Safety control for impedance haptic interfaces

Instability in haptic systems due to behaviors of human operators or unknown virtual environments can present safety issues. For example, a haptic stylus may be ejected at a high speed which may cause injury if manipulated inappropriately. Three safety control approaches are thus proposed in the paper to tackle the issues. The first approach is based on a simple velocity and force dependent controller, while the other two are derived from a grasp force model exploiting the state of the haptic stylus, including acceleration, velocity and penetration force, to generate the grasp force. Based on the force model, safety indicators called “safety observers” are proposed to monitor system instability caused by sudden release of haptic stylus by a user during interactions with virtual objects. Experimental results show that the proposed models can effectively reduce the velocities of the haptic stylus and suppress jerking movement, and stabilize the haptics system even in presence of active forces. The proposed safety controllers can respond to the “safety observers” and generate appropriate damping forces to counteract the instability incurred, whereas traditional control methods fail to trigger a protection mechanism under the same situation. As prior knowledge of virtual environments and additional hardware sensors are not required, the proposed approaches have the potential to be widely adopted in haptics-enabled applications.