Auto-load impedance matching device of a microwave oven

This invention relates to a microwave oven which can generate and transfer maximum microwave power to food being cooked irrespective of the weight and size of that food being cooked, thereby shortening cooking time and improving the efficiency of cooking. The microwave oven includes a sensor for sensing the food load on a turntable in a heating chamber, a microcomputer for controlling the oven in response to key signals input by the user and in response to the load sensor, a power supply for supplying power to a magnetron to operate the magnetron under the control of the microcomputer, an impedance matching device for matching impedance by moving a reflector in a waveguide fixed to the heating chamber, a match driver for controlling the impedance matching device under the control of the microcomputer, a variable vane mounted in an opening of the heating chamber, and a variable vane driving means for rotating the variable vane under the control of the microcomputer.

FIELD OF THE INVENTION 
This invention relates to an auto-load impedance matching device of a 
microwave oven, more particularly to an auto-load impedance matching 
device of a microwave oven which can generate maximum power transfer to 
the food by impedance matching according to the weight and size of food. 
BACKGROUND OF THE INVENTION 
As shown in FIG. 1, a conventional microwave oven includes a heating 
chamber 1 for cooking food, a turntable 2 positioned in the chamber for 
supporting the food, a turntable motor 3 for turning the turntable, a key 
application part 9 having keys for a user to select cooking functions, a 
microcomputer 4 for controlling the microwave oven according to the key 
signals applied from the key application part 9, a magnetron 6 for 
generating microwaves, a power supply part 5 for supplying power to the 
magnetron 6 to operate the magnetron 6 under control of the microcomputer 
4, and a waveguide 8 for transmitting the microwaves generated in the 
magnetron 6 to the heating chamber 1 through an opening 7 formed in the 
wall of the heating chamber 1. 
Operation of the foregoing conventional microwave oven is as follows. 
First, when a user puts food on the turntable 2 in the heating chamber 1 
and presses keys selected to carry out a desired cooking function through 
the key application part 9 for cooking, the microcomputer 4 controls the 
turntable motor 3 to turn the turntable 2 and, at the same time, controls 
the power supply part 5 to supply power to the magnetron 6. 
Upon supplying power to the magnetron 6, microwaves are generated and 
transmitted to the heating chamber 1 through the opening 7 and the 
waveguide 8 to cook the food. 
However, the conventional microwave oven generates microwaves at different 
outputs and with different efficiencies depending on the load of food in 
the oven. 
For example, as shown in FIG. 2, if a microwave oven is designed to have an 
output of 700 W and an efficiency of 50% for 2000 cc of food, the output 
of the microwave oven will fall to 660 W and the efficiency will drop to 
46% for a food load of 1000 cc, and the output of the microwave oven will 
fall to 610 W and the efficiency will drop to 38% for a food load of 500 
cc. Therefore, there has been a problem that an optimum cooking condition 
can not be obtained when the food load in the oven varies. 
SUMMARY OF THE INVENTION 
The object of this invention is to solve the foregoing problem. The 
solution to the problem is to provide an auto-load impedance matching 
device for a microwave oven which can provide optimum cooking conditions 
irrespective of the weight and size of food by impedance matching for 
maximum power transfer of microwaves from the microwave source to the food 
in the heating chamber. 
This and other objects and features of this invention can be achieved by 
providing an auto-load matching impedance device of a microwave oven 
including: a load sensing means for sensing the load of food put on a 
turntable in a heating chamber; a microcomputer for controlling the oven 
in response to key signals applied by a user and to an output of the load 
sensing means; a power supply means for supplying power to a magnetron to 
operate the magnetron under the control of the microcomputer; an impedance 
matching means for impedance matching of source/load impedances by moving 
the position of the impedance matching means in a waveguide fixed to the 
heating chamber to thereby transfer maximum power from the microwave 
source to the food; a match driving means for controlling driving of the 
impedance matching means under the control of the microcomputer;a variable 
vane, mounted in an opening of the heating chamber, which is rotated for 
uniform heating of the food; and a variable vane driving means for 
rotating the variable vane under control of the microcomputer. 
