sponsoredMEASUREMENT SYSTEMS Optical RPM & angle sensors in a nutshell

RPM and angle sensors are used to measure the motion, speed, or angular position of a rotating object – usually a shaft, axle or disc. Such sensors are used in a wide range of machines – from small printers to big manufacturing machines, cars, aircrafts and many more. And also, the intended purpose always varies.

What are optical RPM & angle sensors and how do they work?

Optical rotation sensors are devices which measure rotational characteristics of an object based on optical principles. Such sensors for instance use periodically reflected light to detect changes in position and motion.

In general, an infrared (IR) laser usually sends light onto the spinning object. The object itself is marked with an adhesive tape or another similar reflective material which reflects the incident beam back to the sensor. To measure the reflected light, a photodiode is typically used to convert the absorbed light into measurable current. Every peak in the light intensity due to reflection, is paralleled by a peak in the electrical signal and thus allows to determine the frequency, period, rotational speed, and angular position.

From simple signals to significant characteristics

The before described optical and inductive sensors can be summarized as incremental encoders. The signals provided by these sensor elements are usually converted into square-wave signals by threshold switches. The output pulses are transmitted to the evaluation electronics and counted. The resulting number of pulses counted per time interval represents the measure of change in position.

To obtain information about the direction of movement two slightly shifted sensors are used, which results in two asynchronous signals. From the sequence of the leading and trailing edge of the two signals, it is possible to determine the direction of movement. Finally, for the absolute determination of position, a zero pulse is implemented once per revolution to mark the start/end of a revolution.

Application and implementation

As already noted, RPM sensors are used in a wide range of machines. The intended purpose however always varies – sometimes it is only needed as additional information to other measurement channels and on other occasions, it may be needed for specific analyses. Fields of application are for instance:

• order tracking

• field balancing

• rotational and torsional vibration analysis

• supplementary RPM measurements

• etc.

Depending on the application of the sensor, the mounting and installation are sometimes straightforward and sometimes not. Figure 3 shows an example from practice. Here a DEWETRON Tape Sensor is used for rotation monitoring of a drive test bench with the reflective tape attached to the spinning wheel. The sensor itself is equipped with a LEMO connector and directly linked to a LEMO counter input of a DEWETRON measurement system. DEWETRONs measurement software OXYGEN then displays the measured rotation frequency and calculated velocity.

Looking ahead

The world of optical RPM and angle sensors is continually evolving. Advances in materials and technologies are improving precision and reliability. And as industry is demanding ever more accurate measurements, the role of these sensors will continue to grow.

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