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Magnetic field sensors are used to measure magnetic flux and/or the strength and direction of a magnetic field. They are used mainly in scientific measurement, navigation, and industrial applications. Magnetic field sensors use several types of sensing technologies. Magnetoresistive devices measure electrical resistance as a function of the applied or ambient magnetic field. Flux gate or coil sensors measure differences in the magnetic field at the ends of a vertical rod and then plot this information on a grid. Hall Effect sensors convert the energy stored in a magnetic field to an electrical signal by developing a voltage between the two edges of a current-carrying conductor whose faces are perpendicular to a magnetic field. Magnetoinductive sensors consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth's magnetic field.  Proton precession devices use liquids such as kerosene and methanol that have high densities of hydrogen atoms. Overhauser or nuclear precession devices combine an electron-rich liquid with hydrogen and subject the mixture to a radio frequency (RF) signal. Optically-pumped sensors polarize a gaseous alkali with a specific wavelength of light. Superconducting quantum interference devices (SQUID) are very sensitive to low magnetic fields.

 

Selecting magnetic field sensors requires an analysis of performance specifications. Flux density, the total measurement range in gauss (G), often corresponds to the linear output region of the sensing technology. Resolution, another important specification, is the smallest measurable increment. Accuracy is a percentage of full-scale. Bandwidth is the frequency range over which magnetic field sensors meet their accuracy specifications. The number of axes indicates the number of possible, simultaneous measurements for magnetic field sensors. Single-axis devices are common, but two-axis and three-axis devices are also available. Directional magnetic field sensors measure the direction of the magnetic field and may provide bipolar outputs. Magnitude sensors measure the magnitude of the magnetic field. Maximum shock, maximum vibration and operating temperature are additional specifications.
 

Speed sensors, magnetic rely on a magnet as the sensing element or sensed target to capture rotational or linear speed. They are typically used as gear tooth speed sensors or incorporated into stroboscopes or tachometers. The technology types for magnetic speed sensors include magnetoresistive, inductive, variable reluctance, and Hall effect. In a magnetoresistive sensor the resistance of the sensing element is a function of the direction and magnitude (proximity) of an applied magnetic field. In an inductive sensor an oscillator circuit generates an RF EMF that radiates from a ferrite core and coil assembly. The field is directed at the sensor face. When a metal target enters the field, eddy currents are induced into the surfaces of the target. This causes a reduction in the amplitude of the oscillator circuit (change in inductance). Variable reluctance speed sensors are typically self-generating, meaning they require no external power. When a magnetic surface is passed in close proximity to the sensor, a small voltage is induced. In a Hall effect sensor a current is passed through a semiconductor material. When a magnetic field is applied perpendicularly to the surface of the semiconductor, a voltage is developed. This Hall voltage is proportional to the applied field intensity, driving the magnetic 

Important performance specifications to consider when searching for speed sensors, magnetic include rotary speed range, linear speed range, and accuracy.  Rotary speed and linear speed are two mutually exclusive ranges. In rotary speed sensors, the speed is measured in revolutions per minute, useful for gear speed or belt speeds.  In linear speed sensors, the speed is measured in inches per second, useful for linear speed applications - web processes, robotic arm movement.  The accuracy is measured as a percent of the full-scale range of the sensor range. It is a comparison of the speed sensed and the actual speed. The lower the percentage, the more accurate the device.  Important electrical specifications to consider when searching for speed sensors, magnetic include power requirements and output.  Power requirements can be self-generating, DC powered or AC powered.  Common outputs include resistance, voltage, current, frequency, switch output, serial, and parallel.

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