Design of a hopping mechanism using a voice coil actuator: Linear elastic actuator in parallel (LEAP)

Among legged robots, hopping and running robots are useful because they can traverse terrain at high speeds and are a benchmark platform for locomotion actuators; if an actuator can power a hopping robot, it can power a walking robot. We aim to create a hopping mechanism for a small-scale, one-legged, untethered hopping robot. A parallel-elastic actuator is an efficient way to do this, and enables the actuator to directly inject energy into the spring, but requires a high-speed, low-inertia actuator. Voice coil actuators are electrically-powered direct-drive translational motors that have very low moving inertia, low friction, can produce force at high speeds, and have a linear force output. These qualities make them ideal candidate motors for a linear elastic actuator in parallel (“LEAP”). Here, we derive an electromechanical model of the LEAP mechanism, develop a simple bang-bang hopping controller, and simulate hopping with a range of spring parameters to find an optimal spring stiffness that maximizes hopping height. We detail our implemented design, and characterize its performance through a series of experiments. We test our robot with different spring stiffnesses, and demonstrate hopping at a maximum steady-state of 3.5 cm ground-clearance (approx. 20% leg length). Our results suggest that the LEAP mechanism may serve the weight-bearing functions of a robot leg.