PID Function Block

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LabVIEW 2009 Real-Time Module Help

Edition Date: June 2009

Part Number: 370622G-01

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Owning Palette: Control Function Block

Requires: RT Module

PID algorithm for PID applications or high speed control applications that require an efficient algorithm. The PID algorithm features control output range limiting with integrator anti-windup and bumpless controller output for PID gain changes. This function block also features manual mode control with bumpless manual to automatic transitions, non-linear integral action, two degree of freedom control, and error-squared control.

You can use this function block to operate on one input value at a time.

Dialog Box Options

Block Diagram Inputs

Block Diagram Outputs

Dialog Box Options

Parameter	Description
Inputs	Contains the following configuration options: Terminal name—Contains the name of the block diagram terminal to configure. Visible?—Specifies whether to display the terminal on the block diagram. Variable scope—Specifies the availability and scope of the variable associated with the terminal. Select Single-process to use a single-process shared variable. Select Network-published to use a network-published shared variable. Select No Variable if you do not want to use a terminal variable. To minimize overhead, include function block terminal variables only when necessary. If you change the Variable scope to No Variable after creating the function block instance, LabVIEW deletes the terminal variable. Data source—Specifies the data source of the input. Select Terminal to wire the input value to the terminal on the block diagram. Select Variable to supply the input value to the function block through the associated terminal variable, if available. Select Default value to use the Default value. The Variable data source option is not available if you select No Variable under Variable Scope. If you select Terminal as the data source and leave the block diagram terminal unwired, LabVIEW uses the Default value. Default value—Specifies the default value of the input. If you select Variable as the data source, LabVIEW uses this value as the initial value of the variable. If you select Default value as the data source, LabVIEW uses this value for every iteration of the function block.
Outputs	Contains the following configuration options: Terminal name—Contains the name of the block diagram terminal to configure. Visible?—Specifies whether to display the terminal on the block diagram. Variable scope—Specifies the availability and scope of the variable associated with the terminal. Select Single-process to use a single-process shared variable. Select Network-published to use a network-published shared variable. Select No Variable if you do not want to use a terminal variable. To minimize overhead, include function block terminal variables only when necessary. If you change the Variable scope to No Variable after creating the function block instance, LabVIEW deletes the terminal variable.

Parameter

Description

Inputs

Contains the following configuration options:

Terminal name—Contains the name of the block diagram terminal to configure.
Visible?—Specifies whether to display the terminal on the block diagram.
Variable scope—Specifies the availability and scope of the variable associated with the terminal. Select Single-process to use a single-process shared variable. Select Network-published to use a network-published shared variable. Select No Variable if you do not want to use a terminal variable. To minimize overhead, include function block terminal variables only when necessary. If you change the Variable scope to No Variable after creating the function block instance, LabVIEW deletes the terminal variable.
Data source—Specifies the data source of the input. Select Terminal to wire the input value to the terminal on the block diagram. Select Variable to supply the input value to the function block through the associated terminal variable, if available. Select Default value to use the Default value. The Variable data source option is not available if you select No Variable under Variable Scope. If you select Terminal as the data source and leave the block diagram terminal unwired, LabVIEW uses the Default value.
Default value—Specifies the default value of the input. If you select Variable as the data source, LabVIEW uses this value as the initial value of the variable. If you select Default value as the data source, LabVIEW uses this value for every iteration of the function block.

Outputs

Contains the following configuration options:

Terminal name—Contains the name of the block diagram terminal to configure.
Visible?—Specifies whether to display the terminal on the block diagram.
Variable scope—Specifies the availability and scope of the variable associated with the terminal. Select Single-process to use a single-process shared variable. Select Network-published to use a network-published shared variable. Select No Variable if you do not want to use a terminal variable. To minimize overhead, include function block terminal variables only when necessary. If you change the Variable scope to No Variable after creating the function block instance, LabVIEW deletes the terminal variable.

Block Diagram Inputs

Parameter	Description
error in (no error)	Describes error conditions that occur before this node runs.
setpoint	Specifies the setpoint value of the process variable that you are controlling. This is the desired value for the process variable.
process variable	Specifies the current value of the variable that you are controlling.
Proportional Gain	Specifies the proportional gain (K_c) of the PID algorithm. The default value is 1.
integral time	Specifies the PID parameter (T_i) that adjusts the effect of the error integral term on the output.
derivative time	Specifies the PID parameter (T_d) that adjusts the effect of the error derivative term on the output.
auto? (T)	TRUE selects automatic control (default), FALSE selects manual mode. The function block uses bumpless transfer to transition from manual mode to automatic control.
setpoint high	Specifies the maximum value of the setpoint range.
setpoint low	Specifies the minimum value of the setpoint range.
dt (s)	Specifies the interval (in seconds) at which this function block is called. This value is used in PID calculations. If you specify a value less than or equal to zero, this function block calculates the value since last called using an internal timer with 1 millisecond resolution.
output high	Specifies the maximum value of output. This value is represented by a fixed-point number with the same fixed-point type as process variable.
reinitialize (F)	Specifies whether to reinitialize internal parameters, such as integrated error. Setting this input to TRUE sets internal parameters to default values of 0.0.
output low	Specifies the minimum value of output.
beta	Specifies the relative emphasis of disturbance rejection to setpoint tracking. The default value of 1 is appropriate for most applications. A smaller value between 0 and 1 can be used to specify emphasis on disturbance rejection (such as process load changes).
linearity	Specifies the linearity of the error response. The valid range of linearity values is 0 through 1.0. A linearity value of 1.0 generates a normal linear response, while a value of 0.1 generates an approximately parabolic (square law) response.
manual control	Specifies the value of output while in manual mode.

Block Diagram Outputs

Parameter	Description
error out	Contains error information. This output provides standard error out functionality.
output	Returns the control output of the PID algorithm that is applied to the controlled process.
dt out (s)	Returns the actual time interval in seconds. Indicates either the value specified for dt (s) or the computed interval if dt (s) was specified as -1.0.