UNITEST - A new device for the static and dynamic testing of unconventional machine structures K.E. Meitz Department of Production Engineering, Technical University Graz, Kopernikusgasse 24, A-8010 Graz, Austria Email: meitz@ift.tu-graz.ac.at Abstract A new measuring device for volumetric measurement of machine tools developed at the Department of Production Engineering at the Technical University Graz integrates static and dynamic test principles in a single instrument. Linear positioning test, circular test and freeform test can be applied to unconventional machine structures like industrial robots and hexapods, measuring simultaneously position and orientation of the machine tool's end effector during the motion. 1 Introduction The development of industrial robots and, in particular, of hexapods with their potential in high speed cutting has made rapid progress. Due to the need of attaining reasonable accuracy even at elevated speeds, dynamic measurement is gaining more and more importance in order to check the performance at high speeds, where dynamic effects cause deviations in addition to geometry errors of the machine tool's structure. On the other hand software compensation of geometric errors of the mechanical structure of parallel mechanisms like hexapods can only be based on a complete knowledge of the geometric errors, requiring static measuring procedures. A simultaneous measurement of position and orientation can be useful for both static and dynamic test procedures.
448 Laser Metrology and Machine Performance 2 Measuring Devices Due to the grouping of the links of robots and hexapods, each axis of the machine tool has influence on position and orientation of the end effector, the errors resulting from the single axes are superimposed. Furthermore the joint motion necessary to perform a circular test with an industrial robot is complex, even for the case of a horizontal plane, which makes volumetric measurement desirable. As a simple instrument for the simultaneous measurement of position and orientation of the end effector, the Robot Circular Test Arrangement ROCITA [1] (see fig. 1 and fig. 2) has been developed at the Institute of Production Engineering in Graz. It consists of a rotatable arm fixed to a stable base and a measuring head attached to the robot. A reference sphere fixed to the measuring head and guided on a radial tracing surface moves the arm, whose angular position is measured by an angular encoder. Probes touching the sphere in radial and z-direction detect the position of the measuring head. The roll and yaw angles are measured by probes mounted on the measuring head, which touch a planar reference surface orthogonal to the main axis of rotation. The system can measure 5 degrees of freedom and can be handled easily. Fig. 1: Cross section of circular test device ROCITA
Laser Metrology and Machine Performance 449 Fig. 2: Circular test arrangement In order to measure all 6 components of a motion, a first prototype of the universal device for automated acceptance tests, UNITES! [2], has been developed at the Institute of Production Engineering in Graz. Within a measuring diameter of 570 [mm] and a height of 5 [mm] a simultaneous measurement of x, y and z coordinates together with roll, pitch and yaw angles is possible. It can be used for circular tests and freeform tests as well as for linear positioning tests (see fig. 3, 4 and 5)., V«GUHG/f B^" fie*,** Fig. 3: Circular test Fig. 4: Freeform test
450 Laser Metrology and Machine Performance EKX Fig. 5: Linear positioning test This version of UNITEST has been used for first experiments with the measurement of industrial robots (see fig. 6). Circular and freeform tests were performed at various velocities. The deviations detected during the circular tests (seefig.7 ) reached the order of millimetres, and showed a strong dependency on the velocity of motion (see fig. 8). As a result of the experience gained during these tests a completely new version of UNITEST has been designed. Fig. 6: Circular test and freeform test for industrial robots
Laser Metrology and Machine Performance 451 500 pm / div Fig. 7: Polar diagram at a velocity of 2400 mm/min and R = 200 mm 1000 ~ E k_ QQ - re snn - oq 600 " ^ X S X" %1 ^ x^ y ^% x^ X X x" 10 30 20 DO 30 30 Vel< Deity [mrn / mln] 40 30 5000 Fig. 8: Circularity and the influence of velocity at R = 200 mm
452 Laser Metrology and Machine Performance 3 UNITEST - A new device for static and dynamic tests In order to cope with the higher speeds, the bigger deviations, the lighter structure and the different geometry of unconventional machine structures in comparison to conventional machine tools some aspects regarding the design of the measuring instrument were of major importance. The mass and the dimensions of UNITEST have been significantly reduced, whereas the stiffness and the precision of the device could even be improved. Furthermore it was important to allow the machine tool big deviations without decreasing the quality of measurement or damaging the test equipment. A standardization of the measuring systems helps to avoid software troubles and to increase the measuring frequency, which is precondition for dynamic measurement. The system consists basically of a stable base body which hosts a high precision bearing and an angular measurement system, which detects the rotation of a linear guidance with integrated cross grid measuring system [3]. The measuring sledge contains optical sensors for simultaneous bidirectional measurement, which allows detection of deviations in the traverse direction during linear positioning tests. The measuring head, which is fixed to the spindle, contains a second angular encoder for the measurement of the yaw angle, whose variation is not limited by the measuring device. 4 incremental probes touching a reference plane detect the roll and pitch angle, allowing a considerable pivoting during the motion, and the deviation in z-direction. Within a diameter of 500 mm and a height of 10 mm various static and dynamic tests can be performed in horizontal as well as in vertical planes. Fig. 9: View of UNITEST
Laser Metrology and Machine Performance 453 4 Summary The simultaneous measurement of position and orientation makes the new measuring device UNITES! a powerful instrument for dynamic and static tests, which can be specially useful for the geometrical calibration of hexapods. The integration of several measurement principles in a single device allows efficient and time saving tests. The static and dynamic measurements defined in ISO 230-2 [4] and in ISO 230-4 [5] can be performed with a single measuring device, which has to be adjusted only once. Acknowledgements A word of thanks to Prof. Adolf Frank, Dr. Franz Haas, Jouni Holsa, the Institute of Production Engineering of the Tampere University of Technology, the Dr. Johannes Heidenhain GmbH, and especially to the people in the shopfloor of the Department of Production Engineering at the Technical University Graz for their friendly help and contribution to the entire project. References 1. Haas, F., Frank, A., Circular Test: New Method for Testing Industrial Robots. Proceedings of the 2"<* International Workshop (RAA '93), Springer Verlag, pp. 203-207, 1993. 2. Haas, F., Entwicklung eines neuen Meflgerates zur automatisierten Schnellabnahmeprufung von Werkzeugmaschinen, Dissertation am Institut fur Fertigungstechnik, TU-Graz, 1996. 3. N.N., Product Information VM 182, Dr. Johannes Heidenhain GmbH, Traunreut, Germany, 1998. 4. ISO 230-2: Test code for machine tools - determination of accuracy and repeatability of positioning of numerically controlled machine tools, 1998. 5. ISO 230-4: Test code for machine tools - circular test for numerically controlled machine tools, 1996.