PID Inputs, Outputs, and Properties - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 16.0

Controller Tool Help

Brand
Johnson Controls
Metasys
Product name
Controller Configuration Tool
Document type
User Guide
Document number
LIT-12011147
Version
16.0
Revision date
2023-10-02
Language
English

Table 1 provides a description of the inputs, outputs, and properties of the PID in CCT.

Table 1. Inputs, Outputs, and Properties

Parameter

Description

Process Variable

The value from the area that is being controlled; the Process Variable typically originates at a sensor and after some signal processing is passed to the PID.

Setpoint

The value to which the user expects the PID to control the Process Variable.

Minimum Tune Band

The region around Setpoint within which PRAC+ does not attempt to tune. The PID Pre-Processor and the PMAC Pre-Processor calculate the Minimum Tune Band based on the outputs being controlled. The value calculated is equal to 0.04 * Process Range for proportional outputs and is larger for staged outputs. For custom modules that may manually configure this value, yet keep PRAC+ active (Adaptive Tuning = True and Manual Tuning = False), the minimum allowed value is restricted to 0.01 * Process Range to prevent PRAC+ from tuning on insignificant fluctuations in the noise of the Process Variable.

Startup Value

In case of a switch from one control strategy to another, the PID in the target control strategy uses this parameter to get the latest value of the output, Present Value. This is required to ensure a bumpless transition from one control strategy to another, as long as the time constant is less than 600 seconds. If integration is being used, this value is used to set the Present Value at startup and at restart.
Note: This value is evaluated each time the PID is enabled. Once the PID is operational, this value is not calculated until the next time the PID is enabled.

Low Limit

The minimum value the PID Present Value can assume. The Low Limit must be less than or equal to the High Limit. If the Low Limit is greater than the High Limit, the reliability is set to Input Out of Range. The logic sets the Present Value to the Low Limit and otherwise behaves the same as if the limits are equal.

High Limit

The maximum value the PID Present Value can assume. If the High Limit is set to a value less than the Low Limit, the reliability is set to Input Out of Range and the logic continues to use the last reliable pair of values.

Offset

A value added to the sum of the proportional term, integral term, and derivative term used to generate the Present Value. The Present Value equals the Offset when the proportional, integral, and derivative terms are equal to zero. Offset has no effect for PI or PID control (its effect is negated by bumpless transfer to the Startup Value).

Direct Acting

Signals the action of the PID, where True = Direct Acting and False = Reverse Acting. This output determines a negative or positive error.

Adaptive Tuning

This input determines whether Adaptive Tuning is allowed to operate. When set to False, Adaptive Tuning is disabled. When set to True and the Manual Tuning property is False, PRAC+ is enabled, which may result in runtime changes to the Eff Proportional Band and Eff Integral Time.

Process Range

The magnitude of the range that the Process Variable traverses as the Present Value of the PID 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, Process Range is set to 30.

Time Constant

The time required for a first-order system to reach 63% of its final magnitude given a 100% step change (also known as tau). The input is used to turn bumpless transfer on and off.

Note: 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, this input is used to calculate the initial tuning setup when the Manual Tuning property is False.

Process Dead Time

The time required for the process to begin to reflect the results of a step change. See Delay Time Dominant Systems for information on how dead time can affect PRAC+ performance.

Present Value

The current output of the PID. Its value is typically passed to an actuator to provide repositioning information or as the setpoint to another PID when using cascaded control loops. The Present Value equals the sum of the proportional, integral, derivative, and offset terms.

Error

The difference between the Setpoint and Process Variable (offset by DB/2 if the Eff Deadband is not zero). The Error parameter is updated each execution.

Saturation Status

Shows the current operational status of the PID, based on the output and the amount of time that the output is at the High or Low Limits. The Saturation Status is set to Normal the first execution after the PID is Enabled (or Re-Enabled). Saturation Status is Normal for at least one execution whenever returning from a non-Normal state. For example, if Saturation Status is Timing High and Present Value becomes equal to Low Limit, Saturation Status is Normal for one execution and then transition the next execution to Timing Low.

Note: The Enum Set for Saturation Status contains the Overridden state even though the PID never sets it to that value. The Overridden state is included for use by the standard applications.
Absolute Effort EWMA

An EWMA value of 100 * ABS (Current Present Value - Last Present Value) / (High Limit - Low Limit). This parameter shows the average percent by which the actuator moves on each execution of the PID. See Exponentially Weighted Moving Averages (EWMAs).

Absolute Error EWMA

An EWMA value of absolute value of (Setpoint - Process Variable). This parameter shows the average absolute control error. See Exponentially Weighted Moving Averages (EWMAs).

Error EWMA

An EWMA value of difference (Setpoint - Process Variable). This parameter shows the average control error. A negative value indicates the process variable is above the setpoint. See Exponentially Weighted Moving Averages (EWMAs).

