Abstract:
A thermostat ( 10 ) for sensing the ambient temperature in an environment includes an enclosure ( 100 ) housing a temperature sensor ( 20 ) for sensing the temperature within the enclosure, a temperature display device ( 30 ), a liquid crystal display (LCD) backlight ( 40 ) operative in its on state to illuminate the temperature display. A controller ( 50 ) operatively associated with the thermostat monitors the current on/off state of the backlight and the length of time the backlight has been in its current state, estimates a temperature correction factor using a simplified Discrete Kalman Filter estimator, and applies the temperature correction factor to correct the sensed temperature for the heat generated by the backlight ( 40 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 60/647,095, filed Jan. 26, 2005, and entitled DYNAMIC CORRECTION OF SENSED TEMPERATURE, which application is incorporated herein by this reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to the correction of the temperature displayed by a device and, more particularly, to the correction of the displayed temperature due to the effects of heat generated by a liquid crystal display (LCD) backlighting the display.  
         [0003]     In certain applications, including in commercial heating, ventilating and air conditioning (HVAC) systems, temperature display devices, such as for example thermostats, are employed that display the sensed temperature on a display screen. Often, the display screens are backlighted with an LCD to improve illumination. Typically, the temperature sensor is housed in the same enclosure with the display screen and the LCD backlight. The heat generated by the LCD backlighting effects the temperature within the enclosure, resulting in the temperature sensor transmitting a temperature to be displayed that is not the true temperature of the environment being monitored and serviced by the HVAC system. In HVAC applications, the displayed temperature on an LCD backlighted temperature sensing device may incorrectly reflect the true sensed temperature of the room in which the device is disposed by as much as 5 degrees F.  
         [0004]     Further, the temperature signal from the temperature sensor is commonly transmitted not only to the display screen, but also to the HVAC system controller. Therefore, if the temperature signal received by the HVAC controller does not represent the true temperature of the environment with which the HVAC system is associated, but rather represents an incorrect temperature due to the effects of heat from the LCD backlighting, the HVAC system will overcompensate or undercompensate in response to the received temperature signal. In either case, system efficiency is decreased and the comfort of individuals within the environment associated with the HVAC system is less than optimal.  
       SUMMARY OF THE INVENTION  
       [0005]     In one aspect of the invention, a method is provided for correcting a temperature signal from a temperature sensor for sensing an ambient temperature in an environment wherein the sensor may be affected by a local heating/cooling source. The method includes determining whether the local heating/cooling source is in an on or an off state, monitoring the length of time, either continuously or at selected time intervals, the local heating/cooling source has been in its current on/off state, estimating a temperature correction factor based upon the current on/off state of the local heating/cooling source and the length of time that the heating/cooling source has been in its current on/off state, and applying the temperature correction factor to the sensed temperature to correct the sensed temperature for any effect from the local heating/cooling source whereby the corrected temperature more accurately reflects the ambient temperature of the environment. Advantageously, the temperature correction factor is estimated using a simplified Discrete Kalman Filter analysis and the temperature correction factor is dynamically applied to the sensed temperature.  
         [0006]     In another aspect of the invention, a method is provided for correcting a sensed temperature for display on a temperature display device, the device having a liquid crystal backlight. The sensed temperature is received from a temperature sensor that may be affected by heat generated from the liquid crystal backlight. The method includes determining whether the backlight is in an on state or an off state, monitoring the length of time, either continuously or at selected time intervals, the backlight has been in its current on/off state, estimating a temperature correction factor based upon the current on/off state of the backlight and the length of time the backlight has been in its current on/off state, and applying that temperature correction factor to the sensed temperature prior to displaying the temperature. The method provides a dynamic estimation of a temperature correction factor and summing the estimated temperature correction factor with the currently sensed temperature to generate a corrected display temperature that is indicative of the true temperature in the environment uncorrupted by the effects of heat from the backlight. Advantageously, the temperature correction factor is estimated using a simplified Discrete Kalman Filter analysis and the temperature correction factor is dynamically applied to the sensed temperature.  
         [0007]     In a still further aspect of the invention, a thermostat is provided for sensing the ambient temperature in an environment. The thermostat has an enclosure housing a temperature sensor, a temperature display, and a liquid crystal backlight. The liquid crystal backlight is operative in its on state to illuminate the temperature display. When in its on state, the backlight emits heat into the enclosure. The temperature sensor is operative to generate a sensed temperature signal indicative of the temperature within the enclosure. The controller receives the sensed temperature from the temperature sensor, monitors the current on/off state of the backlight and the length of time of the backlight in its current on/off state, and dynamically estimates a temperature correction factor using a simplified Discrete Kalman Filter estimator. The controller applies the temperature correction factor to the sensed temperature to adjust the sensed temperature signal and generate a corrected temperature indicative of the true temperature of the environment without corruption from the heat of the backlight.  
         [0008]     The temperature correction factor may be estimated using a simplified Discrete Kalman Filter analysis may in accord with the relationship: 
 
