Abstract
A method for synthesizing the feedforward torques and reference trajectory for a flexible two-link planar manipulator is proposed. These torques and the trajectory are not far from being time-optimal. The control law for each drive torque consists of two parts: a commanded feedforward torque and a linear angular position and velocity feedback.
Initially, we theoretically prove that so-called 'fluent’ control enables an individual link to reach the desired hub acceleration with little error and avoid unwanted large flexible vibrations. With this control, the dynamics of a flexible link is close to the dynamics of the associated rigid link.
We begin the design of the feedforward torques and reference trajectory by computing the control torques for an arm with two rigid links which are near time-optimal control functions. These torques are discontinuous functions of time. The jumps in the control torques are not acceptable for an arm with flexible links because large elastic vibrations appear in the links. This conclusion follows from a theoretical study of individual links and also from experimentation with one-link and two-link flexible arms. Therefore, fluent control is considered here. Designing this fluent control, we transform the obtained discontinuous control into continuous functions of time, taking into account the elasticity of flexible links. During the experiments with flexible arms, these continuous functions are used as commanded feedforward torques. We also feed on-line these control torques to a mathematical model of the arm with two rigid links and compute the corresponding desired angular velocities and positions of the links as functions of time. Then we feed this reference trajectory to the linear feedback system (usual PD-controller) of the flexible arm. Designed control does not excite large elastic vibrations although, simultaneously, it is not far from being time-optimal. The designed control algorithm has been successfully implemented in the experiments.
Similar content being viewed by others
References
Akulenko, L.D. and Bolotnik, N.N., 'On controlled rotation of an elastic rod', Applied Mathematics and Mechanics 46(4), 1982, 465–471.
Berbyuk, V.E. and Demidyuk, M.V., 'Controlled motion of an elastic manipulator with distributed parameters', Mechanics of Solids 19(2), 1984, 57–66.
Cannon, R.H. and Schmitz, E., 'Initial experiments on the end-point control of a flexible onelink robot', International Journal of Robotics Research 3(3), 1984, 62–75.
Book, W.J., 'New concepts in lightweight arms', in Proceedings 2nd International Symposium on Robotics Research, M. Brady (ed.), MIT Press, Cambridge, MA, 1985, 203–205.
Chedmail, P. and Michel, G., 'Modelization of plane flexible robots', in Proceedings of 15th ISIR, I. Kato and K. Takahashi (eds), Elsevier Scientific Publishers, Amsterdam, 1985, 1083–1090.
Ding, X., Tarn, T.J. and Bejczy, A.K., 'A novel approach to the modelling and control of flexible robot arms', in Proceedings of 27th Conference on Decision and Control, T. Basar (ed.), IEEE Computer Society Press, New York, 1988, 52–57.
Siciliano, B. and Book, W., 'A singular perturbation approach to control of lightweight flexible manipulators', International Journal of Robotics Research 7(4), 1988, 79–90.
De Luca, A. and Siciliano, B., 'Closed-form dynamic model of planar multi-link lightweight robots', IEEE Transactions on Systems, Man and Cybernetics 21(4), 1991, 826–839.
Bayo, E., Papadapoulos, P., Stubbe, J. and Serna, M.A., 'Inverse dynamics and kinematics of multi-link elastic robots: An iterative frequency domain approach', International Journal of Robotics Research 8(6), 1989, 49–62.
Yurkovitch, S., Tzes, A.P. and Hillsley, K.L., 'Modeling and control issues for a manipulator with two flexible links', in Proceedings of 29th Conference on Decision and Control, J. Herget and R.A. DeCarlo (eds), IEEE Computer Society Press, New York, 1990, 1995–2000.
Hillsley, K.L. and Yurkovitch, S., 'Vibration control of a two-link flexible robot arm,’ in Proceedings IEEE Conference on Robotics and Automation, Sacramento, CA, A.K. Bejczy (ed.), IEEE Computer Society Press, New York, 1991, 2121–2126.
Pham, C.M., Khalil, W. and Chevallereau, C., 'A nonlinear model-based control of flexible robots', Robotica 11(1), 1993, 73–82.
