User guide

34 6250 Servo Controller User Guide
# of Axes SSFR Servo Sampling Motion Trajectory Update
Active
(INDAX)
Command
Setting
Frequency
(samples/sec.)
Period
(µsec)
Frequency
(samples/sec.)
Period
(µsec)
1 SSFR1 5555 180 5555 180
1 SSFR2 6667 150 3333 300
1 SSFR4 8000 125 2000 500
1 SSFR8 10000 100 1250 800
2 SSFR1 2667 375 2667 375
2 SSFR2 3125 320 1563 640
Default → 2 SSFR4 3636 275 909 1100
2 SSFR8 4444 225 555 1800
The general rule to determining the proper SSFR value is to first select the slowest servo
sampling frequency that is able to give a satisfactory response. This can be done by experiment
or based on the closed-loop bandwidth requirement for your application. (Keep in mind that
increasing the SSFR value allows for higher bandwidths, but produces a rougher motion
profile; conversely, decreasing the SSFR value provides a smoother profile, but makes the
servo system less stable and slower to respond.)
As an example, if your application requires a closed-loop bandwidth of 350 Hz, you can
determine the minimum servo sampling frequency by using the rule of thumb—setting the
servo sampling frequency at least 8 times higher than the bandwidth frequency—the required
minimum servo sampling frequency would be 2800 Hz. If two axes are running, then you
should try using the SSFR2 setting.
The table below provides guidelines for various application requirements.
Application Requirement SSFR1 SSFR2 SSFR4 SSFR8
XY Linear Interpolation ✔✔
Fast point-to-point motion ✔✔
Regulation (speed, torque, etc.) ✔✔
High natural frequency system ✔
Setting the Sampling Frequency Ratio
Select a sampling ratio appropriate to your system now, before you proceed to tune each
gain. In Motion Architect's Controller Tuner module select the System menu from the
Setup pull-down menu and select the appropriate servo sampling (SSFR) value. (If you are
not using Motion Architect, enter the appropriate SSFR command.)
If you change the sampling frequency ratio (SSFR) after the tuning is complete and the new
servo sampling frequency is lower than the previous one, the response may change and you
may have to re-tune the system.
Step 3 Set up Motion Architect's data gathering functions:
a. Click on the Motion button in the Data Acquisition display to show the Motion
Profile Setup
dialog box. Select the Step profile, set the Distance to 100 steps,
select the Enable Motion box (✓ Enable Motion) for the axis you are tuning (make
sure the other axis is deselected), and click ✓ OK.
b. Click on the Capture button in the Data Acquisition display to show the Data
Capture Setup
dialog box. Select the axis you are currently tuning, select
Commanded Position, Actual Position, and Analog Servo Output (deselect all
other options), select Go Command as the trigger point that initiates data capture, and
click ✓OK.
c. Click on the Graph button in the Data Acquisition display to show the Graph Setup
dialog box. In both graph axis dialog boxes, select the axis you are currently tuning.
Select Graph 1 and use Commanded Position as the vertical axis and Time as the
horizontal axis. Select Graph 2 and use Actual Position as the vertical axis and Time
as the horizontal axis. (Make sure both Graph 1 and Graph 2 are enabled to be
displayed.) Click ✓OK.