英语期末论文

AbstractThere is an increasing demand from industry for intelligent reverse engineering, to create CAD models, or to compare prototypes with CAD models. The number of measurementpoints has to be minimal, so there is no need for a long and difficult modeling phase. A reverse engineering solution that only requires a fraction of the amount of measurement points compared to other techniques is desired.With this application in mind, Robotics Research Group of The University of Texas at Austin has developed the 3D Probe solution. The indoor GPS metrology system is a laser based reverse engineering system. The 3D Probe is a portable measurement tool equipped with three teceivers in optimum locations and a ball prove tip. The operator holds the 3D Probe n his hand and moves the probe io over the object. The X,Y and Z coordinates of the probe tipare measured in real time with high accuracy. Different probe tips of all sizes and shapes can be used on the probe to have access to all areas.Potential applications of the metrology system range from actuator metrology to robot, manufacturing workcell, and mobile platform calibrations. This paper describes this new three-dimensional, non-contact metrology system designed to measure the position and prientation of an object with motions in up to six degrees of freedom. The general concept and kinematics of the metrology system as well as the derivations of an error budget for the general device are described. Experimental results of geometry and compliance error identification on a robot will also be included.Keywords: metrology, indoor GPS, reconfigurable robot 1. INTRODUCTIONMetrology research for robotics involves the development of technologies and methods to measure robot pose with required accuracies. Many measurement systems have been developed with large variations in complexity, measurement speed, Accuracy, cost, and required operator skills. State-of-the-art accuracy is of the order of 0.01 –0.05 mm over the robot measurement workspace[1]. Common sensors used for pose measurement are: theodolite, laser tracker, cable potentiometers, laser scanner, Coordinate Measurement Machine, acoustic sensor, ball bar, articulated arm, and vision systems. Fig. 1 shows the benefits of a 3D motion measurement by a metrology system.Figure 1. Objectives of a metrology system 2. EXPERIMENTThe UTRRG developed and implemented a number of measurement systems in the past. Theodolite systems based on 1 arc-second instruments sighting at 10 m have measurement accuracy near 0.05 mm. The laser tracker has a better measurement accuracy that is close to or less than the repeatability of most commercial robots. These sensors are complicated and require skilled operators. Special tools must be attached to the robot. These systems also require regular calibration and considerable set up time before use. Cable potentiometers measure the amount of cable spooled from drums. Most cable potentiometer systems consist of 3 or 4 cables. Typical accuracy is 0.5 mm. These systems are simple and easy to set up. However, robot movement is often restricted. Unfortunately, many of these measurement systems the UTRRG used showed practical problems of long setup time, easy of use, limited measurement range, and insufficient accuracy.Recently, UTRRG has been researching on a 6DOF metrology system with laser technology [2]. At least three laser heads are needed to measure a position in space. Fig. 2 illustrates the UTRRGIndoor GPS system with 3D probe for precision applicationsconcept and its requirements. The problem is the transmitted laser needs to be returned to the same laser head to measure the distance. A servo-controlled head must be developed. Unfortunately, this is too costly .metrologysystemand its performance requirements.Figure 2. UTRRG Concept of a 6DOFThe concept of UTRRG 3D Probe is shown in Figure 3. This is a portable hand-held measurement device with optimally located three detectors and a ball probe tip to measure not only the position but also the orientation of an object in space. It carries three Position Control Engines (PCE) enclosed in a compact electronics box to amplify and transmit the sensor signal. The 3D Probe is a convenient hand-held device that an operator can carry and measure 6 DOF position and orientation of an object indoor and outdoors. In addition, it can directly be attached to an object, a moving part or a mobile platform, with a slight modification, providing their pose changes from their base frames.Figure 3. 3D Probe as a hand-held device (left) anda tracking unit (right)Fig.4 illustrates the 3D Probe with its sensor and tool frames, a vector triangle, and a triangle circumcenter. The probe frame is the coordinate system whose position and orientation will be acquired through the observation of the three detectors. In accordance with the probe concept, detector positions are converted into the probe frame through its circumcenter calculation.The probe frame is defined by the physical position of the detectors according to following convention:• The origin coincides with the circumcenter of a triangle formed by the three detectors.• The X -axis passes through the origin and is parallel with a line passing through the first and the second detector.