University of Twente Student Theses


Joint level modelling, characterisation and torque control of the SHERPA robotic arm

Jager, J (2017) Joint level modelling, characterisation and torque control of the SHERPA robotic arm.

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Abstract:The SHERPA project is a European project whose goal is to develop a mixed ground and aerial robotic platform to support search and rescue activities in a real-world environment. The following actors compose the basic ’SHERPA team’: a human rescuer, small scale rotary-wing Unmanned Aerial Vehicles (UAVs), a ground rover and long endurance, high-altitude, highpayload aerial vehicles. In order to improve the autonomous capabilities of the robotic platform, a multi-functional robotic arm is installed on the rover(Marconi et al., 2012) (Barrett et al., 2017). The University of Twente research team has developed a 7 active Degrees of Freedom (DOF) robotic arm for the ground rover. The arm is equipped with two variable stiffness actuators. The VSA enables the robotic arm to become compliant and adapt its dynamic behaviour to different tasks. High level control software is implemented in ROS running on a Intel NUC running Ubuntu, to be able to control the robotic arm in the workspace. In case of the SHERPA project the dynamic environment is unknown and interaction tasks with humans or its dynamic environments is required. A particular popular control method to be able to interact with the environment is the impedance control formulation. In order to obtain impedance control on the SHERPA robotic arm, joint level torque control is required. This master thesis focuses on identification, modelling of the joint characteristics and torque control by means of the arm’s variable stiffness actuators. Torque control on the SHERPA robotic arm is achieved by regulating the deflection of the compliant joints. The output position of the robotic arm is fixed, such that during torque control the robotic arm is in contact with the environment. For each joint a dynamic model in bondgraphs is created and implemented in 20-sim. Simulations are performed to determine the behaviour and characteristics of each joint. The joints are characterised in a test set-up and curve fitting is applied to obtain the torque-deflection characteristics. The acquired torquedeflection characteristics are implemented in the high-level software. A control loop with a PIcontroller is written to perform torque control. Joint controllers convert each desired joint position into motor positions, which are sent to the ELMO motor drives. These will perform local position control to achieve the given set-point. Overall the torque control for the variable stiffness joints provides a good tracking of the reference torque and is validated with experiments. With this approach joint level torque control for the variable stiffness joints is accomplished and the objectives for this master thesis are satisfied.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:50 technical science in general, 52 mechanical engineering, 53 electrotechnology, 54 computer science
Programme:Systems and Control MSc (60359)
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