Application of Siemens PLC in IoT Device Production

The production line of IoT devices requires high levels of automation and data integration. Siemens PLC, with its reliability and networking capabilities, has become the ideal control core.

1. Hardware Configuration

Selection of PLC and Expansion Modules

The production line for IoT devices typically uses the Siemens S7-1500 series PLC as the control core, which is particularly suitable due to its processing speed and integrated communication capabilities. Depending on the scale of the production line, you can choose CPU 1515-2 PN for small to medium-sized lines or CPU 1516-3 PN/DP for large and complex systems.

For I/O expansion, the following configurations are recommended:

Digital Input Module SM 521 (16DI): Collects signals from sensors, buttons, limit switches
Digital Output Module SM 522 (16DO): Controls actuators, indicator lights, solenoid valves
Analog Input Module SM 531 (8AI): Collects analog signals such as temperature, pressure, position
Analog Output Module SM 532 (4AO): Controls frequency converters, adjusts proportional valves

I/O Point Allocation Table (Including Addresses)

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2. Control Program Design

Program Architecture Design

The PLC program for the IoT device production line adopts a modular and hierarchical structure design:

├── OB1 (Main Cycle)
│   ├── System Initialization
│   ├── Mode Selection
│   ├── Subsystem Calls
│   └── Data Processing and Communication
├── OB10-17 (Clock Interrupt)
│   └── Timed Data Collection and Processing
├── OB40-47 (Hardware Interrupt)
│   └── Exception Handling
├── FB10-50 (Function Blocks)
│   ├── FB10: Material Conveying Control
│   ├── FB20: Assembly Station Control
│   ├── FB30: Testing Station Control
│   ├── FB40: Packaging Station Control
│   └── FB50: Data Collection and Processing
└── DB10-50 (Data Blocks)
    ├── DB10: System Parameters
    ├── DB20: Production Recipes
    ├── DB30: Operating Status
    ├── DB40: Historical Data
    └── DB50: Communication Interfaces

Variable Definition Specifications

The variable naming convention for the PLC in the IoT device production line follows the structure “Function_Type_Description”:

Prefix Rules: I (Input), Q (Output), M (Intermediate Variable), T (Timer), C (Counter)
Data Types: BOOL, INT, REAL, DINT, etc.
Descriptions should be concise and intuitive, such as “Feed_M_Start” indicating the feed system start flag

// Variable Definition Example
Feed_I_Ready    AT %I0.0  : BOOL;   // Feed system ready signal
Assy_I_Pos      AT %IW64  : INT;    // Assembly position feedback
Test_Q_Start    AT %Q0.1  : BOOL;   // Test start signal
Pack_M_Status             : INT;    // Packaging system status
Sys_DB_Recipe             : "Recipe_DB"; // Recipe data block

Function Block Design

Below is an example of a function block for the testing station of IoT devices:

FUNCTION_BLOCK "FB30_TestStation"
{ S7_Optimized_Access := 'TRUE' }
VAR_INPUT
    Start         : BOOL;   // Test start signal
    Reset         : BOOL;   // Reset signal
    ProductID     : DINT;   // Product ID
    TestParams    : "TestParams_UDT"; // Test parameters structure
END_VAR
VAR_OUTPUT
    Busy          : BOOL;   // Testing flag
    Done          : BOOL;   // Test completion flag
    Error         : BOOL;   // Error flag
    ErrorCode     : WORD;   // Error code
    TestResults   : "TestResults_UDT"; // Test results structure
END_VAR
VAR
    State         : INT;    // Internal state machine
    Timer         : TON;    // Test timer
END_VAR
BEGIN
    // Reset handling
    IF Reset THEN
        State := 0;
        Busy := FALSE;
        Done := FALSE;
        Error := FALSE;
        ErrorCode := 16#0000;
        RETURN;
    END_IF;
    // State machine implementation
    CASE State OF
        0:  // Standby state
            IF Start THEN
                State := 10;
                Busy := TRUE;
                Done := FALSE;
                Error := FALSE;
            END_IF;
        10: // Initialize test
            // Initialize test device
            IF "InitTestDevice"(ProductID, TestParams) THEN
                State := 20;
            ELSE
                State := 100;
                Error := TRUE;
                ErrorCode := 16#0001;
            END_IF;
        20: // Execute test
            Timer(IN := TRUE, PT := TestParams.TestTime);
            IF Timer.Q THEN
                Timer(IN := FALSE);
                State := 30;
            END_IF;
        30: // Read results
            "ReadTestResults"(TestResults);
            State := 40;
        40: // Result evaluation
            IF TestResults.Result >= TestParams.MinValue AND
               TestResults.Result <= TestParams.MaxValue THEN
                TestResults.Pass := TRUE;
                State := 50;
            ELSE
                TestResults.Pass := FALSE;
                State := 50;
            END_IF;
        50: // Completion
            Busy := FALSE;
            Done := TRUE;
            State := 0;
        100: // Error handling
            Busy := FALSE;
            Done := FALSE;
            // Error handling code
    END_CASE;
END_FUNCTION_BLOCK

3. Communication Network Architecture

Fieldbus Selection

The communication network for the IoT device production line adopts a multi-layer architecture:

Device Layer: PROFINET/PROFIBUS connects sensors, actuators, frequency converters, and other field devices, ensuring high real-time performance
Control Layer: Industrial Ethernet connects PLCs, HMIs, industrial PCs, and other control devices, supporting TCP/IP protocols
Management Layer: Standard Ethernet connects MES, ERP, cloud platforms, and other IT systems

