Abstract:
An apparatus and method for measuring the temperature in an oven includes a circuit and software algorithm that reads the voltage across a standard resistive temperature device (RTD) or thermistor to determine temperature measured by the device. Using an unregulated high voltage supply to increase the gain and resolution, it overcomes the problems of small changes in resistance with respect to temperature. An additional input to measure the unregulated supply voltage is used as a reference voltage input. The apparatus includes resistor dividers for both the temperature sensor and reference voltages, a microprocessor having analog inputs, and additional components for noise suppression and open sensor protection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application Serial No. 60/390,511 filed Jun. 21, 2002, having the same title and inventors as identified herein, which is incorporated by reference herein in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to temperature measurement in appliances. Specifically, the invention involves an apparatus and method for high precision temperature measurement.  
         BACKGROUND OF THE INVENTION  
         [0003]    A conventional household oven allows a user to set a temperature for baking or cooking food. The oven heats an oven chamber to the desired temperature and attempts to maintain that temperature in the oven chamber for the duration of the cooking period. To heat the oven and maintain the oven temperature, the conventional household oven includes heating elements, a temperature sensor, and a controller. For the oven&#39;s basic operation, the heating elements are supplied with power to heat the oven chamber. The temperature sensor senses the temperature within the oven chamber and supplies a temperature measurement signal to the controller indicative of the temperature. Based on the temperature measurement signal, the controller compares the measured signal with the desired temperature/setpoint and sends a control signal to a heater drive. The heater drive is operatively connected to the heating elements, and is capable of varying the power to the heating elements to maintain the desired temperature setpoint within the oven chamber.  
           [0004]    Typically temperature measurement using an RTD is done utilizing a regulated voltage supply along with amplifiers, and comparators that increase the gain of the voltage measured across the RTD. These measurements are usually performed using the low regulated power sources as the voltage supply. Regulation of the voltage as well as amplification of the circuit significantly increases the amount of materials required for the temperature measurement, the cost of the component, and the space required for the measurement device. Moreover, to compensate for inaccuracy based on the circuits&#39; calibration values, the offset determined at calibration is typically added to the measured temperature value during operation, which is less accurate than desired.  
           [0005]    The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention relates to an apparatus for measuring the temperature in an appliance. The apparatus comprise a temperature transducer, the temperature transducer comprising a variable resistance that changes in response to the temperature. First and second resistors are coupled in series between a voltage supply and ground to form a first voltage divider. The junction of the first voltage divider is then coupled to an input of a microprocessor so as to provide the microprocessor with a signal indicative of the voltage across the first resistor. A third resistor coupled in series with the temperature transducer between the voltage supply and ground to form a second voltage divider. The junction of this voltage divider is coupled to another input of the microprocessor so as to provide a signal indicative of the voltage across the temperature transducer. The microprocessor then determines a temperature using the voltage across the temperature transducer and the second resistor to determine the resistance of the temperature transducer.  
           [0007]    In another aspect, the apparatus may be constructed so that first and second resistors each comprise one or more individual resistors interconnected by one or more jumpers to provide suitable resistance values corresponding to the supply voltage. In still another aspect, the jumpers may also provides a signal to the microprocessor indicative of the supply voltage or resistance values selected. Alternatively, some other variable signal indicative of the supply voltage may be connected to the microprocessor.  
           [0008]    In one aspect of the present invention, the microprocessor determines the temperature using a look-up table correlating the resistance of the temperature transducer to the temperature. In still another aspect, the temperature determined by the microprocessor is corrected by an offset value determined during a calibration routine and stored in memory of the microprocessor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.  
         [0010]    [0010]FIG. 1 is a block diagram of a typical household electric oven;  
         [0011]    [0011]FIG. 2 is a circuit diagram illustrating an embodiment of the present invention.  
         [0012]    [0012]FIG. 3 is a circuit diagram illustrating an alternative embodiment of the present invention. 
