📡 S7-1500 Advanced Signal Filtering Library

SCL Function Blocks for Siemens TIA Portal

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Overview

This library provides a collection of advanced signal filtering Function Blocks (FBs) written in Structured Control Language (SCL) for the Siemens S7-1500 PLC platform. All blocks share a consistent interface and are designed for real-time analog signal processing in industrial automation environments.

Each filter uses a circular buffer architecture — no memory shifting per scan — keeping CPU load minimal and deterministic.

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Contents

FB Name	Filter Type	Best Use Case
FB_MovingAverage	Simple Moving Average (SMA)	General noise reduction
FB_EMA	Exponential Moving Average	Fast response + smoothing
FB_WMA	Weighted Moving Average	Emphasize recent samples
FB_MedianFilter	Median / Order Statistics	Spike & glitch rejection
FB_RateLimiter	Slew Rate Limiter	Actuator protection
FB_SignalFilter	Unified wrapper (all above)	Single-block deployment

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Requirements

• PLC: Siemens S7-1500 (any CPU)
• TIA Portal: V16 or later
• Language: SCL (Structured Control Language)
• OB: Can be called from any OB (OB1, cyclic OB, etc.)
• No special libraries required

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Interface — Common Pattern

All FBs follow the same interface convention:

VAR_INPUT
    xEnable     : BOOL;     // Enable filtering; FALSE = passthrough + reset
    rInput      : REAL;     // Raw analog input value
    iWindowSize : INT;      // Buffer/window size (filter-specific)
END_VAR

VAR_OUTPUT
    rOutput     : REAL;     // Filtered output value
    xReady      : BOOL;     // TRUE when buffer is full (SMA/WMA/Median)
END_VAR

Note: When xEnable = FALSE, each FB resets its internal state and passes rInput directly to rOutput. This allows safe runtime switching between filter modes without output jumps.

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Filter Reference

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1. FB_MovingAverage — Simple Moving Average (SMA)

Computes the arithmetic mean of the last N samples using a circular buffer and a running sum. No per-scan array iteration — the sum is updated incrementally.

Parameters:

Parameter	Type	Default	Description
xEnable	BOOL	—	Enable filter
rInput	REAL	—	Analog input
iWindowSize	INT	10	Window size (1–100)
rOutput	REAL	—	Filtered output
xReady	BOOL	—	TRUE when buffer full

Formula:

Output(n) = [ Sum of last N samples ] / N

Characteristics:

• Equal weight to all samples in window
• Output lags behind input by N/2 samples
• Larger window = more smoothing = more lag
• Very low CPU load (no division per element)

Typical window sizes:

Signal Type	Recommended Window
Thermocouple / temperature	20–50
Pressure sensor	5–15
Flow meter	10–20
Current / power	5–10

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2. FB_EMA — Exponential Moving Average

Applies exponentially decreasing weights to past samples. No buffer required — only the previous output value is stored. Ideal when fast response with smoothing is needed.

Parameters:

Parameter	Type	Default	Description
xEnable	BOOL	—	Enable filter
rInput	REAL	—	Analog input
rAlpha	REAL	0.1	Smoothing factor (0.0–1.0)
rOutput	REAL	—	Filtered output

Formula:

Output(n) = α × Input(n) + (1 − α) × Output(n−1)

Alpha selection guide:

Alpha	Effect	Equivalent SMA Window
0.05	Heavy smoothing, slow response	~39 samples
0.1	Balanced (default)	~19 samples
0.2	Light smoothing, fast response	~9 samples
0.5	Minimal smoothing	~3 samples

Characteristics:

• Minimal memory footprint (no array)
• Infinite impulse response — past samples never fully forgotten
• Best choice when memory is constrained or window must be very large

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3. FB_WMA — Weighted Moving Average

Similar to SMA but assigns linearly increasing weights to more recent samples. The newest sample gets weight N, the oldest gets weight 1.

Parameters:

Parameter	Type	Default	Description
xEnable	BOOL	—	Enable filter
rInput	REAL	—	Analog input
iWindowSize	INT	8	Window size (1–16)
rOutput	REAL	—	Filtered output

Formula:

Output(n) = Σ [ weight(i) × sample(i) ] / Σ weight(i)
weight(i) = N for newest, 1 for oldest

Characteristics:

• Responds faster to changes than SMA at same window size
• More computationally intensive than SMA or EMA
• Good compromise between lag and smoothing

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4. FB_MedianFilter — Median / Order Statistics Filter

Collects the last N samples, sorts them using insertion sort, and outputs the middle value. Completely immune to impulse spikes regardless of magnitude.

Parameters:

Parameter	Type	Default	Description
xEnable	BOOL	—	Enable filter
rInput	REAL	—	Analog input
iWindowSize	INT	7	Window size, odd recommended (1–15)
rOutput	REAL	—	Filtered output

Characteristics:

• Best filter for rejecting sensor glitches, wire bounce, EMI spikes
• Non-linear filter — does not distort signal shape between spikes
• Use odd window sizes (3, 5, 7, 9...) for a clean median value
• Higher CPU load than SMA/EMA due to in-place sort each scan

Recommended window sizes:

Application	Window
Remove single-sample spikes	3
General spike rejection	5–7
Heavy transient environment	9–11

Tip: Combine Median + EMA for best results: Median removes spikes, EMA smooths residual noise.

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5. FB_RateLimiter — Slew Rate Limiter

Limits the maximum rate of change (rise and fall) of a signal per scan cycle. The output tracks the input but cannot change faster than the configured rate. Protects actuators from sudden setpoint jumps.

