U.S. patent application number 13/331959 was filed with the patent office on 2012-06-21 for control of microwave source efficiency in a microwave heating apparatus.
This patent application is currently assigned to Whirlpool Corporation. Invention is credited to Hakan Carlsson, Fredrik Hallgren, Olle Niklasson, Ulf Nordh.
Application Number | 20120152938 13/331959 |
Document ID | / |
Family ID | 44059016 |
Filed Date | 2012-06-21 |
United States Patent
Application |
20120152938 |
Kind Code |
A1 |
Nordh; Ulf ; et al. |
June 21, 2012 |
CONTROL OF MICROWAVE SOURCE EFFICIENCY IN A MICROWAVE HEATING
APPARATUS
Abstract
A microwave heating apparatus and a method of operating a
microwave heating apparatus are provided. The microwave heating
apparatus includes a microwave source for generating microwaves, a
transmission line for transmitting the generated microwaves from
the microwave source to a cavity, a measuring device that measures
the power of microwaves transmitted from the microwave source and a
receiving device that receives operational data indicative of the
power supplied to the microwave source. The microwave heating
apparatus further includes a control unit adapted to adjust the
impedance of the transmission line and/or the impedance matching
between the microwave source and the transmission line based on the
measured power of the transmitted microwaves and the received
operational data in order to control the efficiency of the
microwave source. The microwave heating apparatus improves control
of the microwave source efficiency.
Inventors: |
Nordh; Ulf; (Norrkoping,
SE) ; Carlsson; Hakan; (Norrkoping, SE) ;
Niklasson; Olle; (Finspong, SE) ; Hallgren;
Fredrik; (Kolmarden, SE) |
Assignee: |
Whirlpool Corporation
Benton Harbor
MI
|
Family ID: |
44059016 |
Appl. No.: |
13/331959 |
Filed: |
December 20, 2011 |
Current U.S.
Class: |
219/702 |
Current CPC
Class: |
Y02B 40/00 20130101;
H05B 6/705 20130101; H05B 6/688 20130101 |
Class at
Publication: |
219/702 |
International
Class: |
H05B 6/68 20060101
H05B006/68 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2010 |
EP |
EP10196108.4 |
Claims
1. A method of operating a microwave heating apparatus comprising a
microwave source adapted to feed microwaves to a cavity via a
transmission line, the method comprising the steps of: measuring
the power of microwaves transmitted from the microwave source to
the cavity; receiving operational data indicative of the power
supplied to the microwave source; and adjusting the impedance of
the transmission line and/or the impedance matching between the
microwave source and the transmission line based on the measured
power of the transmitted microwaves and the received operational
data in order to control the efficiency of the microwave
source.
2. The method of claim 1 further comprising the step of measuring
the power of microwaves reflected back towards the microwave
source.
3. The method of claim 1, wherein the step of adjusting of the
impedance of the transmission line and/or the impedance matching
between the microwave source and the transmission line comprises:
moving, within the transmission line, at least one element of the
group including a slug tuner, a motor driven slug tuner, a plunger
and a motor driven plunger; or changing the geometry or length of
the transmission line.
4. The method of claim 2, wherein the step of adjusting of the
impedance of the transmission line and/or the impedance matching
between the microwave source and the transmission line comprises:
moving, within the transmission line, at least one element of the
group including a slug tuner, a motor driven slug tuner, a plunger
and a motor driven plunger; or changing the geometry or length of
the transmission line.
5. The method of claim 1, wherein the impedance of the transmission
line and/or the impedance matching between the microwave source and
the transmission line is adjusted such that the efficiency of the
microwave source is increased or maximized if the measured power of
the reflected microwaves is below a predetermined threshold and
such that the efficiency of the microwave source is decreased or
minimized if the measured power of the reflected microwaves is
above the predetermined threshold.
6. The method of claim 1, wherein the impedance of the transmission
line and/or the impedance matching between the microwave source and
the transmission line is adjusted as a function of the ratio
between the measured power of the transmitted microwaves and the
power supplied to the microwave source.
7. The method of claim 1 further comprising the step of turning off
the microwave source if the measured power of the reflected
microwaves is above a security threshold.
8. The method of claim 2 further comprising the step of turning off
the microwave source if the measured power of the reflected
microwaves is above a security threshold.
9. The method of claim 3 further comprising the step of turning off
the microwave source if the measured power of the reflected
microwaves is above a security threshold.
10. A microwave heating apparatus comprising: a microwave source
for generating microwaves; a transmission line for transmitting the
generated microwaves from the microwave source to a cavity; a
measuring device capable of measuring the power of microwaves
transmitted from the microwave source; a receiving device capable
of receiving operational data indicative of the power supplied to
the microwave source; and a control unit that adjusts the impedance
of the transmission line and/or the impedance matching between the
microwave source and the transmission line based on the measured
power of the transmitted microwaves and the received operational
data in order to control the efficiency of the microwave
source.
11. The microwave heating apparatus of claim 10 further comprising
an additional measuring device that measures the power of
microwaves reflected back towards the microwave source.
12. The microwave heating apparatus of claim 10, wherein, for
adjusting the impedance of the transmission line and/or the
impedance matching between the microwave source and the
transmission line, the control unit is configured to: move, within
the transmission line, an element being at least one of the group
including a slug tuner, a motor driven slug tuner, a plunger and a
motor driven plunger; or change the geometry or the length of the
transmission line.
