Test and fabrication of piezoresistive sensors for contact pressure measurement

Abstract

ABSTRACT:The use of contact pressure sensors has become popular in various engineering disciplines in recent years. They are used in characterization of vehicle tires, bearings, wind tunnels, prosthesis design, ergonomic analysis among other areas. These sensors are fabricated with materials that have certain properties such as piezoelectricity, piezoresistance and variable capacitance; however, the most used characteristic is the piezoresistive effect. This paper describes the fabrication of three different sensors using piezoresistive materials. Furthermore, a comparative technical study including a commercial sensor as a benchmark is done with the aim of selecting a suitable material when measuring contact pressure. The repeatability and hysteresis of each sensor were evaluated in a response to load test realized several times. A time drift test with a dead load was also performed for evaluating stability. Materials such as piezoresistive fabric or ink show to be suitable for applications where deformation and flexible sensors are required, Velostat is the least accurate but suitable for basic applications and in which a high resolution is not needed. Finally, some recommendations are given regarding the type of material to be used in pressure sensors for engineering applications, particularly in the biomedical field. RESUMEN:El uso de sensores de presión de contacto se ha popularizado en diferentes disciplinas de la ingeniería en los últimos años. Se utilizan en la caracterización de llantas para vehículos, rodamientos, túneles de viento, diseño de prótesis, análisis ergonómicos, entre otras áreas. Estos sensores, son diseñados con materiales que poseen ciertas propiedades tales como piezoelectricidad, piezorresistencia y capacitancia variable; sin embargo, la característica más usada es la piezorresistencia. En este artículo se describe la fabricación de tres sensores de presión diferentes usando materiales piezorresistivos. Adicionalmente, se realizó un estudio técnico comparativo incluyendo un sensor comercial usado como punto de referencia con el fin de seleccionar el material idóneo para medir presión por contacto. La repetibilidad y la histéresis de cada sensor fueron evaluadas en una prueba de respuesta a la carga realizada varias veces. También se llevó a cabo una prueba de desviación en el tiempo para evaluar estabilidad de la medición de un peso muerto. Los materiales como la tela o tinta piezoresistiva muestran ser adecuados para aplicaciones en las que haya deformación y se necesite de sensores flexibles, el Velostat es el menos preciso pero adecuado para aplicaciones básicas y en las cuales no se necesite de mucha resolución. Finalmente se presentan recomendaciones respecto al tipo de material que se debe utilizar en sensores de presión para diversas aplicaciones en ingeniería en general y en el campo biomédico en particular.

Development of a Self-Powered Piezo-Resistive Smart Insole Equipped with Low-Power BLE Connectivity for Remote Gait Monitoring

“…Afterwards, sensor 6 has been subjected to loading/unloading cycles from 0 Kgf to 50 Kgf, leaving a rest–pause between consecutive loads of 8 min before to measure the resistance. In Figure 18 b are reported the characteristic for ascending (curve orange) and descending load (grey curve); as evident, the sensor shows a hysteresis in the two responses, already observed in previous characterizations [ 52 ]. The hysteresis is the maximum difference between the sensor’s responses for a single load once applied in ascending and descending way.…”

“…Further characterization of piezoresistive material was reported in [ 52 ]. Four sensors of different materials were realized and tested, based on the Velostat piezoresistive layer and built with the sandwich structure with 1 cm 2 sensing area, as shown in Figure 13 .…”

A Velostat-Based Pressure-Sensitive Mat for Center-of-Pressure Measurements: A Preliminary Study

“…The whole mat could be sampled at 10 Hz. For the Velostat, conductance and pressure were approximately proportional [20,21].…”

Force-Sensitive Mat for Vertical Jump Measurement to Assess Lower Limb Strength: Validity and Reliability Study

“…In this way, variations on the resistance values on every intersection of rows and columns when pressure is exerted over the mat can be measured. Some typical characteristic problems with Velostat material are repeatability, nonlinearity, and hysteresis [23,24]. For this application, however, these features are not relevant because high precision is not needed; the mat only needs to be able to detect a heavy body placed on it (average human body weight).…”