Magnetostrictive Linear Position Sensors A noncontact absolute reading linear position sensor characterized by low nonlinearity, long measurement ranges, and fast speed of response operates from the momentary interaction between two magnetic fields, one from a position magnet outside the sensor housing and the other surrounding a waveguide inside. A strain pulse is produced whose travel time is detected and processed into an output signal. Dave Nyce, MTS Systems Corp. L inear position sensors are widely used as the feedback element for motion control in commercial and industrial products and systems. They can be incremental or absolute reading, contact or noncontact, and range through various levels of price and performance. Magnetostrictive linear position sensors are noncontact and absolute reading. Noncontact means that the moving part of the sensor, attached to the member whose position is being measured, does not contact the stationary part of the sensor. The coupling between the moving and stationary sensor parts is achieved by means of a magnetic field. Therefore, any number of position changes can be made without causing wear on the sensor parts. This is in contrast to a contact sensor such as a potentiometer, where the wiper slides along the surface of a resistive element. This rubbing action is a source of noise, hysteresis (see Glossary), and limited lifetime. When wear significantly reduces the SNR, or produces dead spots in the resistive element, the sensor must be replaced. This can happen after a few months when the monitored parts have a constant dithering motion over the same area of the resistive element. Some high-quality potentiometers, for example, have a life rating of 100 million cycles. In industrial motion control applications, it is common to have a slight dithering of the measured part at 60 Hz. At this rate, 100 million cycles will be reached in 20 days, producing a dead spot if the mean position of the sensor has not changed. Normally, however, there would be several frequently used positions along the sensor's measuring range, so forming dead spots would take a few months. Because magnetostrictive sensors are absolute reading, the position is accurately known at power-on, without the need for setting a zero position. An incremental sensor such as a linear optical encoder indicates position changes from a set reference. On power-up, or after a corruption of the count in memory by noise or other means, the system must drive to a reference location so that the sensor and the count can be re-zeroed. Figure 1. A magnetizing force, H, causes a dimensional change due to the alignment of magnetic domains. Magnetostrictive linear position sensors are manufactured in lengths from as short as 10 mm full stroke to more than 20 m long. Nonlinearity (without correction in software) is as low as ±0.02% in sensors produced by MTS Systems Corp. Another popular absolute reading, noncontact, linear position sensor is the linear variable differential transformer (LVDT). Standard LVDTs are manufactured with a full stroke measurement range as short as 1 mm and nonlinearity as low as ±0.1%, but have nonlinearity of ±0.2% to ±1% at lengths >25 mm, and are difficult and expensive to produce with measurement ranges >100 mm. 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 (see Figure 1). The physical response of a ferromagnetic material is due to the presence 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. This effect can be optimized by controlling the ordering of the domains through alloy selection, thermal annealing, cold working, and magnetic field strength [1]. Figure 2. The Wiedemann effect describes the twisting due to an axial magnetic field applied to a ferromagnetic wire or tube that is carrying an electric current. The ferromagnetic materials used in magnetostrictive position sensors are transition metals such as iron, nickel, and cobalt. In these metals, the 3d electron shell is not completely filled, which allows the formation of a magnetic moment. (i.e., the shells closer to the nucleus than the 3d shell are complete, and they do not contribute to the magnetic moment). As electron spins are rotated by a magnetic field, coupling between the electron spin and electron orbit causes electron energies to change. The crystal then strains so that electrons at the surface can relax to states of lower energy [2]. When a material has positive magnetostriction, it enlarges when placed in a magnetic field; with negative magnetostriction, the material shrinks. The amount of magnetostriction in base elements and simple alloys is small, on the order of 10–6 m/m. Since applying a magnetic field causes stress that changes the physical properties of a magnetostrictive material, it is interesting to note that the reverse is also true: applying stress to a magnetostrictive material changes its magnetic properties (e.g., magnetic permeability). This is called the Villari effect. Normal magnetostriction and the Villari effect are both used in producing a magnetostrictive position sensor. An important characteristic of a wire made of a magnetostrictive material is the Wiedemann effect (see Figure 2). When an axial magnetic field is applied to a magnetostrictive wire, and a current is passed through the wire, a twisting occurs at the location of the axial magnetic field. 