AU595721B2 - Oven - Google Patents

Description

I n -*0 FIRST APPLICATION COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION (Original) 595 721 FOR OFFICE USE Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: 0 Published: 0 P 0 :.Priority: 0 l ,*,Rplated Art: 000 9 1t c C I C the am ccfor ;'Ut *r Se 1 ,ta^nts 1 ,',Name of Applicant: 0 t C l0 Address of Applicant: Actual Inventor(s): Address for Service: Address for Service: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.

1006, Oaza-Kadoma, Kadoma-shi, Osaka-fu 571,

JAPAN

Kazuho SAKAMOTO Takao SHITAYA Makoto MIHARA DAVIES COLLISON, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

Complete specification for the invention entitled:

"OVEN"

The following statement is a full description of this invention, including the best method of performing it known to 1 1 1 2 FIELD OF THE INVENTION AND RELATED ART STATEMENT 1. FIELD OF THE INVENTION The present invention relates to an oven which includes control means for controlling heating power so as to cook a food material by measuring its weight.

2. DESCRIPTION OF THE RELATED ART In a conventional heating oven shown in FIG. 1, at first a user measures a weight of a food material, and inputs the measured information (weight) by operating a key-board la. And the heating oven automatically controls suitable heating time, heating power, heating mode and the like by such inputted measured weight. FIG. 2 is an enlarged perspective view showing a control part 1 and display of the conventional heating oven shown in FIG. 1.

The control part 1 comprises an indicator 2 for indicating the inputted measured weight of the food material, the heating time, the heating power or the like, selection keys la for selecting a kind of food or cooking menu, and C 0~ 0 0 e V 0 0 900118. C '.'-S0QC."045 r*PA&S 4 o t 4 r 4r 4 4 4 4 4 4 4 4 4 4 4o 4 4 4. 4 4 e~ 3 numeral keys lb for inputting cooking time and weight of the food material.

Cooking operation by such a conventional heating oven needs the steps of measuring the weight of the food material before putting the food material into a heating cavity, and inputting the measured weight of the food material by operating the numeral keys lb. Such troublesome operation of the measuring of the weight is the cause of a failure of the cooking when an incorrect value 10 is input by mis-operation. Furthermore, when a very large eir value is mis-input instead of a real light weight of the S, food material, there are problems that the food material is over-heated and combusted, and the heating oven is broken.

OBJECT AND SUMMARY OF THE INVENTION S 15 The object of the present invention is to improve the above-mentioned problems and to provide an improved heating C"t oven without failure of cooking due to the mis-operation of e, cinput data such as heating time and weight of the food Smaterial.

a heating cavity for containing an object be heated therein, a heater means for applying in use rergy to said e, object to be heated whereby to heat it 25 a tray disposed on an inner rt of said heating cavity for holding said object be heated thereon, I a pair of beam shapedolders disposed on the inner part of said heating cav* y for holding said tray thereby to receive a load cor sponding to the weight of said object to be heat a capaci ve load cell disposed on an outer part of said heati cavity and for receiving load applied thereto, a/air of linkages disposed on the outer part of said -o Wd* ®0 4 1 The present invention provides an oven comprising: 2 a heating cavity for containing an object to be heated 3 therein, 4 a heater means for applying in use energy to said object to be heated whereby to heat it, 6 a tray disposed on an inner part of said heating cavity 7 for holding said object to be heated thereon, 8 a pair of beam shaped holders disposed on the inner 9 part of said heating cavity for holding said tray thereby to receive a load corresponding to the weight of said object to 11 be heated, 12 a capacitive load cell disposed on an outer part of 13 said heating cavity and for receiving load applied thereto, 14 a pair of linkages disposed on the outer part of said heating cavity and parts thereof penetrating through side 16 walls of said heating cavity each linkage contacting with a 17 respective one of said beam shaped holders for conveying 18 said load to said capacitive load cell, 19 a capacitance measuring circuit for measuring a variation of capacitance of said capacitive load cell in 21 consequence of such conveying of said load, and 22 control means for controlling heating of said heater 23 means in response to weight measurement of said object to be 24 heated by measuring of said variation of capacitance of said capacitive load cell.

