Patent Publication Number: US-2019186907-A1

Title: Multi-dimensional measurement system for precise calculation of position and orientation of a dynamic object

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/599,788 filed on Dec. 17, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present disclosure generally relates to systems and methods for gathering information and data for the calculation of the position and orientation of an object. More specifically, the present disclosure relates to a multi-dimensional measurement system for gathering data and information for the precise calculation of the position and orientation of a dynamic object, where multiple measurements are collected to adjust for error to more precisely calculate the position and orientation of the dynamic object. 
     BACKGROUND 
     In many of life&#39;s endeavors, it is useful to accurately measure and determine the precise position and orientation of objects. Accurate position and orientation measurements are useful and even critical in many areas such as manufacturing, industrial research and development, product development, and academic research. There are current systems that are used to determine the position and/or orientation of objects. Such systems can be of varying sophistication, with some systems limited to determining the spatial position of an object (i.e., the object&#39;s position within three dimensional space), but cannot determine the rotational orientation of the object (i.e., the object&#39;s rotation about its three spatial axes, commonly referred to as yaw, pitch and roll axes). Such systems are often referred to as three-dimensional measurement systems. More sophisticated systems can additionally measure an object&#39;s rotational orientation about two of the special axes, commonly the yaw and pitch axes. Such systems are often referred to as five-dimensional measurement systems. Even more sophisticated systems can additionally measure an object&#39;s rotational positioning about all three spatial axes. Such systems are often referred to as six-dimensional measurement systems. Determining both an object&#39;s spatial position and complete orientation is commonly referred to as measuring six-degrees of freedom of the object. 
     Measurement systems can be arranged to use laser beams to determine the position of an object. Such systems typically include a laser-emitting device for emitting laser beams toward the object, one or more reflective elements that are attached to the object to reflect the laser beam, and a laser-detecting device to detect the reflected laser beam. The laser-emitting device and the laser-detecting device are commonly coupled together into one device. The laser-emitting/detecting device is typically secured in a static position such as positioned on a bench or a tri-pod that provides an unobstructed view of the object to be measured. The laser-emitting/detecting device is typically allowed some degree of rotation about two spatial axes so that it can follow the object as it moves. The reflective elements are attached to the object and are arranged to reflect a laser beam back in the direction from which the laser beam is emitted. The laser-emitting/detecting device detects the reflected laser beam and gathers the characteristics of the laser beam, such as flight time and the angles at which the laser beam is emitted and returned. Through mathematical and geometric calculations, the measurement system can determine certain aspects of the position of the object. 
     Such measurement systems have a wide variety of applications. For example, in robotic manufacturing, accurately locating and orienting robotic components is often required. Accurately tracking the movement of robotic components and adjusting the component&#39;s position and orientation when needed can be critical to manufacturing quality products. Current measurement systems are prone to inaccuracies in the measurement of an object&#39;s orientation, this is especially so when the object undergoes substantial rotation about its spatial axes. There is a need for improvements to existing measurement systems to provide for accurate and precise position and orientation measurements of a dynamic object. 
     SUMMARY 
     In one embodiment, a measurement system is provided that accurately calculates the complete position and orientation of a dynamic object in real-time. The measurement system includes a laser unit, a target, a control unit, and a mechanism for contributing to the measurement of rotation about the roll axis. The target is arranged to rotate about all three spatial axes, and includes a reflective element and a gyroscope. The target is attached to the object for which position will be calculated. The laser unit is arranged to rotate about two of its spatial axes, and further arranged to emit a laser beam toward the target attached to the object. The reflective element of the target reflects the laser beam back toward the laser unit, where the laser unit detects the returned laser beam. In certain embodiments, more than one target can be attached to the object. 
     The laser unit and the target are arranged such that when the laser beam is directed toward the target, the system continuously adjusts so that the surface of the reflective element of the target remains perpendicular to the path of the laser beam. The system gathers information on the rotational position of the laser unit and the flight time of the laser beam from the laser unit to the target and back. From this information, the control unit can calculate the spatial position (i.e., the Cartesian coordinates) of the target, which is translated into the spatial position of the object. The system gathers information on the rotational orientation of the target in the form of data generated by servo motors and encoders included in the target. From this servo motor and encoder data, the control unit can calculate the rotational orientation about the yaw and pitch axes of the target, which is translated into the rotational orientation of the object about the yaw and pitch axes. 
     With regard to the rotational orientation about the roll axis, the system gathers information generated by the gyroscope and an additional mechanism. In one embodiment, the additional mechanism comprises a pair of light emitting devices spaced apart from each other and attached to the target and the camera unit positioned to capture images of the target and generate data about the target. From the data generated by the gyroscope and camera unit, the control unit can calculate the rotational orientation of the target about the roll axis, which is translated into the rotational orientation of the object about the roll axes. The system is arranged such that it generally relies on the data from the gyroscope to determine rotational orientation about the roll axis. Additionally, the system relies on information gathered by the camera to adjust for any drift experienced by the gyroscope. The light emitting devices are spaced apart such that each experiences opposite vertical and horizontal displacement when the target experiences roll. The camera senses the light emitted by the pair of light emitting devices and can map the relative vertical and horizontal displacements of the light emitting devices. From such vertical and horizontal displacement, the control unit can calculate the roll of the target and provide any required correction or adjustments to the orientation calculated from data gathered from the gyroscope. 