Alternatively, there is provided an auto-load impedance matching device of 
a microwave oven including a load sensing means for sensing the load of 
food put on a turntable in a heating chamber, a microcomputer for 
controlling the oven in response to key signals input by the user and 
according to the output of the load sensing means, a waveguide for 
transmitting the microwaves generated in the magnetron into the heating 
chamber and a load impedance matching means mounted at one side of the 
waveguide for adjusting, under the control of the microcomputer, the 
impedance of the load by controlling the direction of irradiation of 
microwaves.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 3, an auto-load impedance matching device of a microwave 
oven in accordance with an embodiment of the invention includes a 
microcomputer 14, a power supply part 15, a load sensing part 20, a 
impedance matching part 21, a match driving part 22, a variable vane 23, 
and a variable vane driving part 24. 
The load sensing part 20 includes optical sensors 27 and a weight sensor 28 
for sensing the volume and weight, respectively, of food on the turntable 
12 in the heating chamber 11. A plurality of optical sensors 27 are 
arranged opposite one another in the heating chamber 11 to roughly sense 
the volume of the food on the turntable. 
The microcomputer 14 controls the oven according to the output of the load 
sensing part 20 and key signals applied by the user through the key 
application part 19. 
Power supply part 15 supplies power to the magnetron 16 to operate the 
magnetron 16 under control of the microcomputer 14. 
Impedance matching part 21 is moved within waveguide 18 which is fixed to 
the heating chamber 11 to match the impedance of the load to that of the 
magnetron to generate microwaves at maximum output under control of the 
microcomputer 14. 
As shown in FIG. 4, the impedance matching part 21, having a guide part 25 
for guiding its position, moves in the vertical direction on the waveguide 
18 along the guide part 25. 
The match driving part 22 controls operation of the impedance matching part 
21 under the control of the microcomputer 14. Referring to FIG. 3, the 
variable vane 23, positioned in the opening 17 of the heating chamber 11, 
rotates to cause the microwaves to heat the food uniformly, and the 
variable vane driving part 24 rotates the variable vane 23 under the 
control of the microcomputer 14. 
Operation of the foregoing auto-load impedance matching device of a 
microwave oven in accordance with the above-embodiment of the invention 
will now be described. 
First, when a user presses keys at the key application part 19 to select 
functions for cooking food on the turntable 12 in the heating chamber 11, 
the microcomputer 14 operates the turntable motor 13 to turn the turntable 
12 having the food thereon. At the same time, the microcomputer 14 
controls the power supply part 15 to supply power to the magnetron 16 to 
generate microwaves. 
Accordingly, microwaves pass through opening 17 and are guided to the 
heating chamber 11 by waveguide 18 to heat the food on the turntable 12. 
Sensors 28 of the load sensing part 20 sense the weight or the weight and 
size of the food put on the turntable 12 and apply the result to the 
microcomputer 14. According to the result of the sensing, microcomputer 14 
controls the match driving part 22 and the variable vane driving part 24 
to drive the impedance matching part 21 and the variable vane 23, 
respectively, to impedance match the load to that of the magnetron 16. 
That is, in the case of the weight of the food sensed at the weight sensor 
28 of the load sensing part 20 being 2000 g, the microcomputer 14 controls 
the matching part 21 to move it to a fixed position "A" for maximum output 
of the magnetron 16 with a 2000 g food load, as shown in FIG. 5. And in 
the case of the weight of the food being 1000 g, the impedance matching 
part 21 is moved to a fixed position "B" for maximum output of the 
magnetron 16 with a 1000 g food load, and in the case of the weight of the 
food being 500 g, the impedance matching part 21 is moved to a fixed 
position "C" for maximum output of the magnetron with a 500 g food load. 
Herein, the impedance matching part 16 is provided to move the position of 
matching part 21 through mechanical parts, such as the match driving part 
22 and a cam, and can be provided to mechanically interlock with the 
variable vane 23 by cams and gears. 
