Section: New Results
Achieving virtual stiff walls with a force feedback device based on piezo-electric motor
The rotor of the piezo-electric motors is propelled by friction by a bending wave at the surface of the stator. They have then a high holding torque if not supplied because in that condition rotor is locked on stator. This feature can be used to simulate very stiff walls on a 1-dof force feedback device  : if the device's lever is at the position of the virtual wall, the supply voltages of the motor are set to zero, otherwise the motor is controlled in an elastic mode (figure 8 ). The main advantage is that the wall mode ensures passivity of the interface because no energy is supplied to the motor. This strategy is specific to this kind of motors . Finally, a torque estimator has been designed; it can help to remove the torque sensor of the test setup  .