Microwave heating apparatus

Abstract

An apparatus and a method for heating a load using microwaves is disclosed. The apparatus includes a transmission line, configured to transmit microwaves from a microwave generator to a cavity. A sensing device configured to measure a standing wave for providing information about the phase and the amplitude of a reflection coefficient that represents a ratio between the amount of microwaves reflected back towards the microwave generator and the amount of microwaves transmitted in the transmission line from the microwave generator. A control unit configured to detect whether the measured standing wave correspond to a reflection coefficient having a phase within a certain interval of phases and an amplitude within a certain interval of amplitudes. Additionally, certain intervals of phases and amplitudes correspond to an operating region of the microwave generator. The control unit controls feeding of microwaves to the cavity based on this detection.

Claims

1. A microwave heating apparatus comprising: a cavity arranged to receive a load; a microwave generator arranged to generate microwaves; a transmission line arranged to transmit the generated microwaves to the cavity; a sensing device arranged to measure a standing wave in the transmission line and configured to provide information about the phase and the amplitude of a reflection coefficient, the reflection coefficient being representative of the ratio between the amount of microwaves reflected back towards the microwave generator and the amount of microwaves transmitted in the transmission line from the microwave generator; and a control unit configured to detect whether the measured standing wave corresponds to a reflection coefficient having a phase within a predetermined interval of phases and an amplitude within a predetermined interval of amplitudes, said predetermined intervals of phases and amplitudes corresponding to an operating region of the microwave generator, and to control the microwave generator based on the detection.

2. The microwave heating apparatus according to claim 1, wherein the control unit is configured to, in response to the detection that the measured standing wave corresponds to a reflection coefficient having a phase within said predetermined interval of phases and an amplitude within said predetermined interval of amplitudes, alter control of parameters of the microwave generator.

3. The microwave heating apparatus according to claim 1, wherein the control unit is configured to alter control of parameters of the microwave generator on a condition that a measured time exceeds a time limit.

4. The microwave heating apparatus according to claim 2, wherein the control unit is configured to alter a duty cycle for operating the microwave generator.

5. The microwave heating apparatus according to claim 2, wherein the control unit is configured to alter parameters of the microwave generator by deactivating the microwave generator.

6. The microwave heating apparatus according to claim 1, wherein the operating region of the microwave generator, to which said predetermined intervals of phases and amplitudes correspond, is one of the group comprising sink phase and anti-sink phase, said microwave generator being a magnetron.

7. The microwave heating apparatus according to claim 1, wherein the correspondence between the predetermined intervals of amplitudes and phases of the reflection coefficient and said operating region of the microwave generator is a known intrinsic characteristic of the microwave generator.

8. The microwave heating apparatus according to claim 1, wherein the control unit is adapted to deactivate the microwave generator on a condition that the measured standing wave corresponds to a reflection coefficient having an amplitude above a tolerance level, wherein said predetermined interval of amplitudes is defined by amplitude values below the tolerance level.

9. The microwave heating apparatus according to claim 1, wherein the sensing device is arranged to measure the standing wave at different positions along the transmission line, said positions being selected such that the measured standing waves provide information about the phase and amplitude of the reflection coefficient.

10. The microwave heating apparatus according to claim 1, wherein the sensing device is arranged to measure the standing wave at least at four different positions spaced from each other along the transmission line.

11. The microwave heating apparatus according to claim 10, wherein the spacing between two adjacent positions is approximately equal to λ.sub.g/8+n×λ.sub.g/2, wherein λ.sub.g is the wavelength of the microwaves in the transmission line, and n is an integer.

12. The microwave heating apparatus according to claim 11, wherein the sensing device is configured to obtain information about the phase and amplitude of the reflection coefficient using the difference between the standing waves measured at two of said four different positions, said two positions being separated along the transmission line by approximately λ.sub.g/4+n×λ.sub.g/2, and the difference between the standing waves measured at the two remaining positions.

13. The microwave heating apparatus according to claim 12, wherein the predetermined interval of phases has a range being less than λ.sub.g/2, wherein λ.sub.g is the wavelength of the microwaves in the transmission line.

14. The microwave heating apparatus according to claim 1, wherein the predetermined interval of amplitudes extends from a value corresponding to no reflection of microwaves back towards the microwave generator to a value corresponding to full reflection of microwaves back towards the microwave generator.

