High precision automatic Weighing system is composed of charging device, weighing module and weighing instrument. In the liquid dispensing process of coating liquid dispensing system, the control precision of reagent filling is high. When the reagent needs to be filled in the liquid dispensing process, the host computer (HostPLC) sends the weighing instruction to the weighing module and the amount of reagent to be added. Above the weighing module is a hopper containing the agent. When weighing, the whole weighing system controls the amount of the agent added. When the added amount reaches the set amount, the weighing ends, and the agent is injected into the next level of the tank, mixed with other agents, and the liquid is dispense. In this system, the following two key problems need to be solved: the guarantee of weighing module and feeding accuracy. The weighing module is controlled by the host computer (HostPLC) and communicates with the host computer, dispenses liquid according to the timing and weight specified by the host computer, and feedbacks the actual weight of the upper computer in real time.

Due to the high weighing precision of the weighing module, automatic weighing precision can be guaranteed by controlling the feeding precision. In order to ensure the weighing module, the control method of "fast first, slow then, and finally move" is adopted, and the weight (tare) of the hopper of the weighing module is deducted. After the loading instruction is given by the upper computer, the unloading starts for a period of time, and the unloading starts slowly when the target value of fast feeding is reached. When the advance of the weight of the ingredients reaches the target value is detected, the feed valve is completely closed and the target weight is reached by the weight of the material. In the weighing control, the feed valve is set in three states: "wide open", "small open" and "closed". The "wide open" state is mainly used for coarse precision feeding, when the feeding approaches the set value, the "small open" action, when the set value is fully reached, the feed valve is "closed". According to the above ideas, the software module of feeding speed, fast feeding and point unloading is controlled by programming, and the influence of air blanking on weighing accuracy is eliminated.

In the whole weighing control system, the weighing controller is the core. Weighing and upper computer MelsecQ series PLC adopts RS-422 communication, accepts the batching instruction of upper computer, and feedbacks the actual weight of weighing to upper computer at the same time. The load is controlled by the external I/O(PI/O) feed valve, and the feed valve feedbacks the feed valve status to the PLC through PI/O; The weighing module sends the weighing weight to PLC through the special bus IDNET; The operation panel is connected to the weighing system through PI/O, and the working mode of the weighing system (on-site/remote) can be selected through the operation panel. When the field mode is selected, the batching can be manually operated through the field operation panel, and the operation status information of the weighing board can be fed back to the PLC through PI/O.