26 Preferably said tray is slidable on said beam shaped 27 holders so as to be able to be taken out from said heating 28 cavity.

29 Preferably said linkages are made of a material which is not affected by microwaves.

31 Preferably said beam shaped holders are disposed for 32 avoiding contact with a bottom face of said heating cavity 33 when said object to be heated has the maximum weight to be 34 measured by said capacitive load cell.

Preferably said heater means is disposed below said 36 heating cavity.

37 Preferably two connecting parts are formed on opposite gAI 38 sides of said tray, and C' 0 V S *O U C' V 0 C0 0* I V i 00.. ~N 5 009 1 offset parts are located between said tray and the 2 connecting parts whereby said connecting parts support said 3 tray on said beam shaped holders.

4 Preferably the distance between said two offset parts of said tray for guiding and positioning said tray with 6 respect to said beam shaped holders, is narrower than 7 distance between said two beam shaped holders.

8 Preferably the width of said connecting parts is 9 narrower than the distance from the side wall of said heating cavity to the innermost point of contact of said 11 beam shaped holder with said tray.

12 Preferably the total width of the distance between said 13 two offset parts and width of one of said connecting parts 14 is larger than the distance apart of said two beam shaped holders.

16 Preferably convex parts are formed on said beam shaped 17 holders for positioning said tray in said heating cavity in 18 a direction along the length of said beam shaped holders.

19 In one instance convex parts are formed on said tray in a manner to prevent contacting of said tray with a rear wall 21 of said heating cavity or a front door of the oven by 22 abutment with said convex parts on said tray.

23 In another instance the convex parts are formed on said 24 beam shaped holders in such a manner to prevent contacting of said tray with a rear wall of said heating cavity or a 26 front door of the oven by abutment of said tray with said 27 convex parts of said beam shaped holders.

28 The present invention also provides an oven comprising: 29 a heating cavity for containing an object to be heated therein, 31 a heater means for applying in use energy to said 32 object to be heated whereby to heat it, 33 a tray for holding said object to be heated thereon, 34 a pair of beam shaped holders, with one beam shaped holder disposed on each side of said heating cavity for 36 holding said tray thereby to receive a load corresponding to 37 the weight of said object to be heated, AAN 38 a pair of linkages disposed for penetrating through .pn LL. or L 4i A 9 1I.. L. 9001V3 d35 4. So 0O 0 C S t T C ga a 1.

side walls of said heating cavity, each linkage contacting with a respective one of said beam shaped holders for conveying said load to externally of said heating cavity, holding mechanisms disposed on both sides of said heating cavity for receiving such load conveyed by said linkages, wherein said holding mechanisms being constituted in a parallelogram construction, wherein one side is fixed and another side parallel thereto is movable, a load cell for measuring load conveyed by said holding mechanisms, and control means for controlling said heater means in response to the weight of said object to be heated as measured by said load cell.

ec o C 9OO118 a e O5 Rsp. I I I I I Ir I S S S I S S S S S S& Cull SoS S S 5 o SO S S S S 5 IC) O'l) *r 0 *r S I i C 6 a InAd cP1 I-for measuring load conqvey!ed by 6aidholding mechanisms, and control means forjacrir rolling said heater means in response to thegweight of said object to be heated as Rni~~lrJL-^~ ar Inarl cell._ J- Preferably loads conveyedby said holding mechanisms are measured by concentrating at one point.

In one instance the movable sides of said mechanisms are disposed relatively close to said heating cavity, and e 10 the fixed sides of said mechanisms are disposed ^o ~relatively remote from said heating cavity.

*t In another instance the movable side of one of said t t «.mechanisms is disposed relatively close to said heating °\cavity and the fixed side of said one of said mechanism is C 15 disposed relatively remote from said heating cavity, and the movable side of the other of said mechanisms is disposed relatively remote from said heating cavity and the C tc fixed side of said other of said mechanism disposed C c c relatively close to said heating cavity.

c 20 BRIEF DESCRIPTION OF THE DRAWINGS a 2 FIG. 1 is the perspective view showing the conventional heating oven.