13. The microwave heating apparatus of claim 11, wherein, for
adjusting the impedance of the transmission line and/or the
impedance matching between the microwave source and the
transmission line, the control unit is configured to: move, within
the transmission line, an element being at least one of the group
including a slug tuner, a motor driven slug tuner, a plunger and a
motor driven plunger; or change the geometry or the length of the
transmission line.
14. The microwave heating apparatus of claim 12, wherein the
plunger is positioned proximate the microwave source.
15. The microwave heating apparatus of claim 11, wherein the
control unit is configured to adjust the impedance of the
transmission line and/or the impedance matching between the
microwave source and the transmission line such that the efficiency
of the microwave source is increased or maximized if the measured
power of the reflected microwaves is below a predetermined
threshold and such that the efficiency of the microwave source is
decreased or minimized if the measured power of the reflected
microwaves is above the predetermined threshold.
16. The microwave heating apparatus of claim 11, wherein the
control unit is configured to turn off the microwave source if the
measured power of the reflected microwaves is above a security
threshold.
17. The microwave heating apparatus of claim 11, wherein the
control unit is configured to control the efficiency of the
microwave source as a function of the ratio between the measured
power of the transmitted microwaves and the power supplied to the
microwave source.
18. The microwave heating apparatus of claim 14, wherein the
control unit is configured to control the efficiency of the
microwave source as a function of the ratio between the measured
power of the transmitted microwaves and the power supplied to the
microwave source.
19. The microwave heating apparatus of claim 18 further comprising
a directional coupler adapted to measure the power of the
transmitted microwaves and the power of the reflected
microwaves.
20. The microwave heating apparatus of claim 19, wherein the
microwave source is a magnetron and the operational data is the
anode current of the magnetron.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to European Application No.
EP10196108.4, filed on December 21, 2010, entitled CONTROL OF
MICROWAVE SOURCE EFFICIENCY IN A MICROWAVE HEATING APPARATUS, the
disclosure of which is hereby incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] The art of microwave heating involves feeding microwave
energy into a cavity. There are many aspects to consider when
heating a load in the form of food using a microwave oven. Some of
these aspects may be related to the heating itself wherein it is
generally desired to obtain a uniform heating of the food at the
same time as a maximum amount of available microwave power is
absorbed in the food (to achieve a satisfactory degree of heating
efficiency). However, there may also be other aspects related to
e.g. energy consumption of the microwave oven, wherein an
optimization (or improvement) of the overall efficiency of the
microwave oven is desired. For this purpose, various types of power
regulation of the microwave source have been suggested in prior art
microwave ovens to improve efficiency.
[0003] For example, in a traditional microwave oven equipped with a
regular high-voltage transformer for supplying a magnetron, power
regulation of the magnetron is often accomplished via the use of
duty-cycle operation, wherein the magnetron is alternatively turned
on and off during periods as defined by the duty-cycle. In such
microwave ovens, the magnetron is always operated at full power
(usually a fixed large average anode current) and the average power
of microwaves supplied to a cavity of the microwave oven is
regulated by feeding microwaves using a certain percentage of a
predefined time base during which the magnetron is oscillating. As
another example, in a microwave oven equipped with an inverter
connected to the magnetron, power regulation can be accomplished by
supplying the magnetron with intermediate average anode currents,
i.e. the magnetron is not necessarily operated at full power and
there is no need of a duty-cycle. In both cases, however, the
microwave ovens are often designed and optimized for heating of a
standard 1000 g water load while the magnetron is at full power
(normally 1000 W). As a result, prior art microwave ovens still
suffer from a reduced overall efficiency if the load differs from
the 1000 g water load or, if for a 1000 g water load, the oven (in
particular provided with an inverter) is not operated at a full
power level.
SUMMARY OF THE INVENTION
[0004] Generally, it is an object of the present invention to
provide a microwave heating apparatus with an improved overall
efficiency and, in particular, an improved control of the microwave
source efficiency.
[0005] According to an aspect of the present invention, a method of
operating a microwave heating apparatus including a microwave
source adapted to feed microwaves to a cavity via a transmission
line is provided. The method including the steps of measuring the
power of microwaves transmitted from the microwave source to the
cavity and receiving operational data indicative of the power
supplied to the microwave source. The impedance of the transmission
line and/or the impedance matching between the microwave source and
the transmission line is then adjusted based on the measured power
of the transmitted microwaves and the received operational data in
order to control the efficiency of the microwave source.
[0006] According to another aspect of the present invention, a
microwave heating apparatus is provided. The microwave heating
apparatus includes a microwave source for generating microwaves, a
transmission line for transmitting the generated microwaves from
the microwave source to a cavity, a measuring device that measures
the power of microwaves transmitted from the microwave source and
receiving device that receives operational data indicative of the
power supplied to the microwave source. The microwave heating
apparatus further includes a control unit adapted to adjust the
impedance of the transmission line and/or the impedance matching
between the microwave source and the transmission line based on the
measured power of the transmitted microwaves and the received
operational data in order to control the efficiency of the
microwave source.