     In another embodiment, the additional mechanism comprises one or more levels incorporated into the target, which generate data about the target. From the data generated by the gyroscope and level, the control unit can calculate the rotational orientation of the target about the roll axis, which is translated into the rotational orientation of the object about the roll axis. Again, the system is arranged such that it generally relies on the data from the gyroscope to determine rotational orientation about the roll axis. The system relies on information gathered by a plurality of MEMS levels to determine rotation about the roll axis when the target is in a static state and to adjust for any drift experienced by the gyroscope during dynamic movement. The plurality of levels are positioned within the target such that at least one of the levels can determine the rotational orientation about the roll axis through the entire 360 degree path about the roll axis. From such a determination, the system can make any correction or adjustments to the rotational orientation calculated from data gathered from the gyroscope. 
     Thus, the multi-dimensional measurement systems disclosed herein can accurately calculate the complete position and rotational orientation (i.e., six-degrees of freedom) of a dynamic object in real-time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, structures are illustrated that, together with the detailed description provided below, describe example embodiments of the disclosed systems, methods, and apparatus. Where appropriate, like elements are identified with the same or similar reference numerals. Elements shown as a single component can be replaced with multiple components. Elements shown as multiple components can be replaced with a single component. The drawings may not be to scale. The proportion of certain elements may be exaggerated for the purpose of illustration. 
         FIG. 1  schematically illustrates a perspective view of an exemplary multi-dimensional measurement system in accordance with this disclosure. 
         FIG. 2  schematically illustrates a perspective view of an exemplary target for use with the multi-dimensional measurement system of  FIG. 1 . 
         FIG. 3  schematically illustrates an exemplary perspective view of a gyroscope and a pair of light emitting devices mounted to a shaft for use with the target of  FIG. 2 ; 
         FIG. 4  schematically illustrates a perspective view of the target of  FIG. 2  with the top removed; 
         FIG. 5  schematically illustrates a front-left perspective view of another exemplary multi-dimensional measurement system in accordance with this disclosure. 
         FIG. 6  schematically illustrates a front-right perspective view of the multi-dimensional measurement system of  FIG. 5 . 
         FIG. 7  schematically illustrates a front-left perspective view of yet another exemplary multi-dimensional measurement system in accordance with this disclosure. 
         FIG. 8  schematically illustrates a front-right perspective view of the multi-dimensional measurement system of  FIG. 7 . 
         FIG. 9  schematically illustrates an exemplary perspective view of a target located in a known initial position for use with multi-dimensional measurement systems as disclosed herein. 
         FIG. 10  schematically illustrates a front elevation view of the target of  FIG. 9 . 
         FIG. 11  schematically illustrates the target of  FIG. 9  rotated about the roll axis away from its initial position. 
         FIG. 12  schematically illustrates the target of  FIG. 9  rotated about the roll axis away from its initial position. 
         FIG. 13  schematically illustrates the target of  FIG. 9  rotated about the roll axis away from its initial position. 
         FIG. 14  schematically illustrates the target of  FIG. 9  rotated about the roll axis away from its initial position. 
         FIG. 15  schematically illustrates an arrangement for the placement of three levels within a target for use with multi-dimensional measurement systems as disclosed herein, where the target is located in its known initial position. 
         FIG. 16  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 17  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 18  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 19  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 20  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 21  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 22  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 23  schematically illustrates the arrangement of  FIG. 15  rotated about the roll axis away from its initial position. 
         FIG. 24  schematically illustrates another arrangement for the placement of three levels within a target for use with multi-dimensional measurement systems as disclosed herein. 
         FIG. 25  schematically illustrates an arrangement for the placement of one level within a target for use with multi-dimensional measurement systems as disclosed herein. 
         FIG. 26  schematically illustrates an arrangement for the placement of a pair of levels within a target for use with multi-dimensional measurement systems as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The apparatus, systems, arrangements, and methods disclosed in this document are described in detail by way of examples and with reference to the figures. It will be appreciated that modifications to disclosed and described examples, arrangements, configurations, components, elements, apparatus, methods, materials, etc. can be made and may be desired for a specific application. In this disclosure, any identification of specific techniques, arrangements, method, etc. are either related to a specific example presented or are merely a general description of such a technique, arrangement, method, etc. Identifications of specific details or examples are not intended to be and should not be construed as mandatory or limiting unless specifically designated as such. Selected examples of apparatus, arrangements, and methods for accurately calculating the complete position and orientation of a dynamic object in real-time are hereinafter disclosed and described in detail with reference made to  FIGS. 1-26 . 
     As will be described in detail herein, the multi-dimensional measurement systems and methods for using such measurement systems disclosed herein provide for novel apparatus and methods for accurately determining the complete position and rotational orientation of a dynamic object in real-time. In one embodiment, the multi-dimensional measurement system calculates the rotational orientation of the dynamic object in part through the combined use of a gyroscope, a pair of light emitting devices, and a camera unit that is sensitive to the light emitted from the pair of light emitting devices. In another embodiment, the multi-dimensional measurement system calculates the rotational orientation of the dynamic object in part through the combined use of a gyroscope and one or more sensors arranged to determine an orientation angle with respect to the force of gravity. Such sensors, which can be referred to, for example, as inclinometers, tilt sensors, or slope sensors, will be referred to herein as “levels” for convenience of description. Levels and methods of using levels will be further discussed with reference to  FIGS. 9-25 . In yet another embodiment, the multi-dimensional measurement system calculates the rotational orientation of the dynamic object in part through the combined use of a gyroscope, a pair of light emitting devices, a camera unit that is sensitive to the light emitted from the pair of light emitting devices, and one or more levels. 
