Sensing Large Strains by Measuring Light Attenuation in Elastomeric Waveguides

Abstract

Soft actuators can bend and deform into many shapes and it is advantageous to be able to measure the position of the end of a soft actuator to provide feedback control. Relying solely on input measurements of the quantity that provides the energy for actuations (driving fluid pressure or volume in the case of pneumatic and hydraulic actuators) does not provide enough information to estimate the position considering that external forces will also have an impact on the resulting shape and position. Therefore, more direct position sensing methods such as strain sensing are required. These position sensing methods will need to satisfy a number of requirements to be suitable as a sensor for a soft actuator. The sensor must be able to detect larger strains ideally greater than 200 % to provide coverage for a large variety of movements. The sensor should be able to do this with a high degree of sensitivity and repeatability. In addition, it would be beneficial for the sensor to be relatively cheap and cost effective, and immune to or largely unaffected by electromagnetic interference. After considering different position and strain sensing technologies, a strain sensing method using light attenuation in an elastomeric waveguide was deemed the most appropriate. The light attenuation is characterized with a fit to the Beer-Lambert Law where the light power exponentially decreases as the distance increases from the light source. The elastomeric waveguide itself was inexpensive, easy to manufacture, and could undergo large strains with strains of up to 275 % under test (potentially to strains of up to 900 % as quoted in the product datasheet). Being an optical method, the sensor is immune to electrical or magnetic interference. Testing proved that the sensor was accurate and reliable over low numbers of stretching cycles.