Design Ideas for Industrial Automation Control Systems: A Complete Siemens PLC Solution

Recently, I received an automation renovation project to upgrade an old production line. This made me think of how many beginners often don’t know where to start when facing actual industrial projects. Today, I will discuss how to plan a PLC control system from scratch based on this practical case.

Requirements Analysis and System Planning

1. Determine Control Objects

First, clarify what needs to be controlled. In this project, the production line includes:

• 3 conveyor belt motors

• 2 cylinder positioning devices

• 1 electric gripper

• Multiple limit switches and photoelectric sensors

• 1 touchscreen human-machine interface

2. I/O Point Statistics

This is the most critical first step, as it determines the PLC selection.

• Input points: 16 points (including emergency stop, limit, photoelectric, etc.)

• Output points: 12 points (motor, solenoid valve, indicator light, etc.)

• Analog input: 2 channels (temperature, pressure sensors)

• Analog output: 1 channel (frequency converter control)

Hardware Selection

PLC Host Selection

Select Siemens S7-1200 series, CPU 1214C DC/DC/DC model:

• Built-in 14 inputs/10 outputs

• 2 analog input channels

• Supports Ethernet communication

• Moderate price, strong scalability

Note: At least 20% spare I/O points should be reserved for future renovations.

Expansion Module Configuration

• 1 SM1231 analog input module (4AI)

• 1 SM1232 analog output module (2AO)

• 1 CM1241 serial communication module (connect to frequency converter)

Electrical Design Key Points

Control Cabinet Layout

1. High voltage part
   - Circuit breaker
   - Contactor
   - Thermal relay
   - Frequency converter
2. Low voltage part
   - PLC and expansion modules
   - 24V power supply
   - Terminal block
   - Relay group

Key Reminders:

• High and low voltage must be wired separately.

• Reserve more than 20% installation space.

• Ensure proper waterproofing for the control cabinet’s incoming line.

Program Design Architecture

Main Program Structure

// Program block organization
OB1 Main loop program
 ├─ FC1 System initialization
 ├─ FC2 Safety monitoring
 ├─ FC3 Manual operation
 ├─ FC4 Automatic operation
 └─ FC5 Alarm handling

Key Functional Modules

1. Safety control

// Emergency stop handling
IF "Emergency_Stop" THEN
    // Reset all outputs
    FOR #i := 0 TO 10 DO
        #Output[#i] := FALSE;
    END_FOR;
    // Set alarm flag
    #Alarm_Flag := TRUE;
END_IF;

Debugging and Optimization

System Debugging Process

1. I/O point testing

2. Single component debugging

3. Linkage testing

4. Automatic operation testing

5. Exception handling verification

Common issues and solutions:

1. Sensor false triggering

• Cause: Poor anti-interference capability

• Solution: Install shielding wire, adjust installation position

1. Unstable communication

• Cause: Poor grounding

• Solution: Check grounding system, rewire if necessary

Practical Suggestions

Operational key points:

1. Prepare a detailed I/O allocation table before starting.

2. Leave enough maintenance space when wiring the control cabinet.

3. Add more comments to the program for easier maintenance later.

4. Keep debugging records to document parameter tuning values.

5. Always have someone watching during the first startup.

Spare parts list:

• 2-3 commonly used relay models

• 1 power supply module

• Several commonly used fuses

• 1 row of rail-mounted terminal blocks

This project made me deeply realize that the key to system design is not how complex the program is, but whether the initial planning is thorough, the layout is reasonable, and whether maintenance and expansion needs are fully considered.