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XPS-Q8 Controller
XPSDocumentation V1.4.x (EDH0301En1060 — 10/17)
Parameters
14.3.1.1
FeedForward Method:
• Velocity
• KFeedForwardVelocity is a gain that can be applied to this feed forward.
• When the system is used in open loop, the PID output is not applied and the feed
forward gain is set to 1 (the entire output of the controller is FF gain).
PID corrector:
• Total output of the PID is a speed (units/s), so:
Kp is given in 1/s.
Ki is given in 1/s
Kd has no unit.
Filtering and Limitation:
• ScalingVelocity (units/s) is the theoretical speed resulting from a 10 V input to the
driver.
• VelocityLimit (units/s) is the maximum speed that can be commanded to the driver.
Basics
14.3.1.2
For a "perfect system" (no friction, all performance factors known, no following errors),
a KFeedForwardVelocity value of 1 will generate the exact amount of output required
to reach the TargetPosition.
The Kd parameter is generally redundant when using the speed loop of the driver and is
usually set to zero, but a higher value can be used to improve the "tightness" of the
speed loop.
The proportional gain Kp drives the cut-off frequency of the closed loop.
Due to the integration of the speed command in a position by the encoder, the overall
gain of the proportional path at a given frequency Frq is equal to Kp/2πFrq. This gain is
equal to 1 at Frq P = Kp/2π (close to the cut-off frequency).
This frequency must remain lower than the cut-off frequency of the speed loop of the
driver and lower than the mechanic's natural frequencies to maintain stability.
The integral gain Ki drives the capability of the closed loop to overcome perturbations
and to limit static error.
Due to the integration of the speed command in a position by the stage encoder, the
overall gain of the integral path at a given frequency Frq is:
This gain is equal to one at FrqI:
This frequency FrqI must typically remain lower than the frequency FrqP of the
proportional path to keep the stability of the servo loop.
2
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