     The various components of an exemplary multi-dimensional measurement system will be first described in detail, followed by a detailed description of how such various components interact to provide the system with the data and information required to calculate the complete position and rotational orientation (i.e., six degrees of freedom) of a tracked dynamic object in real-time. For clarity, the term “dynamic object” as used herein refers to an object that can move positionally (i.e., with respect to the Cartesian coordinate system) as well as rotationally about its three spatial axis (yaw, pitch, and roll). 
       FIG. 1  illustrates an exemplary multi-dimensional measurement system  10 . The measurement system  10  includes a laser unit  20 , a target  30 , a camera unit  40 , and a control unit  50 . Although these components are illustrated and described as discrete components, it will be understood that one or more of the disclosed components can be combined into a single component. For example, the laser unit  20  and the camera unit  40  can be combined into one component, the laser unit  20  and control unit  50  can be combined into one component, or the laser unit  20 , camera unit  40 , and control unit  20  can all be combined into one component. Conversely, components that are illustrated or described with multiple functionalities can be separated into multiple discrete components. 
     The laser unit  20  is arranged to emit a laser beam (“outgoing laser beam”) and detect that laser beam when it is reflected back (“incoming laser beam”) toward the laser unit  20 . The laser unit  20  is arranged so that it can rotate about two spatial axis. It will be understood that the terms “spatial axis” or “spatial axes” as used herein refer to one or more of the axis of the traditional Cartesian coordinate system, which provides three perpendicular axes as a reference for three dimensional geometry. As illustrated in  FIG. 1 , the first spatial axis of rotation for the laser unit  20  is about a vertical line (hereinafter referred to azimuth) passing through the laser unit  20 , and the second spatial axis of rotation is about a horizontal line (hereinafter referred to as elevation) passing through the laser unit  20 . Such rotational movement allows for the laser unit  20  to emit a laser beam in many different directions and, thus, follow the target  30  as it moves along with the object. 
     The laser unit  20  includes a number of servo motors to rotate the laser unit  20  about the azimuth and elevation axes. The laser unit  20  also includes a number of encoders to measure the rotation of the laser unit  20  about the azimuth and elevation axes. The laser unit  20  is arranged such that when it emits an outgoing laser beam, and subsequently receives the reflected incoming laser beam, the laser unit  20  can detect and capture information and data regarding the characteristics of the outgoing and incoming laser beams. Such information and data includes, for example, the time of flight between when the outgoing laser beam was emitted and when the incoming laser beam was detected and the angles of the outgoing and incoming laser beams as compared to the laser unit  20 . As will be explained in further detail herein, the information and data gathered by the laser unit  20  can be useful in calculating the position of the an object with respect to the traditional Cartesian coordinate system. 
     As illustrated in  FIGS. 1 and 2 , the target  30  includes a reflective element  60 , a central shaft  70  passing through the target  30 , a gyroscope  80  positioned within the target  30  and secured to the central shaft  70  (as will is further illustrated and described herein), a first light emitting device  90  secured proximate to one end of the central shaft  70 , and a second light emitting device  100  secured proximate to the opposite end of the central shaft  70 . The reflective element  60  is positioned to reflect any laser beam directed to the target  30  back in the direction from which it originated. In one embodiment, the reflective element  60  can be a retroreflector. 
     The target  30  is arranged so that it can rotate about the three spatial axes. As best illustrated in  FIG. 1 , the first spatial axis of rotation for the target  30  is about a vertical line (hereinafter referred to yaw) passing through the target  30 , the second spatial axis of rotation is about a first horizontal line (hereinafter referred to as pitch) passing through the target  30 , and the third spatial axis of rotation is about a second horizontal line, perpendicular to the first horizontal line (hereinafter referred to as roll). Such rotational movement allows the target  30  substantial freedom in its orientation. Such substantial freedom may be required when the target  30  is secured to a highly dynamic system such as an arm of a manufacturing assembly robot. As will be further described, the target  30  can be arranged to generally face in substantially the same direction during operation regardless of the six dimensional movements made by the object to which it is attached. Such functionality can provide for the target  30 , and specifically the reflective element  60 , to consistently face the laser unit  20  during operation. Thus, the laser unit  20  and target  30  can maintain optical communication, through the outgoing and incoming laser beams, throughout the period in which the position and rotational orientation of the object is to be determined. 
     Similar to the laser unit  20 , the target  30  includes a number of servo motors to rotate the target  30  about the yaw, pitch, and roll axes. The target  30  also includes a number of encoders to measure that rotation about the yaw and pitch axes. As will be understood, in the arrangement of the target  30  described herein, rotation about the roll axis is challenging to measure using an encoder. The gyroscope  80  is arranged to measure the rotation about the roll axis. As illustrated in  FIGS. 3 and 4 , the gyroscope  80  can be secured to the central shaft  70  and positioned within the target  30 . In such an arrangement, when the target  30  rotates about the roll axis, thus rotating the central shaft  70  about the roll axis, the gyroscope  80  rotates the same amount about the roll axis. The data generated by the gyroscope  80  can then be used to determine the target&#39;s rotation about the roll axis. In one example, the gyroscope  80  can be what is commonly referred to as a micro-electro-mechanical system (MEMS) gyroscope. 
