A plastic enterprise dosage using loss-in-weight weighing, in order to ensure that each product dosage in the main material and various auxiliary materials to achieve accurate dosage, the entire dosage by more than one set of weighing equipment, weighing and dosage at the same time, the use of screw feeder method of material delivery. All the main and auxiliary materials are finally fed into a weighing hopper with a capacity of about 200Kg for mixing. The customer decided to upgrade the dosage due to the expensive maintenance of the dosage faults and the failure of the dosage performance to meet the requirements. This paper describes the transformation process and effect analysis of this system using Siemens special weighing module.

Original Batching System

The user's original feeding system mainly consists of weighing modules, weighing instruments, scales and other components, the system has fewer human-computer interaction interfaces, maintenance is quite difficult, and the price of board spare parts is expensive. Due to the design exclusivity of the weighing controller, the controller has a single function in the control process of this dosage, only the weight weighing function, there is no other control function in the whole process, and it can't realize the function of square report output and so on. In order to provide production efficiency, reduce the amount of on-site maintenance, users need to transform the original dosage into a weighing, control and management functions, real-time and accurate control of the weighing process, while human-computer interaction is fast and convenient, the information output of a variety of weighing control system. At the same time, the weighing control system is required to be universal, the weighing module accuracy and stability, and the man-machine operation interface is friendly.   

Modification process

The weighing module was selected according to the user's requirements. The weighing module is directly integrated into Siemens S7-300 PLC control system, which can be easily programmed to realize the weighing control function of loss-in-weight feeding and dosing. It can accurately control the instantaneous feeding flow and cumulative amount of powder, granule, flake, strip and lump solids during the production process. In the production process, the accurate control of feeding flow plays an important role in ensuring product quality.

The system consists of a main control PLC CPU 315-2DP with two external modules to control two dosages at the same time. The weighing module adopts Siemens' RN series single sensor, and the upper HMI adopts the latest MP277 10-inch color touch screen, the larger display area is conducive to the setting and displaying of the weight parameters, as well as the recording and displaying of the weighing working status. The function of feeding recipe enhances the flexibility of using the system. The recipe set value can be set directly on the touch screen by password. All the recipe data are saved in a memory card to prevent accidental loss. The module comes with function-specific output points to control some specialized functions of dosing, such as the switching of the dosing valve, the speed control of the screw doser, and other protection functions.

The dosage control system continuously controls the flow of material from the feeding mechanism by frequency conversion of the entire feeding system including the hopper scale and the feeding mechanism. The reduction of the material weight Δd/ Δt per unit of time in the dosage hopper corresponds to the feed flow rate of the dosage. The dv/dt is compared with the preset feed flow rate and automatically corrected by adjusting the feeder speed, so that an accurate feed flow rate can be maintained.

When the weight of the material in the hopper falls below the preset value, the weighing control module locks the feeder speed, keeping the lowering mechanism feeding the material in a volumetric manner. At the same time, the filler valve will open for fast feeding. When the weight of material in the hopper reaches the high preset value, the charging valve will be closed automatically.

Weighing and mixing machine contains touch screen, weighing instrument, measuring module, frequency converter and field control box. And leave the communication interface with the upper computer. The material is conveyed into the mixer. The screw conveyor sends the weight signal from the weighing module to the weighing instrument, which calculates and processes the load weight signal to arrive at the material weight, which is displayed on the weighing instrument respectively. The weighing instrument and frequency converter control the screw conveyor speed to form a closed-loop control. The built-in intelligent software regulator of the touch screen can set high, middle and low feeding to ensure the accuracy of batching. The output of three signals is used to control the frequency converter, which in turn controls the feeding speed of the screw feeder, thus achieving the purpose of accurate dosing. The weighing meter can set the feeding quantity manually or set the proportion through the two-way communication between computer and meter.

Screw conveyor can be seamlessly and confidentially connected with ERP system, integrating computer technology, weighing technology, automatic control, communication technology as a whole, its control function is perfect, display, operation is centralized, installation, maintenance is simple, data processing is convenient, the system is easy to expand. With powerful data management function and summarization, it can detect, calculate and control the industrial control machine monitoring and PLC is the most advanced and reliable industrial automation control mode at present. Siemens PLC's average continuous trouble-free operation time of up to tens of thousands of hours, and the most suitable for strong interference, harsh environment of the industrial site, industrial control computers are also designed for industrial site-specific, continuous, safe and reliable operation, and the industrial computer is one of the best on-site measurement and control of the ideal choice.