The PID Pre-Processor logic block provides a way to look up key process data information for the PID block. If controlling a proportional device, the outputs of the PID Pre-Processor are connected to the PID directly.
See the Cooling Proportional Valve v51, Heating Proportional Valve v51, Cooling Staged Outputs v51, and Heating Staged Outputs v51 sections for examples of how this block is used. For more in depth information on PID function, see PID PRAC Commissioning Overview.
The following table describes the inputs and outputs of the PID Pre-Processor logic block. Also, see the Process IDs Predefined Parameters in Appendix: Process IDs.
The PID Pre-Processor block provides a reliability value of Input Out of Range.
|
Name |
Data Type |
Exposed for Connection by Default |
Description |
Default Value |
|---|---|---|---|---|
|
Inputs |
||||
|
Integral Time |
Float |
No |
Specifies the time required by the PID’s integral action to contribute the same amount to the controller output as the proportional term for a constant error. This value is used when Standard Tuning is False. |
729 Seconds |
|
Interval |
Float |
No |
Specifies the period that the process should run when Standard Tuning is False. |
60 Seconds |
|
Process Dead Time |
Float |
No |
Specifies the time required for the process to begin to reflect the results of a step change. |
72 Seconds |
|
Process ID 1 |
Enum |
No |
Specifies the process being controlled. For each Process Identifier, only a certain set of units is allowed (see Appendix: Process IDs). If the Process Identifier is set to a value that does align with the current Units, the block’s Reliability is set to Input Out of Range and the logic continues to use the last reliable value. However, if Standard Tuning is False, the Process ID and Units inputs are ignored, the block is Reliable, and the outputs are calculated based on the current values of the Process Time Constant, Process Dead Time, and Process Range inputs. If a process ID is not available for the particular process being controlled, then choose a process ID with a similar dynamic response and units. |
ZN-T |
|
Process Range |
Float |
No |
Indicates the range that the PID Process Variable traverses as the PID Present Value varies between its minimum and maximum values. For example, when the Process Variable ranges between 60 and 90 degrees due to the PID Present Value varying from its Low Limit to its High Limit, then Process Range is set to 30 (|60 - 90|). |
22, 40 |
|
Proportional Band |
Float |
No |
Specifies the amount of change in the Process Variable that produces a full range change (High Limit - Low Limit) in the Present Value. This value may not be a negative number. This value is used when Standard Tuning is False. |
16 |
|
Standard Tuning |
Boolean |
No |
Indicates whether to use standard process data or to directly pass the process data inputs to the corresponding outputs. If the Standard Tuning input is False or the Process Identifier is Other, the block passes the Time Constant, Process Dead Time, Process Range, Proportional Band, Integral Time, Saturation Time, and Interval directly to their corresponding effective outputs. |
True |
|
Saturation Time |
Float |
No |
Specifies time required for the process to saturate (high or low) when the output of the PID reaches its High Limit or its Low Limit, when Standard Tuning is False. |
600 Seconds |
|
Time Constant |
Float |
No |
Indicates the time required for a system to reach 63% of its final magnitude given a 100% step change. |
720 Seconds |
|
Units |
Enum |
No |
Specifies the units used for the process being controlled. For each Process Identifier, only a certain set of units is allowed (see Appendix: Process IDs). If the Process Identifier is set to a value that does align with the current Units, the block’s Reliability is set to Input Out of Range and the logic continues to use the last reliable value. However, if Standard Tuning is False, the Process ID and Units inputs are ignored, the block is Reliable, and the outputs are calculated based on the current values of the Process Time Constant, Process Dead Time, and Process Range inputs. |
°C, °F |
|
Outputs |
||||
|
Eff Integral Time |
Float |
Yes |
Indicates the integral time determined by the block to pass to the PID. |
729 Seconds |
|
Eff Period |
Float |
Yes |
Indicates the interval/period determined by the block to pass to the PID. |
60 Seconds |
|
Eff Process Dead Time |
Float |
Yes |
Indicates the dead time determined by the block to pass to the PID. |
72 Seconds |
|
Eff Process Range |
Float |
Yes |
Indicates the process range determined by the block to pass to the PID. |
22, 40 |
|
Eff Proportional Band |
Float |
Yes |
Indicates the proportional band determined by the block to pass to the PID. |
16 |
|
Eff Saturation Time |
Float |
Yes |
Indicates the saturation time determined by the block to pass to the PID. |
600 Seconds |
|
Eff Time Constant |
Float |
Yes |
Indicates the time constant determined by the block to pass to the PID. |
720 Seconds |
|
Minimum Tune Band |
Float |
Yes |
Indicates the region around the PID setpoint that the PRAC+ does not attempt to tune. |
0.88, 1.6 |