➃
Servo Tuning
37
S t e p 5
Use the Integral Feedback Gain (
SGI
) to reduce steady state error:
☞
Steady state position
error is described
earlier in the
Performance
Measurements
section.
a.
Determine the steady state position error (the difference between the commanded position
and the actual encoder position). You can determine this error value by using Motion
Architect's
Graph
feature, or by issuing the
TPER
command when the motor is not
moving, or by viewing the
Motion
Display
(selected from the
View
pull-down menu).
NOTE
If the steady state position error is zero or so small that it is acceptable for your
application, you do not need to use the integral gain.
b .
If you have to enter the integral feedback gain to reduce the steady error, start out with a
small value (e.g.,
SGIØ.1
). After the gain is entered, observe two things from the
response:
❏
Whether or not the magnitude of steady state error reduces
❏
Whether or not the steady state error reduces to zero at a faster rate
c.
Keep increasing the gain to further improve these two measurements until the overshoot
starts to increase and the response becomes oscillatory.
d.
There are three things you can do at this point (If these three things do not work, that
means the integral gain is too high and you have to lower it.):
1
st
Lower the integral gain (
SGI
) value to reduce the overshoot.
2
nd
Check whether the 6250's analog output saturates the
±
10V limit; you can do this by
either using Motion Architect's data gathering feature (in the
Graph Setup
dialog
box, select
Analog Servo Output
versus
Time
for
Graph 1
and cancel the display
for
Graph 2
), or by observing the signal from a digital oscilloscope. If it saturates,
then lower the integral output limit by using the
SGILIM
command. This should
help reduce the overshoot and shorten the settling time. Sometimes, even if the
analog output is not saturated, you can still reduce the overshoot by lowering
SGILIM
to a value less than 10V. However, lowering it too much can impair the effectiveness
of the integral feedback.
3
rd
You can still increase the velocity feedback gain (
SGV
value) further, provided that it
is not already at the highest possible setting (causing the motor to chatter).
S t e p 6
Use the Velocity Feedforward Gain (
SGVF
) to reduce position error at constant speed:
If you are not using Motion Architect:
➀
Execute a continuous (
MC1
command) move, setting the acceleration, deceleration and
velocity values appropriate to your application. Set the
SGVF
value to be the product of
SGP
∗
SGV
(if
SGV
= zero, set
SGVF
equal to
SGP
).
➁
Check the position error at constant velocity by issuing the
TPER
command.
➂
Increase
SGVF
to reduce the position error (repeat steps
➀
and
➁
as necessary).
a.
In the
Graph Setup
dialog box (via the
Graph
button), set Graph 1 to display
commanded position versus time, and set Graph 2 to display actual encoder position
versus time.
Alternative setup if velocity error is critical to your application:
Set up the Graph feature to compare commanded velocity versus time and actual
velocity versus time, and set up the Capture feature to include commanded velocity.
b .
In the
Motion Profile Setup
dialog box (via the
Motion
button), select a trapezoidal
or s-curve profile and set the acceleration, deceleration and velocity to the values
appropriate to your application.
c.
In the
Tuning Gains
panel, set the initial value for
SGVF
as the product of
SGP
∗
SGV
. For example, if
SGP
=
1.2
and
SGV
=
0.5
, then
S G V F
=
1.2
∗
0.5
=
0.6
. If
S G V
= zero, then just set
S G V F
=
S G P
.