     
    
       [0013]    While the invention is susceptible to various modifications and alternative forms, certain specific embodiments thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular forms described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0014]    Although the following description is in terms of a control system for an oven, it will be understood by those skilled in the art that it is applicable to all types of appliances including all types of ovens, refrigerators, freezers, washers, dryers and dishwashers.  
         [0015]    [0015]FIG. 1 is a block diagram of a household electric oven  10  according to one embodiment of the present invention. The oven comprises an oven chamber  42  having at least one heating element  41  and at least one temperature sensor  40 . The oven  10  also has a user interface  18  that allows the user to control the operation of the oven  10 . The user interface  18  is a typical interface on the front of a typical household oven. The interface  18  comprises a keypad with keys and/or dials that turn the oven on and off. Additionally, the keys and/or dials present on the user interface  18  instruct the oven to operate at particular temperature set point and operational mode. For example, the user selects the appropriate set point temperature for the oven chamber  42 , such as 350° F., and selects the operating mode, such as bake mode and self-cleaning mode with the user interface  18 .  
         [0016]    The user interface  18  generates signals indicating pressed keys and/or dial positions. These signals are transmitted from the user interface  18  to a control unit  20  through an analog-to-digital converter  22 . The analog-to-digital converter  22  receives the analog signals from the user interface  18  and transforms them into digital signals that are readable by the control unit  20 . Although shown as separate elements, the analog-to-digital conversion can be done internally at the control unit  20  if it is the type of microcomputer or microprocessor equipped for such a purpose.  
         [0017]    The control unit  20  receives and processes the signals from the user interface  18  through the analog-to-digital converter  22 . The processing results in a series of control signals being sent from the control unit  20  to other elements of the oven to operate the oven at the desired oven temperature and in the desired oven mode. The control unit  20  sends control signals to a heater drive  24  that transmits power from a power source  26  to the heater elements  41 . The control unit  20  may also send control signals to other elements of the oven, such as a fan, depending on the oven mode.  
         [0018]    The control unit  20  also receives signals representing information stored in a memory  28 . The memory  28  transmits its stored information signals over a data bus that is coupled to the control unit  20 . In an alternative embodiment, the control unit  20  includes nonvolatile memory. The memory  28  stores information representing various heat settings in the oven&#39;s modes of operation. The control unit  20  requests the information stored in memory  28  based on the signal inputs received from the user interface  18 . For example, if the user has selected the self-cleaning mode with the user interface  18 , the control unit  20  obtains information from the memory  28  relating to the self-cleaning mode.  
         [0019]    The control unit  20  also receives a signal representing an oven cavity temperature from the temperature sensor  40 . The temperature sensor  40  is a standard resistive temperature device (RTD) sensor or any other temperature sensor known to those skilled in the art. The temperature signal is transmitted from the temperature sensor  40  to the control unit  20 . In an alternative embodiment, the temperature signal from the temperature sensor  40  passes through an analog-to-digital converter (not shown). The analog-to-digital converter transforms the analog signals into digital signals for reading by the control unit  20  if the control unit  20  is only equipped to read digital signals.  
         [0020]    The present invention relates to a circuit and algorithm for measuring the temperature of the oven chamber using the standard RTD temperature sensor.  
         [0021]    [0021]FIG. 2 illustrates one embodiment of the temperature measuring circuit of the present invention. The circuit includes an unregulated power source  11 , a microprocessor  12 , resistive temperature device (RTD)  17 , resistors R 1 , R 2 , R 3 , diode  14 , RC filter circuits  15  and  16 , and ground connection  13 .  
         [0022]    The unregulated power source is a high voltage DC supply that can have nominal values of 24V, 32V, or 40V DC. Because the voltage supply is unregulated the actual voltage supply can have a variety of ranges.  
