Practical use case: Hysteresis

Hysteresis is a control technique where two different thresholds are used - one for switching ON and one another for switching OFF. This prevents rapid toggling (chatter) when the input signal fluctuates around a single setpoint.

Common use cases are:

1. Temperature control (heaters, ovens)

2. Pressure systems (compressors)

3. Motor control with feedback signals

4. Tank level control (pump start/stop)

Why it’s needed?

Analog signals often fluctuate slightly. Without hysteresis:

• A value near a threshold (e.g., 50.0°C) may bounce between 49.9 and 50.1

• This causes outputs (like heaters, pumps, relays) to switch rapidly

• That leads to wear, instability, and poor control

  
  

A typical implementation looks like this:

FUNCTION_BLOCK "MAIN"
VERSION:1.0
VAR_INPUT
	input : LREAL;		// signal value
	low_limit : LREAL;	// lower threshold
	high_limit : LREAL;	// upper threshold
END_VAR
VAR_OUTPUT
	output : BOOL;		// ON/OFF 
END_VAR
BEGIN
IF #input < #low_limit THEN
	#output := true;
ELSIF #input > #high_limit THEN
     #output := false;
END_IF;
END_FUNCTION_BLOCK

Think of a home thermostat:

• Heater turns on when it’s too cold

• Turns off when it’s warm enough

• Doesn’t constantly toggle at one exact temperature

Design tips:

1. Choose hysteresis band wide enough to handle noise

2. A band too wide will affect system performance