Page 233 - Electric Drives and Electromechanical Systems
P. 233

Chapter 9   Related motors and actuators  229


                 magnetic flux destiny, B, the current, i, hence for a winding of N turns, the resultant force
                 is given by,
                                                     F ¼ BLiN                                (9.1)
                   In its simplest form, a linear voice coil actuator is a tubular coil of wire situated within
                 a radially oriented magnetic field, as shown in Fig. 9.1. The field is produced by
                 permanent magnets embedded on the inside of a ferromagnetic cylinder. The inner core
                 of ferromagnetic material is aligned along the axial centre-line of the coil and joined
                 at one end to the permanent magnet assembly, is used to complete the magnetic circuit.
                 The force generated axially upon the coil when current flows through the coil will
                 produce relative motion between the field assembly and the coil, provided the force is
                 large enough to overcome friction, inertia, and any other forces from loads attached to
                 the coil. For a specific operating displacement of the actuator, the axial lengths of the coil
                 and the magnet assemblies can be chosen such that the force vs displacement curve can
                 be optimised, resulting in the reduction of force at the mid-stroke force being limited to
                 less than 5% of the maximum force.
                   The sizing and selection of a voice coil actuator is no different from any other linear
                 application, the process discussed in Section 3.8.4 should be followed.



                 9.2 Limited-angle torque motors

                 Limited-angle torque motors are a range of special-purpose motors that are capable of
                 giving controllable motion in the range  90 degrees from their rest position. While
                 brushless motors, as discussed in Chapter 7, have many benefits, they have the penalty


























                         FIG. 9.1 Cross section of a voice coil; the dimensions of the air gap have been exaggerated.
   228   229   230   231   232   233   234   235   236   237   238