High-precision force control of short-stroke reluctance actuators with an air gap observer

A short-stroke reluctance actuator linearization scheme that simultaneously achieves high linearity, high bandwidth, and low stiffness is demonstrated. These properties are required in high speed and high precision motion systems. They are achieved by combining various control schemes, namely flux feedforward and analog sensing coil feedback for high bandwidth, Hall probe feedback to stabilize the drift, and an air gap observer together with gain scheduling to reduce the remaining stiffness. Using the presented scheme, the attractive force of the actuator can be controlled with high precision without the need for a position or force sensor. Experiments indicate that a linearization error of 50mN for second-order 200 N force reference profiles is obtained. This translates into force predictability of 99.98%. Furthermore, absolute actuator stiffness below 500 N/m at force levels of 100 N is achieved, which is comparable to more linear Lorentz actuators.