RPM Measurement: Sensors and Techniques

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Low-cost, high-value sensor options are available for revolutions-per-minute (RPM) measurement applications. Most of these sensors provide simple, reliable and inexpensive transducers for highly sophisticated control systems. Take a look at different types of tachometers for RPM measurement, advanced sensor modules, and frequency measurement and period measurement methods.

T.K. Hareendran


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Detecting the stoppage of a rotating shaft (zero-speed detection) is significant in many applications, for example, to determine whether a conveyor has failed. Revolutions per minute (RPM) measurement is crucial when controlling or monitoring the speed of motors, conveyors, turbines, etc. RPM is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis. It is used as a measure of rotational speed of a mechanical component. Advanced electronics technology now offers low-cost, high-value sensor options for RPM measurement applications. Most of these sensors provide simple, reliable and inexpensive transducers for highly sophisticated control systems.

RPM sensors
A sensor is essential to sense shaft speed. Typically, devices used for this purpose are shaft (rotary type) encoders, photoelectric (optical type) sensors and magnetic rotational speed (proximity type) sensors. All of these sensors send speed data in the form of electrical pulses. Shaft encoders offer a high resolution of typically 1-5000 pulses per revolution (PPR) and clearly defined, symmetrical pulses. Proximity sensors provide medium (or low) resolution sensing, depending on the number of pulses measured per revolution. Photoelectric sensors usually sense a reflective target on the rotating shaft. Magnetic rotational speed sensors use various magnetic proximity measuring principles to monitor the speeds of machine components in a range between 0 and 30,000 rpm.

In principle, RPM sensors convert mechanical motion into electric pulses with or without direct contact when positioned near a turning rotor, gear, shaft or other regularly moving device. The resultant output signals are then fed to a digital counter, totaliser, tachometer, or other monitoring and control device.

Tachometers
The word ‘tachometer’ is derived from the Greek words ‘tachos,’ meaning ‘speed,’ and ‘metron,’ meaning ‘to measure.’ Tachometer is a device designed to measure rotational or surface speed either in contact or non-contact way. Tachometers are widely used to measure rotational speed of motors, conveyor belts, and other moving or rotating systems. RPM measurement with a tachometer can simply be divided into three popular methods:

Mechanical. In this method, revolutions in the mechanical sensor are electronically analysed in the instrument. This good old method is still commonly used for low resolutions between 20 and 20,000 rpm. Mechanical tachometers utilise the fact that the centrifugal force on a rotating mass depends on the speed of rotation.

Optical. Here rotation is transmitted to the measuring instrument via an infrared (IR) light beam/laser beam coming from the instrument, which is then reflected by a reflective tape on the object.

As explained, traditional tachometers require physical contact between the instruments and the objects being monitored. Laser tachometer is a powerful choice where this type of direct-contact type measurement is not workable for technical or safety reasons. Laser tachometers work by pulsing a laser beam against the rotating element. The rotating element will have one reflective spot, and the tachometer measures the rate at which the light beam is reflected back.

Fig. 1: Block diagrams of the KMI 15/x and KMI 16/x rotational speed sensors with integrated signal conditioning circuit, from Philips Semiconductors. The KMI 16/x sensor modules provide an opencollector output, while KMI 15/x sensor modules have a current interface that requires only two wires to connect them in the application
Fig. 1: Block diagrams of the KMI 15/x and KMI 16/x rotational speed sensors with integrated signal conditioning circuit, from Philips Semiconductors. The KMI 16/x sensor modules provide an opencollector output, while KMI 15/x sensor modules have a current interface that requires only two wires to connect them in the application

Stroboscopic. It has clear advantages over other measuring methods using mechanical or optical sensors. According to the stroboscopic principle, objects are stationary in the eyes of the observer when the frequency of high-speed flashes is in synchronisation with the RPM of the object. A stroboscopic light source provides high-intensity flashes of light, which can be caused to occur at a precise frequency. When this light source is made to fall on an object with periodic motion, it appears that the motion has slowed down or stopped when both frequencies bear a definite relationship.

Stroboscopic tachometers employ this effect for RPM measurement. Mechanical RPM measurement is not good for small objects. Although optical method is superior to mechanical method, practically it is not always possible to bond reflective tapes on the objects. But with stroboscopic method, it is always possible to measure the RPM of even very small objects.

1 COMMENT

  1. Hello , We have a requirement of RPM sensor for motor shaft , Can you please suggest the right one along with specs and pricing

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