Batching process is an important link in the production of cement, chemical, food, building materials and other industrial and agricultural production, but also the source of production process, while the quantitative weighing and batching system is also widely used in a variety of bag packaging measurement of the production process. The traditional quantitative weighing method has been unable to meet the requirements of measurement accuracy and work efficiency in many fields. This paper introduces the quantitative weighing and dosage system, is based on the development of intelligent microcontroller devices, not only for industrial quantitative weighing can also be used for industrial dosage.

1 System working principle

The upper computer of this system adopts weighing instrument to set the weight of various ingredients, which is transmitted to microcontroller through serial communication. At the beginning of a dosage process, the microcontroller opens the first motorized valve to start the first bin of material, and at the same time collects the signal from the weighing module on the hopper scale, compares it with the preset weight of the corresponding dosage, and closes the motorized valve when the preset weight is reached. When the preset weight is reached, the motorized valve will be closed. Repeat the same operation for the next channel until all channels have finished discharging materials, and then transfer the dosage to the next dosage. The structure and working principle of the system are shown in Fig. 1 and Fig. 2 respectively.

2.1 Microcontroller

This system microcontroller selection of serial converter for Freescale's 08 series of products, the product is widely used because of its high stability, short development cycle, low cost, more models, good compatibility. At the same time the high reliability of electromagnetic compatibility EMC and EDC so that it can be used in various types of harsh environments, the applicable temperature range is very wide, suitable for a variety of work environments.

2.2 Hopper scale

The installation position of the hopper adopts the cantilever beam metering module for hopper weighing, which is a foil strain gauge pasted on the elastomer made of alloy steel, featuring high precision, small temperature drift, etc. It can be used for the load force detection of electronic belt scales and silos. Multiple sensors are installed symmetrically, and the voltage outputs are connected in parallel, so the installation position of the weighing elastomer force elements on the hopper has a great influence on the measurement accuracy. In order to minimize the additional error of the total output voltage, the weighing module should be installed in such a way that each element is subjected to the same force in order to guarantee the accuracy of the measurement, i.e., it is necessary to make the centers of polygons constituted by all sensors and the gravity center of the weighing hopper be on the same vertical axis. On the center of gravity of the weighing hopper. At the same time, the sensitivity, internal resistance, linearity and other properties of multiple sensors should be basically the same, in order to ensure that the weighing results are accurate! Meanwhile, in order to ensure the smooth discharging of materials, the bin and hopper scales and all pipelines should be made of plastic materials with very high finish.

Sensor 4 is a four-group parallel connection of resistance strain type weighing module, due to the strain gauge type sensor output is mV signal, amplified by the instrumentation amplifier, input AD conversion chip.

A/D conversion chip using 12-bit analog-to-digital converter TCL2543, with SPI interface capability of MC908GP32 can be directly connected (as shown in Figure 5). TCL2543 is the United States Texas Instruments TI introduced a cost-effective and better chip, with fast conversion, good stability, and microprocessor interface simple, low programming prices and other advantages.

Hopper scales in production will have two forms of error: static error and dynamic error. Static error refers to the weighing accuracy of the weighing part of the automatic batching system in the non-production state. Depends on the hopper weighing design, load cell accuracy, linearity, consistency of all the sensors on the same hopper weighing and whether the sensor installation is reasonable and other factors. Dynamic error refers to the actual dosage error generated after the whole dosage production system is operated. In the actual production process, the dosage accuracy is affected by many factors. For example, there is a distance between the silo and the hopper weighing (or drop), so the material into the hopper weighing always need a period of time, at the same time will produce a certain impact on the hopper weighing, due to the impact of gravity of the material and sudden stop, the hopper weighing will produce jittery vibration, the instrument will detect the weight data is greater than the actual weight, resulting in error. In addition, when unloading due to the adhesion of materials and residual part, will also produce errors. So when the beginning of the hopper weighing feeding or end of the feeding, should avoid this period of instability, can be designed to prohibit the beginning of the comparison time and the end of the time to prohibit the comparison of time, in this period of time does not carry out the comparison of the measured value and the preset value, in order to avoid the occurrence of misjudgment. The portion of material that is still in the air for a split second when the feed gate is closed is called the overshoot. In batching production, the most important parameter to control the accuracy is the overshoot, which is affected by the humidity, adhesion, block size and other factors of the material, and the randomness is great, so the overshoot is usually set by test.

