Abstract:
A thermostat ( 800 ) operates continuously in a current mode ( 304 ) that is one of a heating mode and a cooling mode, and completes ( 520, 610 ) a demand for one of heating and cooling in accordance with the current mode. After completing the demand and until another demand occurs for one of heating and cooling, the thermostat repeatedly makes ( 302 ) measurements of a sensed room temperature, and determines ( 310, 414 ), from the measurements, whether the sensed room temperature has finished a post-demand overshoot. In response to determining that the sensed room temperature has finished the post-demand overshoot, the thermostat records ( 316, 418 ) an evaluation temperature, and decides whether to make an automatic changeover from the current mode to a new mode by periodically comparing ( 318, 420 ) the sensed room temperature with the evaluation temperature.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates in general to temperature controllers for heating and cooling systems, and more specifically to a thermostat and method for adaptively providing a changeover between heat and cool.  
       BACKGROUND OF THE INVENTION  
       [0002]     Thermostats for use with a building heating and cooling system are well known. A typical prior-art thermostat provides a mode switch having at least two positions for allowing a user to changeover manually between a heating mode, in which the thermostat controls the heating system; and a cooling mode, in which the thermostat controls the cooling system. Such thermostats generally have used a single setpoint temperature. Unfortunately, these thermostats require frequent user attention to the mode switch during temperate seasons such as spring and fall, in which cooling may be desired during the day, and heating at night.  
         [0003]     In an attempt to automate the changeover between heating and cooling, manufacturers of prior-art thermostats have constructed “automatic-changeover” thermostats, which have used first and second setpoint temperatures, respectively, for heating and cooling. In such prior-art thermostats, the first and second setpoint temperatures are not independent of each other, because, in effect, both are active simultaneously. The first setpoint temperature is required to be less than the second setpoint temperature by a predetermined number of degrees, e.g., 3 degrees F., to prevent excessive cycling of the thermostat between heating and cooling due to a demand for heating causing the sensed room temperature to move into the cooling operational range, and vice versa. Unfortunately, without manual intervention, this type of prior-art thermostat forces the average room temperature when using heat to be at least 3 degrees F. cooler than the average room temperature when using cooling, which some people find uncomfortable.  
         [0004]     Thus, what is needed is an automatic changeover thermostat in which the first and second setpoint temperatures can be set independently of each other, without concern for excessive cycling between heating and cooling. Such a thermostat preferably will allow the use of a single setpoint temperature for both heating and cooling, if desired, without requiring manual user intervention to select between the heating and cooling modes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a flow diagram depicting operation of a prior-art thermostat when in a heating mode.  
         [0006]      FIG. 2  is a flow diagram depicting operation of a prior-art thermostat when in a cooling mode.  
         [0007]     FIGS.  3  to  7  are flow diagrams depicting operation of a thermostat in accordance with the present invention.  
         [0008]      FIG. 8  is an electrical block diagram of the thermostat in accordance with the present invention.  
         [0009]      FIG. 9  is a graphical depiction of the performance measured on a working model of the thermostat in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]     U.S. Pat. No. 6,681,848 issued Jan. 27, 2004 to Breeden is hereby incorporated herein by reference. Referring to  FIG. 1  of the instant disclosure, a flow chart  100  depicts operation of a prior-art thermostat when in a heating mode. The flow begins with defining  102  a user-programmed setpoint temperature Tsh to be the target temperature when in the heating mode, and a temperature tolerance Tt (preferably pre-programmed by the manufacturer of the thermostat) within which the temperature is to be maintained, centered about the setpoint temperature Tsh. For example, if a user sets Tsh at 75, and Tt is pre-programmed at 0.5, the thermostat will attempt to maintain the sensed room temperature between 74.5 and 75.5 degrees F. Next, the room temperature Tr sensed by the thermostat is measured  103  and recorded. At step  104 , a first comparison is made to determine whether Tr is less than Tsh minus Tt. If so, a demand for heat is activated  106 , and the flow then moves to step  108 . If not, step  106  is skipped, and the flow moves directly to step  108 . At step  108 , a second comparison is made to determine whether Tr is greater than Tsh plus Tt. If so, any existing demand for heat is inactivated  110 , and the flow returns to step  103 . If not, the flow returns directly to step  103 .  
         [0011]     Referring to  FIG. 2 , a flow chart  200  depicts operation of a prior-art thermostat when in a cooling mode. The flow begins with defining  202  a user-programmed setpoint temperature Tsc to be the target temperature when in the cooling mode, and a temperature tolerance Tt (preferably pre-programmed by the manufacturer of the thermostat) within which the temperature is to be maintained, centered about the setpoint temperature Tsc. For example, if a user sets Tsc at 76, and Tt is pre-programmed at 0.5, the thermostat will attempt to maintain the sensed room temperature between 75.5 and 76.5 degrees F. Next, the room temperature Tr sensed by the thermostat is measured  203  and recorded. At step  204 , a first comparison is made to determine whether Tr is greater than Tsc plus Tt. If so, a demand for cooling is activated  206 , and the flow then moves to step  208 . If not, step  206  is skipped, and the flow moves directly to step  208 . At step  208 , a second comparison is made to determine whether Tr is less than Tsc minus Tt. If so, any existing demand for cooling is inactivated  210 , and the flow returns to step  203 . If not, the flow returns directly to step  203 .  
