Force display using a hybrid haptic device composed of motors and brakes

In the virtual environment, force feedback to the human operator makes virtual experiences more realistic. However, the force feedback using active actuators such as motors can make the system active and sometimes unstable. To ensure the safe operation and enhance the haptic feeling, system stability should be guaranteed. Both active actuators such as motors and passive ones such as brakes are commonly used for haptic devices. Motors can generate a torque in any direction, but they can make the system active and thus, sometimes unstable during operation. On the other hand, brakes can generate a torque only against their rotation, but they dissipate energy during operation and this dissipation makes the system intrinsically stable. Consequently, motors and brakes are complementary to each other. In this research, a two degree-of-freedom (DOF) haptic device equipped with motors and brakes is designed, in which each DOF is actuated by a pair of motor and brake. Simultaneous operation of motors and brakes is analyzed. Models for some environments, virtual wall contact and frictional effect, are proposed. The results for the hybrid haptic system are compared with those for the active haptic system and the passivity based control system. The experimental results show that the hybrid haptic device is more suited to some applications than the other haptic systems.