Automatic Control of Rice Milling and Polishing Machine Using Single Chip Microcomputer

1 Introduction
After multiple milling processes, the rice bran has been stripped, but the surface of the rice grains is relatively rough. In order to ensure the quality and appearance of the rice, the last process of rice processing is required: polishing. The polishing of rice grains is completed by a rice milling and polishing machine. Polishing can make the surface of the rice grains dense and smooth, which not only ensures the quality of the rice, but also improves the appearance of the rice grains. Polishing is completed under certain temperature and humidity conditions and after a certain period of grinding. Therefore, some important process parameters must be met during the polishing process. First, the temperature of the added water must be strictly controlled. Too low or too high a temperature will make the polished rice grains black, affecting the quality of polishing; second, the amount of water added must be strictly controlled. If too much water is added, the humidity after polishing is too high, which affects both the quality of polishing and future storage and transportation; if too little water is added, the humidity during polishing is too low, and the smoothness and density of the rice grains after polishing will not meet the requirements. The existing polishing methods of domestic small and medium-sized polishing machines all adopt semi-manual methods: the temperature of the water tank is controlled by a temperature controller, and the amount of water added is achieved by a manual regulating valve. On the one hand, manual adjustment makes it difficult to ensure quality, and on the other hand, it increases the labor intensity of workers.

Therefore, it has become an urgent need for polishing machine manufacturers to adopt advanced automatic control technology to realize the automation of rice milling and polishing machine control.

2 Principle and composition of automatic temperature and humidity control

After many contacts with manufacturers, we learned some basic requirements for the automation of the polishing machine production process. According to some specific requirements of the manufacturer, we designed an automatic control system for rice polishing machine with 89C51 as the core. (1) Automatic control of water temperature

The water temperature is an important parameter in this process. To realize the automatic temperature control, an 8-bit A/D conversion chip ADC0809 is expanded. The temperature sensor and transmitter are designed by ourselves, so that the water temperature of 0-100℃ corresponds to the AD0809 input of 0-5V. The single-chip microcomputer controls the switch of the heating wire after sampling and comparison, realizing the automatic control of the water temperature.

(2) Achieve uniform humidification

In order to ensure the humidification effect of the polishing machine, the original water injection humidification is changed to atomization humidification. The humidification method of existing domestic small and medium-sized polishing machines is water injection. Since the humidification amount of the polishing machine is not large, this method can easily cause uneven humidification, thereby affecting the quality of polishing. Using atomization to inject water can basically eliminate the uneven humidification and greatly improve the polishing effect. The atomization of water is achieved by high-frequency vibration. This kind of atomization device can be easily purchased on the market.

(3) Automatically control the humidity in the polishing machine chamber

We use the humidity sensor of Guangdong Fenghua Company. This sensor integrates sensing and transmission, is small in size and weight, moderately priced, and relatively easy to install. The standard voltage signal output by the sensor is sent to another channel of AD0809. After sampling by the single-chip microcomputer, it is converted into a digital signal, which is compared with the set humidity value to control the opening and closing of the humidification solenoid valve. There are three solenoid valves that control the humidification amount: large, medium, and small. The application of this method avoids the complexity of the system and ensures the reliability of the system and the accuracy of control.

(4) Ensure that the feed door can only be opened when the water temperature reaches the set value, and the atomizing vibrator is turned on at the same time. This requirement is to prevent the addition of water that does not meet the temperature requirements to the polishing machine when the water temperature has not reached the set value. To meet this requirement, a solenoid valve switch is installed at the feed port. When the water temperature starts to work, when it is lower than the set value, the switch is locked to prohibit the entry of raw materials; only when the water temperature reaches the set value can the solenoid switch be turned on. In practice, the water temperature cannot be strictly equal to the set value. It will fluctuate around the set value and can be limited to a certain range. As long as the temperature difference is within the limited range, it is considered to have met the requirements.

3. Hardware composition of single chip microcomputer system

In order to reduce the cost of the system, the temperature sensor probe uses the cheap thermal resistor CU50, and the transmitter is designed by ourselves. First, the temperature signal is converted into a corresponding analog electrical signal through a bridge circuit. The voltage signal output by this bridge circuit is relatively weak and must be amplified accordingly. This system uses an ordinary operational amplifier to form an instrument measurement amplifier with a relatively high input resistance and a relatively strong common-mode suppression effect to realize the function of the transmitter and meet the needs of the system. The adjustable resistor brother can realize the zero adjustment of the temperature signal, and the adjustable resistor Ro can realize the adjustment of the temperature signal amplification factor. These two adjustable resistors are used for the calibration of the temperature signal.