FIG. 2 is the enlarged perspective view showing the control part of the conventional heating oven.

FIG. 3 is a cross-sectional front view of a heating oven having a weighing apparatus.

FIG. 4 is a perspective view of the principal part of I the weighing apparatus.

FIG. 5 and FIG. 6 are partial cross-sectional side views of the heating oven, respectively.

FIG. 7(a) and FIG. 7(b) are cross-sectional front views showing the principal part of a holding mechanism for holding the weight of the measured object in accordance with the present invention and the prior art, respectively.

7 FIG. 8 is a cross-sectional front view showing an embodiment of the holding mechanism of the apparatus of FIG. 7(a).

FIG. 9(a) and FIG. 9(b) are cross-sectional front views showing other embodiments of the holding mechanism, respectively.

FIG. 10 is a perspective view showing the principal part of a weight measuring unit.

FIG. 11 is a cross-sectional front view showing te o I0 detailed configuration of a pressure sensor.

~FIG. 12 is a circuit diagram showing an embodiment of t oscillator to be used in the embodiment.

FIG. 13(a) and FIG. 13(b) are diagrams showing output wave forms of the embodiment.

S 115 FIG. 14 is a characteristic curve showing a relation between the oscillation frequency and the weight of the measurement object.

FIG. 15 is a circuit diagram of control circuit of ta the embodiment of the present invention.

til 20 DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention is described in the followings with reference to FIG. 3.

FIG. 3 shows a cross-sectional front view of a heating oven having a weight measuring apparatus in S 25 accordance with the present invention. A food stuff 3 is placed on a tray 4 and put into a heating cavity 5. The tray 4 is put on rails 6a and 6b on the right and left side of the heating cavity 5. Both right and left rails 6a and 6b are disposed along the side walls of the heating cavity 30 5 to extend from the front to the back. The tray 4 can slide on the rails 6a and 6b, as it is taken out from or placed into the heating cavity 5. The weight of the food stuff 3 and the tray 4 which are placed on the rails 6a and 6b, is transmitted to outside the heating cavity 5 by m m I. m I 8 coupling members 7 which project through holes in the side walls of the heating cavity 5. Such coupling members 7 are to be made of a material which is not affected by microwaves in the case that the oven is a microwave oven.

The coupling members 7 also serve to shield microwave leakage to external of the heating cavity 5. The weight is transmitted by the coupling members 7, and further by a link mechanism so called as Roberval mechanism. Such Roberval mechanism constitutes a parallelogram, and one C O 10 side of the parallelogram is fixed and the opposing side C moves retaining the parellelism. Namely, the parallelogram oo@ poa is constituted by a vertical rod 8, a vertical holder 9 and two link members 10, and such elements are rotatably cc connected by coupling pins 11 on the four corners of the parallelogram. The holders 9 are fixed on the external side wall of the heating cavity 5, and the rods 8 are C(disposed alongside the side walls of the heating cavity C r so as to be able to move in parallelogram. The coupling C C C aC Cmembers 7 are respectively fixed on the rods 8. The weight S 20 is vertically transmitted to the region of the upper part O of the heating cavity 5 via the rods 8 and vertical rods 12. And the weight transmitted from the right and the left sides of the heating cavity 5 are transformed into a force o "of horizontal direction by converters 13 onto a horizontal S6 25 rod 15. The converters 13 are respectively rotatably held on holders 14 by pins 11', and link the vertical rods 12 and the horizontal rod 15 at an angle of about 90 0 The force transmitted from both sides of the heating cavity are concentrated on the horizontal rod 15, and the concentrated force is directed from left to right in FIG. 3 to a weight measuring unit 16 for effectively measuring the lateral displacement of the horizontal rod As a heating source, a heater 17 can be disposed below the heating cavity 5, so that effective and good 9 thermally distributed cooking can be achieved. For example, the tray 4 may be made of a metal plate and heated from below by the heater 17 so that heat is transmitted to the food stuff 3 by passing through the tray 4. As heater 17 is disposed below the tray 4, the whole of the heating cavity 5 is heated by convection and heat distribution of the heating cavity 5 is uniform. Accordingly, the food stuff 3 will be wholly heated, and the result of cooking should be good.

