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Rev. 1.0
0006302011
SSM23C User Manual
+86-400-820-9661
5.1.1 Gain: The Velocity Proportional Term (VP)
The simplest part of the Velocity Loop is the proportional,
or VP, term. The drive applies current to the motor in
direct proportion to the error. For example, if a motor is
not moving, and the shaft is turned by hand or some other
force, the drive will increase the motor current until the
motor returns to “0” speed. The faster the motor is moved from “0” velocity, the more the opposing
torque will increase. The VP term (also called VP gain) governs how much torque will be applied
for a given amount of velocity error (Vn). In general, more load inertia or load friction, requires
more torque and therefore a higher VP gain. The torque provided by the VP term is:
T = VP * Vn
5.1.2 IntegGain: The Velocity Integral Term (VI)
In the previous example, applying the VP term alone will not result in perfect velocity control. If
one ounce-inch of torque were applied to the motor, it would move at a slower speed. The VP term
will increase the motor torque until it is producing as much torque as the force attempting to move
it. The motor may slow down or even stop moving but there will still be error. The VI term adds up
all the error the velocity calculation has reported and produces a torque that is added to the torque
command from the VP term. The equation for this is:
T = VP * Vn + VI∑(V)
In the example, the VP term allowed the motor to reach equilibrium at a speed where the applied
torque equaled the torque of the VP term. Thus, the error was not zero. But the VI term continues
adding up the error and increasing the torque until the motor returns to the true target position.
5.1.3 FF Gain: Acceleration Feed-forward Term (KK)
Larger loads typically generate larger load Inertia. These larger inertias can be more easily
controlled by anticipating the system’s torque need. The Acceleration Feed-forward term does this
by adding an acceleration value to the Torque command. The acceleration value is derived from
the Trajectory Calculation during the acceleration and deceleration phase. As can be seen in the
equation below this increased Torque command is added with the VP and VI torque command
values:
T = KK * A + VP * Vn + VI∑(V)
5.1.4 PID Filter: Torque Command Filter Term (KC)
This final term in the Velocity control loop can be considered an over-all filter term. In fact this term
is always used even when the drive has been placed in the Torque Control Mode where only the
Current control loop is active. The filter is a very simple single-pole low pass filter that is used to
limit the high frequency response of the Velocity and therefore the Position control loops.
