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Control Loops
6.3
Section 6 — Motion Control Tutorial
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In any case, special consideration should be given when setting the accel-
eration. Though in most cases no harm will be done in setting a high
acceleration value, avoid doing so if the application does not require it. The
driver, motor, motion device and load undergo maximum stress during
high acceleration.
6.2.16 Combined Parameters
Very often a user looks at an application and concludes that he needs a
certain overall accuracy. This usually means that he is combining a number
of individual terms (error parameters) into a single one. Some combined
parameters even have their own name, even though not all people mean
the same thing by them: Absolute Accuracy, Bi-directional 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.
The best approach is to carefully study the application, extract from the
specification sheet the applicable discrete error parameters and combine
them (usually add them) to get the worst-case general error applicable to
the specific case. This method not only offers a more accurate value but
also gives a better understanding of the motion control system perfor-
mance and helps pinpoint problems.
Also, due to the integrated nature of the ESP6000 system, many basic
errors can be significantly corrected by another component of the loop.
Backlash, Accuracy and Velocity Regulation are just a few examples where
the controller can improve motion device performance.
When talking about motion control systems, one of the most important
questions is the type of servo loop implemented. The first major distinction
is between open and closed loops. Of course, this is of particular interest
when driving stepper motors. As far as the DC servo loops, the PID type is
by far the most widely used.
The ESP6000 controller card implements a PID servo loop with velocity
feed-forward for both DC and stepper-motor motion devices. It is not just a
static closed loop, when the motion is stopped, but a fully dynamic one.
The basic diagram of a servo loop is shown in Figure 6.3-1. Besides the com-
mand interpreter, the two main parts of a motion controller are the trajec-
tory generator and the servo controller. The first generates the desired
trajectory and the second one controls the motor to follow it as closely as
possible.
Command
Trajectory
Interpreter
Generator
Motion Controller
Servo
Driver
Controller
Figure 6.3-1— Servo Loop
Motor
Encoder
6 - 1 1
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