Voice coil actuators are direct-drive electric motors that provide limited linear or rotary motion. They are often used in applications requiring proportional and tight servo control, such as mirror and lens positioning in optical systems, high-accuracy machine tool positioning, precision valve control in medical and industrial devices, and linear drives for cryogenic coolers and other pumps.

These non-commutated actuators use a permanent-magnet field assembly with a coil winding to produce a force proportional to the current applied to the coil. Both linear and rotary versions meet the acceleration, frequency actuation, and linear force or torque-versus-stroke output requirements needed in many linear and rotary motion applications. Some of the linear versions, Figure 1, provide continuous force (stall) from 1.15 to 9.7 lb, with a peak force for 10 sec from 2.5 to 32.5 lb. Some rotary versions provide continuous torque (stall) from 4.05 to 480 ozin, with 10-sec peak torque from 11 to 1,536 oz-in.

A linear voice coil actuator consists of a two-terminal tubular coil of wire in a radially oriented dc magnetic field, Figure 2. Permanent magnets embedded on the inside diameter of a ferromagnetic cylinder produce a constant field. The side of the magnets that faces the ferromagnetic cylinder has the same polarity as the cylinder. The opposite side of the magnets facing the coil has the opposite polarity. Completing the magnetic circuit is an inner core of ferromagnetic material along the axial centerline of the coil that is joined at one end to the permanent magnet assembly.

The actuator is a single-phase device. It operates using the cross product of a magnetic field and a current flow through the coil winding. Applying voltage across the two coil leads generates a current in the winding. This current produces a proportional force on the coil, causing the coil to move axially along the air gap. The direction of the current flow in the wire determines the direction of movement. The speed of actuation depends on the magnitude of the current flowing through and the voltage applied to the coil.

From a straight line to a curve

If you “flatten” a linear voice coil actuator, Figure 2, from a round tube to a flat tube, then bend the two ends to form a planar arc, you will have a rotary voice coil actuator, Figure 3A and 3B. Rotary voice coil actuators generate force identical to their linear counterparts. However, the ratings are in units of torque, instead of force, because force is generated along the circumference of an arc.

Material composition

Both types of actuators are available in several materials, depending on required system performance, operating environment, manufacturing considerations, and cost. Typically, the coil is wound with copper or aluminum-magnet wire coated with a thin polymer film for electrical insulation.

The most common permanent magnet materials are hard-magnetic ferrites, Neodymium Iron Boron, and Samarium Cobalt. The steel flux return, or “backiron” steel, can be any high permeability ferromagnetic material, and need not be laminated. The fasteners and bonding agents must be able to survive the required operating environment.

Control

In precise servo control applications, voice coil actuators require feedback for closed-loop control. Many position, velocity, and force transducers can function as feedback devices. Most common are optical encoders, contact and magneto-resistive potentiometers, LVDT’s, and load cells.

The actuator’s power supply must have sufficient current to meet an application’s force requirements. It must have a high enough voltage rating to overcome the back EMF at maximum coil velocity and the resistive and inductive voltage drops across the winding.

The coil resistance (Ohms) and force sensitivity (Newtons/Ampere) of these actuators can be impedance matched to most dc power supplies. Common amplifiers include H-bridge and PWM types.

Mechanical systems

Voice coil actuators are usually sold as a set consisting of the magnet and coil assemblies. The minimum air gap clearance between these assemblies is 0.010- 0.015 inches; sometimes more if required by the application. The user must provide a guidance system for full range of motion and to prevent the coil winding from rubbing against or crashing into the magnet assembly. In most cases, the load is connected to the coil assembly because the coil has a lower mass (often by an order of magnitude) than the field assembly. In cases where the load is sensitive to applied heat, it can be connected to the magnet assembly.

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