         [0023]    As is known in the art, the resistance of the RTD  17  varies in proportion to the temperature being measured by the RTD  17 . As shown in FIG. 2, the resistor R 3  and RTD  17  are series connected to form a voltage divider for the measuring the RTD  17  resistance. The voltage divider connection  21  measures the voltage across the RTD  17  and provides the value of the measurement to input  18  of microprocessor  12 . The RC filter circuitry  15  filters any noise. Because the voltage supply is unregulated, to compensate for supply fluctuations, resistors R 2  and R 1  are connected in series to form a second voltage divider that represents the reference voltage. Voltage divider connection  22  has RC noise filter circuitry  16  and is connected to another input of the microprocessor. A diode  14  protects the microprocessor input  18  from over-voltage when the RTD  17  is disconnected.  
         [0024]    Resistors R 1  and R 2  provide a signal below 5V DC for measuring the unregulated voltage supply. The values of resistors R 1 , R 2 , and R 3  depend on the range of the unregulated supply for a given application, which in turn depends on the voltage requirements of any other devices connected within the circuitry. The microprocessor  12  as an analog-to-digital-converter (ADC) that converts the analog voltage readings into corresponding digital values.  
         [0025]    It should be understood that any suitable values of supply voltage and component values can be used. However for illustrative purposes, the nominal values for a typical supply voltage have been used, as well as corresponding components. The microprocessor uses the voltage input  18  across the RTD  17  and the reference voltage input  19  across resistor R 1  to compute the resistance of RTD  17 . The microprocessor  12  then converts the resistance of the RTD  17  to a temperature value. Based on the measured voltage values across the RTD  17  and across the resistor R 1  an equation is developed for a value for the resistance of the RTD  17 , which eliminates the unregulated voltage supply value. As is known in the art, the resistance of the RTD  17  can be represented by equation 1 below:  
               R   RTD     =         R   1     ·     R   2     ·     V   RTD             (       R   1     +     R   2       )     ·     V   REF       -       R   1     ·     V   RTD                   Equation                 1                               
 
         [0026]    where: R 1  is the resistance of resistor R 1 ; R 2  is the resistance of resistor R 2 ; R 3  is the resistance of resistor R 3 ; V RTD  is the measured voltage across RTD  17 ; and V REF  is the measured voltage across resistor R 1 .  
         [0027]    Typical nominal values of the unregulated high voltage DC power source are 24V, 32V, or 40V DC. Because the voltage supply is unregulated the actual voltage supply can have a variety of ranges for these nominal values, as indicated in table 1 below. Moreover, based on the nominal voltage values of the power source, resistor values for the resistors R 1 , R 2 , R 3  have been chosen for illustrative purposes to maximize the analog input values and improve the resolution. These resistive values are also indicated in Table 1 below.  
                               TABLE 1                           Actual                   Nominal   Voltage       Voltage   Range   R1   R2   R3                   24 V   16-28 V   41.2 kΩ ± 1%   200 kΩ ± 1%   14.0 kΩ ± 1%       32 V   22-38 V   41.2 kΩ ± 1%   287 kΩ ± 1%   20.0 kΩ ± 1%       40 V   28-48 V   41.2 kΩ ± 1%   347 kΩ ± 1%   26.1 kΩ ± 1%                  
 
         [0028]    As shown in Table 1, based on these nominal voltage values  11 , the resistance of R 1  is a constant value of 41.2 kΩ. Because the typical nominal voltage can vary based on the appliance manufacturer&#39;s standards, in one embodiment of this invention, the circuit design includes three resistors for R 2  having the computed resistive values for each nominal voltage value, and three resistors for R 3  having the computed resistive values for each nominal value. In this embodiment, a jumper is installed on the printed circuit board of the device and is used to indicate the manufacturer&#39;s nominal voltage supply. The manufacturer places the jumper across the correct pins of the printed circuit board to indicate the corresponding nominal voltage. Also in this embodiment, placement of the jumper also sends an input to the microprocessor  12 , triggering the corresponding resistors R 1 , R 2 , R 3  value data stored within the memory of the microprocessor  12 . In a tive embodiment, rather than having a hardwired input signal sent to the rocessor  12 , based on the jumper position, the microprocessor  12  has a dial with le positions, indicative of the nominal voltage values. The dial is adjustable to the manufacture to select the desired nominal voltage value. Because the costs of itional resistors and jumper components is minuscule, this design allows for lower cturing cost, by enabling the manufacture to produce one device that is end use urable based on the end users requirements.  
