encoder_ratio - an electronic gear to synchronize two axes


loadrt encoder_ratio [num_chan=num | names=name1[,name2…​]]


encoder_ratio can be used to synchronize two axes (like an "electronic gear"). It counts encoder pulses from both axes in software, and produces an error value that can be used with a PID loop to make the slave encoder track the master encoder with a specific ratio.

This module supports up to eight axis pairs. The number of pairs is set by the module parameter num_chan. Alternatively, specify names= and unique names separated by commas.

The num_chan= and names= specifiers are mutually exclusive. If neither num_chan= nor names= are specified, the default value is one.



Read all input pins. Must be called at twice the maximum desired count rate.

encoder-ratio.update (uses floating-point)

Updates all output pins. May be called from a slower thread.


The names for pins and parameters are prefixed as:

encoder-ratio.N. for N=0,1,…​,num-1 when using num_chan=num nameN. for nameN=name1,name2,…​ when using names=name1,name2,…​

The encoder-ratio.N. format is shown in the following descriptions.


encoder-ratio.N.master-A bit in

encoder-ratio.N.master-B bit in

encoder-ratio.N.slave-A bit in

encoder-ratio.N.slave-B bit in

The encoder channels of the master and slave axes

encoder-ratio.N.enable bit in

When the enable pin is FALSE, the error pin simply reports the slave axis position, in revolutions. As such, it would normally be connected to the feedback pin of a PID block for closed loop control of the slave axis. Normally the command input of the PID block is left unconnected (zero), so the slave axis simply sits still. However when the enable input goes TRUE, the error pin becomes the slave position minus the scaled master position. The scale factor is the ratio of master teeth to slave teeth. As the master moves, error becomes non-zero, and the PID loop will drive the slave axis to track the master.

encoder-ratio.N.error float out

The error in the position of the slave (in revolutions)


encoder-ratio.N.master-ppr unsigned rw

encoder-ratio.N.slave-ppr unsigned rw

The number of pulses per revolution of the master and slave axes

encoder-ratio.N.master-teeth unsigned rw

encoder-ratio.N.slave-teeth unsigned rw

The number of "teeth" on the master and slave gears.