Abstract:
A method and device for controlling and limiting the cooking temperature of any selected utensil for cooking on a surface burner element of a range are shown. A temperature sensor is provided either on a heat transfer plate which is mounted to the burner element or in a remote-controlled unit mounted directly to the utensil. The actual temperature of the utensil is compared with the selected cooking temperature, and it is then adjusted to maintain the two temperatures close to each other. The device also automatically turns off the burner element in a potential runaway condition or in the absence of a utensil placed thereon.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the method and the device for controlling the temperature of all utensils for cooking on a burner element particularly on the surface burner element of a range. There are many types of ranges equipped with surface burner elements presently in use. Some surface burner elements of electric ranges are in the form of spiral sheathed heating elements, while others have a flat heat transfer top made of either glass, metal alloy, or ceramic with the heating elements located underneath such top. Others use natural gas for heating. The challenge is to create a regulating device which can provide safe control of the cooking temperature of the utensil placed on the heating element. Most electric ranges in use nowadays do not have any cooking temperature control per se; a device commonly referred to as a heat switch is used to regulate the current to the burner element so as to vary its heating power. The heat switch may be in the form of a mechanical switch provided with a current coil or a current control electronic circuit which operates to vary the heating power of the burner element. Malfunction of such current control would inevitably result in a run away operation of the burner element, namely the burner element would operate continuously with increasing power to result ultimately in a fire hazard. In cooking, it is, in fact, the temperature of the utensil that actually cooks the food contained within it. However, by controlling the heating power output of the burner element, the actual cooking temperature is unknown. Since utensils of various sizes and volumes may be used for cooking and also various types and volumes of food are being cooked, it is necessary to re-adjust the heat switch when using utensils of different sizes and volumes for cooking various types and amount of food. The user has to estimate the appropriate setting of the heating power of a burner element, depending on the size of the utensil, and the amount of food to be cooked in the utensil. The heat switch varies the current to the burner element with a series of ON and OFF cycles so as, in turn, to vary its heating power. Such method of merely varying the heating power does not provide a satisfactory cooking operation when using utensils of different shapes and sizes for cooking foods having various masses of liquids and/or solids so that the heat losses, as well as the thermal capacities may vary widely; therefore, the same amount of heat applied to two very different utensils will result in two very different actual cooking temperatures. Furthermore, the heat required to raise the temperature of the utensil is often much higher than the heat required to maintain the utensil at a desired cooking temperature. Most users would turn the heat switch control dial to the maximum heat setting in order to save time in heating the utensil to the desired cooking temperature quickly; and it is intended to turn the control dial down to a lower maintenance heat setting after the utensil has attained the desired temperature. However, often time, the user forgets to do so, resulting in dire consequences in which the cooking pot may become overheated, or even melted, or the cooking oil or similar flammable substance inside the pot may be ignited. Kitchens and whole houses have been known to burn down in many cases due to such negligence. 
     In U.S. Pat. No. 4,492,336 to Takata et al and U.S. Pat. No. 4,470,888 to Ceste et al, temperature controls for particular types of cooking vessels are shown. Such temperature controls do not cater to universal applications for cooking with different types of cooking utensils such as boiling water in a kettle, or heating a large stew pot; and they cannot be used for controlling common cooking utensils intended for use in cooking with different types of conventional surface burner elements. 
     There have been attempts to sense directly the temperature of the utensil during cooking by means of infrared devices, such as those shown in U.S. Pat. 4,734,553 to Noda and U.S. Pat. No. 4,499,357 to Kojima. However, these methods and devices suffer the critical drawback that the infrared waves can be blocked by the lid, handle, or other parts of the utensil to render them inoperative. Thus, there is a great demand that the temperature sensing method and device must be capable of sensing directly the actual cooking temperature of any cooking utensil which is heated on all types of conventional burner elements so that the application of the method and device is universal. Furthermore, the method and device must be capable of turning off the burner element when there is no cooking utensil placed thereon or to prevent the burner element from operating in a runaway condition. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the present invention to provide a method and device for controlling the actual cooking temperature of any utensil to be heated on a common burner element of a cooking heater. 
