Piezoresistive pressure sensors based on elastomer-conductive material composite is particularly promising due to their many advantages such as simple readout circuit, low crosstalk, low susceptibility to electromagnetic pick-up, and low-cost and simple fabrication process. [5a,6] Various works have been reported to improve the performance of piezoresistive pressure sensors, most of which have been focused on increasing the sensitivity. [3a,7] For instance, microstructuring of the piezoresistive element into porous structure, [4b,8] pyramids, [7a,9] microdomes [3d,10] have been demonstrated to improve the sensitivity, which has been attributed to the decrease in the compressive modulus. [7a,11] Porous structures, in particular, was utilized in various pressure sensors due to their facile fabrication process and scalability. Porous structure can be fabricated either by filling a 3D template such as sugar, [12] nickel foam [13] with an elastomer and subsequently etching away the template, or by mixing aqueous and oil solutions to form an emulsion and removing the solvents. [4b,14] Despite its significance, maximizing sensitivity in composite-based piezoresistive pressure sensors is not necessary for many applications (i.e., often moderate levels are sufficient). On the other hand, sensor-to-sensor uniformity and hysteresis are two properties that are of critical importance to realize any application. In fact, without assuring high uniformity and low hysteresis, using the sensor in a practical setting is unrealistic. However, there is currently a lack of reported work that specifically addresses these issues. As far as it is known, no quantitative assessment of sensor-to-sensor uniformity (error bars are sometimes included in the sensor performance plots but are not specifically addressed) or hysteresis was reported in composite-based piezoresistive pressure sensors. The importance of sensor-to-sensor uniformity is obvious. If sensors with largely varying characteristics are used together as an array, each sensor has to be individually calibrated, making accurate measurement impractical with increasing number of sensors. Hysteresis, which is the difference in the output signal under loading and unloading of pressure, also causes inaccuracy in measurement. Hysteresis is especially problematic in piezoresistive sensors, which originates from weak interactions Sensor-to-sensor variability and high hysteresis of composite-based piezoresistive pressure sensors are two critical issues that need to be solved to enable their practical applicability. In this work, a piezoresistive pressure sensor composed of an elastomer template with uniformly sized and arranged pores, and a chemically grafted conductive polymer film on the surface of the pores is presented. Compared to sensors composed of randomly sized pores, which had a coefficient of variation (CV) in relative resistance change of 69.65%, our sensors exhibit much higher uniformity with a CV of 2.43%. This result is corroborated with finite element simulation, w...
Involuntary movements such as heart beating in surgical environment and surgeon's tremor disturb a micro surgical manipulation and cause a risk of patient wound. Although the delicate operation is performed by a skilled surgeon, the sensitivity of the surgeon is limited to quantify the range of safe contact forces. In this paper, we developed a compact hand-held surgical device to maintain a required contact force to maintain a required contact force using a custom force sensor and a linear delta mechanism. The custom optical force sensor measured the contact force of the device tip and the linear delta mechanism compensated undesired forces to maintain a consistent contact force. The proposed device is consisted of force sensing unit and actuating unit. The device was improved from our previous Linear Delta mechanism based prototype in terms of size, weight, and force sensing capability. The developed device was validated by investigation of contact force accuracy in a fixed condition and a hand-held condition. In hand-held condition, the visual feedback of the current contact force was provided, and the performance of the contact force regulation was investigated by comparing the root mean square (RMS) contact force errors and standard deviation in with and without control cases. The fluctuation (less than 50 mN) of the force regulation control of the device showed the feasibility of the device for the use in delicate operations.
The use of power assistive devices that use surface electromyography (SEMG) signals may be limited by the noisy nature of SEMG signals. The aim of this study was to investigate the variation in human movement stability while the amount of SEMG-based assistive power was changed. A robotic device provided a torque that was proportional to the torque estimated by SEMG for assisting human movements, and 12 volunteers participated in the elbow flexion experiments. The maximum finite-time Lyapunov exponent (MFTLE), the average logarithmic rate of the divergence of neighboring trajectories, and the variability of the kinematic data were used to quantify the stability of the assisted elbow movements. The stability provided by the MFTLE decreased as the amount of assistive torque increased with respect to the amount of human torque. The kinematic variability increased with the increase in assistive torque. Therefore, by ensuring that the amount of SEMG-based assistive torque is less than the amount of human torque, the assistance may provide relatively natural movements. This study is the first to quantify movement stability as SEMG-based assistive power is applied. This study can provide a foundation for determining the appropriate amount of SEMG-based assistive power.
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