     While a gyroscope can generally be used to measure rotation about the roll axis, gyroscopes can be subject to a known phenomenon often referred to as “drift.” Gyroscope drift causes rotational measurements gathered by gyroscopes to become inaccurate over time. One solution is to regularly “zero out” the gyroscope by returning it to a known initial position. However, for certain applications, such as those that required continuous measurement over time, such a process may be impractical or inefficient. Therefore, the multi-dimensional measurement systems disclosed and described herein includes novel arrangements and processes for correcting in real-time errors in rotational measurement cause by gyroscope drift. 
     One novel arrangement and process for correcting errors in roll measurements in real-time includes the use of the first and second light emitting devices ( 90 ,  100 ) and the camera unit  40 . In one embodiment, the first and second light emitting devices ( 90 ,  100 ) can be light emitting diodes (“LED”). As illustrated in  FIGS. 1 through 4 , the first and second light emitting devices ( 90 ,  100 ) can be secured to the central shaft  70  and can be spaced equally apart relative to the roll axis. In the illustrated embodiment, the first and second light emitting devices ( 90 ,  100 ) are positioned so there is as much separation as allowed by the length of the central shaft  70 . As will be understood, as the target  30  rotates about the roll axis, the first and second light emitting devices ( 90 ,  100 ) will move in opposite directions with respect to both the vertical and horizontal directions. Therefore, when the light emitted from the first and second light emitting devices ( 90 ,  100 ) is detected, the vertical displacement, horizontal displacement, or both can be used to calculate the rotation of the target  30  about the roll axis. As will be explained in further detail herein, the information and data gathered from the servo motors, encoders, and the detection of the position of the first and second light emitting devices ( 90 ,  100 ) can be useful in calculating the position and rotational orientation of the object to which the target  30  is attached. 
     The camera unit  40  is arranged to be light sensitive. Thus, the camera unit  40  can detect and record light emitted from devices such as the first and second light emitting devices ( 90 ,  100 ). As will be understood, information and data gathered by the camera unit  40  can be useful in calculating the position and rotational orientation of the object. As illustrated in  FIG. 1 , the camera unit  40  is positioned so the lens  110  or other image capturing apparatus is directed toward the target  30  and, specifically, toward the first and second light emitting devices ( 90 ,  100 ). The camera unit  40  is arranged so that its field of view  120 , i.e., the field across which the camera unit  40  can capture images, includes the possible locations for the first and second light emitting devices ( 90 ,  100 ) as the target  30  moves with the object. It will be understood that the field of view  120  is a three dimensional space, where the cross-sectional size of the field of view  120  increases as the field of view  120  projects farther away from the camera unit  40  and toward the target  30 . The field of view  120  includes an optical axis passing through the center of the cross-sectional areas of the field of view  120 . Provided the light emitting devices ( 90 ,  100 ) are in the field of view  120  of the camera unit  40 , information and data gathered from the detection of the light emitting devices ( 90 ,  100 ) can be included in a determination position and orientation of a dynamic object in real-time. In one example, the light emitting devices ( 90 ,  100 ) can each include a unique property, such as intensity of light, color or light, etc., that allows the system to distinguish between the two light emitting devices ( 90 ,  100 ) when analyzing data and information captured by the camera unit  40 . 
     The control unit  50  can be placed in communication with the laser unit  20 , the target  30 , and/or the camera unit  40  by either wired methods or wireless methods to access information and data gathered by the components of the system  10 . The control unit  50  can be arranged to use such information and data to calculate the position and rotational orientation of the target  30 . This position and rotational orientation of the target  30  can then be translated to calculate the position and rotational orientation of the object. The control unit  50  can also be arranged to understand the desired or correct position of the object over time, and, if the position or rotational orientation of the object is incorrect, the control unit  50  can send information and data via wired or wireless signal to the mechanism controlling the position and rotational orientation of the object to correct the object&#39;s position and rotational orientation. In the example of the object being a welding head affixed to the end of a robotic arm, the control unit  50  can include data and information on the desired position and orientation of a weld bead formed by the welding head over time. If the real-time position and rotational orientation of the weld head does not correspond the desired position and orientation of the weld bead, the control unit  50  can send a signal to the mechanism controlling the robotic arm to adjust the position and/or rotational orientation of the weld head to correspond to the desired position and orientation of the weld bead. 
     The following discloses a method for utilizing the system  10  and components detailed above to calculate the six degrees of freedom of the position and rotational orientation of an object. As an initial matter, the target  30  is secured to an object that will be tracked. In one example, the object would be an arm of a manufacturing robot, such as a robot designed to weld a precise metal seam on an automobile assembly line. As will be appreciated, with advancements in automobile design and manufacturing processes, automobile manufacturers are demanding more precise positioning from welding robots. Therefore, the task of tracking the welding robot and correcting any errors in its positioning and rotational orientation has become more important. The target  30  is secured to the object to be tracked in such a manner that exposes its reflective element  60  and first and second light emitting devices ( 90 ,  100 ) to other components of the system  10 , such as the laser unit  20  and the camera unit  40 . 