     It is another object of the present invention to provide a heat control device having an integral fail safe circuits which is operative to turn off the burner element in case of some critical components failure in the device due to unforeseeable circumstances. 
     It is another object of the present invention to provide a reliable temperature control device which may be incorporated into existing cooking heaters with simple modification to the latter. 
     It is another object of the present invention to provide a heat control device which has a protective cover for the heating elements of the cooking heaters so as to alleviate the likelihood of any fire hazard. 
     It is yet another object of the present invention to provide a method and device which is capable of sensing the presence or absence of a utensil placed on the cooking heater and to switch off the heating elements automatically when the utensil is absent. 
     Briefly, the device according to the present invention includes a temperature sensing means mounted in close contact with the cooking utensil such as on a heat transfer plate mounted to the burner element on which the utensil is located. A switching means is connected in series with the temperature sensing means. The switching means has switching elements connected in series to the current control switch and the surface burner element. The switching means has actuation element therein connected to the temperature sensing means. The actuation means is operative in response to the actual temperature of the cooking utensil sensed by the temperature sensing means for selectively operating the switching elements for energizing the burner element. 
     The device may have a battery powered transmitter mounted directly on the utensil and the switching means is located in a receiver means. The actual cooking temperature of the utensil is sensed by a thermocouple having a heat sensing junction which is either in direct contact with the side wall of the utensil or mounted to the heat transfer plate. The thermocouple will generate a voltage signal relative to the actual cooking temperature of the utensil. A temperature selector in provided in the transmitter for selecting the desired cooking temperature. The transmitter operates to compare the actual cooking temperature and the selected temperature to transmit a control signal to the receiver for actuating the switching means for turning the burner element in a series of on and off cycles so that the actual cooking temperature is equal to the desired cooking temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a bottom elevation of the burner element of a cooking heater having the component of the heat control according to the present invention mounted thereon. 
     FIG. 2 is an exploded elevation view of FIG. 1 showing the various components therein. 
     FIG. 3 is a schematic circuit diagram of the electrical control circuit of the device according to the present invention. 
     FIG. 4 is a schematic diagram of a preferred embodiment of the heat control electrical circuit in the device according to present invention. 
     FIG. 5 is a schematic diagram of an alternate electrical circuit of the control according to the present invention using a battery operated remote sensing and control transmitter. 
     FIG. 6 is a schematic diagram of the electrical circuit of the receiver when the remote sensing transmitter is used. 
     FIG. 7 is a side elevation view of the temperature remote control unit according to the present invention. 
     FIG. 8 is a top elevation view of the temperature remote control unit. 
     FIG. 9 is a side elevation showing the mounting of the temperature remote control unit to a cooking utensil. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described with reference to the accompanying drawings wherein corresponding parts are identified with the same reference numerals and/or alphanumerals. The heat sensing assembly of the present device for mounting to a conventional burner element is best shown in FIGS. 1 and 2. Only one heat sensing assembly is shown for the simplicity of illustration purposes. It can be appreciated that the assembly is similar for all surface burner elements in an electric range in which, commonly, four surface burner elements are provided. The heat sensing assembly of the present invention includes a heat transfer plate  10  mounted to the burner element  11  such as a spiral sheath-type surface burner element. A removable burner element is shown as an example. The assembly may similarly be applied to tiltable burner elements which are permanently mounted to the range top but they may be tilted upwards to facilitate cleaning of any food spillage to a burner box area located below the burner elements. The heat transfer plate  10  has a diameter larger than the opening in the range top surrounding the burner element such that any spillage due to boil over during cooking will be diverted by the heat transfer plate  10  to the range top rather than running into the burner box so as to facilitate easy cleaning up in the event of such spillage. The heat transfer plate  10  may be molded of aluminum or other metal or alloy having a high thermal conductivity and its surface may be anodized to resist abrasion. The heat transfer plate  10  is mounted to the burner element  11  by a Y-shaped mounting plate  12  which is secured to the heat transfer plate  10  by three bolts  13 ,  14  and  15  affixed to the under side of the heat transfer plate  10  together with associated nuts  16 ,  17  and  18  respectively. 
