Pressure Control System: PLC Programming Practical Analysis

The pressure control system is widely used in automation production. Today, let’s discuss how to use Siemens PLC to create a simple pressure control system. This project seems simple, but in practice, we often encounter some issues, and today we will clarify these step by step.

Project Background and Objectives

The purpose of this pressure control system is to control the on/off of pressure equipment through PLC. When the pressure reaches the set upper and lower limits, the PLC should accurately start or stop the related equipment. The main goal is to achieve automated control and avoid human intervention.

Common Issues Encountered

In practice, there are several common issues with the pressure control system, such as:

1. 1. Unstable Pressure: After the equipment starts, the pressure may fluctuate, leading to inaccurate control.
2. 2. Signal Loss: The signal between the PLC and the sensor may be lost, causing the control system to not respond.
3. 3. Overpressure Protection: If the set pressure value is too high, it may trigger the safety valve, requiring proper handling of these boundary conditions.

System Hardware and Equipment

To implement this system, you will need:

• • Siemens PLC (e.g., S7-1200): Main control logic.
• • Pressure Sensor: Used to monitor pressure in real-time.
• • Solenoid Valve: Used to control the opening and closing of the gas source.
• • Pressure Switch: Used to implement upper and lower pressure control.

Programming Approach

What we need to do is read the signal from the pressure sensor through the PLC and then control the opening and closing of the solenoid valve based on this signal. The core idea is to set the upper and lower pressure values, and when the pressure exceeds the range, the PLC takes action automatically.

Detailed Code Explanation

Next, let’s see how to write the control program for the Siemens PLC. We assume that the pressure sensor’s signal has been connected to the PLC’s analog input module, and the output module controls the solenoid valve.

// Define input and output signals
Inputs: 
  Pressure_Sensor : AI 1 // Pressure sensor signal
Outputs: 
  Valve_Control : Q 1    // Solenoid valve control signal

// Set pressure upper and lower limits
Pressure_Low_Limit  := 200  // Low pressure set value
Pressure_High_Limit := 800  // High pressure set value

// Control logic
IF Pressure_Sensor < Pressure_Low_Limit THEN
  Valve_Control := 1  // Start solenoid valve for pressurization
ELSEIF Pressure_Sensor > Pressure_High_Limit THEN
  Valve_Control := 0  // Close solenoid valve for depressurization
ELSE
  Valve_Control := 0  // No action within normal range
ENDIF

Code Analysis

• • Pressure_Sensor is the input signal from the pressure sensor, assumed to be an analog input.
• • Valve_Control is the output signal for the solenoid valve, controlling the opening and closing of the gas source.
• • Pressure_Low_Limit and Pressure_High_Limit are the upper and lower limits for pressure, which can be set according to actual needs.
• • In the program, the <span>IF</span> statement is used to determine whether the pressure exceeds the upper and lower limits, and then control the solenoid valve accordingly.

Common Optimization Solutions

1. 1. Add Delay Control: In cases of significant pressure fluctuations, a delay can be added to avoid frequent starting and stopping of the solenoid valve, reducing equipment wear. For example, using TON (timer) to control the solenoid valve action, ensuring the pressure stabilizes before switching states.
2. 2. Fault Handling Mechanism: If the signal between the PLC and the sensor is lost, it may cause the system to not function properly. A fault detection module can be added to the program; if sensor failure is detected, an alarm or backup plan can be triggered.

   IF NOT Sensor_OK THEN
     Alarm := 1  // Trigger alarm
   ELSE
     Alarm := 0
   ENDIF

3. 3. Dynamic Adjustment of Set Values: Depending on changes in the production environment, the upper and lower pressure values may need to be dynamically adjusted. This can be done through HMI or external inputs to modify the set values in the PLC.

Tips

• • Sensor Selection: Choose an appropriate pressure sensor to ensure measurement accuracy meets requirements, avoiding inaccurate control due to sensor issues.
• • Be Cautious During Debugging: During initial debugging, set the upper and lower limits slightly higher to avoid premature alarms or shutdowns. Adjust to appropriate values after the system stabilizes.

Conclusion

After reading this article, I believe everyone has a general understanding of the pressure control system. The code is simple, but the key lies in debugging and optimization. If you have any questions, feel free to leave a message, and I will help you solve them!