Newport ESP6000, Руководство пользователя

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Section 6 — Motion Control Tutorial 6 - 1 5
There is another 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
amplifier or current (torque) driver. A special case is when the driver has
its own velocity feedback loop from a tachometer (Figure
6.3-7).
Trajectory
Generator
Motion Controller
Servo Controller
Driver
Motor
Tachometer
Encoder
Kp
Ki
∫
e
Kd
Kvff de
dt
e
Figure 6.3-7— Tachometer-Driven PIDF Loop
The tachometer is a device that outputs a voltage proportional with the
velocity. Using its signal, the driver can maintain a velocity proportional to
the control signal. If such a driver is used with a velocity feed-forward
algorithm, by properly tuning the
K
vff
parameter, the feed-forward signal
could perform an excellent job, leaving very little for the PID loop to do.
6.4 Motion Profiles
When talking about motion commands we refer to certain strings sent to a
motion controller that will initiate a certain action, usually a motion. There
are a number of common motion commands which are identified by name.
The following paragraphs describe a few of them.
6.4.1 Move
A move is a point-to-point motion. On execution of a move motion com-
mand, the stage moves from the current position to a desired destination.
The destination can be specified either as an absolute position or as a
relative distance from the current position.
When executing a move command, the stage will accelerate until the
velocity reaches a pre-defined value. Then, at the proper time, it will start
decelerating so that when the motor stops, the stage is at the correct
position. The velocity plot of this type of motion will have a trapezoidal
shape (Figure
6.4-1). For this reason, this type of motion is called a trapezoi-
dal motion .
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