Voice-coil actuators in adaptive optics

Voice-coil actuators are a special form of electric motor, capable of moving an inertial load at extremely high accelerations and relocating it to an accuracy of millionths of an inch over a limited range of travel. Motion may be in a straight line (linear actuators) or in an arc (rotary, or swing-arm actuators)\cite{emdesignandactuators}. They are called "voice coil actuators" because of their common use in loudspeakers.


When electric current flows in a conducting, wire which is in a magnetic field, a Lorentz force is produced on the conductor which is at right angles to both the direction of current and magnetic field. The voice coil actuators are nothing but the technical manifestation of the Lorentz force principle, according to which "power current carrying windings of electrical conductivity is directly proportional to the strength of the magnetic field and current."


The electromechanical conversion mechanism of a voice coil actuator is governed by the Lorentz Force: if a current-carrying conductor is placed in a magnetic field, a force of magnitude
$F = kBLIN$
will act upon it, where:

• k - Constant
• F - Force
• B - Magnetic flux density
• I - Current
• L - Length of a conductor
• N - Number of conductors

In its simplest form, a linear voice coil actuator is a tubular coil of wire situated within a radially oriented magnetic field. The field is produced by permanent magnets embedded on the inside diameter of a ferromagnetic cylinder, arranged so that the magnets ``facing'' the coil are all of the same polarity.



Note that when the current direction is reversed, then (assuming that the direction of B is unchanged), the direction of the force is reversed. The reversible force of a voice coil actuator is an advantage over the force of moving iron actuators of the preceding sections, which is directed to attract the iron armature toward the iron stator regardless of the direction of the current. Another advantage over solenoid actuators is that voice coil force is much more independent of armature position and is proportional to current. A minor disadvantage is that the voice coil current must be supplied via a flexible lead, and thus the proper stranded wire must be selected for reliable long-term operation\cite{brauer2006magnetic}.

The electromagnetic (EM) driver consists of a coil moving in a magnetic field and driving a piston against a cylindrical spring. A permanent magnet with pole pieces is used to transmit the force to the backplate\cite{dmforallseasons}. Voice-coil actuators are electromagnetic devices which produce accurately controllable forces over a limited stroke with a single coil or phase. They are also often called linear actuators, a name also used for other types of motors\cite{voicecoilsbasics}. Because the moving parts of the speaker must be of low mass (to accurately reproduce high-frequency sounds), voice coils are usually made as light weight as possible, making them delicate. Passing too much power through the coil can cause it to overheat (caused by ohmic heating).

The single phase linear voice coil actuator allows direct, cog-free linear motion that is free from the backlash, irregularity, and energy loss that results from converting rotary to linear motion\cite{voicecoilsactbasics}.


References:

\begin{thebibliography}{1}

\bibitem{emdesignandactuators}
Jr. George P.~Gogue, Joseph J.~Stupak.
\newblock {\em Theory \& Practice of Electromagnetic Design of DC Motors \&
Actuators (CHAPTER 11, ACTUATORS)}.
\newblock G2 Consulting, Beaverton, OR 97007.

\bibitem{brauer2006magnetic}
J.R. Brauer.
\newblock {Magnetic actuators and sensors}.
\newblock 2006.

\bibitem{dmforallseasons}
R.~H. Freeman and J.~E. Pearson.
\newblock Deformable mirrors for all seasons and reasons.
\newblock {\em Applied Optics}, 21(4):580--588, 1982.

\bibitem{voicecoilsbasics}
George~P. Gogue and Jr. Joseph J.~Stupak.
\newblock Voice-coil actuators.
\newblock Technical report, G2 Consulting, Beaverton, OR 97005, 2007.

\bibitem{voicecoilsactbasics}
B.~Black, M.~Lopez, and A.~Morcos.
\newblock {Basics of Voice Coil Actuators}.
\newblock {\em PCIM-VENTURA CA-}, 19:44--44, 1993.

\end{thebibliography}