FB_Hysteresis (Function Block)

Implements a hysteresis controller with two thresholds and latching behaviour.

The output switches low when the input rises above UpperThreshold and switches high when the input falls below LowerThreshold. Between the two thresholds the output latches at its current state.

This convention suits heating applications, the output drives a heater that turns on when the process falls below the lower threshold and off when it exceeds the upper threshold.

Note

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

Extends: FB_SiComponent

Properties

LowerThreshold

Type: LREAL

Gets or sets the lower threshold.

When the input falls below this value the output switches high. Must be less than UpperThreshold for correct operation.

Output

Type: BOOL

Gets the current output state.

TRUE when the process is below LowerThreshold. FALSE when above UpperThreshold.

UpperThreshold

Type: LREAL

Gets or sets the upper threshold.

When the input rises above this value the output switches low. Must be greater than LowerThreshold for correct operation.

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)
`fUpperThreshold`	`LREAL`	Input level above which the output switches low.
`fLowerThreshold`	`LREAL`	Input level below which the output switches high.

Reset

Evaluates the current input against the thresholds and sets the output immediately, rather than waiting for the input to cross a threshold.

If the input is above UpperThreshold the output is set low. If below LowerThreshold the output is set high. If between the two thresholds the output defaults to low as a safe state.

Run

Evaluates the input against the thresholds and updates the output.