Siemens S7-1200 PLC Lifting Control System

1. System Overview

This case study designs a lifting control system based on the Siemens S7-1200 PLC, suitable for material lifting equipment in industrial settings. The system adopts a modular programming approach, integrating position closed-loop control, safety interlock protection, and human-machine interaction functions, enabling manual/automatic mode switching, precise positioning, and fault alarm functionalities.

2. System Hardware Composition

Main Hardware Configuration:

• Controller: Siemens S7-1215C DC/DC/DC (6ES7 215-1AG40-0XB0)
• Drive System: Siemens V90 servo drive + 1.5kW servo motor (with encoder feedback)
• Detection Elements: Laser distance sensor (for real-time position feedback), upper/lower limit switches (mechanical redundancy design)
• Human-Machine Interface: TP1200 intelligent panel (12-inch touchscreen)
• Safety Devices: Emergency stop button (dual-circuit design), fall prevention status sensor, chain slack detection switch

3. I/O Address Allocation

Input Signals (DI):

Signal Name	Address	Type	Description
Emergency Stop Button	I0.0	Normally Closed	Hardware safety loop + software detection
Up Button	I0.1	Normally Open	Valid in manual mode
Down Button	I0.2	Normally Open	Valid in manual mode
Auto/Manual Switch	I0.3	Normally Open	High level for automatic mode
Upper Limit Switch	I1.0	Normally Closed	Mechanical + photoelectric dual redundancy
Lower Limit Switch	I1.1	Normally Closed	Mechanical + photoelectric dual redundancy
Chain Slack Detection	I1.2	Normally Open	Chain tension anomaly detection
Laser Sensor Ready	I1.3	Normally Open	Position feedback signal normal

Output Signals (DO):

Signal Name	Address	Type	Description
Up Contactor	Q0.0	Relay	Controls motor forward rotation
Down Contactor	Q0.1	Relay	Controls motor reverse rotation
Brake Coil	Q0.2	Transistor	Motor brake control
Running Indicator Light	Q0.3	LED	Indicates system is running normally
Alarm Indicator Light	Q0.4	LED	Red flashing indicates fault
HMI Alarm Buzzer	Q0.5	Transistor	Sounds alarm on fault

4. Control Program Design

4.1 Program Architecture

The system program adopts a modular design, mainly consisting of the following organization blocks (OB) and function blocks (FB):

• OB1: Main cycle organization block, responsible for calling various function modules
• OB100: Initialization organization block, system power-on initialization
• FB10: <span>FB_Positioning</span> Position control function block
• FB20: <span>FB_SafetyCheck</span> Safety interlock function block
• FB30: <span>FB_HMI_Comm</span> HMI data interaction function block

4.2 Control Flow Chart

Main Control Flow:

1. System Initialization (OB100): Reset axis status, initialize variables, check hardware readiness
2. Safety Interlock (FB20): Real-time monitoring of emergency stop, limit, chain slack, and other safety signals
3. Mode Selection

• Manual Mode: Directly responds to up/down buttons, jog control
• Automatic Mode: Receives target position set by HMI, automatically completes positioning

Position Control (FB10): Achieves high-precision position closed-loop control through PID algorithm

Fault Handling: Immediately stops the motor and triggers an alarm upon detecting anomalies

4.3 Core Function Block Code Example

FB_Positioning Position Control Function Block:

FUNCTION_BLOCK FB_Positioning
VAR_INPUT
    CurrentHeight : REAL;    // Current height measurement (mm)
    TargetHeight : REAL;     // Target height (mm)
    Tolerance : REAL := 5.0; // Allowable error (mm)
    Enable : BOOL;           // Function block enable
END_VAR
VAR_OUTPUT
    MoveUp : BOOL;           // Up command
    MoveDown : BOOL;         // Down command
    PositionOK : BOOL;       // Position signal
    Error : BOOL;            // Positioning error
END_VAR
VAR
    ErrorCount : INT;        // Out-of-tolerance count
    TimeoutTimer : TON;      // Position timeout timer
END_VAR

// Position timeout detection
TimeoutTimer(IN := Enable AND NOT PositionOK, PT := T#5S);

// Position closed-loop control logic
IF Enable THEN
    IF ABS(CurrentHeight - TargetHeight) > Tolerance THEN
        MoveUp := CurrentHeight < TargetHeight;
        MoveDown := CurrentHeight > TargetHeight;
        ErrorCount := 0;
        Error := FALSE;

        // Timeout judgment
        IF TimeoutTimer.Q THEN
            Error := TRUE;
            MoveUp := FALSE;
            MoveDown := FALSE;
        END_IF;
    ELSE
        // Confirm position only after 3 consecutive successful detections
        ErrorCount := ErrorCount + 1;
        IF ErrorCount >= 3 THEN
            PositionOK := TRUE;
            MoveUp := FALSE;
            MoveDown := FALSE;
        END_IF;
    END_IF;
ELSE
    // Reset outputs when function block is not enabled
    MoveUp := FALSE;
    MoveDown := FALSE;
    PositionOK := FALSE;
    ErrorCount := 0;
    TimeoutTimer(IN := FALSE);
END_IF;

5. Safety Design Points

1. Dual Safety Loop:

• The emergency stop button is connected to both PLC input and safety relay, achieving dual protection through hardware and software
• The limit switches use normally closed contacts in series to ensure protection is triggered in case of a disconnection

Motor Safety Control:

• The brake coil is linked with the motor drive contactor, immediately braking upon power loss
• The program includes power loss delay detection (500ms) to prevent brake failure from causing slipping

Fault Diagnosis:

• Real-time monitoring of sensor status, triggering alarms on anomalies
• The HMI displays detailed fault codes and troubleshooting guides

6. Debugging and Optimization

1. Sensor Debouncing:

• Laser distance sensor data uses 3-sample average filtering
• Position signal includes 3 consecutive successful judgments to avoid false triggers

PID Parameter Tuning:

• Proportional coefficient (Kp) = 2.0, Integral time (Ti) = 10s, Derivative time (Td) = 1s
• Soft start is used at startup to avoid mechanical shock

HMI Interface Design:

• Real-time display of current height, target height, and operating status
• Trend chart control records 24-hour position change curve
• Hierarchical alarm display: emergency fault (red flashing), warning (yellow), prompt (blue)

7. Application Expansion

This system can be functionally expanded in the following ways:

1. Add PROFINET bus interface for multi-axis coordinated control
2. Integrate RFID identification for automatic material tracking
3. Connect to cloud platform for remote monitoring and data analysis
4. Add visual recognition system for automatic material positioning compensation

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