     A thermostat  19  is also mounted to the heat transfer plate  10  by the bolts  14  and  15  with separate nuts so that it abuts tightly against the surface of the heat transfer plate  10  for sensing its surface temperature. Alternatively, a thermocouple may be mounted to either one of the bolts  14  and  15  by welding to a washer which is secured to the bolt by a nut so that it is held tightly against the heat transfer plate  10  for sensing its surface temperature. The opening in the bifurcation of the mounting plate  12  permits ready access to the electrical connection terminals of the thermostat  19 . 
     The electrical circuit for use with the heat sensor of the device for controlling the temperature of all four burner elements is shown in FIG.  3 . The supply of power from the power lines L 1  and L 2  to the four burner elements EL 1 , EL 2 , EL 3  and EL 4  are controlled by four heat switches HC 1 , HC 2 , HC 3 , and HC 4  respectively in a conventional electric ranges, which, in fact, only control the current flowing through the burner elements. The actuation of the burner elements are indicated by pilot lights PL 1  and PL 2 . According to the present invention, four relays RL 1 , RL 2 , RL 3  and RL 4  are incorporated to control the operation of the burner elements EL 1 , EL 2 , EL 3  and EL 4  respectively. The terminals of the switching contacts RS 1 , RS 2 , RS 3  and RS 4  of the relays RL 1 , RL 2 , RL 3  and RL 4  respectively are connected in series with the burner elements EL 1 , EL 2 , EL 3  and EL 4 . The direct current operating power for the relays EL 1 , EL 2 , EL 3  and EL 4  is supplied from the power lines L 1  and L 2  by a rectification circuit consisting of a stepdown transformer TX, rectifier diodes D 1  and D 2 , and modulating capacitor C 1 . Temperature control thermostats TS 1 , TS 2 , TS 2  and TS 4  are connected in series with the operating coils of the relays. The thermostats TS 1 , TS 2 , TS 3  and TS 4  correspond to the thermostat  12  mounted to the heat transfer plate  10  of each of the burner elements EL 1 , EL 2 , EL 3  and EL 4  respectively as shown in FIGS. 1 and 2. Since the cooking utensil is placed on the heat transfer plate  10 , its mass becomes integral with the cooking utensil such that the temperature of the heat transfer plate  10  is equal to the actual cooking temperature of the utensil. As long as the thermostats are closed, the relays are energized and power can be applied to the burner elements. The thermostats are preset so that their contacts will be opened at a safe temperature which is below the ignition temperature of oil, paper or such similar substances commonly present in the kitchen thereby reducing the risk of fire. 
     In the beginning of the cooking operation, the thermostat is closed. When the temperature of the heat transfer plate  10  is increased to higher than the preset opening temperature of the thermostat, the thermostat contacts become opened thereby deactivating the relay and, in turn, turning off the associated burner element so that the heat transfer plate as well as the utensil will cool down in a short period of time. 
     An electrical control circuit for use with thermocouple sensors mounted to the heat transfer plate is best shown in FIG. 4 in which L 1  and L 2  are incoming AC line voltages to which most electric ranges are commonly connected. The primary PR of a stepdown transformer TX is connected to the power lines L 1  and L 2 . The low voltage secondary SR of the transformer TX is connected to diodes D 1  and D 2  for rectification of the AC voltage so as to produce a DC positive voltage V 1  which is modulated by a capacitor C 7 . The center tap CT of the secondary of the transformer TX is connected to the negative pin of the capacitor which, in turn, is connected to a common negative voltage ground line G. The positive voltage V 1  is applied to the input of a regulator REG which has one pin grounded and has a regulated output voltage V 2 . The regulated output V 2  which powers most of the circuit is switched to the circuit when the potentiometer P 2  is turned on for setting a required temperature of the utensil located on the burner element, by the switch S 1  which is a part of a potentiometer P 2 . A capacitor C 1  is provided to reduce the possibility of spurious oscillation of the integrated circuits present in the circuit. As soon as the potentiometer P 2  is turned on, the light emitting diodes D 3  and D 4  will be turned on by the resistors R 20  and R 21  to emit a light of mixed color from this diode assembly which has two diodes, D 3  and D 4  each emitting a different color. The integrated circuits A 1 , A 2 , B 1  and B 2  have their positive voltage terminal connected to a regulated output line V 2  and their negative voltage terminal connected to the ground line G. The resistors