On preshaped reference inputs to reduce swing of suspended objects transported with robot manipulators

One of the application areas of robot manipulators is pick-and-place operations where some objects may not be grasped by robot manipulators due to the task constraints. Such objects, for example those simply suspended, need to be carried by a hook or a similar device attached to the manipulator endpoint. This paper presents the results and implications of a study into preshaped reference inputs to reduce the swing of suspended objects at the end of a desired trajectory. Two reference input preshaping methods are considered; (i) superimposing a ramp function onto another function, and (ii) convolving a sequence of impulses with a desired reference input to generate a shaped input. A hydraulically actuated robot manipulator carrying a compound pendulum was employed as an experimental system to test the methods. Simulation and experimental results are presented to demonstrate the feasibility of both methods. It is concluded that while superimposing a ramp function onto another function such as a cycloid does not increase the transportation time while reducing the swing, whereas the latter, convolving a sequence of impulses with a cycloid, increases the transportation time. In addition to this, the robustness of the first method to the uncertainties in the natural frequency of the suspended object is better than that of the second method. The simulation and experimental results prove that by properly preshaping the reference input of a robot manipulator, a swing-free stop is obtainable. The proposed approaches are simple and easy to implement.