Electromagnetic Linear Actuators For Active Vibration Control.
F. T. van Namen

Electromagnetic linear force transducers produce a shaft output force proportional to the input current and nearly independent of the position of the armature. Inertial force transducers use action and reaction forces in a sealed package to produce forces over a wide range of frequencies. Low inductance and flux modulation is achieved by linking the flux of multiple coils. Customizing for specific output and environmental requirements is facilitated by the use of computer simulation in combination with FEA.

Since the invention and publication of the virtues of active noise and vibration control, significant progress has been made in signal processing and technical implementation into industrial applications. All major obstacles seem to be scaled except for one: the force producing element.

Availability of transducers which produce a force via a linear output shaft are known in the form of solenoids and shakers. The well known shaker tables are mainly used in experimental settings, they are bulky and expensive, not suitable for large scale commercial application. The less expensive, widely available solenoid has major disadvantages for the use in servo controlled systems. The force output of a solenoid is not constant over the stroke and only one directional; the armature must be returned with a spring. The large amount of iron in the changing magnetic field creates a high inductance and electrical time constant, making them virtually useless at high frequencies. Alternative actuator materials like piezo and magnetostrictive alloys produce very high linear forces, but at extremely small amplitudes.

In essence there are very few electro magnetic transducers available off the shelf that can be used in an active controlled system. The requirements for this kind of transducer are slowly being developed. A specification should provide the following:

Two types of transducers were developed and are being applied in widely varying industries ranging from automotive to household appliances.

Transducers with a linear shaft output apply a direct force between a mounting surface and a driven element often parallel with passive elements.

Inertial transducers use the reaction forces on an internal mass and spring to produce a force on the mounting surface. These units are completely sealed with no external moving parts. They can be mounted on any surface and produce mass reaction forces onto that structure, they can be placed in series or in parallel with passive elements, they can be placed tangential on rotating equipment in order to create torque reaction.

A wide variety of these transducers can now be produced using the standard production techniques of the loudspeaker industry. Computer simulation and FEA assist in the customization for special applications.

The inertial transducer relies on the action and reaction of internally produced electromagnetic forces on "voice coils". The reaction force and the spring force, resulting from the displacement of the internal mass, work together to produce the resulting axial force on the housing of the unit following well known formulas, creating a magnification of the magnetic force when the forcing frequency is above the resonance frequency. A typical force vs. frequency response is shown in fig. 1. The unit that produced this output is described in exhibit A.

In order for this type of transducer to produce a force which closely follows the input current it is necessary to avoid modulation of the magnetic field as a result of the field produced by the coils. This is obtained by using a plurality of coils, which produce closely linked opposing fluxes, not only avoiding flux modulation, but also providing a strong reduction in inductance. An example of flux modulation is shown in fig. 2 for a unit using a single coil. Fig. 3 shows the result of a unit with two coils.

A smooth, distortion free motion of the mass is obtained by the use of flexures. Molded flexures are used in units operating at lower temperatures, while units operating at 300¡C are outfitted with metal supports.

The closed housing of the inertial transducer can withstand and transfer very high external forces and therefore is ideally suited to be used in series with passive elements like engine mounts.

The Axial Force Transducer uses the same coil and armature structure as the inertial transducer, but here the armature is mounted on a shaft, which protrudes through the endbells of the unit. Here the amplification of force is not available and the reaction force on the case becomes equal to the force of the output shaft.

The operating frequency range is very wide, only limited on the high end by the resonance frequency of the armature on the shaft or the stiffness of the mounting and on the low end by the amplitude of the armature motion.

A typical output chart is shown in fig. 4 for the transducer specified in exhibit B.

Optimizing a force transducer requires the ability to change all design parameters quickly, with instantaneous answers on specific performance characteristics. The use of finite element analysis tools are not very practical and much too slow for fast manipulation of parameters, but they are excellent for the verification of a final design.

To zoom in on a design a special simulation program is developed, which calculate all the magnetic, electric and mechanical characteristics, including power dissipation, temperature sensitivity and material cost. Magnet material can be changed on the spot.

It is possible to optimize a design for maximum force per dollar, or for a specific mechanical impedance. The design can be customized for a specific performance within a certain temperature range.

The inertial force transducer is especially suitable in the area of active vibration control, where vibration isolation can not be accoplished with passive elements only. These areas are in automotive, aircraft, marine, indutrial and applicances where rotary engines and motors crate unwanted vibrations. These actuators can be used in series or in parallel with any type of suspension or they can be used as an addition to an existing structure to create local vibration damping, like in aircraft structures or panels.

Limitation in the use of inertial transducers are a result of the natural frequency of the internal mass which limits the low end of the operating frequency range. Since the amplitude of the mass is proportional to the output force at a certain frequency it follows that the maximum force is restricted by the dimensions and weight of the unit.
This is not the case for the axial force transducer. Here the limitaions are caused by the use of linear bearings to support any radial load. These bearings will cause low amplitude high frequency distortion, as a result there will be a low end force limit. However for small amplitudes the axial unit can also be outfitted with flexures.