Control of electro-magnets

A support actuator comprises an electro-magnet having an input current , provided through a current controller , and which is arranged to generate a magnetic field having a variable intensity proportional to a variable current supplied by the controller . The intensity of the field is controlled to ensure that the magnet is separated from a support armature arranged to carry a load by an operational gap A load cell is connected between the armature and a mounting position for the armature . The cell produces a control signal representing variations in force generated by the magnetic field which acts on the load, and the signal is fed backward, along a feedback control path to the controller so as to control the variable current such that it is made substantially equal to a gain demand force D.