Newport ESP6000, Руководство пользователя

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The average velocity is low but the velocity ripple is very high. Depending
on the application, this may be acceptable or not. With increasing velocity,
the
ripple decreases and the velocity becomes smoother.
This example is even more true in the case of a stepper motor driven stage.
The typical
noise comes from a very fast transition from one step position
to another. The velocity ripple in that case is significantly higher.
In the case of a DC motor, adjusting the PID parameters to get a softer
response will reduce the velocity ripple but care must be taken not to
negatively affect other desirable motion characteristics.
6.2.14 Velocity Regulation
In some applications, for example scanning, it is important for the velocity
to be very constant. In reality, there are a number of factors besides the
controller that affect the velocity.
As described in the Minimum Velocity definition, the speed plays a signifi-
cant role in the amount of ripple generated, especially at low values.
Even if the controller does a perfect job by running with zero following
error, imperfections in the mechanics (friction variation, transmission
ripple, etc.) will generate some velocity ripple that can be translated to
Velocity Regulation problems.
Depending on the specific application, one motor technology can be
preferrable to another.
As far as the controller is concerned, the stepper motor version is the ideal
case for a good
average Velocity Regulation because the motor inherently
follows the desired trajectory precisely. The only problem is the ripple
caused by the actual stepping process.
The best a DC motor controller can do is to approach the stepper motor’s
performance in
average Velocity Regulation , but it has the advantage of
significantly reduced velocity ripple, inherently and through PID tuning. If
the DC motor driver implements a velocity-closed loop through the use of a
tachometer, the overall servo performance increases and one of the biggest
beneficiaries is the
Velocity Regulation . Usually only higher-end motion
control systems use this technology and the ESP6000 controller card is one
of them. Since having a real tachometer is very expensive and in some
cases close to impossible to implement, the ESP6000 controller card can
both use or
simulate a tachometer through special circuitry and obtain the
same result.
6.2.15 Maximum Acceleration
The Maximum Acceleration is a complex parameter that depends as much
on the motion control system as it does on application requirements. For
stepper motors, the main concern is not to loose steps (or synchronization)
during the acceleration. Besides the motor and driver performance, the
load inertia plays a significant role.
For DC motor systems the situation is different. If the size of the following error
is of no concern during the acceleration, high Maximum Acceleration values
can be entered. The motion device will move with the highest natural accel-
eration it can (determined by the motor, driver, load inertia, etc.) and the
errors will consist of just a temporary larger following error and a velocity
overshoot.
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