Enhanced force control using force estimation and nonlinearity compensation for the Universal Haptic Pantograph

The design of a stable and robust force controller is one of the most important and difficult tasks in rehabilitation robotics. In previous works, the Universal Haptic Pantograph (UHP) was presented as an alternative to conventional arm rehabilitation after a stroke. This robot is composed by a Series Elastic Actuator (SEA) and a Pantograph. In this work an enhanced force control for the UHP is presented. The proposed controller uses the robot model to estimate the contact force without direct measurement and to compensate nonlinearities in the actuators. In order to prove the effectiveness of the approach, several tests are carried out in simulation and experimentally. Results reveal that mean of tracking errors between desired and actual force is smaller than 0.1 N, which is significantly improved compare to that (around 2.5 N) shown in previous results of UHP, indicating that the proposed force control is likely to enhance haptic performance of the UHP.