FB_PID (Function Block)

Ideal-form PID controller with clamped output, comparator-based anti-windup, and bumpless manual-to-auto transfer.

The control output is computed as:

\[u(t) = K_p \left( e(t) + \frac{1}{T_n} \int e(t)\, dt + T_v \frac{d(-y)}{dt} \right)\]

where \(e(t) = SP - PV\) is the error, \(-y\) is the negated process variable (derivative on measurement, avoiding derivative kick on setpoint steps).

In Manual mode the output tracks Setpoint directly through the output clamp. The integrator is continuously primed so that switching to Auto produces no jump in output.

Note

Set Maximum before Minimum to ensure the clamp guard applies correctly. The same applies to IntegratorBounds.

Use FB_init to set fKp, tTn, tTv and tTd, fMaximum, and fMinimum at declaration time.

Extends: FB_SoComponent

Properties

AntiWindupActive

Type: BOOL

Gets whether the integrator is currently saturated and integration is halted or disabled.

AntiWindupMode

Type: E_AntiWindupMode

Gets or sets the anti-windup mode for the integrator.

See E_AntiWindupMode for available modes.

DerivativeTerm

Type: LREAL

Gets the contribution of the derivative term to the output.

Enable

Type: BOOL

Gets or sets whether the controller is active.

When FALSE all internal components are reset and the output is held at zero. The controller resumes from a clean state when re-enabled. Defaults to TRUE.

Error

Type: LREAL

Gets the current error (Setpoint - Feedback, SP - PV).

Feedback

Type: LREAL

Gets or sets the process variable (PV).

Used to compute the error and the derivative on measurement.

IntegralTerm

Type: LREAL

Gets the contribution of the integral term to the output.

IntegratorBounds

Type: I_Bounded

Gets the integrator bounds interface.

Use this to set independent upper and lower limits on the integrator output, separate from the output clamp. This is useful when you want tighter integrator limits than the output clamp.

Kp

Type: LREAL

Gets or sets the proportional gain (Kp).

Scales the response to the current error. A higher value reacts more strongly to error but risks overshoot and oscillation. A value of 0 disables the P term.

Maximum

Type: LREAL

Gets or sets the upper output clamp.

Internally clamped to be greater than or equal to Minimum. Set this before Minimum to ensure the guard applies correctly.

Minimum

Type: LREAL

Gets or sets the lower output clamp.

Internally clamped to be less than or equal to Maximum. Set Maximum before this property to ensure the guard applies correctly.

Mode

Type: E_ControllerMode

Gets or sets the operating mode.

In Auto mode the output is computed from Setpoint and Feedback. In Manual mode the output tracks Setpoint directly through the output clamp. The integrator is continuously primed in manual mode so switching to auto produces no jump in output.

ProportionalTerm

Type: LREAL

Gets the contribution of the proportional term to the output.

Setpoint

Type: LREAL

Gets or sets the setpoint.

In Manual mode this is used as the manual output value, routed directly through the output clamp.

Td

Type: LTIME

Gets or sets the derivative damping time (Td).

Filters the derivative signal to reduce sensitivity to measurement noise. A longer value gives more filtering but weakens derivative action. LTIME#0 removes the filter entirely, not recommended when the measurement is noisy.

Tn

Type: LTIME

Gets or sets the integral action time (Tn).

Controls how quickly accumulated error is corrected. A shorter value eliminates steady-state error, faster but risks overshoot. A longer value is more stable but slower to correct. LTIME#0 disables the I term entirely.

Tv

Type: LTIME

Gets or sets the derivative action time (Tv).

Controls how strongly the controller reacts to changes in the process variable (Feedback). A longer value dampens sudden changes and reduces overshoot, but too much can cause instability. LTIME#0 disables the D term entirely.

Methods

FB_init

Inputs

Name	Type	Description
`bInitRetains`	`BOOL`	if TRUE, the retain variables are initialized (warm start / cold start)
`bInCopyCode`	`BOOL`	if TRUE, the instance afterwards gets moved into the copy code (online change)
`fKp`	`LREAL`	Proportional gain.
`tTn`	`LTIME`	Integral action time. LTIME#0 disables the I term.
`tTv`	`LTIME`	Rate time. LTIME#0 disables the D term.
`tTd`	`LTIME`	Derivative damping time.
`fMaximum`	`LREAL`	Upper output clamp.
`fMinimum`	`LREAL`	Lower output clamp.

Run

Advances the PID controller by one time step.

Note

Must be called once per cycle on a single PLC task.

SyncOutput

Sets the controller output to a given value on the next Run() call.

The integrator is back-calculated from the current P and D terms so the three terms add up to fValue. Use it to start from a known operating point instead of from zero.

Note

Does nothing if Tn is LTIME#0, or in Manual mode, where the output already follows Setpoint. If IntegratorBounds cuts the integrator short, the output stops below the target.

Inputs

Name	Type	Description
`fValue`	`LREAL`	Target controller output.

SyncTn

Presets the integrator state to match a given output value on the next Run() call.

Use this when switching from manual to automatic mode with a known starting output, or when initialising the controller to a steady-state value to avoid an initial transient.

Inputs

Name	Type	Description
`fValue`	`LREAL`	Target integrator state.