[0007] According to an aspect of the invention, the impedance
matching between the microwave source and the transmission line may
be adjusted in order to control the efficiency of the microwave
source. For this purpose, the efficiency of the microwave source
may be evaluated or determined based on measurement, or monitoring,
of the power level of the microwaves transmitted (in the
transmission line) from the microwave source to the cavity and
based on operational data indicative of the power supplied to the
microwave source. The efficiency of the microwave source may then
be controlled by adjusting the impedance matching between the
microwave source and the transmission line and/or the impedance of
the transmission line via which the microwaves are transmitted from
the microwave source to the cavity. Advantageously, the impedance
of the transmission line and/or the impedance matching between the
microwave source and the transmission line may be adjusted such
that the power level of the microwaves transmitted from the
microwave source to the cavity is increased, thereby increasing the
efficiency of the microwave source (i.e. operating the magnetron,
if the microwave source is a magnetron, in the so-called sink
phase). However, in some cases, the impedance of the transmission
line and/or the impedance matching between the microwave source and
the transmission line may be adjusted such that the power level of
the microwaves transmitted from the microwave source to the cavity
is decreased, thereby decreasing the efficiency of the microwave
source (i.e. operating the magnetron, if the microwave source is a
magnetron, in anti-sink phase). The present invention is thus
advantageous in that it provides a microwave heating apparatus with
improved control of the microwave source efficiency. Further, an
improved control of the microwave source efficiency contributes
positively to the overall energy efficiency of the microwave
heating apparatus as a whole.
[0008] The present invention is particularly advantageous in that
the efficiency of the microwave source may be controlled depending
on dynamical changes occurring in the microwave heating apparatus.
Indeed, the efficiency of the microwave source is dependent on the
impedance of a system defined by the microwave source, the
transmission line and the cavity. In its turn, the impedance of
such a system is dependent on a number of parameters such as the
form, size and phase of a load arranged in the cavity, the form and
size of the transmission line and the form and size of the cavity.
In particular, the impedance may vary because of a change in size,
form or phase of the load like at a transformation from frozen to
thawed (due to the microwave heating). With the present invention,
by adjusting the impedance of the transmission line and/or the
impedance matching between the microwave source and the
transmission line, it is thus possible to control the efficiency of
the microwave source while taking into account any changes
occurring in the load (change in size/geometry or change in
temperature which alters the dielectric data of the load).
Similarly, with the present invention, it is possible to account
for changes occurring in the microwave source, e.g. a magnetron,
such as a change of the anode current or a change in
temperature.
[0009] The present invention is also advantageous in that the
efficiency of the microwave source, in particular a magnetron (but
also for other types of microwave sources), can automatically be
kept at a certain level of efficiency, and preferably in the sink
phase, during operation of the microwave heating apparatus. Indeed,
the efficiency of the microwave source can be controlled during
operation by adjusting or tuning the impedance of the transmission
line (and/or the impedance matching between the microwave source
and the transmission line, also referred to as impedance
adjustments in the following) independently of what has influenced
the need for an adjustment (in particular an increase) of the
efficiency of the microwave source. The efficiency of the microwave
source may be kept at a high level by dynamic impedance
adjustments.
[0010] The present invention is also advantageous in that the
microwave heating apparatus can operate at optimal (or at least
improved) conditions independently of the load arranged in the
cavity (and in particular not only for a standard 1000 g water
load). Indeed, as compared to prior art microwave ovens in which
the impedance is tuned so as to have sink phase operation for large
loads, normally a standard 1000 g water load, the microwave heating
apparatus of the present invention is more flexible. In the present
invention, sink phase operation for the magnetron may be obtained
in principle for any kind of load sizes (or weights) and at least
for other (smaller) loads than the standard load normally used for
design of microwave ovens. In prior art microwave ovens, if a load
is different, and in particular smaller, than the standard load,
the efficiency of the microwave source is often not optimal.
[0011] In addition, the present invention is advantageous in that
it provides a microwave heating apparatus and a method of operating
a microwave heating apparatus with reduced energy consumption since
the magnetron may be controlled to operate in the sink phase, i.e.
at a high efficiency, and an improved overall efficiency of the
microwave heating apparatus is obtained.
[0012] As mentioned above, the microwave source may be a magnetron
such as e.g. a main voltage transformer powered magnetron or an
inverter-powered magnetron.
[0013] The method may further include the step of measuring the
power of microwaves reflected back towards the microwave source.
For this purpose, the microwave heating apparatus may further
include additional measuring devices that measure the power of the
reflected microwaves. The efficiency of the microwave source may be
controlled based on both the power level of the microwaves
transmitted from the microwave source to the cavity and the power
level of the microwaves reflected back towards the microwave
source. The power level of the reflected microwaves is generally
representative of the amount of microwaves absorbed by the cavity
and, in particular, a load arranged in the cavity. The impedance of
the transmission line and/or the impedance matching between the
microwave source and the transmission line may be adjusted with
respect to both the efficiency of the microwave source and the
heating efficiency of the microwave heating apparatus.
[0014] It will be appreciated that the additional measuring
device(s) may be provided as part of a single measuring device
adapted to measure the power of the transmitted microwaves or as a
separate unit specifically dedicated to the measurement of the
power level of the reflected microwaves. For example, the measuring
device and the additional measuring device(s) may be a directional
coupler, i.e. a single entity, adapted to separately measure the
power of the transmitted microwaves and the power of the reflected
microwaves.
