Add SCL-low-pass-filter.md

SCL-low-pass-filter.md

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+```
+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:  
+> $$\alpha = e^{-2\pi \cdot f_c \cdot T_s}$$