S7-1200 Peak Detection: Finding Max Values from Analog Sensors

Capturing the maximum value reached by an analog sensor on a SIMATIC S7-1200 PLC is a recurring requirement in machine building: an accelerometer measuring peak shock on a press, a load cell capturing spike forces on a stamping die, a pressure sensor recording water hammer, or a vibration sensor monitoring bearing health. This reference covers three engineering-grade approaches - native MAX instruction, the LGF SearchMinMax function block, and a slope-based transient peak detector FB - together with the analog scaling, filtering, and cycle-time decisions that determine whether the recorded peak is real or noise.

1. Overview

An S7-1200 CPU (firmware V4.2 or higher for full SCL feature set, V4.4+ recommended) running TIA Portal V17 or V18 exposes three building blocks for peak detection:

• MAX / MIN instructions in the "Basic instructions > Comparator operations" folder - scalar comparators that return the larger of two REAL or INT values per scan.
• LGF_SearchMinMax from the official Siemens Library of General Functions (LGF) for S7-1200/S7-1500 - a parameterizable FB that scans an ARRAY[*] OF REAL and returns the minimum, maximum, and the array index where each occurred.
• Custom FB using slope (first derivative) analysis - a three-sample window detects the apex of a transient that may last only a few milliseconds, which a single-scan MAX may miss entirely.

The choice between them is driven by the signal class: slowly varying process values (level, temperature, position) suit MAX; windowed/buffered measurement histories suit LGF_SearchMinMax; impulsive events (acceleration, pressure spikes, strain bursts) require a slope detector plus a fast cyclic interrupt OB.

2. Prerequisites

3. Analog Input Signal Conditioning on the S7-1200

The SM 1231 module presents the raw input as an INT in the process image. The standard range is:

Convert raw to engineering units using the dedicated SCALE_X and NORM_X instructions in TIA Portal under Basic instructions > Converter operations:

// SCL - call SCALE_X in a function block
#EngineeringValue := SCALE_X(
    MIN   := 0,                // Engineering low  (e.g. 0.0 g)
    MAX   := 50.0,             // Engineering high (e.g. 50.0 g)
    VALUE := NORM_X(
        MIN := 0,              // Raw low  = 0
        MAX := 27648,          // Raw high = 27648
        VALUE := %IW64         // SM 1231 channel 0
    )
);

4. Approach 1 - Native MAX / MIN Instructions

The simplest peak latcher is a feedback loop: compare the current scaled value with a retained maximum and overwrite the retained value whenever the current value is higher. Because the retained value must survive across scan cycles, the logic must live in a function block (FB) with a static PeakValue variable - never in an FC.

FUNCTION_BLOCK "FB_PeakLatch_R"
{ S7_Optimized_Access := 'TRUE' }
VAR
    PeakValue : REAL := 0.0;          // Static, retained
    SampleCnt : DINT;                 // Number of samples examined
END_VAR
VAR_INPUT
    Reset     : BOOL;                 // TRUE clears PeakValue
    Enable    : BOOL;                 // FALSE freezes the latch
    Input     : REAL;                 // Scaled engineering value
END_VAR
BEGIN
    IF #Reset THEN
        #PeakValue := 0.0;
        #SampleCnt := 0;
    ELSIF #Enable THEN
        IF #Input > #PeakValue THEN
            #PeakValue := #Input;
        END_IF;
        #SampleCnt := #SampleCnt + 1;
    END_IF;
END_FUNCTION_BLOCK

Call the FB in OB1 (or in a cyclic interrupt OB - see Section 7):

// OB1 - cyclic call from main program
"FB_PeakLatch_R_DB"(
    Reset  := "HMI".ResetPeak OR "StartupFirstScan",
    Enable := TRUE,
    Input  := "ScaleAccel".Output   // scaled g value
);
"HMI".PeakValue := "FB_PeakLatch_R_DB".PeakValue;

Limitations of pure MAX latching:

• No time stamp of the peak - you know the value but not when it occurred.
• If the scan period is slower than the transient (e.g., 50 ms OB1 scanning a 5 ms pulse), the peak may be missed entirely between samples.
• No valley / minimum recording - extend the FB symmetrically if needed.

5. Approach 2 - LGF SearchMinMax Function Block

The Siemens LGF provides LGF_SearchMinMax (V3.0.0+), a parameterizable FB that scans an array of samples in one call and returns the minimum, maximum, and their indices. This is the correct choice when you want the peak over a defined measurement window (e.g., the last 1000 samples, or the samples taken between Start and Stop inputs).