Aoustin, Y. and Chevallereau, C., 'The singular perturbation control of a two-flexible-link robot', in Proceedings IEEE International Conference on Robotics and Automation, Atlanta, GA, W. Book (ed.), IEEE Computer Society Press, New York, 1993, 737–742.
Khorrami, F. and Jain, S., 'Nonlinear control with end-point acceleration feedback for a twolink flexible manipulator: Experimental results', Journal of Robotic Systems 10, 1993, 729–736.
Moallem, M., Khorosani, K. and Patel, R.V., 'Optimum structure design for flexible-link manipulators', in Proceedings IEEE Conference on Robotics and Automation, Minneapolis, MN, Vol. 1, T.J. Tarn (ed.), IEEE Computer Society Press, New York, 1996, 798–803.
Zhao, H. and Chen, D., 'Optimal motion planning for flexible space robots', in Proceedings IEEE Conference on Robotics and Automation, Minneapolis, MN, Vol. 1, T.J. Tarn (ed.), IEEE Computer Society Press, New York, 1996, 393–398.
Lin, L.C. and Yeh, S.L., 'A composite adaptive control with flexible quantity feedback for flexible-link manipulators', Journal of Robotic Systems 13, 1996, 289–302.
Pontryagin, L.S., Boltyanskii, V.G., Gamkrelidze, R.V. and Mischenko, E.F., TheMathematical Theory of Optimal Processes, Wiley-Interscience, New York, 1962.
Aoustin, Y. and Formal'sky, A., 'On the synthesis of a nominal trajectory for control law of a one-link flexible arm', International Journal of Robotics Research 16(1), 1997, 36–46.
Dombre, E. and Khalil, W., Modélisation et Commande des Robots, Editions Hermes, Paris, 1988.
Formal'sky, A.M. and Osipov, S.N., 'On the problem of the time-optimal manipulator arm turning', IEEE Transactions on Automic Control AC-35, 1990, 714–719.
Formal'sky, A.M., 'On the time-optimal control of the bending of a plane two-link mechanism rotation', Applied Mathematics and Mechanics 60(2), 1996, 243–251.
Geering, H.P., Guzzella, L., Hepner, S. and Onder, C.H., 'Time-optimal motions of robots in assembly tasks', IEEE Transactions on Automic Control AC-31, 1986, 512–518.
Zelikin, M.I. and Borisov, V.F., Theory of Chattering Control with Applications to Astronautics, Robotics, Economics, and Engineering, Birkhäuser, Boston, 1994.
Akulenko, L.D., 'Quasi stationary finite motion control of hybrid oscillatory systems', Applied Mathematics and Mechanics 55(2), 1991, 183–192.
De Luca, A., Isidori, A. and Nicolo F., 'Control of robot arm with elastic joints via nonlinear dynamic feedback', in Proceedings of 24th IEEE Conference on Decision and Control, Ft. Lauderdale, FL, C.I. Byrnes (ed.), IEEE Computer Society Press, New York, 1985, 1671–1679.
Bayo, E., 'A finite-element approach to control the end-point motion of a single-link flexible robot', Journal of Robotic Systems 4(1), 1987, 63–75.
Bayo, E., Movaghar, R. and Medus, M., 'Inverse dynamics of a single-link flexible robot. Analytical and experimental results', International Journal of Robotics and Automation 3(3), 1988, 150–157.
Spong, M.W., 'Modeling and control of elastic joint robots', Transactions ASME 109(4), 1987, 310–319.
Formal'sky, A.M. and Lavrovsky, E.K., 'Stabilization of flexible one-link arm position: Stability domains in the space of feedback gains', International Journal of Robotics Research 15(5), 1996, 492–504.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Aoustin, Y., Formal'sky, A. On the Feedforward Torques and Reference Trajectory for Flexible Two-Link Arm. Multibody System Dynamics 3, 241–265 (1999). https://doi.org/10.1023/A:1009875908235
Issue Date:
DOI: https://doi.org/10.1023/A:1009875908235