While the weight sensor 28 of the load sensing part 20 is sensing weight of 
the food on the turntable 12, the optical sensors 27 roughly sense the 
volume of the food, as shown in FIG. 6 and apply the results of the 
sensing to the microcomputer 14. 
That is, light emission parts D, E, and F, and light reception parts D', 
E', and F' are provided at respective, opposite sides of the heating 
chamber 11 in the vertical direction, wherein light emitted from the light 
emission parts D, E, and F at one side of the heating chamber are received 
by the respective light reception parts D', E', and F' at the opposite 
side thereof to categorize the volume of the food on the turntable 12. 
According to the outputs of optical sensors 27 of the load sensing part 20, 
the microcomputer 14 controls the variable vane driving part 24, either to 
hold fixed or to rotate the variable vane 23, and thereby to heat the food 
uniformly which, thereby, improves the efficiency of heating. 
The load sensing part 20 (FIG. 8) may instead comprise a dielectric 
constant sensor S, which utilizes the output of the dielectric constant 
sensor S to infer the quantity of the food in the heating chamber, as 
shown in FIG. 7(b). Since the intensity of the microwaves which can be 
sensed by the dielectric constant sensor S varies with quantity of foods 
in the heating chamber, quantity of the food in the heating chamber can be 
calculated from the output of sensor S. 
Therefore, the microcomputer 14 converts the signals sensed at, and 
received from, the dielectric constant sensor S into weight of the food, 
and, utilizing this, controls the impedance matching part 21 and the 
variable vane 23 to move to an impedance matching position so that 
microwaves can be generated at maximum output. 
As shown in FIG. 8, an auto-load impedance matching device of a microwave 
oven in accordance with another embodiment of the invention includes a 
microcomputer 14, a power supply part 15, a load sensing part 20, a 
impedance matching part 21, a match driving part 22, a variable vane 23, 
and variable vane driving part 24. 
Herein, the systems and operations of the microcomputer 14, the power 
supply part 15, the impedance matching part 21, the match driving part 22, 
the variable vane 23, and the variable vane driving part 24 are the same 
as that of the auto-load impedance matching device of the first embodiment 
of the invention. 
As shown in FIG. 9, the load sensing part 20 includes a current transformer 
CT for sensing change of a load current "ip" supplied from the power 
supply part 15 to the magnetron 16, and a rectification circuit 26 for 
rectifying a load current "ip" sensed at the current transformer CT into 
direct current and supplying the rectified current to the microcomputer 
14, thereby sensing change of load by detecting change of the current 
supplied from the power supply part 15 to the magnetron 16, and applying 
the detected current change to the microcomputer 14 (FIG. 8). 
Operation of the auto-load impedance matching device of a microwave oven in 
accordance with another embodiment of the invention will now be explained. 
First, when a user presses keys at the key application part 19 (FIG. 3) to 
select functions for cooking the food on the turntable 12 in the heating 
chamber 11, the microcomputer 14 operates the turntable motor 13 to turn 
the turntable 12 having the food thereon. At the same time, the 
microcomputer 14 controls the power supply part 15 to supply power to the 
magnetron 16 to generate microwaves. The microwaves, generated in the 
magnetron 16, are transmitted to the heating chamber 11 through the 
opening 17 and guided by the waveguide 18 to heat the food on the 
turntable 12. 
While cooking proceeds, the load sensing part 20 senses change of load in 
the heating chamber (based on the kind and weight of the food) and applies 
the load change data to the microcomputer 14. The microcomputer 14, based 
on the result of the sensing of load change data applied thereto, controls 
the match driving part 22 and the variable vane driving part 24 to always 
supply a constant maximum output by minimizing the change of impedance 
(i.e., the change in output of microwaves irrespective of the kind and/or 
weight of the food), thereby providing the impedance matching part 21 and 
the variable vane driving part 24 with an optimum impedance for maximum 
power transfer irrespective of food load. 
Processes for sensing a load at the load sensing part 20 will now be 
explained in detail, referring to FIG. 9. 
