IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 3, NO. 2, JUNE 1995 189 Positioning of Unknown Flexible Payloads for Robotic Arrns Using a Wrist-Mounted Forcemorque Sensor Sandeep Jain, Student Member, IEEE, and Farshad Khorrami, Member, IEEE Abstruct-The problem of control design for robotic arms manipulating unknown flexible payloads is considered in this paper. A novel scheme utilizing a forcdtorque sensor at the wrist of the manipulator is utilized. This choice of sensor is adequate to glean information about payload vibrations without outfitting the payload with sensors and offers the additional advantage of furnishing a minimum phase system, thus simplifying the control design. A controller utilizing wrist torque feedback from the sensor is designed as a function of the system parameters. An adaptive scheme for on-line identification of these parameters and adaptation of controller parameters is discussed. The pro- posed scheme is generic in regard to the nature of the flexible payload, as is shown through simulation studies. The scheme is implemented on a single-link manipulator carrying an unknown flexible payload. Simulation and experimental results validate the fact that the adaptive control maintains a robust performance for high speed slewing and varying payloads. I. INTRODUCTION N many applications, vibration suppression of the payload I being carried by manipulators is crucial. High speed slews may result in excitation of lightly damped vibrational modes in payloads, which nullifies the primary objective of rapid task execution and thus increased efficiency, since the time taken for the payload oscillations to settle adds to the task execution time. At the same time, the characteristics of payloads to be handled vary with application. These considerations make it impractical to outfit the payload with sensors. In space applications, manipulators typically handle spacecrafts and satellites which contain flexible appendages. Earth-based ap- plications include: 1) vibration damping of the umbilical cord providing power to the manipulator/positioner (such as high speed probes), 2) manipulators carrying payloads containing fluids, 3) vibration damping of oscillations occurring in pay- loads attached to cranes with long ropes (in general tethered payloads exhibiting pendulum type motion), and 4) precision positioning of payloads that exhibit significant flexural modes. Manuscript received February 25, 1994; revised October 5, 1994. Recom- mended by Associate Editor, D. W. Repperger. This work was supported in part by U.S. Army Research Office Grants DAAHM-93-2-0009 and DAAH04-93-G-0209. An earlier version of this paper was presented at the IEEE Intemational Conference on Robotics and Automation, May 1994, San Diego, CA. The authors are with the ControliRobotics Research Laboratory (CRRL), School of Electrical Engineering and Computer Science, Polytechnic Univer- sity, Brooklyn, NY 11201 USA. IEEE Log Number 9410676. Several efforts have been reported on control of manip- ulators with joint and link flexibility [1]-[16], to name a few. Handling of flexible payloads, however, has not received as much attention. This is possibly partially because much effort is still underway on control of multilink flexible ma- nipulators. Another constraint associated with this problem is that it is impractical to outfit the payload with sensors. Earlier works on vibration-free motion of suspended objects exhibiting pendulum-type motion have been reported [ 171, [ 181. These techniques basically reduce to the preshaping idea introduced in [19] and later utilized by others [20], [21]. Recently, studies have been done to deal with the flexural motion of payloads. Chen and Zheng utilized a similar technique to damp out the vibration of a cantilevered beam [22]. The technique advocated in [22] is to synthesize the end- effector motion trajectory such that the vibrations are damped. This requires the knowledge of the flexural modes of the payload a priori. Another recent work is by Alder and Rock [23]. The authors considered a pendulum type payload attached at the tip of a one-link flexible arm. The measured quantities of the payload were the inertial position and orientation of the payload. Finally, recent work on dynamics and control issues in space robotics using forcekorque sensing is reported in [24]. In this paper, a novel scheme for vibration damping of flexible payloads utilizing only the measurements from a forcekorque (FE) sensor mounted at the wrist of the ma- nipulator is considered. The advantages of this measurement scheme over other schemes for sensing payload vibrations are discussed. The effectiveness of the advocated controller is shown for vibration damping of a flexible beam carried by a manipulator. Simulation and experimental results are presented to illustrate the validity of the proposed scheme. Furthermore, an adaptive version of the controller has been introduced to handle unknown payloads. Therefore, no prior knowledge of the payload dynamics needs to be assumed. The proposed scheme is generic in regard to the nature of the flexible payload and also the nature of the rigid-link manipulator used for task execution. 11. EXPERIMENTAL SETUP The experimental setup at the ControYRobotics Research Laboratory (CRRL) has been designed to simulate remote manipulator systems. The initial experimental effort reported here is toward the design of a single-link arm manipulating 10634536/95$04.00 0 1995 IEEE