Capacitive strain gauges on flexible polymer substrates for wireless, intelligent systems

Abstract

Abstract. This paper presents a novel capacitive strain gauge with interdigital electrodes, which was processed on polyimide and LCP (liquid crystal polymer) foil substrates. The metallization is deposited and patterned using thin-film technology with structure sizes down to 15 µm. We determined linear strain sensitivities for our sensor configuration and identified the most influencing parameters on the output signal by means of an analytical approach. Finite-element method (FEM) simulations of the strain gauge indicated the complex interaction of mechanical strains within the sensitive structure and their effect on the capacitance. The influence of geometry and material parameters on the strain sensitivity was investigated and optimized. We implemented thin films on 50 µm thick standard polymer foils by means of a temporary bonding process of the foils on carrier wafers. The characterization of the strain sensors after fabrication revealed the gauge factor as well as the cross sensitivities on temperatures up to 100• C and relative humidity up to 100 %. The gauge factor of a sensor with an electrode width of 45 µm and a clearance of 15 µm was −1.38 at a capacitance of 48 pF. Furthermore, we achieved a substantial reduction of the cross sensitivity against humidity from 1435 to 55 ppm % −1 RH when LCP was used for the sensor substrate and the encapsulation instead of polyimide. The gauge factor of a sensor half-bridge consisting of two orthogonal capacitors was 2.3 and the cross sensitivity on temperature was reduced to 240 ppm K −1 . Finally, a sensor system was presented that utilizes a special instrumentation Integrated Circuit (IC). For this system, performance data comprising cross sensitivities and power consumption are given.

Microstrip Patch Strain Sensor Miniaturization Using Sierpinski Curve Fractal Geometry

“…These sensors are especially useful for measuring stresses in applications with high elasticity. They consume about ten times less energy than resistive sensors, which is very important in case of wireless systems [4]. Thus, capacitive strain sensors can be applied in the measurement of skin or tire strain [4,5,6].…”

“…The parallel-plate strain sensor is assembled by sandwiching a dielectric layer between two electrodes [6]. Whereas, the second type has in-plane electrodes parallel to their substrate [4,5]. Sensors with in-plane electrodes proved higher sensitivity, sensing linearity and lower hysteresis than parallel-plate-type capacitive strain sensors [5].…”

Gold/Polyimide-Based Resistive Strain Sensors

“…Along with the advantages of these sensors described above in terms of their fabrication and processed materials, these sensors exhibited a reasonably high G.F. The G.F. value was higher than other strain sensors that were polyimide-based [53,54], silicon-based [55] and fabric-based [56] used for biomedical applications. Figure 11 shows the change in the relative resistance of the sensors with respect to the induced strain in the vertical direction.…”

Aerosol jet printed capacitive strain gauge for soft structural materials

“…Currently, commercial CSGs are available, and typically consist of a parallel plate design restricting strain measurements to those that are perpendicular to the gauge direction, are limited in the geometries they can monitor, or require bulky electromechanical devices for mechanical attachment 2,12 . These issues can be overcome by using CSGs having an interdigitated electrode design 13,24 . Interdigitated electrodes preserve the advantages associated with CSGs vs RSGs in harsh environments, while also providing a vehicle by which CSGs can integrate directly onto structural components.…”