Pneumatic Valve Control System
The pneumatic valve control system is a core technology in the field of automation, capable of achieving precise flow regulation and rapid switching, widely used in industries such as chemical and pharmaceutical.
1. Hardware Configuration
Selection of PLC and Expansion Modules
In the pneumatic valve control system, we typically use the Siemens S7-1200 series PLC as the control core. For medium and small systems, the CPU 1214C model (14 digital inputs, 10 digital outputs, 2 analog inputs) is sufficient to meet basic needs. If multiple pneumatic valves need to be controlled, the SM 1223 digital expansion module (16DI/16DO) can be added. For applications requiring precise control of pneumatic proportional valves, it is recommended to configure the SM 1234 analog module (4AI/2AO).
I/O Point Allocation Table
Address Function Description Signal Type
I0.0 System Start Button Digital Input
I0.1 System Stop Button Digital Input
I0.2 Air Source Pressure Switch Digital Input
I0.3-I1.2 Pneumatic Valve Limit Feedback 1-8 Digital Input
Q0.0-Q0.7 Pneumatic Valve Control Output 1-8 Digital Output
IW64 Flow Sensor 1 Analog Input (4-20mA)
IW66 Pressure Sensor 1 Analog Input (4-20mA)
QW80 Proportional Control Valve Control Analog Output (4-20mA)
Key Points for System Wiring
All digital signals are powered by 24VDC, and all analog signals follow the 4-20mA standard.
The electromagnetic valve control circuit must be equipped with a flyback diode to prevent back EMF from damaging the PLC output.
Analog signal wires should use shielded twisted pairs, grounded at one end to reduce interference.
Separate wiring for strong and weak currents in the control cabinet, with 90° crossings at intersections.
2. Control Program Design
Program Architecture Design
The program architecture of the pneumatic valve control system adopts a three-layer structure:
OB1: Main loop program, responsible for calling various function blocks.
FB series: Function processing blocks, such as valve control FB, pressure monitoring FB, etc.
DB series: Data blocks, storing system parameters and operating states.
The main program is divided into the following sections by function:
OB1 Main Loop
|– System Initialization
|– Operating Status Monitoring
|– Valve Control Logic
|– Alarm Handling
|– Human-Machine Interaction Handling
Function Block Design
Taking the valve control function block as an example, we design a general pneumatic valve control FB:
FB1: Pneumatic Valve Control Block
Parameters:
Input:
– Control Command (Boolean)
– Open Position Feedback (Boolean)
– Close Position Feedback (Boolean)
– Operation Timeout (Time)
Output:
– Valve Output (Boolean)
– Valve Status (Word)
– Valve Fault (Boolean)
Static Variables:
– Operation Timer
– Current State
This function block implements control, status monitoring, and fault diagnosis of a single pneumatic valve. By reusing this FB, a multi-valve control system can be quickly constructed.
Status Control Design
The pneumatic valve control adopts a state machine method, with each valve having the following states:
Initialization State: Initial check when the system starts.
Closed State: Valve is fully closed.
Open State: Valve is fully open.
Opening Process: Valve is in the process of opening.
Closing Process: Valve is in the process of closing.
Fault State: Abnormal operation of the valve.
