1 Introduction:
Automation control systems are an indispensable component of industrial production. As the core of automation control systems, Programmable Logic Controllers (PLCs) are widely utilized in various industrial sectors due to their flexibility, reliability, and economic benefits. The control algorithms in PLC programming are crucial in achieving automation control, directly determining the performance and effectiveness of the control system. This article will introduce several commonly used control algorithms in PLC programming, combined with specific cases and ST programming examples, to help readers gain a deeper understanding of the working principles of these algorithms and their applications in actual production.
2 The Importance of Control Algorithms:
Control algorithms are the core of PLC programs, whose primary function is to calculate and output control signals based on input signals and predefined control strategies to achieve control over the controlled object. The choice and application of control algorithms have a direct impact on system performance and reliability, including:
Control accuracy: The precision of the control algorithm determines whether the system can accurately track the target value and effectively eliminate interference.
Response speed: The response speed of the control algorithm determines the system's reaction to input signals and its ability to handle unexpected events.
Stability: The stability of the control algorithm ensures the system's stable operation under various working conditions and robustness to parameter changes.
Reliability: The reliability of the control algorithm determines the system's reliability during long-term operation and its fault tolerance capability.
3 Common Control Algorithms:
3.1ON/OFF Control Algorithm:
The ON/OFF control algorithm is one of the most basic control algorithms, utilizing ON and OFF states to control system operation. When the system reaches a setpoint, the PLC issues an OFF command to turn off the system; when the system exceeds the setpoint, the PLC issues an ON command to turn on the system. The ON/OFF control algorithm is commonly used for simple control tasks, such as switches, valves, and door controls.
Case Study:
In a beverage production line, the PLC utilizes the ON/OFF control algorithm to control the operation of a water pump. When the water tank is full, the PLC issues an OFF command to turn off the pump; when the water tank is empty, the PLC issues an ON command to turn on the pump.
ST Program:
IF (Level < Setpoint) THEN
Pump:= ON;
ELSE
Pump:= OFF;
END_IF;
3.2 Proportional Control Algorithm:
The proportional control algorithm calculates the control output as a proportion of the error signal. This algorithm is useful when the system requires a fast response to deviations.
Case Study:
Let's assume we want to control the flow rate near a setpoint by adjusting the valve opening. Based on the error value, the control signal for the valve opening is calculated using the proportional control formula. Adjust the proportional gain (Kp) value to optimize the system's response speed and stability.
ST Program:
Error := Setpoint - ActualValue; // Calculate the error signal
Output := Error * Kp; // Calculate the control output
3.3 PID Control Algorithm:
PID (Proportional, Integral, Derivative) control algorithm is one of the most commonly used control algorithms. It can control system operation by adjusting three parameters: proportional, integral, and derivative. PID control algorithms are typically used for complex control tasks such as temperature control, position control, and speed control.
Case Study:
In a chemical plant, the PLC can control the temperature of a reaction vessel using a PID control algorithm. When the temperature of the reaction vessel exceeds the setpoint, the PLC reduces the controller's output to lower the temperature; when it falls below the setpoint, the PLC increases the output to raise the temperature. Adjust the proportional gain (Kp), integral gain (Ki), and derivative gain (Kd) values to optimize the system's response speed and stability.
ST Program:
Err := Setpoint - Temp;
PID := Kp * Err + Ki * Integral(Err) + Kd * Derivative(Err);
Output := Limit(PID, 0, 100);
3.4 State Machine Control Algorithm:
The state machine control algorithm is a state-based control algorithm that controls system operation by defining system states and state transitions. It is commonly used for complex control tasks such as robot control, elevator control, and vehicle control.
Case Study:
In a production line, the PLC can control the operation of a conveyor belt using a state machine control algorithm. When the conveyor belt is in an idle state, the PLC sends a start command; when it is running, the PLC sends a stop command to halt the belt.
ST Program:
CASE Status OF
Idle:
IF (Start_Button) THEN
Status := Running;
CONVEYOR_BELT := ON;
END_IF;
Running:
IF (Stop_Button) THEN
Status := Idle;
CONVEYOR_BELT := OFF;
END_IF;
END_CASE;
3.5 Fuzzy Control Algorithm
The fuzzy control algorithm is a control algorithm based on fuzzy logic. Its principle utilizes fuzzy language and fuzzy rules to describe control strategies, and outputs fuzzified control signals based on the fuzzification of input signals and the reasoning of fuzzy rules. The fuzzy control algorithm can effectively handle uncertainty and nonlinear systems, suitable for systems that are difficult to describe with precise mathematical models.
Case Study:
When a robot grasps an object, the fuzzy control algorithm can fuzzily describe the grasping strategy based on information such as the object's shape, weight, and position. It then performs reasoning based on fuzzy rules to output control signals, achieving precise control of the robot.
ST Program:
IF Error IS NegativeBig THEN
Output := NegativeBig;
ELSIF Error IS NegativeSmall THEN
Output := NegativeSmall;
ELSIF Error IS Zero THEN
Output := Zero;
ELSIF Error IS PositiveSmall THEN
Output := PositiveSmall;
ELSIF Error IS PositiveBig THEN
Output := PositiveBig;
END_IF
4 Conclusion:
Control algorithms in PLC programming are crucial for implementing automated control systems. They simplify and effectively complete complex control tasks. In practical applications, the design and selection of control algorithms require consideration of the system's characteristics and requirements to ensure its reliability and efficiency. ON/OFF control algorithm, proportional algorithm, PID control algorithm, state machine control algorithm, and fuzzy algorithm are the most commonly used control algorithms in PLC programming, applicable to various automated control systems.
However, it is important to note that the design and selection of control algorithms require relevant knowledge and experience, as they can lead to system instability and inefficiency if not done correctly. Therefore, in practical applications, it is necessary to select the appropriate control algorithm based on the system's requirements and characteristics, and conduct careful design and testing.
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