The present disclosure 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 angle-measuring device 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. Exemplary laser tracker systems are described by U.S. Pat. No. 4,790,651 to Brown et al., the contents of which is incorporated by reference herein, and U.S. Pat. No. 4,714,339 to Lau et al.
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 the 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. It is possible to place a glass window on the top of the SMR to prevent dust or dirt from contaminating the glass surfaces.
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 encoders attached to the mechanical azimuth and zenith axes of the tracker may measure 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 performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
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. Such an ADM is described in U.S. Pat. No. 7,352,446 to Bridges et al., the contents of which is incorporated by reference herein.
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. By default, following the beam break, the beam remains fixed at the point of the beam break or at the last commanded position. The operator may 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.
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. Exemplary systems are described in U.S. Pat. No. 6,166,809 to Pettersen et al., and U.S. Pat. No. 7,800,758 to Bridges et al., the contents of which are incorporated by reference herein.
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. Exemplary systems are described in U.S. Published Patent Application No. 2010/0128259 to Bridges, the contents of which is incorporated by reference herein; U.S. Pat. No. 7,800,758 to Bridges et al., U.S. Pat. No. 5,973,788 to Pettersen et al.; and U.S. Pat. No. 7,230,689 to Lau.
Two common modes of operation of the laser tracker are tracking mode and profiling mode. In tracking mode, the laser beam from the tracker follows the retroreflector as the operator moves it around. In profiling mode, the laser beam from the tracker goes in the direction given by the operator, either through computer commands or manual action.
Besides these modes of operation that control the basic tracking and pointing behavior of the tracker, there are also other option modes that enable the tracker to respond in a manner selected by the operator ahead of time. The desired option mode is typically selected in software that controls the laser tracker. Such software may reside in an external computer attached to the tracker (possibly through a network cable) or within the tracker itself. In the latter case, the software may be accessed through console functionality built into the tracker.
An example of an option mode is the Auto Reset mode in which the laser beam is driven to a preset reference point whenever the laser beam is broken. One popular reference point for the Auto Reset option mode is the tracker Home Position, which is the position of a magnetic nest mounted on the tracker body. The alternative to Auto Reset is the No Reset option mode. In this case, the laser beam continues pointing in the original direction whenever the laser beam is broken. A description of the tracker home position is given in U.S. Pat. No. 7,327,446 to Cramer et al., the contents of which is incorporated by reference herein.
Another example of an option mode is sometimes referred to as “PowerLock”. In the PowerLock option mode, the location of the retroreflector is found by a tracker camera whenever the tracker laser beam is broken. The camera immediately sends the angular coordinates of the retroreflector to the tracker control system, thereby causing the tracker to point the laser beam back at the retroreflector. Methods involving automatic acquisition of a retroreflector are given in international application WO 2007/079601 to Dold et al. and U.S. Pat. No. 7,055,253 to Kaneko.
Some option modes are slightly more complex in their operation. An example is the Stability Criterion mode, which may be invoked whenever an SMR is stationary for a given period of time. The operator may track an SMR to a magnetic nest and set it down. If a stability criterion is active, the software will begin to look at the stability of the three-dimensional coordinate readings of the tracker. For instance, the user may decide to judge the SMR to be stable if the peak-to-peak deviation in the distance reading of the SMR is less than two micrometers over a one second interval. After the stability criterion is satisfied, the tracker measures the 3D coordinates and the software records the data.
More complex modes of operation are possible through computer programs. For example, software is available to measure part surfaces and fit these to geometrical shapes. Software will instruct the operator to move the SMR over the surface and then, when finished collecting data points, to raise the SMR off the surface of the object to end the measurement. Moving the SMR off the surface not only indicates that the measurement is completed; it also indicates the position of the SMR in relation to the object surface. This position information is needed by the application software to account for the offset caused by the SMR radius.
A second example of computer control is a tracker survey. In the survey, the tracker is driven sequentially to each of several target locations according to a prearranged schedule. The operator may teach these positions prior to the survey by carrying the SMR to each of the desired positions.
A third example of software control is tracker directed measurement. The software directs the operator to move the SMR to a desired location. It does this using a graphic display to show the direction and distance to the desired location. When the operator is at the desired position, the color on the computer monitor might, for example, turn from red to green.
The element common to the tracker actions described above is that the operator is limited in his ability to control the behavior of the tracker. On the one hand, option modes selected in the software may enable the operator to preset certain behaviors of the tracker. However, once the option modes have been selected by the user, the behavior of the tracker is established and cannot be changed unless the operator returns to the computer console. On the other hand, the computer program may direct the operator to carry out complicated operations that the software analyzes in a sophisticated way. In either case, the operator is limited in his ability to control the tracker and the data collected by the tracker.
A laser tracker operator performs two different functions. The operator positions an SMR during a measurement, and sends commands through the control computer to the tracker. However, it is not easy for a single operator to perform both of these measurement functions because the computer is usually remotely located from the measurement location. Various methods have been tried to get around this limitation, but none is completely satisfactory.
One method sometimes used is for a single operator to set the retroreflector in place and walk back to the instrument control keyboard to execute a measurement instruction. However, this is an inefficient use of operator and instrument time. In cases where the operator must hold the retroreflector for the measurement, single operator control is only possible when the operator is very close to the keyboard.
A second method is to add a second operator. One operator stands by the computer and a second operator moves the SMR. This is labor intensive and an expensive method. Further, verbal communication over large distances can be a problem.