15. A method of heating a load in a cavity using microwaves transmitted in a transmission line from a microwave generator, the method comprising the steps of: measuring a standing wave for providing information about the phase and the amplitude of a reflection coefficient, the reflection coefficient being representative of the ratio between the amount of microwaves reflected back towards the microwave generator and the amount of microwaves transmitted in the transmission line from the microwave generator; detecting whether the measured standing wave corresponds to a reflection coefficient having a phase within a predetermined interval of phases and an amplitude within a predetermined interval of amplitudes, said certain intervals of phases and amplitudes corresponding to an operating region of the microwave generator; and controlling the microwave generator based on the detection.

16. The microwave heating apparatus according to claim 1, wherein the control unit is configured to measure time during a single visit in the operating region.

17. The microwave heating apparatus according to claim 1, wherein the control unit is configured to measure time accumulated during several visits in the operation region during a heating procedure.

18. The microwave heating apparatus according to claim 1, wherein the sensing device is further arranged to measure a standing wave strength in the transmission line, and wherein the control unit is further configured to detect whether the measured standing wave strength corresponds to a reflection coefficient having a phase within a predetermined interval of phases and an amplitude within a predetermined interval of amplitudes.

19. The microwave heating apparatus according to claim 18 wherein the standing wave strength corresponds to a maxima or minima of the standing wave.

20. The microwave heating apparatus according to claim 18 wherein the phase corresponds to a distance from a reference plan in the transmission line to the standing wave strength in the transmission line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present disclosure, will be better understood through the following illustrative and non-limiting detailed description of preferred aspects of the present disclosure, with reference to the appended drawings, in which:

(2) FIG. 1 is a schematic perspective view of a microwave heating apparatus according to an aspect of the present disclosure;

(3) FIG. 2 illustrates a transmission line of a microwave heating apparatus in accordance with an aspect of the present disclosure;

(4) FIGS. 3a-b illustrate examples of certain intervals of amplitudes and phases with respect to a Smith chart;

(5) FIGS. 4a-c illustrate the correspondence between certain intervals of amplitudes and phases and operating regions of microwave generators in Rieke diagrams;

(6) FIG. 5 illustrates the extraction of the phase of the reflection coefficient from measured electromagnetic field strengths in accordance with an aspect of the present disclosure;

(7) FIG. 6 illustrates the relationship between certain intervals of amplitudes and phases and an amplitude tolerance level; and

(8) FIG. 7 shows the outline of a method of heating a load using microwaves in accordance with an aspect of the present disclosure.

(9) All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the disclosure, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

(10) With reference to FIGS. 1 and 3, a microwave heating apparatus 100 according to an aspect of the present disclosure will be described.

(11) The microwave heating apparatus 100 comprises a cavity 101 arranged to receive a load, a microwave generator 102 arranged to generate microwaves and a transmission line 103 arranged to transmit the generated microwaves to the cavity 101. A sensing device 104 is arranged to measure electromagnetic field strengths for providing information about the phase and the amplitude of a reflection coefficient being representative of the ratio between the amount of microwaves reflected back towards the microwave generator 102 and the amount of microwaves transmitted in the transmission line 103 from the microwave generator 102.

(12) The microwave heating apparatus may further comprise a control unit 105 configured to detect whether the measured electromagnetic field strengths correspond to a reflection coefficient having a phase within a certain interval of phases 304a-d and an amplitude within a certain interval of amplitudes 303a-d, wherein the certain intervals of phases and amplitudes correspond to an operating region of the microwave generator 101. The phase and the amplitude may be illustrated using a circle (or polar coordinates) with the amplitude representing a distance from the center of the circle to a working point and the phase representing an angle counted clockwise from a reference position 301 located e.g. in the upper part of the circle to the intersection of the circle with a line joining the working point and the center of the circle. Several different scales may be used to measure the amplitude. For example, the amplitude may be represented by the reflection factor ρ having values between 0 and 1. Alternatively, the amplitude may be represented via the voltage standing wave ratio VSWR, which can be expressed as: VSWR=(1+ρ)/(1−ρ) having values between 1 and infinity.