[0015] The impedance of the transmission line and/or the impedance
matching between the microwave source and the transmission line may
be adjusted by moving, within the transmission line, at least one
element of the group including a slug tuner (i.e. a pin projecting
into the transmission line), a motor driven slug tuner, a plunger
and a motor driven plunger. A plunger enables a change of the
geometry or length of the transmission line (e.g. via a movable end
or wall of the transmission line), thereby adjusting the impedance
of the transmission line and, in particular, the impedance matching
between the microwave source and the transmission line. A plunger
may preferably include a quarter-wave choke (or seal). The movable
element may also be designed to be a matching element adapted to
match or mismatch, depending on the desired control, the impedance
between the microwave source and the transmission line. Such
movable elements or matching elements may be impedance transformers
that have the capability of introducing a variable shunt
susceptance into the transmission line. A slug tuner or plunger may
e.g. be implemented as a (longitudinally) adjustable or movable
piece of metal arranged at a wall of the transmission line or into
the transmission line. Advantageously, the movable element may be
motor-driven such that an automatic impedance adjustment, and
thereby an automatic control of the microwave source efficiency, is
obtained.
[0016] In particular, if the movable element is a plunger, the
movable element may advantageously be arranged in proximity to the
magnetron (or an antenna of the microwave source) since its effect
on the impedance then becomes even more increased and, in addition,
it strongly influences the magnetron-to-waveguide matching. On the
other hand, a slug tuner may advantageously be arranged at a
certain distance (normally in the order of about half to one
wavelength of the microwaves) from the microwave source. The
desired distance between the slug tuner and the magnetron may be
selected to reduce the likelihood for propagation of evanescent
modes or higher order modes present locally around the magnetron
antenna.
[0017] To further illustrate how the impedance may be adjusted,
considering a tubular (e.g. with a rectangular cross section)
waveguide where the microwave source is arranged at a first
extremity, or end, of such a waveguide and the cavity, or a feeding
port of the cavity, is arranged at an opposite (second) extremity
of the waveguide, the movable element may be arranged at the
extremity corresponding to the microwave source. For example, this
extremity may be replaced, or equipped, with a movable plug (or
plunger) having about the same cross section as the waveguide such
that if the distance between the movable plug and the microwave
source (or antenna of the microwave source) varies, the coupling
between the magnetron and the waveguide varies. Thus, the
magnetron's efficiency may be adjusted by moving the plunger, which
effectively changes the overall system impedance.
[0018] The impedance of the transmission line and/or the impedance
matching between the microwave source and the transmission line may
be adjusted such that the efficiency of the microwave source is
increased or maximized if the measured power of the reflected
microwaves is below a predetermined threshold. Further, the
impedance of the transmission line and/or the impedance matching
between the microwave source and the transmission line may be
adjusted such that the efficiency of the microwave source is
decreased or minimized if the measured power of the reflected
microwaves is above the predetermined threshold. The present
embodiment is advantageous in that it provides a safe manner of
operating the microwave heating apparatus (and thereby controlling
the efficiency of the microwave source) in that, on the one hand,
the efficiency of the microwave source may be increased on a
condition that the power level of reflected microwaves remains
under a predetermined threshold, and, on the other hand, the
efficiency of the microwave source may be decreased if the power
level of the reflected microwaves is above the predetermined
threshold. Indeed, an increase of the efficiency of the microwave
source while the power level of the reflected microwaves has
reached the predetermined threshold would deteriorate the microwave
source as the amount of microwaves reflected back to the microwave
source is already too high. Thus, even if the magnetron operates in
anti-sink phase, the impedance is advantageously not adjusted to
make the magnetron operate in sink phase if the reflection is too
high.
[0019] For example, for a magnetron equipped with a ceramic window,
although it normally can withstand a significant amount of
reflected microwaves, if the amount of reflected microwaves is very
high in anti-sink phase (also called the thermal region due to poor
efficiency and high anode current), the very large temperature
derivative may in fact lead to a crack in the ceramic window
(mainly because of the difference in thermal expansion coefficient
between the metallic magnetron's antenna and the ceramic window).
On the other hand, a very high reflection in the sink phase usually
results in reduced oscillation stability and/or antenna to
waveguide flashover.
[0020] The power level measured for the reflected microwaves may
therefore determine how the microwave source is to be controlled
and, in particular, whether the efficiency of the microwave source
is to be increased. The predetermined threshold may be selected
with respect to an amount of reflected microwaves that the
microwave source of the microwave heating apparatus can withstand.
Advantageously, the efficiency of the microwave source is increased
as long as the power level of the reflected microwaves is below the
predetermined threshold. If the power level of the reflected
microwaves is above the predetermined threshold, the efficiency of
the microwave source (and thereby the amount of microwaves
transmitted to the cavity) is decreased, i.e. leaving the sink
phase (in case the microwave source was already operated in the
sink phase), which is advantageous due to the risk for moding
and/or arcing if the reflection is very high (i.e. above the
threshold).
[0021] In particular, the impedance of the transmission line and/or
the impedance matching between the microwave source and the
transmission line may be adjusted as a function of the ratio
between the measured power of the transmitted microwaves and the
power supplied to the microwave source. More specifically, if the
microwave source is a magnetron, the operational data is the anode
current of the magnetron. In such a case, the impedance of the
transmission line and/or the impedance matching between the
microwave source and the transmission line may be adjusted as a
function of the ratio between the measured power of the transmitted
microwaves and the anode current, wherein a high ratio (and in
particular the highest ratio) corresponds to a high efficiency of
the microwave source (i.e. the sink phase) and a low or lower ratio
correspond to a low or lower efficiency (i.e. the anti-sink
phase).
[0022] The microwave source may be turned off if the measured power
of the reflected microwaves is above a second predetermined
threshold or security threshold, which is advantageous in that it
further improves the life-time of the microwave source.