R 2  and R 3  and the capacitor C 2  produce a divided voltage V 3  which is arranged to be substantially equal to the voltage across the collector and emitter of the transistor Q 1 . A thermocouple TC 1  is provided for sensing the temperature of the heat transfer plate PT which is substantially identical to the temperature of the utensil placed on the heat transfer plate. When the hot junction of the thermocouple TC 1  is heated up with respect to the cold junction CON 1  and CON 2 , there is a voltage generated at the cold junction. The amount of voltage generated is proportional to the temperature difference between the hot and cold junctions. The negative wire of the thermocouple is connected to the divided voltage V 3  and the positive voltage of the thermocouple is applied to the non-inverting input NIV of an operational amplifier A 1  via the resistor R 5 . A capacitor C 5  is connected between NIV and V 3  while the feedback capacitor C 3  attenuates unwanted pick-up voltage at the output of the amplifier. To compensate for the variation in temperature of the cold junction, the transistor Q 1  with its base and collector are joined together to output line V 2 . The base and collector volt is applied to the inverting voltage IV of the amplifier A 1  by the resistor R 11 ; and by selecting appropriate value for the resistor R 11 , the effect of the variation of the cold junction temperature can be greatly reduced. The resistor R 4  is connected between CON 1  and V 2  so that if there is an open circuit in the thermocouple, the NIV voltage of A 1  will go high, resulting in a high voltage at the output of integrated circuit A 1 , thus switching off the relay RL 1  and resulting in the removal of the power to the burner element EL 1 . The resistors R 7  and R 6  and the potentiometer are used to adjust the output of the operational amplifier so that its output is zero when the temperature of the thermocouple is at zero. The resistors R 9  and R 8  and the potentiometer P 4  sets the gain of the amplifier. Resistor R 12  and the capacitor C 8  roll off the frequency response of the circuit and further reduce the value of pick-up voltage; and the output of the integrated circuit A 1  is applied to the IV input of the amplifier which serves as a comparator. Resistor R 13  provides a voltage drop across the control potentiometer P 2  so that the maximum required temperature which is set as voltage on potentiometer P 2  does not exceed the selected safe level. Resistors R 14 , R 15 , R 16  and R 17  provide a positive feedback to allow a small hysteresis in the operation of this comparator. If the selected voltage of slider of the potentiometer representing the required temperature, which is applied to the non-inverting input NIV of the comparator, is higher than the voltage of the output of amplifier representing the utensil temperature, the output of the comparator is high. In this condition, the transistor Q 2  is turned on by the resistor R 18 , while the resistor R 19  acts as a shunt. When the transistor Q 2  turns on, it applies an operating voltage to the coil of the relay RL 1  to energize it, and the contact RS 1  of relay RL 1  will become closed so that power is applied to the element EL 1  for heating up the heat transfer plate and the utensil placed thereon. When the transistor Q 2  is on, it shorts out the diode D 4  to draw current away from it, which changes the color output of the light emitting diode assembly. The diode D 6  ensures that the diode D 4  is properly turned off. Diode D 8  is across the relay to limit the back EMF generated by the coil of the relay RL 1  in order to protect the transistor from breaking down. If for any reason the temperature of the heat transfer plate rises too high, the thermostat TS 1  will become opened, thus terminating the current to the coil of the relay and, in turn, the switch RS 1  becomes opened so that power is removed from the burner element EL 1 . If the temperature of the utensil is higher than the required temperature set by the potentiometer P 2 , the output of the integrated circuit A 2  is low, and it will turn off the transistor so that the relay is de-energized and the power to the burner element is again removed. The output of the amplifier is connected to the capacitor C 4 , while the other end of which is connected to the NIV input of the amplifier B 1  and also to the resistor R 24 . In this arrangement the voltage at the NIV input terminal of the amplifier is proportional to the rate of rise of temperature of the heat transfer plate. The gain of this amplifier is set by the resistors R 26  and R 25 . R 23 , R 22  and the potentiometer P 3  which is adjusted to substantially zero for the output of the amplifier when there rate of rise of temperature is substantially zero. This circuit functions to switch off the burner element EL 1  in