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Machine Design

Sensor Sense: Measuring rpms

The monitoring of rotation can involve such sensor types as proximity, photoelectric, resolvers, encoders, and tachometer generators.

Most determine direction of rotation, speed, and angular position of their rotating targets by generating a series of pulses. Tachometer generators and many other conventional rotation monitors require a physical connection to the driving element. Proximity sensors, in contrast, do not.

The proper size and configuration of proximity sensors used for measuring rotation depends on whether the sensor is or is not an embeddable type. For embeddable sensors, the spacing of each tooth and gap, M, must equal or exceed the diameter of the sensor, D. For nonembeddable types, M must equal or exceed D or 3 Sn, whichever is larger.

In many cases proximity sensors directly monitor rotating parts of a machine such as shafts, gears, or cams. They need no special actuators or additional connecting elements. A caveat is that the gap or air space between targets must equal or exceed the diameter of any proximity sensor used to sense rotation.

The time needed to acquire a value for rotation speed depends on how fast the sensor generates pulses. A single-lobe cam needs enough time to complete one revolution before a sensor can determine its rotational speed. However, a cam with four equalspaced lobes only needs to complete one-quarter of a revolution.

Mild steel targets demand that sensors be positioned at half the nominal sensing range. Nonferrous targets require the use of correction factors. Neither statement applies for Factor 1 sensors that sense all metal at the same distance.

Turck Inc. ( provided information for this column.

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