10 In case of heating by microwave, a supply rout 18 of the microwave can be disposed below the heating cavity Stvo and the supply rout 18 is connected with a magnetron aa In this case, the tray 4 is made of ceramics or glass, and the microwave is emitted to the lower part of the food stuff 3. Accordingly, the food stuff 3 is heated from below, and especially in case that the food stuff 3 is c fluid, convection occurs and the food stuff is uniformly C and effectively heated. As mentioned above, such

C

C C SC embodiment can be made ai that having the heater 17 or S 20 microwave supply rout I- as a heat source, and thermally effective cooking can be made without fear of lack of uniform heating.

In the above-mentioned embodiment, when the food

C

C C stuff 3 is put on the tray 4, a displacement of parts C C25 charged with the force occurs and the tray 4 slightly moves down. Accordingly, it is necessary to have sufficient vertical room so that a contact of the bottom of the tray 4 with the bottom of the heating cavity 5 as avoided when a designed maximum weight is on the tray 4.

In the above-mentioned configuration, the tray 4 needs to be stable when put on the rails 6a and 6b, and also should be able to easily be taken out and put into the heating cavity 5. Accordingly, the below-mentioned configuration is desirable. Offset or step parts 4a and 4a ~Y~c Ij=-~~uull--r- dl~ 10 are formed on the peripheral part of the tray 4 for limiting the horizontal position of the tray 4, and for putting it into the heating cavity 5 along the rails 6a and 6b. And further, horizontal connecting parts 4b and 4b are formed outside the offset parts 4a and 4a for lying on the rails 6a and 6b. Then, for achieving putting the tray 4 on, the rails with stability, the distance A between both offset parts 4a and 4a and the distance B between both rails 6a and 6b should be as A<B. Under the relation Q 10 A>B, the horizontal connecting parts 4b and 4b can be s stably held on the rails 6a and 6b, and the tray 4 is stable. If the relation is A>B, one of the inclined part of the offset part 4a will be over one of the rails 6a or o~c 6b and the tray 4 may be inclined and unstable.

C C C 15 Next, for preventing the tray 4 falling from the rails 6a or 6b, the distance A and the width of one co, connecting part 4b, namely the length A+C shown in FIG. 3 and the distance B are to be as If the relation c c is the tray 4 may drop at one side between the c, 20 rails 6a and 6b. Further, if one of the connecting parts

S

cce 4b touches the side wall of the heating cavity 5, the weight measuring may be disturbed and the precise measurement may not be achieved. So as to prevent such 300 touching of the connecting parts 4b with the side wall of the heating cavity 5, the distance D which is from the contact part of the rail 6a or 6b to the side wall of the heating cavity 5, and the width C of the connecting parts 4b are to be as D>C. When the distance and width from A to D in FIG. 3 are set as mentioned above, the tray 4 is stable on the rails 6a and 6b, and precise weight measuring can usually be achieved. And also the taking out and putting in the tray 4 from and to the heating cavity 5 can be smoothly done.

FIG. 4 is a perspective view showing part of the 1I heating oven having weight measuring apparatus in accordance with the present invention. In FIG. 4, the food stuff 3 is placed on the tray 4, and the weight of the food stuff 3 is carried on rails 6a and 6b. The rails 6a and 6b on both side walls of the heating cavity 5 are disposed along the side walls from the front to the back. The tray 4 can slide on the rails 6a and 6b and can be taken out from or put into the heating cavity 5, The rails 6a and 6b are supported at two points, in the front part and the rear O qo part, by coupling members 7 and two Roberval mechanisms.

30 Rods 8 of the front and rear Roberval mechanisms are formed Pn in one piece, and an intermediate position of such rod 8 is connected to a vertical rod 12 outside of the heating cavity 5. The forces transmitted by two vertical rods 12 Son both sides of the heating cavity 5 (one of which is not shown in FIG. 4) are concentrated into a displacement. of r the horizontal rod 15 and the concentrated force is measured by the weight measuring unit 16, 3 CI€t In the above-mentioned configuration, if the tray 4 is deeply put into the heating cavity 5, the tray 4 may collide with a rear wall of the heating cavity 5, and the tray 4 or the rear wall of the heating cavity 5 may be broken. And if the tray 4 is not fully put into the heating cavity 5, the tray 4 may collide with an inner side 0 "C of the door (which is not shown in FIG. and the tray 4 or the door may be broken. And also, when the tray 4 contacts the rear wall of the heating cavity 5 or the door the precise maeasuring of the weight may not be possible.

