Magnetic Sensing

Nov. 15, 2002
Magnetic sensors detect moving ferrous metal.

Magnetic sensors detect moving ferrous metal. The simplest magnetic sensor consists of a wire coiled around a permanent magnet. A ferrous object approaching the sensor changes magnetic flux through the coil, generating a voltage at the coil terminals.

Magnetic pickups sense linear or rotary motion without an external power source. They have high resolution, generating many pulses/in. of target travel and can sense very small ferrous objects. For example, one sensor responds to 96-pitch gears, while Hall-effect sensors can only register 16-pitch gear teeth. In these applications, magnetic transducers can be accurate to hundredths of a mechanical degree. For sensing rotating shaft speed, on the other hand, output-pulse frequency is converted to rpm at an accuracy of 0.1%.

Magnetic sensors measure speeds up to 600,000 rpm. Maximum sensor frequency is in the megahertz region, with usable frequencies limited by internal sensor impedance and external load.

Speeds near zero, however, cannot be measured because output voltage depends on the rate of change of flux through the coils. As frequency approaches zero, sensor output drops to the millivolt range.

The absence of electronic elements in magnetic sensors allows operation beyond temperatures (-65 to 300°F) associated with solid-state devices. Magnetic sensors built with special materials operate at cyrogenic temperatures and withstand temperatures excursions greater than 400°F. Magnetic sensors are almost impervious to shock, operating at levels exceeding 30,000 g.

Since magnetic sensors can detect ferrous discontinuities through nonferrous metals, they can be hermetically sealed within nonferrous housings to withstand 100% humidity or complete immersion in water and oil. Sensors enclosed in stainless steel can operate in salt spray or sand and dust environments, and under differential pressures up to 20,000 psi.

Eddy Currents: Eddy-current sensors detect ferrous and nonferrous metals. A high-frequency magnetic field induces eddy currents in metal targets. The eddy currents generally change the sensor's oscillation amplitude, which is sensed by a coil to create an output signal. For measuring speed, these sensors register metallic discontinuities in a moving target at a rate of about 5 kHz, but some models respond up to about 20 kHz.

Maximum response speed is determined by the method used to sense oscillator amplitude. Devices that sense amplitude changes with conventional demodulator/integrator circuits are slower than those that convert oscillator amplitude into a string of pulses whose widths vary with frequency.

Eddy-current devices also produce pulses with high positional accuracy. Because eddy-current sensors do not depend on a time rate of change to register motion, their response does not diminish near zero speed like that of magnetic pickups.

Eddy-current devices are seldom contaminated by dirt or metal particles, but sensing distance is typically limited to the diameter of the sensor (usually 0.06 to 3 in.). These sensors must also be enclosed in nonmetallic packages because they cannot sense through metals.

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