High voltage current induced on the secondary winding of a power 
transformer T is rectified and smoothed through a condenser C and a diode 
D and supplied to the magnetron 16. The load current "ip" supplied to the 
magnetron 16 is sensed at a current transformer CT and transmitted to a 
rectification circuit 26. That is, the load current "ip" supplied to the 
magnetron 16 through the power transformer T is rectified through the 
diode D, and current "ip'" in a rectification loop received from the diode 
D is sensed at the current transformer CT to sense change of the load 
current "ip". 
The reason for detecting the current "ip'" flowing through the diode D is 
that the load current "ip" flowing through the magnetron 16 is 
proportional to the current "ip'" flowing through the diode D. That is, 
since the load current "ip" flowing through the magnetron 16 is 
proportional to the load current "ip'" flowing through the diode, even 
though the current "ip'" flowing through the diode D is measured with the 
current transformer CT, this is equivalent to measuring the load current 
"ip" flowing through the magnetron 16. 
The current "ip'" received from the current transformer CT is rectified 
into direct current at a level corresponding to an increment of load 
change at the rectification circuit 26 and applied to the microcomputer 
14. 
The microcomputer 14 detects the level of direct current received from the 
rectification circuit 26 to determine increment of load change and, based 
on the determined increment of load change, controls the match driving 
part 22 and the variable vane driving part 24 to achieve an optimum 
efficiency through driving the impedance matching part 21 and the variable 
vane 23. 
As shown in FIGS. 10-12, one embodiment of the variable vane driving part 
24 for driving the variable vane 23 includes a motor shaft 32 penetrating 
the waveguide 18 (the waveguide being fixed to the heating chamber 11 and 
having the variable vane 23 fixed at one end thereof). A motor 36 is 
connected to the motor shaft 32 for transmitting power, and a cam 30 is 
mounted on the motor shaft 32 between the motor 36 and the waveguide 18. A 
microswitch 35 is attached to cam 30 for sensing turn on/off, or lapse of 
a certain period of time after turn off of the motor, to stop the motor 
shaft 32 at a particular angle upon operation of the motor 36. A movable 
stub 33 having a follower gear 34 attached thereon is engaged to a driving 
gear 31 mounted on the motor shaft 32 for moving the movable stub back and 
forth. 
That is, a motor shaft 32 connected to a motor 36 is provided penetrating a 
waveguide 18 fixed to one side of a heating chamber 11. A variable vane 
23, rotated by the motor shaft 32, is provided at the end thereof to 
position the variable vane in an opening 17 provided at a lower part of 
one side of the heating chamber 11. Both a cam 30 and a driving gear 31, 
rotated by the motor 36, are mounted on the motor shaft 32 between the 
motor 36 and the waveguide 18. 
At the end of a movable stub 33, which moves back and forth and is 
positioned above the motor shaft 32, there is provided a follower gear 34 
rotatably engaged to a driving gear 31. Also, a microswitch 35 is provided 
that senses turn on/off, or lapse of a certain period of time after turn 
off, of the motor 36 and stops the motor 36 at a particular angle 
following operation of the motor 36. 
Shown in FIGS. 13(a)-13(c) is another embodiment of a variable vane driving 
part 24, including a cam 30 mounted on a motor shaft 32 between the 
waveguide 18 and a motor 36, a movable stub 33 positioned above the cam 30 
and penetrating waveguide 18 above the motor shaft 32, a spring 37 mounted 
on the movable stub 33 between the waveguide 18 and the cam 30 for biasing 
the moveable stub 33 toward the cam 30, and a microswitch 35 positioned 
above the cam 30. 
Shown in FIGS. 14(a)-14(b) is another embodiment of a variable vane driving 
part 24, including a cam 30 integrated with a driving gear mounted on a 
motor shaft 32 between the waveguide 18 and the motor 36. A follower gear 
34 is positioned at the end of the movable stub 33 and both the gear 34 
and movable stub 33 are positioned above the motor shaft 32 penetrating 
the waveguide 18. Rotatably engaged to the cam 30, there is a microswitch 
35 positioned above the cam 30. 