Present Value EWMA

An EWMA of the current Present Value scaled to percent based on the relation Percent Present Value = 100 * (Present Value - Low Limit) / (High Limit - Low Limit). This parameter shows the average output to the actuator. See Exponentially Weighted Moving Averages (EWMAs).

Execution Count

The number of executions performed by the PID. The execution count is incremented by 1 every execution and rolled over to zero after reaching the maximum floating-point value.

PRAC Status

Indicates what the PRAC+ calculation did during its most recent execution. See PRAC Status Output.

Eff Proportional Band

The value the PID is currently using for the proportional tuning value. This value can be updated by PRAC+ during execution.

Eff Integral Time

The value the PID is currently using for the integral tuning value. This value can be updated by PRAC+ during execution.

Manual Tuning

This input allows the user to specify whether to use the process-based tuning calculations described previously or the manual tuning properties. When True, the Setpoint Differential is used to decide when a large setpoint change occurs and the Eff Proportional Band, Eff Integral Time, Eff Derivative Time, Eff Saturation Time, and Eff Period are set based on the corresponding configurable properties. When False, these tuning parameters and some additional parameters required by PRAC+ (Eff Min/Max Proportional Band) are calculated based on the Process Range, Time Constant, and Process Dead Time. PRAC+ is disabled when Manual Tuning is True.

Proportional Band

Used to set the initial Eff Proportional Band. Proportional Band is the amount of change in the Process Variable that produces a full range change (High Limit - Low Limit) in the Present Value. This parameter cannot be a negative number, and it must be greater than zero (the action is set using the Direct-Acting input rather than by the sign of the Proportional Band). Changing this value causes the Eff Proportional Band to be updated, and PRAC+ is reinitialized.

Integral Time

This input is used to set the initial Eff Integral Time. It is the time required by the integral action to contribute the same amount to the controller output as the proportional term for a constant error. The Integral Time must be zero (to disable integral action) or must be between 2 and 30 times the Interval for adaptive tuning. (If manual turning, the Integral Time can be much larger.) This rule is to ensure the numerical stability of the trapezoidal integration that is used to represent the integration. Integration is inherently numerically stable, so the minor variation of actual period between executions does not require a factor of safety (unlike the derivative time). PRAC+ follows the same constraints in its automatic adjustment of the Eff Integral Time. If the value is set outside the range, the object returns a write error indicating the value is out of range. Changing this value causes the Eff Proportional Band to be updated, and PRAC+ is reinitialized.

Derivative Time

When Manual Tuning is set to True, this input is used to set the Eff Derivative Time during runtime. The Derivative Time must be either zero (to disable derivative action) or must be greater than 10 times the Interval. This setting is necessary because the derivative term is calculated including a first-order finite-difference filter. In order for the filter to be stable, the Derivative Time must be greater than the time constant chosen for the filter. The filter time constant chosen for the PID is 8.0 * Actual Period, but the limit is set for 10 times to give a factor of safety for stability in cases where the Actual Period is greater than the Interval. If the value is set outside the range, the object returns a write error indicating the value is out of range.

Setpoint Differential

When Manual Tuning is set to True, this property is used to determine when a setpoint change is large enough for immediate execution.

Saturation Time

This input is used to set the Eff Saturation Time. Saturation time is the duration the Present Value must meet or exceed either the Low or High Limit for the saturation status to be set to either Low or High. The Eff Saturation Time is updated whenever this input changes.

Deadband

This input is used to set the Eff Deadband. This parameter is a band centered around the Setpoint where the error is considered zero (+/- DB/2). The Eff Deadband is updated whenever this input changes.

Interval

This input is used to set the Eff Period. Interval is the time period between normal PID algorithm executions.

Tuning Updated

A Boolean flag used to indicate that PRAC+ has updated the tuning parameters.

PRAC Prop Band

This property is used to store the last PRAC+ update to Eff Proportional Band.

PRAC Integral Time

This property is used to store the last PRAC+ update to Eff Integral Time.

Eff Derivative Time

The value the PID is using for the derivative tuning value. When Manual Tuning is False, Eff Derivative Time is set to zero. Otherwise, it is set to the value of the configurable Derivative Time input.

Eff Saturation Time

The value the PID is using for the saturation time limit; see Saturation Time for more information. This is set to the Saturation Time input whenever the Saturation Time input changes.

Eff Deadband

The value the PID is using for the Deadband. This parameter is set to the value of the Deadband input whenever the Deadband input changes.

Eff Period

The value the PID uses for the period.

Actual Period

The actual instantaneous period of execution, measured as the actual time between successive executions of the PID algorithm. The Actual Period is not computed after the first periodic execution (startup, restart, enable after disable), in which case it is assigned the value of Eff Period.