     The laser unit  20  is secured at a stationary location, with the laser beam emitting and detecting functions of the laser unit  20  directed toward and exposed to the target  30 . The camera unit  40  is secured at a stationary location, with the image receiving function of the camera unit  40  directed toward and exposed to the target  30 . In anticipation of the object moving, the laser unit  20  emits an outgoing laser beam  130  toward the target  30 . The laser unit  20  and target  30  are arranged so that the servo motors maintain the surface of the reflective element  60  generally perpendicular to the path of the laser beam  130 . Such an arrangement can be referred to as an “active target.” The reflective element  60  reflects an incoming laser beam  130  back toward the laser unit  20 , where the laser unit  20  detects the incoming laser beam  130 . The first and second light emitting devices ( 90 ,  100 ) emit light within the field of view  120  of the camera unit  40 , and the camera unit  40  detects that light. 
     During these processes, the system  10  is gathering information and data. For example, the system  10  continuously gathers real-time information and data from servo motors and/or encoders of the laser unit  20  for use in determining the laser unit&#39;s  20  rotation about both the azimuth and elevation axes. The system  10  gathers real-time information and data on the outgoing and incoming laser beams  130  for use in calculating the distance between the laser unit  20  and the target  30 . The laser beam  130  can generate information and data through techniques, such as, for example, pulsed laser configuration, repetitive time of flight pulses, phase/intensity modulation of the laser beam, or so on. The system  10  continuously gathers real-time information and data from servo motors and/or encoders of the target  30  for use in determining the target&#39;s  30  rotation about both the yaw and pitch axes. The system  10  continuously gathers real-time information and data from the gyroscope  80  for use in determining the target&#39;s  30  rotation about the roll axis. Finally, the system  10  continuously gathers information and data regarding the positions of the first and second light emitting devices ( 90 ,  100 ) for use in refining and confirming the target&#39;s  30  rotation about the roll axis. 
     With regard to the determination of the target&#39;s  30  rotation about the roll axis, it will be understood that the system  10  can use both the information and data from the gyroscope  80  and the first and second light emitting devices ( 90 ,  100 ) captured by the camera unit  40  to determine the target&#39;s  30  rotational orientation about the roll axis. In one exemplary method, the system  10  continuously calculates the target&#39;s  30  rotation about the roll axis using information and data gathered by the gyroscope  80 . However, periodically, the system  10  calculates rotation about the target&#39;s  30  rotation about the roll axis using information and data gathered by the camera unit  40  based on light emitted from the first and second light emitting devices ( 90 ,  100 ). The system  10  then uses the calculated rotation about the roll to adjust or “zero out” the gyroscope  80  to correct for drift, resulting in precise calculation of the position and rotational orientation of the object. In such an example, the camera unit  40  can be an approximately 12 hertz camera unit that gathers information and data several times a second, and the system  10  applies that information and data several times a second to insure accurate and precise calculation of the position and rotational orientation of the object. In another example, the camera unit  40  may be an approximately 100 hertz, which produces even more rapid feedback from the first and second light emitting devices ( 90 ,  100 ) and camera unit  40  to accurately and precisely calculate the position and orientation of the object. 
     As noted herein, once the system  10  precisely calculates the position and rotational orientation of the object, the system can provide feedback to the mechanism controlling the movement of the object. Turning again to the example of a robotic welding arm, a welding head can be secured at the end of the welding arm. The target  30  can be attached near the welding head so that the position and rotational orientation of the target  30  can be associated or translated to the position and rotational orientation of the welding head. The system  10  can be provided with information and data that defines where the welding head should be positioned and how it should be oriented during each time increment of the welding processes. The system  10  can establish a feedback link or loop with the robotic welding machine. As the system  10  calculates the position and orientation of the welding head, the system  10  can compare that position and orientation with the optimal position and orientation of the welding head. The system  10  can provide continuous, real-time feedback to the robotic welding machine that either the welding head is where it should be or, if not, provide instructions to the robot welding machine on the difference between actual position and orientation and optimal position and orientation so that the robotic welding machine can make an appropriate adjustment. 
       FIGS. 5 through 8  illustrate other embodiments of a measurement system.  FIGS. 5 and 6  illustrate an embodiments of a measurement system  200  that joins the camera unit  40  and the control unit  50  and secures that assembly to the laser unit  20 . As is illustrated, similar to the system  10  illustrated in  FIG. 1 , the system  200  includes a laser unit  20 , a target  30 , a camera unit  40  and control unit  50 . Generally, the operation of the system  200  of  FIGS. 5 and 6  is similar to that of the system  10  of  FIG. 1 . However, in the system  200  of  FIGS. 3 and 4 , the camera unit  40  is coupled to or otherwise affixed to the laser unit  20 . The laser unit  20  and camera unit  40  are coupled to each other along the elevation axis. In one embodiment, the laser unit  20  and camera unit  40  can both be secured to a shaft  210  aligned with the elevation axis such that the laser unit  20  and camera unit  40  move together about the elevation axis. It will also be understood that when the camera unit  40  is coupled to the laser unit  20 , the laser unit  20  and camera unit  40  move together about the azimuth axis as well. As a result, the optical axis of the camera unit  40  and the path of the laser beam  130  are in essence both pointed toward the reflective element  60  of the target  30 . Thus, when the “active” target  30  continuously rotates and moves such that the reflective element  60  is perpendicular to the path of the outgoing laser beam  130 , the light emitting devices ( 90  and  100 ) are always in the field of view  120  of the camera unit  40 . As will be understood, with such an arrangement, the data and information generated by the camera unit  40  capturing images of the light emitting devices ( 90  and  100 ) will be consistently and continuously available to the control unit  50  for use in determining the position and orientation of the object. 
