Know Your Magnetic Field Sensors: Reed, Hall, and Beyond

When you’re dealing with automated pneumatic machinery, having reliable sensors that detect the position of your pistons is an absolute necessity. The most common sensors for accomplishing that goal are magnetic field sensors that sense simple magnets implanted on the inside of the pneumatic pistons.

Magnetically actuated switches mounted on the sides of a cylinder will detect the magnets as they come within range. Generally the magnets are placed so that the field sensor activates at one or both extreme ends of the actuator’s stroke, though there are applications where an actuator may need to be stopped at any of several points along its route and this is easily accomplished by arranging several magnetic field sensors at different points along the stroke.

Reed Switches, the simplest kind of magnetic field sensor, contain two ferrous nickel-and-rion ‘reed’ elements (thin wafers) inside of a vacuum-filled glass tube. The reeds generally don’t touch, but when a strong enough magnetic field comes close, the reeds come together, closing the circuit and thus activating the switch. Reed switches are inexpensive, simple, and require close proximity to the activating magnet. They require no standby power, but they respond relatively slowly and they have a finite life expectancy.

Hall-Effect Sensors, on the other hand, are solid-state devices with no moving parts, and thus a much longer life expectancy. They require continuous power, as they’re electronic, but they activate (and deactivate) much more quickly than reed switches. A Hall-effect sensor is a thin wafer that, under normal conditions, allows electricity to flow directly across it. But when a magnet approaches, the field and the wafer interact in such a way that the electricity flowing into the wafer instead veers off to both sides, into different circuitry.

Giant Magneto-resistance Sensors, or GMR sensors, are actually quite tiny — they’re molecular in nature. In a GMR sensor, two layers of ferrous conductive material are laid out on either side of a thin layer of nonconductive material. As long no magnetic field exists, the ‘top’ conductive layer directs electricity in one direction, while the ‘bottom’ layer directs it in the opposite direction — both 90 degrees to the input. When even a weak magnetic field appears on-scene, however, the field causes the molecules in the ferrous layers to align with the field, causing both layers to ‘point’ 180 degrees away from the input, forming a straight line and thus activating the circuit. Because GMR sensors are literally a few dozen molecules thick, they can be placed almost anywhere. Because there’s no contact between moving parts, they have a lifetime measurable in centuries or more. And because they respond well to even weak magnetic fields, they are much more sensitive and rapidly-actuating then Hall-effect sensors.

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