Input / Output Devices

Position Sensors & Magnetic Sensors

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

Magnetic sensors feature pickups, which are able to 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, some position sensors respond to 96-pitch gears, while Hall-effect position sensors can only register 16-pitch gear teeth. In these applications, magnetic transducers can be accurate to hundredths of a mechanical degree. Magnetic sensors are often used to determine a rotating shaft's speed. The output-pulse frequency is converted to RPM at an accuracy of 0.1%.

Magnetic sensors measure speeds up to 600,000 rpm. Most magnetic sensors feature a maximum sensor frequency, which 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, magnetic sensors often experience a drop in output, which goes down 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.

Sensors: Position Sensors & Magnetic Sensors

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