Advantageously, the security threshold is at a level above the
above mentioned (first) predetermined threshold such that, if the
power level of the reflected microwaves has (suddenly) reached or
is above the security threshold, the control unit of the microwave
heating apparatus will not attempt to initiate a decrease of the
efficiency of the microwave source by impedance adjustment (which
may require some time) but, instead, immediately shut down or turn
off the microwave source. It will be appreciated that this is
advantageous both if the magnetron is operated in the anti-sink
phase or in the sink phase. In other words, the measured power of
the reflected microwaves is preferably monitored even if the system
is tuned for sink phase oscillation.
[0023] Further objectives of, features of, and advantages with, the
present invention will become apparent when studying the following
detailed disclosure, the drawings and the appended claims. Those
skilled in the art will realize that different features of the
present invention can be combined to create embodiments other than
those described in the following.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The above, as well as additional objects, features and
advantages of the present invention, will be better understood
through the following illustrative and non-limiting detailed
description of preferred embodiments of the present invention, with
reference to the appended drawings, in which:
[0025] FIG. 1 schematically shows a microwave heating apparatus
according to an embodiment of the present invention;
[0026] FIG. 2 schematically shows a microwave heating apparatus
according to another embodiment of the present invention;
[0027] FIG. 3 shows a block diagram illustrating the general
functions of a microwave heating apparatus according td an
embodiment of the present invention; and
[0028] FIG. 4 is a general outline of a method of operating a
microwave heating apparatus in accordance with an embodiment of the
present invention.
[0029] All the figures are schematic, not necessarily to scale, and
generally only show parts which are necessary in order to elucidate
the invention, wherein other parts may be omitted or merely
suggested.
DETAILED DESCRIPTION
[0030] The present invention generally relates to the field of
microwave heating, and in particular to a microwave heating
apparatus for heating a load by means of microwaves.
[0031] With reference to FIG. 1, there is shown a schematic view of
a microwave heating apparatus according to an embodiment of the
present invention. The microwave heating apparatus 100 comprises a
microwave source 110 (e.g. a magnetron), a transmission line 120
and a cavity 130. The microwave source 110 is arranged at a first
end, or extremity, of the transmission line 120 while the cavity
130 is arranged at a second end, opposite to the first end, of the
transmission line 120. The microwave source 110 is adapted to
generate microwaves, e.g. via an antenna 112, and the transmission
line 120 is configured to transmit the generated microwaves from
the (antenna 112 of the) microwave source 110 to the cavity
130.
[0032] The transmission line 120 may also be equipped with a
movable element, namely a plunger 180, arranged at the extremity of
the transmission line 120 which is closest to the microwave source
110. The plunger 180 is movable such that its position may vary in
the transmission line 120, thereby varying the impedance matching
between the microwave source 110 and the transmission line 120. As
depicted in FIG. 1, the antenna 112 of the microwave source 110 is
inserted into the transmission line or waveguide 120 such that its
extremity is at a certain position between the waveguide back
shorting wall close to the microwave source and the end of the
waveguide connecting to the cavity 130, usually at a distance of
about 18-26 mm from the back shorting wall. The back shorting wall
of the waveguide 120 closest to the microwave source is the wall
that is preferably used as the plunger wall. In FIG. 1, the plunger
180 acts as the lateral shorting wall of the transmission line
120.
[0033] Alternatively, or in addition, the transmission line 120 may
also be equipped with further movable element such as a post or pin
185 arranged at a wall of the transmission line 120, preferably one
of the broadest wall in the case of a rectangular waveguide
(wherein the movable element is inserted in the transmission line
120 via a slot for sliding the pin back and forth). The slot(s) are
also preferably symmetric in order to reduce any potential
microwave leakage (some choking might be needed). The post 185
extends in the transmission line 120 and is movable such that its
displacement can alter the impedance of the transmission line 120.
It will be appreciated that both the lateral position of the
movable element 185 along a wall of the transmission line 120 and
the penetration depth of the movable element 185 in the
transmission line 120 may be adjusted.
[0034] The movable elements, such as the plunger 180 and the post
185, may be controlled by a control unit 170 via e.g. an
intermediate "servo" unit 172. In particular, the servo-unit 172
may be connected to a motor (not shown) associated with the plunger
180 and another motor (not shown) associated with the post 185 such
that the plunger 180 or the post 180 may be automatically moved in
accordance with instructions received from the control unit
170.
[0035] The control unit 170 may determine the need of impedance
adjustment and the level of impedance adjustment (e.g. a
displacement of any one or both of the movable elements 180 and
185) based on the power level of the microwaves transmitted from
the microwave source 110 to the cavity 130 and operational data
indicative of the power supplied to the microwave source 110. For
this purpose, the control unit 170 may be connected to a measuring
device 140 that is adapted to measure and measures the power of the
transmitted microwaves and a receiving device 150 that is adapted
to receive and receives the operational data.
[0036] In accordance with the present invention, the impedance of
the transmission line 120 and/or the impedance matching between the
microwave source 110 and the transmission line 120 is adjusted in
order to control the efficiency of the microwave source 110. Thus,
if based on the power level of the transmitted microwaves measured
at the measuring device 140 and the operational data received at
the receiving device 150, it is determined that the efficiency of
the microwave source 110 is not optimal (which may either be that
the magnetron 110 operates in the sink phase while the anti-sink
phase would be preferred or vice versa), the control unit 170 is
then configured to adjust the impedance of the transmission line
120 and/or the impedance matching between the microwave source 110
and the transmission line 120. The receiving device is typically
linked to information about the power fed to the source itself. In
the case of magnetron, this is, for example, the magnetron anode
current. The magnetron anode current can be readily measured in the
power supply feeding the magnetron either by using anode current
data directly accessible in the case of an inventor or with an
additional current clamp circuitry if a half-wave voltage doubler
power supply is used.
