Design of an Optical Soft Sensor for Measuring Fingertip Force and Contact Recognition

Among the various ways to estimate user intention in hand exoskeletons, a contact force measurement is definitely the most straightforward and intuitive method. A force sensor, located at the center of a fingertip usually, hinders the tactile sensation of the user by blocking the contact between an object and the fingertip. To overcome this problem, a soft force sensor with horse shoe shape is utilized to measure the contact force and provide the tactile sensation to the user. This work presents the mechanical design, implementation and evaluation of a soft fingertip force sensor. To maximize tactile sensation of the user, we adopted a horse shoe shape structure to leave the finger pad exposed. An optical sensing mechanism was selected for its relatively fast response compared to other soft sensors. The whole sensor system has a soft exterior providing flexibility and a user-friendly interface. To evaluate the sensor's performance, we carried out sensor optimization process and calibration experiment with a customized test bed. Then, we investigated both static and dynamic response and observed the mechanical behavior and light intensity changes caused by the cross sectional shape and base/agent ratio of PDMS. Lastly, we applied the proposed sensor to the glove type fingertip force monitoring system. The sensor estimates the index finger tip force with high accuracy (R-2 = 0:96) within 5N range.