Effects of thermal protection methods on haptic perception

Electric DC motors that convert electric current to torque are the most common type of actuators used in haptic interfaces. However, high currents necessary to deliver large haptic forces can generate heat in the electromagnetic coils, and in the extreme can cause malfunctions due to overheating. It is therefore necessary to add thermal protection mechanisms to limit the output current. While this is a common feature of haptic devices, it is not clear what makes a superior current limiting method or what effect such limits have on haptic perception. The current work evaluates three methods for current limiting: limit output to zero, limit output to a safe steady-state current, limit output current in linear proportion to temperature. Using both quantitative and qualitative metrics, these are compared to a control case with no current limit. Human subjects use one finger of a multi finger haptic device to perform a psychophysical thresholding experiment designed to measure human perception of small haptic effects, while forcing the device to heat up. All methods are shown to effectively regulate temperature and show no statistically significant difference in psychophysical threshold value. Users show a slight preference for the linear method, while the zero output method requires less time spent interacting with current limited conditions.