Provided is an actuator, comprising a motor/generator element and a clutch/brake element. The motor/generator element comprises a stator made of a high magnetic permeable material, an outer coil being wound around the stator; a rotor made of a high magnetic permeable material and with a plurality of permanent magnets fixed thereon; and a cavity configured to receive a fluid. The clutch/brake element is arranged in the cavity. An inner coil is wound around the clutch/brake element. The fluid is capable of producing a shear stress in response to an electromagnetic field induced from the inner coil.