The Ultimate PLC Flow Accumulation Calculation Tool: A Detailed Explanation of the SCL Function Block

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In the field of industrial automation, the cumulative flow statistics of flow meters is a common and important requirement. How can we accurately and reliably calculate the total flow in the face of fluctuating instantaneous flow signals? Today, we bring you a practical SCL function block solution.

Problem Analysis

Flow meters typically output a 4-20mA analog signal, which the PLC reads and converts into an instantaneous flow value. Due to the dynamic characteristics of the process, this instantaneous flow is often fluctuating. To calculate the total flow, we need to perform a time integral of the instantaneous flow:

Total Flow = ∫ Instantaneous Flow × dt

Complete Code for the SCL Function Block

FUNCTION_BLOCK FB_FlowAccumulator
VAR_INPUT
    b_Reset: BOOL;  // Reset signal for cumulative value
    r_InstantaneousFlow: REAL;  // Instantaneous flow input signal
    t_SamplingTime: TIME := T#1S;  // Calculation sampling time
END_VAR
VAR_OUTPUT
    r_CumulativeFlow: REAL;  // Cumulative flow value
    r_CurrentFlow: REAL;  // Current instantaneous flow value
    w_StatusWord: WORD;  // System status indication
    i_LoopCount: UDINT;  // Number of execution loops
END_VAR
VAR
    t_LastTime: TIME;  // Last calculation timestamp
    b_InitializationComplete: BOOL;  // Initialization status flag
    r_TemporaryCumulative: LREAL;  // High precision temporary cumulative value
    i_InternalCount: UDINT;  // Internal loop counter
END_VAR
VAR CONSTANT
    r_MaxFlow: REAL := 10000.0;  // Upper limit of reasonable flow range
END_VAR

// ========== Flow Accumulation Calculation Method ==========METHOD CalculateFlowTotal : BOOL
VAR_INPUT
    r_InputFlow: REAL;
END_VAR
VAR
    t_TimeDifference: TIME;
    r_TimeDifferenceSeconds: REAL;
    r_FlowIncrement: LREAL;
    t_CurrentTime: TIME;
END_VAR
// Initialization Check
IF NOT b_InitializationComplete THEN
    t_LastTime := TIME();
    b_InitializationComplete := TRUE;
    r_CumulativeFlow := 0.0;
    r_TemporaryCumulative := 0.0;
    RETURN TRUE;
END_IF;

// Get Current System Time
t_CurrentTime := TIME();
// Calculate Time Interval
IF t_CurrentTime >= t_LastTime THEN
    t_TimeDifference := t_CurrentTime - t_LastTime;
ELSE
    // Handle system time anomaly
    t_TimeDifference := t_LastTime - t_CurrentTime;
END_IF;
// Convert Time Unit to Seconds
r_TimeDifferenceSeconds := TIME_TO_REAL(t_TimeDifference) / 1000.0;
// Input Validity Check
IF r_InputFlow < 0.0 THEN
    w_StatusWord := 16#0001;  // Negative flow warning
    r_InputFlow := 0.0;
ELSIF r_InputFlow > r_MaxFlow THEN
    w_StatusWord := 16#0002;  // Overrange warning
    r_InputFlow := r_MaxFlow;
ELSE
    w_StatusWord := 0;        // Normal status
END_IF;

// Calculate Flow Increment: Flow × Time
r_FlowIncrement := r_InputFlow * r_TimeDifferenceSeconds;
// Accumulate to Total Flow
r_TemporaryCumulative := r_TemporaryCumulative + r_FlowIncrement;
// Update Output Values
r_CumulativeFlow := LREAL_TO_REAL(r_TemporaryCumulative);
r_CurrentFlow := r_InputFlow;
// Update Timestamp and Counter
t_LastTime := t_CurrentTime;
i_InternalCount := i_InternalCount + 1;
i_LoopCount := i_InternalCount;
RETURN TRUE;
END_METHOD

// ========== Main Program ==========IF b_Reset THEN
    // Reset all cumulative values and status
    r_CumulativeFlow := 0.0;
    r_TemporaryCumulative := 0.0;
    r_CurrentFlow := 0.0;
    t_LastTime := TIME();
    i_InternalCount := 0;
    i_LoopCount := 0;
    w_StatusWord := 0;
    b_InitializationComplete := FALSE;
ELSE
    // Execute Flow Accumulation Calculation
    CalculateFlowTotal(r_InstantaneousFlow);
END_IF;
END_FUNCTION_BLOCK

Function Block Usage Example

// Instantiate Function Block
VAR
    FB_FlowCalculation : FB_FlowAccumulator;
END_VAR
// Call in Main Loop
FB_FlowCalculation(
    b_Reset := "Reset Button",           // Connect reset button
    r_InstantaneousFlow := "AI_FlowMeter",      // Connect flow meter analog input
    t_SamplingTime := T#100MS          // Set 100ms sampling period
);
// Read Calculation Results
"Display_CumulativeFlow" := FB_FlowCalculation.r_CumulativeFlow;
"Display_InstantaneousFlow" := FB_FlowCalculation.r_CurrentFlow;
"Display_Status" := FB_FlowCalculation.w_StatusWord;

Function Block Features Explained

1. High Precision Calculation

Uses r_TemporaryCumulative (LREAL type) for intermediate calculations to avoid cumulative errors
Automatic time compensation to adapt to different PLC scan cycles

2. Comprehensive Exception Handling

Negative Flow Detection: Automatically resets to zero and alarms when negative flow is detected
Overrange Protection: Automatically limits flow when it exceeds the set upper limit
Time Anomaly Handling: Handles special cases such as system time reversal

3. Flexible Reset Mechanism

Can reset and re-accumulate at any time via b_Reset signal
All related variables are correctly initialized upon reset

4. Status Monitoring

w_StatusWord provides detailed system status information
i_LoopCount records the number of function block executions for debugging purposes

Project Application Recommendations

1. Sampling Time Selection

// Choose appropriate sampling time based on flow fluctuation frequency
t_SamplingTime := T#50MS;   // For rapid fluctuations
t_SamplingTime := T#1S;     // For general applications
t_SamplingTime := T#5S;     // For slow changes

2. Power Failure Protection Implementation

// Store critical data in retain memory
VAR RETAIN
    r_CumulativeFlow_Retain: REAL;
END_VAR

3. Flow Filtering Processing

If the flow signal has significant noise, it is recommended to add a moving average filter before input:

// Simple moving average filter example
r_InstantaneousFlow_Filtered := (r_InstantaneousFlow + r_LastFlow1 + r_LastFlow2) / 3.0;

Conclusion

This SCL function block provides a stable and reliable flow accumulation solution with the following advantages:

✅ High Precision: Reduces cumulative errors
✅ Robust: Comprehensive exception handling mechanism
✅ Easy to Use: Clear interfaces and status indications
✅ Scalable: Facilitates functional expansion and customization

Suitable for various industrial scenarios requiring precise flow accumulation, such as water treatment, chemical processing, and energy management.

Try this function block now to make your flow statistics more accurate and reliable!

Feel free to leave your experiences in the comments or share your improvement suggestions!

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