       FIGS. 5 and 6  illustrate another embodiments of a measurement system  300  that joins the camera unit  40 , control unit  50  to the laser unit  20 . As is illustrated, similar to the systems  10  and  200  illustrated in prior figures, the system  300  includes a laser unit  20 , a target  30 , a camera unit  40  and control unit  50 . Generally, the operation of the system  300  of  FIGS. 7 and 8  is similar to that of the systems  10  and  200 . However, in the system  300 , the camera unit  40  is coupled directly or otherwise affixed to the laser unit  20 . The laser unit  20  and camera unit  40  are coupled to each other such that the camera unit  40  movements are in sync with the movements of the laser unit  20 . It will also be understood that when the camera unit  40  is coupled to the laser unit  20 , the laser unit  20  and camera unit  40  move together. As a result, the optical axis of the camera unit  40  and the path of the outgoing laser beam  130  are in essence both pointed toward the reflective element  60  of the target  30 . Thus, when the “active” target  30  continuously rotates and moves such that the reflective element  60  is perpendicular to the path of the outgoing laser beam  130 , the light emitting devices ( 90  and  100 ) are always in the field of view  120  of the camera unit  40 . As will be understood, with such an arrangement, the data and information generated by the camera unit  40  capturing images of the light emitting devices ( 90  and  100 ) will be consistently and continuously available to the control unit  50  for use in determining the position and orientation of the object. 
     Additional embodiments of a novel arrangements and processes for correcting in real-time errors in rotational measurement cause by gyroscope drift are illustrated in  FIGS. 9-25 . These novel arrangements and processes include one or more levels. In one example, the one or more levels can be MEMS levels. Levels are typically very accurate and a good complement to the use of a gyroscope to correct for drift. As will be understood, it is desirable to track objects through all possible ranges of motion, including a full 360 degree rotation about the roll axis. For example, it is not uncommon for a robotic welding head to rotate through the full 360 degrees about the roll axis while preforming welding operations on a large and complicated assembly. 
     Throughout the description of these embodiments and figures, several points of reference will be used to facilitate such description. For example, when discussing rotational movement about the roll axis, it will be assumed for simplicity that the target and object only rotate about the roll axis. This is to say that the rotation of the target and object are in the plane defined be the pitch and yaw axes. Additionally, the rotational movement of the target and object to be tracked about the roll axis are described with reference to the direction of the force of gravity, which will be referred to herein as the “gravitational vector” and be represented by the symbol “g.” For example, as illustrated in  FIGS. 9  (perspective view) and  10  (front plan view), the target  810  is in an “initial” position. The initial position is defined as, from a front elevation view of the target  810 , a centerline (CL v ) passing through the “height” of the target is parallel to both the gravitational vector g and the yaw axis, and a centerline (CL h ) passing through the “width” of the target is perpendicular to the gravitational vector g. Both CL v  and CL h  will be subsequently used as references to describe the rotation of the target  810  and object about the roll axis. 
     In its initial position, illustrated in  FIGS. 9 and 10 , the target  810  will be described as rotated 0 degrees with respect to the gravitational vector g. Additionally, the convention of “clockwise” and “counterclockwise” will be used for convenience for further description. For example, in  FIG. 11 , the target  810  is described as rotated 25 degrees counterclockwise with respect to the gravitation vector g, and in  FIG. 12 , the target is described as rotated 25 degrees clockwise with respect to the gravitational vector g. 
     With reference to  FIGS. 10-14 , an assembly  800  can include a target  810  secured to a welding head  820  (shown schematically as a block for convenience). As illustrated in  FIGS. 9 and 10 , the target  810  is positioned in the initial position. During operation, the welding head  820  and the attached target  810  can rotate about the roll axis. As discussed previously, in  FIG. 11 , the welding head  820  and target  810  have rotated approximately 25 degrees counterclockwise about the roll axis, and in  FIG. 12 , the welding head  820  and target  810  have rotated approximately 25 degrees clockwise about the roll axis, both with respect to the gravitational vector g. Similarly, as illustrated in  FIG. 13 , the welding head  820  and target  810  have rotated approximately 135 degrees clockwise about the roll axis, and as illustrated in  FIG. 14 , the welding head  820  and target  810  have rotated approximately 145 degrees counterclockwise about the roll axis. For any application where the object tracked experiences such ranges of rotation about the roll axis, in order to fully track the movement of the object, the multi-dimensional measurement system must be arranged to measure the angular rotation about the roll axis (which can be accomplished by the gyroscope) and further correct for any drift in the gyroscope over time. 
