Influence of Internal Oscillations on Force Sensing in Coil Springs

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.