Δ T   BL ( t )=−0.00075(Δ T   BL ( t ))+0.001425( u   BL ( t )); 
 
 where: 
 
         [0009]     ΔT BL (t) is a time incremental function representing the temperature change due to the backlight effect; and  
         [0010]     u BL (t) is a function of the backlight status, equal to 1 if backlight  40  is on and equal to 0 if backlight  40  is off; and the sensed temperature corrected in accord with the relationship: 
 
 T ( t )= T   raw ( t )−Δ T   BL ( t ) 
 
 where: 
 
         [0011]     T(t) is the corrected temperature, degrees F;  
         [0012]     T raw (t) is the sensed temperature, degrees F.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a diagram illustrating a thermostat employing a basic aspect of the invention; and  
         [0014]      FIG. 2  is a chart showing representative temperature to time traces for uncorrected sensed temperature and true temperature. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The invention will be described herein with reference to a temperature sensing and display device, commonly known as a thermostat, as applied to a heating, ventilating and air conditioning system. It is to be understood, however, that the basic concept of the present invention may be applied for the correction of any sensed temperature that has been corrupted by the presence of a local heat source or cooling source.  
         [0016]     Referring now to  FIG. 1 , the thermostat  10  includes a temperature sensor  20 , a temperature display device  30 , a liquid crystal display (LCD) backlight  40  and a controller  50 , all housed in a common enclosure  100 . The temperature display device  30  functions in a conventional manner to display a temperature corresponding to a temperature signal  55  received from the controller  50 . The LCD backlight  40  is provided in operative association with the temperature display device  30  to backlight the display device  30 , thereby improving the illumination of the display device.  
         [0017]     The function of the temperature sensor  20  is to sense the temperature of the local environment external of the enclosure  100 , that is the environment associated with and controlled by the HVAC system (not shown). The particular type of temperature sensor employed is not relevant to the invention. In a conventional manner, the temperature sensor  20  generates a temperature signal  25  indicative of the sensed temperature and transmits that temperature signal to the controller  50 .  
         [0018]     In operation, the LCD backlight  30  may be on for varying periods of time and off for varying periods of time. When the LCD backlight  40  is on, heat generated in the light production process is emitted from the LCD backlight. As a result of the heat being emitted into the common enclosure  100 , the temperature sensed by the temperature sensor  20  is corrupted as it does not accurately reflect the true temperature of the environment exterior to the enclosure  100 . However, when the LCD backlight  40  is turned off for a period of time sufficient for the temperature interior of the enclosure  100  to come to an equilibrium with the temperature exterior to the enclosure  100 , the sensed temperature does indeed reflect the true temperature of environment exterior of the enclosure  100 .  
         [0019]     To compensate for the rise in sensed temperature caused by the heat emitted into the enclosure  100  by the LCD backlight  40 , the controller  50  estimates a correction factor based on a simplified Discrete Kalman Filter analysis and dynamically corrects the temperature signal  25  received from the temperature sensor  20  accordingly. After applying the generated correction factor to the sensed temperature derived from the temperature signal  25 , the controller  50  generates the temperature signal  55  and transmits the temperature signal  55  indicative of the corrected temperature to both the temperature display device  30  and an HVAC system controller (not shown).  
         [0020]     In operation, the controller  50  monitors the on/off status of the LCD backlight  40 , and, either continuously or at selected time intervals, as desired, receives a backlight on/off status signal  45 . The controller also receives the temperature signal  25  from the temperature sensor  20 , again either continuously or at selected time intervals coordinated with the backlight on/off status signal  45 . With both the sensed temperature signal  25  and the backlight on/off status signal  45  present, the controller  50  estimates a correction factor that is summed with the sensed temperature signal to generate the temperature signal  55  that represents the corrected temperature without corruption from heat from the backlight  40 .  
         [0021]     In another aspect of the invention, the controller  20  dynamically calculates the correction factor using a simplified Discrete Kalman Filter analysis in accord with the following formulas: 
 
