Feedback not at the motor shaft, odd-numbered ratio

Question:

How can an incremental encoder be used as speed feedback, if it is not directly mounted to the motor shaft and has a certain ratio between the encoder speed and the motor speed?

Answer:

First, it should be clarified, if the operation is also possible without encoder feedback. If the incremental encoder is not directly mounted to the motor shaft (e. g. on the gearbox output side), an improvement of the drive's control features is almost impossible due to the eventually not torsionally stiff coupling. In this case a feedback has positive influence on the static precision of the speed.

Function:

In case of an even-numbered ratio the number of increments per revolution can be multiplied by the ratio (i) via the signal source constant (incr./rev.) and can be parameterized as encoder constant under C0420.

If an odd-numbered ratio results in no even-numbered encoder constant or if the setting range of C0420 (1...8192 incr./rev.) is not sufficient, the signal at the encoder input X9 (DFIN) is to be read and to be converted into an analog signal (CONV3). Finally, a converter (CONV1) adjusts the ratio. In order to increase the speed precision the signal is compared with the setpoint speed via the process controller PCTRL1 and is connected as correction via ADD1 with the speed set value (see figure).

According to our experience the influence of the process controller output signal (PCTRL1-INFLU) is to be set via FCODE472/1 to maximum 5 - 10 %.