Newport MM4006, Руководство пользователя

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MM4006 M o t i o n Control Tutorial
4.2.14 Ve l o c i t y 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, specially at low values.
Even if the controller does a perfect job by running with zero following
error, imperfections in the mechanics (friction variation, transmission rip-
ple, etc.) will generate some velocity ripple that can be translated t o
Velocity Regulation problems.
Depending on the specific application, one motor technology can be pre-
ferred over the other.
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 precisely the desired trajectory. 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 sig-
nificantly 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 beneficiary is the Velocity Regulation. Usually only higher end
motion control systems use this technology and the MM4006 is one of
them. Since having a real tachometer is very expensive and in some cases
close to impossible to implement, the MM4006 can both use or simulate a
tachometer through special circuitry and obtains the same result.
4.2.15 M a x i m u m 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 m o t o r s , t h e m a i n c o n c e r n i s n o t t o l o o s e s t e p s ( o r
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 natur-
al acceleration it can (determined by the motor, driver, load inertia, etc.)
and the errors will he just a temporary larger following error and a velocity
overshoot.
In any case, special consideration should be given when setting the acceler-
ation. Though in most cases no harm will be done in setting a high acceler-
ation value, avoid doing so if the application does not require it. The
driver, motor, motion device and load undergo maximum stress during
high acceleration.
4.2.16 C o m b i n e d Parameters
Very often a user looks at an application and concludes that he needs a cer-
tain overall accuracy. This usually means that he is combining a number of
individual terms (error parameters) into a single one. Some of this com-
bined parameters even have their own name, even though not all people
mean t h e same t h i n g b y t h e m : A b s o l u t e A c c u r a c y, B i d i r e c t i o n a l
Repeatability, etc. The problem with these generalizations is that, unless
the term is well defined and the testing closely simulates the application,
the numbers could be of little value.
ED110181En1020 — 03/03
4.12 N e w p o r t .