^I In order to reduce the above-mentioned problems, offset parts in the direction along the side walls or rails are formed on the tray 4 and the rails 6a and 6b, which limit the position of the tray 4 along the rails 6a and 6b. FIG. 5 shows an embodiment in which the tray 4 has projecting parts 4C on its front and rear ends, and is -a 12 r Cr T t r t r r C (L limited with regard to the position it may take up by them. FIG. 6 shows another embodiment in which the rails 6a and 6b have projecting parts 6C on their front and rear ends to achieve a similar effect.

FIG. 7(a) shows a configuration in which the weight of the tray is supported by the left rail 6b, and transmitted through the coupling member 7 and to the external part of the heating cavity 5, and supported by the Roberval mechanism. The reason for preferably adopting the 10 Roberval mechanism in the present invention is described in the followings with reference to FIG. 7(b) which shows an alternative configuration excluding a parellelogram link.

In FIG. a near horizontal lever 21 is used in place of the Roberval mechanism. The first rea:-n to adopt the Roberval mechanism is that, in FIG. the rail 6b, the coupling member 7 and the vertical rod 8 are moved in vertical direction without changing its attitude. And therefore the tray 4 is laid on the rail 6b with stability. Alternatively, in FIG. the lever 21, the coupling member 7 and the rail 6b are rotated around a coupling pin 11 as a fulcrum C, and so, the top face of the rail 6b becomes inclined and the tray 4 encounters a danger of slipping down. The second reason is that a slight movement of the point A on which the weight of the tray 4 is carried does not affect the weight measuring in FIG.

but affects in FIG. Namely, in FIG. the force applied to the vertical rod 12 does not change at all irrespective of changing the position of point A, and such merit is a characteristic of the Roberval mechanism. On the other hand, in FIG. a lever mechanism determined by a ratio of the length between points C and A versus the length between points C and B is formed around the point C as the fulcrum. Accordingly, the force on the point B is changed by the movement right and left of the point A. The

J

'C

*4 13 Roberval mechanism is adopted because of the above-mentioned two reasons.

FIG. 8 shows another embodiment. In FIG. 3, the Roberval mechanism is configurated in symmetry with respect to both sides of the heating cavity 5. However, in order to attain more precise weight measuring, the embodimentof FIG. 8 comprises a pair of Roberval mechanism disposed in another way. Namely, the right and left Roberval mechanisms of FIG. 8 are different in shape and position of 10 each elements. In the left hand Roberval mechanism, a rod 8b which moves vertically is disposed nearer to the side wall of the heating cavity 5 and a holder 9c is disposed rfarther from the side wall. On the contrary, a rod 8b on

C

t the right hand is disposed farther from the side wall of the right hand and upper and lower holders 9a and 9b which are of equal length to the holder 9c, are respectively disposed nearer to the Fide wall on the right hand side.

The reason why such non-symmetrical configuration of the Roberval mechanism can measure more precise force is 20 described in the followings with reference to FIG. 9(a) and FIG. 9(b) FIG. 9(a) shows the symmetrical configuration and FIG. 9(b) shows the non-symmetrical configuration of the Roberval mechanism. In FIG. when the food stuff 3 is put on the tray 4, the rails 6a and 6b are moved down by the weight of the food stuff 3, and positions the rails 6a and 6b may take up are shown by dotted line in the figure. The distance between the right rail 6a and the left rail 6b at points of contact with the tray 4 is changed from E to F shown in the figure. Relation of such distances E and F is E,F, the distance between both the rails is widened when the force is applied. And friction occurs at the points of contact of the rails 6a and 6b with the tray 4. Such friction adversely influences the weight measuring. That is, due to the friction, the 14 right and left rods 8a and 8b may not be precisely charged of the weight in vertical direction. The embodiment shown in FIG. in which the Roberval mechanisms are symmetrically disposed on the right and left hand of the heating cavity, is usually sufficient to make cooking for ordinary use. But, when the more precise weight measuring is attained, in special purposes, the embodiment shown in FIG. 9(b) is more suitable. In this embodiment, the distance G between both rails 6a and 6b when the food stuff is not placed on the tray 4, and the distance H when the its: food stuff 3 is placed on the tray 4 are equal to each other; namely G=H. This means that there is no friction on S I the contact point of the tray 4 and the rails 6a and 6b.