Import the LGF into TIA Portal via Options > Global libraries > Open library, then drag the FB into your project. The interface of LGF_SearchMinMax is:

Typical use - capture the peak in a 1-second window at 100 Hz (100 samples):

// Cyclic OB30 every 10 ms, with 100-element ring buffer
"LGF_SearchMinMax_DB"(
    execute    := "Trigger".OneSecondTick,   // 1 Hz pulse
    arrayOfReal := "DataLog".Buffer,         // 100 x REAL ring buffer
    mode       := 2,                          // 0=min, 1=max, 2=both
    minValue   => "HMI".WindowMin,
    maxValue   => "HMI".WindowMax,
    minIndex   => "HMI".WindowMinIdx,
    maxIndex   => "HMI".WindowMaxIdx,
    error      => "Diag".LgfErr,
    status     => "Diag".LgfStatus
);

Advantages over a single MAX instruction:

• Returns index, so a peak can be correlated with the time stamp stored in a parallel array.
• Detects both extremes from one scan, useful for closed-loop valves that need to know max and min over a stroke.
• Battle-tested FB with documented status codes from the LGF manual.

6. Approach 3 - Slope-Based Transient Peak Detection

For impulsive events - the peak g of a stamping press, the water-hammer spike in a hydraulic line, the impact force of a tool engaging a workpiece - a single comparator misses the apex unless the scan period is shorter than the event itself. The robust solution is a three-sample slope detector that flags a peak when the signal was rising on the previous sample and falling on the current one.

FUNCTION_BLOCK "FB_PeakDetector_Transient"
{ S7_Optimized_Access := 'TRUE' }
VAR
    x_n0 : REAL;    // current sample
    x_n1 : REAL;    // previous sample
    x_n2 : REAL;    // pre-previous sample
    ts   : DINT;    // peak time stamp (ms since PLC start)
END_VAR
VAR_INPUT
    Input        : REAL;     // scaled engineering value
    Threshold    : REAL := 0.001;  // min rise/fall slope
    Enable       : BOOL;
    Reset        : BOOL;
END_VAR
VAR_OUTPUT
    PeakDetected : BOOL;
    PeakValue    : REAL;
    PeakTime_ms  : DINT;
END_VAR
VAR_TEMP
    slopeUp   : BOOL;
    slopeDown : BOOL;
END_VAR
BEGIN
    IF #Reset THEN
        #PeakValue := 0.0;
        #PeakDetected := FALSE;
        #x_n0 := #Input;
        #x_n1 := #Input;
        #x_n2 := #Input;
        RETURN;
    END_IF;

    IF NOT #Enable THEN RETURN; END_IF;

    // Shift history
    #x_n2 := #x_n1;
    #x_n1 := #x_n0;
    #x_n0 := #Input;

    // Rising then falling = peak
    #slopeUp   := (#x_n0 - #x_n1) >  #Threshold;
    #slopeDown := (#x_n1 - #x_n2) >  #Threshold;

    IF #slopeUp AND #slopeDown THEN
        #PeakValue    := #x_n1;          // apex is the middle sample
        #PeakTime_ms  := TIME_TCK();     // system tick in ms
        #PeakDetected := TRUE;            // sticky flag for HMI
    END_IF;
END_FUNCTION_BLOCK

Why three samples? Two samples tell you the slope direction now; the third sample lets you know what the slope was previously. The combination up then down is a local maximum. Setting Threshold above the noise floor (use 3-5× the standard deviation of the resting signal) prevents the detector from firing on quantisation noise. TIME_TCK() returns a 100 ns tick; divide by 10,000 to convert to milliseconds.

7. Cycle Time and Sampling Considerations

The S7-1200 default scan in OB1 is 10-50 ms. A 5 ms accelerometer pulse will be aliased or missed. To sample fast, move the peak logic into a cyclic interrupt OB:

For a 1 kHz vibration sample, set the OB30 time to 1 ms. The S7-1200 hardware filters the analog input to ~50 Hz on most SM 1231 channels, so a 1 kHz event is heavily attenuated - a true 1 kHz capture requires an external sample-and-hold or a third-party IEPE digitiser. For press-impact peaks of ~10-50 ms duration, a 1 ms OB30 is more than sufficient.

