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01 February 2003

Magnetostriction positions the ride

Chassis, transmission shifting, and steering controls leaving rotary transducers.

By Jesse Russell

When placed in a magnetic field, some materials change size and/or shape. This phenomenon-magnetostriction-is a property of ferromagnetic materials such as iron, nickel, and cobalt.

This physical response of a ferromagnetic material is due to the pres ence of magnetic moments. Understand this by considering that the material is a collection of tiny permanent magnets, or domains. Each domain consists of many atoms.

In a nonmagnetized material, the domains arrange themselves randomly. Upon magnetizing, the material's domains orient with their axes approximately paral lel to one another. Interaction of an external magnetic field with the domains causes this magnetostrictive effect.

Orienting magnetic domains cause shape change.

A current-induced magnetic field plus a permanent magnet causes a twist and pulse.

When a material has positive magnetostriction, it enlarges in a magnetic field; with negative magnetostric tion, the material shrinks. When an axial magnetic field applies to a magnetostrictive wire and a current passes through the wire, a twisting occurs at the location of the axial magnetic field.

The twisting results from the interaction between the axial magnetic field, usually from a permanent magnet, and the magnetic field along the magnetostrictive wire, which is present due to an interrogation current in the wire.

Because the current is a pulse, the mechanical twisting travels in the wire as an ultrasonic wave, making the wire a waveguide. The wave travels at the speed of sound (approximately 3,000 meters per second) in the waveguide material.

A magnetostrictive posi tion sensor uses a position marker magnet to propagate the axial magnetic field. The position marker magnet is attached to whatever is being measured-perhaps the piston in a strut. The waveguide wire is inside a protective cover and attaches to the sta tionary part of the machine-perhaps the strut cylinder body.

One determines the location of the marker mag net by first applying a current pulse to the waveguide and starting a timer counter. The current pulse causes a sonic wave to generate at the location of the marker magnet. The wave then travels along the waveguide until it hits the pickup, which generates a voltage pulse. The voltage pulse stops the counter timer. The elapsed time translates to the distance be tween the position magnet and the pickup.

The higher the counter's frequency, the finer the resolution of the measurement.

The interval between interrogation and pulse detection passes through conditioning into any of several outputs. Available outputs are DC voltage and pulse width modulation in automotive. But magnetostrictive sensors also have a history of start/stop digital pulses, CANbus, Profibus, serial syn chronous interface, HART, and others.

Magnetostrictive effect

Previously, magnetostrictive linear position measurement was limited in the number of applications it could address because each sensor, though modular, required handwork to assemble. Industrial versions of the sensors come in a myriad of types and sizes to suit the wide range of industrial applications.

Consequently, the industrial sensors are typically built to order and application specific. That necessitates manual or semiautomatic assembly, limiting potential for cost reduction.

Now, a novel automated manufacturing process reduces unit cost into a whole new realm compatible with the cost demands of high-volume products. This technology advancement, which offers high reliability and high performance, is suddenly very attractive for high-volume use in automotive and other extremely cost-sensitive applications.

The implementation costs are very competitive with other technologies normally associated with high-volume applications but with some very significant advantages.

The automotive market is increasingly measuring linear displacements and positions in the suspension, steering, transmission shifting, and other chassis control applications. Up until now, rotary transducers and reversing mechanisms or gear units covered these measurements.

This technique also reduces the measurement accuracy due to conversion of the linear into a rotary motion, which extends the tolerance chain and stack-up errors considerably. Rotary solutions increase space requirements, mechanical complexity, and cost.

AUTOMOTIVE APPLICATIONS

The new Mercedes CL is in production now with an active body control (ABC) system. Each strut has an integrated magnetostrictive sensor. The operator can set the suspension for a sporty ride or choose a softer setting for higher comfort and to control ride height.

The main advantage is the safety the system provides: The ABC system stabilizes the body of the car within milliseconds. The roll angle, for example, reduces by 68% using this system.

Hydraulic servocylinders control body movements using a high-pressure hy draulic system driven by microcomputers that calculate the amount of pressure and the duration applied to each spring, depending on the information the magnetostrictive sensors receive. Other sensors monitor acceleration.

The cylinders move at frequencies up to 5 hertz to react to vibrations. The system also includes an automatic self-leveling system based on the load of the car. In addition, the driver can change the height of the car at low speed by 25-50 millimeters (mm). At high speed, the car automatically sinks back, and at more than 140 kilometers per hour, the body lowers an additional 10 mm to reduce the air resistance.

Although this system is fairly sophisticated, these sensors are cost-effective enough to consider their use in less complex and cheaper systems to control the damp in active or semiactive systems or ride height. Sport utility vehicles, vans, pickups, and off-road vehicles may benefit greatly using a self-leveling system or terrain adaptive control.

Other applications include electrically assisted power steering systems and robotic manual transmissions (automated manual transmissions).

Power steering systems are migrating toward electric assist from the traditional hydraulic. Magnetostrictive sensors can apply in two ways to measure position and velocity of the motion.

One houses a linear version within the linear electric motor along its axis. Another houses the linear sensor along the motion of the connecting linkages. The sensor can be curved if the motion forms an arc or a complete circle to form rotary sensor.

In transmissions, there is a trend toward automating manual transmissions to shift using actuators. This saves weight over traditional automatic transmissions and costs less to manufacture. Magnetostrictive sensors can simultaneously measure the linear and rotary motions necessary with one sensor as the transmission shifts through the traditional H pattern.

This happens by using two magnets instead of one to mark positions. One magnet is in the shape of a helix, allowing the sensor to measure shaft rotation as well as shaft linear motion with the traditional ring magnet. IT

Behind the byline

Jesse Russell has worked with industrial sensors at MTS Systems for 28 years. He is the development manager for commercial high-volume sensor applications at the company's division in Cary, N.C. Write him at jesse.russell@mts.com.