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Posted 06 April 2005 - 04:23 AM
Few prototypes have been made thus far, but those that were have been given exceptionally high acclaim by those who have experienced them. My goal is to work with TC Sounds to continue this saga in appropriate fashion, bringing to market a truly awesome driver, capable of massive excursion, huge output, high power handling, all with incredibly low distortion and a beastliness that will amaze any onlooker (and of course every listener!).
So you might be asking yourself, "What is this Linear Motor technology all about? I've been hearing the term thrown around forever now but could never get my hands on it.. Is this a dream?"
.. A dream come true maybe!
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Variable Density Coil - White Paper
By Thilo Stompler
Property of TC Sounds, Inc.
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Principle
Electromechanical systems are made up of coupled electric and magnetic circuits. By a magnetic circuit, we mean a path for magnetic flux, just as an electric circuit provides path for the flow of electric current. In electromechanical devices, current-carrying conductors interact with magnetic fields (themselves arising from electric currents in conductors or from permanent magnets), resulting in electromechanical energy conversion.
Consider a conductor of length "L" placed between the poles of a permanent magnet producing a magnetic flux density B in the "air gap" between the poles N - S (ref. Image 1). Let the conductor carry a current "I" and be at right angles to the magnetic flux lines. The conductor experiences a force F, in the direction of which is perpendicular to both the conductor and to the magnetic filed lines, and the magnitude of which is given by F=(B)(I)(L). For N conductors (or turns), F becomes (N)(B)(I)(L).
A more general statement which holds for an arbitrary orientation of the conductor with respect to the flux lines is the cross product of the two vectors B and L, i.e. F=(I)(L) x B (Ampere’s Law).
Note: If L is expressed in Meters, B in Tesla and I in Amperes, the Force is given in Newtons.
Standard voice coil implementation
Voice coil actuators are direct drive, limited motion devices that use a coil winding within a permanent magnet field to produce a force proportional to the current applied to the coil. These non-commutated electromagnetic devices are used in linear applications requiring linear force and high acceleration.
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 Image 2.
Based upon the required operating stroke of the actuator, the axial lengths of the coil and the magnet assemblies can be chosen so that the force versus stroke curve stays very flat. Since the coil density is uniform, this is somewhat possible if the magnetic field in the air gap of the permanent magnet circuit remains constant over the rated stroke.
In some cases the axial length of the coils exceeds that of the magnet by the amount of coil travel. In other cases the magnet is longer than the coil by the travel length. Compared to the short-coil configuration, the long-coil configuration provides a superior force-to-power ratio and dissipates heat better. The short-coil, however, has a lower electrical time constant, smaller mass, and produces less armature reaction. Neither arrangement provides a perfectly linear force-versus-travel characteristic. Most of the force acting on a bobbin is generated in the "air gap" of the magnetic circuit (as can be seen in Images 3, 4, & 5), where the magnetic field magnitude B is highest and fairly constant. However a long coil will also be subjected to the leaking fields above and below the gap region. Since such leakages are not symmetrical (ref. Image 5), they are an inherent cause for non-linearity when using a uniform density coil.
In some implementations, the magnetic steel return can be notched and/or the permanent magnet shaped in order to provide a more uniformed field. This operation is not always effective, especially when dealing with a long stroke device. Drivers utilizing this method are much more sensitive to a slightly offset coil, which will result in an undesirable manipulation of the BL curve. Furthermore, it is not very economical.
As the turns of the coil are located more remotely from the gap region, their contribution to the total force becomes lesser. The magnetic field seen by these turns decays quite rapidly, but is still significant (ref. Image 5).
When the coil travels along its stroke (ref. Image 4), a uniform density bobbin would be "missing" the contribution from the field located below the gap region since there are no turns interacting with such field. The same statement can be made for the case when the coil is at the opposite end of its stroke.
The uniform density bobbin is "missing" the contribution from the field located below the gap region since there are no turns interacting with such field. The result is a force-versus-displacement curve shaped as a bell as depicted in Image 6.
Variable coil density
An alternative to shaping the magnetic flux density is to shape the coil density in order to keep the product (N)(B)(L) constant. The variable density coil offers three bulges (ref. Images 7 & 8) at the center and at the extremities of the bobbin, in order to compensate for the lack of active turns when the coil travels near the end of its stroke.
Consequently the product (N)(B)(I)(L) remains nearly constant for the total stroke of the coil.
In theory, it is possible to shape the coil with many "steps" in order to get a perfectly flat response. Realistically, this might not be practical due to manufacturing issues as well as an economy. Based on the geometry (ratio of gap size to coil length mostly), five to seven segments are sufficient in order to provide linearity better than just a few percent (Image 9).
The shape of the coil is strongly linked to the magnetic structure hosting it. Such a shape is derived from a combination of Finite Element Analysis (FEA to compute the magnetic field) and self-consistent equations (solved within MathCAD) to represent the electro-mechanical behavior of the system.
Conclusion
Near perfect force linearity can be easily obtained by shaping the turn distribution of the coil of a voice coil actuator. The main drawback of such a concept is that the peak force obtained is less than that produced by a uniform winding. It is a trade-off between linearity and peak force. In a practical loudspeaker, this simply means that more magnet is required to obtain the same BL-product at rest, or small excursions. In an actual subwoofer system, the result is typically a threefold reduction in distortion, resulting in a significant easily audible subjective improvement in sound quality.
// End of document. Reproduced and edited by Michael Grynick with permission. April 6 2005.
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Posted 06 April 2005 - 04:30 AM
