381 lines
12 KiB
Markdown
381 lines
12 KiB
Markdown
# 📡 S7-1500 Advanced Signal Filtering Library
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### SCL Function Blocks for Siemens TIA Portal
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---
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## Overview
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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.
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Each filter uses a **circular buffer** architecture — no memory shifting per scan — keeping CPU load minimal and deterministic.
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---
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## Contents
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| FB Name | Filter Type | Best Use Case |
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|---|---|---|
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| `FB_MovingAverage` | Simple Moving Average (SMA) | General noise reduction |
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| `FB_EMA` | Exponential Moving Average | Fast response + smoothing |
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| `FB_WMA` | Weighted Moving Average | Emphasize recent samples |
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| `FB_MedianFilter` | Median / Order Statistics | Spike & glitch rejection |
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| `FB_RateLimiter` | Slew Rate Limiter | Actuator protection |
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| `FB_SignalFilter` | Unified wrapper (all above) | Single-block deployment |
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---
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## Requirements
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- **PLC:** Siemens S7-1500 (any CPU)
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- **TIA Portal:** V16 or later
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- **Language:** SCL (Structured Control Language)
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- **OB:** Can be called from any OB (OB1, cyclic OB, etc.)
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- **No special libraries required**
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---
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## Interface — Common Pattern
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All FBs follow the same interface convention:
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```pascal
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VAR_INPUT
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xEnable : BOOL; // Enable filtering; FALSE = passthrough + reset
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rInput : REAL; // Raw analog input value
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iWindowSize : INT; // Buffer/window size (filter-specific)
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END_VAR
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VAR_OUTPUT
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rOutput : REAL; // Filtered output value
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xReady : BOOL; // TRUE when buffer is full (SMA/WMA/Median)
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END_VAR
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```
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> **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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---
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## Filter Reference
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---
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### 1. `FB_MovingAverage` — Simple Moving Average (SMA)
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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.
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**Parameters:**
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| Parameter | Type | Default | Description |
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|---|---|---|---|
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| `xEnable` | BOOL | — | Enable filter |
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| `rInput` | REAL | — | Analog input |
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| `iWindowSize` | INT | 10 | Window size (1–100) |
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| `rOutput` | REAL | — | Filtered output |
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| `xReady` | BOOL | — | TRUE when buffer full |
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**Formula:**
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```
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Output(n) = [ Sum of last N samples ] / N
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```
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**Characteristics:**
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- Equal weight to all samples in window
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- Output lags behind input by N/2 samples
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- Larger window = more smoothing = more lag
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- Very low CPU load (no division per element)
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**Typical window sizes:**
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| Signal Type | Recommended Window |
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|---|---|
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| Thermocouple / temperature | 20–50 |
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| Pressure sensor | 5–15 |
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| Flow meter | 10–20 |
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| Current / power | 5–10 |
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---
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### 2. `FB_EMA` — Exponential Moving Average
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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.
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**Parameters:**
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| Parameter | Type | Default | Description |
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|---|---|---|---|
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| `xEnable` | BOOL | — | Enable filter |
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| `rInput` | REAL | — | Analog input |
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| `rAlpha` | REAL | 0.1 | Smoothing factor (0.0–1.0) |
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| `rOutput` | REAL | — | Filtered output |
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**Formula:**
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```
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Output(n) = α × Input(n) + (1 − α) × Output(n−1)
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```
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**Alpha selection guide:**
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| Alpha | Effect | Equivalent SMA Window |
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|---|---|---|
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| 0.05 | Heavy smoothing, slow response | ~39 samples |
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| 0.1 | Balanced (default) | ~19 samples |
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| 0.2 | Light smoothing, fast response | ~9 samples |
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| 0.5 | Minimal smoothing | ~3 samples |
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**Characteristics:**
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- Minimal memory footprint (no array)
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- Infinite impulse response — past samples never fully forgotten
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- Best choice when memory is constrained or window must be very large
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---
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### 3. `FB_WMA` — Weighted Moving Average
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Similar to SMA but assigns linearly increasing weights to more recent samples. The newest sample gets weight *N*, the oldest gets weight *1*.
