EP0209201B1

EP0209201B1 - A method for heating in oven and microwave oven utilizing the method

Info

Publication number: EP0209201B1
Application number: EP19860201243
Authority: EP
Inventors: Per Olov Gustav Risman
Original assignee: Philips Gloeilampenfabrieken NV; Koninklijke Philips Electronics NV; Philips Norden AB
Current assignee: Whirlpool Europe BV
Priority date: 1985-07-18
Filing date: 1986-07-15
Publication date: 1991-05-02
Also published as: DE3679000D1; SE8503510D0; EP0209201A1; SE8503510L; SE452838B; JPS6266025A

Description

The invention relates to a method of automatically cooking food in an oven in which a signal representative of at least one parameter indicative of the extent to which cooking has been completed is fed back to a device controlling a source of the energy employed in cooking, the weight of the food being cooked being measured as cooking proceeds.
In DE-C2-32 05 124 a microwave oven is described in which during cooking at several time intervals the weight of the food is measured and compared with a calculated final or desired weight in order to control the process. Such a weighing measurement only is not sufficient for a reliable control of the cooking process. Therefore to compensate for intervening factors sensors are located in the inlet and outlet air paths of the cooking chamber, the signals therof being used for additional control of the process.
The object of the present invention is to improve the above described method in which use is made only of weight sensing to control the process.
According to the invention the method is characterized in that the weight decrease rate of the food is the parameter fed back to the control device.
It turned out that the weight decrease rate is a very reliable parameter for sensing the condition of the food. For example, the weight decrease will not be constant until a moment when the food has been heated down to an appreciable depth below the surface so that its cooling due to heat convection inwardly is small. The surface temperature then does not need to be 100°C because the power balance due to convective and evaporative surface cooling can occur earlier.
In one embodiment of the invention a final heating power and time are initiated when said weight decrease rate reaches a predetermined level. Initiating the final heating is inhibited if the running heating time is greater than a predetermined heating time.
A further embodiment of the invention is characterized in that after starting an initial heating power another parameter representing the difficulty of heating said food based upon the time required for said weight to reach a predetermined weight is determined, said parameter being used for the selection of a heating power for continued heating.
The invention also relates to a microwave oven for carrying out the above described method comprising a microwave source for feeding microwave energy into the oven cavity where the food to be cooked is placed, a weighing device for weighing the food inside the cavity and delivering an analogue signal indicative of the measured weight, an analogue-to-digital converter for converting the analogue signal into a digital signal, characterized in that it comprises a control device responsive to said digital signal for controlling the microwave source in respect of power level and/or remaining cooking time in dependence on the weight decrease rate of the food as cooking proceeds.
The invention is illustrated by means of the accompanying drawings, in which
Figure 1 shows the power balance during heating in a microwave oven,
Figure 2 shows the corresponding power balance in case of cooking, i.e. when the temperature no longer increases,
Figure 3 shows some curves of the weight decrease for different food products as function of the time in case of heating in microwave oven,
Figure 4 shows a basic block diagram for a microwave oven according to the invention, and
Figure 5 shows a flow diagram for the process in case of automatic control of a microwave oven according to the principles of the invention.
In Fig. 1, showing the power balance during heating at a temperature of 40°C, P0 represents supplied power, P1 represents the evaporative losses, P2 represents the convective losses, P3 represents losses due to heating of the vessel, while P4 represents the utilized power, i.e., the power which is dissipated in the food and which causes temperature increase therein. It is evident that the main part of the supplied power is utilized in the food, while the evaporative loss power as well as remaining loss powers are relatively small as compared with the utilized power.
In Fig. 2 showing the power balance of cooking (about 100°C), P0 again represents supplied power, while P1' represents the evaporative losses and P2' the convective losses. The utilized power in the food is zero including loss due to heating of the vessel. In this case,all supplied power also must be removed, which usually takes place by evaporation. This will result in a constant weight decrease rate. An evaporative loss power of 300 W,for example, corresponds to a weight decrease rate of 8g/minute at a surface temperature of +80°C.
During the transition from the heating condition according to Fig. 1 to the cooking condition according to Fig. 2, the evaporative losses will vary with time, resulting in a weight decrease rate which varies with time. By measuring the weight decrease or weight decrease rate, it is thus possible to obtain a reliable indication of how far the heating has advanced.
Fig. 3 shows some curves over the weight decrease S as a function of the time t in some different heating cases. For example, the curve 1a relates to about 400 g water or soup in an open pan, the curve 1b the same quantity in a covered pan, the curve 2 relates to the same quantity of compact food, such as pudding, in a covered pan,and the curve 3 relates to a larger quantity (1000 g) of compact food. The initial temperature in all cases is normal room temperature and the oven is a microwave oven with about 600 W output power.
It is evident that the weight decrease as such not always gives the best indication on how far the heating has advanced. It is furthermore evident that a humidity sensor supplying a signal at a given -relatively low- derivata (see Fig. 3) should produce values which must be corrected due to inner convection, covering, if any, and quantity in order to be fully usuable. However, if the initial weight and the variation rate of the weight are known,an appreciably improved system can be constructed. According to the invention, this may be done in the following manner, reference being made to Fig. 3:

A. The initial weight M is stored in the electronic memory of the system (taring is presumed). The initial temperature T1 (frozen, refrigerator temperature, room temperature) is set by the user as well as the intended process (heating, cooking).
B. The time t₁ until a weight decrease of for example 2 g has occurred is stored. In Fig. 3,a weight decrease of 2 g is indicated by the dashed horizontal line and two values of t₁ marked with an arrow; more closely t₁ (1a) in the case 1a and t₁ (2) in case 2. The program now can decide what type of food it is question of: if t₁/M (with correction for T1) is small then the food is bulky and difficult to heat and should in the following be heated at a low power; if the said magnitude is large the further heating can be effected at a high power; if M is small, t₁/M is large and heating (not cooking) is desired then the process can be interrupted directly.
C. The weight is determined at even intervals and the decrease rate v is calculated. - The program now compares v with the (power dependent) maximal value V in case of temperature equilibrium. When v/V has reached a certain value the final heating is initiated; its duration depends upon the total time reached until then, possibly also t₁, and the set process type. - If M is large the remaining time in case of reheating can be 0; the same is valid if M is small and cooking is desired.

In the marked case t₁ (1a):

t₁/M is rather large and
M is rather small.

If heating is concerned the heating process can be interrupted immediately (the temperature is ca. 65°C).
If desired, the heating time can be elongated with ca. 30% which results in a final temperature of 75°C.
In the case t₁ (2):

t₁/M is relatively small.

