How do Sensors Measure Gear Tooth Speed and Direction?

Measuring speed of gears is an important factor in various industries, especially in pharmaceutical, tobacco, printing, woodworking, paper, textile, food and others where rotational machinery predominates. Gear speed measurements also necessary in pumps, blowers, mixers, exhaust and ventilation fans, wheel-slip measurement on autos and locomotives, flow measurement on turbine meters and many more.

The most common gear tooth sensors detect a change in the magnetic field for determining the speed and direction. Usually, these are of three types – the Hall Effect, magneto-resistive and the Variable Reluctance. There are optical types of sensors as well, detecting a change in light levels as the gear rotates past the sensor.

Sensors using magnetic properties are good for measuring speed and direction of gears made of ferrous metals. All these sensors are non-contact type and sensitive to detect the presence of gear teeth passing in front of the sensor. As a gear tooth comes close to the magnetic sensor, its output flips and the electrical level at its output changes state. The output remains steady as long as the gear tooth is within the detectors sensing zone. As the tooth passes out of this zone, the output flips back. Therefore, a magnetic sensor placed in front of a rotating gear, the output from the sensor will be a series of electrical pulses.

There are several advantages when using magnetic sensors. Apart from the sensors being non-contact type, they are robust, hermetically sealed and can withstand unregulated power supply. Most manufacturers make then RoHS and IP67 compliant. That means no lead or other toxic materials are used for manufacturing these sensors and dust or liquid will not enter their enclosure. That makes such sensors suitable for use in food processing industries.

For measuring the speed of gears made of non-magnetic material, engineers often use optical sensors. The most common sensor of this type is the optical interrupters. Gear teeth interrupt a light beam from an LED source and the detector produces a corresponding electrical output. A continuously rotating gear in front of the sensor therefore, creates a similar series of electrical pulses as the output from magnetic sensors do.

The functioning of optical speed or proximity sensors is dependent of the dust and dirt level of the environment where they are used. Therefore, their range of applications is somewhat restricted as compared to magnetic sensors.

Measurement of direction involves a reference point, which means two sensors need to be used, with one of them being the reference sensor. An electronic circuit measures the time gap between the responses from each sensor. As the gear tooth passes in front of both sensors, one of them will change output before the other. If sensor A happens to trigger before sensor B does, the electronic circuit determines the gear is moving from A towards B. In case the output of sensor B switches before sensor A does, then the gear is moving from B towards A.

Usually, the sensors provide separate digital outputs for speed and direction. Their measuring capability may extend from detecting near zero speed up y 15 kHz.