Delta Tau GEO BRICK LV User Manual

Turbo PMAC User Manual

192

Setting Up the Servo Loop

Backlash Table Example

Imagine the calibration of an axis assigned to Motor 3 had been performed against an accurate linear
measurement device on the load, working in both directions, and the following readings of the linear
reference device for set positions of the motor encoder (expressed in units of the motor encoder):

Motor Pos. (cts)

0 500 1000 1500 2000 2500 3000 3500

Load Pos.+ (cts)

0* 510

995 1492.5 1994 2497.5 3003.5 3500.5

Load Pos.- (cts)

5 516 998.5 1494 2000 2501 3010.5

3508.5

* Reference point; zero by definition

Only the compensation table works in the positive direction, so the entries in the compensation table
should be the negative of the difference between positive-going load position and motor position,
expressed in 1/16 counts:

Motor Pos. (cts)

0 500 1000 1500 2000 2500 3000 3500

Load - Motor (cts)

0* +10 -5 -7.5 -6 -2.5 +3.5

+0.5

Motor - Load (1/16 cts)

0* -160 +80 +120 +96 +40 -56 -8

* Reference point; zero by definition

The position (leadscrew) compensation table definition to create these corrections would be:

#3 DEFINE COMP 8,4000

-160 80 120 96 40 -56 -8 0

Note that the first entry is for the correction at 500 counts, and the added last entry is 0, for the correction
at 4000 counts and 0 counts.

There is a 5-count backlash at motor position 0, so Ixx86 should be set to 5*16, or 80.

Next, the backlash compensation table should contain the differences between negative-going load
position and positive-going load position, minus Ixx86:

Motor Pos. (cts)

0 500 1000 1500 2000 2500 3000 3500

Load(-) - Load(+) (cts)

5 6 3.5 1.5 6 3.5 7 8

Load(-) - Load(+)-
Ix86 (cts)

0* 1 -1.5 -3.5 1 -1.5 2 3

Load(-) - Load(+)-
Ix86 (1/16 cts)

0* 16 -24 -56 16 -24 32 48

* Reference point; zero by definition

The backlash table definition to create these corrections would be:

#3 DEFINE BLCOMP 8,4000

16 -24 -56 16 -24 32 48 0

Note that the first entry is for the correction at 500 counts, and the added last entry is 0, for the correction
at 4000 counts and 0 counts.

Note:

While the range and spacing of a backlash table will typically be the same as for
the leadscrew compensation table for the same motor, this is not required. Even
the presence of a leadscrew compensation table for a motor is not required to have
a backlash table for that motor.