[0037] Normally, the control unit 170 is configured to optimize, or
at least increase, the efficiency of the microwave source 110.
Thus, if because of a change in e.g. the geometry of a load 138
arranged in the cavity 130, it is determined that the efficiency of
the microwave source 110 has decreased since the preceding
measurement (i.e. the last measurement) of the power level of the
transmitted microwaves, the control unit 170 may be configured to
adjust the impedance of the transmission line 120 and/or the
impedance matching between the microwave source 110 and the
transmission line 120 such that the efficiency of the microwave
source 110 is increased. For example, the control unit 170 may send
instructions to the servo-unit 172 to displace or move at least one
of the movable elements 180 and 185 in the transmission line 120,
thereby altering the impedance of the transmission line 120 and/or
the magnetron to waveguide matching. However, in other
circumstances, as will be further described in the following, the
control unit 170 may also be configured to decrease the efficiency
of the microwave source 110.
[0038] Alternatively, if it is determined by the control unit 170
that the impedance of the transmission line 120 and/or the
impedance matching between the microwave source 110 and the
transmission line 120 is to be adjusted, the movable elements 180
and 185 may be directly activated. In other words, the functions of
the servo-unit 172 controlling a motor, or indicating an amount of
displacement by which a movable element is to be moved, may be
integrated in the movable element itself. In such a case, a direct
connection between the control unit 170 and the movable elements
180 and 185 may be established.
[0039] Although the various connections between the control unit
170, the servo-unit 172, the receiving device 150, the measuring
device 140 and the movable elements 180 and 185 are represented by
continuous lines in FIG. 1, it will be appreciated that these
connections may be either wire connections or wireless connections
(e.g. using RF signals).
[0040] According to an embodiment, the microwave heating apparatus
100 may comprise additional measuring device 145 configured to
measure the power level of microwaves reflected back towards the
microwave source 110. In FIG. 1, the measuring device 140 and the
additional measuring device 145 are integrated in a single entity.
Generally, microwaves transmitted to a cavity may be either
absorbed by a load arranged in the cavity, absorbed by elements of
the cavity (or other objects present in the cavity), or reflected
back from the cavity (or feeding port). Indeed, if the coupling to
the cavity 130 is not perfect, some microwave power may be
reflected, e.g. through a feeding port, back into the transmission
line 120 towards the microwave source 110. An advantageous, and
thus preferred, way to control whether there is a satisfactory
coupling to the cavity 130, is by measuring the power that is
reflected from a feeding port of the cavity 130. In the example
schematically shown in FIG. 1, the power of the reflected
microwaves may be measured at the extremity of the transmission
line 120 which is closest to the cavity 130. The powers of the
reflected microwaves are, at least partly, representative of the
amount of microwaves absorbed by the load 138 arranged in the
cavity 130.
[0041] The control unit 170 may then be configured to adjust the
impedance of the transmission line 120 and/or the impedance
matching between the microwave source 110 and the transmission line
120 such that the efficiency of the microwave source 110 is
increased or maximized if the power of the reflected microwaves is
below a predetermined threshold. The measured power level of
reflected microwaves represents an amount of microwaves absorbed by
the cavity (and thus the load) such that, if the amount of
reflected microwaves is below the predetermined threshold, the load
impedance is not completely suitable to the transmission line
impedance. As a result, the amount of microwaves transmitted to the
cavity may be increased. The predetermined threshold may be
selected such that it represents an amount of microwaves that is
relatively high but, still, that the microwave source 110 can
withstand, which is advantageous in that it provides some kind of
protection for the microwave source 110, thereby increasing its
lifetime.
[0042] In particular, the control unit 170 may be configured to
turn off the microwave source if the power level of the reflected
microwaves measured at the additional measuring unit 145 exceeds a
second predetermined threshold, namely a security threshold. The
security threshold may be selected such that it represents an
amount of microwaves that is slightly below an amount of microwaves
that the microwave source could not withstand, thereby avoiding (or
at least limiting the risk of) any serious deterioration of the
microwave source 110.
[0043] The impedance of the transmission line 120 and/or the
impedance matching between the microwave source 110 and the
transmission line 120 may be adjusted as a function of the ratio
between the measured power of the transmitted microwaves and the
power supplied to the microwave source 110, which ratio is
representative of the efficiency of the microwave source 110. In
particular, if the microwave source is a magnetron, the impedance
of the transmission line 120 and/or the impedance matching between
the microwave source 110 and the transmission line 120 may be
adjusted as a function of the ratio between the measured power of
the transmitted microwaves and the anode current of the magnetron
(wherein the anode current is representative of the power supplied
to the magnetron 110). It will be appreciated that for microwave
ovens provided with inverters for controlling the anode current of
the magnetron, such information may be directly obtained, normally
via the inverter, by the control unit. However, it is also
contemplated to apply the present invention to microwave ovens not
comprising any inverter and for which the anode current may be
derived via e.g. an external current meter. Measurements of the
anode current in microwave ovens provided with regular high voltage
transformers is preferably performed "outside" the tube of the
magnetron itself, e.g. in the supply circuit.