8. Filtering the Sensor Signal

Three filter strategies are commonly used before peak detection, each available as either a Siemens standard instruction or a hand-rolled SCL block:

8.1 Moving Average (N-tap FIR)

// 8-tap moving average ring buffer
IF #Index > 7 THEN #Index := 0; END_IF;
#Sum := #Sum - #Buf[#Index] + #Input;
#Buf[#Index] := #Input;
#Index := #Index + 1;
#Average := #Sum / 8.0;

An 8-tap moving average at 1 ms OB30 gives a flat passband to ~125 Hz with linear phase - ideal for a press-impact peak that lasts 10-50 ms.

8.2 PT1 First-Order Low-Pass

Siemens provides the standard CTRL_PT1 instruction, or use LGF_Filter_PT1 from the LGF library. Cutoff frequency f_c is set by:

f_c = 1 / (2π · T_const)

For a 10 Hz cutoff: T_const = 0.0159 s.

8.3 Exponential Smoothing

#Filtered := #Alpha * #Input + (1.0 - #Alpha) * #Filtered;

With α = 0.1-0.3, exponential smoothing is computationally trivial and effective for slowly varying DC levels. It is not a true low-pass filter (no defined cutoff), but its single-coefficient footprint makes it the default on memory-constrained S7-1200 CPUs.

9. FB vs FC - Choosing the Right Block Type

A function (FC) is stateless; an function block (FB) carries a static data block (DB) that retains its values between calls. The peak-detection logic must remember the previous peak value, so it cannot live in an FC unless the previous value is passed in as an IN_OUT parameter from a global variable or another FB's static memory.

If you convert an FB to an FC because you only want a stateless function (e.g., to read the peak out of a buffer), declare the previous peak value as VAR_IN_OUT instead of VAR_OUTPUT. The compiler will not permit a VAR (static) inside an FC.

10. HMI Display and Data Logging

Typical WinCC / HMI tag mapping for a peak detection application:

For archiving, use the DataLog function on a S7-1200 with a 4 GB SD card (max 32 GB supported). Open a CSV log per shift, write the peak value, timestamp, and sample count on every OB30 tick. Retention: 2-3 months at 1 s logging is feasible on a 4 GB card.

11. Verification and Commissioning

1. Force a known input. Apply a 5.000 V reference (or a calibrated 12.000 mA current source) to the SM 1231 input. Verify the scaled value matches the expected engineering unit to within 0.1 %.
2. Step test. Connect a function generator producing a 10 Hz, 4 V_pp square wave to the analog input. The expected peak is +2 V and -2 V (bipolar) or +2 V and 0 V (unipolar). Confirm the FB's PeakValue stabilises within 2-3 samples of the true amplitude.
3. Burst test. Drive a single impulse (e.g., tap the accelerometer with a calibration hammer) and verify that the slope detector's PeakDetected flag pulses for one OB30 cycle and PeakValue latches at the expected g value within ±5 %.
4. Reset verification. From the HMI, trigger Reset and confirm PeakValue returns to 0.0 (or to the configured idle value) within one OB1 cycle.
5. Long-duration test. Log the peak value for 24 h. Drift, saturation, and noise are easier to spot in a trend than a single number.

12. Troubleshooting Matrix

13. Field-Proven Caveats

• Sample-and-hold mismatch. If your OB1 cycle is 30 ms but the analog event is 5 ms, the analog read may already be returning the tail of the pulse. Insert a hardware peak-detect op-amp (e.g., a diode + capacitor with FET reset) on the analog front-end, or move the entire capture into a 1 ms OB30.
• Retain behaviour. Mark the instance DB as Non-optimised with retain or Optimised with retain in TIA Portal if you need the peak value to survive a power cycle. Without retain, the peak resets to 0 on every restart.
• First-scan reset. Tie the FB Reset input to FirstScan (system bit) so the latched peak does not contain a stale value from a previous run on the first cycle after PLC startup.
• Float math overhead. REAL division on the S7-1200 takes ~6 µs of CPU time at firmware V4.4. At OB30 1 ms, a 100-tap moving average with 100 divisions/sec is well within budget (~0.6 % of cycle), but on the smaller CPU 1212C at 1 ms OB30 it can starve communication. Use DINT fixed-point if cycle time is tight.
• Ground loops on accelerometers. If the peak value drifts at mains frequency, the accelerometer shell is not grounded. Use a shielded cable with the shield bonded at the PLC end only and an isolated signal conditioner.

14. Choosing the Approach - Quick Selection