The heating continues with a relatively low power (this has not been done in the curve). After a while v/V will be > a given pre-programmed value (perhaps after 3 minutes). Then the heating continues further 30% of the total heating time until then, whereafter the food is ready.
Fig. 4 shows schematically a microwave oven with magnetron and a basic diagram for a control circuit, by means of which the principles of the invention can be realized. In the drawing A designates an oven cavity, B is a magnetron which via a waveguide connection (not shown) feeds microwave energy into the cavity and C is a start-stop circuit for the magnetron. In C is included a timer and an intermittently operating switch arrangement, as a cam follower device, whereby the average power delivered by the magnetron can be set.
A weighing scale D is,according to the invention,placed in the bottom of the cavity and continuously measures the weight of the introduced food. The scale, which may be of strain gauge type, delivers an electrical signal which represents the instantaneous value of the measured weight. This signal is fed to an analogue-to-digital converter E, in which it is converted to a digital signal, and is thereafter applied to a control device F. At a second input the control device F receives signals from a keyboard G and delivers its output signal to the start-stop circuit for the magnetron. The keyboard G can also be directly connected to the start-stop circuit for pure manual setting.
When operating with automatic control, the heating or cooking process is according to the invention controlled with signals derived from the weight indicating signal delivered by the scale D. The control device F comprises for this purpose memory means, in which the initial weight of the introduced food with reduction for the weight of the vessel is stored. Furthermore the control device F has calculating means which from the weight indicating signal derives magnitudes representing the weight decrease and/or the weight decrease rate. By means of these magnitudes: initial weight, weight decrease and/or weight decrease rate, the heating or cooking process is controlled such that an optimal result is obtained in each individual heating case.
The control device F can suitably comprise a microprocessor or the like, which is pre-programmed to perform a desired function. An example of a flow diagram for a program which is executed by a microprocessor included in the control device is given in Fig. 5.
The process is started by pushing an "on"-button, represented by the block 10 in Fig. 5, whereby the oven is made clear for use. Thereafter taring is effected by putting the empty vessel into the oven and pushing a button marked "taring" represented by the block 11 in Fig. 5, whereby the vessel is weighed and the weight of the vessel M_T is stored. Then the vessel is filled with food to be heated and weighing of the food plus the vessel is initiated by pushing a corresponding button on the keyboard. This operation is represented by the block 12 in Fig. 5. The initial weight M of the food is then calculated by subtracting the weight M_T of the vessel from the total weight determined in the block 12 and the value of M is stored in order to serve as a control parameter during the whole heating process. The calculation and storing of the initial weight M is represented by the block 13 in Fig. 5. In the block 14 it is checked if M is smaller than 50 g. If the answer is "yes" then the process is interrupted, the block 15, and the oven assumes ready state for manual heating. This is because small quantities should not be heated automatically. If the answer is "no" then the process continues and a parameter T1, representing the initial temperature of the food is set, in block 16. T1 which is set by means of buttons on the keyboard can,for example, assume one of three values representing "freezer temperature", "refrigerator temperature" and "room temperature", respectively. The desired process is selected, in block 17, also by means of buttons on the keyboard. For the selection in block 17 there are for example two alternatives: "heating" and "cooking". The heating process is then started by pushing a start button, in block 18, whereby the magnetron is connected to its operation voltage. Simultanously the timer is started for indicating the running time t from the start of the magnetron, in block 19. In the block 20 the absolute decrease of weight M-M_p is determined and the following question "is M-M_p larger than 2 g ?" is made, M_p being the weight of the food during the heating. If the answer to the question in the block 20 is "no" then repeatedly a new calculation of the absolute decrease of weight M-M_p and comparison with the absolute value 2 g is effected. If the answer in the block 20 is "yes" then the timer is read and the time t₁ required to reach the weight decrease 2 g is stored, in block 21. Now the program continues by forming a parameter A, in block 22, which is defined by the formula:
A = f (t₁/M + k T1)
where f is an empirically obtained function, t₁, M and T1 have the previously mentioned meanings and k is a scale factor. The value of the parameter A is an indication on how difficult it is to heat the food; the smaller A the more difficult it is to heat the object.
In the block 23 the following question "is M small, A large and heating desired?" is made. If the answer to this question is "yes" then the heating process can be interrupted, the block 24, and oven returns to the ready position for new heating. If the answer in the block 23 is "no" then the process will continue, which is effected by selection of power.
The power selection is made in dependence of the parameter A and in the block 25 the question "is A larger than x?" is made, x being a stored constant. If the answer is "yes" then the oven is set to a high power P = P_h, the block 26. If the answer in the block 25 is "no" then the program continues to the block 27. Here the question "is A larger than y?" is made, y also being a stored constant (y < x). If the answer in the block 27 is "yes" then the oven is set to a mean power P = P_m, the block 28. If the answer in the block 27 is "no" then the program continues to the block 29. Here the question "is A larger than z?" is made, z (z < y) being a stored constant. If the answer in the block 29 is "yes" then the oven is set to a low power P = P_l, the block 30.
The constants x, y and z are empirically determined in such manner that the power P is adapted to the load in each individual operation case.
Immediatley after the selection of power level for the continued heating a parameter V is determined, which parameter is defined as the maximal weight decrease per time unit, i.e., the weight decrease rate at temperature equilibrium and for the selected power. This is effected in the block 31 in Fig. 5. During the continued heating with the selected power a measuring procedure takes place, which leads to final heating and switching-off of the oven. This is effected by means of the weight decrease rate, which is determined intermittently with a time interval of t_v, e.g. 20 seconds, the blocks 32 and 33. In the block 32 the running time t' from the foregoing weight measurement is determined and the question "is t' equal to tv?" is made. If the answer "no" then the time measurement continues. If the answer is "yes" then the instantaneously prevailing weight M_pi is measured, in block 33. In the block 34 the weight decrease rate v_i is calculated according to the formula:
v₁ = △M₁/t_v
where △M_i = M_pi - M_p (i-1) is the weight decrease and i is the running order number of the weight measurement.
In the block 35 the question "is v_i smaller than v_i-1?" is made. If the answer is "yes" this can indicate the start of "dry-boiling" or a similar abnormal happening, and heating is immediately interrupted, in block 36. If the answer in the block 35 is "no" then the program proceeds to block 37, in which the ratio between actual measured weight decrease rate v_i and the previously defined maximum weight decrease rate V is calculated.
This ratio v_i/V is in most cases a good indication on how far the heating has advanced and is used for initiating final heating. Before this is made, however, a check is made of the total heating time t, in block 38. Here the question "is t larger than f' (t, P, M)?" is made, where f' is a function of the time t, the power P and the initial weight M. If the answer in the block 38 is "yes" then the remaining heating time t_r in this case is set equal to f' (t, P, M), in block 39, whereafter the heating is interrupted, in block 40. The function f' (t, P, M) is then such that very large quantities of food, which could result in a value for v_i/V, which normally is used to initiate final heating, never will be reached, and instead are finally heated according to the rule given in the block 39.
If the answer in the block 38 is "no" then in block 41 the question "is v_i/V larger than α?" is made, α being a stored constant. If the answer is "no" the program returns to the beginning of the block 32 and a new determination of the weight decrease rate v_i and thereby of v_i/V is effected. When the weight decrease rate has reached such a high value that the answer in the block 41 is "yes" then the program proceeds to the block 42. Here the question "is M smaller than m?" is made, m representing a relatively small quantity of food. If the answer in the block 42 is "yes" then the heating is interrupted immediately, in block 43, and the oven returns to the ready state. If the answer in the block 42 is "no", which is valid for medium-sized quantities of food, then the program proceeds to the block 44 where the final time t_s is determined according to the formula:
t_s = f'' (s . t + r . t₁)
where f'' is an empirically determined function of t and t₁ and s, r are scale factors.
At the same time the final power P _s is determined, in block 45, according to the formula :
P_s = f''' (s . M)
where f''' is an empirically determined function of M and s is a scale factor.
After setting of the final power P_s the question "is t equal to t_s?" is made in the block 46. As long as the answer in the block 46 is "no" the final heating continues with the determined power. When the answer in the block 46 is "yes" then the heating is interrupted, in block 47, and the oven returns to the ready state.