         [0029]    In one embodiment a data look-up table of degree Fahrenheit values an ponding resistor values, shown in Table 2, is stored in the microprocessor&#39;s I read-only memory (ROM). In this embodiment, based on Equation. 1, the microprocessor  12  tes the resistance of the RTD, and then using the stored ROM values indicated in 2, and interpolation, the microprocessor  12  calculates the temperature measured by D  17 .  
                                         TABLE 2                                   ° F.   Ω                                        0   932.060           10   953.340           20   974.572           30   995.766           40   1016.922           50   1038.042           60   1050.124           70   1080.169           80   1101.177           90   1122.148           100   1143.081           110   1163.978           120   1184.837           130   1205.659           140   1226.445           150   1247.192           160   1267.903           170   1288.577           180   1309.213           190   1329.812           200   1350.374           210   1370.899           220   1391.387           230   1411.838           240   1432.251           250   1452.628           260   1472.967           270   1493.269           280   1513.534           290   1533.762           300   1553.952           310   1574.106           320   1594.222           330   1614.301           340   1634.348           350   1654.343           360   1674.316           370   1694.246           380   1714.140           390   1733.996           400   1753.815           410   1773.597           420   1793.341           430   1813.049           440   1832.720           450   1852.353           460   1871.949           470   1891.508           480   1911.030           490   1930.514           500   1949.962           510   1969.372           520   1988.746           530   2008.082           540   2027.381           550   2046.642           560   2065.867           570   2085.054           580   2104.205           590   2123.318           600   2142.392           610   2161.433           620   2180.435           630   2199.399           640   2218.326           650   2237.217           660   2256.070           670   2274.886           680   2293.665           690   2312.406           700   2331.111           710   2349.778           720   2368.408           730   2387.001           740   2405.557           750   2424.076           760   2442.557           770   2461.002           780   2479.409           790   2497.779           800   2516.112           810   2534.408           820   2552.666           830   2570.888           840   2589.072           850   2607.219           860   2625.330           870   2643.402           880   2661.438           890   2679.437           900   2697.398           910   2715.322           920   2733.210           930   2751.059           940   2768.872           950   2786.648           960   2804.386           970   2822.088           980   2839.752           990   2857.379                      
 
         [0030]    It is known to those skilled in the art that deviations in the circuit&#39;s components compromise the accuracy of the RTD  17  temperature measurement and creates an offset in measured value. To compensate for this offset, calibration of the circuit is required. Calibration is performed by replacing the RTD  17  with a known resistance, representative of an ideal temperature. The microprocessor  12  is put in calibration mode and prompts the programmer to input the known resistance value. Based on the known resistance value, the microprocessor  12  chooses a temperature value corresponding to the known resistance, referred to as an ideal temperature. The microprocessor  12  then using Eq. 1 calculates the actual measured resistance and the corresponding temperature value. The microprocessor  12  then subtracts the measured temperature from the ideal temperature; the resulting value is the circuit&#39;s offset. This offset is stored in the ROM of the microprocessor  12 .  
         [0031]    In one embodiment of this invention, during normal operation of the circuit, the offset value is added to the measured temperature value, to provide a more accurate representation of the actual measured temperature. In a further aspect of this embodiment, for increased accuracy, the offset value is multiplied by the resistance of the ideal temperature used for calibration purposes and this value is then divided by the actual measured temperature, resulting in a percentage offset value. In this embodiment, rather than add the entire offset amount to the measured temperature value, the percentage offset value is. added to the measured temperature value, providing a more accurate representation of the actual measured temperature.  
