An improved human-robot interface by measurement of muscle stiffness

Human contact with haptic devices introduces instabilities due to human operators´ attemps to stiffen their arm to stabilize the system. Controllers often cannot measure arm stiffness and do not typically account for this. A method to effectively adjust the controller of a robotic force assist device to compensate for changes in operator arm stiffness was established. It was expected to achieve reduced oscillations and increased performance than one with fixed gains. The results could be used to design human-robot interfaces for force assisting devices. The compensating system used EMG signals to measure muscle activity, then estimated the stiffness of the human´s arm. This was used to adjust the parameters of a haptic device´s impedance controller based on a threshold. The system was then implemented on a small haptic device to study the effects with a human subjects. EMG signals were experimentally validated as an effective prediction of the stiffness of an operator´s arm. The system was assessed in terms of performance and was found to provide improved stability and demonstrated the potential for increased performance.