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**Parameters:**
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| Parameter | Type | Default | Description |
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|---|---|---|---|
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| `xEnable` | BOOL | — | Enable filter |
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| `rInput` | REAL | — | Analog input |
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| `iWindowSize` | INT | 8 | Window size (1–16) |
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| `rOutput` | REAL | — | Filtered output |
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**Formula:**
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```
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Output(n) = Σ [ weight(i) × sample(i) ] / Σ weight(i)
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weight(i) = N for newest, 1 for oldest
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```
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**Characteristics:**
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- Responds faster to changes than SMA at same window size
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- More computationally intensive than SMA or EMA
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- Good compromise between lag and smoothing
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---
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### 4. `FB_MedianFilter` — Median / Order Statistics Filter
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Collects the last *N* samples, sorts them using insertion sort, and outputs the middle value. Completely immune to impulse spikes regardless of magnitude.
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**Parameters:**
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| Parameter | Type | Default | Description |
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|---|---|---|---|
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| `xEnable` | BOOL | — | Enable filter |
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| `rInput` | REAL | — | Analog input |
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| `iWindowSize` | INT | 7 | Window size, odd recommended (1–15) |
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| `rOutput` | REAL | — | Filtered output |
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**Characteristics:**
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- **Best** filter for rejecting sensor glitches, wire bounce, EMI spikes
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- Non-linear filter — does not distort signal shape between spikes
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- Use **odd** window sizes (3, 5, 7, 9...) for a clean median value
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- Higher CPU load than SMA/EMA due to in-place sort each scan
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**Recommended window sizes:**
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| Application | Window |
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|---|---|
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| Remove single-sample spikes | 3 |
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| General spike rejection | 5–7 |
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| Heavy transient environment | 9–11 |
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> **Tip:** Combine Median + EMA for best results: Median removes spikes, EMA smooths residual noise.
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---
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### 5. `FB_RateLimiter` — Slew Rate Limiter
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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.
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**Parameters:**
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| Parameter | Type | Default | Description |
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|---|---|---|---|
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| `xEnable` | BOOL | — | Enable filter |
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| `rInput` | REAL | — | Analog input |
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| `rMaxRiseRate` | REAL | 10.0 | Max units/second rising |
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| `rMaxFallRate` | REAL | 10.0 | Max units/second falling |
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| `rCycleTimeS` | REAL | 0.1 | OB cycle time in seconds |
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| `rOutput` | REAL | — | Rate-limited output |
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**Formula:**
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```
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MaxStep = MaxRate × CycleTime
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Output(n) = Output(n-1) + clamp(Input(n) − Output(n-1), −MaxStep, +MaxStep)
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```
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**Characteristics:**
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- Not a noise filter — purely a change-rate constraint
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- Asymmetric rise/fall rates supported (e.g. slow open, fast close)
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- Must be configured with correct `rCycleTimeS` to function correctly
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- Does not affect steady-state accuracy
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> **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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---
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### 6. `FB_SignalFilter` — Unified Wrapper
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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.
