Using S7-1200 for Intelligent Monitoring of Laboratory Environments

Click the blue text to follow usThe laboratory environment monitoring system ensures the stability of precision experimental environments, relying on multi-parameter real-time monitoring and intelligent linkage control to improve the reliability of experimental data.

01 Hardware Configuration

The laboratory environment monitoring system requires a precise and reliable hardware foundation. To ensure stable operation of the system, the hardware configuration is as follows:

PLC and Expansion Module Selection

For small to medium-sized laboratories, it is recommended to use the S7-1200 series PLC (such as CPU 1214C) as the main controller, which has the following advantages:

• Built-in 14 digital inputs / 10 digital outputs
• 2 analog inputs to meet basic monitoring needs
• Supports Profinet communication for easy system expansion
• Cost-effective, suitable for laboratories with limited budgets

For laboratories with many parameter monitoring points, it is advisable to add the following expansion modules:

• SM 1231 TC (8-channel thermocouple module): for precise multi-point temperature measurement
• SM 1231 AI (8-channel analog input): for collecting signals from humidity, air pressure, gas concentration sensors
• SM 1222 DQ (16 digital outputs): for controlling fans, solenoid valves, alarms, and other actuators

I/O Point Allocation Table

Address	Description	Signal Type	Range
%I0.0	System Start Button	Digital Input	0/1
%I0.1	System Stop Button	Digital Input	0/1
%I0.2	Emergency Stop Button	Digital Input	0/1
%I0.3	Fresh Air System Feedback	Digital Input	0/1
%IW64	Temperature Sensor 1	Analog Input	0-27648(0-100℃)
%IW66	Humidity Sensor 1	Analog Input	0-27648(0-100%)
%IW68	CO2 Concentration Sensor	Analog Input	0-27648(0-5000ppm)
%Q0.0	Fresh Air System Start	Digital Output	0/1
%Q0.1	Humidifier Control	Digital Output	0/1
%Q0.2	Dehumidifier Control	Digital Output	0/1
%Q0.3	Sound and Light Alarm	Digital Output	0/1
%QW64	Temperature Control Valve Control	Analog Output	0-27648(0-100%)

System Wiring Points

• Analog sensors should use shielded twisted pairs, grounded at one end to reduce electromagnetic interference
• Sensor power supply should be separate from the PLC power supply to avoid interference
• Important control circuits should use relay isolation to improve system reliability
• All terminal connections should use spring-loaded terminals to reduce the risk of loosening

02 Control Program Design

Variable Definition Standards

Good variable naming is the foundation of program maintainability. The following standards are recommended:

// Global variable naming conventions
g_        // Global variable prefix
b_        // Boolean variable prefix
i_        // Integer variable prefix
r_        // Real variable prefix
s_        // String variable prefix
t_        // Time variable prefix

// Example
g_bSysRunning    // System running status
g_rTempSetPoint  // Temperature setpoint
g_iTempAlarmHigh // High temperature alarm value

Program Architecture Design

The program adopts the following hierarchical structure:

1. OB1: Main program loop, calling various function blocks

2. • System initialization
   • Data acquisition FB call
   • Control logic processing
   • Communication processing
   • Alarm processing

3. OB100: Power-on initialization, loading default parameters

4. OB82: Diagnostic interrupt, handling module faults

5. Function Block Design:

6. • FB1: Data acquisition and processing
   • FB2: Temperature and humidity control
   • FB3: Gas concentration control
   • FB4: Alarm management
   • FB5: Data logging

Status Control Design

The laboratory environment monitoring system includes multiple operating states, managed using a state machine:

// System state definitions
#define SYS_INIT       0  // Initialization state
#define SYS_STANDBY    1  // Standby state
#define SYS_RUNNING    2  // Running state
#define SYS_ALARM      3  // Alarm state
#define SYS_EMERGENCY  4  // Emergency state
#define SYS_MAINTAIN   5  // Maintenance state

State transition logic example (SCL language):

CASE g_iSysState OF
    SYS_INIT:
        // Initialization tasks
        IF Init_Done THEN
            g_iSysState := SYS_STANDBY;
        END_IF;
        
    SYS_STANDBY:
        // Standby tasks
        IF Start_Command THEN
            g_iSysState := SYS_RUNNING;
        END_IF;
        
    SYS_RUNNING:
        // Running tasks
        IF Stop_Command THEN
            g_iSysState := SYS_STANDBY;
        ELSIF Alarm_Active THEN
            g_iSysState := SYS_ALARM;
        END_IF;
        