• The Y -axis passes through the origin, orthogonal to the X-axis and in the plane formed by the three detectors.• The Z -axis is orthogonal to the two previous axes.Figure 4. Frame assignments of 3D ProbeThe 3D Probe is based on indoor GPS technology. Figure 5 shows system components of indoor GPS [3]. They are; three or more transmitters, control center, detectors, wireless electronics, and a scale bar. A battery operated transmitter uses laser and infrared light to create one-way position information, the relative azimuth and elevation from the transmitter to the receiver. The receiver has photodiodes inside its module and senses the transmitted laser and infrared light signals. With the addition of a second transmitter of known location and orientation, users can calculate the position of the receiver in the base coordinate system. By adding two more transmitters, the system will have four laser transmitters having its accuracy maximized. The signal is transferred through a wireless network connection providing mobility to the operator.Figure 5. Indoor GPS componentsAs in satellite-based GPS, a one-way signal path is created from transmitters to the receiver, allowing an unlimited number of receivers to continuously and independently calculate positions whenever two or more transmitters are in view. The indoor GPS metrology system has an accuracy of 4-8 parts-per-million under controlled conditions.A comparable laser-based system cost to measure 6 DOF information of an object would be significantly more expensive. The indoor GPS metrology system also eliminates the recurring problem of accidentally interrupting a laser beam during measurement action that requires the operator to begin the measurement again.An error budget needs to be developed for the 3D Probe to determine the achievable accuracy and to identify the principal sources of error. Error budgets have evolved over the past several years as critical players in the design of instrumentation and machinery, and they have taken on a number of different forms. Slocum [4] discusses the propagation of errors in the homogeneous transformation matrices (HTMs) of a serial kinematic chain. The errors of one component are transmitted into all of the components downstream. The analysis is simply one of identifying the HTMs for each component with errors, deriving the final position of the end-effector, and subtracting that position from the ideal position of a system with no errors. This method is well suited for the 3D Probe where the forward kinematic problem is easily solved. The forward kinematic analysis is used to determine the predicted position and orientation of the object being measured. It is shown in Fig. 6 with three detector position vectors. The error budget is determined by measuring the difference between the calculated position with errors and the original assumed position over the working envelope of the metrology system. The error sources affecting the final accuracy of the metrology system are: the sensor uncertainty, styli and ball deflections, calibration error, user-introduced error, best-line-of-sight error, number-of-line-of-sight error, environmental error, etc.Figure 6. Forward kinematic analysis with errors indetector positionNow the question is how the vector P from the sensor frame to the tool frame may be determined? When the 3D Probe is attached to an object we want to measure, there will be an error between the sensor frame and the interested frame. Thus, how can we identify the difference between the sensor frame and the frame that we want to know? Special methods are developed to answer these3D Probe must be calibrated since both machining tolerance and assembly errors will have an effect on the geometrical relationship between the sensors and the too point. The nominal “as de signed” p arameters are not accurate due to these error sources. The actual value of the vector from the circum center to the styli ball center needs to be identified. A series of calibration procedures are developed for this purpose. The calibration procedures are as follows.1. Obtain a triangle geometry measuring the distances between the three detectors.2. Place the tool on the right hole of the calibration tool and rotate the tool following its spherical surface.3. Measure and record the detector positions and obtain three spheres of data.4. Perform least squares fitting on spheres usingthe recorded data and determine the radiuses of three spheres.6. PROTOTYPE 3D PROBE DEVELOPMENTThe 3D Probe designed in this paper is prototyped and is shown in Fig. 7. The body of the 3D Probe is constructed by aluminum to reduce its weight. It is ideal to have non-reflective color around detectors. It is, therefore, anodized with black color to reduce any reflection. Two different ball probes are shown in Fig.7. One is 10mm diameter ball probe and the other is 3mm ball probe. The 3mm ball probe is connected to an extension with an adaptor. It may be used for small objects, holes, and crowded areas. If the ball probe is exchanged, a calibration procedure must be performed to identify the vector from the sensor frame to the tool frame as described in Section 5.1. The 3D Probe is battery operated. Each detector uses a separate battery. The measured data is transferred to the control center using wireless communication. Thus, wireless electronics unit is used for data transfer. They are stored inside a portable case. An operator can conveniently carry the 3D Probe and the electronics and measures 6 DOF position and orientation of an object in space.Figure 7. Prototype 3D Probe and its components 3. RESULTFig. 