         [0032]    In a further embodiment of this invention, efficiency in calculating the measured temperature and simplification of the software is achieved by manipulating Equation 1 to include a constant K and developing a value termed ‘internal value’, which can be used to determine the measured temperature. Equation 1 can be manipulated to include the constant K resulting in Equation 2:  
                 (       R   1     ·     R   3       )       K   ·     R   RTD         =           (       R   1     +     R   2       )     ·     V   REF       -       R   1     ·     V   RTD           K   ·     V   RTD                 Equation                 2                               
 
         [0033]    Equation 2 above represents the ‘internal value’ as well as Equation 7 below. The ‘internal value’ is inversely proportional to sensor resistance. In Equation 2, the voltage units cancel each other out. This allows the raw 10-bit analog input values to be used directly for V REF  and V RTD  without actually converting them to volts.  
             InternalValue   =             K   R     ·     V   REF       -       K   S     ·     V   RTD           V   RTD       =         R   1     ·     R   3         K   ·     R   RTD                   Equation                 7                               
 
         [0034]    where: K R =(R 1 +R 2 )/K; K S =R 1 /K; and K=(R 1 ·R 3 )/(InternalValue·R RTD ).  
         [0035]    Equation 3 represents the value of the constant K. Because the value of K depends upon the values of R 1  and R 3 , a value for K has to be determined for each of the unregulated nominal voltage supply values. Because the internal value is inversely proportional to the resistance of the RTD, and the resistance of the RTD increases as the temperature increases, the value of 12288 decimal or  3000  hex is selected as the maximum to indicate a temperature of 0 deg F., which has an ideal resistance value of 963.63 Ω, based on Table 2. In binary form, this 3000 hex maximum value is much lower than the maximum 16-bit value. Based on this, the maximum ‘internal value’ can be determined using Equation 3.  
             K   =       (       R   1     ·     R   3       )       InternalValue   ·     R   RTD                 Equation                 3                               
 
         [0036]    Using these values, a value for K at each nominal voltage value can be determined as shown in Table 3 below. Also values for K R  and K S  can be computed as shown above. Therefore using these equations, values for K, K R  and K S  are computed and shown in Table 3 below.  
                                   TABLE 3                                   Nominal Voltage Supply   K   K R     K S                             24 V   50.36119   4789   818           32 V   74.94456   4562   573           40 V   93.88765   4422   439                      
 
         [0037]    The values of K, K R  and K S  shown in Table 3 are programmed into the microprocessor&#39;s  12  ROM. Based on the circuit&#39;s nominal voltage supply  11 , determinable by a jumper connection and/or a dial setting on the microprocessor  12 , the microprocessor selects the correct values to calculate the RTD  17  temperature measurement.  
         [0038]    on the ‘internal values’ for various resistance and equivalent temperature ok-up table, illustrated in Table 4, is generated and stored in the r&#39;s  12  ROM. Based on a 256 value decimal decrement being subtracted the maximum ‘internal value’ 12288, and each iteration thereafter, Table 4 nce points termed i, that range consecutively from 0 to 33. Each i value o a specific ‘internal value’ and corresponding temperature value in degrees s illustrated in Table 4.  
                                                           TABLE 4                           ROM Table for Degrees Fahrenheit Conversion                i       Internal Value   Table                            0   3840   0F00 hex   1056           1   4096   1000 hex   955           2   4352   1100 hex   864           3   4608   1200 hex   783           4   4864   1300 hex   713           5   5120   1400 hex   650           6   5376   1500 hex   594           7   5632   1600 hex   543           8   5888   1700 hex   498           9   6144   1800 hex   456           10   6400   1900 hex   418           11   6656   1A00 hex   383           12   6912   1B00 hex   351           13   7168   1C00 hex   322           14   7424   1D00 hex   294           15   7680   1E00 hex   269           16   7936   1F00 hex   245           17   8192   2000 hex   223           18   8448   2100 hex   203           19   8704   2200 hex   183           20   8960   2300 hex   165           21   9216   2400 hex   148           22   9472   2500 hex   132           23   9728   2600 hex   116           24   9984   2700 hex   102           25   10240   2800 hex   88           26   10496   2900 hex   75           27   10752   2A00 hex   63           28   11008   2B00 hex   51           29   11264   2C00 hex   40           30   11520   2D00 hex   29           31   11776   2E00 hex   19           32   12032   2F00 hex   9           33   12288   3000 hex   0                      
 
         [0039]    An example of operation of the circuit is illustrated below, using the following values and referring to FIG. 2. As illustrated by this example, the RTD  17  actual temperature is 350° F.  
