The need to integrate robots in society grows, as several socioeconomic issues put pressure on our current level of productivity and prosperity. This requires robots to safely interact with unpredictable and fragile stakeholders, such as humans. Compliant actuation can facilitate such safe physical interaction. The Series Elastic Actuator (SEA) and the Twisted and Coiled PolymerMuscle (TCPM) constitute two compliant actuators with favorable properties. However, both need sensors to be able to performclosed-loop control. This complicates design and integration of SEAs, and negates two major benefits of TCPMs. This problem can be solved by determining the state of the actuator via structures ormaterials that are already part of the actuator, i.e. self-sensing....@en

The twisted and coiled polymer muscle (TCPM) has two major benefits: low weight and low cost. Therefore, this new type of actuator is increasingly used in robotic applications where these benefits are relevant. Closed-loop control of these muscles, however, requires additional sensors that add weight and cost, negating the muscles' intrinsic benefits. Self-sensing enables feedback without added sensors. In this article, we investigate the feasibility of using self-sensing in closed-loop control of a Joule-heated muscle. We use a hardware module that is capable of driving the muscle, and simultaneously providing sensor measurements based on inductance. A mathematical model relates the measurements to the deflection. In combination with a simple force model, we can estimate both deflection and force, and control either of them. For a muscle that operates within deflections of [10, 30] mm and forces of [0.32, 0.51] N, our self-sensing method exhibited a 95% confidence interval of 2.14 mm around a mean estimation error of −0.27 mm and 29.0 mN around a mean estimation error of 7.5 mN, for the estimation of, respectively, deflection and force. We conclude that self-sensing in closed-loop control of Joule-heated TCPMs is feasible and may facilitate further deployment of such actuators in applications where low cost and weight are critical.@en

Coil springs are a common element in compliant actuators. For closed-loop control, the force of the coil spring has to be measured. Typically, deflection sensors indirectly measure this force. Implicitly, this assumes that the coil spring is a pure stiffness, without any mass. In reality, internal oscillations can occur due to impacts or other excitations of the spring’s resonance frequencies. This letter investigates the reliability of different force-sensing methods for coil springs that are oscillating internally. In addition to standard sensing via strain gauges or deflection sensors, also a new type of sensing is included, namely force estimation via the spring’s own electrical inductance. First, a lumped-mass model is used in simulations of three realistic conditions a coil spring might be subjected to in robotic applications. Second, a hardware experiment is conducted for one condition. Key effects predicted by the model are also found in the experiment, confirming the model’s validity. Results show that for all sensors, the increase in measuring uncertainty due to internal oscillations is of the same order of magnitude as typical sensors’ measuring uncertainty.@en

The recently introduced twisted and coiled polymer muscle is an inexpensive and lightweight compliant actuator. Incorporation of themuscle in applications that rely on feedback creates the need for deflection and force sensing. In this paper, we explore a sensing principle that does not require any bulky or expensive additional hardware: Self-sensing via electrical impedance. To this end, we characterize the relation between electrical impedance on the one hand, and deflection, force, and temperature on the other hand for the Joule-heated version of this muscle. Investigation of the theoretical relations provides potential fit functions that are verified experimentally. Using these fit functions results in an average estimation error of 0.8%, 7.6%, and 0.5%, respectively, for estimating deflection, force, and temperature. This indicates the suitability of this self-sensing principle in the Joule-heated twisted and coiled polymer muscle.

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