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**Parameters:**
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| Parameter | Type | Default | Description |
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|---|---|---|---|
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| `xEnable` | BOOL | — | Enable filter |
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| `rInput` | REAL | — | Analog input |
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| `iMode` | INT | 0 | Filter selection (see below) |
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| `iWindowSize` | INT | 10 | Window for SMA / WMA / Median |
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| `rAlpha` | REAL | 0.1 | Alpha for EMA |
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| `rMaxRate` | REAL | 10.0 | Rate for RateLimiter |
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| `rCycleTimeS` | REAL | 0.1 | Cycle time for RateLimiter |
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| `rOutput` | REAL | — | Filtered output |
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| `xReady` | BOOL | — | Buffer ready flag |
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**Mode selection:**
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| `iMode` | Filter |
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| 0 | Passthrough (no filtering) |
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| 1 | Simple Moving Average |
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| 2 | Exponential Moving Average |
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| 3 | Weighted Moving Average |
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| 4 | Median Filter |
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| 5 | Rate Limiter |
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---
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## Performance Comparison
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| Filter | Noise Reduction | Spike Rejection | Response Speed | CPU Load | Memory |
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| SMA | ★★★★☆ | ★★☆☆☆ | ★★☆☆☆ | Low | Medium |
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| EMA | ★★★☆☆ | ★★☆☆☆ | ★★★★☆ | Very Low | Minimal |
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| WMA | ★★★★☆ | ★★☆☆☆ | ★★★☆☆ | Medium | Medium |
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| Median | ★★★☆☆ | ★★★★★ | ★★★☆☆ | Medium-High | Medium |
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| Rate Limiter | ★☆☆☆☆ | ★★★☆☆ | ★★★☆☆ | Very Low | Minimal |
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---
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## Recommended Filter Combinations
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For best results in demanding environments, chain two filters:
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```
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Raw Signal → Median (spike removal) → EMA (noise smoothing) → Control loop
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```
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```pascal
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// Example: Median pre-filter + EMA smoothing
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fbMedian(xEnable := TRUE, rInput := rRawAnalog, iWindowSize := 5);
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fbEMA(xEnable := TRUE, rInput := fbMedian.rOutput, rAlpha := 0.15);
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rCleanSignal := fbEMA.rOutput;
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```
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---
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## Usage Example
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```pascal
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// Declarations
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VAR
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fbFilter : FB_SignalFilter;
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rRawAnalog : REAL; // From analog input card (e.g. scaled 0.0–100.0)
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rFiltered : REAL;
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END_VAR
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// In OB1 or cyclic FC
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fbFilter(
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xEnable := TRUE,
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rInput := rRawAnalog,
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iMode := 1, // 1 = SMA
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iWindowSize := 15,
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rCycleTimeS := 0.1 // 100ms OB cycle
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);
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rFiltered := fbFilter.rOutput;
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```
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---
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## Circular Buffer Architecture
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All array-based filters use a **circular (ring) buffer** pattern:
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```
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Buffer: [ 12.4 | 13.1 | 12.8 | 13.0 | 12.6 ]
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↑ Index pointer (oldest slot, next to overwrite)
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New sample arrives → overwrite oldest → advance index
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No shifting, O(1) write per scan
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```
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This ensures constant, predictable scan-time contribution regardless of window size.
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---
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## Signal Flow Diagram
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```
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Analog Input Card
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│
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▼
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┌─────────────┐
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│ Raw REAL │ e.g. 0.0 – 100.0 (engineering units, already scaled)
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└──────┬──────┘
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│
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▼
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┌─────────────────────────────────────┐
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│ FB_SignalFilter │
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│ │
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│ iMode=1 → FB_MovingAverage │
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│ iMode=2 → FB_EMA │
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│ iMode=3 → FB_WMA │
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│ iMode=4 → FB_MedianFilter │
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│ iMode=5 → FB_RateLimiter │
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└──────────────────┬──────────────────┘
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│
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▼
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rOutput (REAL)
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│
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┌─────────┴──────────┐
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▼ ▼
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PID Controller HMI / SCADA
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```
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---
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## Integration Notes
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- **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.
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- **First scan:** On the first scan after `xEnable` goes TRUE, output equals input. No startup spike.
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- **Mode switching:** Changing `iMode` at runtime resets the newly selected filter on its next call. A brief transient (1–2 scan cycles) may occur.
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- **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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---
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## Related Libraries
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This library pairs naturally with:
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- **`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.
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- **`FB_CUSUM`** *(planned)* — Cumulative sum control chart for slow drift detection
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- **`FB_KalmanFilter`** *(planned)* — State estimation and sensor validation
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---
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## License
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Internal use. Adapt freely for your project.
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Tested on: **S7-1500 CPU 1515-2 PN**, TIA Portal V18.
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---
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*Generated for S7-1500 SCL Signal Filtering Library — Advanced Process Control Series* |