Sensorless force control for a manipulator with flexure joints

Sensorless force control aims at controlling the force or torque applied by a manipulator on its environment without the need of including a dedicated force or torque sensor. To enable this control concept the dynamic relation between drives and interaction with the environment needs to be established accurately. From that perspective manipulators with flexure joints may offer advantages. In these manipulators the relative rotation of links is enabled by elastic deformations of flexible elements in the joints which results in reproducible behaviour due to the low amount of friction and hysteresis.

In this paper the development of such force control is addressed. It depends heavily on a simulation that evaluates the non-linear dynamic behaviour of the manipulator to estimate the end-effector forces from the actuator torques in realtime. Main ingredients of the model are the finite compliance of the flexure joints and the mass properties of the rigid links. Next to setting up the model structure the involved mass and stiffness parameters need to be determined from experimental identification of the system.

The concept is demonstrated for a manipulator with two degrees of freedom. For a limited range of motion the compliance can be described by a (constant) stiffness matrix. For larger displacements non-linear behaviour is observed which can be modelled with polynomial fits of the actuator torques as functions of the displacement. It is assumed that the mass properties can be captured with a (constant) mass matrix and velocity dependent inertia terms. Currently, the control concept has mainly been evaluated in simulations, but it is expected that preliminary experimental data can be presented during the online symposium.