The position of the movable stub 33 or the angle of the variable vane 23 
can be controlled according to various classifications of weight and size 
of the food on the turntable 12 in the heating chamber 11. For example, 3 
to 4 categories of positions for backward/or forward movement of the 
movable stub 33, or various combinations of positions of the moveable stub 
and angles of the variable vane 23, can be stored in a memory in advance 
to perform the impedance matching needed to provide optimum power transfer 
between the microwave source and the load. Thus, the microcomputer 14 
utilizes the scored memory data to achieve maximum power for cooking, 
irrespective of the cooking load. 
That is, by making the microcomputer 14 control the motor 36 to match the 
position of the movable stub 33, or to match the angle of the variable 
vane 23 and by using the microswitch 35 to signal a step corresponding to 
the weight and size of food sensed at the load sensing part 20, it is 
possible to maintain the maximum transfer of microwaves to the food in the 
oven, thereby improving the cooking efficiency. 
Shown In FIG. 15 is an auto-load impedance matching device of a microwave 
oven in accordance with another embodiment of the invention, including a 
load sensing part 20, a microcomputer 14, a waveguide 18, and a load 
impedance matching part 40. 
The load sensing part 20 senses the load of food put on a turntable 12 in a 
heating chamber 11, and the microcomputer 14 controls the oven according 
to the result of the load sensing of the load sensing part 20 and key 
signals applied by the user through a key application part 19. 
The waveguide 18 transmits microwaves generated in a magnetron 16 into the 
heating chamber 11 under the control of the microcomputer 14. 
The load impedance matching part 40, as shown in FIGS. 16-17(a)-17(c), 
positioned at one side of the waveguide 18, includes a deflection plate 41 
for reflecting microwaves, and a control part 45 for controlling the angle 
of the deflection plate 41. Thus the direction of irradiation of 
microwaves is controlled by the microcomputer 14. 
Herein, as shown in FIG. 17(b), the deflection plate 41 includes a body 
plate 46, a turning shaft 47 formed at one side of the body plate 46 and 
rotatably fixed to a fixing groove 18" formed in the waveguide 18, and a 
position controlling plate 49 having a position controlling groove 48 
inserted into the control part 45 for turning the body plate 46. 
And, as shown in FIG. 17(c), the control part 45 includes a motor 42, and a 
rotator 44 having a projection 43 formed thereon to be inserted into the 
position controlling groove 48 for rotating deflection plate 41 at a 
preset angle by the motor 42. 
The waveguide 18 has the fixing groove 18" at one side thereof for 
inserting the rotator 44, and a bottom part formed in an arc for enabling 
the deflection plate 41 to be rotated therein. 
Operation of the foregoing auto-load impedance matching device of a 
microwave oven in accordance with another embodiment of the invention will 
now be described. 
When the keys are pressed to select functions for cooking using the key 
application part 19, the microcomputer 14 operates the turntable motor 13 
to rotate the turntable 12 having the food thereon. 
The load sensing part 20 then senses the food load on the turntable and 
applies the result to the microcomputer 14. Accordingly, the microcomputer 
14 controls the number of revolutions of the motor 42 through the rotation 
of the rotator 44, according to the angle of the deflection plate 41 that 
provides optimum impedance match for transferring power to the load. 
As shown in FIG. 18, following rotation of the rotator 44, the projection 
43 of the rotator 44 moves within the position controlling groove 48 of 
the deflection plate 41, thereby moving the body plate 46 of the 
deflection plate 41 at designated angles .theta.1, .theta.2, and .theta.3, 
thereby controlling the position of the deflection plate for an optimum 
impedance matching condition for the load. 
As has been explained, since this invention facilitates achieving maximum 
microwave output for the load, it provides advantages such as improving 
cooking efficiency and shortening cooking time and, consequently, it saves 
energy and improves the reliability of microwave oven cooking. 
Although the invention has been described in conjunction with specific 
embodiments, it is evident that many alternatives and variation s will be 
apparent to those skilled in the art in light of the foregoing 
description. Accordingly, the invention is intended to embrace all of the 
alternatives and variations that fall within the spirit and scope of the 
appended claims.