Example of State Transition Logic:
// Network 1: State Transition Logic
CASE #CurrentState OF
0: // Initialization State
IF #InitializationComplete THEN
#CurrentState := 1; // Transition to Closed State
END_IF;
1: // Closed State
IF #ControlCommand THEN
#ValveOutput := TRUE;
#OperationTimer(IN := TRUE, PT := #OperationTimeout);
#CurrentState := 3; // Transition to Opening Process
END_IF;
2: // Open State
IF NOT #ControlCommand THEN
#ValveOutput := FALSE;
#OperationTimer(IN := TRUE, PT := #OperationTimeout);
#CurrentState := 4; // Transition to Closing Process
END_IF;
3: // Opening Process
IF #OpenPositionFeedback THEN
#OperationTimer(IN := FALSE);
#CurrentState := 2; // Transition to Open State
ELSIF #OperationTimer.Q THEN
#ValveOutput := FALSE;
#ValveFault := TRUE;
#CurrentState := 5; // Transition to Fault State
END_IF;
4: // Closing Process
IF #ClosePositionFeedback THEN
#OperationTimer(IN := FALSE);
#CurrentState := 1; // Transition to Closed State
ELSIF #OperationTimer.Q THEN
#ValveFault := TRUE;
#CurrentState := 5; // Transition to Fault State
END_IF;
5: // Fault State
IF #ResetCommand THEN
#ValveFault := FALSE;
#CurrentState := 0; // Transition to Initialization State
END_IF;
END_CASE;
3. System Debugging Methods
Step-by-Step Debugging Method
The debugging of the pneumatic valve control system should adopt a step-by-step debugging method from point to surface:
I/O Point Testing: Test each input and output point one by one to confirm correct wiring.
Single Valve Function Testing: Test the opening and closing functions and limit feedback of a single valve.
Interlocking Logic Testing: Test the interlocking logic of multiple valves.
Exception Handling Testing: Simulate various exception scenarios to verify system response.
Performance Indicator Testing: Test valve response time, switching accuracy, and other indicators.
Exception Simulation Testing
To verify system stability, the following exception scenarios need to be simulated:
Air Source Failure: Shut off the air source to verify system alarm and safety handling logic.
Valve Sticking: Manually prevent valve action to test timeout alarm function.
Limit Switch Failure: Disconnect the limit switch to verify system diagnostic capability.
Power Fluctuation: Simulate power fluctuations to test system stability.
Communication Interruption: Disconnect network connection to verify local control logic.
4. Fault Diagnosis and Troubleshooting
Common Fault Analysis
Common faults in the pneumatic valve control system and their solutions:
Valve Movement is Slow
Cause: Insufficient air source pressure, air pipe blockage, mechanical failure of the valve.
Solution: Check air source pressure, clean air path, maintain mechanical parts of the valve.
Valve Cannot Reach Position
Cause: Limit switch position deviation, valve sticking, insufficient driving force.
Solution: Adjust limit switch, check if the valve is stuck, increase driving air pressure.
PLC Output is Normal but Valve Does Not Act
Cause: Electromagnetic valve failure, loose wiring, air source issues.
Solution: Replace the electromagnetic valve, check wiring, confirm air source is normal.
Valve Shaking
Cause: Air source pressure fluctuations, control signal interference, poor sealing.
Solution: Install pressure stabilizing device, check signal line shielding, replace sealing components.
Use of Diagnostic Tools
Effective fault diagnosis can be aided by the following tools:
PLC Status Monitoring: Use TIA Portal’s online diagnostic function to monitor I/O status in real-time.
Oscilloscope: Analyze control signal quality and detect interference issues.
Pressure Gauge: Monitor air source and actuator pressure to identify pneumatic issues.
Thermal Imaging Camera: Detect abnormal heating of electrical components.
5. Operation Interface Design
Interface Layout Description
The HMI interface of the pneumatic valve control system should include the following areas:
System Overview Area: Display the entire system P&ID diagram, visually showing valve status.
Valve Operation Area: Provide manual/automatic control interface for each valve.
Parameter Setting Area: Configure valve action time, interlocking conditions, and other parameters.
Alarm Display Area: Real-time display of system alarm information.
Trend Curve Area: Display historical trends of key parameters such as flow and pressure.
Operation Monitoring Description
The operation monitoring interface should display the following information:
Valve Status: Use different colors to indicate valve open/close/fault status.
Operation Mode: Display current manual/automatic/maintenance mode.
Air Source Status: Display air source pressure and status.
Operation Count: Record the number of actions for each valve for maintenance purposes.
System Running Time: Record continuous running time.
Conclusion
The pneumatic valve control system is a key link in industrial automation, and correct design and implementation can significantly improve production efficiency and safety. Feel free to share your practical experiences in the comments!