FB_LADRC (Function Block)

Linear Active Disturbance Rejection Controller (LADRC) with integral action.

An alternative to PID control that actively estimates and cancels disturbances, unmodelled dynamics, and plant nonlinearities in real time using an Extended State Observer (ESO). The integral term eliminates steady-state offset.

Tuning procedure:

Estimate B0 from a step test by applying a known control output and measuring the initial rate of change of the process variable.
Set Wc to roughly the desired closed-loop bandwidth. For a process that should settle in 10 seconds, start with Wc = 0.5 rad/s.
Set Wo to 3–5 times Wc and increase if disturbance rejection is too slow.
By default AutoTn is FALSE, when it is TRUE, the integral action time is derived automatically as \(T_n = 1 / \omega_c^2\). Set AutoTn to FALSE and tune Tn manually if the automatic value causes overshoot or instability. LTIME#0 disables integral action entirely.

In Manual mode the output tracks Setpoint directly through the output clamp. The observer and integrator continue running in manual mode so 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.

Uses Forward Euler discretisation. For stability ensure Wo \* T < 1 where T is the task cycle time in seconds.

Use FB_init to set fWc, fWo, fB0, fMinimum, and fMaximum at declaration time.

Extends: FB_SoComponent

Properties

AutoTn

Type: BOOL

Gets or sets whether the integral action time is derived automatically.

When TRUE (default), Tn is computed as \(1 / \omega_c^2\). Set to FALSE to tune Tn manually if the automatic value causes overshoot or instability.

B0

Type: LREAL

Gets or sets the estimated plant gain (b0).

This is the ratio of how much the output changes per unit of control signal. For example, if a 10% heater output raises the temperature at 2°C/s, then b0 = 0.2. This does not need to be exact because the observer treats inaccuracies as disturbance and cancels them. Must be non-zero.

Sets both the observer and control law gain simultaneously.

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. Defaults to TRUE.

Feedback

Type: LREAL

Gets or sets the process variable (PV).

Used by the observer to estimate the process state and total disturbance.

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. Useful when the actuator range differs from the process range.

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 observer continues running in manual mode so switching to auto produces no jump in 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.

Tn

Type: LTIME

Gets or sets the integral action time (Tn).

Only used when AutoTn is FALSE. Controls how aggressively the integral term corrects steady-state offset. A shorter value eliminates offset faster but risks overshoot. LTIME#0 disables the integral term entirely.

Wc

Type: LREAL

Gets or sets the controller bandwidth in rad/s.

Controls the speed of the closed-loop response. A higher value gives a faster response but risks overshoot and noise sensitivity. For a process that should settle in roughly 10 seconds, start with Wc = 0.5 rad/s and increase gradually.

Wo

Type: LREAL

Gets or sets the observer bandwidth in rad/s.

Controls how quickly disturbances are estimated and rejected. Set to 3–5 times Wc. For stability ensure Wo \* T < 1 where T is the task cycle time in seconds. For a 10ms task, keep Wo below 100 rad/s.

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)
`fB0`	`LREAL`	Estimated plant gain.
`fWc`	`LREAL`	Controller bandwidth in rad/s.
`fWo`	`LREAL`	Observer bandwidth in rad/s. Typically 3–5 times Wc.
`fMaximum`	`LREAL`	Upper output clamp.
`fMinimum`	`LREAL`	Lower output clamp.

Run

Advances the ADRC by one time step.

Note

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