Application Design Method of PLC in Beverage Filling Machine Control System

0 Introduction

The beverage filling machine is used to fill all kinds of bottled drinks and is suitable for large and medium-sized beverage manufacturers. Most of the early filling machines used positive displacement pump and peristaltic pump as metering methods. These methods have some disadvantages, for example, it is difficult to ensure the filling accuracy and stability, and it is difficult to change the filling specifications. The beverage sub packaging metering used in this system is determined by time and flow per unit time. The metering accuracy is controlled and determined by programmable logic controller (PLC). The operation status is monitored through man-machine interface - touch screen, and the operation parameters can be modified online. PLC control has the characteristics of simple programming, reliable operation and convenient use. It is widely used in the field of industrial automation control. Touch screen is a programmable terminal (PT) integrating display and touch switch. It is a new generation of advanced man-machine interface products. The touch screen specially designed for PLC application integrates host, input and output equipment, which is suitable for use in harsh industrial environment.

1 working principle

The beverage filling machine mainly includes three parts: constant pressure liquid storage tank, bottle clamping and filling head, frequency conversion and speed regulation conveyor belt. The upper part of the main engine is a constant pressure liquid storage tank, in which there are upper limit and lower limit liquid level sensors, which are in 1 state when submerged. When the liquid level is lower than the lower limit, the constant pressure reservoir is empty. The beverage flows into the constant pressure liquid storage tank through the liquid inlet solenoid valve. When the liquid level reaches the upper limit, the liquid inlet solenoid valve is powered off and closed to keep the liquid level stable. The production process flow of filling equipment is shown in Figure 1.

Fig. 1 process flow chart

Below the constant pressure liquid storage tank are bottle clamping and filling heads, with a total of 24 filling heads. The bottle clamping device is driven and lowered by the air cylinder 1. After it is lowered in place, the bottle clamping device is clamped and positioned by the air cylinder 2, and the lowering and clamping are controlled by the travel switch. After positioning and clamping, the filling head is driven down by the pneumatic cylinder 3. After it is in place, the solenoid valve of the filling head opens and starts filling liquid. After the delay, the solenoid valve closes, and the filling capacity is controlled by controlling the opening time of the solenoid valve.

The conveyor belt motor is controlled by frequency converter to realize stepless speed change and achieve the purpose of economic operation of the system. After the motor is started for 1 s, the bottle inlet cylinder retracts and begins to enter the bottle. After 3 s, the cylinder 4 at the bottle outlet extends to block the empty bottle. A photoelectric switch is set at the bottle inlet to detect the number of bottles. When 24 bottles are detected, the cylinder 5 at the bottle outlet will not enter the bottle, and the conveyor belt motor will stop. At this time, the filling head drops to the bottle mouth, and the PLC controls the opening time of the filling head by the time input through the touch screen. After filling, the filling head rises, and the bottle clamping device relaxes and rises. The cylinder at the bottle outlet retracts, the conveyor belt motor starts to rotate again, 1 s later, the cylinder 5 at the bottle inlet retracts, and the photoelectric switch starts to detect the number of bottles.

2 hardware system design2.1 system framework

The system has both switching value control and analog variable frequency speed regulation control. The equipment can operate automatically and continuously, and each moving point can be jogged manually, so there are many input points corresponding to various operations and output points of action status information to be displayed. If these I / O signals are displayed by electrical buttons and indicators, the hardware modules and electrical connections will be greatly increased, and the corresponding failure rate will also be increased. We adopt the scheme of combining PLC and touch screen. The screen of the touch screen is designed with special configuration software, and then downloaded to the touch screen through the RS - 232C serial communication port of the computer. PLC and touch screen communicate through serial interface, and the connection is simple. The system framework is shown in Figure 2.

Fig. 2 hardware system block diagram2.2 design of I / O controlThe filling equipment is designed with 13 digital input points, including 10 cylinder motion sensors, 2 liquid level sensors and 1 photoelectric switch. There are 35 digital output points, including 24 filling head solenoid valves, 10 cylinder motion solenoid valves and 1 liquid storage tank solenoid valve.

The variable frequency speed regulation system needs one analog input point and one analog output point. The speed measuring motor measures the speed of the motor, and the voltage value signal is connected to the analog input point. After comparison with the given value and PID operation, the operation result is output from the analog output point as the control signal of the frequency converter to realize variable frequency speed regulation.The main control unit adopts the S7 - 200 series PLC product cpu224 of Siemens, plus two digital expansion modules em223 and one analog expansion module EM235. The touch screen adopts pws3260 made in Taiwan.3 software implementation

3.1 overall software functions

Fig. 3 is a block diagram of touch screen software control program. The control program is compiled in the form of menu. Automatic functions include: operation, pause, end, reset, etc. Manual functions include: forward, backward, start, stop, etc. of all moving parts. Use the alternating button function programming in adp3 configuration software. When the button is pressed and lifted, the corresponding intermediate relay of PLC is set respectively, so that PLC can realize the forward and backward control of a moving part. Initial value setting: arbitrarily set the speed, count value and other parameters according to the user's needs, monitor the upper limit of the parameters, and give a prompt once it exceeds the limit. Operation monitoring: monitor the status of various components of the system, such as the abnormal status of frequency converter and motor, timely power off protection, and give alarm prompt.

Fig. 3 program block diagram3.2 PLC programmingSiemens STEP7 - micro / w in32 software is used to program according to the control flow and control algorithm on the upper computer using a more intuitive ladder diagram or statement table. After the program is compiled successfully, the program is downloaded to the PLC through the PC / PPI cable connecting the upper computer and PLC.

4 ConclusionThe electrical control scheme combined with PLC touch screen and combined with mechanical, pneumatic and sensor technology make the filling equipment simple to operate, reliable in performance, and significantly improve the maintainability and flexibility of the equipment.