Magnetostriction in Automotive Position Measurement

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1 l MTS Systems Corporation Sensors Division 3001 Sheldon Drive Cary, NC Phone , Fax TECHNICAL PAPER Part Number: M1163 Revision A Magnetostriction in Automotive Position Measurement Jesse Russell Figure 1. Automotive magnetostrictive sensing elements. Magnetostriction is a property of ferromagnetic materials such as iron, nickel, and cobalt. When placed in a magnetic field, these materials change size and/or shape (Figure 2). This physical response of a ferromagnetic material is due to the pres-ence of magnetic moments, and can be understood by considering the material as a collection of tiny permanent magnets, or domains. Each domain consists of many atoms. When a material is not magnetized, the domains are randomly arranged. When the material is magnetized, the domains are oriented with their axes approximately parallel to one another. Interaction of an external magnetic field with the domains causes the magnetostrictive effect. Figure 2, Orienting the magnetic domains in a ferromagnetic material causes a change in shape, either elongation or swelling. When a material has positive magnetostriction, it enlarges when placed in a magnetic field; with negative magnetostric-tion, the material shrinks. When an axial magnetic field is applied to a magnetostrictive wire, and a current is passed through the wire, a twist- 1

2 ing occurs at the location of the axial magnetic field (see Figure 3). The twisting is caused by interaction of the axial magnetic field, usually from a permanent magnet, with the magnetic field along the magnetostrictive wire, which is present due to an interrogation current in the wire. Figure 3, The combination of current induced magnetic field with the permanent magnet field causes a twist in the sensor s element that travels in the material as a strain pulse. Since the current is applied as 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 in the waveguide material, approximately at 3000 m/s. The operation of a magnetostrictive posi-tion sensor is shown in Figure 4. The axial magnetic field is formed by a position marker magnet. The position marker magnet is attached to whatever is being measured, perhaps a piston in a strut. The waveguide wire is enclosed within a protective cover and is attached to the sta-tionary part of the machine, perhaps a strut cylinder body. Figure 4, In a Magnetostrictive sensor, the Magnetostrictive effect is used to launch a strain pulse at the position of a market magnet. The interval of time between launch of the strain pulse and its arrival at a pickup is used to determine position. 2

3 The location of the marker mag-net is determined by first applying a current pulse to the waveguide and starting a timer counter. The current pulse causes a sonic wave to be generated at the location of the marker magnet which then travels along the waveguide until it is detected by the pickup, which generates a voltage pulse. The voltage pulse also stops the counter timer. The elapsed time indicated by the timer then represents the distance be-tween the position magnet and the pickup. The higher counter frequency, the finer the resolution. The interval between interrogation and pulse detection is conditioned into the desired output. Many outputs are available, most particularly 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 linear position measurement was limited however 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 according to the configura-tion specified for the particular application. That necessitates manual or semi-automatic 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. Complete integration is possible No additional wearing parts Well-proven, long term stability measurement principle High accuracy High temperature stability Easy matching of stroke length The Automotive market is increasing requirements for measuring linear displacements and positions in the fields of suspension, steering, transmission shifting and other chassis control applications. So far, these measurements are covered mainly with rotary transducers and reversing mechanisms or gear units. 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 also increase space requirements and mechanical complexity, as well as cost. 3

4 AUTOMOTIVE APPLICATIONS The new Mercedes CL is in production now with an Active Body Control system (ABC). Each strut has an integrated MTS Temposonics magnetostrictive sensor (figure 5). The operator can set the suspension for a sporty ride or a softer setting for a higher comfort, and control ride height. But beyond this the main advantage is the safety advantage that the system provides: the ABC system stabilizes the body of the car within milliseconds. The roll angle, for example, is reduced by a 68% when this system is used. Body movements are controlled by hydraulic servocylinders using a high-pressure hydraulic system driven by microcomputers that calculate the amount of pressure and the duration applied to each spring depending on the information received by the magnetostrictive sensors. Other sensors are also used to monitor the acceleration. Sensor Input Circuit Time Pulse Converter to Desired Output Interrogation Pulse Timer Waveguide Interrogation Pluse Driver Sensor DATA FORMATS Start/Stop Digital: PWM CANbus SSI Analog: 0 to 5 Vdc Ratiometric The cylinders can move at frequencies up to 5 Hz to react to vibrations. The system also includes an automatic self-leveling system based on the load of the car. Also the driver can change the height of the car at low speed (25 or 50 mm difference). At high speed the car automatically sinks back and exceeding 140 Km/h the body lowers to an additional 10 mm to reduce the air resistance. Though 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 semi-active systems, or ride height. Sport utility vehicles (SUV s), vans, pick-ups and off road vehicles may achieve high benefit using a self-leveling system or terrain adaptive control. 4

5 Figure 5, Mercedes Strut showing Temposonics automotive applications sensor in the center of the assembly. Other applications include electrically assisted power steering systems and robotic manual transmissions (automated manual transmissions). Power steering systems are migrating toward electric assist rather than the traditional hydraulic. Magnetostrictive sensors can be applied 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 into a complete circle to form rotary sensor. If a rotary instead of a linear motor is used, this version of the sensor could be considered (figure 6). Figure 6, The types of sensors that can be configured with Magnetostriction is broad. One version, on the right in the photo, can be used to monitor rotary steering position. In transmissions, there is trend toward automating manual transmissions to shift using actuators. This saves weight over traditional automatic transmissions and costs less to manufacture. Magnetostrictive sensors can measure simultaneously the linear and rotary motions necessary with one sensor as the 5

6 transmission shifts through the traditional H pattern. This is done with 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 (figure 7). About the Author Jesse Russell is New Markets Development Manager for Automotive and High Volume Products Unit of MTS System Corporation, Sensors Division in Cary, NC (USA). His address is: jesse.russell@mts.com. For further information contact: MTS Systems Corporation Sensors Division Tel: , Fax: Part Number: M1163 Revision A