Process IDs - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 15.0

Characteristics that are common to numerous standard control loops are grouped together into process IDs. The process ID sets the initial tuning parameters and the expected time constant for the control loop. The initial system parameters include dead time, process range, and time constant. Prior to Release 5.1, or in certain upgrades where the PID pre-processor is not connected to the PID inputs of interval, proportional band, integral time, and saturation time, the following equations determine the initial values of proportional band and integral time from the values for dead time, process range, and time constant. At Release 5.1 and later, the proportional band and integral time values are determined by what is on their inputs.

CCT/PCT process IDs and associated standard tuning parameters have been selected based on typical mechanical system design criteria. When a mechanical system's design and performance is different from what was considered typical when the CCT/PCT standards were created, the PRAC+ adaptive tuning may not perform as expected. Sometimes the system can benefit from changing the process ID to a different ID that is more representative of the system. For example, a lab or operating room with a high air change rate may more closely resemble a wide space in a duct than it does a room, and may benefit from changing the process ID from Zone Temperature (ZN-T) to Discharge Air Temperature (DA-T). When you compare the performance characteristics defined by these two process IDs in Table 1, you can see that the DA-T is much quicker to detect changes (shorter dead time) and to detect most of the change much quicker (time constant). As a result, the values for saturation time and period are much shorter.