Voice coil actuators (VCAs) are direct drive, limited motion devices that utilize a permanent magnet field and coil winding (conductor) to produce a force that is proportional to the current applied to the coil. These non-commutated electromagnet devices are used in linear and rotary motion applications that require linear force or torque output, and high acceleration, or high frequency oscillation. Originally used in radio loud speakers, voice coil actuators are gaining popularity in applications where proportional or tight servo control is a necessity.
The electromechanical conversion mechanism of a voice coil actuator is governed by the Lorentz Force Principle. This law of physics states that if a current-carrying conductor is placed in a magnetic field, a force, “F”, will act upon it. The magnitude of this force is determined by the magnetic flux destiny, “B”, the current, “I”, and the orientation of the field and the current vectors. Furthermore, if a total of “N” conductors (in series) of length “L” are placed in the magnetic field, the force acting upon those conductors is shown by Equation (1):
Where k equals a constant. Figure 1 is a simplified illustration of this law of physics.
( Figure 1 )
In Figure 1, the direction of the force generated is a function of the direction of current and magnetic field vectors. Specifically, it is the cross-product of the two vectors. If current flow is reversed, the direction of the force on the conductor will also reverse. If the magnetic field and the conductor length are constant, as they are in a voice coil actuator, then the generated force is directly proportional to the input current.
Furthermore, a conductor moving through a magnetic field will have a voltage induced across the conductor. The magnitude of the voltage, E, is dependent on the magnetic flux density, B, the length of the conductor, L, and the speed of the conductor traverses the field. The voltage potential induced in the conductor (i.e., the back EMF) is shown by Equation (2):
E=kBLvN, ( Equation 2 )
Where k equals a constant and N equals the total number of conductors of length L.
Equations (1) and (2) can be restated as follows: a device that contains a permanent magnet field and coil winding moving in the field will produce a force proportional to current [carried in the coil] and a voltage proportional to velocity [of the coil].
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 Figure 2. The field is produced by permanent magnets embedded on the inside diameter of a ferromagnetic cylinder, arranged so that the magnets that are “facing” the coil are all of the same polarity. An inner core of ferromagnetic material set along the axial centerline of the coil, 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.
( Figure 2 )
Based upon the required operation stroke of the actuator, the axial lengths of the coil and the magnet assemblies can be chosen such that the force vs. stroke curve is extremely flat. The degradation of force at the two travel extremes with respect to the mid-stroke force can often be kept below 5%. This is possible, because the working air gap of the permanent magnet circuit remains constant over the rated stroke.
If one were to “flatten” the linear voice coil actuator from a round tube to a flat tube, then bend the two ends to form a planar arc, such as a sector of an annulus, one would have a rotary voice coil actuator. This device can also be referred to as a Limited Angle Torquer or a Sector Torquer. Its principle of operation and force generation is analogous to that of the linear counterpart; however, rating are in units of torque, instead of force, because force is generated along the circumference of an arc (i.e., Torque = Force X Radius). Figure 3 depicts a typical rotary voice coil actuator.
( Figure 3 )
The voice coil actuator is a single phase device. Application of a voltage across the two coil leads will generate a current in the coil, causing the coil to move axially along the air gap. The direction of movement is determined by the direction of a current flow in the wire.
The single phase linear voice coil actuator allows direct, cog-free linear motion which is free from the backlash, irregularity, and energy loss that result from converting rotary to linear motion. Rotary versions of voice coils provide such smooth motion that they are the preferred device in applications requiring quick response, limited angle actuation, such as gimbal assembles.