William_B

USING A SENSING ELEMENT.. ENCODER (part 1)
Refer back to the original post to see the image of the example system.

Several encoder types exists, but for this discussion we are ignoring these multitude of variations. Here will be made mention only of the discrete connected incremental encoders type.. and only in broad terms.

An incremental encoder is a sensor that gives out a set pulse count for each rotation of its own shaft. An encoder is rated in pulses per revolution (ppr). The PLC (in our case) is responsible for counting the pulses as generated by the rotating encoder.

Three sub variations to mention are..
(a) single pulse
(b) two single pulse... generate a pulse depending of travel direction
(c) double pulse...... two pulses with pulses 90^o off phase from one another.

All of these may also include an added signal that gives a short pulse burst for each rotation of the encoder.. also called the Z pulse. It is worth to mention that (c) can work as (a) if only one pulse phase is connected. There is no need to wire and use all provided signals.

The hardware of the PLC should be configured to measure the encoder pulses correctly. The PLC needs to know which of the categories (a) to (c) does the PLC need to measure. Alternatively a program needs to be coded if hardware configuration is not possible.

Furthermore the PLC input(s) should be configured to not cancel fast pulses by aggressive input filtering (hardware configuration). The designer must also be aware of what pulse rate the input can handle (rated in kHz). Knowing the PLC hardware specification is essential.

Measurement of (a) has the most basic set-up. It is important to note that no direction change can be detected by this configuration, while (b) and (c) can detect direction change. In our example that fact might not apply, as only one feed direction is probable. If direction change is required, internal PLC coding should regulate the counting direction internally.

It is VERY important to note that (c) is special. In Siemens hardware this encoder may be further be configured to measure x1 the rated pulses per revolution (ppr) specification of the encoder or x4 the ppr. The x4 is the default for most systems. .........!

This x4 rate is also referred to as quadrature encoder measurement. This result is achieved by measuring all the changes from off-on and on-off. There are x4 such transitions for each rated encoder pulse. That ultimately means that the PLC registers..
PLC counts per revolution = 4 x (encoder specified pulse per revolutions)

(to be continued..)

 does anyone have made a sample project for B type (two single pulse... generate a pulse depending of travel direction)? I am working on the same

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Split from Calculation Example: Paper feed speed from motor RPM.