Delta Tau GEO BRICK LV User Manual

 

PMAC Compensation Tables

Standard leadscrew compensation

e.g. 

∆x = f(x)

y

Get linear encoder accuracy (almost!) with rotary encoder

y

Characterize system with linear sensor

y

Enter errors in PMAC

M

E

∆x

∆x

∆x

∆x

∆x

DEFINE COMP 200, #1, #1, 100000

Table Length

Source Motor

Table Span in Counts
Target Motor

 

Uses of Cross-Axis Compensation 

The ability to have separate source and target motors for a table has several uses.  The first is the 
traditional compensation for imperfect geometry, as in a bowed leadscrew.  For instance, on an XY table, 
if the X-axis leadscrew is bowed, the Y-axis position should receive a correction as a function of X-axis 
position.  If motor #1 is the X-axis, and motor #2 is the Y-axis, the table holding this correction would 
have motor #1 as the source motor, and motor #2 as the target motor. 

PMAC Compensation Tables

Cross-axis compensation

e.g. 

∆y = f(x)

y

Useful for bowed leadscrews

y

Can be used to build electronic cam tables

DEFINE COMP 100, #1, #2, 100000

Table Length

Source Motor

Table Span in Counts
Target Motor

∆y

∆y

∆y

∆y

∆y

∆y

M

E

 

A second use for cross-axis compensation is what is often known as the electronic cam.  In this case, the 
entire movement of the target motor is caused by the entries in the compensation table, not just the 
corrections.  This method of implementing electronic cam operation has two significant advantages over 
Turbo PMAC’s time-base following, the other method of creating electronic cams: the compensation 
table is bidirectional – the master can turn in either direction – and it is absolute, so the phasing in is 
simply a matter of homing the axes.   

The time-base method, in which the motion program of the slave motors defines the motion, retains the 
advantage of being able to change on the fly through math and logic in the program, and of second or 
third order interpolation between points, rather than the compensation table’s 1

st

-order interpolation.  

Refer to the Synchronizing Turbo PMAC to External Events section of this manual for details. 

Turbo PMAC presently supports one-dimensional (1D) and two-dimensional (2D) position compensation 
tables.  Three-dimensional (3D) tables may be supported in the future.  1D tables have a single source 
motor; 2D tables have two source motors.  Note that a table with a single source motor that is different 
from the target motor (a cross-axis compensation table) is still a 1D table.