Modeling and Feasibility of an Elastomer-Based Series Elastic Actuator as a Haptic Interaction Sensor for Exoskeleton Robotics

Safe and compliant physical human-robot interaction (HRI) is a crucial consideration in exoskeleton design. This paper presents a model for an elastomer-based series elastic actuator (eSEA) and tests its feasibility to provide torque sensing as a haptic interface for soft, comfortable HRI in exoskeletons. The design of the eSEA and a model accounting for elasticity, rate dependency, and rate-independent hysteresis is presented. It is fitted with a worst-case root-mean-square (RMS) error of 1.6% of the full-scale sensor range (FSR) of 4.5 Nm and validated with both multitone sinusoid signals and new elastomers constructed from the same material. We then investigate the feasibility of using the sensor for sustained periods of time, with rehabilitation as a case study. An 8-9 h experiment was designed to mimic how the eSEA might be used in a physiotherapy clinic. The model accuracy displayed some variation in these trials; however, this was limited to a variation in RMS error of 3.3% of FSR. Additionally, any permanent changes appeared to be minor. The results hold promise for obtaining a reliable prediction of HRI torque in an exoskeleton, leading to improved interaction between man and machine in various haptic applications.