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6250 Servo Controller User Guide
When velocity feedback control is used, the control signal is proportional to the encoder's
velocity (rate of change of the actual position). The Servo Gain Velocity (
SGV
) command sets
the gain, which is in turn multiplied by the encoder's velocity to produce the control signal.
Since the velocity feedback acts upon the encoder's velocity, its control action essentially
anticipates the position error and corrects it before it becomes too large. Such control tends to
increase damping and improve the stability of the system.
A high velocity feedback gain (
SGV
) can also increase the position tracking error when
traveling at constant velocity. In addition, setting the velocity feedback gain too high tends to
slow down (overdamp) the response to a commanded position change. If a high velocity
feedback gain is needed for adequate damping, you can balance the tracking error by applying
velocity feedforward control (increasing the
SGVF
value—discussed below).
Since the encoder's velocity is derived by differentiating the encoder's position with a finite
resolution, the finite word truncation effect and any fluctuation of the encoder's position would
be highly magnified in the velocity value, and even more so when multiplied by a high
velocity feedback gain. When the value of the velocity feedback gain has reached such a limit,
the motor will chatter (high-frequency, low-amplitude oscillation) at steady state.
Velocity Feedforward Control (
SGVF
)
The purpose of velocity feedforward control is to improve tracking performance; that is, reduce
the position error when the system is commanded to move at constant velocity. The tracking
error is mainly attributed to three sources—friction, torque load, and velocity feedback control
(
SGV
).
Velocity feedforward control is directed by the Servo Gain Velocity Feedforward (
SGVF
)
setting, which is in turn multiplied by the rate of change (velocity) of the commanded position
to produce the control signal. Consequently, because the control signal is now proportional to
the velocity of the commanded position, the 6250 essentially anticipates the commanded
position and initiates a control signal ahead of time to more closely follow (track) the
commanded position.
Applications requiring linear interpolation can benefit from improved tracking performance;
however, if your application only requires short, point-to-point moves, velocity feedforward
control is not necessary.
Because velocity feedforward control is not in the servo feedback loop (see Servo Control
Algorithm drawing above), it does not affect the servo system's stability. Therefore, there is
no limit on how high the velocity feedforward gain (
SGVF
) can be set, except when it
saturates the control output (tries to exceed the 6250's
±
10V analog control signal range).
Acceleration Feedforward Control (
SGAF
)
The purpose of acceleration feedforward control is to improve position tracking performance
when the system is commanded to accelerate or decelerate.
Acceleration feedforward control is directed by the Servo Gain Acceleration Feedforward
(
SGAF
) setting, which is in turn multiplied by the acceleration of the commanded position to
produce the control signal. Consequently, because the control signal is now proportional to
the acceleration of the commanded position, the 6250 essentially anticipates the velocity of the
commanded position and initiates a control signal ahead of time to more closely follow (track)
the commanded position.