r' C CAccordingly, a force is accurately applied on both rods 8a and 8b, which are disposed on the right and left side and move parallel, so that precise weight measuring is achievable.

*FIG. 10 is a perspective view showing the principal part of the weight measuring unit 16. As described with tI 20 respect to the embodiment shown in FIG. 3, the forces induced by the weight of the object are applied to effect displacement of the horizontal rod 15. In the following, C c the weight measuring mechanism is described.

The horizontal rod 15 is displaced by a force appli.ed by the weight of the food stuff in the direction shown by a white arrow, and a knife edge 152 of a protrusion 151, abutting a notch pushes an intermediate part of a lever 22. One end of the lever 22 at a projection 221 abuts a pressure sensor 24 and another end of the lever is supported by a fulcrum member 23. Accordingly, the pressure applied on the pressure sernsor 24 is a value dependent lever ratio of the lever 22. The fulcrum member 23 and the pressure sensor 24 are precisely posLtioned by being fixed on a sensor base 25. The shapes of the edge 15 4, *4 99 9,, 4 Ic I 4t 44 9 r* 9 44 9 I @9 152 of the horizontal rod 15 and the lever 22, and the fulcrum member 23 and the projection 221 of the lever 22 by which the pressure sensor 24 is pressed, are respectively made knife-edges. Especially the lever 22 and the pressure sensor 24 are configured to avoid discrepancy of the projection 221.

FIG. 11 is a cross-sectional side view showing the constitution of the pressure sensor 24. A sensor plate 26 is a flat plate made of a resilient insulating material and 10 a substrate 27a is also a flat plate made of an insulating material but is not necessarily resilient and may be rigid. As the insulating material, to satisfy the above-mentioned condition, ceramics or alumina (A1 2 0 3 are suitable. The sensor plate 26 and the substrate 27a 15 are bonded parellel to have a predetermined gap by providing spacers 28 at peripheral parts. On opposing surfaces of the sensor plate 26 and the substrate 27a, electrodes 29a are printed, and terminals 29b and 29b extend out from the electrodes 29a and 29a. When a force 20 due to the weight of the object is impressed of the sensor plate 26 as shown by a white arrow mark 221 in FIG. 11, the sensor plate 26 becomes bent, and the average distance between the sensor plate 26 and the substrate 27 changes.

Accordingly, the distance between the two electrodes 29a which are disposed on the sensor plate 26 and the substrate 27a, respectively, is changed and the capacitance is changed. As the bending of the sensor plate 26 is roughly in proportion to the impressed pressure, by measuring such change of the capacitance the weight of the food stuff 3 in the heating cavity is measurable.

Generally, vapor is discharged from the food stuff in the oven, and further the oven is usually placed in the kitchen, and so; the humidity change is large and average value of the humidity is high for the oven. Accordingly, /4 16 dew may form on the electrodes 29a, and further, absorbed dust on the electrodes may become damp. By such phenomena, the capacitance between the electrodes 29a may be changed.

Accordingly, it is preferable to shield the air between the electrodes 29a from the open air. That is, the sensor plate 26, the substrate 27a and the spacers 28 are preferably in an air tight shield. However, since the heating oven may come to a high temperature, the air between the electrodes may expand. In the case in which i0 the space around the electrodes 29a is shielded by a hard material, the pressure between the electrodes may be tzrr trr greatly raised by thermal expansion, and a strain may be Sgenerated on the pressure plate 26. In order to reduce the above-mentioned two problems, an air duct 27c is formed on the substrate 27a for ventilating thermally expanded air, and the opening of the air duct is made small enough not to absorb dust or dew, as shown in FIG. 11.

