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Better motion control: New linear motors

April 1, 2000
The number of choices for converting electrical energy into linear motion recently increased. Welcome the linear motor.

This new design in linear motors, as well as claiming the above features, also almost eliminates mechanical wear. Developed by Avcon Inc., all of the force generated by these moving magnet linear motors is applied directly to perform work. Little force is lost to side vectors in the plane of motion as can happen with rotary to linear conversion.

Motion choices

There are three basic types of electromagnetic linear actuators (motors):

A voice coil actuator uses a coil in a magnetic field to generate a force when a current passes through the coil. These actuators generally have low armature mass and can therefore generate high accelerations. Voice coils are used in such applications as audio speakers and disk drive positioners. Because the coil must be located in the air gap of the magnetic circuit, the air gap is typically larger than 0.40 in. The efficiency for this type of actuator is low, less than 0.4 in.-lb of work per watt of electrical input. However, the response is fast, with a range of 10 to 20,000 Hz.

A moving magnet motor uses a stator with many coils sequentially energized to move a permanent magnet with repulsion and attraction forces. In some applications, the magnets remain fixed and the stator is moved. The actuator coil does not have to be located in the air gap. Therefore, the gap is smaller than with voice coil actuators, typically 0.010 in. or less. This small air gap increases motor efficiency and force capacity.

An induction motor uses a stator similar to a moving magnet motor, but the armature is made of magnetic material — no permanent magnets. This type of motor relies on the induction of currents that are generated in the armature, which in turn create magnetic fields that are attracted and repulsed by the stator fields as the coils are sequentially energized.

This type of linear motor is the simplest, and generally the least expensive to manufacture. Because of their low manufacturing costs, these motors are well suited to applications that require long strokes or long travel, such as motors used for trains and monorails.

However, of the three basic linear motor types, the induction motor is the least efficient. In some applications, encoder feedback is necessary for precise positioning.

Advantages and disadvantages of moving magnet linear motors

The design of the moving magnet linear motor helps its speed approach that of voice coil actuators. This linear motor uses an unconventional magnetic circuit in a cylindrical armature, similar to the shape of hydraulic and pneumatic actuators, and a three-phase wound stator assembly. This configuration is spatially efficient and accommodates an increase in the amount of magnet material and coil windings in the actuator. Mininum magnetic circuit lengths and short air gaps aid the efficiency and force capability.

The acceleration of a motor is a function of its force capability and the mass of the moving armature. Voice coil actuators have low mass because they do not use a metal core to wind the moving coil. However, they require large air gaps because the coil is located in the gap. The larger air gap needs a larger magnet and magnetic circuit to achieve high field strengths in the gap. Voice coil actuators can achieve force to mass ratios of better than 5,000 ft/sec2.

The acceleration of the moving magnet linear motors is its force divided by the mass (force/mass ratio), and is large due to the cylindrical design. The working gap length, or circumference of the armature, is long versus the amount of material in the armature (mass). These linear motors accelerations range from 1,000 to 1,500 ft/sec2. A 600 lb force output version demonstrated speeds to 90 in./sec. The force/mass ratio of this linear motor is 1,360 ft/sec2. The maximum velocity of moving magnet linear motors is limited by the drive voltage available and the travel distance.

Moving magnet motors can be synchronized with the electrical sequencing of the field coils. This ability provides controlled velocity, acceleration, and positioning without the use of additional sensors.

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However, compared with induction motors, the cost of the magnets in the moving portion of these moving magnet motors can be high, especially for long travel actuators. In order to achieve desired performance, the linear motors use small air gaps and tight fits to obtain the needed volumetric efficiency. Tight tolerances and complex shapes, though, increase the fabrication costs of the armature.

Potential applications for moving magnet linear actuators include those that require:
• High acceleration — high ratio of force to moving mass.
• Synchronized speed or acceleration.
• High detent force to maintain position with power off.
• High reliability.

And where power efficiency and force to weight ratios are important. The moving magnet linear motor achieves high detent forces through the permanent magnet design and very small air gaps mentioned earlier. Detent forces are the result of changes in the reluctance of the magnetic circuit in the linear motor. Permanent magnets, even with the coils off, still produce flux. The armature seeks a position of least reluctance and attempts to remain in that position. The force needed to move the armature, with the coils off, is the detent force. Some applications rely on the detent force to hold the armature in position when power is off or in stand by.

In moving magnet linear motor designs, detent force cannot be completely eliminated. It can be varied over a narrow range, about 5 to 15% of the maximum force capability.

In applications that require high detent, the magnetic circuit produces a change in reluctance with position. If engineers wish to move the armature when power is off, low detent motors are available. Reducing the detent involves the same techniques used in dc motors to reduce cogging; i.e. use of smaller pitch for the coils and magnets, a larger pole width relative to pitch, a larger air gap, or skewing the stator poles relative to the armature. Generally, however, lowering the detent force increases the size and cost of a linear motor. These techniques tend to reduce the force capability of the motor, requiring the use of a larger motor for a given force.

In operation

One equipment manufacturer has already begun the conversion to these linear actuators. For the capital expense of the motor, this manufacturer’s equipment now operates faster — at a speed that is more than double what it was with previous motors. Fewer moving parts of the linear motor have also lowered maintenance costs.

Another potential application involves using these linear motors in a virtual-reality amusement park ride. The rides consist of small rooms that are moved under computer control to mimic various virtual situations, such as flight simulation. Currently hydraulics are used in these applications. While hydraulic systems have good force to moving mass ratios, the viscosity of the hydraulic fluid dictates the maximum speed. To achieve high speeds, hydraulic systems need high pressures, big pumps, large valves, and large lines to the cylinders. The cylindershaped configuration of the linear actuator can develop 1,800 lb of force with a travel of 32 in.

The U.S. Air Force is interested in using moving magnet linear motors in “more electric” aircraft that will replace conventional hydraulic actuators for flight control surfaces. A goal of this Air Force program is to replace single point failure mode systems with high-reliability electrical systems.

The loss of a hydraulic system — from a pump failure, a broken actuator seal, or a hole in a hydraulic line — can disable an aircraft. The hydraulic actuators that move aircraft flight control surfaces require high-pressure — 2,000 psi — hydraulic lines. A pinhole leak from such a line can release a stream of liquid capable of cutting through metal, electrical wiring, or people. Hydraulic systems in aircraft also require a backup system. This redundancy can add expense and weight to the aircraft.

With electrical actuators in a flight control system, the failure of any single electromagnetic actuator would affect only one control surface. Electric actuators do not have a single-point failure mode, which can improve aircraft survivability. Generally, electrical systems need less maintenance and are known for high reliability. However, redundancy will still be necessary in critical areas, to help elminate risk to the aircraft in the event of an actuator failure.

Avcon Inc., Agoura Hills, Calif. is a developer of motion control and servo electronic systems. For more information on the moving magnet linear motors discussed in this article, circle 415 on the reader service card.

If this article is helpful, please circle 416 on the reader service card.

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