     One method of correcting for gyroscope drift includes the positioning of one or more levels in the target  810 . As will be described in detail, the positioning and use of the one or more levels can provide correction for gyroscope drift regardless of the rotational position of the tracked object about the roll axis.  FIG. 15  schematically illustrates the positioning of three levels  910 ,  920 , and  930  within the body of the target  810 . As illustrated, all three levels  910 ,  920 , and  930  are positioned generally in the plane defined by the yaw and pitch axes and positioned generally symmetrically about the roll axis (i.e., about 120 degrees apart about the roll axis). As will be understood, a level includes a weight that is acted on by gravity as the level rotates, and each level has a “zero” orientation, where it is in line with the gravitational vector g. When the target  810  is in the initial position, as illustrated in  FIG. 15 , a first level  910  is positioned such that generally in line with the gravitational vector g (i.e., in its zero orientation), a second level  920  is positioned approximately 120 degrees clockwise about the roll axis from its zero orientation, and a third level  930  is positioned approximately 120 degrees counterclockwise about the roll axis from its zero orientation. As will be further discussed, the three levels  910 ,  920 , and  930  work cooperatively with each other and the gyroscope to accurately measure rotation about the full 360 degrees the roll axis 
     The operation of the levels  910 ,  920 , and  930  will be discussed with reference to six 60 degree “rotational paths” about the roll axis. As illustrated in  FIG. 15 , a first rotational path α 1  is defined as a path that is between 30 degrees clockwise and 30 degrees counterclockwise with respect to the gravitational vector g; a second rotational path α 2  is defined as a path that is between 30 degrees counterclockwise and 90 degrees counterclockwise with respect to the gravitational vector g; a third rotational path α 3  is defined as a path that is between 90 degrees counterclockwise and 150 degrees counterclockwise with respect to the gravitational vector g; a fourth rotational path α 4  is defined as a path that is between 150 degrees counterclockwise and 210 degrees counterclockwise (also 150 degrees clockwise) with respect to the gravitational vector g; a fifth rotational path α 5  is defined as a path that is between 90 degrees clockwise and 150 degrees clockwise with respect to the gravitational vector g; and a sixth rotational path α 6  is defined as a path that is between 30 degrees clockwise and 90 degrees clockwise with respect to the gravitational vector g. 
     Levels can typically have a working range of at least 60 degrees, and can include a working range of about 70 degrees. The term “working range” refers to a rotational range where the level provides a valid reading for rotational orientation when the level is rotated no more than half its working range either clockwise or counterclockwise relative to the gravitational vector g. For a level with a 60 degree working range, it can provide a valid rotational orientation when it is rotated between 30 degrees clockwise and 30 degrees counterclockwise relative to the gravitational vector g. For a level with a 70 degree working range, it can provide a valid rotational orientation when it is rotated between 35 degrees clockwise and 35 degrees counterclockwise relative to the gravitational vector g. 
     With reference to  FIGS. 15-17 , it will be understood that as the target  810  rotates through the first rotational path α 1  (i.e., between clockwise 30 degrees and counterclockwise 30 degrees with respect to the gravitational vector g), the first level  910  (assuming a 60 degree working range) can determine the target&#39;s rotational orientation about the roll axis. For example, as shown in  FIGS. 16 and 17 , when the target is rotated 25 degrees counterclockwise (as in  FIG. 11  as well) or 25 degrees clockwise (as in  FIG. 12  as well), the first level  910  can determine that the rotational orientations of the target are in fact 25 degrees counterclockwise and 25 degrees clockwise respectively. It will also be understood that when the target  810  rotates more than 30 degrees clockwise or 30 degrees counterclockwise with respect to the gravitational vector g, the first level  910  may be ineffective because it is outside of its working range. However, when the target  810  rotates more than 150 degrees clockwise or 150 degrees counterclockwise (the fourth rotational path α 4 ), as illustrated in  FIGS. 18 and 19 , the first level  910  again is positioned within its working range, i.e., its zero orientation is within 30 degrees rotation of the gravitational vector g. For example, in  FIG. 18 , the target  810  has rotated 155 degrees counterclockwise from the initial position, which positions the first level  910  within 30 degrees of its zero orientation. Thus, the first level  910  can determine that the rotational orientation of the target is in fact 155 degrees counterclockwise from its initial position. In  FIG. 19 , the target  810  has rotated 155 degrees clockwise from the initial position, which positions the first level  910  within 30 degrees of its zero orientation. Thus, the first level  910  can determine that the rotation of the target is in fact 155 degrees clockwise from the initial position. From the two examples provided regarding the first level  910 , it will be understood that for the levels described herein, there are two rotational paths where the level&#39;s 60 degree working range is effective in determining rotational orientation, and these two rotational paths are generally located opposite to one another along the full 360 degree path of travel about the roll axis (i.e., for the first level  910 , its working range is effective in rotational paths α 1  and α 4 ). 
     As will be understood, for the second level  920  and the third level  930 , there are two rotational paths for which the level&#39;s working range is effective. As illustrated in  FIG. 20 , when the target  810  is rotated between 30 degrees and 90 degrees counterclockwise (rotational path α 6 ), the working range of the second level  920  is effective, and the second level  920  can determine the rotational orientation of the target  810  about the roll axis. As illustrated in  FIG. 21 , when the target  810  is rotated between 90 degrees and 150 degrees clockwise (rotational path α 3 ), the working range of the second level  920  is again effective, and the second level  920  can determine the rotation of the target  810  about the roll axis. 
     As illustrated in  FIG. 22 , when the target  810  is rotated between 30 degrees and 90 degrees clockwise (rotational path α 2 ), the working range of the third level  930  is effective, and the third level  930  can determine the rotation of the target  810  about the roll axis. As illustrated in  FIG. 23 , when the target  810  is rotated between 90 degrees and 150 degrees counterclockwise (rotational path α 5 ), the working range of the third level  930  is again effective, and the third level  930  can determine the rotation of the target  810  about the roll axis. 
     Another embodiment of a target with three levels incorporated into the body of the target is illustrated in  FIG. 24 . The first level  910  is positioned in the same location as in  FIGS. 15-23 . The second level  920  is positioned approximately 60 degrees clockwise to the first level  910 , and the third level  930  is positioned approximately 60 degrees counterclockwise of the first level  910 . In such an arrangement, the working range of the first level  910  is effective in rotational paths α 1  and α 4 , the working range of the second level  920  is effective in rotational paths α 2  and α 5 , and the working range of the third level  930  is effective in rotational paths α 3  and α 6 . 
