Speed sensors - AI-Tek - #5

The internal construction of the typical Al-Tek variable reluctance sensor is a magnet, pole piece and coil (See figure 1). A magnetic field (lines of flux) extends from the magnet, through the pole piece and coil out into the air space at the end of the sensor. The return path of the magnetic field is from the air space to the other end of the magnet. As a ferrous object ap-

proaches the tip of the pole piece, the magnetic field increases and then decreases as the object

moves away from the pole piece. The snap or the rapid change in the magnetic field induces an AC

Figure 1 - Internal configuration of typical sensors.

voltage signal in the coil. With an ideal target and matching sensor, the induced voltage is in the shape of a sine wave.

As can be seen, the generated frequency signal is directly proportional to the number of ferrous objects passing the pole piece per unit time. The amplitude of the voltage output is proportional to the speed of the ferrous objects passing the pole piece.

Many applications of Al-Tek magnetic sensors use gears as targets. Typical sensor output wave

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ferrous discontinuities such as sprockets, keyways, boltheads, etc. is because the output predictable and repeatable. See Figure 2 for commonly used gear terminology.

n other

is

Diametral Pitch = No. of Teeth + 2

Outside Dia. of Gear (in.)

PICKUP MEDIUM L^J TOOTH GEAR

AA

A

PICKUP COARSE L-o-l TOOTH GEAR

B

PICKUP

WIDELY SEPARATED SMALL TEETH OR

FINE

TOOTH

GEAR

_A_A_ C

PROJECTIONS

D

Figure 2 - Common terms used in defining gears.

The performance of a sensor can be easily defined when using a gear for a target; it also allows for estimated performance with alternate targets. Al-Tek sensors are tested with AGMA standard gears; the performance curves are i ncluded in this catalog.

Al-Tek Instruments differs from most sensor manufacturers in the presentation of performance curves and test parameters. Most existing data is specified at a surface speed of 1000 in/sec and 0.005 in. air gap; we feel that a 0.030 in. air gap and 500 in sec. surface speed (1800 RPM motor with 5 to 6 in. dia. gear) are more realistic parameters to specify performance.

<

PICKUP ,.„„,_,„ CAM MOTION PICKUP

SEPARATED ^CL™^SE | | LARGE TEETH

_E OR p

PROJECTIONS

Z

a O

E □

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PICKUP

CAM MOTION COUNTER-

3 - Generated voltage waveforms.