The stator assembly (left) shows the windings that are sequentially activated to move the armature (right). The actuator, including its own weight, can accelerate at a rate of 8,600 in./sec² when driving a load of 23 lb. The stator dimensions are 9.0 in. OD X 4.9 in. ID X 5 in. long. The armature is just under 4.9 in. in diameter X 8 in. in length. Armatures can be made to almost any length to accommodate the needed travel distance.

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 design of moving magnet linear actuators is similar to the design of brushless motors or step motors. The actuator’s controller is programmable for various positions, acceleration rates, and velocity profiles, and is capable of over 100 ips. Position accuracy depends on the kind of sensor used, including Hall sensors and LVDTs. With a suitable optical encoder, accuracy can be better than 0.0005 in. The system can also operate open loop, by counting steps.

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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