the absence of the utensil on the heat transfer plate. When there is no utensil on the plate, the rate of rise of temperature of the plate is high, this results in a high voltage level at the output of amplifier B 1 , and this voltage is applied to the NIV input terminal of the amplifier B 2  which serves as a comparator. The resistor R 29  and potentiometer P 5  are used to set the rate of rise of temperature to detect the absence of a utensil and this voltage is applied to input terminal IV of the amplifier B 2 . If the rate of rise of the temperature of the plate is higher than the set level, it makes the output of amplifier high which turns on the transistor Q 3  via the resistor R 30 , and the resistor R 31  acts as a shunt. When the transistor Q 3  is turned on, it places a short circuit on the base emitter junction of the transistor Q 2 , thus de-energizing the relay and removing the power from the burner element EL 1 . The capacitor C 6  and resistor R 28  is selected to have a sufficient time constant so that the plate can cool down adequately until the next attempt to switch it on A battery-operated remote-controlled circuit for the device of the present invention is best shown in FIG. 5 in which the power supply voltage is provided by batteries BAT. The battery voltage V 4  is applied to the circuit by operating a control switch S 8 . The required temperature of the utensil is set by adjusting the potentiometer P 7 , and the switch S 8  forms a part of the potentiometer P 7 . When the switch S 8  is closed, it applies the supply voltage to the power line V 5 , which will activate the light emitting diode D 21  to turn on briefly via the capacitor C 14  and resistor R 59 . The regulated voltage for the circuit is provided by the reference diode RF and the resistor R 33 . The control unit operates with the heat sensing thermocouple TCR. The operation of the circuit for this unit is similar to that described in FIG.  4 . Resistors R 37 , R 39  and transistor Q 4  provide the cold junction compensation circuit. Resistors R 34 , R 35  and capacitor C 17  provide the voltage divider whose voltage is substantially equal to the voltage across Q 4 . C 11  is the smoothing capacitor. Resistors R 36 , R 38  and capacitors C 18  and C 19  provide the attenuation of spurious signals and open circuit detection for the thermocouple TCR. R 61  and P 6  provide the zeroing circuit for the amplifier AA 1 , while resistors R 40 , R 41 , R 42  and R 43  set the gain for the amplifier. Resistor R 44  and capacitor provides a further attenuation for unwanted signals. The required temperature is set on the potentiometer P 7 , and the set operating voltage is applied to the positive or NIV input terminal of the comparator AA 2  via the resistor R 45 . This voltage is compared with the output of AA 1  which represents the temperature of the utensil which is applied to the negative or inverting input terminal of the comparator AA 2 . R 48 , R 49 , R 47 , R 46  act as voltage selection means for the positive feedback for the comparator AA 2 . When the utensil temperature is higher than the required set temperature, the output of the comparator is low. The output of the comparator is inverted by the inverter H 6 . H 1 -H 6  are CMOS inverters. When the comparator is low, the output of the inverter H 6  is high and it back biases a diode D 23  to activate an oscillator composed of inverters H 1  and H 2  and resistor R 60  and capacitor C 15 . During the sample time of inverter H 1 , the diode D 22  is also back biased. An amplifier E 1  is used to provide an oscillator having a relatively long operating time period. The resistors R 50 , R 51  and R 52  provide a positive feedback and the resistor R 53  and capacitor C 12  provide a negative feedback to produce the required oscillation. Each time the output of amplifier E 1  goes from negative to positive the amplifier E 2  emits a negative pulse at is output. The period of this pulse depends upon the time constant of the capacitor C 13  and resistor R 54  and is chosen to be small to conserve power, as its period determines the length of time of the transmission. The resistors R 57 , R 56  and R 55  provide a positive feedback so that the edges of the pulse are rapid in transition. The negative pulse is inverted by the inverter H 1 . If the duration the pulse is high at the output of the inverter H 1 , the light emitting diode D 21  is turned on via the resistor R 58  for indicating that the unit is in operation. When there is a positive pulse at the output of the inverter H 1 , the diode D 23  is back biassed, and the oscillator composed of inverters H 1  and H 2  plus other associated components can oscillate. This oscillation is amplified by the inverters H 4  and H 5  and applied to the induction coil COIL 2  or ultra-sonic piezo-electric transducer UT for transmission. 
