Abstract:
Many motion sensing applications, especially for movement analysis in sports and physiotherapy, favour wearable technology over costly stationary camera based systems. Because of their compliance with soft tissue and fabric, DE (dielectric elastomer) sensors are well suited for integration into motion sensing garments. They are essentially soft capacitors with a stretch dependent capacitance that is measured by applying a harmonic excitation signal. Inexpensive materials, such as silicone dielectrics and electrodes made from a compound of silicone and carbon black, keep the production cost low. However, the resistance of such compounds increases drastically under stretch, thus compromising the accuracy of stretch measurement. Furthermore, connecting each sensor to its own channel increases the footprint of the sensing circuitry, and adds bulky wiring; both are undesirable in a soft sensing garment. The impact of the resistance dynamics of carbon black-silicone electrodes on the capacitance readout was investigated experimentally by periodically stretching a DE sensor, and measuring its capacitance at different signal frequencies. Dips and transient changes in the measured capacitance were more pronounced at higher strain rates, and higher sensing signal frequencies. The second area of interest, the reduction of wiring and sensing channels, was addressed with the development of a method capable of detecting multiple DE sensors through one channel. In the final embodiment of the system, DE sensors were interconnected with conductive conductive elastomer resistors, thus demonstrating the possibility of having an entirely soft sensor transmission line. This work has found that the excitation frequency has to be adjusted to the dynamics of electrode resistance caused by stretching in DE sensors with resistive electrodes. Furthermore, a frequency-based method capable of detecting multiple DE sensors through a single set of wires was developed. Both contributions aim to enable the high-volume production of comfortable, inexpensive motion garments.