    SYS_ALARM:
        // Alarm handling tasks
        IF Emergency_Button THEN
            g_iSysState := SYS_EMERGENCY;
        ELSIF Alarm_Reset AND NOT Alarm_Active THEN
            g_iSysState := SYS_RUNNING;
        END_IF;
        
    SYS_EMERGENCY:
        // Emergency state handling
        IF Emergency_Reset THEN
            g_iSysState := SYS_STANDBY;
        END_IF;
        
    SYS_MAINTAIN:
        // Maintenance state handling
        IF Maintain_Done THEN
            g_iSysState := SYS_STANDBY;
        END_IF;
END_CASE;

03 Data Management and Storage

Parameter Configuration Table

Parameter configuration information is stored in a global data block for unified management:

// Create parameter configuration data block
DATA_BLOCK "DB_Parameters"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
NON_RETAIN
   STRUCT 
      // Temperature control parameters
      TempSetPoint : Real := 23.0;    // Temperature setpoint (℃)
      TempHighLimit : Real := 25.0;   // Temperature upper limit (℃)
      TempLowLimit : Real := 21.0;    // Temperature lower limit (℃)
      TempAlarmHigh : Real := 28.0;   // High temperature alarm (℃)
      TempAlarmLow : Real := 18.0;    // Low temperature alarm (℃)
      
      // Humidity control parameters
      HumidSetPoint : Real := 50.0;   // Humidity setpoint (%)
      HumidHighLimit : Real := 55.0;  // Humidity upper limit (%)
      HumidLowLimit : Real := 45.0;   // Humidity lower limit (%)
      HumidAlarmHigh : Real := 60.0;  // High humidity alarm (%)
      HumidAlarmLow : Real := 40.0;   // Low humidity alarm (%)
      
      // CO2 concentration control parameters
      CO2SetPoint : Real := 800.0;    // CO2 setpoint (ppm)
      CO2AlarmHigh : Real := 1200.0;  // High CO2 alarm (ppm)
      
      // PID control parameters
      TempPID_P : Real := 1.0;       // Temperature PID-P
      TempPID_I : Real := 120.0;     // Temperature PID-I (s)
      TempPID_D : Real := 0.0;       // Temperature PID-D (s)
      HumidPID_P : Real := 0.8;      // Humidity PID-P
      HumidPID_I : Real := 180.0;    // Humidity PID-I (s)
      HumidPID_D : Real := 0.0;      // Humidity PID-D (s)
   END_STRUCT;
END_DATA_BLOCK

Runtime Data Logging

System runtime data is stored in a circular buffer data block, overwriting the oldest data:

DATA_BLOCK "DB_RunData"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
NON_RETAIN
   STRUCT 
      // Record management
      CurrentIndex : Int := 0;      // Current record index
      MaxRecords : Int := 1000;     // Maximum number of records
      
      // Data record arrays
      TimeStamp : Array[0..999] of Date_And_Time;  // Timestamps
      Temp : Array[0..999] of Real;  // Temperature values
      Humid : Array[0..999] of Real; // Humidity values
      CO2 : Array[0..999] of Real;   // CO2 concentrations
      AlarmBits : Array[0..999] of Word; // Alarm bits
   END_STRUCT;
END_DATA_BLOCK

Alarm Information Management

Alarm information is managed using bits, with each bit representing a type of alarm:

// Alarm bit definitions
#define ALARM_TEMP_HIGH    16#0001  // High temperature alarm
#define ALARM_TEMP_LOW     16#0002  // Low temperature alarm
#define ALARM_HUMID_HIGH   16#0004  // High humidity alarm
#define ALARM_HUMID_LOW    16#0008  // Low humidity alarm
#define ALARM_CO2_HIGH     16#0010  // High CO2 concentration alarm
#define ALARM_SENSOR_FAULT 16#0020  // Sensor fault alarm
#define ALARM_SYSTEM_FAULT 16#0040  // System fault alarm

Example of alarm handling function block:

FUNCTION_BLOCK "FB_AlarmManagement"
   VAR_INPUT 
      Temp : Real;              // Current temperature
      Humid : Real;             // Current humidity
      CO2 : Real;               // Current CO2 concentration
      SensorStatus : Word;      // Sensor status
   END_VAR

   VAR_OUTPUT
      AlarmWord : Word;         // Alarm word
      AlarmActive : Bool;       // Alarm active
      AlarmSound : Bool;        // Sound alarm
   END_VAR

   VAR 
      OldAlarmWord : Word;      // Previous alarm word
      AlarmChangeTrigger : Bool; // Alarm change trigger
   END_VAR

BEGIN
   // Initialize alarm word
   AlarmWord := 16#0000;
   
