This work describes a self-sensing technique for a piezoelectrically driven MRI-compatible tweezer style end effector, suitable for robot assisted, MRI guided surgery. Nested strain amplification mechanisms are used to amplify the displacement of the piezo actuators to practical levels for robotics. By using a hysteretic piezoelectric model and a two port network model for the compliant nested strain amplifiers, it is shown that force and displacement at the tweezer tip can be estimated if the input voltage and charge are measured. One piezo unit is used simultaneously as a sensor and an actuator, preserving the full actuation capability of the device. An on site calibration procedure is proposed that calibrates the combined electromechanical model without requiring specific loading conditions on the inner piezoelectric actuators. Experimental validation shows an average of 12% error between the self-sensed and true values.

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. I. INTRODUCTIONHaptic devices require physical contact between the operator and the machine, using force feedback and creating a coupled system including the operator and the robot. This introduces inherent instabilities due to the typical response of human operators [1][2][3][4]. Instability in large haptically controlled force assisting devices which aid human operators in manipulating heavy or bulky loads can pose undesired risks to the load and the operator. Human operators often attempt to control oscillation by stiffening their arm,leading to a stiffer system with more instability. Controllers do not typically account for this, as the level of stiffness is often not directly measurable.This research will design a control scheme that adjusts to changes in the way an operator grips a haptic controller to attempt to correct for this. It will adjust the control parameters based on the overall stiffness of the operator's arm, which is expected to achieve higher performance in robotic devices than a fixed control system. This study will consider the effects of such a system on haptic force assisting devices. The control scheme must acquire some metric to estimate the operator's arm stiffness, and electromyogram (EMG) signals are expected to be suitable, as they provide information about the internal state of an operator's muscles. This will be validated by testing if EMG signals can be used to accurately indicate the stiffness level of a human arm. It should then be possible to design a control scheme with

This paper presents the application of a piezoelectric self-sensing technique based on discharged current to robotic tweezers incorporating a rhombus strain amplification mechanism driven by serially connected piezoelectric stack actuators. Connecting a shunt resistor in series with a piezoelectric element allows it to be used simultaneously as an actuator and a sensor by measuring the current generated by the piezoelectric element. This allows the displacement and force to be measured without extra sensors or the loss of actuation capability. Applying an inverse model of the nested structure allows the force and displacement at the tip of the tweezers to be determined. The accuracy of this method is then examined by experiment for the case of free displacement.