Master/Slave FU/LU Roundoff with RATIO_GR and RATIO_SYN

This article refers to G&L PiC application note document AN000006 and describes Master/Slave, FU/LU roundoff with RATIO_GR and RATIOSYN.

When the feedback unit to ladder unit ratio specified for a slave axis is not a whole number, the magnitude of the numbers specifying the master and slave distances (MDST and SDST inputs to

RATIO_GR and RATIOSYN) has an effect on the actual master:slave ratio used.  Internally, the FU to LU values are used to convert the MDST and SDST values from LU to FU.  The resulting FU values are used to command a slave axis.

The equations are:

       (slave FU * SDST) / slave LU = slave distance in FU

       (master FU * MDST) / master LU = master distance in FU

        where “/” represents integer division

Note the difference when specifying 1:1or 1000:1000 with MDST and SDST:

       Slave FU = 10               actual slave distance used = (10 * 1) / 3 = 3 slave FU

       Slave LU = 3                 actual master distance used = (1*1) / 1 = 1 master FU

       Master FU = 1

       Master LU = 1

       MDST = 1

       SDST = 1

       Slave FU = 10               actual slave distance used = (10 * 1000) / 3 = 3333 slave FU

       Slave LU = 3                 actual master distance used = (1 * 1000) /  1 = 1000 master FU

       Master FU = 1

       Master LU = 1

       MDST = 1000

       SDST = 1000

Note that in the first example the slave will move 3000 FU when the master moves 1000 FU.  In the second example the slave will move 3333 FU when the master moves 1000 FU.

Also, when Master FU is less than Master LU, the magnitude of MDST will determine whether or not a master distance is accepted. For example:

       Master FU = 1              actual master distance used = (1 * 1) / 5 = 0 master FU

       Master LU = 5

       MDST = 1

       SDST = 1

       Master FU = 1              actual master distance used = (1 * 10) / 5 = 2 master FU

       Master LU = 5

       MDST = 10

       SDST = 10

Note that the computed master distance in the first example is 0.  This will result in an “Invalid  Master Distance” error despite the fact that MDST = 1.