86
APEX615n Installation Guide
Step 6
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 position). You can determine this error value by the TPER
command when the load is not moving.
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. For hydraulic
applications, it is usually best to use a small SGI value, or use SGIØ while moving and
use SGIn when stopped. The use of the Target Zone Settling Mode (STRGTE) is
recommended.
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.
☞
If you are using current control,
convert the offset from
milliamps to volts and enter the
result in the
SGILIM
command.
2
nd
Check whether the 615n's analog output saturates the ±10V limit; you can do this 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 the maximum output
value. 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 or valve to chatter).
Step 7
Use the Velocity Feedforward Gain (SGVF) to reduce position error at
constant speed:
a. 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).
b. Check the position error at constant velocity by issuing the TPER command.
c. Increase SGVF to reduce the position error (repeat steps a and b as necessary).
Step 8
Use the Acceleration Feedforward Gain (SGAF) to reduce position error
during acceleration:
a. Execute a continuous (MC1 command) move, setting the acceleration, deceleration and
velocity values appropriate to your application. Set SGAF to 0.01 (SGAFØ.Ø1).
b. Check the position error during acceleration by issuing the TPER command.
c. Increase SGAF to reduce the position error (repeat steps a and b as necessary).
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