Abstract: This paper introduces the design and implementation of Siemens S7-200 programmable logic controller (PLC) in an automatic control system for a wind pressure balancer. Combining the cigarette production process, the necessity of applying a wind pressure balancer in cigarette production is analyzed. Based on the characteristics of the S7-200 PLC, the control principle, software, and hardware implementation methods of its application in controlling wind pressure are detailed. The designed system is reliable, easy to operate, and highly practical. Keywords: Wind pressure balancer, PID regulation, programmable logic controller, stepper motor, sensor 1. Introduction The process airflow of the cigarette making unit is mainly used for lifting and dust removal of tobacco during the cigarette making process, and for drawing the cigarette and filter rod onto the filter rod roller through the negative pressure generated by the airflow during the cigarette making process. The process airflow parameters are designed according to the number of cigarette making units, and the airflow branch pipes of each unit are connected to the main airflow pipe in parallel. Because the process airflow parameters of each cigarette-making unit differ, the pressure of the air duct branch pipes for each unit should be determined according to the unit's operating status. Alternatively, when a unit malfunctions or production scheduling requires certain units to be shut down or the cigarette brand to be changed, subjective or objective shutdowns can occur, causing changes in the air pressure of normally operating units and affecting their operation. A feasible solution is to install an air pressure balancer on the air duct branch pipe of each cigarette-making unit to control the air pressure. The air valve of the air pressure balancer changes within a certain range, adjusting the air pressure without changing the airflow. Therefore, using an air pressure balancer can keep the air pressure on the branch pipes of the cigarette-making unit stable, ensuring optimal conditions for tobacco lifting and dust removal, and for the cigarette nozzle process. Due to the superior performance, high reliability, and industrial control characteristics of programmable logic controllers (PLCs), using a PLC in the control system of the air pressure balancer is a reasonable choice. Practice has proven that the air pressure control system designed based on a PLC has advantages such as high control accuracy, stable operation, and short development cycle, meeting production requirements. 2. Hardware Composition of the Control System [align=center] Figure 1 [/align] As shown in Figure 1, the wind pressure balancer control system mainly consists of a programmable logic controller (S7-200 CPU212), an analog input module (EM231), a human-machine interface device (TD200), a fieldbus adapter card (EM277), limit switches, wind pressure sensors, stepper motors, stepper motor drivers, dedicated power supplies for the drivers, and special wind pressure balancer regulating dampers. The central processing unit uses a CPU212 with transistor output, which has functions such as pulse output, high-speed counting, and power-off protection. The CPU module itself has 8 built-in digital input points and 6 digital output points, and is connected to the expansion function modules via a dedicated internal bus. Input quantities include digital input from push-button switches, digital input from position detection switches, and external digital input from the winding unit's operating status. Digital input parameters determine the initial, state transition, and termination operating conditions of the system; digital outputs include alarm outputs, stepper motor direction control, and stepper motor enable control; pulse outputs use pulse width modulation (PWM) to control the rotation and speed of the stepper motor. The stepper motor drives a special regulating damper in the wind pressure balancer to open and close within a certain range via a screw drive to regulate wind pressure. The analog input module (EM231) is an analog input module with four current or voltage differential inputs. This system has two wind pressure analog inputs. The wind pressure sensor has a 4-20mA current output, while the EM231's measurement range is 0-20mA. Therefore, the wind pressure signal acquired by the system needs to undergo linear transformation to obtain its true value. Analog inputs are the basis for system adjustment. The fieldbus adapter card (EM277) allows the S7-200 CPU to be connected to the PROFIBUS fieldbus as a substation, communicating with the master station on the bus for monitoring. In fact, winding and unwinding units often consist of multiple units, and the CPU212 on each unit can be connected to the bus for equipment status monitoring and production management. The human-machine interface device (TD200) is used to monitor the working status of a single unit and set control parameters. It exchanges data with the CPU212 using PPI communication. When multiple CPU212s are connected to the fieldbus, a human-machine interface device using MPI or DP communication can be connected to the bus to replace the TD200, enabling a single HMI device to monitor all CPU212s. 3. Control Principle of the Control System As shown in Figure 2, the control system of the wind pressure balancer uses a PID control algorithm with dead time. In the figure, p(k) represents the deviation signal between the set wind pressure difference r(k) and the actual wind pressure difference y(k). This deviation signal is the input for PID control and satisfies the following conditions: When |r(k) - y(k)| > e, p(k) = e(k); When |r(k) - y(k)| <= e, p(k) = 0; ±e is the dead zone. Using a PID algorithm with a dead zone avoids excessively frequent damper operation of the wind pressure balancer, preventing system oscillations and hindering control. The dead zone is adjustable; a suitable value can be input via the TD200 HMI connected to the CPU212, depending on the