     If the operational requirements of a tracking system are such that the object to be tracked will travel less than the full 360 degrees about the roll axis, then a system can be arranged with less than three levels. In one embodiment, for example, if the object to be tracked will not rotate more than 60 degrees about the roll axis, then only one level can be used. As illustrated in  FIG. 25 , one level  910  is positioned in the target and can determine the rotational position of the target from 30 degrees counterclockwise to 30 degrees clockwise with respect to the gravitational vector g (illustrated as rotational path α 7  in  FIG. 25 ). In another embodiment, for example, if the object to be tracked will not rotate more than 120 degrees about the roll axis, then two level can be used. As illustrated in  FIG. 26 , a first level  910  and a second level  920  are positioned in the target and can determine the rotational position of the target from 60 degrees counterclockwise to 60 degrees clockwise with respect to the gravitational vector g (illustrated as rotational paths α 8  and α 9  in  FIG. 26 ). 
     In order for a multi-dimensional measurement system to properly determine the rotation about the roll axis, the system must be able to distinguish between the two rotational paths where the level&#39;s working range is effective. In one example, the system can make such distinctions by closely tracking the rotational movement of the target over time. For example, as the target rotates about the roll axis, the system can determine and store data on rotational positions at short discrete time increments. The system can compare each newly determined and stored data point with the previous data point and determine if the movement is clockwise or counterclockwise or if there has been a change in the rotational movement from clockwise to counterclockwise or vice versa. 
     With reference to  FIG. 15 , if the target is rotating in a clockwise manner, as the target approaches the edge of the working range of the first level  910  (i.e., the edge of rotational path α 1 ), once the system detects a valid reading from the third level  930 , the system can determine that the target is in rotational path as and not rotational path α 5 . Similarly, if the target is rotating in a counterclockwise manner, as the target approaches the edge of the working range of the first level  910  (i.e., edge of rotational path α 1 ), once the system detects a valid reading from the second level  920 , the system can determine that the target is in rotational path α 6  and not rotational path α 3 . As will be understood, the system can continuously track the rotational orientation of the target as it rotates through the full 360 degree angle of the roll axis and dynamically determine the appropriate level to rely on and which rotational path (α 1 -α 6 ) the target is passing through in determining the precise rotational position of the target, and thus the tracked object, about the roll axis. 
     As previously noted, the working range of a level can be greater than 60 degrees. In one example, the working range can be approximately 70 degrees. The additional working range can be used by the system to determine which level to rely on and which of the two rotational paths is the correct rotational path. For the arrangements illustrated in  FIG. 15  and/or  FIG. 24 , it will be understood that the additional working range can create an overlap of about 10 degrees between adjacent rotational paths (α 1  through α 6 ). Such overlap can be considered by the system in determining the appropriate level to use in determining precise rotational position about the roll axis. With reference to  FIG. 15 , if the target is rotating in a clockwise manner, as the target approaches the edge of the working range of the first level  910  (i.e., the edge of rotational path α 1 ), the system also detects a valid reading from the third level  930 . Thus, as the target continues to rotate clockwise, the system can determine that the target is in rotational path as and not rotational path α 5 . Similarly, if the target is rotating in a counterclockwise manner, as the target approaches the edge of the working range of the first level  910  (i.e., the edge of rotational path α 1 ), the system also detects a valid reading from the second level  920 . Thus, as the target continues to rotate counterclockwise, the system can determine that the target is in rotational path α 6  and not rotational path α 3 . As will be understood, the system can continuously track the rotational orientation of the target as it rotates through the full 360 degree angle of the roll axis and dynamically determine the appropriate level and the appropriate rotational path to rely on in determining the precise rotational position of the target, and thus the tracked object, about the roll axis. 
     In another embodiment, the light emitting devices and the camera can be used to identify the appropriate level and rotational path to rely on when determining the rotational orientation of the target about the roll axis. As previously described, the camera can capture images of the target, including the light emitting devices to determine the rotational orientation of the target. It will be understood that the system can use the information and data generate by the camera to determine the general rotational orientation of the target over time, and use such information to identify the appropriate level and rotational path to rely on. To the extent that the level provides more precise readings than can be ascertained from the information and data generated by the camera, such a method can result in more precise determination of the rotational orientation of the target, and thus the tracked object, about the roll axis. 
     While numerous embodiments of exemplary multi-dimensional measurement systems are described and illustrated herein, the examples are not exhaustive. The components of multi-dimensional measurement systems can be arranged in any number of ways and combinations. However, in any arrangements where data and information derived from light emitting devices is to be considered in determining the rotation of a target about its roll axis, and thus determining the position and orientation of the target, the light emitting devices associated with the target are to be in the field of view of a camera unit when such data and information is to be considered. 
     The foregoing description of examples has been presented for purposes of illustration and description. It is not intended to be exhaustive or limiting to the forms described. Numerous modifications are possible in light of the above teachings. Some of those modifications have been discussed, and others will be understood by those skilled in the art. The examples were chosen and described in order to best illustrate principles of various examples as are suited to particular uses contemplated. The scope is, of course, not limited to the examples set forth herein, but can be employed in any number of applications and equivalent devices by those of ordinary skill in the art.