Pressure and Angle Sensors with Optical Fiber for Instrumentation of the PrHand Hand Prosthesis

Abstract

The principal cause of upper limb amputations is due to traumatism. The prosthesis is an assistive device to help in the activities of daily for the amputee person. However, one of the latest reports shows that in developing countries there are around 30 million people without assistive devices. This work presents the development of two kinds of sensors for the PrHand, an upper limb prosthesis based on compliant mechanism and soft-robotics. The sensors are made with polymeric optical fiber (POF), due to their flexibility and low cost, and the working principle is based on intensity variation. The angle sensors are used for monitoring the interphalangeal joint of the fingers, and for the assessment were made cycles of closing and opening each finger. On the other hand, the force sensors are located at the tip of three fingers to track the force made over the objects. Before encoring the sensors were evaluated making five cycles of compressing and decompressing each sensor. The results show a linear behavior between the angle and the voltage variation, one most remarkable angle sensor result was with a sensibility of 0.0357 V/° and an R2 of 99 % closing and 0.0483 V/° opening. In the case of the force sensor, a polynomial relation was found between the voltage changes and the pressure over the sensor; in some cases, the relation between voltage changes and pressure could be linear but that depends on the construction of the sensor. Regarding the obtained R2 of 99 %, its sensibility was 0.0361 V/N compression and 0.0368 V/N decompression. In conclusion, was successfully developed two kinds of sensors for the instrumentation of PrHand prosthesis. It is expected to use angle and sensor variables as input in algorithms of Machine Learning to improve the detection of objects. One aspect to improve is to control in a better way the sensor construction parameters due to the considerable influence over the sensor behavior.

Soft-Sensor System for Grasp Type Recognition in Underactuated Hand Prostheses

“…The characterization of the first sensor (side polish) was described in previous works [ 47 , 48 , 49 ]. This sensor was already used to instrument the PrHand prosthesis, and it is used in this study for comparison purposes.…”

“…A 3D-printed mold was used for the sensor construction ( Figure 6 a). The chosen fiber polish was the lateral one since, in [ 49 ], it could measure the contact force in the desired range, between 0 N and 3 N. The CNC machine shown in Figure 3 a was used for construction. To fabricate the sensor head, the LED WS2812 (WorkdSemi Co., Dongguan, China) and the fiber were located in the mold, with the side polish of the POF aligned with the LED’s lens; see Figure 6 a.…”