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 the current in the wire. The current is applied as a short-duration pulse, ~1 or 2 ms; the minimum current density is along the center of the wire and the maximum at the wire surface. This is due to the skin effect. The magnetic field intensity is also greatest at the wire surface. This aids in developing the waveguide twist. Since the current is applied as a pulse, the mechanical twisting travels in the wire as an ultrasonic wave. The magnetostrictive wire is therefore called the waveguide. The wave travels at the speed of sound in the waveguide material, ~3000 m/s. Figure 3. The interaction of a current pulse with the position magnet generates a strain pulse that travels down the waveguide and is detected by the pickup element. Table 1 Linear Position Sensor Performance Features s Color LVDT Magnetostrictive Optical Encoder Potentiometer Auto-SE Contact s s s X s Noncontact X X X s X Absolute X X s X X Incremental s s X s s Nonlinearity ±0.1%–1% ±0.02% ±0.01% ±0.05% ±0.1% F.S. ranges 1–100 mm 10 mm to 20 m 50 mm to 2 m 50 mm to 1 m 80–250 mm Cost Medium Medium Medium Low Low The operation of a magnetostrictive position sensor is shown in Figure 3. The axial magnetic field is provided by a position magnet Photo 1. This magnetostrictive position sensor uses a floating magnet that is attached by the user to a suitable location on the moving part to be measured. . The position magnet is attached to the machine tool, hydraulic cylinder, or whatever is being measured. The waveguide wire is enclosed within a protective cover and is attached to the stationary part of the machine, hydraulic cylinder, etc. The location of the position magnet is determined by first applying a current pulse to the waveguide. At the same time, a timer is started. The current pulse causes a sonic wave to be generated at the location of the position magnet (Wiedemann effect). The sonic wave travels along the waveguide until it is detected by the pickup. This stops the timer. The elapsed time indicated by the timer then represents the distance between the position magnet and the pickup. The sonic wave also travels in the direction away from the pickup. In order to avoid an interfering signal from waves traveling in this direction, their energy is absorbed by a damping device (called the damp). The pickup makes use of the Villari effect. A small piece of magnetostrictive material, called the tape, is welded to the waveguide near one end of the waveguide. This tape passes through a coil and is magnetized by a small permanent magnet called the bias magnet. When a sonic wave propagates down the waveguide and then down the tape, the stress induced by the wave causes a wave of changed permeability (Villari effect) in the tape. This in turn causes a change in the tape magnetic flux density, Photo 2. A captive magnet enclosed in a sliding bearing can be more easily adapted to existing equipment. and thus a voltage output pulse is produced from the coil (Faraday effect). The voltage pulse is detected by the electronic circuitry and conditioned into the desired output. MTS magnetostrictive sensors are available with many outputs, including DC voltage, current, pulse width modulation, start-stop digital pulses, CANbus, Profibus, serial synchronous interface, HART, and others. Magnetostrictive position sensors have many different form factors. One important physical feature relates to the way in which the position magnet is applied. Photo 1 shows a floating magnet, which is attached by the user to a suitable location on the part to be measured. Photo 2 shows a captive magnet, which is enclosed in a sliding bearing to more easily adapt it to existing equipment. Table 1 compares performance features of several types of linear position sensors, including standard Temposonics magnetostrictive sensors and the Temposonics Auto-SE. There has recently been an important new development in magnetostrictive sensors: low-cost sensors for high-volume applications. Magnetostrictive linear position sensors have typically required a substantial amount of manual assembly by skilled workers to produce the sensing element, even though various jigs, fixtures, semiautomated tools, and automated testers are used in their manufacture. In response to requests for this technology for use in high-volume automotive applications, MTS has developed a fully automated manufacturing facility. One operator makes sure all parts feeders have a supply of parts. The fully automated production line assembles and tests all parts and final tests the completed sensors, then packages them. Assembly techniques include robotic devices, resistance welders, laser welders, vision systems, and computer analysis of assembly and performance data. The final product, the automated assembly sensor element, is called the Temposonics Auto-SE. Historical Note The magnetostrictive linear position sensor was invented by Jacob (Jack) Tellerman in 1975. He was developing delay lines for use in computer memory devices when it occurred to him to use similar technology to produce a position sensor. In the magnetostrictive memory device, digital ones and zeroes were represented by pulses of ultrasonic waves impressed onto one end of the waveguide (a magnetostrictive wire). Additional data could be pulsed into the waveguide