Development and applications of a laser micromachined inter-digitated capacitive strain sensor

Reliable strain measurement plays a very important role in the damage detection of the mechanical and civil structures. It can also have a significant contribution in assisting the proper functioning of human body and organs. The current strain sensors and measurement systems have significant drawbacks in terms of high sensitivity to temperature, low gauge factor, low biocompatibility and bulky data acquisition/conditioning circuitry. These factors make the strain measurement unreliable and expensive. In this work, a laser micro-machined inter-digitated capacitive strain sensor was developed which overcame the limitations of existing strain sensing technologies. The structural health monitoring along with bladder volume sensing applications were targeted and the suitability of the sensor to these applications was verified. The capacitive strain sensor was made up of a low cost metal sheet which was micro-machined with a laser to form an inter-digitated structure. This structure was then encapsulated in a flexible, bio-compatible material which also acted as the dielectric. The connections were made on the metal with the help of a conductive paste to acquire the data. Design of the Interdigitated capacitors with the highest possible capacitance value, precision and repeatability was chosen for the desired applications. The inter-digitated structure was sensitive to stretching and bending resulting in capacitance changes. Modifications and amendments were made in the initial device to enhance its suitability and performance for structural health monitoring applications. The sensor was attached to load bearing structures such as cantilevers and the variation in initial capacitance value was checked experimentally for different strains. A simple circuit was implemented to convert the capacitance changes into frequency changes for ease of remote data transmission. Devices with flexible inter-digitated fingers were fabricated through laser micro-machining in order to improve their performance for in-vivo applications. The suitability of the new devices was shown experimentally for monitoring strain changes of structures such as the urinary bladder. The testing of these devices is done with a wireless, batteryless circuit to verify the working of the strain sensors for inside body applications. In this work, the design, fabrication, observations and results for the desired applications have been discussed.