 T ( t )= T   raw ( t )−Δ T   BL ( t ); and 
 
Δ T   BL ( t )=−0.00075(Δ T   BL ( t ))+0.001425( u   BL ( t )); 
 
 where: 
 
         [0022]     T(t) is the corrected temperature, degrees F  
         [0023]     T raw (t) is the sensed temperature, degrees F  
         [0024]     ΔT BL (t) is a time incremental function, also referred to as the correction factor, representing the temperature change due to the backlight effect; and  
         [0025]     u BL (t) is a function of the backlight status, =1 if backlight  40  is on 
        =0 if backlight  40  is off.        
 
         [0027]     The value of the time incremental function ΔT BL (t) depends on the on/off status of the backlight  40  and also upon the time that the backlight has been continuously on or off.  
         [0028]     The time incremental function ΔT BL (t) is used by the controller  50  to estimate the temperature correction factor to be summed with the sensed temperature  25  to generate the corrected temperature signal  55 .  
         [0029]     To develop the function ΔT BL (t) for a particular device, such as thermostat  10 , the device is operated without any temperature correction being applied to the sensed temperature signal  25  and with the backlight  40  being cycled through off/on/off/on operation to provide a data trace  75  of sensed temperature over temperature. This data trace  75  is then compared to a similar data trace  85  obtained from an identical sensor except without any backlighting on, and therefore unaffected by the heat from the backlight  40  and representative of the true temperature. For illustration purposes, a comparison of representative data traces  75  and  85  is shown in  FIG. 2 .  
         [0030]     To facilitate a Discrete Kalman Filter analysis, the following model was selected to represent the displayed temperature behavior as exemplified by the data trace  75  shown in  FIG. 2 . 
 
 T   true ( t )= T   nominal (t)+ε 1 ( t );   Eq. 1 
 
Δ T   BL ( t )=−0.00075(Δ T   BL ( t ))+0.001425( u   BL ( t ))+ε 2 ( t );   Eq. 2 
 
 T   disp ( t )= T   true ( t )+Δ T   BL ( t )+ε 3 ( t );   Eq. 3 
 
 where: 
 
         [0031]     T true (t) is the true temperature, degrees F  
         [0032]     ε 1 (t) is the normally distributed temperature noise associated with sensor temperature fluctuations;  
         [0033]     ΔT BL (t) is a time incremental function representing the temperature change due to the backlight effect;  
         [0034]     u BL (t) is a function of the backlight status, =1 if backlight  40  is on 
        =0 if backlight  40  is off.        
 