Claims

A method of automatically cooking food in an oven in which a signal representative of at least one parameter indicative of the extent to which cooking has been completed is fed back to a device controlling a source of the energy employed in cooking, the weight of the food being cooked being measured as cooking proceeds, characterized in that the weight decrease rate of the food is the parameter fed back to the control device.
A method as claimed in Claim 1, characterized in that a final heating power and time are initiated when said weight decrease rate reaches a predetermined level.
A method as claimed in Claim 2, characterized in that initiating the final heating is inhibited if the running heating time is greater than a predetermined heating time.
A method as claimed in Claim 3, characterized in that said predetermined heating time is based upon an initial weight of the food and a previous continued heating power.
A method as claimed in Claim 1, characterized in that after starting an initial heating power another parameter representing the difficulty of heating said food based upon the time required for said weight to reach a predetermined weight is determined, said parameter being used for the selection of a heating power for continued heating.
A microwave oven for carrying out the method according to any one of Claims 1 to 5 comprising a microwave source for feeding microwave energy into the oven cavity where the food to be cooked is placed, a weighing device for weighing the food inside the cavity and delivering an analogue signal indicative of the measured weight, an analogue-to-digital converter for converting the analogue signal into a digital signal, characterized in that it comprises a control device responsive to the said digital signal for controlling the microwave source in respect of power level and/or remaining cooking time in dependence on the weight decrease rate of the food as cooking proceeds.