Dynamic modelling and control of a normal-force actuator

The authors describe the dynamic modelling and control of a simple form of electromagnetic actuator comprising a laminated core with a coil and a `mover´ section of the magnetic circuit separated by a small airgap. The modelling of the ideal case, in which a constant voltage is applied to the coil, is considered first. The important idea of control using freewheeling is then introduced. The model is developed to incorporate saturation, eddy currents and realistic circuit components and measured performance is compared with model results. The comparison demonstrates that this relatively simple model is capable of describing the behaviour of an actual device with good accuracy. The behaviour of the actuator when lifting a range of masses is then presented and it is shown to have a high force density and be capable of lifting loads up to 500 N. Finally, several control strategies are discussed which link the load to the length of freewheeling time with the aim of achieving an optimum balance between speed of action and efficient operation. One technique is recommended for further work