of light reflects light back to one or more cameras  52  (step  725  in  FIG. 7 ). A lens included within each locator camera forms an image of each illuminated retroreflector onto a photosensitive array that is a part of the camera (step  730  in  FIG. 7 ).  FIG. 5  shows a nearby retroreflector  26  sending light  510  back to the locator cameras  54 . 
     In a step  732 , software determines which retroreflector meets the retroreflector criterion. For example, if the retroreflector criterion were selected by the operator to be the retroreflector target nearest the tracker beam  46 , the software would evaluate the position of the images on the photosensitive array(s) of the camera(s) to determine whether the retroreflector criterion was being met. The decision of step  732  is made based on an evaluation of two conditions. First, the software notes whether the position detector is receiving a retroreflected tracker beam  46 . Such a beam will be received by the position detector if the beam strikes relatively near the center of a retroreflector target. The determination of whether the position detector has received the retroreflector light is made on the basis of a signal level provided by the position detector. For example, one type of position detector is a lateral type position sensitive detector having four electrodes. By adding the voltage levels at each of the four electrodes, the total optical power present on the position detector may be determined. If the optical power exceeds a pre-established level, presence of a retroreflected portion of the beam  46  is indicated. In other words, in this case, the laser tracker has locked onto the target. Second, the software notes whether the image obtained on the photosensitive array(s) of the camera(s)  52  corresponds to the position of the retroreflector that meets the retroreflector target criterion. If this is the case and if a retroreflected portion of the tracker beam  46  is being received by the position detector, the procedure continues to track on the retroreflector target, as indicated in step  745 . Otherwise, in step  740  the azimuth (AZ) and zenith (ZE) motors are activated to drive the tracker beam toward the selected retroreflector target. The steps  720 - 740  are then repeated until an exit condition of step  735  is satisfied. 
     At the step  745 , tracking of the beam on the retroreflector target is initiated by activating the AZ and ZEN motors to keep the beam approximately centered on the position detector. The retroreflector may be tracked by the operator to a location of interest, at which point the tracker may be used measure distance and two angles to determine three-dimensional coordinates of an object under test (step  750  in  FIG. 7 ). 
     The use of the selection criterion of step  710  is illustrated for the situation shown in  FIG. 8  in which two retroreflectors  26 ,  28  are within the cone of light emitted by the illuminators  54 . In an embodiment, a target criterion is chosen by the operator, which might for example provided on IMU soft keys of a smart phone as shown in  FIG. 9 . In the example choices of  FIG. 9 , the operator may select the retroreflector target that is nearest the current beam direction, farthest from the nearest beam direction (which could be used to change the direction of the tracker by “dragging” the SMR around the outer edge of the cone of light), fastest (which could be used to select the a moving targets from among a collection of stationary targets), closest to an inertial measurement unit (IMU, which might be the IMU within a smart phone, as discussed hereinbelow), in the direction to which an IMU is pointed (as discussed hereinbelow), of selected from an image. There are many other possibilities for choices that may be offered. 
     In another embodiment, a mode of selection is chosen ahead of time as a setting value or default value. For example, the operator may want to always have the tracker beam lock onto the retroreflector target closest to the IMU or the operator may always want to select a retroreflector from an image. Such a setting may be chosen by the operator on a computer program used with the laser tracker, for example. 
     One choice the user may make is whether the method discussed herein applies only to the case in which the beam is broken (not tracking on the retroreflector) or whether it also should apply to the case in which a beam is tracking on a target and the operator wishes to direct the retroreflector to another target. 
     As an example, we consider the situation of  FIG. 10  but first give some background on an inertial measurement unit (IMU)  525 , which is part of the handheld appliance  510 . IMUs are devices that provide information about linear or rotational motion or position as a result of inertial sensors. Examples of inertial sensors are accelerometers (which are the same thing as inclinometers), gyroscopes, magnetometers (compasses), and global positioning system (GPS) devices. It is commonplace today for smart phones to contain a three-dimensional accelerometer, a three-dimensional gyroscope, a compass, and a GPS. By establishing an initial position of an IMU in relation to a laser tracker, it is possible to get a relatively good estimate of position of the IMU over some time. For example, an initial position of an IMU in a cell phone may easily be obtained if an operator has the smart phone in a shirt pocket while a retroreflector target is being measured by the tracker. The smart phone then has the approximate three-dimensional coordinates of the measured retroreflector target. As the operator moves around, the IMU in the cell phone can provide an updated estimate of the retroreflector position. 
     In  FIG. 10 , a beam  46  is locked onto the retroreflector  26 , but the operator wants to track on a different retroreflector  28  that the operator is holding. The operator is using a handheld appliance  510  that contains an inertial measurement unit  525  and a transceiver  515 . The operator chooses “Closest IMU” in the UI of  FIG. 9  or by some other means (such as by making “Closest IMU” the default choice). When the operator actuates the handheld appliance to emit a wireless signal from the transceiver  515 , the wireless signal received by transceivers  65 A,  65 B, or  65 C may begin software that executes the steps of  FIG. 9 , driving the beam  46  to the retroreflector target  28  as shown in  FIG. 11 . As discussed above, for the case depicted in  FIGS. 10 ,  11 , the beam was initially locked onto one target but directed to another target by the operator. 
     The IMUs found in smart phones and other handheld devices provide information not only about the position of the smart phone but also the direction in which the smart phone is pointing. The capability can be used to provide a useful measurement technique illustrated in  FIG. 12 . In the case shown in  FIG. 12 , the approximate position and direction of the handheld appliance  610  is known because of the IMU  625  within the appliance. By pointing the handheld appliance at the desired target  28 , a rear-facing camera  645  may be used to obtain an image  638  near the center of a display screen  635 . With the handheld appliance oriented in this way, a line in space may be drawn from the handheld appliance  610  to each of the cameras  52 . This line may be projected mathematically on each of the photosensitive arrays of the cameras  52  and software used to determine those retroreflector targets closest to the projected line. 
     Another possibility is to have the cameras  52  send an image showing the relative positions of the retroreflector targets from the perspective of the laser tracker. The operator may then select the retroreflector target of interest. 
     The handheld appliances described hereinabove may be any of several different types. They might be remote controls, mobile phones (including smart phones), electronics pads, or keypads. Although wireless communication is advantageous in most cases, it is also possible to use the method described herein with a wired method—in other words, with the handheld appliance communication with the laser tracker or associated computer through a wired connection. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention being indicated by the appended claims. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.