US Patent 10114300 Lithographic apparatus and device manufacturing method

An electromagnetic actuator includes a coil assembly including a coil; a magnet assembly including a first and a second magnet unit, each magnet unit including a magnetic yoke and a plurality of permanent magnets mounted to the magnetic yoke, the first and second magnet unit forming a magnetic circuit for receiving the coil assembly and, upon energizing the coil, generating a force in a first direction; and a holder for holding the magnet units, wherein a weight ratio of the magnet assembly over the coil assembly is smaller than the weight ratio of the magnet assembly over the coil assembly when the ratio of force over electrical power is maximized.