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PID
PID.gif

Description

The PID label stands for Proportional, Integral, and Derivative (3-mode) control action. This block is part of the Loop Blocks category.

Function

Provides Proportional (P, Integral (I and Derivative (D, (3-mode) control action based on the deviation or error signal created by the difference between the setpoint (SP) and the Process Variable analog input value (PV).

It provides two digital output signals for alarms based on configured parameters.

The PID function block provides for Feedforward, Cascade, and Ratio control.

Automatic tuning with Fuzzy Logic Overshoot Suppression can be configured.

Digital inputs may be used to set control mode, select the setpoint source, change control action plus other discrete actions.

Inputs

PV = Process Variable Analog Input value in Engineering Units to be controlled

RSP = Remote Setpoint Analog Input value in Engineering Units or Percent to provide external setpoint

FFV = Feedforward value in percent. The Feedforward value is multiplied by the Feedforward Gain, then directly summed into the output of the PID block

TRV = Output Track value in Percentage (PID Output = TRV Input when TRC = ON.)

TRC = Output Track Command [ON, OFF (On -Enables TRV.) (Mode = Local Override)

BIAS = Remote Bias value for Ratio PID

SWI = Switch Inputs (from SWO on LPSW function block)
 0 = No Change
 1 = Initiate Autotuning
 2 = Change Control Action
 4 = Force Bumpless Transfer
 8 = Switch to Tune Set 1
 16 = Switch to Tune Set 2

MDRQI = External Mode request (typically connected to the MDRQO output of a MDSW function block that encoded discrete switch inputs).

 0 = No Change
 1 = Manual Mode Request
 2 = Auto Mode Request
 4 = Local Mode Request
 8 = Remote Mode Request

BCI = Back Calculation Input (for blocks used as Cascade Primary

Outputs

OUT = Control Output

WSP = Working Setpoint in Engineering Units for monitoring

AL1 = Alarm 1 - Digital Signal

AL2 = Alarm 2 - Digital Signal

DIRECT = Direct Acting control (ON = Direct, OFF = Reverse Acting)

ATI = Autotune Indicator (ON = Autotune in Progress)

MODE = Loop mode status (typically connected to the Mode Flags block for encoding). Value indicates modes as follows:
 0.0 RSP AUTO
 1.0 RSP MAN
 2.0 RSP Initialization Manual (See Note 1)
 3.0 RSP Local Override (See Note 1)
 4.0 LSP AUTO
 5.0 LSP MAN
 6.0 LSP Initialization Manual (See Note 1)
 7.0 LSP Local Override (See Note 1)

BCO - Back Calculation Output (for blocks used as Cascade Secondary)—See Note 2.

NOTE 1.
When a request to change from Auto to manual is received and:
 - the request comes from the operator Interface, the request is ignored.

 - the request comes from the Mode Switch (MDSW) function block, the request is retained
 and when leaving the Initialization Mode or Local Override Mode the loop will go to manual.

NOTE 2.
BCO output is provided for applications where the block is used as a cascade secondary.
BCI input is provided for applications where the block is used as a cascade primary. When the BCO output of a secondary loop is connected to the BCI input of a primary loop, bumpless transfer is achieved when the secondary is switched into remote setpoint (i.e., cascade) mode. In addition, the primary loop is prevented from reset windup when the secondary is de-coupled from the process. The secondary is de-coupled from the process when it is in local setpoint mode or manual output mode or has reached a setpoint or output limit or is integral limiting because of its BCI input.

 

Operation details

The PV Hi/Lo range values configured in the PID-Range/Limit Tab determine the points at which the block status changes to a fail condition, driving the output to the configured failsafe value. There is no dead band for these PID block limits. To prevent the loop from going to failsafe, the user can adjust the PV Hi/Lo settings to allow for slight variations of the PV value from an AI channel that operates at or near these limits. Additionally, if the PV value exceeds the configured limits, the PID block will indicate a PV out of range status and will cause the bad block pin of the system monitor block to energize.

When the control mode is switched from Manual to Automatic, the mode switchover is bumpless and the PID loop's integration time is set to zero. Control Action is then determined by the control loop configuration and tuning.

In version 4.X controller firmware, the system default is set to cause a manual mode to override the Track command; the user has the option to change this setting in HC Designer to allow the Track command to override the Manual mode output. This action is a master setting and cannot be configured per loop. Loop Mode Priority

When the output of a PID loop is driven to the Hi or Lo Output limit, the integral value is clamped to prevent reset wind up.

 

Configurable Parameters

The PID properties dialog box is divided into 7 tab cards

Click on the tab name below to access the Configurable Parameters for that tab.

GENERAL 
START/RESTART 
RSP 
RANGE/LIMIT 
TUNING 
ACCUTUNEIII 
ALARMS

 

Other Control Blocks

CARB - Carbon Potential

TPSC - Three Position Step

ONOFF -

AMB - Auto/Manual Bias

 

Associated Blocks

Setpoint Programmer Blocks

Setpoint Scheduler Blocks
