User's Manual

PMAC User Manual
Closing the Servo Loop 107
Reference
Position
IM
Secondary
Position
("Velocity")
Feedback
(usually
the same)
Primary
Position
Feedback
-1
1-z
K
i
-1
-
1-z
Big Step/
Deadband
Filter
Notch Filter
d
-
IM
: Proportional Gain (Ix30)
: Derivative Gain (Ix31)
: Velocity Feedforward Gain (Ix32)
: Integral Gain (Ix33)
: Integration Mode (Ix34)
: Acceleration Feedforward Gain (Ix35)
K
p
K
d
K
vff
K
i
K
aff
n
n
d
d
1
2
:Ix36
:Ix37
:Ix38
:Ix39
1
2
K
vff
(1-z )
-1
K
aff
-1
(1-2z +z )
-2
Notch Coefficients
-1
-2
12
1+d z +d z
-1
-2
1
2
1+n z +n z
K
p
K
+
+
+
+
Figure 18 PMAC PID and NOTCH Servo Filter
In addition, velocity feedforward gain (Ix32) reduces following errors introduced by damping (which are
proportional to velocity), and acceleration feedforward gain (Ix35) reduces or eliminates following errors
due to system inertia (which are proportional to acceleration).
Tuning the PID Filter
The PMAC Executive Program for PC-compatible computers provides an easy means of tuning the PID
filter. It allows simple commands that automatically perform standard moves, gather the response data,
plot this data to the screen, and compute important statistics for the response. This permits even
inexperienced users to make some judgments according to simple (provided) rules to optimize the tuning.
Detailed instructions and examples are provided in the Executive Program manual.
The Executive program also has an auto-tuning function that stimulates the motor, evaluates the response,
and calculates the gains required for the desired response level. This function allows the computer to
make all of the decisions as to what the proper gains should be.
Command
Position
Deadband
Input
"Big Step"
Limit
Ix64,65 Ix67
Integration
Mode
Cmd Vel = 0?
Ix34
Actual
Position
I
Ix63
Integration
Limit
D
P
Ix69
Output
Limit
Ix79
Ix29
Ix29
Ix79
Ix29
DAC Bias
Phase 2 Bias
Phase 1 Bias
SIN
Q
SIN (
Q- F
)
(Ix01=1)
Ix02
Bit 16
=1
+
-
+
-
+
+
Motor Position
Load Position
Ix68
"Friction
Feedforward"
Signum
Function
1
1-Z
-1
-1
Command
Velocity
+
+
Figure 19 PMAC PID Servo Loop Modifiers