Newport MM4006, Руководство пользователя

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MM4006 M o t i o n Control Tutorial
4.3.2 F e e d -Forward Loops
As described in the previous paragraph, the main driving force in a PID
loop is the proportional term. The other two correct static and dynamic
errors associated with the closed loop.
Taking a closer look at the desired and actual motion parameters and at
the characteristics of the DC motors, some interesting observations can be
made. For a constant load, the velocity of a DC motor is approximately pro-
portional with the voltage. This means that for a trapezoidal velocity pro-
file, for instance, the motor voltage will have also a trapezoidal shape (Fig.
4.17).
Desired Velocity
Time
Fig. 4.17 — Trapezoidal Velocity Profile.
The second observation is that the desired velocity is calculated by the tra-
jectory generator and is known ahead of time. The obvious conclusion is
that we could take this velocity information, scale it by a Kvff factor and
feed it to the motor driver. If the scaling is done properly, the right amount
of voltage is sent to the motor to get the desired velocities, without the
need for a closed loop. Because the signal is derived from the velocity pro-
file and it is being sent directly to motor driver, the procedure is called
velocity feed-forward.
Of course, this looks like an open loop, and it is (Fig. 4.18). But, adding this
signal to the closed loop has the effect of significantly reducing the "work"
the PID has to do, thus reducing the overall following error. The PID now
has to correct only for the residual error left over by the feed-forward sig-
nal.
Servo Controller
Trajectory
Generator
x
Motion Controller
•
xKp
xK
•
de
• , x Kddt
Fig. 4.18 — PID Loop with Feed-Forward.
Encoder •
There is an other special note that has to be made about the feed-forward
method. The velocity is approximately proportional to the voltage and only
for constant loads. but this is true only if the driver is a simple voltage
ED1-10181E01020 — 03/03
4.16
Newport.