SPIRA Coils: Soft Self-Sensing Pneumatic Integrated Retractable Actuator Coils

Abstract

Abstract This paper introduces a novel self-sensing coiled actuator called a “SPIRA Coil.” This soft pneumatic actuator is formed from thin sheets of PET plastic that are laminated into a coil shape that unfurls like a “party horn” when inflated. Many soft actuators require large pressure to create only modest strains. SPIRA Coils, in contrast, can easily be designed and fabricated to extend over dramatic distances with relatively low working pressures. In this work, some of the actuators with a coiled outer diameter of approximately 2 cm were able to extend 60 cm with less than 5 kPa. The internal structure of a SPIRA Coil is formed by two layers of metalized PET that separate in the extended portion of the actuator. Where the coiled portion of the actuator begins, the sheets come together, closing an electrical circuit. By measuring the resistance of the circuit, the extended length of the actuator can be determined. The paper presents and experimentally validates easy-to-use design models for the actuator’s self-retracting spring stiffness, its pneumatic extension force, and its internal length-sensing electrical resistance. Testing of the self-sensing capabilities demonstrated that the embedded sensor can be used repeatably and accurately to determine the actuator length, with virtually no hysteresis (R2 = 0.996).