[0044] In particular, in microwave ovens, the frequency of the
microwaves varies as a function of the anode current (or as a
function of a current from some power supply connected to the
magnetron). Thus, if the anode current varies (for any reasons such
as a change in output power from e.g. 900 W to 400 W), the
oscillating frequency of the magnetron may vary (also refers to as
the pushing factor), which may affect the efficiency of the
magnetron. As the oscillation frequency is changed, the microwave
source may then operate in sink phase without the need for retuning
the impedance/source matching. The pushing factor (i.e. a change in
oscillating frequency because of a change in the average anode
current) may make the magnetron operate in anti-sink phase so that
a mechanical adjustment of e.g. a plunger is needed or in sink
phase such that no impedance adjustment is needed. The present
invention takes care of the pushing factor in that the need for
impedance adjustment is based on the power level of the transmitted
microwaves and the anode current.
[0045] Turning again to the above example, changing the output
power level from 900 W to 400 W may for instance lead to a change
from sink phase to anti-sink phase. The microwave heating apparatus
according to the present invention may then be configured to
determine whether the impedance of the transmission line and/or the
impedance matching between the microwave source and the
transmission line needs to be adjusted in order to control the
efficiency of the magnetron. The efficiency of the magnetron may
e.g. be regulated with respect to a preferred (or maximal) value of
the ratio between the power level of the transmitted microwaves and
the anode current. Alternatively, as mentioned above, the
efficiency of the magnetron may be increased or decreased with
respect to a predetermined threshold for the power level of the
reflected microwaves.
[0046] With reference to FIG. 2, there is shown a microwave heating
apparatus 200, e.g. a microwave oven, having features and functions
according to another embodiment of the present invention.
[0047] The microwave oven 200 includes a cavity 230 defined by an
enclosing surface. One of the side walls of the cavity 230 may be
equipped with a door 235 for enabling the introduction of a load,
e.g. food, in the cavity 230. Further, the cavity 230 may be
provided with a feeding port (or antenna) 233 through which
microwaves are fed to the cavity 230 of the microwave oven 200. The
feeding port may for instance be an antenna, such as a patch
antenna or a H-loop antenna, or even an aperture in a wall
(including sidewalls, the bottom and the ceiling) of the cavity
230. In the following, reference is made to the term "feeding
port".
[0048] The microwave oven 200 further typically includes a
microwave source 210, e.g. a magnetron, connected to the feeding
port 233 of the cavity 230 by use of a transmission line or
waveguide 220. The transmission line 220 may for instance be a
coaxial cable. The transmission line 220 may include a plurality of
movable elements 280, such as those described above with reference
to FIG. 1, for adjusting the impedance of the transmission line 220
and/or the impedance matching between the microwave source 210 and
the transmission line 220.
[0049] Further, the microwave oven 200 includes a first measuring
unit (or measuring means) 240 for obtaining, or being adapted to
obtain, a signal representative of the power transmitted from the
microwave source 210.
[0050] Further, the microwave oven 200 may also include a second
measuring unit (or measuring means) 245 for obtaining, or being
adapted to obtain, a signal representative of the reflected
microwaves from the cavity 230 at the feeding port 233. Both the
first and the second measuring devices 240 and 245 may be arranged
at the feeding port 233.
[0051] Further, the microwave oven 200 includes a receiving device
250 adapted to receive and that receives operational data (i.e.
information) indicative of the power supplied to the microwave
source 210.
[0052] Further, the microwave oven 200 includes a control unit 270
operatively connected to the first measuring unit 240, the second
measuring unit 245 and the receiving device 250. The result of the
measurement performed by the first measuring unit 240, the second
measuring unit 245 and the information received by the receiving
device 250 are transmitted to the control device or unit 270. The
control unit 270 is then configured to adjust the impedance of the
transmission line 220 and/or the impedance matching between the
microwave source 210 and the transmission line 220 based on the
measured power of the transmitted microwaves, the received
operational data and, optionally, the measured power of the
reflected microwaves.
[0053] Either one, or both, of the first measuring unit 240 and the
second measuring unit 245 may be integrated as sub-units in the
control unit 270. Alternatively, the measuring units 240 and 245
may be arranged as separate units connected to the control unit
270. The sensing part(s) of the first measuring unit 240 and the
second measuring unit 245 may be a probe including a field-sensor
at its extremity for sensing the energy transmitted to or reflected
from the cavity, respectively. As another example, the first
measuring unit 240 and the second measuring unit 245 may be a
directional coupler arranged in proximity to the feeding port 233
and in proximity to, or in connection with, the transmission line
220 connecting the microwave source 210 with the feeding port
233.
[0054] It will be appreciated that the receiving device 250,
although it is represented as a separate entity in FIG. 2, may be
an integrated part of either one of the microwave source 210 or the
control unit 270.
[0055] Further, the respective powers of the transmitted, and
optionally the reflected, microwaves may be measured by the
measuring units 240 and 245 at various time points during an
operation cycle (for instance used for heating a load arranged in
the cavity) of the microwave heating apparatus 200 and the
impedance of the transmission line 220 adjusted in accordance with
any one of the above described embodiments. It is therefore
contemplated that the first and second measuring units 240 and 245
may be adapted to, continuously or periodically, monitor the
signals representative of the powers of the transmitted and
reflected microwaves in order to dynamically (i.e., during an
operation cycle) control the efficiency of the microwave source
210. For the synchronization of the power measurements in relation
to, or within, the operation cycle, the microwave oven 200 may
further comprise a clock system (not shown in FIG. 1 but in FIG.