(13) FIGS. 3a and 3b illustrate several examples of certain intervals of phases 304a-d and amplitudes 303a-d. In one example, a first region denoted 302a is defined by an interval of amplitudes 303a of the form C<ρ<1, where C is a positive constant, and an interval of phases 304a including the reference position 301 and thereby comprising two parts of the form A<Φ≤λ.sub.g/2 and 0≤Φ<B, where Φ is the phase and A and B are positive constants. In another example, a second region denoted 302b is defined by an interval of amplitudes 303b of the form C<ρ<1, where C is a positive constant and an interval of phases 304b of the form A<Φ<B. In yet another example, a third region denoted 302c is defined by an interval of amplitudes 303c of the form C<ρ<D, where C and D are positive constants, and an interval of phases 304c still of the form A<Φ<B. In yet a further example, a fourth region 302d is defined by an interval of amplitudes 303d including all possible amplitudes, i.e. of the form 0<ρ<1, and an interval of phases still of the form A<Φ<B. The region denoted 302d corresponds to a sector of the circle.

(14) FIG. 4a is a Rieke diagram illustrating the properties of a magnetron having a nominal power of 1 kW. The Rieke diagram shows how the output power and the frequency of the generated microwaves are affected by the amplitude (represented by the voltage standing wave ratio, VSWR) and the phase of the reflection coefficient.

(15) FIG. 4b shows the Rieke diagram of FIG. 4a, with two regions 401 and 402 corresponding to certain intervals of phases and amplitudes as defined in FIGS. 3a-b. In the present example, the region 402, located around the phase 0.25×λ.sub.g, corresponds to a sink phase of the magnetron. The sink phase may be recognized in the Rieke diagram by a region in which the curves corresponding to constant frequency converge. In the present example, the region 401, located around the phase 0×λ.sub.g (i.e. around the reference plane), corresponds to an anti-sink phase of the magnetron. The anti-sink phase may be recognized in the Rieke diagram by a region in which the curves corresponding to constant frequency diverge.

(16) More specifically, for the magnetron selected as an example here, there are three regions which may advantageously be avoided if the VSWR is larger than a threshold. The first region and the second region may be combined into a single region consisting of the high antenna current phase (phase 0.1×λ.sub.g−0.2×λ.sub.g) and the sink phase (0.2×λ.sub.g−0.3×.sub.g). The third region is also called the thermal region (corresponding to anti-sink phase) which surrounds phase 0×λ.sub.g (˜0.47×λ.sub.g−0.03×λ.sub.g for the present example magnetron). Via dynamic impedance measurement capable of sensing if the magnetron is being operated at or above the maximum rating for the VSWR in one of these phase regions (i.e. via the electromagnetic field measurements along the transmission line), the magnetron may either be shut off or its power output be decreased.

(17) As described above, the sink phase (electronic instability region) is defined as the phase where the frequency contours converge and the anti-sink phase (thermal region) is the phase where they diverge. It is therefore preferable to detect whether the microwave generator operate in these regions.

(18) In addition, there are two other regions that may be of interest. Due to the fact that the magnetron reference plane is set to be coaxial with the output antenna and that phases are calculated as distances from the reference plane to the standing wave voltage minimum, the phase 0.25×λ.sub.g means that the voltage maximum is at the reference plane, i.e. at the antenna. This in turn means that the electric field strength at the antenna may be very large and that the magnetron may be prone to electric field breakdown, i.e. flashover at the antenna. Such an operating region or condition corresponds to the antenna high electric field region. If the electric field minimum is “moved” towards the reference plane by changing phase of the standing wave, the electric field maximum moves backward into the antenna, thereby creating conditions of very large electric field strength in the antenna, which may create overheating and, in some cases, cause the centre conductor of the antenna to melt. When the field maximum has “moved” approximately from 0.25×λ.sub.g to 0.1×λ.sub.g, the maximum field strength will be at the bottom of the resonator output end space. Thus, it may be advantageous to monitor whether, for a certain level of amplitudes, the phase of the reflection coefficient is in the range of 0.1-0.3×λ.sub.g.

(19) It will be appreciated that different maximum values for the amplitude of the reflection coefficient may be used to define various areas or regions of interest. Different values for the maximal amplitude may be defined for different phase region. For example, the region corresponding to the sink phase usually needs lower amplitude values of the reflection coefficient than other areas. The region may be defined using logical expressions, which may be programmed into the microwave heating apparatus (e.g. in the control unit or some kind of microwave oven control system).

(20) FIG. 4c shows a Rieke diagram for a magnetron having a higher nominal power, such as e.g. 2 kW. In the present example, the magnetron is affected somewhat differently by the reflection coefficient than the magnetron described with reference to FIG. 4a. Indeed, as compared to FIGS. 4a-b, the Rieke diagram shown in FIG. 4c is rotated such that the sink phase 402 is located around phase 0.17×λ.sub.g and the anti-sink phase 401 is located around 0.4×λ.sub.g.