A third method is to equip a laser tracker with a remote control. However, remote controls have several limitations. Many facilities do not allow the use of remote controls for safety or security reasons. Even if remote controls are allowed, interference among wireless channels may be an issue. Some remote control signals do not reach the full range of the laser tracker. In some situations, such as working from a ladder, the second hand may not be free to operate the remote control. Before a remote control can be used, it is usually necessary to set up the computer and remote control to work together, and then only a small subset of tracker commands can ordinarily be accessed at any given time. An example of a system based on this idea is given in U.S. Pat. No. 7,233,316 to Smith et al.
A fourth method is to interface a cell phone to a laser tracker. Commands are entered remotely by calling the instrument from the cell phone and entering numbers from the cell phone keypad or by means of voice recognition. This method also has many shortcomings. Some facilities do not allow cell phones to be used, and cell phones may not be available in rural areas. Service requires a monthly service provider fee. A cell phone interface requires additional hardware interfacing to the computer or laser tracker. Cell phone technology changes fast and may require upgrades. As in the case of remote controls, the computer and remote control must be set up to work together, and only a small subset of tracker commands can ordinarily be accessed at a given time.
A fifth method is to equip a laser tracker with internet or wireless network capabilities and use a wireless portable computer or personal digital assistant (PDA) to communicate commands to the laser tracker. However, this method has limitations similar to a cell phone. This method is often used with total stations. Examples of systems that use this method include U.S. Published Patent Application No. 2009/017618 to Kumagai et al., U.S. Pat. No. 6,034,722 to Viney et al., U.S. Pat. No. 7,423,742 to Gatsios et al., U.S. Pat. No. 7,307,710 to Gatsios et al., U.S. Pat. No. 7,552,539 to Piekutowski, and U.S. Pat. No. 6,133,998 to Monz et al. This method has also been used to control appliances by a method described in U.S. Pat. No. 7,541,965 to Ouchi et al.
A sixth method is to use a pointer to indicate a particular location in which a measurement is to be made. An example of this method is given in U.S. Pat. No. 7,022,971 to Ura et al. It might be possible to adapt this method to give commands to a laser tracker, but it is not usually very easy to find a suitable surface upon which to project the pointer beam pattern.
A seventh method is to devise a complex target structure containing at least a retroreflector, transmitter, and receiver. Such systems may be used with total stations to transmit precise target information to the operator and also to transmit global positioning system (GPS) information to the total station. An example of such a system is given in U.S. Published Patent Application No. 2008/0229592 to Hinderling et al. In this case, no method is provided to enable the operator to send commands to the measurement device (total station).
An eighth method is to devise a complex target structure containing at least a retroreflector, transmitter and receiver, where the transmitter has the ability to send modulated light signals to a total station. A keypad can be used to send commands to the total station by means of the modulated light. These commands are decoded by the total station. Examples of such systems are given in U.S. Pat. No. 6,023,326 to Katayama et al., U.S. Pat. No. 6,462,810 to Muraoka et al., U.S. Pat. No. 6,295,174 to Ishinabe et al., and U.S. Pat. No. 6,587,244 to Ishinabe et al. This method is particularly appropriate for surveying applications in which the complex target and keypad are mounted on a large staff. Such a method is not suitable for use with a laser tracker, where it is advantageous to use a small target not tethered to a large control pad. Also it is desirable to have the ability to send commands even when the tracker is not locked onto a retroreflector target.
A ninth method is to include both a wireless transmitter and a modulated light source on the target to send information to a total station. The wireless transmitter primarily sends information on the angular pose of the target so that the total station can turn in the proper direction to send its laser beam to the target retroreflector. The modulated light source is placed near the retroreflector so that it will be picked up by the detector in the total station. In this way, the operator can be assured that the total station is pointed in the right direction, thereby avoiding false reflections that do not come from the target retroreflector. An exemplary system based on this approach is given in U.S. Pat. No. 5,313,409 to Wiklund et al. However, this method does not offer the ability to send general purpose commands to a laser tracker.
A tenth method is to include a combination of wireless transmitter, compass assembly in both target and total station, and guide light transmitter. The compass assembly in the target and total station are used to enable alignment of the azimuth angle of the total station to the target. The guide light transmitter is a horizontal fan of light that the target can pan in the vertical direction until a signal is received on the detector within the total station. Once the guide light has been centered on the detector, the total station adjusts its orientation slightly to maximize the retroreflected signal. The wireless transmitter communicates information entered by the operator on a keypad located at the target. An exemplary system based on this method is given in U.S. Pat. No. 7,304,729 to Wasutomi et al. However, this method does not offer the ability to send general purpose commands to a laser tracker.
An eleventh method is to modify the retroreflector to enable temporal modulation to be imposed on the retroreflected light, thereby transmitting data. The inventive retroreflector comprises a cube corner having a truncated apex, an optical switch attached to the front face of the cube corner, and electronics to transmit or receive data. An exemplary system of this type is given in U.S. Pat. No. 5,121,242 to Kennedy. This type of retroreflector is complex and expensive. It degrades the quality of the retroreflected light because of the switch (which might be a ferro-electric light crystal material) and because of the truncated apex. Also, the light returned to a laser tracker is already modulated for use in measuring the ADM beam, and switching the light on and off would be a problem, not only for the ADM, but also for the tracker interferometer and position detector.
A twelfth method is to use a measuring device that contains bidirectional transmitter for communicating with a target and an active retroreflector to assist in identifying the retroreflector. The bidirectional transmitter may be wireless or optical and is part of a complex target staff that includes the retroreflector, transmitter, and control unit. An exemplary system of this type is described in U.S. Pat. No. 5,828,057 to Hertzman et al. Such a method is not suitable for use with a laser tracker, where it is advantageous to use a small target not tethered to a large control pad. Also the method of identifying the retroreflector target of interest is complicated and expensive.
Accordingly, while existing laser trackers are suitable for their intended purposes the need for improvement remains. In particular there is a need for a simple method for an operator to communicate commands to a laser tracker from a distance.