Experimental research of force/pressure sensor static and dynamic accuracy and repeatability

Abstract

The progress observed in 'soft robotics' brought some promising research in flexible tactile, pressure and force sensors, which can be based on polymeric composite materials. Therefore, in this paper, we intend to evaluate the characteristics of a force-sensitive material – polyethene-carbon composite (Velostat®) by implementing this material into the design of the flexible tactile sensor [1]. Velostat® is a composite polymer material consisting of carbon-impregnated polyethene. Entrapped carbon powder turns initially dielectric polyethene into electrically conducting composite material which belongs to the group of piezoresistive materials. Simply speaking by applying force or pressure on this material, its resistance is changed. The main problem is an insufficiency of research of Velostat®, in order to successfully predict its reaction to particular mechanical irritation and therefore implement it as a reliable sensor in a mechatronic system. During the research, the velostat® film was placed between two conductive copper electrodes which allowed to measure the resistance of the material in respect of time and evaluate sensors' dynamic response. Dynamic load acting on the sensor's upper surface was created using a motor carrying an eccentric load (Fig. 1.). By changing the mass and adjusting the loading fixture offset from motor shaft different magnitude mechanical loads were exerted. The loading frequency was controlled by the stepper motor's varying rotation speed. From the Fig. 1 you can observe a velostat® film (1) being place between two conductive strips (2), to connect it to measuring device, placed between a lower (3) and upper (4) plates to hold it in place. The upper plate is represented transparent, in order to show the interconnection between the elements. The upper plate is firmly connected to the motor (5), which is going to create an eccentric load for force-sensitive material. On the shaft of the motor a pulley (6) with slots is mounted, in order to allow attachment and adjustment of different weights making testing device more versatile also an accelerometer (7) is joined with the upper plate to monitor the magnitude of excited force and create a closedloop system. When the experiment is carried out, data is obtained and saved on the memory device. Correlation between frequency and amplitude of applied dynamic load and change in resistance of velostat® defines how precisely sensor can react to varying load. Obtained data allows defining the sensor's main dynamic characteristics such as response time and bandwidth. After result evaluation, precise velostat® reaction is established, promoting the further implementation of forcesensitive material into "soft robotics" applications.