2.3 Motorized valve drive circuit

This system selects the electric valve as the hopper discharge electronic control device, the operating voltage 24VDC, the mode of operation for the current regulation type, the input signal 4 ~ 20mADC, the electric valve is suitable for powder media. As shown in Figure 6 using DAC8571 as a DA converter chip, the chip for the IIC bus, 16-bit conversion, SCL is used to synchronize the communication with the microprocessor, SDA is used for the transmission of data, the 5V reference voltage generated by the REFO2 to provide. v/i converter chip using AnalogDevices high-performance converters produced by the company AD694, the current generated to meet the Motorized valve 4~20mADC requirements.

2.4 Stepping motor drive circuit

When all the ingredients are discharged as required, the hopper scale motorized valve opens and discharges the material to the pulley, and the stepper motor is controlled by the microcontroller to transfer the material to continue the next process. The system stepper motor driver chip using L6208, the chip integrated by eight power DMOS transistors composed of two full-bridge, a centralized logic circuit to perform the phase generation function and the shutdown time is constant pulse width modulation current controller, the controller uses the principle of internal chopper circuit, to solve the stepper motor frequent phase changes when the current fluctuates sharply and caused by torque changes.

The chip ENABLE for the L6208 enable signal, high level to enable the work. CW / CCW is used to control the stepping motor steering, high level, the motor is forward, low level, the motor reversed. CLOCK rising edge trigger is valid, can be adjusted through the frequency of the CLOCK pulse to control the frequency of the stepping sequence generated by the phase sequence generator, in order to achieve the stepping motor speed control. When RST is low, the phase sequence generator is initialized. HALF/FULL is used to set the mode of the stepping sequence, which can be set to half-step mode, single-phase full-step mode, and two-phase full-step mode, etc. The COMTROL signal is used to set the attenuation mode, which is a slow attenuation mode when it is high level and a fast attenuation mode when it is low level.

2.5 TTL/RS232 Level Conversion Circuitry

In order to facilitate the setting of the preset weight of the dosage, this design adopts the serial communication between PC and MCU to transmit the preset weight of the dosage from PC to MCU through the weighing instrument. Since the level standard of the RS232 serial port of the PC is not the same as the TTL level of the MCU, the serial communication between the MCU and the PC must adopt the RS232/TTL level conversion circuit.

3 software system design

3.1 Microcontroller control process

When a dosage process is completed, the microcontroller scans whether the material reset button on the interface is pressed or not, if there is a material reset command, then start the material reset subroutine, when the material reset is completed, the worker presses the material reset button on the microcontroller interface to confirm that the material reset is completed, at this time the microcontroller communicates with the upper computer, and the upper computer sends a new preset value of the dosage to the microcontroller. The microcontroller will control the dosage system to implement the new dosage process.

3.2 Serial Data Transfer Based on Weighing Instruments

Weigh Meter is a graphical programming language that creates applications with icons instead of lines of text, widely accepted in industry, academia and research laboratories as a standard data acquisition and instrument control software. The Weigh Meter integrates all the functions of hardware and data acquisition card communication with GPIB, VXIRS-232 and RS-485 protocols.

In this paper, the microcontroller and the weighing instrument for serial communication, using a common instrument driver VISA standard programming, VISA that is, the virtual instrument architecture is a standard to ensure that different manufacturers, different interface standards of the instrument can be compatible with each other, communication and data exchange, and to provide users with easy-to-use driver. Such as VISA configuration serial port function, VISA write function VISA read function. Communicate to the serial port initialization, set the serial port baud rate, data bits, stop bits and other parameters.VISA read the control word sent by the microcontroller, analyze whether the dosage reset is complete, such as the dosage reset is complete, through the VISA write, the dosage of 1 ~ 8 data, sent to the microcontroller.

4 Summary

In this paper, a microcontroller is used to control an automatic dosage process, where the host computer transmits the preset dosage values from the serial port to the microcontroller through a weighing instrument. This system can improve the accuracy work efficiency of the dosage process and has good practical value.