94 lines
2.7 KiB
Markdown
94 lines
2.7 KiB
Markdown
```
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FUNCTION "FC_LowPassFilter" : Void
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{ S7_Optimized_Access := 'TRUE' }
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VERSION : 0.1
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VAR_INPUT
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analog_input : Real; // Raw analog input signal
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alpha : Real := 0.8; // Smoothing factor (0.0 to 1.0)
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enable : Bool := TRUE; // Enable filter
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reset : Bool := FALSE; // Reset filter output to current input
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END_VAR
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VAR_OUTPUT
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low_passed_output : Real; // Filtered output signal
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END_VAR
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VAR_IN_OUT
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END_VAR
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VAR
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prev_output : Real; // Previous filtered output (retains value)
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prev_input : Real; // Previous raw input
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initialized : Bool := FALSE; // First-run flag
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END_VAR
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VAR_TEMP
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END_VAR
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BEGIN
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// Clamp alpha to valid range [0.0, 1.0]
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IF #alpha < 0.0 THEN
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#alpha := 0.0;
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ELSIF #alpha > 1.0 THEN
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#alpha := 1.0;
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END_IF;
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// Handle reset
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IF #reset THEN
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#low_passed_output := #analog_input;
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#prev_output := #analog_input;
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#prev_input := #analog_input;
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#initialized := TRUE;
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RETURN;
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END_IF;
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// Skip if disabled
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IF NOT #enable THEN
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#low_passed_output := #analog_input;
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RETURN;
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END_IF;
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// Initialize on first run to prevent startup transient
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IF NOT #initialized THEN
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#low_passed_output := #analog_input;
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#prev_output := #analog_input;
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#prev_input := #analog_input;
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#initialized := TRUE;
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RETURN;
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END_IF;
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// First-order IIR low-pass filter: Y(k) = alpha * Y(k-1) + (1 - alpha) * X(k-1)
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#low_passed_output := (#alpha * #prev_output) + ((1.0 - #alpha) * #prev_input);
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// Update previous values for next cycle
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#prev_output := #low_passed_output;
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#prev_input := #analog_input;
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END_FUNCTION
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```
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# Usage Example (in another block)
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```
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scl
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// Call the filter with alpha = 0.9 (heavy smoothing)
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"FC_LowPassFilter"(
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analog_input := "Raw_Pressure_Sensor",
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alpha := 0.9,
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enable := TRUE,
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reset := "Sensor_Fault",
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low_passed_output => "Filtered_Pressure"
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);
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```
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## Tuning Guidelines
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### Tuning Guidelines
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| Alpha | Behavior | Use Case |
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|-------------|-----------------------------------|---------------------------------------|
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| 0.95 – 0.99 | Very heavy smoothing, slow response | Very noisy signals, slow processes |
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| 0.80 – 0.90 | Moderate smoothing | General purpose (default 0.8) |
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| 0.50 – 0.70 | Light smoothing, responsive | Faster processes, moderate noise |
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| 0.10 – 0.40 | Minimal smoothing, very responsive | Clean signals needing slight damping |
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> **Tip:** For a given cutoff frequency $f_c$ and sample time $T_s$, you can calculate alpha as:
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> $$\alpha = e^{-2\pi \cdot f_c \cdot T_s}$$
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