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Servo Tuning
33
Before you tune the 6250:
Be sure to complete the
Tuning Setup Procedure (and the Drive Tuning Procedure, if you are
using a velocity drive) before proceeding with the following tuning procedure. Unlike the
Tuning Setup Procedure, you must tune one axis at a time; therefore, you will have to repeat
Steps 3 through 7 below for the other axis.
S t e p 1
Launch Motion Architect's Controller Tuner Module:
To effectively tune the 6250, you should use Motion Architect's interactive tuning feature
(this is available only if you have the Servo Tuner option for Motion
Architect, part number 95-013714-01). It greatly improves your efficiency and gives
you powerful graphical tools to measure the performance of the system. The rest of this
tuning procedure is based primarily on using Motion Architect's Controller Tuner module.
Use the following steps to engage the Controller Tuner module:
a.
Launch Motion Architect.
b .
Under the
Product
pull-down menu, choose
Selection
, select the 6250 and click
O k a y
.
c.
Under the
Utilities
pull-down menu, choose
Controller Tuner
.
d.
Under the
Communication
pull-down menu, choose
Connect
to initiate the RS-232C
link with the 6250 over the COM port.
Tuning without Motion Architect®
If you do not use Motion Architect, use a computer (with a terminal emulator) or a dumb
terminal to enter the commands noted in these procedures. Without Motion Architect, the
only method of monitoring system performance is by visual inspection, or by using an analog
type position transducer (potentiometer, LVDT, RVDT, etc.) to pick up the load's or motor's
position displacement and monitoring the transducer output on a digital storage
oscilloscope.
S t e p 2
Select the 6250's sampling frequency ratios (
SSFR
):
The 6250's control signal is computed by the digital signal processor (DSP). The velocity of
the commanded position, the velocity of the encoder position, and the integral of the position
error are used for various control actions. These measurements are derived by the DSP from
the position values sampled periodically at a fixed rate; this sampling rate is called the servo
sampling frequency (samples/second).
Higher sampling frequencies improve the accuracy of the velocity and integral values derived.
A higher sampling frequency can also improve the tracking of a rapidly changing or oscillating
position. Therefore, the servo sampling frequency is a key parameter that influences the servo
system's stability and closed loop bandwidth.
In addition to computing the 6250's control signal, the DSP also computes the commanded
position trajectory. When the servo sampling frequency is increased, the motion trajectory
update rate has to be decreased, and vice versa. The ratio between the servo sampling frequency
and the trajectory update rate, called the sampling frequency ratio, depends on the requirements of
your application and/or the dynamic characteristics of the system. The Servo Sampling
Frequency Ratio (
SSFR
) command offers four selectable ratio settings. These four ratios and the
actual sampling frequencies and sampling periods (reciprocal of sampling frequency) are shown
below.
NOTE
Changing the active axes with the
INDAX
command will change the
SSFR
ratio.
