A first order transfer function to balance the workload in brake-motor hybrid actuators

The paper proposes a control algorithm to share the torque load between a brake and a motor (hybrid actuation) suitable for an impedance controlled haptic device. Unlike several other methods, it does not include the need for a force or torque sensor, allowing to design a more compact and cheaper haptic device. The proposed algorithm solves the problem of the so called “sticky wall” effect. This problem occurs when, during a virtual wall collision, the brake is still engaged when the user wants to get off from the virtual wall. When this happens, the user can sense such virtual wall as “sticky”. This problem is emphasized when using multiple DOF brakes such as the spherical MR brake built in our research laboratory. In fact, since the brake locks all the DOF simultaneously, operations like sliding on a wall could not be performed without a force/torque sensor. The idea behind the proposed algorithm is to use the brake when passive (braking) torque is necessary, and slowly decaying brake activation by compensating it using motors according to a first order transfer function. The transfer function allows an exponential decay of the brake activation, with a faster response compared to linear transition. The algorithm we developed resulted to be more energy efficient than using just motors during virtual wall collision. Furthermore, the impact against a virtual wall provided less vibrations for stiffness values up to 5000 Nm/rad.