         [0040]    Nominal Voltage Supply=32 V  
         [0041]    Actual Voltage Supply=31 V  
         [0042]    R RTD =1654.3  
         [0043]    V REF =3.852 V=788 after ADC  
         [0044]    V RTF =2.379 V=487 after ADC  
         [0045]    Using these values as well as the values for K R  and K S  shown in Table 3, the internal value is calculated using Equation 7 to be  6809  (rounded).  
         [0046]    As shown in Table 4, the smallest ‘internal value’ is  3840 , which represents the largest temperature value of 1056, indicated as Table 1 . Hence calculation of i is computed as follows: i=(InternalValue=8340)/256 (truncated)=11. Once i has been calculated, the Degree Measurement (Deg.Meas.) is computed using the equation below, and referring back to Table 4. The microprocessor. 12 interpolates to determine the Deg. Meas. value.  
         Deg   .   Meas   .     =         Table   i     -       (       Table   i     -     Table     i   +   1         )     ·   InternalValue     -   3840   -     256   ·   i       256                           
 
         [0047]    Using the above equation and the example above, the Deg.Meas. value is 364° F., whereas the actual temperature of the RTD  17  is 350° F. As previously mentioned, a further embodiment of this invention includes adjustment of the temperature for the offset determined by the initial calibration. In this embodiment of the invention, for increased accuracy, the calibration adjustment to the temperature measurement is proportional to the actual resistance measured. To further increase the accuracy, the calibration is performed at a resistance that corresponds to a relatively high temperature, so that the adjustment can be proportionally reduced for lower temperatures. The internal value is inversely proportional to the resistance, therefore the internal value is in the denominator of the calibration adjustment equation.  
         [0048]    An example of the calibration, to determine to offset is illustrated below. In this example, the RTD  17  is replaced with a resistance that has a value of 2199.4 Ω that represents an ideal temperature of 630° F. During calibration the following is an example of the actual supply voltage and measured RTD  17  and reference voltages used:  
         [0049]    Nominal Voltage Supply=32 V  
         [0050]    Actual Voltage Supply=34V  
         [0051]    R RTD =2199.4  
         [0052]    V REF =4.224 V=864 after ADC  
         [0053]    V RTF =3.384 V=692 after ADC  
         [0054]    The internal value, i, and Deg. Meas. are calculated using the same method used for calculating the example actual operational measurement. Thus the following values are computed by the microprocessor  12 :  
         [0055]    Calibrated InternalValue=5123  
         [0056]    Calibrated i=4  
         [0057]    Calibrated Deg.Meas=649° F.  
         [0058]    As indicated by the calibration, the offset of the circuit is −19° F., determined by subtracting the Cal. Deg. Meas. from the actual value (equivalent temperature value based on the resistance of the calibrating resistor).  
         [0059]    Using the offset value of −19° F., the Compensated Deg. Meas. is more accurately determined using the Deg. Meas. of 364° F. computed earlier and the offset. Using the offset calculation, which compensates for the error determined at calibration, the Compensated Deg. Meas. of 349° F. is much closer to the Actual Deg. Meas. of 350° F.  
         [0060]    Although the embodiments have discussed the use of only one RTD  17 , in a further embodiment of the present invention, multiple RTDs  17 ,  17 ′ are used along with multiple series resistors R 3 , R 3 ′, as illustrated in FIG. 3  
         [0061]    The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants or defined in the appended claims. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. It is intended that the inventive concepts defined by the appended claims include all modifications and alterations to the full extent that such modifications or alterations come within the scope of the appended claims or the equivalents thereof.