Turbo PMAC Users Manual Manual

Delta Tau Turbo PMAC User Manual
Reference
Position
+
PMAC PID + NOTCH Servo Filter
-
K aff
Big Step/
Deadband
Filter
IM
K (1-z -1)
vff
-1 -2
(1-2z +z )
K : Proportional Gain (Ix30)
p
K : Derivative Gain (Ix31)
d
K : Velocity Feedforward Gain (Ix32)
vff
K : Integral Gain (Ix33)
i
IM : Integration Mode (Ix34)
K : Acceleration Feedforward Gain (Ix35)
aff
K i
+
-1
1-z
+
K
d
+
-
K p
Notch Filter
-1 -2
1+n z +n z
1 2
-1 -2
1+d z +d z
1 2
Notch Coefficients
n1:
Ix36
n2:
Ix37
d1:
Ix38
d2:
Ix39
1-z-1
Secondary
Position
("Velocity")
Feedback
(usually
the same)
Primary
Position
Feedback
PID Feedback Filter
The PID feedback filter consists of proportional (“P”), integral (“I”), and derivative (“D”) terms, each
with its own contribution to the control effort.
Ixx30 Proportional Gain Term
The proportional gain term set by Ixx30 provides the basic corrective action for position errors, providing
a control effort proportional to the size of the error to try to reduce the error. Proportional gain alone acts
like a spring, and the magnitude of the proportional gain term is the “spring constant;” the higher this gain
term, the stiffer the spring action.
In Turbo PMAC, Ixx30 is an overall loop gain term, post-multiplying the other gain terms, and not just a
proportional gain term. This makes it possible to change Ixx30 alone if an external gain term (e.g.
encoder resolution or amplifier gain) is changed; it also “normalizes” the derivative and integral gain
terms into a time constant and inverse time constant, respectively.
Ixx31 Derivative Gain Term
The derivative gain term set by Ixx31 provides a damping effect by providing a contribution to the control
effort proportional to the actual velocity acting against that velocity. In this respect it acts much like a
“dashpot” or the shock absorber of a vehicle’s suspension. The higher the derivative gain term, the
heavier the damping action.
Some form of derivative action – effectively a velocity loop – is required for a stable position loop. If a
well-tuned velocity loop is closed in the amplifier, the Ixx31 derivative gain term in Turbo PMAC can be
set to 0. However, if there is no velocity loop closed externally, a positive value of Ixx31 will be required
for stable operation.
Setting Up the Servo Loop 8-5