Parameters:

Parameter	Type	Default	Description
xEnable	BOOL	—	Enable filter
rInput	REAL	—	Analog input
rMaxRiseRate	REAL	10.0	Max units/second rising
rMaxFallRate	REAL	10.0	Max units/second falling
rCycleTimeS	REAL	0.1	OB cycle time in seconds
rOutput	REAL	—	Rate-limited output

Formula:

MaxStep = MaxRate × CycleTime
Output(n) = Output(n-1) + clamp(Input(n) − Output(n-1), −MaxStep, +MaxStep)

Characteristics:

• Not a noise filter — purely a change-rate constraint
• Asymmetric rise/fall rates supported (e.g. slow open, fast close)
• Must be configured with correct rCycleTimeS to function correctly
• Does not affect steady-state accuracy

Important: Set rCycleTimeS to match your OB cycle time. For OB1 at 100ms, use 0.1. You can read this from OB_CYCL.CYCLE_TIME if needed.

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6. FB_SignalFilter — Unified Wrapper

A single FB that encapsulates all five filters. Select the active filter at runtime via iMode. Internally instantiates all FBs; only the selected one processes each scan.

Parameters:

Parameter	Type	Default	Description
xEnable	BOOL	—	Enable filter
rInput	REAL	—	Analog input
iMode	INT	0	Filter selection (see below)
iWindowSize	INT	10	Window for SMA / WMA / Median
rAlpha	REAL	0.1	Alpha for EMA
rMaxRate	REAL	10.0	Rate for RateLimiter
rCycleTimeS	REAL	0.1	Cycle time for RateLimiter
rOutput	REAL	—	Filtered output
xReady	BOOL	—	Buffer ready flag

Mode selection:

iMode	Filter
0	Passthrough (no filtering)
1	Simple Moving Average
2	Exponential Moving Average
3	Weighted Moving Average
4	Median Filter
5	Rate Limiter

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Performance Comparison

Filter	Noise Reduction	Spike Rejection	Response Speed	CPU Load	Memory
SMA	★★★★☆	★★☆☆☆	★★☆☆☆	Low	Medium
EMA	★★★☆☆	★★☆☆☆	★★★★☆	Very Low	Minimal
WMA	★★★★☆	★★☆☆☆	★★★☆☆	Medium	Medium
Median	★★★☆☆	★★★★★	★★★☆☆	Medium-High	Medium
Rate Limiter	★☆☆☆☆	★★★☆☆	★★★☆☆	Very Low	Minimal

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Recommended Filter Combinations

For best results in demanding environments, chain two filters:

Raw Signal → Median (spike removal) → EMA (noise smoothing) → Control loop

// Example: Median pre-filter + EMA smoothing
fbMedian(xEnable := TRUE, rInput := rRawAnalog, iWindowSize := 5);
fbEMA(xEnable := TRUE, rInput := fbMedian.rOutput, rAlpha := 0.15);
rCleanSignal := fbEMA.rOutput;

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Usage Example

// Declarations
VAR
    fbFilter     : FB_SignalFilter;
    rRawAnalog   : REAL;    // From analog input card (e.g. scaled 0.0–100.0)
    rFiltered    : REAL;
END_VAR

// In OB1 or cyclic FC
fbFilter(
    xEnable      := TRUE,
    rInput       := rRawAnalog,
    iMode        := 1,          // 1 = SMA
    iWindowSize  := 15,
    rCycleTimeS  := 0.1         // 100ms OB cycle
);

rFiltered := fbFilter.rOutput;

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Circular Buffer Architecture

All array-based filters use a circular (ring) buffer pattern:

Buffer: [ 12.4 | 13.1 | 12.8 | 13.0 | 12.6 ]
                  ↑ Index pointer (oldest slot, next to overwrite)

New sample arrives → overwrite oldest → advance index
No shifting, O(1) write per scan

This ensures constant, predictable scan-time contribution regardless of window size.

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Signal Flow Diagram

Analog Input Card
      │
      ▼
 ┌─────────────┐
 │  Raw REAL   │  e.g. 0.0 – 100.0 (engineering units, already scaled)
 └──────┬──────┘
        │
        ▼
 ┌─────────────────────────────────────┐
 │         FB_SignalFilter             │
 │                                     │
 │  iMode=1 → FB_MovingAverage         │
 │  iMode=2 → FB_EMA                   │
 │  iMode=3 → FB_WMA                   │
 │  iMode=4 → FB_MedianFilter          │
 │  iMode=5 → FB_RateLimiter           │
 └──────────────────┬──────────────────┘
                    │
                    ▼
             rOutput (REAL)
                    │
          ┌─────────┴──────────┐
          ▼                    ▼
     PID Controller        HMI / SCADA

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Integration Notes

• Analog scaling: All FBs operate on pre-scaled REAL values in engineering units (°C, bar, %, m³/h, etc.). Scale your analog input before passing to any filter FB.
• First scan: On the first scan after xEnable goes TRUE, output equals input. No startup spike.
• Mode switching: Changing iMode at runtime resets the newly selected filter on its next call. A brief transient (1–2 scan cycles) may occur.
• Cycle time: Only FB_RateLimiter is cycle-time sensitive. All other filters are cycle-time agnostic — window size implicitly defines the time constant based on your OB period.

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Related Libraries

This library pairs naturally with:

• FB_StateAlarmManager — State-based alarm management with bitmask suppression, hysteresis, on-delay, and priority. Feed filtered signals into alarm manager for dramatically fewer nuisance alarms.
• FB_CUSUM (planned) — Cumulative sum control chart for slow drift detection
• FB_KalmanFilter (planned) — State estimation and sensor validation

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License

Internal use. Adapt freely for your project.
Tested on: S7-1500 CPU 1515-2 PN, TIA Portal V18.

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Generated for S7-1500 SCL Signal Filtering Library — Advanced Process Control Series