DIY Encoder for RPM and Torsion Vibration Analysis

Torsion vibration can be difficult to detect in rotating machinery as vibration occurs in a rotating component or drive train. Torsion vibration may result in equipment damage as loads increase and machine internal timing is affected.

This post is about optic sensors and the use of a Do It Yourself (DIY) encoder to detect torsion vibration.

A tacho signal can consist of one or more pulses per revolution. It can be picked up using e.g. a rotary encoder placed at a free shaft end, magnetic or optic sensors for the purpose of torsion analysis. Torsion analysis can be made using other techniques, such as torsion lasers, torsion meters (strain gauges that rotate with the shaft), or be implicitly determined based on the current oscillation when a motor or generator is part of the assembly.

The optic sensor usually is a laser that detects a reflected or not reflected signal and communicates this as a TTL signal, i.e. a switched 0 Volt or + 5 Volt signal. The target illuminated by the laser can be reflecting tape or paint or, in this case, a piece of black and white printed paper taped onto the shaft. The end result can look like this. The extra BW tags are 10mm marks that are used to ensure correct scaling during the printout (see Figure 1):

Figure 1. Example of figure used for printing a simple encoder pattern. A reference length mark is included to allow correct sizing of the printed encoder strip. (Click figure to expand)

The encoder pattern can be printed on ordinary paper using a laser or inkjet printer and then be taped to the rotating shaft. This kind of installation is never perfect and always leaves the encoder pattern with some degree of distortion – in particular at the interface where there is an overlap. If required, this distortion can be compensated for in a post-processing of the signal.

Dividing the shaft circumference, C, into N intervals produces an angular resolution of 360/N degrees, or a spatial resolution of CN = C/2N mm. What it does not provide is the ability to accurately detect angular vibration, because the sensor cannot determine if the laser triggers on the same pulse twice, e.g. if there is a backlash in the rotation.

To detect direction, a second 90-degree phase shifted channel is introduced. The first channel is commonly referred to as the A channel and the second channel is the B channel. This is what an A/B pattern looks like (see Figure 2):

Figure 2. DIY encoder with A/B pattern and reference length mark for scaling of printed encoder. The B pattern is shifted 90 degrees relative to the A pattern to allow the detection of back and forth rotation. (Click figure to expand)

To detect absolute angle, a third channel is introduced. This third channel, the Z-channel, provides one pulse per revolution. This is what an A/B/Z pattern looks like (see Figure 3):

Figure 3. DIY encoder with A/B/Z pattern and reference length mark for scaling of printed encoder. The Z pattern is used to count the number of revolutions. (Click figure to expand)

The laser that is used to illuminate the encoder pattern has a finite size, d mm. The laser spot diameter must fit well into the separation distance CN. It logically follows that for a high spatial resolution, i.e. small values of CN, the laser diameter d must be small. Small laser spots are achieved by using focused lasers where the laser spot diameter can be of sub millimeter size (see Figure 4).

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Figure 4. An example pattern and a focused spot laser. The laser spot diameter is 0.25 mm.

Using a DIY encoder with a narrowly-focused laser beam is not always easy, as small errors or small relative motion between the shaft target position and the laser suspension position can misalign the laser so that it does not detect the pattern. The more encoder channels that are in use, the more complicated the setup.

A single laser (encoder channel) is required to monitor the pattern shown in Figure 1. To monitor the pattern shown in Figure 3, three lasers (encoder channels) are required.