8 shows the laboratory implementation of the indoor GPS metrology system. Four transmitters are installed on rigid steel posts to reduce noise. For a reverse engineering purpose, a C-shaped arrangement is selected rather than a square shape. 3D Probe has a hardwire connection with a case containing wireless electronics and power supply. Between 3D Probe and a control center, wireless communication is used for ease of use. A scale bar and a setup tool are used to setup the system.Figure 8. Laboratory implementation of indoor GPSmetrology systemA system diagram of the metrology system is shown in Fig. 9 (a). Figures 9 (b)-(e) shows its technological impacts on many industrial applications such as reverse engineering of industrial and modular actuators, robot calibration, real-time calibration of manufacturing workcells, and pose tracking of mobile platforms. A back plate may be attached to the 3D Probe with its ball probe tip detached. Then, it may be attached to a robot providing spatial information of the robot. For manufacturing workcells, it can compensate guide rail errors and warm-up drift errors, mechanical wear, temperature gradient dependencies, mechanical play at individual axes, and non-linearity errors such as mechanical friction maximizing the workcell accuracy and enabling flexible programming. When attached to the mobile platform, such as the PowerBot from ActiveMedia (Fig.9(e)), the current trajectory can be corrected [5].Figure 9. Indoor GPS metrology system with 3DProbe and application areas4 .APPLICATIONA modular reconfigurable robot is a collection of individual link and joint modules withstandardized connection interfaces that can be assembled into different robot geometries. The basic building blocks of such systems are modular actuators that have embedded intelligence and can be used to assemble on demand a wide variety of robotic configurations ranging from mobile platforms and 6 DOF manipulator arms to 40 DOF manufacturing robotic cells. The key to the success of modular robotics is the ability to replace modules efficiently. This requirement of frequent calibrations can diminish the benefits of modular robots and thus can be a potential obstacle for the development of modular robotics in the long run. The success of modular robotics is based on two key technologies; the first key is an accurate and interchangeable standardized interface and the second key is Indoor GPS metrology system with 3D Probe. Figure 10 shows how the 3D Probe may be attached to a reconfigurable modular robot. Then, the robot end-effector is moved to a number of preset positions around its workspace. By comparing the measured and nominal positions, the robot errors are determined and compensated. On the other hand, if a reconfigurable robot is constructed using standardized interface, 3D Probe is used for reverse engineering of individual modules. The obtained geometric and compliance information may then be used to calibrate reconfigurable robots after its assembly.Figure 10. 3D Probe installed on a reconfigurablerobot5. CONCLUSIONA unique metrology system utilizing Indoor GPS technology is introduced. A hand-held tool, 3D Probe is designed and a prototype is made. 3D Probe enables 6DOF position and orientation measurement of an object in three-dimensional space. It can also be used as a reverse engineering tool for a manufactured part or a machine. Four transmitters are installed on top of rigid steel posts to reduce noise. The system uses multiple triangulation principles to calculate position of each detector.The kinematic solutions for 3D Probe are developed. Both position and orientation of 3D Probe ball probe tip are determined using the kinematic solutions. The selected method minimizes the uncertainty effect of each detector on the pose of the ball probe tip. Several different ways to calibrate 3D Probe are presented in this paper. Both manufacturing and assembly errors are eliminated by identifying a vector from each detector to ball probe tip after components of 3D Probe are assembled. Currently, 3D Probe is being used for reverse engineering of manufactured machines such as actuators and linkages. It is also being used to test a set of robot performance parameters. By testing accuracy and repeatability of a robot, its errors can be compensated. The developed metrology system willeventually be used for real-time measurement and calibration of an open-architecture,flexible manufacturing workcells. REFERENCES[1] Mooring, B.W., Roth, Z.S., and Driels, M.R., 1991, Fundamentals ofManipulator Calibration, John Wiley and Sons, New York, NY.[2] S. Kang and D. Tesar, 2000, “Metrology Test Environment for ModularActuators”, Master’s Thesis, The University of Texas at Austin.[3] ARC Second, Inc., 2004, Product Literature, Company Web Site URL:.[4] Slocum, A. H., 1992, Precision Machine Design, Englewood Cliffs, NJ:Prentice Hall, pp. 61-80.[5] ActivMedia Robotics, 2004, Product Literature, Company Web Site URL。