FB_Differentiator (Function Block)

Differentiates the input signal with an optional first-order damping filter.

The transfer function is:

\[D(s) = \frac{T_v \cdot s}{T_d \cdot s + 1}\]

where \(T_v\) is Tv and \(T_d\) is Td. The filter is discretised using the Tustin (bilinear) method.

Setting Tv to LTIME#0 disables the derivative term entirely. Setting Td to LTIME#0 removes the damping filter, giving a pure derivative. This is not recommended in practice due to noise sensitivity.

Note

If derivative kick on setpoint steps is undesirable, pass the negated process variable as Input rather than the full error. The integrator should still receive the full error signal.

The block is FINAL and must be instantiated, not extended. Use FB_init to set tTv and tTd at declaration time.

Extends: FB_SisoComponent

Properties

Td

Type: LTIME

Gets or sets the damping time (Td).

This filters the derivative signal to reduce sensitivity to noise. A longer value gives more filtering but reduces derivative effectiveness. Setting it to LTIME#0 removes the filter entirely, giving a pure derivative (not recommended in practice due to noise sensitivity).

Tv

Type: LTIME

Gets or sets the rate time (Tv).

This controls how strongly the block reacts to changes in the input. A longer value increases derivative action. Setting it to LTIME#0 disables the derivative term entirely, holding the output at zero.

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)
`tTv`	`LTIME`	Rate time. T#0S disables the derivative term entirely.
`tTd`	`LTIME`	Damping time. LTIME#0 means no filtering (pure derivative).

Reset

Clears the filter state and output. Sets the previous input to the current input to avoid a spike on the next Run() call.

Run

Advances the differentiator by one time step.

On the first call the cycle time is measured, the filter state is initialised, and the method returns early without producing an output. This prevents a spike caused by an uninitialised previous input.

Note

Must be called once per cycle on a single PLC task. If Tv is LTIME#0 the output is held at zero.