(21) The rotation angle may be governed by the magnetron pushing factor, which relates the operating behavior and the anode current. For magnetrons intended for use in microwave ovens, the rotation may be approximately 0.05×λ.sub.g per 30 mA (milliamperes) of average anode current. If the average anode current is increased from its nominal value, the Rieke diagram rotates anti-clockwise and for lower current than the nominal average anode current, it rotates clockwise. Such information may be used by the control unit to locate the various operating regions of the magnetron in case the anode current is changed.

(22) Turning back to FIG. 1, the control unit 105 may be adapted to control feeding of microwaves to the cavity 101 based on the detection whether the measured electromagnetic field strengths correspond to a reflection coefficient having a phase within a certain interval of phases 304a-d and an amplitude within a certain interval of amplitudes 303a-d.

(23) With reference to FIGS. 1 and 2, the microwave generator 102 may be a magnetron connected to the transmission line via an antenna 203. The sensing device 104 is arranged to measure electromagnetic field strengths at four different positions 201a-d spaced from each other along the transmission line 103. The positions in the transmission line 103 may be measured from a reference plane 202 located at the position at which the magnetron antenna 203 enters the transmission line 103. The first position 201a may be located at a distance λ.sub.g/4 from the reference plane 202, wherein λ.sub.g is the wavelength of the transmitted microwaves. The second position 201b may be located at λ.sub.g/8 further away from the reference plane 202 and so on, the spacing between two adjacent positions (at which measurements are performed) being λ.sub.g/8. The measured field strengths at the first 201a, second 201b, third 201c and fourth 201d positions will be referred to as Y1, X1, Y2 and X2, respectively.

(24) The electromagnetic field strengths measured along the transmission line 103 originate from the microwaves generated by the microwave generator 102. The field strengths tend to be periodic with a periodicity being the double of the wavelength of the transmitted microwaves, i.e. periodic with the period λ.sub.g/2. Therefore, any of the positions 201a-d at which the field strengths are measured may in general be translated along the transmission line 103 by e.g. λ/2, 2×λ/ 2, 3×λ/ 2 or 4×λ/ 2 without significantly affecting the results of the measurements.

(25) The sensing device 104 may be configured to obtain information about the phase and amplitude of the reflection coefficient using the differences between the electromagnetic field strength measured at the first 201a and third 201c positions (i.e. Y1−Y2), and at the second 201b and fourth 201d positions (i.e. X1−X2). In an exemplifying aspect, the sensing device 104 may be configured to obtain the amplitude as

(26) $\rho =\frac{\psi }{{\psi }^{\inf }},$
wherein ψ==√{square root over ((Y1−Y2).sup.2+(X1−X2).sup.2)} and ψ.sup.inf is a rescaling factor. The rescaling factor may be obtained by operating the microwave heating apparatus at full reflection, measuring field strengths X1.sup.inf, Y1.sup.inf, X2.sup.inf and Y2.sup.inf at the same positions 201a-d and calculating √{square root over ((Y1.sup.inf−Y2.sup.inf).sup.2+(X1.sup.inf−X2.sup.inf).sup.2)}.

(27) Referring now to FIG. 5, as the phase of the reflection coefficient may be defined as the distance from the reference plane 202 to the first voltage minima of the standing wave in the transmission line 103, the phase may be obtained by representing the values X1−X2 and Y1−Y2 as a point 501 in a plane coordinate system (such as e.g. in a Smith chart or a Rieke diagram). The difference X1−X2 defines the x-coordinate of the working point 501 and the difference Y1−Y2 defines the y-coordinate. The phase is then obtained as the angle (from 0 to λ.sub.g/2 corresponding to an angle between 0 and 360 degrees) counted clockwise from the y-axis to a ray 502 from the origin 503 of the coordinate system to the working point 501.

(28) Depending on the phase of the reflection coefficient, the measurements made at the four different positions will provide field strengths as different parts of the standing wave. Table 1 lists examples where maxima (indicated by “max”) and minima (indicated by “min”) are located at different measurement positions. Table 1 shows coordinates achieved from measured field strengths as well as the obtained phase, for these examples.

