In this chapter, we will explore how to implement PID loop control using the GForce-200PLCCPU222.
2. Overview of PID Control
The GForce-200 series PLC is equipped with PID control capabilities, and its CPU can support up to 8 independent PID loops. PID stands for Proportional-Integral-Derivative, a widely used algorithm in closed-loop control systems. It combines three components—proportional, integral, and derivative—to adjust the output based on the difference between the setpoint and the actual process value. This feedback mechanism allows the system to continuously correct itself, making it a form of negative feedback control. The proportional term is determined by the gain (Kc) multiplied by the error. The integral term accounts for the accumulated error over time, while the derivative term reacts to the rate of change of the error.
PID control is implemented through the PID instruction function block. In the S7-200 system, the PID loop instruction uses input data and configuration parameters from the loop table to perform the necessary calculations and data exchange. The programming is straightforward, but it's important to note that the instruction affects the special memory flag SM1.1 (overflow). The PID operation can only be executed when the top of the logical stack is set to 1. The instruction has two operands: TBL, which specifies the starting address of the loop table (limited to VB area and byte type), and LOOP, which represents the loop number (ranging from 0 to 7). Up to 8 PID instructions can be used in a program, but if multiple instructions use the same loop number, even with different loop tables, unpredictable results may occur. Before using the PID instruction, real-world values such as gain (Kc), sampling time (Ts), integral time (TI), and derivative time (Td) must be normalized to the range of 0.0 to 1.0. This ensures proper conversion of physical quantities into data that the PID instruction can process.
3. PID Control Programming and Debugging
In this system, the steam pressure in the boiler needs to be maintained within a specific range of 0.85–1.0 MPa. A pressure transmitter detects the pressure, which has an output range of 4–20 mA corresponding to 0–2.5 MPa. At 0.85 MPa, the current output is approximately 9.44 mA, and at 1.0 MPa, it is around 10.4 mA. The normalized scale is shown in the figure below.
[Image: Pressure Transmitter Output Range]
The process variable is a unipolar analog signal detected by the pressure transmitter, and the loop output is also a unipolar analog signal used to control the blower speed. Both signals are normalized to a range of 0.0–1.0 with a resolution of 1/32000. Initially, the parameters were set as Kc = 0.06, Ts = 0.2, TI = 10.0, with no derivative action applied. The program was then developed accordingly.
[Images: Program Structure and Function Block Diagrams]
The program utilizes a main program, subroutine, and interrupt program structure, making it clear and efficient, which helps reduce the scan cycle time significantly.
4. Debugger
Proper tuning of PID parameters is essential for stable control. If the sampling time is too short, the system might not detect changes in the input signal effectively. On the other hand, a sampling time that is too long can lead to poor control accuracy. Similarly, an excessively high gain can cause oscillations in the system. During debugging, it’s crucial to carefully adjust these parameters and gradually fine-tune them to achieve a stable and responsive control system.
5. Conclusion
PID control is a fundamental technique in closed-loop systems. In this example, it was used to regulate the blower speed to maintain constant steam pressure in the boiler. Similar applications, such as maintaining consistent negative pressure in the boiler, follow the same principle and are not detailed here.
[Image: PID Control Application in Boiler System]
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