National Instruments FP-3000 - Manual (Page 51)

Chapter 3 Example Applications
FieldPoint FP-3000 User Manual 3-16 www.ni.com
Download and Bring the Loop into Auto
1. To download this schedule to the device, select Configure»Download
Configuration, or click on the Download button. This establishes all
the linkages that you “wired” in the previous step. It also schedules the
PID function block, which has not yet been scheduled to execute.
A dialog box appears that enables the configuration to be downloaded.
Select the Clear Devices and Automatic Mode Handling
checkboxes, then click on the Download button. Go through the
download process, as described in the documentation that came with
your configurator. When the download is finished, click on Close.
2. Double-click on the PID block to open the block configuration
window.
3. Go to the MODE_BLK parameter on the Process tab of the PID block
you created and set the TARGET to Auto , or click on the Auto button
in the block configuration window.
4. Read the MODE_BLK parameter. The ACTUAL mode should read IMan ,
for Initialization Manual. This means the PID is not able to enter Auto
because the AO block is not in Cascade mode.
5. Go to the MODE_BLK parameter on the Process tab of the AO block you
created and set the TARGET to Cas and Auto (check both the Cas and
Auto boxes). This tells the AO to operate in Cascade if possible, and
otherwise, to fall back to Auto .
6. If the Periodic Updates checkbox is checked, the MODE_BLK.ACTUAL
parameter should change after a few seconds. Otherwise, re-read the
parameter by clicking on the Read Selected or Read All button with
MODE_BLK selected. The ACTUAL field should be Cas . If it does not,
refer to Appendix F, Troubleshooting , for more information.
Now, your loop should be running under automatic control. Verify this by
reading the
ACTUAL mode of the PID block. If it is Auto , the PID is trying
to control the temperature. You can change the desired temperature by
changing the Set Point (SP) parameter of the PID. Remember that the units
of SP are the same as
PV_SCALE for the PID, which in our example is
degrees Celsius.
Tune the PID
Adjust the PID tuning constants to match the dynamics of your temperature
process. A general description of tuning PID loops is beyond the scope of
this document. The parameters to change in the PID block are
GAIN ,
RESET , and RATE . You can adjust these constants, change the PID Set
Point, and watch how the temperature changes over time.