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Rugged, Reliable Detector Monitors Motion
System sounds an alarm or cuts power when rotary or linear motion stops
By Robert N. Boggs, Southern Technical Editor -- Design News, March 26, 1995
Martinsville, VA-In many industrial operations, it's important to know whether a mechanism is rotating or not. Unfortunately, some customary motion-detecting sensor arrangements produce a false signal if motion stops at the specific actuation point of the sensor. Sensor reliability also suffers from harsh operating conditions such as heat, dust, and voltage fluctuations.
Faced with the problem of keeping conveyors running in a rock quarry, David Minter developed a high-reliablity motion detector. His patented Zero-Motion Detection system cuts conveyor-motor power quickly if jam-ups occur. It has contributed to record production levels at the quarry, which employs eight of Minter's systems.
Each of Minter's systems includes a rotating metal target, one or two proximity switches, and two specialized timer relays. The metal target has one or more cutouts or voids that pass by the proximity sensors. In the two-proximity-switch version, switch 1 detects the void and starts timer relay 1. When switch 2 detects the void, it starts timer relay 2 and resets timer relay 1. The process repeats with neither timer timing out as long as the target keeps rotating.
If the target stops, whichever timer is running will time-out, triggering an alarm or shutting off power to the system. The user sets the time-out limits to fit his system's needs.
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Machine tools
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Conveyors
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Materials handling gear
The switches and relays operate at normal line voltage. Connecting a tachometer to the output of a proximity sensor adds speed-control capabilities to the system. Optional counters permit count-up, count-down, reverse, speed change, and other functions.
To demonstrate the system for this writer, Minter tossed a two-by-four into the auger of a screw conveyor. The timer cut power to the motor and the system stopped without such consequences as sheared keys, broken chains, tripped thermal overloads, or other trauma. He removed the two-by-four, pressed Reset, and the system was up and running.
"This system is not affected by dust, dirt, and grit such as we encounter every day in the quarry," says Minter. "We even have a unit operating under water."
Additional details...Contact David Minter, RPM Detection, Inc., Box 3091, Martinsville, VA 24115, (703) 632-8718.