         [0036]     ε 2 (t) is the normally distributed temperature noise associated with backlight temperature fluctuations;  
         [0037]     T disp (t) is the displayed temperature, degrees F  
         [0038]     ε 3 (t) is the normally distributed display temperature noise associated with backlight temperature fluctuations and other effects.  
         [0039]     Equation 1 represents the true temperature of the environment as a constant nominal value plus noise. For purposes of this analysis, a nominal value of 72 degrees F. with a 5 degree F. variation symmetric about the nominal value was assumed to accommodate day and night temperature variations for a typical thermostat application. Further, assuming the variation to be normally distributed enables ε 1 (t) to be defined as a random process with zero mean, a standard deviation of σ=5, and a variance computed as σ 2   ε1 =25.  
         [0040]     Equation 2 represents the backlight effect on display temperature behavior. The two numerical coefficients were calculated from the data traces shown in  FIG. 2 . The noise signal, ε 2 (t), was also estimated from the data by first selecting a time range where the backlighting was on, calculating the mean value, subtracting the mean to produce a zero mean random process, and finally calculating the variance of the random process numerically as σ 2   ε2 =0.1443.  
         [0041]     Equation 3 defines the displayed temperature as the sum of the true temperatures plus the backlight temperature correction plus a noise signal. The noise signal, ε 3 (t), was estimated from the data by first selecting a time range where the backlighting was on, calculating the mean value, subtracting the mean to produce a zero mean random process, and finally calculating the variance of the random process numerically as σ 2   ε3 =0.1443.  
         [0042]     With this information, a Discrete Kalman Filter analysis was applied to the system equations 1 through 3. The systems equations were discretized using a bilinear transform using an update time of 1 second, i.e. ΔT=1. 
 
 T   k+1   =T   k +ε 1k ;   Eq. 4 
 
Δ T   BL/k+1 =0.99925(Δ T   BL/k )+0.0014254( u   BL k )+ε 2k ;   Eq. 5 
 
 T   disp k   =T   k   +ΔT   BL k +ε 3k .   Eq. 6 
 
         [0043]     With the realization that the Kalman gain vector elements are approximately 1 and 0, respectfully, a simplified Discrete Kalman Filter analysis is applied executing only the state predictor and state corrector calculations. The covariance and gain calculations do not need to be made because they stabilize to constant values rapidly. The resulting difference equation for the true temperature becomes: 
 
 T   k+1/k+1   =T   k/k +0.99146( T   raw k   −T   k/k   −ΔT   BL/k ).   Eq. 7 
 
         [0044]     Applying a bilinear transform to Eq. 7, coverts it to the following differential equation: 
 
1.00854 T ( t )=−1.9829 T ( t )+1.9829  T   raw ( t )−1.9829 ΔT   BL ( t ).   Eq. 8 
 
         [0045]     Recalling that the backlighting temperature correction behavior, presented in Eq. 2, can be estimated as: 
 
Δ T   BL ( t )=−0.00075 ΔT   B ( t )+0.001425 u   BL ( t ).   Eq. 9 
 
         [0046]     Observing that the time constants in Eq. 8 and 9 are separated by over 3 orders of magnitude, it is permissible to consider Eq. 8 as having reached steady state equilibrium, while Eq. 9 is still in a dynamic range. Therefore, in steady state, Eq. 8 can be written as 
 
0=−1.9829 T ( t )+1.9829 T   raw ( t )−1.9829 ΔT   BL ( t ); which can be rewritten as: 
 
 T ( t )= T   raw ( t )−Δ T   BL ( t ).   Eq. 10 
 
         [0047]     Equations 9 and 10 form a set of the simplified filter equations that when programmed into the controller  50  permit the controller  50  to dynamically correct the raw temperature sensed by the temperature sensor  20  for the effects of backlighting whereby the temperature displayed on the temperature display  30  of the thermostat  10 , will reflect the true temperature of the environment associated with the HVAC system.  
         [0048]     While the invention has been described in connection with a thermostat in an HVAC System, it is to be understood that those skilled in the art will recognize that the invention may be applied to other temperature display devices in other applications within the spirit and scope of the present invention.