Decentralised control of an electro-pneumatic adjustable impedance actuator

We present a decentralised controller design for the novel ElectroPneumatic Adjustable Impedance Actuator (EPAIA). EPAIA is designed as a rotational, back-drivable actuator, applying a rotary pneumatic element, used as a pneumatic spring in series to a brushless direct-current motor in the gear train. The actuator control consists of two coupled loops. Following a squaring down procedure, an impedance controller is presented for the torque loop using an ℋ -optimal controller design employing additional constraints on passivity. The stiffness between motor and load is controlled by a PID-type controller including a decoupling pre-filter. Robust stability is verified by structured singular value analysis and the decentralised control strategy and the actuator are validated in simulations and on a test-bench.