Material Properties and Related Considerations for Magnetomechanical Sensing Devices
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IVAN J. GARSHELIS Magnetoelastic Devices Inc., 17 Downing Three, Pittsfield, MA 01201 ABSTRACT Suitability of specific materials for use in magnetomechanical sensors more often depends on combinations of properties than on an outstanding value of a single property. These combinations include ratios and products of properties from different physical families. While magnetic, magnetoelastic, electrical and strength properties often play key roles in determining the performance of the sensor, non-physical characteristics such as manufacturability, available forms, environmental compatibility, toxicity and costs strongly influence the device design and may well dominate the final choice of material. The development of materials specifically for use in this class of sensors is encouraged by their already broad and growing range of applicability.
INTRODUCTION The mechanical forces exerted by and on the various members constituting a machine are often fundamental to the performance of its function. Immediate and accurate information concerning the most functionally important forces can be used to rationalize the control of the machine leading to its optimal performance and to also supervise other important aspects of operation such as safety and efficiency. These forces of interest are not limited to simple pushes and pulls but often appears as twisting or bending moments. They may also be distributed rather than localized, appearing e.g., as a pressure. Moreover, a force of interest need not be constant but may vary with time over a wide range of rates and over almost any conceivable interval. Sometimes the important information is singular in nature, concerned only with identifying whether some action involving force is taking place, as for example that the driver of a motor vehicle is applying some force on the brake pedal. More often it is important to maintain a continuous flow of information concerning the magnitude of the force of interest (for example to improve the operation of an anti-skid braking system by considering the amount of force being applied to the brake pedal). In either case, processing, distribution and utilization of the information is made more convenient and immediate if it appears as an electrical signal. Devices that provide these electrical signals in response to mechanical force inputs fall into the general category of devices called sensors. Sensors that perform a measuring function by providing electrical signals having some feature that is proportional to a particular force quantity of interest are known as transducers. A sensor that provides an electrical signal that merely toggles between two values at a preset magnitude of the force of interest is known simply as a switch. While a variety of physical phenomena are readily utilizable in the construction of force sensors [1], attention here is to magnetomechanicalsensors, i.e., devices based on the "inverse magnetostriction" manifestation of the magnetoelastic interaction [2] found in ferromagnetic materials. Magnetomechanical sensors a
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