   // Check temperature alarm
   IF Temp > "DB_Parameters".TempAlarmHigh THEN
      AlarmWord := AlarmWord OR ALARM_TEMP_HIGH;
   END_IF;
   
   IF Temp < "DB_Parameters".TempAlarmLow THEN
      AlarmWord := AlarmWord OR ALARM_TEMP_LOW;
   END_IF;
   
   // Check humidity alarm
   IF Humid > "DB_Parameters".HumidAlarmHigh THEN
      AlarmWord := AlarmWord OR ALARM_HUMID_HIGH;
   END_IF;
   
   IF Humid < "DB_Parameters".HumidAlarmLow THEN
      AlarmWord := AlarmWord OR ALARM_HUMID_LOW;
   END_IF;
   
   // Check CO2 alarm
   IF CO2 > "DB_Parameters".CO2AlarmHigh THEN
      AlarmWord := AlarmWord OR ALARM_CO2_HIGH;
   END_IF;
   
   // Check sensor fault
   IF SensorStatus <> 16#0000 THEN
      AlarmWord := AlarmWord OR ALARM_SENSOR_FAULT;
   END_IF;
   
   // Detect alarm change
   IF AlarmWord <> OldAlarmWord THEN
      AlarmChangeTrigger := TRUE;
   ELSE
      AlarmChangeTrigger := FALSE;
   END_IF;
   
   // Update alarm output
   AlarmActive := (AlarmWord <> 16#0000);
   AlarmSound := AlarmChangeTrigger AND AlarmActive;
   
   // Save current alarm state
   OldAlarmWord := AlarmWord;
END_FUNCTION_BLOCK

04 User Interface Design

Interface Layout Description

The HMI interface configured using Siemens WinCC has a reasonable and intuitive layout:

1. Main Screen: Displays the overall laboratory floor plan and key parameters

2. • Left side: Laboratory floor plan, graphically displaying the status of each area
   • Right side: Real-time values and set values of key parameters displayed side by side
   • Bottom: System status bar and scrolling alarm information

3. Parameter Setting Screen:

4. • Setting parameters such as temperature, humidity, and CO2 concentration
   • PID parameter tuning interface
   • Alarm limit settings

5. Trend Curve Screen:

6. • Multi-parameter trend curves, supporting zoom and historical queries
   • Data export function

7. Alarm Screen:

8. • Current alarms displayed
   • Historical alarm records
   • Alarm confirmation function

Parameter Setting Description

To ensure operational safety, parameter settings adopt a hierarchical permission management system:

1. Operator Level:

2. • Can only view operating parameters
   • Can confirm alarms
   • Can export historical data

3. Engineer Level:

4. • Can modify control set values
   • Can adjust alarm limits
   • Can switch system operating modes

5. Administrator Level:

6. • Can modify PID parameters
   • Can set system configuration parameters
   • Can manage user permissions

Parameter modifications require a double confirmation mechanism to avoid misoperation:

1. Input target value
2. Confirmation dialog
3. Actual write to PLC
4. Parameter change record

05 System Debugging Methods

Step-by-Step Debugging Method

The debugging of the laboratory environment monitoring system adopts a bottom-up approach:

1. I/O Point Testing:

2. • Test digital input points one by one
   • Calibrate analog input points
   • Test digital and analog output points

3. Function Module Testing:

4. • Temperature control loop testing
   • Humidity control loop testing
   • CO2 concentration control loop testing
   • Alarm function testing

5. Joint Debugging:

6. • Cooperative operation testing of each control loop
   • Simulate system response under various working conditions
   • HMI and PLC communication testing

7. System Testing:

8. • Long-term stability testing (over 24 hours)
   • Power recovery testing
   • Abnormal handling testing

Abnormal Simulation Testing

To verify the system’s handling capability under abnormal conditions, the following tests need to be conducted:

1. Sensor Fault Testing:

2. • Temperature sensor disconnection/short circuit testing
   • Humidity sensor signal anomaly testing
   • CO2 sensor failure testing

3. Actuator Fault Testing:

4. • Fan startup failure testing
   • Temperature control valve jamming testing
   • Humidifier/dehumidifier failure testing

5. Communication Abnormality Testing:

6. • HMI communication interruption testing
   • Network communication disconnection testing
   • Remote access failure testing

7. Power Abnormality Testing:

8. • Short-term power recovery testing
   • Long-term power recovery testing
   • Power fluctuation testing

Each test must record the system’s response and verify whether the fault safety mechanisms are effective.

06 Summary and Reflections

The laboratory environment monitoring system is a typical application of precision environmental control, achieved through Siemens PLC for multi-parameter monitoring and control, ensuring the stability and reliability of the experimental environment. Feel free to share your project experiences in the comments!

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