         [0012]     Activation and inactivation of a demand for heating or cooling by a thermostat in accordance with the present invention is similar to that depicted in the flow charts  100  and  200 , respectively, when in the heating mode or in the cooling mode. What is different is the method employed by the thermostat in accordance with the present invention for deciding whether and when to switch into the heating mode or into the cooling mode.  
         [0013]     Referring to  FIG. 3 , a flow chart depicts operation of a thermostat in accordance with the present invention. The flow begins with measuring  302  the sensed room temperature Tr. Then at step  304  the mode of the thermostat is checked. The thermostat is arranged such that it operates continuously in one of the heating mode, in which the thermostat controls the heating system, and the cooling mode, in which the thermostat controls the cooling system. When the mode is heating, the flow moves to step  502  ( FIG. 5 ) to determine whether a demand for heat is active. If so, the thermostat attempts  508 , through well-known techniques, to find the minimum sensed room temperature reached during the demand. This is done because the sensed room temperature Tr is subject to substantial undershoot and overshoot in the heating mode. In the heating mode, undershoot is defined herein as a drop in the sensed room temperature when the demand for heat begins, due to cooler air being circulated around the thermostat by the heating, ventilation, and air conditioning (HVAC) system fan. Overshoot is defined herein as an increase in the sensed room temperature when the demand for heat is inactivated and the HVAC system fan stops. Overshoot is believed to be caused by poorly-mixed pockets of warm and cool air, which redistribute themselves after the fan stops, the warm air rising and the cool air falling. Whatever the causes, undershoot and overshoot are problems that need to be dealt with in an automatic changeover thermostat. Undershoot causes the heating system to operate for longer than is desirable, temporarily making the heated area warmer than desired at the completion of the demand. Overshoot, on the other hand, increases the difficulty of making an accurate decision as to whether the thermostat should switch from the heating mode to the cooling mode.  
         [0014]     In the cooling mode, overshoot is defined herein as a further drop in the sensed room temperature after the demand for cooling is inactivated. Undershoot is defined herein in the cooling mode as a further rise in the sensed room temperature after a demand for cooling begins. In the particular installation in which an embodiment of the present invention was evaluated, neither overshoot nor undershoot was large enough in the cooling mode to require any special handling.  
         [0015]     Again referring to  FIG. 5 , after step  508  the thermostat checks  512  whether it has found the minimum temperature during the demand. If not, the flow moves to step  306  ( FIG. 3 ) to check whether the demand is still active. If so, the flow returns to step  302  to again measure the sensed room temperature Tr. If, on the other hand, at step  512  the minimum Tr has been found, then the thermostat checks  516  whether Tr is greater than a heat limit. To minimize the effect of the undershoot on the run time of the heating system, the heat limit is preferably less than the temperature at which the demand was started. Empirical observation has indicated that a reasonable value for the heat limit is 0.1 degree F. below the temperature at which the demand was started. If at step  516  the sensed room temperature is not greater than the heat limit, the flow again returns to step  306 . If, on the other hand, the sensed room temperature is greater than the heat limit, the thermostat then inactivates  520  the demand, leaving the fan turned on. The fan preferably is allowed to remain in operation until a peak in Tr is detected, or until fifteen minutes have passed, whichever happens first. The reason for leaving the fan on is to better mix the air in the heated area, which will reduce the overshoot. In addition, the thermostat temporarily holds  522  the demand off. This is necessary at this point, because the sensed room temperature is below the temperature at which the demand was started, and we do not want the demand for heat to be reactivated. The thermostat also sets  524  the evaluation temperature Te to a big value, e.g., 600 degrees F., in preparation for some post-demand calculations to follow. The flow then returns to step  306 .  
         [0016]     If, on the other hand, at step  502  the demand is not active, the thermostat then checks  504  whether the demand is held off. If the demand is held off, the thermostat checks  506  whether the sensed room temperature Tr is greater than the heating setpoint temperature Tsh minus the temperature tolerance for heat Tth plus a temperature variation Tv. Tth is preferably a small value, e.g., 0.1 degree F., to further reduce the undershoot and overshoot. Tv is also preferably a small value, e.g., 0.05 degrees F., which provides sufficient margin for any temperature variations in the A/D converter of the temperature sensor. In essence, step  506  is simply checking whether the sensed room temperature is high enough to allow removing the hold-off of the demand without any risk of reactivating the demand. If so, the thermostat removes  510  the hold-off of the demand and the flow moves to step  514 . If, on the other hand, at step  506  the temperature is not high enough, the flow returns to step  306 . If, on the other hand, at step  504  the demand is not held off, the flow moves to step  514  to check whether the sensed room temperature is less than the heat setpoint temperature Tsh minus the temperature tolerance for heat Tth. If so, the thermostat activates  518  a demand for heat and turns the fan on, and the flow returns to step  306 . If not, the flow simply returns to step  306 .  
