Application of single chip microcomputer fuzzy control in electric rice cooker

At present, most of the rice cookers on the market use fixed power to heat, with low energy utilization and single functions, which makes it difficult to meet people's growing living needs. It is very necessary to develop a microcomputer rice cooker with complete functions, low cost, energy saving, safety and reliability.

1 Working principle and hardware composition of rice cooker

The system uses a control circuit with a low-cost, low-power, and good-performance 8-bit A/D type HT46R47 microcontroller as the control core. The pins are shown in Figure 1.

Its main features are as follows:

·Operating voltage: fSYS=4MHz: 2.2～5.5V;
fSYS= 8MHz; 3.3-5.5 V;
·13-bit bidirectional input/output port;
·8-bit programmable timer/counter with overflow interrupt, with 7-level prescaler;
·Quartz crystal or RC oscillator;
·2 048×14-bit program memory PROM;
·64×8-bit data memory RAM;
·When VDD=5V and the system clock is 8MHz, the instruction clock is 0.5μs;
·s four-channel 9-bit A/D converter;
·The instruction execution time is 1 or 2 instruction cycles with low voltage reset function.

1.1 Working Principle

The working principle of the rice cooker is shown in Figure 2. After power is turned on, the system enters the standby state. At this time, the system can receive the user's function selection, and the user's selected function is displayed through the display circuit. When the user presses the OK button, the MCU starts to monitor the temperature and perform corresponding heating control on various functions until the function ends and a sound alarm prompts.

1.2 Hardware circuit design

(1)MCU

MCU is the core part of the rice cooker, completing functions such as data collection, input, processing, output, and display. [page]

(2) Temperature measuring element

The temperature measuring element is the key to accurately detect the temperature. A thermistor with a negative temperature coefficient is used. Since the change in the thermistor value is nonlinear with the change in temperature, in order to improve the temperature measurement resolution and the anti-interference performance of the system, the circuit is designed as shown in Figure 3.

In Figure 3, Rt is a thermistor with a negative temperature coefficient; after being connected in parallel with R1, the resistance value changes with the temperature in a nearly linear relationship, which improves the resolution; R2 acts as a voltage divider; Point O is the measurement point: when the temperature changes, the resistance value of Rt changes, and the voltage at Point O also changes accordingly. Measuring Point O can measure the temperature change; C1 prevents interference from causing a sudden change in the voltage at Point O.

(3) The heating execution circuit MCU outputs a square wave control signal through PBl, which is coupled by a capacitor, rectified, and sent to the B pole of the transistor. After amplification, it drives the relay to work. In this way, when there is a square wave output, the relay connects the power supply of the heating plate, and when there is no square wave output, the power supply of the heating plate is disconnected. The square wave signal can only be output when the MCU is working normally. When the single-chip computer crashes, it cannot be output. In other words, when the single-chip computer crashes due to serious interference, PBl cannot output square waves, and the power supply of the heating plate will be automatically disconnected. In this way, the safety of the system can be ensured.

(4) Sound alarm circuit

The MCU outputs a square wave signal through the PD0 port, which is amplified by the transistor and drives the AC buzzer to sound an alarm.

(5) Display key multiplexing circuit

The display circuit uses time-sharing scanning to output, 3 common ports and 7 display segments, in the form of 3×7 display output. The key reuses 4 of the 7 display segments. When scanning the key, all displays are turned off, and the I/O port connected to the key is set as the input port. When the scan is completed, it is changed to the output port.

(6) The clock power supply circuit generates the necessary working conditions for the MCU and is used to provide the clock and power required for the MCU to operate.

2 Software Design

The software flow of fuzzy control is shown in Figure 4.

In FIG4 , t1, t2, t3, t01, and t02 are time parameters, which are set according to different powers of the rice cooker, wherein t01

In a typical 750W rice cooker application, the values ​​are as follows:

t1=2 min，t2=30 s，t3=5 min，
t01=4 min，t02=8 min

The main steps are described as follows:

(1) After power is turned on, heat to 60 degrees at full power and enter (a).

(2) (a) Stage: Record the time t required to heat the rice cooker from 60 to 70 degrees, where t is proportional to the mass of rice and water in the pot;

(3) (b) stage: taking t1 as the time unit, measuring the start and end temperatures T1, T2 is the boiling state when T1 = T2;

(4) (c) stage: according to the time t obtained by measuring the amount of rice in (a), compare it with the set parameters t01 and t02, and select the appropriate heating power for heating;

(5) Taking t2 as the time unit, measure the start and end temperatures T1 and T2. When T1 = T2, the water is dry and heating is stopped;

(6) The rice is heated with residual heat for t3 seconds, and an alarm is sounded when the rice is cooked.

3 Conclusion

Because the electric rice cooker adopts fuzzy logic control, imitates the way of thinking of people, and combines accurate condition judgment, this electric rice cooker can accurately detect the boiling of water in the electric rice cooker even at different altitudes (with different boiling points), and can never overflow; because of the use of fuzzy logic, it can accurately detect whether the water in the electric rice cooker has been boiled dry, and accurately cut off the heating power supply, so that the cooked rice is soft and not burnt. On the basis of fuzzy logic, plus time control, this fuzzy control electric rice cooker has the functions of scheduled cooking, soup, porridge, etc. In addition, the circuit design of this electric rice cooker is simple and reliable, and the hardware cost of all control circuits is only more than ten yuan. At present, this control circuit designed by us has been adopted by many manufacturers and has been mass-produced. The wide application of this single-chip fuzzy control technology in electric rice cookers will have a far-reaching impact on improving people's lives.