The above-mentioned configuration is sufficient for most uses. But in the case that more accurate measuring is t, 20 demanded, an additional shielded space in which the air between the electrodes can freely flow in and out is formed and the shielding is made from the open air. For example, e a cover 27b can be bonded on the substrate 27a in airtight manner by adhesive, and the cover 27b may be made of a soft or flexible material such as gum or plastic film. The air between the electrodes 29a moves freely to the additional space defined by the cover 27b and substrate 27a by passing through the air duct 27c, and hence the strain of the sensor plate 26 by the change of the temperature is not substantially affected, and adverse influence of humidity is reduced. Accordingly, an accurate weight measuring can be obtained.

FIG. 12 shows an oscillator 30 for obtaining oscillation frequency which changes corresponding to the 17 change of the capacitance of the pressure sensor 24. The oscillator 30 uses two operational amplifiers (hereinafter abbreviated as OP-amp) OP1 and OP2, and transforms the change of the capacitance of the pressure sensor 24 to an oscillation frequency change. Output waveforms of the OP-amps OP1 and OP2 are respectively shown in FIG. 13(b) and FIG. 13(a). FIG. 13(a) shows the waveform of point B which is output voltage of the OP-amp OP2, and FIG. 13(b) Sr Ishows the waveform of point A which is the output voltage t 10 of the OP-amp OP1. The OP-amp OP2 functions as an ,7 integration circuit, and its input signal is the output S t t signal of the OP-amp OP1. The OP-amp OP1 functions as a comparator circuit with a hysterisis characteristic, and C its input signal is the output signal of the OP-amp OP2.

The oscillator is constituted by the above-mentioned group, and the oscillation frequency f is given as: iR 4

R

6 C(R +R 2 11 2 FIG. 14 is a graph showing the relation of the oscillation frequency f and the weight W of the food stuff tE 3, and the dotted line is straight. As shown in the 20 figure, the characteristic curve is curving. If the distance between the electrodes 29a of the pressure sensor 24 changes while maintaining their parallelism, the capacitance C may be given as

S

d and also the oscillation frequency f may be given as dR4R f

S(R

1

R

2

I

18 and the relation of the capacitance c and the oscillation frequency f may be shown by the straight line which is the dotted line is FIG. 14. But in practice, as circumference of the sensor plate 26 is supported by the spacer 28, the parallel state can not be maintained. Therefore, the strain of the sensor plate 26 is curved in a manner that displacement becomes gradually smaller from the center to -the circumference. Accordingly, the characteristic curve becomes as shown by the curved solid line in FIG. 14.

p Oe 10 FIG. 15 is a circuit diagram showing an embodiment of

I

a control circuit of the heating oven having the weighing apparatus in accordance with the present invention. Main I part of the control circuit is constituted by a micro-computer 31, and commands which are inputted on input terminals I

I

to 14 of the micro-computer 31 from an operational input part 32, are decoded in the micro-computer 31 and indicated on an indicator 33. The pressure sensor 24 is impressed with a force induced by the weight of the food stuff 3 and the capacitance of the pressure sensor 24 changes in proportion to the weight.

The capacitance is transformed to the oscillation frequency t tf by the oscillator 30, and the micro-computer 31 counts the frequency f. In the micro-computer 31, the relation of the frequency f and the weight w shown in FIG. 14 is memorized. Such relation is approximated as a quadratic equation so that the weight w can be calculated as W Af+Bf+C (4) In the micro-computer 31, most suitable heating power, heating time and heating mode corresponding to the weight of the food stuff have been memorized, and the micro-computer 31 outputs control signals to a driver 19 for controlling the magnetron 20 and the heater 34. The driver 35 is connected to a relay 36 for controlling the heater 34 and a relay 37 for controlling the magnetron The magnetron 20 is oscillated by an oscillation circuit comprising a high-voltage transformer 38, a high-voltage capacitor 39 and a high-voltage diode 1 4 0 a *a emo 04 o &0 4 V9, 9*t i .4