Remote Communication Solutions

Siemens PLC provides various remote communication capabilities for IoT device production:

OPC UA Server: The S7-1500 CPU has a built-in OPC UA server function, enabling cross-platform and cross-vendor data integration
WebServer: The built-in WebServer function allows remote monitoring of PLC status via a browser
MQTT Client: The SIMATIC IoT function library enables direct communication between PLC and cloud platforms
Industrial Cloud Connector: Uses edge devices (such as SIMATIC IoT2040) to build a secure bridge between the production line and cloud platforms

// MQTT Message Publishing Example Code
FUNCTION "PublishDeviceData" : VOID
VAR_INPUT
    DeviceID      : STRING;  // Device ID
    Temperature   : REAL;    // Temperature data
    Pressure      : REAL;    // Pressure data
    Status        : INT;     // Status data
END_VAR
VAR
    mqttCON       : "IotMqtt_Connect";
    mqttPUB       : "IotMqtt_Publish";
    payload       : STRING;
END_VAR
BEGIN
    // Build JSON data
    payload := '{';
    payload := CONCAT(IN1 := payload, IN2 := '"deviceId":"');
    payload := CONCAT(IN1 := payload, IN2 := DeviceID);
    payload := CONCAT(IN1 := payload, IN2 := '",');
    payload := CONCAT(IN1 := payload, IN2 := '"temperature":');
    payload := CONCAT(IN1 := payload, IN2 := REAL_TO_STRING(Temperature));
    payload := CONCAT(IN1 := payload, IN2 := ',');
    payload := CONCAT(IN1 := payload, IN2 := '"pressure":');
    payload := CONCAT(IN1 := payload, IN2 := REAL_TO_STRING(Pressure));
    payload := CONCAT(IN1 := payload, IN2 := ',');
    payload := CONCAT(IN1 := payload, IN2 := '"status":');
    payload := CONCAT(IN1 := payload, IN2 := INT_TO_STRING(Status));
    payload := CONCAT(IN1 := payload, IN2 := '}');
    // Connect to MQTT broker
    mqttCON(REQ := TRUE,
            BROKER := 'mqtt.cloud-platform.com',
            PORT := 1883,
            CLIENTID := 'PLC_' + DeviceID,
            USERNAME := 'plc_user',
            PASSWORD := 'plc_password');
    // Publish data
    IF mqttCON.DONE THEN
        mqttPUB(REQ := TRUE,
                TOPIC := 'devices/' + DeviceID + '/data',
                PAYLOAD := payload,
                QOS := 1,
                RETAIN := FALSE);
    END_IF;
END_FUNCTION

4. Fault Diagnosis and Troubleshooting

Common Fault Analysis

Faults in the IoT device production line are typically categorized as follows:

Hardware Faults: Sensor failures, actuator jams, damaged communication cables, etc.
Program Logic Faults: State machine transition errors, improper parameter settings, inadequate boundary condition handling
Communication Faults: Network congestion, protocol mismatches, address conflicts, etc.
Data Processing Faults: Data overflow, type conversion errors, pointer access out of bounds, etc.

Using Diagnostic Tools

Siemens PLC provides various fault diagnosis tools:

System Diagnostic Buffer: Records system events and faults, viewable via TIA Portal or Web server
ProDiag: Programmable diagnostic functions for custom fault monitoring and reporting
GRAPH Diagnostics: Visual diagnostics for sequential control programs
Trace Function: Records variable changes to analyze timing issues

Troubleshooting Methodology

The troubleshooting steps for faults in the IoT device production line are as follows:

Observe Phenomena: Record fault manifestations, occurrence times, preceding operations, etc.
Collect Information: Check alarm information, status indicators, log data, etc.
Locate Range: Identify the subsystem or module where the fault may have occurred
Analyze Causes: Infer possible causes based on phenomena and information
Validate Hypotheses: Verify fault causes through testing or monitoring
Develop Solutions: Formulate temporary and permanent solutions
Implement Solutions: Apply solutions and verify their effectiveness
Summarize Experience: Document fault cases and update maintenance documentation

5. System Maintenance and Management

Key Points for Daily Maintenance

Daily maintenance of the PLC system in the IoT device production line includes:

Hardware Inspection: Cleaning fan filters, tightening terminal connections, checking battery status
Program Backup: Regularly perform complete backups of PLC programs and parameters
Log Analysis: Regularly check system logs to identify potential issues
Communication Quality: Monitor network communication quality, including packet loss rate, response time, etc.

Backup and Recovery Strategy

The backup strategy for the PLC system in the IoT device production line includes:

Cyclic Backup: Perform a complete program backup once a week
Change Backup: Immediately back up after each program modification
Multi-level Backup: Retain the last 10 backup versions
Off-site Backup: Store important backups in the cloud or off-site servers
Backup Testing: Test backup recovery functionality every quarter

System Upgrade Methods

The steps for upgrading the PLC system in the IoT device production line are as follows:

Upgrade Assessment: Assess the necessity and impact scope of the upgrade
Complete Backup: Perform a complete backup of the current system before the upgrade
Testing and Validation: Validate the upgrade effects in a testing environment
Step-by-Step Implementation: Implement upgrades module by module to avoid overall risk
Rollback Preparation: Prepare rollback plans and preset fault handling processes
Upgrade Execution: Execute the upgrade during planned downtime
Function Verification: Fully verify system functions after the upgrade
Document Update: Update system documentation to record upgrade content

Conclusion

Siemens PLC plays a core role in the production of IoT devices, achieving data integration and remote management through its networking capabilities. We welcome you to share your application experiences!