User's Manual

PMAC User Manual
82 Setting Up a Motor
S1=L1/N
S2=L2/M
C =CORRECTION
1
L1
S1
(0)
S2
L2
C
1
C
2
C
N-1
C
N
C
N+1
C
N+2
C
N+3
C
2N
C
2N+1
C
2N+2
C
2N+3
C
2N+4
C
3N+1
C
3N+2
+SOURCE 1
POSITION
+SOURCE 2
POSITION
C
MN+N
C
MN+M+1
C
MN+M+2
C
MN+M+N-1
C
MN+M+N
ROWS
SHOULD
MATCH
COLUMNS
SHOULD
MATCH
Figure 14 Two Dimensional Compensation Table
Backlash Compensation
PMAC can perform sophisticated backlash compensation for all motors. On reversal of the direction of
the commanded velocity, a pre-programmed backlash distance is added to or subtracted from the
commanded position. This backlash distance can be constant over the travel of the motor, or it can be a
function of motor position. The rate at which the backlash is introduced or removed is programmable, as
is the magnitude of the reversal required for backlash to be introduced or removed.
Constant Backlash
Ix86 for motor x is the constant backlash distance parameter. When the direction of the motor’s
commanded movement changes from positive to negative, this value is introduced into the active
backlash compensation register, which is subtracted from the nominal commanded position. When the
direction of the motor’s commanded movement changes from negative to positive, the value of the
backlash compensation register is reduced to zero.
Note:
A positive value of Ix86 adds extra distance to the travel of the motor on reversal,
which is what is desired to compensate for true physical backlash. The units of
Ix86 are 1/16 of a count, so the value should be 16 times the number of counts of
backlash compensation required.
Backlash Take-Up Rate
Ix85 controls the rate at which backlash is introduced or removed upon reversal for motor x. This helps
to optimize for swift but smooth backlash compensation. When reversal is detected, each background
cycle (between each scan of each PLC) an amount equal to Ix85 is added to or subtracted from the active
backlash compensation register, as appropriate, until a value Ix86 or 0 in that register is reached. In
general, the highest value of Ix85 that produces smooth transitions should be used.
Backlash Hysteresis
I99 controls for all motors on PMAC the number of counts in the new direction of the commanded
position that must be seen before PMAC determines that a reversal has occurred and the backlash must be
changed. As such, it acts as a hysteresis term. It is particularly important if a master encoder is used to
drive the motor, so slight dithering in the master encoder does not cause repeated introduction and
removal of backlash. I99 has units of 1/16 count, so the default value of 64 provides a 4-count hysteresis.





⋅⋅
−
⋅
+
⋅+⋅
+⋅⋅=
−
1
(
A0
Ix31Ix
2
)n(IE33Ix
128
)n(CA35Ix)n(CV32