3).
[0056] Any of the embodiments described above with reference to
FIG. 1 for controlling the efficiency of the microwave source 110
is applicable to the microwave heating apparatus described with
reference to FIG. 2.
[0057] The general function of a microwave heating apparatus, e.g.
a microwave oven, according to an embodiment of the present
invention is further illustrated in FIG. 3 in the form of a block
diagram.
[0058] A microwave generator 310 feed microwaves to a cavity 330
via a transmission line 320 and a feeding port 333. The signals
representative of the power of the transmitted microwaves are
obtained by a first measuring unit 340 and are then transmitted to
a control unit 370. Operational data indicative of the power
supplied to the microwave generator 310 is received by a receiving
device 350 and then transmitted to the control unit 370. The
control unit 370 may include a processor or processing device 375,
such as a microprocessor, for analyzing the signal obtained from
the first measuring device 340 and the operational data received
from the receiving device 350.
[0059] Further, the microwave heating apparatus may include an
additional measuring device 345 for measuring the power of the
microwaves reflected back towards the microwave generator 310. The
signal representative of the power of the reflected microwaves is
then transmitted to the control unit 370.
[0060] Based on the signals received from the first measuring
device 340, the additional measuring device 345, and the
operational data received from the receiving device 350, the
control unit 370 is configured to control the efficiency of the
microwave generator 310 by adjustment of the impedance in the
transmission line 320 and/or the impedance matching between the
microwave generator 310 and the transmission line 320.
[0061] As mentioned above in connection to FIG. 2, the control unit
370 may further include comprise a clock system 376 for
synchronizing the period of measurements and the period during
which the impedance is being adjusted.
[0062] The first and second measuring units 340 and 345 and the
control unit 370 may be configured to operate in a similar manner
as the first measuring devices 140 and 240, the second measuring
units 145 and 245, and the control units 170 and 270 described
above with reference to FIGS. 1 and 2, respectively.
[0063] With reference to FIG. 4, a method of operating a microwave
heating apparatus including a microwave source adapted to feed
microwaves to a cavity via a transmission line is described. The
method includes the step of measuring 4100 the power of microwaves
transmitted from the microwave source to the cavity and the step of
receiving 4200 operational data indicative of the power supplied to
the microwave source. Optionally, the method includes also the step
of measuring 4300 the power of microwaves reflected back towards
the microwave source. The method then typically includes the step
of adjusting 4400 the impedance of the transmission line and/or the
impedance matching between the microwave source and the
transmission line based on the measured power of the transmitted
microwaves and the received operational data in order to control
the efficiency of the microwave source.
[0064] The impedance adjustment(s) at step 4400 includes moving of
at least one element within the transmission line. Once the movable
element has been displaced, the power measurements performed at
steps 4100 and 4300 for measuring the power level of the
transmitted microwaves and the power level of the reflected
microwaves, respectively, may be repeated, as indicated in FIG. 4.
In this manner, a continuous regulation of the efficiency of the
microwave source is provided.
[0065] It will be appreciated that the measurements and the
adjustment of the impedance are advantageously performed at a
sufficient rate such that the magnetron efficiency in principle
remains unaffected of changes, e.g. thermal changes, in either the
load or the magnetron itself.
[0066] Further, it will be appreciated that any one of the
embodiments described above with reference to FIGS. 1-3 is
combinable and applicable to the method described herein with
reference to FIG. 4.
[0067] The present invention is applicable for domestic appliances
such as an oven, typically a microwave oven, using microwaves for
heating. The present invention is also applicable for larger
industrial appliances found in e.g. food operation. The present
invention is also applicable for vending machines or any other
dedicated applicators.
[0068] While specific embodiments have been described, the skilled
person will understand that various modifications and alterations
are conceivable within the scope as defined in the appended
claims.
[0069] For example, although the microwave ovens 100 and 200
described with reference to FIGS. 1 and 2 have a rectangular
enclosing surface, it will be appreciated that, in the present
invention, the cavity of the microwave oven is not limited to such
a shape and may, for instance, have a circular cross section.
[0070] For example, the steps of the method described with
reference to FIG. 4 may be performed in another order than that
described above, in particular for steps 4100-4300.
[0071] It will be appreciated that the present invention is not
limited to any specific range of frequencies for operation of the
microwave heating apparatus. The present invention is therefore
applicable for any standard microwave sources having mid-band
frequencies of 915 MHz, 2450 MHz, 5800 MHz and 22.125 GHz.
[0072] Further, it will be appreciated that the present invention
is not limited to a microwave source being a magnetron. The
microwave source may for example be a solid state microwave
generator (or semiconductor-based microwave generator) including
e.g. a varactor diode (having a voltage-controlled
capacitance).
[0073] Although a microwave heating apparatus including a single
microwave source is described in the present application, it will
be appreciated that the present invention may also be applied to a
microwave heating apparatus including a plurality of microwave
sources wherein each of the microwave source is associated with a
dedicated transmission line and dedicated feeding port such that
the power of the microwaves transmitted from each of the microwave
sources and, optionally, the power of the microwaves reflected to
each one of the microwave sources can be separately monitored
(while reducing any crosstalk disturbance). In particular, the
feeding ports may be arranged to provide an orthogonal feeding of
the microwaves in the cavity.
* * * * *