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➃

Servo Tuning

Step 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.):

Lower the integral gain (SGI) value to reduce the overshoot.

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.

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).

Step 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

SGVF = 1.2 ∗ 0.5 = 0.6. If SGV = zero, then just set SGVF = SGP.