➃
Servo Tuning
35
S t e p 4
Optimize the Proportional (
SGP
) and Velocity (
SGV
) gains (
see illustration for tuning process):
If you are not using Motion Architect:
➀
Enter the following commands to create a step input profile
(use a comma in the first data
field when tuning axis 2—e.g.,
D,5Ø
):
Co mma n d
De s c r i p t i o n
>
A999
Set acceleration to 999 revs/sec
2
>
AD999
Set deceleration to 999 revs/sec
2
>
V3Ø
Set velocity to 30 revs/sec
>
D1ØØ
Set distance to 100 steps
➁
Enter the
SGPØ.5
command as a starting point for tuning (this is the Motion Architect's
default SGP starting point).
Motion Architect Automatic Gain Selection Option
As an alternative to Steps 4.a. through 4.e. below, you may use Motion Architect's
Automatic Gain Selection feature to automatically select
SGP
and
SGV
gains that should
greatly shorten the iterative nature of the controller tuning process.
To use this feature, select the Tune... menu item to display the Automatic Gain
Selection dialog box. For each axis, provide the following information:
➀
Under Control Law, select PV.
➁
Under Drive Type, select either Velocity or Torque.
➂
Enter the drive data:
❏
For a velocity drive, fill in the Drive Scale Factor and Step Response data. If
you select the Get Data button, these data fields will be filled in with the data
captured from the last drive tuning session.
❏
For a torque drive, fill in the Total Inertia and Motor/Drive Constants data.
➃
Click the
✓
OK button. Notice that the data fields in the Tuning Gains display show
the new gain settings.
a.
In the
Data Acquisition
display, select the
Start
button to trigger the step input
move and gather data. (If you are not using Motion Architect, enter
GO1
or
GO,1
depending on which axis is being tuned at the time.)
b .
Observe the plot of the commanded position versus the actual position in the
Graph
Display
area (or on the oscilloscope). If the response is already very oscillatory, lower
the gain (
SGP
); if it is sluggish (overdamped), increase the
SGP
gain. Repeat Steps 4.a.
and 4.b. until the response is slightly under-damped.
c.
In the
Tuning Gains
panel, set the initial
SGV
value to
0.1
(or enter the
SGVØ.1
command if you are not using Motion Architect).
d.
As you did in Step 4.a., press the
Start
button (or enter
GO1
or
GO,1
).
e.
Observe the plot in the
Graph Display
area (or on the oscilloscope). If the response is
sluggish (overdamped), reduce the
SGV
gain. Repeat Steps 4.d. and 4.e. until the
response is slightly under-damped.
☞
Refer to the Tuning
Scenario
section
later in this chapter
for a case example.
f.
The flow diagram below shows you how to get the values of the proportional and velocity
feedback gains for the fastest, well-damped response in a step-by-step fashion. The tuning
principle here is based on these four characteristics:
❏
Increasing the proportional gain (
SGP
) can speed up the response time and increase the
damping.
❏
Increasing the velocity feedback gain (
SGV
) can increase the damping more so than the
proportional gain can, but also may slow down the response time.
❏
When the
SGP
gain is too high, it can cause instability.
❏
When the
SGV
gain is too high, it can cause the motor to chatter.
