Linear Voice Coil Actuators VM

Description
Voice coil motors offer excellent control characteristics where linear actuation is required over short distances with electronic control systems. Comparison of the force characteristics vs displacement, and vs current, for voice coil devices and solenoid actuators shows the difference between these devices. The flat force characteristic exhibited by the voice coil motor lends this to applications requiring precise control of force or position such as control valves, or lens and mirror positioning systems, whereas the sharp increase in developed force as the pole faces approach one another in a solenoid device makes these difficult to control. The voice coil motor can develop force in either direction by reversing polarity of the excitation. In a solenoid, a spring is typically required to produce force in the return direction. This spring force subtracts from the magnetic force developed, reducing force and speed in the energized direction. Combined with low inductance this makes possible cycle times <1ms in certain cases, typically an order of magnitude faster than solenoid devices.

Structure
Voice Coil Motors of VM series incorporate shafts and bearings to ensure accurate guidance of the coil assembly within the magnet assembly, and to facilitate easy installation in your application. For wear resistance and good surface finish the shaft material is a hardened stainless steel which is magnetic and will have a slight influence on the force characteristic. Shafts of non-magnetic stainless steel can be supplied to special order but are much more susceptible to mechanical damage.
The magnet assembly of VM series is designed for good volumetric efficiency at the expense of some loss of linearity. High Energy Density magnets drive the material of the voice coil pot (housing) close to magnetic saturation to develop the strongest possible magnetic field. Custom designs can demonstrate better linearity at the expense of increased size / weight & cost. Coils of standard VM series are normally designed to use the full depth of the pot assembly. This results in maximum mechanical work output capability but may also result in a force characteristic which is not ideally suited to a given application. The portion of the coil which lies outside the airgap field dissipates power (as heat) but develops no useful force. The linear range of a voice coil (the range within which developed force is >90% of peak force) will normally be roughly equal to the difference between the coil length, and the length of the pole. For maximum force, the coil length and pole length should be approximately , but the linear range with this configuration will be small. For best linearity, one of the coil and polepiece should be longer than the other by the linear range required. It is usually more cost effective to make the coil longer than the magnet assembly – making the coil shorter than the polepiece results in lower moving mass and faster dynamic response, but this may require a more massive and expensive magnet assembly to produce a required force characteristic.

Mechanical Integrity
The design of VM series devices ensures good concentricity and mechanical integrity of the complete device. Accurate fixtures are used in assembly to control assembly dimensions, and coil assemblies are individually measured to ensure concentricity and clearance with the magnet assembly. All devices are designed to ensure that finite clearances are maintained throughout an operating range from 0ºC to 130ºC.

Electrical Termination
Connection to the moving coil of a voice coil motor must be implemented with care to ensure reliable operation. Flexible cable with many fine strands and Silicone Rubber insulation can provide reliable termination. Care should be taken that the leads are mechanically secured to the moving assembly preferably at some distance from the soldered joints (solder fuses the strands together and leads to large stresses being applied to the termination pins, or to fatigue adjacent to the fused portion of the wire). The leads should be carefully routed to minimize stress. A more consistent means of termination is to use a flexible circuit. This is offered as a standard option in the VM5050 device.

Behavior
The behavior of a voice coil motor can be explained by reference to the classical physics problem of a current carrying wire supported in a magnetic field. Where the magnetic field strength is B, the current carried by the wire is I, and the length of the portion of wire cut by the field is l, a force F is developed according to the equation F = B x I x l
The force developed is perpendicular to both the magnetic field, and to the current flowing in the wire.

Hysteresis
Hysteresis is analogous to backlash in mechanical systems which can give rise to position or force errors. The graph shows how hysteresis is manifested in a positioning system - as current is varied to change the direction of a move it needs to change back significantly to achieve any movement. Hysteresis in solenoid devices can be as great as 10% or more of the developed force, whereas in voice coil motors it is typically <<1% of the developed force. Low hysteresis enables precise and repeatable position control to be realized.