The single-chip microcomputer uses ATMEI's 89C51. This chip has a 4kFlash memory, which is enough to meet the needs of this system, and no external memory expansion is required. The A/D converter uses ADC0809, which has 8 input channels and fully meets the requirements of this system. The A/D converter is connected to the CPU in a query mode. The single-chip microcomputer continuously collects the digital quantity sent by the A/D, and sends it to the 7-segment LED for real-time temperature and humidity display after corresponding processing. The display circuit uses the 7-segment code decoder 4513 to complete the segment selection of the LED, and the 1413 to complete the bit selection. These two chips are driven by a parallel chip 74HC273 that is expanded by the single-chip microcomputer. The system setting is completed using 4 independent keyboards: one is the setting key, pressing this key to enter the setting mode, each time it is pressed, it switches back and forth between the two setting modes of temperature and humidity; one is the working setting key, pressing this key to enter the working mode; the other two keys realize the functions of plus 1 and minus 1. The plus 1 and minus 1 keys can only work in the setting mode. In the working mode, the system does not respond to pressing these two keys. The system also has 5 switch outputs, P1.3 pin is used to control the feed solenoid valve, P1.4 pin is used for temperature control, P1.5, P1.6, P1.7 pins are used to control the mist vibration switch.

Considering that this single chip microcomputer has no power-off protection memory, the system setting value will return to zero in the case of power failure or crash, and it needs to be set every time the computer is turned on, which is quite cumbersome. For this reason, the system has expanded an E2PROM memory. We choose 12C memory--CAT24C021, which is used to store the system setting value.

CAT24C021 is a product of CATALYST, an American company. It integrates three functions: E2PROM memory, watchdog timer, and reset controller. Its application, on the one hand, provides power-off protection memory; on the other hand, it greatly increases the reliability of the system. Its built-in watchdog timer provides an independent protection system for the microprocessor. Once the system fails, the reset signal will respond after the timeout. In addition, its internal Vcc detection circuit can protect the system from low voltage. When the minimum operating voltage is detected, the system will reset until Vcc returns to the normal value. Its memory part uses 256B serial E2PROM with a simple 2-wire I2C interface. It can transmit data with only one bidirectional data line SDA and one clock control line SCL. In addition, when used as an input port, the electronic jump on this pin will reset the watchdog timer.

The application of I2C interface technology has greatly reduced the system cost and its requirement for circuit board space. This system uses P1.0 and P1.1 to realize data transmission between it and the microcontroller.

The write protection pin WP of 24C021 is connected to P3.0 of the microcontroller. When WP is high, the E2PROM is in write protection (read-only) state; when it is low, the microcontroller can rewrite the E2PROM.

To further ensure the reliability of the system, photoelectric isolators and 12V relays are used to isolate the output port of the microcontroller from the high-voltage contactor.

4 System software design

(1) Setting module

The single chip microcomputer first reads the temperature and humidity setting value of the keyboard and then stores it in 24C021. This module mainly includes key value reading, software delay debounce subroutine and I2C read and write subroutine.

(2) Sampling display module

This module mainly realizes data acquisition and display. According to the timing requirements of A/D, start A/D conversion, then check whether the conversion is completed, and then read the conversion result of A/D. The collected 2-way digital quantity is the corresponding value of the temperature and humidity analog signal. In order to further increase the stability of the system, the collected values ​​are smoothed and median filtered. Finally, the binary number corresponding to the actual temperature and humidity is obtained, and the binary data is converted into the corresponding temperature and humidity values ​​through a certain algorithm, and sent to the LED for display.

(3) Control module

Another major task of the system is to realize automatic control of temperature and humidity. When the machine is turned on, the microcontroller collects the temperature value and compares it with the set value. If it is not within the specified range, heating will continue; if it meets the requirements, the material level switch and the atomizing vibrator will be turned on to enter the normal operation state: the microcontroller reads the temperature and humidity set values ​​in 24C021, and compares them with the samples, and performs heating and humidification control according to the comparison results
.

This system was used in the automatic control transformation of a polishing machine in a grain machinery factory last year. It has been running well since it was put into use and has created certain economic benefits for the manufacturer. Practice has shown that the design of this system is successful and reliable.