University of Twente Student Theses


External force estimation on an non-linear compliant 2DOF manipulator system

Holst, T.H.J. van der (2022) External force estimation on an non-linear compliant 2DOF manipulator system.

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Abstract:In some applications, the external forces acting on a system might be required (e.g. for the purposes of force control). It might, however, be difficult to implement a force sensor on the interaction surface or it may be too costly to do so. In this report, estimation of the external forces acting on a 2DOF manipulator with flexure hinges is considered. The estimation of the external forces is based on the system dynamics and the actuator inputs of the system. The joints of the manipulator are based on flexures, which reduce the uncertainties due to friction and hysteresis. Nevertheless, the estimation process is complicated by the non-linear nature of the 2DOF manipulator. An estimation method is developed based on linearized Kalman filtering, whereby a linearized dynamic model of the system is derived and applied to a Kalman filter, and the remaining Kalman filter parameters are set based on the expected disturbances caused by errors in the identification of the system’s parameters. The method is first evaluated on a double pendulum model through simulations. The double pendulum is a simpler system in comparison, but has key similarities with the 2DOF manipulator. From testing simulations, it is found that the accuracy of the model used by the linearized Kalman filter has a significant influence on the quality of the estimates. Any difference in the dynamic behavior of the real system with respect to the model will result in fictitious forces being estimated. This cannot be avoided by considering the modelling errors when setting the parameters of the Kalman filter. Despite the importance of the model’s accuracy, neglecting the non-linear behavior in the model does not have a significant influence on the estimated external forces. The method is then tested on an existing setup of the 2DOF manipulator. The results are agreeable, although an estimation error appears to be present based on the sensor measurements. The magnitude of the estimation errors are in general less than 10% of the magnitude of the applied external forces. Estimation errors also occur when only actuator forces are applied, again with a magnitude of at most 10% of the actuator forces. It is observed that disturbances not measured by the sensors, such as friction and hysteresis, act on the system. It is difficult to determine exactly to what extent the estimation errors are caused by these unmeasured disturbances using the current setup
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Programme:Systems and Control MSc (60359)
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