s7-SCL-scripts/SCL-low-pass-filter.md

FUNCTION "FC_LowPassFilter" : Void
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
   VAR_INPUT 
      analog_input : Real;           // Raw analog input signal
      alpha : Real := 0.8;           // Smoothing factor (0.0 to 1.0)
      enable : Bool := TRUE;         // Enable filter
      reset : Bool := FALSE;         // Reset filter output to current input
   END_VAR

   VAR_OUTPUT 
      low_passed_output : Real;      // Filtered output signal
   END_VAR

   VAR_IN_OUT 
   END_VAR

   VAR 
      prev_output : Real;            // Previous filtered output (retains value)
      prev_input : Real;             // Previous raw input
      initialized : Bool := FALSE;   // First-run flag
   END_VAR

   VAR_TEMP 
   END_VAR

BEGIN
	// Clamp alpha to valid range [0.0, 1.0]
	IF #alpha < 0.0 THEN
		#alpha := 0.0;
	ELSIF #alpha > 1.0 THEN
		#alpha := 1.0;
	END_IF;
	
	// Handle reset
	IF #reset THEN
		#low_passed_output := #analog_input;
		#prev_output := #analog_input;
		#prev_input := #analog_input;
		#initialized := TRUE;
		RETURN;
	END_IF;
	
	// Skip if disabled
	IF NOT #enable THEN
		#low_passed_output := #analog_input;
		RETURN;
	END_IF;
	
	// Initialize on first run to prevent startup transient
	IF NOT #initialized THEN
		#low_passed_output := #analog_input;
		#prev_output := #analog_input;
		#prev_input := #analog_input;
		#initialized := TRUE;
		RETURN;
	END_IF;
	
	// First-order IIR low-pass filter: Y(k) = alpha * Y(k-1) + (1 - alpha) * X(k-1)
	#low_passed_output := (#alpha * #prev_output) + ((1.0 - #alpha) * #prev_input);
	
	// Update previous values for next cycle
	#prev_output := #low_passed_output;
	#prev_input := #analog_input;
	
END_FUNCTION

Usage Example (in another block)

scl
// Call the filter with alpha = 0.9 (heavy smoothing)
"FC_LowPassFilter"(
    analog_input := "Raw_Pressure_Sensor",
    alpha := 0.9,
    enable := TRUE,
    reset := "Sensor_Fault",
    low_passed_output => "Filtered_Pressure"
);

Tuning Guidelines

Tuning Guidelines

Alpha	Behavior	Use Case
0.95 – 0.99	Very heavy smoothing, slow response	Very noisy signals, slow processes
0.80 – 0.90	Moderate smoothing	General purpose (default 0.8)
0.50 – 0.70	Light smoothing, responsive	Faster processes, moderate noise
0.10 – 0.40	Minimal smoothing, very responsive	Clean signals needing slight damping

Tip: For a given cutoff frequency f_c and sample time T_s, you can calculate alpha as:

\alpha = e^{-2\pi \cdot f_c \cdot T_s}