Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
2000-02-03
2001-10-09
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S492000
Reexamination Certificate
active
06300606
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of detection of a boil dry condition of a utensil placed on a glass-ceramic cooking surface of a cooking unit, which has at least one cooking area, which is associated with a heating element, whose energy input is adjusted by means of a heating element power control device, and in which the temperature, or a variable dependent on it, such as a signal (temperature signal), is measured in operation and is limited by means of a temperature limiting protective device with an adjustable cutoff temperature.
The invention also relates to a device or apparatus for performing this method.
2. Prior Art
The moisture or liquid present in cooking material in a cooking utensil or vessel can be completely evaporated during cooking when too much heat energy is supplied or when it is heated for too long a time interval. This situation is called a boil dry condition. It can lead to damage of the cooking apparatus in the case of aqueous cooking materials, especially of the glass-ceramic cooking surface and the vessel. With other cooking materials there is a danger that the cooking material can ignite or burn and damage surrounding devices. In this case the energy supply is preferably turned off and the operator is at least warned. Typically this error occurs when high heat energy input used for a rapid cooking phase is not turned off because the operator has forgotten that high heat energy input is being used.
Of course the surface temperature of the cooking area on glass-ceramic cooking surfaces of cooking units is typically monitored and protected by means of rod-like mechanical temperature limiting protective devices. These devices measure the average temperature under the cooking area and limit it. The shutoff temperature, also its so-called threshold value, is established on the basis of the properties of the standard cooking vessels or utensils and the glass-ceramic material. Also an electronic device for limiting the temperature is known. However only the behavior of the mechanical temperature limiting protective devices is sufficient sensitive, so that an additional evaluation of temperature information is not required.
The boil dry condition of a cooking utensil cannot be detected only by means of the typical temperature limiting protective device provided with a glass-ceramic cooking surface of a cooking unit.
A method and corresponding device for detection of the so-called boil dry condition is disclosed in U.S. Pat. No. 4,493,981. In this method the first derivative of the temperature with respect to time is used as the indicator for this condition. When it exceeds a certain predetermined limiting value, then a boil dry condition is detected. The limiting value is dependent among other things on the type of cooking unit and the actual power set. Furthermore a boiled dry vessel or utensil is detected, when the temperature in the cooking vessel or utensil exceeds a certain limiting value. Also this value depends on the type of cooking unit and the actual input power. When a boil dry condition is detected, the operator is warned or the cooking unit is turned off.
In this known method it is disadvantageous that the detection of the critical operating condition depends on the actual input power. Also in practice this has the consequence that no uniform limit for the first time derivative of the temperature can be used to reliably detect the boil dry condition for different types of cooking utensils. More rapid increases in temperature can occur in filled poorly heat-conducting utensils than in empty, heat conductive utensils.
The boil dry condition is detected by measurement of the temperature in the interior and exterior of a two-circuit heating element in the device described in DE 40 22 846 C2. This known device is however limited to glass-ceramic cooking surfaces with two-circuit heating elements.
A method for detection of the boil dry condition of a vessel on a glass-ceramic cooking surface of a cooking unit and an associated apparatus are disclosed in DE 43 36 752 A1. In this method the temperature is measured and its first derivative is determined. When the value of the first derivative first exceeds a predetermined boundary or limiting value, the initial heating power is reduced by 100%. This reference also discloses that empty metal vessels being heated are detected because of the reduced electrical heating power required to reach the same temperature and the current supply is shut off when the input power drops below a certain predetermined value. This method has the same disadvantage as described above for the U.S. Patent reference.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus of the above-described kind for detecting a boil dry condition, wherein a reliable detection of this critical operating condition of a cooking vessel or utensil is possible independently of the actual input power and also independently of the type of cooking vessel or utensil, so that reliable protection of the associated cooking area is possible during this critical operating state.
According to the following different embodiments of the invention, which differ according to the relative magnitude of the actual temperature in comparison to the shutoff temperature, with a first temperature pattern or relationship the method comprises the following steps:
a) comparing a measured temperature with a shutoff temperature;
b) forming both the first and also the second time derivative of a temperature signal and detecting operation of a heating element power control device, when the measured temperature is outside of a predetermined temperature range at the shutoff temperature as determined by the comparing; and
c) generating a control signal, when both time derivatives are positive and the heating element power control device has not been operated within a predetermined time interval to increase the input power; or
with a second temperature pattern or relationship
a) comparing the measured temperature with a shutoff temperature;
b) determining the energy input to the heating element and detecting operation of the heating element power control device when the measured temperature corresponds to the shutoff temperature according to the comparing; and
c) generating a control signal when the energy input is reduced a predetermined amount and the heating element power control device has not been operated within a predetermined time interval to reduce the input power; or
with a third temperature pattern or relationship
a) comparing the measured temperature with a shutoff temperature;
b) lowering the shutoff temperature to the measured temperature, if a temperature equilibrium exists within a predetermined temperature range at the shutoff temperature during the comparing;
c) determining the energy input to the heating element and detecting operation of the heating element power control device; and
d) generating a control signal when the energy input is reduced a predetermined amount and the heating element power control device has not been operated to reduce the input power within a predetermined time interval.
The foregoing features of the method according to the invention very advantageously provide a reliable detection of a boil dry condition of a cooking utensil or vessel independently of the actual power input and also independently of the cooking utensil type. The disclosed sequence of process steps produces no transient or spurious results, but describes the conditions for detection of the boil dry condition. This criteria can temporarily supercede the operation of the heating element power control device prior to testing of the temperature threshold.
The individual temperature patterns or relationships are coupled with each other, since the temperature of the cooking region is not constant, chiefly the heating element power control device can be operated at any time.
In the case of the second temperature pattern the detection of the critical operating state acco
Engelmann Harry
Schaupert Kurt
Glas Schott
Hoang Tu Ba
Striker Michael J.
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