Tachometric measurements

Tachogenerators cover a wide range of activities, from the direct measurement of rotational speeds to the implementation of speed or position servo-controls for motors. In the latter case, the measurement function is directly integrated into the servo loop. In this case, the measurement function is directly integrated into the control loop. The measurements made are indeed speed measurements, but can also be position or acceleration measurements. As the processing of such servo loops could be the subject of a separate document, it will not be discussed here.

As the field of motor servo-controls is excluded, most speed measurements carried out in industrial environments use either tachometers or stroboscopes. As far as tachometers are concerned, two technologies dominate: photoreflective tachometers and contact tachometers. Stroboscopes have the added advantage of being able to monitor vibrations or slippage.

In the first part, we present the different types of equipment, how they are used, their respective fields of application, and the factors of influence and uncertainty associated with measurements.

Although frequently found in servo loops, incremental encoders may be necessary for certain speed measurement applications, particularly where low uncertainty is required. In such cases, the incremental encoder acts as a sensor, which needs to be combined with suitable electronics. This is why the end of Part 1 is devoted to this type of encoder.

Calibration is essential to ensure traceability of measurements, and to provide the information needed to identify measurement errors and assess measurement uncertainties. For this reason, the second part of the dossier presents procedures for calibrating rotational speed measuring devices, as well as elements for assessing the associated uncertainties.

As the methodology for assessing measurement uncertainties is now relatively well described in standards documents and in a number of books, it is not repeated here; it is directly applied to the problem at hand.

Finally, in order to keep the text as short as possible, we have chosen to describe as precisely as possible all the information for applications involving photoreflective tachometers, since this information can easily be transposed to other devices. For the latter, only the specific features are described.