(29) TABLE-US-00001 TABLE 1 Examples of detected field strengths together with associated coordinates and phases x- y- 201a 201b 201c 201d coordinate coordinate phase max min 0 positive 0 max min positive 0 0.125 min max 0 negative 0.25 min max negative 0 0.375

(30) Although the formulas may be different for different quadrants in the coordinate system, the angle (providing the phase of the reflection coefficient) may be calculated using trigonometry.

(31) According to an aspect, the certain intervals of phases and amplitudes are known characteristics of the microwave heating apparatus (or microwave generator). Such known characteristics may be obtained from the supplier of the magnetron or may be measured. For example, the certain intervals and amplitudes associated with an operating region of a microwave generator may be obtained by test-running the microwave generator. Returning to FIG. 2, a capacitive post (not shown) may be introduced in the transmission line and be adjusted so that the phase and amplitude of the reflection coefficient takes different values. The output power and the frequency of the generated microwaves may be measured for different values of phases and amplitudes and a Rieke diagram may be drawn. Certain intervals of phases and amplitudes associated with for example sink and/or anti-sink phase may then be identified in the Rieke diagram.

(32) FIG. 6 shows a certain interval of phases 602 and a certain interval of amplitudes 601 according to an aspect. In the present aspect, the control unit is configured to deactivate the magnetron if the amplitude is above a tolerance level 603 in order to e.g. protect the magnetron from being overheated by microwaves reflected back in the transmission line. According to the present aspect, the operating region of the magnetron to be detected, i.e. the region in the Rieke diagram in which the reflection coefficient is to be detected, may be defined by an interval of amplitudes in the form of C<ρ<D, with C and D being two constants, and an interval of phases in the form of A<Φ≤B, with A and B being two constants. In the present example, the operating region is defined by an interval of amplitudes being lower than the tolerance level 603 (corresponding to D in the present example). In such a microwave heating apparatus, if it is detected that the reflection coefficient is in the region as defined above, the control unit may be configured to alter parameters (e.g. by modifying a parameter of the microwave generator such as the anode current or the transmission line such as its impedance) such that the reflection coefficient is shifted outside this region, thereby avoiding the microwave generator to operate in this particular operating region. Further, the control unit may be configured to deactivate (i.e. turn off) the microwave generator if the amplitude of the reflection coefficient is detected to be above the tolerance level 603.

(33) With reference to FIG. 7, a method of heating a load in a cavity using microwaves transmitted in a transmission line from a microwave generator is described in accordance with an aspect of the present disclosure. The same reference numbers as for the features of the microwave heating apparatus described with reference to FIGS. 1 and 2 are used in the following.

(34) The method comprises the steps of measuring 701 electromagnetic field strengths for providing information about the phase and amplitude of a reflection coefficient and detecting 702 whether the measured electromagnetic field strengths correspond to a reflection coefficient having a phase within a certain interval of phases and an amplitude within a certain interval of amplitudes, wherein the certain intervals of phases and amplitudes correspond to an operating region of the microwave generator 102. The method further comprises the step of controlling 703 feeding of microwaves to the cavity 101 based on the detection.

(35) It will be appreciated that any one of the aspects described above with reference to FIGS. 1-6 is combinable and applicable to the method described herein with reference to FIG. 7.

(36) The present disclosure is applicable for domestic appliances such as a microwave oven using microwaves for heating. The present disclosure is also applicable for heating in industrial appliances. The present disclosure is also applicable for vending machines or any other dedicated applications.

(37) While specific aspects have been described, the skilled person will understand that various modifications and alterations are conceivable within the scope as defined in the appended claims.

(38) For example, although the cavity may preferably be rectangular, with e.g. one or several rectangular parts, the cavity may also be cylindrical or have any other shape suitable for heating a load via microwaves.

(39) Further, the microwave generator may be of any suitable type, such as e.g. a magnetron. The microwave heating apparatus may comprise several microwave generators of one type, or of several different types and these may be connected to the cavity by one or more transmission lines. The transmission line(s) may be at least one of a coaxial structure (such as a coaxial cable), a waveguide, a microstrip and a stripline. The microwave heating apparatus may include several transmission lines, among which some are of one type and some are of another.

(40) Further, it will be appreciated that the positions along the transmission line, at which the electromagnetic field strengths are measured, may preferably be selected such that the measured field strengths are usable for extraction of the phase of the reflection coefficient with good accuracy. Accuracy of the phase of the reflection coefficient may depend on the positions at which the measurements are made and the accuracy of the actual values recorded during these measurements.