Chapter 3 Example Applications©National Instruments Corporation3-15FieldPoint FP-3000 User Manual4. Read the MODE_BLK parameter. The ACTUAL mode should read IMan,for Initialization Manual. This means the PID is not able to enter Autobecause the AO block is not in Cascade mode.5. Go to the AO block, and look at its MODE_BLK.6. Set the TARGET to Cas and Auto (check both the Cascade and Autoboxes). This tells the AO to operate in Cascade if possible, andotherwise, to fall back to Auto.7. Re-read the MODE_BLK parameter. ACTUAL should be Cas. If this doesnot happen, refer to Appendix C, Fieldbus Parameters, for moreinformation.Now, your loop should be running under automatic control. Verify this byreading the ACTUAL mode of the PID block. If it is Auto, the PID is tryingto control the temperature. You can change the desired temperature bychanging the Set Point (SP) parameter of the PID. Remember that the unitsof SP are the same as PV_SCALE for the PID, which in our example isdegrees Celsius.Tune the PIDAdjust the PID tuning constants to match the dynamics of your temperatureprocess. A general description of how to tune a PID is beyond the scope ofthis document. However, the parameters to change in the PID block areGAIN, RESET, and RATE, and for temperature control, which is often fairlyslow, the RATE parameter need not be used. You can adjust these constants,change the PID Set Point, and watch how the temperature changesover time.AlarmingIn the above example, it might be convenient to have FP-3000 generate analarm whenever the temperature goes above 40° C. This behavior can beconfigured from the PID block or the AI block. This example uses the PIDblock.To set a high limit alarm, follow these steps:1. Open the PID block and locate the HI_PRI parameter. This is thepriority of the high limit alarm.2. Set the HI_PRI parameter to 2. Fieldbus alarms can range in priorityfrom 0-15, with 0 being disabled and 1 meaning that the alarm isdetected but not reported. All other priorities cause the alarm to bereported.
