Application of single chip microcomputer in power control of electromagnetic cooker

introduction

In recent years, with the gradual improvement of environmental protection and energy-saving awareness, a new "green kitchenware" - induction cooker is becoming popular in households. It has changed the traditional open flame cooking method. It uses the principle of electromagnetic induction to generate a magnetic field when the current passes through the built-in coil. The magnetic lines of force in the magnetic field are induced to the iron utensils, generating countless small eddy currents moving at high speed. The huge circulation energy generated by the eddy current is converted into effective heat energy, which makes the pot heat up at high speed by itself and finally directly heats the food. The thermal efficiency of the induction cooker reaches more than 90%. At the same time, it is smokeless, ash-free, pollution-free, does not raise the room temperature, does not produce harmful substances such as carbon monoxide, and is safe and environmentally friendly. The induction cooker also uses microcomputer control and can control the temperature at will. It is precisely because of the above advantages that the penetration rate of induction cookers in developed countries has reached more than 80%.

In order to improve the electric heat conversion rate, household induction cookers generally use high-frequency induction cookers. The industrial frequency electricity must be rectified into direct current and then inverted into a high-frequency oscillating current above 20kHz. At high frequencies, stable power output and real-time detection become the difficulty and key of the design. The use of the A/D type microcontroller HT46R22 produced by Holtek can easily realize constant temperature control, real-time detection, alarm detection and power control. This article focuses on the implementation of power control. 1 Power control design principle

1.1 Working principle of induction cooker

The working principle of the induction cooker is shown in Figure 1. It consists of a main circuit and a control circuit. The 220V AC in the main circuit is rectified and filtered before being added to a voltage resonant converter consisting of capacitors C1 and C2, inductor L1 and IGBT tube S. The converter has two main functions: one is to convert the power frequency into a high-frequency AC of 20 to 40kHz, and the other is to convert electrical energy into magnetic energy. The high-frequency AC is added to the excitation coil and acts directly on the bottom of the pot through electromagnetic induction, generating eddy currents and rapidly heating the pot. The control circuit uses the HT46R22 microcontroller as the main control chip, which can realize many necessary detection and protection functions. At the same time, it generates the PWM signal required for control to control the output power of the induction cooker.

1.2 Power control principle of induction cooker

Power control means that when the industrial frequency AC power is stable, the induction cooker can output the set power stably so that the induction cooker can work stably. Because the load current can reflect the size of the output power, the object of control can be converted into the load current. At the same time, the size of the load current is designed to be determined by the duty cycle of the PWM signal. The larger the duty cycle, the larger the load current.

The fire level (power level) set by the user is 1600W, 1400W, 1200W, 1000W, and 800W respectively. The relationship between the measured output power (W), load current (A), AC voltage (V), load current detection value CRU voltage (V) and PWM duty cycle of the original induction cooker is shown in Table 1.

Table 1 Relationship between output power and load current duty cycle

The control circuit adopts double closed-loop control, and the schematic diagram is shown in Figure 4.

Pin 14 (PD0) of the HT46R22 microcontroller can output a channel of PWM signal, which forms a DC control level with the current negative feedback signal through the DC control level forming circuit. At the same time, the current detection signal and voltage detection signal from the main circuit form a sawtooth wave after passing through the sawtooth wave generating circuit. The sawtooth wave is compared with the DC control level to obtain the control pulse for controlling the on and off of S in the main circuit of the induction cooker.

The comparison voltage V+ is constant, while V- varies with the output power. When the output power is greater than the set power, a low-level control level is obtained, a narrow on pulse is output, and the output power is reduced, and vice versa. In this way, the purpose of stabilizing the output power is achieved through a closed-loop regulation control.

2.3 Software Design

The software design of power control is the main part of the induction cooker software design, which is programmed in assembly language. The variables are described as follows:

1) Heating/fixed temperature state switching flag heatflg 1 for heating, 0 for fixed temperature, initialized to 1 when powered on, this flag can only be changed by the keyboard;

2) The high 5 bits of the heating gear register heatreg and the counter heatcnt heatreg are valid at low level, each bit corresponds to a heating gear, heatreg7 is "keep warm" and the control display is more convenient; heatcnt counts 5 gears, 0 is "fry", and the control table is more convenient; the power-on initialization heatreg=f7h, heatcnt=0, is the "fry" gear, and only the keyboard can change this sign;

3) The minimum power control change timing counter minpowcnt is reduced to 0 in the timer interrupt and then transmitted to the main program. When in "heating-keeping", the main program resets this counter and reverses the control line.

The source program and explanation are as follows:

; Power control

powctrl:

mov r0, a ; The following is the calculation of PWM = 128 * Ptab / VOL - 28

mov a, 80h

mov r1, a

call mul ;ro,r1=128* Ptab

mov a, VOLADN

add a, offset adcbfr

mov mp, a ; point to the mains voltage ADC value

mov a, jar

mov r2, a

call div ;r1=r0,r1/VOL

mov a, r1

sub a, 28 ;PWM=r1－28, value in A

mov r0, a

sub a, 127

snz cy ;PWM exceeds upper limit or not

jmp powctrl1 ;No

mov a, 126

jmp powctrl3

mov a, r0

sub a, 38

sz cy ; PWM exceeds the lower limit or not

jmp powctrl2; No

mov a, 38 ; yes

jmp powctrl3

powctrl2:

mov a, r0

powctrl3:

r1 acc

mov pwm, a

; Control line control

snz heatflg ; heating status no

jmp keephdl; No, switch to constant temperature control

snz heatreg.7; yes, yes "keep warm" mode no

jmp chgctrl; yes

clr control; No, open the control line

jmp endputdl

chgctrl:

sz minpowcnt; yes, 3 seconds to no

jmp chgctrl1; no

mov a, MINPTIME* 4 ； yes

mov minpowcnt a ； reset the minimum power counter

sz control ; Is the current control line output valid?

jmp chgctrl2; No

set control ; yes, set the control line to be valid

mov a, 50

mov ctrlcnt, a

jmp chgctrl1

chgctrl2:

clr control ; Enable control line valid

chgctrl1:

jmp endputdl

3 Conclusion

The power control method is used to control the power output of the induction cooker, and its power floating range can be effectively controlled within 50 W. Through actual product verification, the performance of the induction cooker is stable. Therefore, the power control method is feasible and is a method worth promoting.