until just before the first pulses would begin to arrive at the other end. Then that particular memory element was considered full. When the first pulses arrived at the other end, the data signals would be amplified and resent as ultrasonic waves into the first end of the waveguide. This could continue indefinitely, keeping the data stored on the waveguide until the information was needed again by the computer. Then, new data could be "written" to the waveguide. Jack had the idea of generating the ultrasonic wave at locations along the waveguide by using a permanent magnet. Then the time taken until an ultrasonic pulse reached one end of the waveguide would indicate the position of the magnet. He co-founded a company called Tempo-sonics, which was further developed after being acquired in 1987 by MTS Systems Corp. Sensors assembled on the MTS automated production line are not at present so accurate as those that are hand assembled for industrial use, but they are very low cost. Automotive and other high-volume applications use these sensors because of their unlimited lifetime, their stability over time and temperature, and especially because of their low cost. With automated assembly, a whole new field of application requirements can now be met with magnetostrictive position sensors. References 1. David S. Nyce. Apr. 1994. "Magnetostriction-Based Linear Position Sensors," Sensors, Vol. 11, No. 4. 2. Robert Philippe. 1988. Electrical and Magnetic Properties of Materials, Artech House, Inc. 3. O. Esbach. 1975. Handbook of Engineering Fundamentals, New York, NY, John Wiley & Sons:957. 4. R. Lerner and G. Trigg. 1990. Encyclopedia of Physics, New York, NY, VCH Publishers, Inc.:529. 5. P. Neelakanta 1995. Handbook of Electromagnetic Materials, New York, NY, CRC Press:333. For Further Reading Boll, R. 1977. Soft Magnetic Materials, London, Heyden & Son Ltd. Burke, H. 1986. Handbook of Magnetic Phenomena, New York, NY, Van Nostrand Reinhold Co. Carstens, J.R. 1992. Electrical Sensors and Transducers, Regents/Prentice Hall:125. Craik, D. 1995. Magnetism Principles and Applications, New York, NY, John Wiley & Sons. Cullity, B.D. 1972. Introduction to Magnetic Materials, Reading, MA, Addison-Wesley Publishing Co. Lorrain, P. and D. Corson. 1962. Electromagnetic Fields and Waves, San Francisco, W.H. Freeman and Co. Norton, H. 1989. Handbook of Transducers, New Jersey:106-112. Dave Nyce is Director of Technology, Sensors Group, MTS Systems Corp., Sensors Div., 3001 Sheldon Dr., Cary, NC 27513; 919-677-0100, fax 919-677-0200, info@mtssensors .com Glossary of Magnetic Properties Faraday effect. The generation of a voltage by a coil of wire when the coil is subjected to a changing magnetic field. Hysteresis. A phenomenon in which the state of a system does not reversibly follow changes in an external parameter [4]. In a linear position sensor, it is the difference in output readings obtained at a given point when approaching that point from upscale and downscale readings. Magnetic field intensity (H). The force that drives the generation of magnetic flux in a material. It is also called magnetizing force and can be produced by the application of an electric current. H is measured in amperes/meter. Magnetic flux density (B). The amount of magnetic flux that results from the applied magnetizing force. B is measured in newtons/ampere-meter. Magnetic hysteresis. When a ferromagnetic material is placed in an alternating magnetic field, the flux density (B) lags behind the magnetizing force (H) that causes it. The area under the hysteresis loop is the hysteresis loss per cycle, and is high for permanent magnets and low for high-permeability, low-loss magnetic materials [5]. Magnetic permeability (µ). This indicates the ability of a material to support magnetic lines of flux. The µ of a material is the product of the relative permeability of that material and the permeability of free space. The relative permeability of most nonferrous materials is near unity. In free space, magnetic flux density is related to magnetic field intensity by the formula: B = µ0 H where: µ0 = permeability of free space, having the value 4p3 3 10–7 henry/m. In other materials, the magnetic flux density at a point is related to the magnetic intensity at the same point by: B = µH where: µ = µ0µr and µr is the relative permeability [3] Magnetic saturation. The upper limit of the ability of a ferromagnetic material to carry flux. Magnetization curve. Shows the amount of magnetizing force required to saturate a ferromagnetic material. It is normally shown as a graph with B as the ordinate and H as the abscissa, and is known as the B-H curve. Magnetostriction. The change of size and/or shape of a ferromagnetic material due to the application of a magnetic field. Skin effect. The nonuniform distribution of high-frequency current in a conductor, where current is concentrated in the outer layer rather than in the center of the conductor. Villari effect. The change in magnetic properties of a ferromagnetic material in response to the presence of stress in the ferromagnetic material. Wiedemann effect. The mechanical torsion that occurs when an electric current is passed along or through a long, thin ferromagnetic material while it is subjected to an axial magnetic field.

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