Abstract

In the paper, numerical analysis of dynamics of a variable mass system is considered. Its reference example is a serial-like manipulator composed of revolute joints and rigid bodies. Payload of its gripper is considered as its variable mass element (mass and inertia irregularly between subsequent payloads). For the rest of the system, inertia parameters are considered as known (precisely identified during the assembling process). As a revolute joint manipulator is considered, its dynamics depends on its pose, i.e., when driven over some significant range, nonlinear terms are present. In this case, classical linear controllers may not guarantee satisfactory performances of the control. In the paper proposed control algorithm, a nonlinear model of the plant is prepared and implemented in its feedforward controller. In the initial tests, the control is restricted to the open-loop feedforward model-based controller, only. Behaviour of such system is tested numerically. Identical equations for the two sub-models are implemented in two separate blocks of the program (for the controlled plant and the for the model based controller). Both models are prepared according to the classical rules of the multi-rigid-body modelling. To test the influence of the payload variability, different sets of the inertia parameters are proposed for these blocks. When the two model differ (incorrect inertia identification), observed trajectory errors are significant. To eliminate them, the controller is enlarged. Feedback loops are introduced and proportional-derivative controllers are instilled in the feedback paths. Finally, possibility of mass identification is examined. To arrange it, at beginning of each of the operational motion, a test run is performed, and output signals of the feedback controllers are analysed. Identified mass and its centre position are used to correct data in the model-based controller. With the improved model, the feedback signal decreases and realisation of the required path is corrected, as well.

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