actual situation. The control principle of the PID algorithm is based on the following equation: M(t) is a function of the proportional, integral, and derivative terms; Kc is the loop amplification factor; e is the deviation value; and Minitial is the initial value of the loop output. To facilitate control by a digital computer, numerical approximation methods must be used. When the sampling period is very short, summation replaces integration, and backward difference replaces differentiation, transforming the analog PID discretization into a difference equation. Therefore, the above equation can be rewritten as a continuous function quantized into periodic sampling of error values, simultaneously calculating the output value Mn, where en and en-1 are the error sampling values ​​at times n and n-1 respectively, Kc is the loop amplification factor, Ki is the integral term constant, and Kd is the differential term constant. The above equation can be rewritten as follows: where MX is the value of the integral term at time n-1. Alternatively, it can be rewritten as follows: where SPn is the setpoint, PVn is the process value (actual pressure difference), Ts is the sampling period, Ti is the integration time, and SPn = SPn-1, so Ts is the sampling period, Td is the differential time, SPn and SPn-1 are the setpoints, and PVn is the process value (actual pressure difference). Since SPn = SPn-1, we have: Ts is the sampling period, Td is the differential time, SPn and SPn-1 are the setpoints, and PVn is the process value (actual pressure difference). The actual analog value is linearized, R is the linearization value, R is the actual analog value, and Span is the range. When the analog input is unipolar, Span=32000, Offset=0. 4. Software Implementation of the Control System The design uses Siemens' STEP7 MICROWIN V3.2 integrated development platform. After the hardware is configured, the CPU212 can be programmed directly. The software design mainly includes sampling interrupt program, pulse generation program, TD200 text display configuration, etc. 4.1 Sampling Interrupt Program: After initializing SMB34 (special memory byte), a sampling interrupt service routine is written in the interrupt program block. The analog signal value is obtained by reading the peripheral word AIWx. The analog input adopts a single-stage input method. After linear transformation, the pressure value can be obtained. In the sampling interrupt service routine, the pressure signal values ​​at both ends of the wind pressure balancer on the wind duct are read respectively, and the absolute value of the two pressure differences is calculated. This value is used as the process value. Changing the SMB34 value changes the sampling period. 4.2 Pulse Generation: In the CPU 22X series, the embedded digital output ports Q0.0 and Q0.1 of the CPU module can be used to output high-speed pulse signals. There are two pulse output modes: PTO and PWM. PTO is a pulse train output mode, where the number of pulses and the pulse period are controllable. PWM is a pulse modulation output mode, where the pulse width and pulse period are controllable. This can be achieved through the INSTRUCTION WIZARD in STEP7 IDE or by writing a pulse generation program. 4.3 Implementation of PID Control: Pure Proportional Control: After adjusting the stepper motor's microstepping factor and number of steps, use the INSTRUCTION WIZARD in STEP7 IDE to set Q0.0 to PWM pulse output mode. Carefully adjust the output pulse width and period; this is equivalent to adjusting the gain of the proportional term to adapt to the changes in the controlled variable, thereby achieving the goal of stabilizing the wind pressure differential. The PWM pulse output of Q0.0 is determined by whether the deviation between the set wind pressure difference r(k) and the actual wind pressure difference y(k) exceeds the set dead zone range. If the deviation is within the set dead zone range, the output of Q0.0 is disabled; otherwise, Q0.0 is enabled, controlling the stepper motor to rotate. The direction of rotation of the stepper motor is determined by the sign of the deviation value. When the wind pressure balancer damper reaches the upper or lower limit, a high level is output through Q0.x to prevent the stepper motor from continuing to rotate in the direction it has reached its limit. PID control mode: The output is still a PWM pulse, thus realizing the digitization of analog output. The method is: using the PID WIZARD in the INSTRUCTION WIZARD of STEP7 IDE, select digital output mode when designing the output mode, input the appropriate square wave duty cycle parameter, and fill in the address of the data block required for PID calculation. When the deviation exceeds the set dead zone range, Q0.x will output a pulse width modulated square wave to control the stepper motor. The control of other control points is the same as in the pure proportional control mode. PID control improves the quality factor of the control system by eliminating steady-state error and enhancing dynamic performance. However, PID parameter tuning is relatively complicated, requiring a period of trial and error to obtain suitable parameters. In engineering, common methods for PID parameter tuning include: extended critical proportional gain method; extended response curve method; normalized parameter tuning method; optimization method; and trial and error method. These will not be elaborated further due to space limitations. The wind pressure balancer control system based on a programmable logic controller (PLC) adopts a modular and open design philosophy for both hardware and software. The system features high cost-effectiveness, high control precision, stable and reliable operation, convenient expansion, low failure rate, and simple maintenance, meeting the requirements of cigarette production. The wind pressure balancer and its control system designed by our company have been applied in cigarette factories, and the system has significant potential for wider application.