     A receiver unit for operating with the remote-controlled unit of FIG. 5 is best shown in FIG. 6. A regular control signal is received by the receiver to turn off a particular burner element as long as the temperature of the utensil is higher than the required set temperature. In the absence of the control signal, the burner element is turned on. The control signal is received by the coil COIL 2  or the ultra-sonic piezo-electric transducer VR. This control signal is amplified by a CMOS inverter J 1 , where R 60  and C 21  are the coupling components and R 61  and C 22  are the feedback components. The following stages of amplification are of the type commonly termed Sallen-Key filter amplifiers. Three stages are shown in the FIG. 6; however, more stages may be added for greater filtering. In the first stage resistors R 76  ,R 63  R 62  and associated capacitors C 24  and C 23  are provided. The gain, bandwidth and the center frequency of the receiver can be set by selecting the values of these components. Sirnilarly, resistors R 64 , R 65 , R 66 , R 67 , R 68 , R 69  and associated capacitors C 24 , C 23 , C 26 , C 25 , C 28  and C 27  are used as the components for the other two stages. The output voltage of the inverter J 4  is applied to rectifying diodes D 14  and D 15  by a capacitor C 29 . The resultant DC voltage is smoothed by a capacitor C 30 , and a resistor R 70  is used as a bleeder resistor. The rectified DC voltage is applied to the base of the transistor Q 5 , the emitter of which is connected to the capacitor C 31  and the resistor R 71 , and the time constant of which is chosen so that if the turn off pulse does not arrive in time the capacitor is discharged and the output is a low voltage. If the pulse are received at regular time the voltage at the input terminal IV of the amplifier K is low. R 74 , R 73  and R 72  are positive feedback resistors. The output of amplifier K is coupled to the base of transistor Q 6  ,and when the output of the amplifier K is low, the transistor Q 6  is turned off. In such condition, the relay RL 1  is de-energized, and the burner elements are turned off. When a signal is not received in time, the output of the amplifier K is high which energizes the relay RL 1  so that the burner elements are turned on. D 11  is the diode across the relay RL 1  for breakdown protection. As described previously, the transformer provides the rectified low voltage which is smoothed by the capacitor C 33 . A regulator REG provides the regulated voltage for the receiver unit. A capacitor C 32  is connected across the regulated voltage for stability. The transformer and the regulator may be selectively shared by all the controls or may be used individually. 
     The transmitter box  20  of the remote-controlled device ofthe present invention is best shown in FIGS. 7 and 8. The transmitter box  20  has a rectangular enclosure  21 . An adjustable knob  22  located at the top of the unit is mounted to the adjustable shaft of the temperature setting potentiometer in the unit. The knob  22  is operative rotatably to set the required temperature of the cooking utensil shown on a scale  23 . The operation indicator of the device is provided by a light emitting diode  24 . The enclosure has an extension base  25 , and the top of the enclosure  21  has an upstanding ridge  26  therein adjacent to the extension base  25 . A substantially T-shaped clamping member  27  is provided. The clamping member  27  has an upstanding arm  28 , and the base portion perpendicular to the clamping member  27  which consists of a longer side arm  29  and a shorter side arm  30 . The shorter side arm  30  has an L-shaped side edge  31 . The clamping member  27  is coupled to the transmitter box  20  with the L-shaped side edge  31  engaging with the upstanding ridge  26  at the top of the enclosure  21  and the longer arm  29  extending downwards in a sloping manner down to the extension base  25 . The clamping member  27  is pivotally retained in place by a torsion spring  30  such that the extension base  25  and the longer arm  29  form a clamp which is operative by pulling the upstanding arm  28  against the spring force of the torsion spring  30  for mounting the transmitter unit  20  on the handle of a cooking utensil as best shown in FIG.  9 . The thermocouple  32  provided with a flexible tube extending outwards from the transmitter box  20  may be positioned with its thermal junction free end  33  contacting the side of the cooking utensil to sense its temperature. The extension base  25  and the longer arm  29  of the clamping member  27  may be curved in shaped and may include a flexible moderately high temperature material surface liner so as to provide a better grip of the handle of the utensil. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention is defined only by the claims that follow: