Optical Incremental Encoder Systems

Monday, April 30th, 2018 - Photoconductors, Velocity

Optical incremental encoder systems is used mostly for position feedback and its direct output is a pulse train whose frequency is proportional to the shaft angular speed. Counting hardware needs therefore to be used in order to extract the shaft angular position or, as in this case, its angular velocity. This principle is illustrated in Figure 1:

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Figure 1 Velocity transducer system based on an optical incremental encoder

Optical Incremental Encoder Systems For Velocity Transducer

The light source is usually an Infra Red Light Emitting Diode (IR LED) whereas the light detector is a spectrally matched photodiode (IR PD).

The modulation of the light from the source to the detector can be achieved in different ways, as applies also to position transducers; the perforated disk shown in Figure 1 can in fact be substituted by a transparent (i.e. glass or clear plastic) disk with a pattern of light and dark stripes printed on it. These light stripes can, in turn, be transparent or reflective which, in the latter case, permits the use of single packaged LED-PD pairs. A comparison of these two alternatives is shown in Figure 2:

Comparison of transmissive and reflective incremental encoder disks

Figure 2 Comparison of transmissive and reflective incremental encoder disks

The advantage of using optical reflection rather than optical transmis¬≠sion as the light modulation principle is twofold; a more compact transducer due to the single LED-PD package and a more versatile transducer due to the opportunity to discard the encoder disk and coat (or etch) the required single pattern of alternate light and dark stripes directly on the surface to be monitored, such as the shaft of a gear box shown in Figure 3. In most cases, in fact, the encoder disk used in velocity transducers, unlike the position transducer’s one, does not need a second phase¬≠ offset light modulating pattern and a reference slot so as to determine the direction of rotation and number of revolutions. We can, in fact, measure the speed, a scalar quantity, with a single disk pattern and convert it to velocity, a vector quantity, by adding the direction of rotation.

Example of direct load shaft speed monitoring using reflective optical transducer

Figure 3 Example of direct load shaft speed monitoring using reflective optical transducer

This can be done within the computer either as a measurement from the position transducer (that is the velocity transducer supplies the vector modulus and the position transducer the direction) or as an estimated data (that is the computer predicts the direction of rotation from the coordinate it generated for the motor operation). This latter technique, however, is not suitable for robot systems where load position oscillation is likely to occur, as is the case when driving large inertial loads.

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