Claims (2)

1. 5. An oven in accordance with claim 1, wherein said heater 6 means is disposed below said heating cavity. 7 8 6. An oven in accordance with claim i, wherein two 9 connecting parts are formed on opposite sides of said tray, and 11 offset parts are located between said tray and the 12 connecting parts whereby said connecting parts support said 13 tray on said beam shaped holders. 14
2. 7. An oven in accordance with claim 6, wherein the 16 distance between said two offset parts of said tray for 17 guiding and positioning said tray with respect to said beam 18 shaped holders, is narrower than distance between said two 19 beam shaped holders. 21 8. An oven in accordance with claim 6, wherein the width 22 of said connecting parts is narrower than the distance from 23 the side wall of said heating cavity to the innermost point 24 of contact of said beam shaped holder with said tray. 26 9. An oven in accordance with claim 6, wherein the total 27 width of the distance between said two offset parts and 28 width of one of said connecting parts is larger than the 29 distance apart of said two beam shaped holders. 31 10. An oven in accordance with claim i, wherein convex 32 parts are formed on said beam shaped holders for positioning 33 said tray in said heating cavity in a direction along the 34 length of said beam shaped holders. 36 11. An oven in accordance with claim 1, wherein convex 37 parts are formed on said tray in a manner to prevent \38 contacting of said tray with a ro3r wall of said heating .900118.c_.sspe. 5.a. :C a S Q S 5 22 1 cavity or a front door of the oven by abutment with said 2 convex parts on said tray. 3 4 12. An oven in accordance with claim 10, wherein the convex parts are formed on said beam shaped holders in such a 6 manner to prevent contacting of said tray with a rear wall 7 of said heating cavity or a front door of the oven by 8 abutment of said tray with said convex parts of said beam 9 shaped holders. 11 13. An oven comprising: 12 a heating cavity for containing an object to be heated 13 therein, 14 a heater means for applying in use energy to said object to be heated whereby to heat it, 16 a tray for holding said object to be heated thereon, 17 a pair of beam shaped holders, with one beam shaped 18 holder disposed on each side of said heating cavity for 19 holding said tray thereby to receive a load corresponding to the weight of said object to be heated, 21 a pair of linkages disposed for penetrating through 22 side walls of said heating cavity, each linkage contacting 23 with a respective one of said beam shaped holders for 24 conveying said load to externally of said heating cavity, holding mechanisms disposed on both sides of said 26 heating cavity for receiving such load conveyed by said 27 linkages, wherein said holding mechanisms being constituted 28 in a parallelogram construction, wherein one side is fixed 29 and another side parallel thereto is movable, a load cell for measuring load conveyed by said holding 31 mechanisms, and 32 control means for controlling said heater means in 33 response to the weight of said object to be heated as 34 measured by said load cell. 36 14. An oven in accordance with claim 13, wherein loads 37 conveyed by said holding mechanisms are measured by 38 concentrating at one point. /4 a a t I a I a II I I I I r a I a1 a S k, a a a a 23 1 15. An oven in accordance with claim 13, wherein the 2 movable sides of said mechanisms are disposed relatively 3 close to said heating cavity, and 4 the fixed sides of said mechanisms are disposed relatively remote from said heating cavity. 6 7 16. An oven in accordance with claim 13, wherein the 8 movable side of one of said mechanisms is disposed 9 relatively close to said heating cavity and the fixed side of said one of said mechanism is disposed relatively remote 11 from said heating cavity, and 12 the movable side of the other of said mechanisms is 13 disposed relatively remote from said heating cavity and the 14 fixed side of said other of said mechanism disposed relatively close to said heating cavity. 16 17 17. An oven substantially as hereinbefore described with 18 reference to FIGS. 3-15 of the accompanying drawings 19 excluding FIG.7(b). 21 22 23 DATED THIS 19th January, 1990 24 DAVIES COLLISON Fellows Institute of Patent 26 Attorneys of Australia. 27 Patent Attorneys for the Applicant 28 29 31 32 33 34 36 37 900118, c.sspe.O03R.irat .i .sp"'.20 o o 0 O t