Moving-magnet linear motors

Feb. 1, 2008
Linear motors are becoming increasingly practical for general motion. Many are used for long strokes of a few feet or more. One new design, however, where

Linear motors are becoming increasingly practical for general motion. Many are used for long strokes of a few feet or more. One new design, however, where the magnets move relative to a stationary coil set, is finding a niche because of its fast response.

How it works

The magnetic portion of a moving magnet motor is comprised of three basic elements: magnet, coils, and core. The magnet moves back and forth, while the coils and core remain stationary. The magnet is charged (magnetized) with two polarities. The coil-core stator assembly carries and modulates flux across the gap where the magnet resides. The two sections of core are connected to complete a magnetic path.

The coils are wound and connected so that magnetic flux generated by the current in each coil adds constructively in the motor gap. This flux interacts with the flux generated by the magnet, causing the magnet to react with a force in one of two directions, depending on coil flux polarity.

In other words, the core/coil combination produces south and north poles as a function of current. When current is applied, the appropriate poles of the magnet are either attracted or repelled to produce force. The stronger the current, the stronger the resulting force; in fact, the structure of this design produces a force directly proportional to the current in the coils.

Maximum force is only limited by the thermal performance of the overall system design. To meet heavier requirements, some motors include Neodymium Iron Boron (NdFeB) magnets. These magnets have an energy product of 36 mega Gauss Oersteads (MGOes) — ten times more energetic than typical ferrite magnets. NdFeB magnets also make the motor's moving portion very light, for acceleration capabilities exceeding 150 gs.

What's unique about it

To optimize performance, the gap between the magnet and core must be very small compared to the magnet thickness. This is accomplished with a mechanical suspension.

The motor's suspension is a low-mass flexure that's stiff in the lateral direction because the web is tall like an I beam — a geometry that produces a ratio of lateral to axial stiffness greater than 1,000 to 1. Also, because the flexure is thin, the suspension is very compliant in the direction of motion. The flexure itself is made of a special stainless steel alloy that resists fatigue. By design, stresses are kept below the alloy's fatigue limit, for infinite flexure life.

The suspension serves other important functions as well. It allows the magnet to move along the desired axial path with minimal use of motor force, and it keeps the magnet from crashing against the face of the core. (Any contact would produce friction and nonlinear behavior.) Finally, the suspension has low moving mass so as not to diminish acceleration capabilities.

Other benefits

First, the stationary drive coils are directly mounted to a structure that provides a more effective heat sink; this facilitates the removal of the resistive losses in the coils, and allows for much higher forces. Second, because the coils are stationary, their mass does not affect maximum velocity, permitting the use of a larger (more efficient) coil.

Third, with stationary coils, there are no flying leads; this improves motor longevity. Finally, as with all linear motors, the moving magnet does not contact any of the motor's stationary elements, so there is none of the frictional wear associated with mechanical power transmission.

Testing, 1, 2, 3

One area where moving-magnet motors are already making an impact is test instruments. The motors are bridging the gap between traditional servo-hydraulic and DMTA technologies. Dynamic mechanical analysis tests are one use; they're conducted on materials from elastomers, plastics, and tissue to gels and emulsions. Properties are measured as a function of changing frequency, temperature, strain, amplitude, mean level, or preconditioning.

Fatigue and fracture studies represent another area where moving-magnet linear motors excel. Mechanical tests like these take advantage of the high frequency response, which accelerates crack growth in materials, for example, as well as the application of millions of fatigue cycles. The flexure makes sure that the damage is directed to the specimens, not the test system's seals and bearings.

Another application is nonlinear material modeling. Most finite element material models require the measurement of force along with the deflection behavior of materials under various conditions. Moving-magnet motor instruments measure cyclic and time-dependent properties of elastomeric materials under realistic conditions. This includes strain levels greater than 100% and frequencies from one cycle per day to 400 cycles per second to simulate the critical conditions for components being modeled. Samples can be tested in simple tension, compression, shear, pure shear (planar tension), and three or four-point bend configurations under single or multi-axial loading — to measure all critical material responses.

Flexure-based moving magnet actuators in testing equipment are designed for high acceleration (more than 1,500 m/sec2), high bandwidth (static to more than 400 Hz), and near-infinite life, for testing beyond 15 billion cycles. These units need only standard electrical power, and are much more efficient than servo-hydraulic test systems.

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Suspension systems

An electromagnetic suspension system based on the moving-magnet motor is proving useful for automobiles.

Every automotive suspension has two goals: passenger comfort and vehicle control. Isolating the vehicle's passengers from road disturbances provides comfort. Control comes with keeping the car body from rolling and pitching excessively, and maintaining good contact between the tire and road. Unfortunately, these objectives conflict. In luxury sedans, suspensions are usually designed for comfort; in sports cars, comfort is sacrificed for more control. No conventional variable spring or hydraulic addresses all of these issues. But electromagnetic solutions offer promise.

One such system consists of four linear motors (one on each wheel) and independent power amplifiers and control algorithms. When power is applied to the motor coils, the motor retracts and extends, creating motion between the wheel and car body.

Continually increasing processor speed is making the design ever more viable, too. In fact, prototypes of the motor-based suspension have been installed in standard production vehicles that have been tested on a variety of roads, tracks, and durability courses under day-to-day and racing-type driving. Aggressive cornering maneuvers like a lane change are free of body roll; hard braking and acceleration are free of body pitch. Over bumpy roads, overall body motion and jarring vibrations are dramatically reduced. On typical U.S. roads, moving-magnet linear motor suspension systems are fast enough to counter the effects of bumps and potholes at speed, maintaining a comfortable ride.

As far as how it works, on each wheel, a power amplifier delivers drive current to the motor in response to sensor signals and subsequent inputs from the control algorithms. Because the power amplifiers are regenerative, they allow power to flow into as well as out of the motor (operating in generator mode). When the suspension encounters a pothole, for example, power is used to extend the motor and isolate the vehicle's occupants from the disturbance. On the far side of the pothole, the motor switches to generator mode and returns power (to the battery) through the amplifier. As a result, the suspension requires less than a third of the power of a typical vehicle's air conditioning system.

The suspension system is controlled by a set of mathematical algorithms that operate by observing sensor measurements taken from around the car and sending commands to the power amplifiers installed at each corner of the vehicle. Unlike conventional dampers, the system operates without pushing against the car body, to maintain passenger comfort.

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