<< Various and Sundry

Voice Coil Actuators



Introduction

A voice coil is a kind of linear actuator, it moves a mass along a line. To be precise, it pushes and pulls a mass wherever it is along a line, the mass then moves according to this force acting on it. Thus a voice coil doesn’t primarily move things, although that is usually a consequence of the force it applies.

The name “voice coil” is due to one of its first applications: vibrating the paper cone of a loudspeaker. The name stuck and voice coils designed to move larger masses in other applications are called “voice coils.”

It is sometimes said that voice coils are intended for fast motion but they work just as well for slow motion. A voice coil exerts a force, the consequence of that force depends on the circumstances.


Principle of Operation

The voice coil works because of the force between a static magnetic field and an electric current perpendicular to the field, what is called the Lorentz force. Below is a schematic diagram of a current carrying wire and a magnet. Arrows indicate the direction of the electric current (to the right) and of the magnetic field (up):


The Lorentz force is proportional to the product of the magnetic field and the current, in a direction perpendicular to both of them. In the diagram this direction would be directly toward you. If the current were reversed it would be directly away from you.

If the magnet is fixed you can think of the magnet as pushing on the wire, and if the wire were free to move it would accelerate toward you all along its length.

If the magnetic field strength is constant, the magnitude of the force it exerts on the wire is proportional to the magnitude of the current through it.


From Principle to Voice Coil

Now imagine the magnet above as a rectangular slab extending into the page, and the single wire replaced by multiple wires repeated into the page. Bend both magnet and wires up and around until the left and right edges meet at the top forming one cylinder inside another. However, when the left and right ends of the wire segments meet at the top, offset them so they form a single coil with only two wire ends. Voilà, the voice coil !


The inner cylinder pushes out of or into the page depending on the direction of the current. The magnitude of the force is proportional to the magnitude of the current.

To help focus the magnetic field where it belongs the permanent magnet is surrounded and held by “keeper” material – soft iron – capped at one end and penetrating the middle of the coil. It helps complete the magnetic circuit, in the course of which the field lines out of the North face become field lines into the South face. The coil is wrapped about a non-conducting cylinder capped at the other end. Below is a cross-section of the side view, the magnet pushes the coil (which in turn pushes the coil holder) to the right and left:



Advantages of Voice Coils

Voice coils have two advantages over other kinds of actuators:

  • Simplicity of construction.
  • No gears. They are completely silent and there is no backlash when direction is reversed (they are “hysteresis free”).
  • They are force actuators, they naturally provide a force, specified by the current through the coil – or what amounts to the same thing (because of the fixed electrical resistance of the coil) the voltage across the coil. Acceleration and subsequent velocity and position are consequences of the specified force.

    ... That was three advantages you moron.

    All right three advantages. Typically voice coils are used in conjunction with position and/or velocity feedback. Force feedback from a strain gauge might also be used.


    Complications

    Voice coils have some disadvantages, or at any rate different characteristics that must be taken into account, compared to other types of actuators. In those other types (stepper, DC) a motor is considerably geared down to move the working end of the actuator. Thus the motor can move the actuator end but because of gear friction the actuator end cannot move the motor. The actuator can maintain a force against the load while drawing no current. With a voice coil, a position that is intended to be fixed must be actively maintained against any force that would change it. If a force acts on the actuator to move it from a desired position, it will draw current to oppose that force.

    This must especially be taken into account with a vertically mounted voice coil. Current is required merely to counteract the static weight of the load. If that is a problem, the static weight can more or less be counterbalanced by either springs or counterweights, like the window sash in some window mechanisms and the counterweights in elevators (lifts). Then the actuator need only (more or less) draw current when accelerating the load.

    Also the design must account for a sudden power outage, that is, it must fail safely when the actuator force summarily drops to zero.

    Another possible complication is that when the coil moves rapidly the magnet induces a current in it opposing the current due to the driving voltage – a “back EMF.” However this is negligible when the operation is slow. The inductance of the coil, which introduces a time lag between applied voltage and current, is negligible when the movement changes slowly.


    Voice Coil Parameters

    Several numbers describe a particular voice coil. Here are the most important ones.

    Mechanical parameters:

  • Size of housing (diameter and length, or height and width and length).
  • Length of stroke, measured either starting from full retraction or mid-stroke. (The second is plus or minus half the first.)
  • Protruding length of shaft at either full retraction or mid-stroke.

    Electrical parameters:
  • The force exerted per amp of current.
  • Coil resistance.
  • Coil inductance.
  • Back EMF (volts per unit velocity)

    Electrical extremes and ratings:
  • Allowed peak (transient) current.
  • Allowed steady (“stall”) current.
  • Force at peak current.


    Types of Voice Coils

    Voice coils come in two shapes: cylindrical (circular as viewed along the axis) just described, and rectangular (as viewed along the axis). Another variation is to bend the voice coil sideways into an arc, like a macaroni noodle (in that case the actuator is no longer linear).

    Bearings can be added between the coil and magnet/keeper so that they maintain alignment and don’t touch one another. Such voice coils are called “housed.” Voice coils are also made without bearings and called “unhoused.”

    The alignment for unhoused voice coils is provided by the particular application. The magnet part and electric part are rigidly attached to two other parts of the assembly (one fixed, the other movable) which are themselves aligned and consequently keep the voice coil aligned. This is useful for not over-determining the assembly (having to align two alignments).


    Further Reading

    Links to publications available online (all are PDF files except the last which is HTML):

    Design and Control of a Voice Coil Actuated Robot Arm ...
    by John McBean, Mechanical Engineering Masters thesis, 2004.

    Basics of Voice Coil Actuators
    by Bill Black, et al in PCIM Power Electronics Systems, July 1993.

    Voice Coil Actuators: Applications & Product Selection Guide
    BEI Kimco Magnetics, company information.

    Voice-coil actuators
    by George Gogue & Joseph Stupak, G2 Consulting.