         [0017]     When at step  304  ( FIG. 3 ) the mode is cooling, the flow moves to step  602  to check whether a demand for cooling is active. If not, the thermostat checks  604  whether the sensed room temperature Tr is greater than the cooling setpoint temperature Tsc plus the temperature tolerance for cooling Ttc, e.g., 0.5 degrees F. If so, the thermostat activates  608  a demand for cooling, and the flow then returns to step  306 . If not, the flow simply returns to step  306 . If, on the other hand, at step  602  the demand is active, then the thermostat checks  606  whether Tr is less than Tsc minus Ttc. If so, the thermostat inactivates  610  the demand and turns the fan off. In addition, the evaluation temperature is set  612  to a small value, e.g., 10 degrees F. The flow then returns to step  306 . If, on the other hand, step  606  produces a negative result, the flow returns immediately to step  306 .  
         [0018]     If at step  306  the demand is not active, then at step  308  the mode is checked. If the mode is cooling then the thermostat checks whether the sensed room temperature Tr is less than the cooling setpoint temperature Tsc minus a force-switchover temperature Tfs, e.g., 1.25 degrees F. If not, the thermostat then checks  310  whether the minimum post-demand temperature been found. This would signify that the bottom of any overshoot past the lower cooling limit has been reached, and Tr is now rising. If not, the thermostat continues to attempt  312  to find the minimum post-demand temperature, through well-known techniques, and the flow returns to step  302 .  
         [0019]     If, on the other hand, step  310  produces an affirmative result, the thermostat checks  314  whether Tr is greater than Te. If so, at step  316  Te is set equal to Tr up to a maximum limit preferably defined by the setpoint temperature for cooling Tsc. It will be appreciated that, alternatively, a maximum limit higher or lower than Tsc can be substituted for Tsc, if desired. If at step  314  Tr is not greater than Te, then step  316  is skipped. In either case, flow then moves to step  318 , to check whether Tr is less than Te minus Tm, the temperature margin for mode switching. If so, the thermostat checks  320  whether Tr is also less than Tsh-Tth. In other words, the thermostat is checking whether Tr is low enough to cause a demand for heat in the heating mode. If so, the thermostat switches  322  to the heating mode and records the new mode in EEPROM. In addition, the thermostat turns the fan on  324  and demands heat. The flow then returns to step  302 . If either step  318  or step  320  produces a negative result, the flow returns immediately to step  302 . If, on the other hand, at step  326  Tr is less than Tsc minus Tfs, the flow skips immediately to step  320 . This advantageously allows a user to force a mode change from the cooling mode to the heating mode by increasing the heating and cooling setpoints by about two degrees F. above their current settings.  
         [0020]     If, on the other hand, at step  308  the mode is heating, then the flow moves to step  402  ( FIG. 4 ) to check whether the fan is on. If so, the thermostat checks  430  whether the sensed room temperature Tr is greater than the setpoint temperature for heating Tsh plus the force-switchover temperature Tfs. If not, the thermostat attempts  404  to find a peak in Tr (due to overshoot after the demand ends), through well-known techniques. The thermostat then checks  406  whether the peak has been found. If so, the thermostat turns the fan off  408 , and the flow returns to step  302  for another temperature measurement. If at step  406  the peak has not been found, the flow returns immediately to step  302 . If, on the other hand, at step  430  an affirmative result is produced, the flow goes immediately to step  408  to turn the fan off. It will be appreciated that, as a backup, a timer can be used to turn off the fan if it operates for too long, e.g., more than fifteen minutes, after the demand for heat has ended.  
         [0021]     If, on the other hand, at step  402  the fan is not on, then the thermostat checks  428  whether the sensed room temperature Tr is greater than the setpoint temperature for heating Tsh plus the force-switchover temperature Tfs. If not, the thermostat checks  410  whether a second peak (due to stopping the fan) has been found in Tr. If not, the thermostat attempts  412  to find the second peak through well-known techniques. If at step  414  the thermostat has found the peak, that fact is recorded, so that the thermostat will not continue testing for the peak, and the flow moves to step  416 . If not, the flow returns to step  302  for another temperature measurement. If, on the other hand, at step  410  the thermostat determines that the second peak has already been found, then the flow skips immediately to step  416 .  
         [0022]     At step  416  the thermostat checks whether Tr is less than the evaluation temperature Te. If so, Te is set  418  equal to Tr down to a minimum value preferably equal to the setpoint temperature for heat Tsh, and the flow moves to step  420 . It will appreciated that, alternatively, another minimum value different from Tsh can be used instead, if desired. If at step  416  Tr is not less than Te, then the flow skips immediately to step  420 , where the thermostat checks whether Tr is greater than Te plus Tm, the temperature margin for switching modes. If so, the thermostat checks  422  whether Tr is also greater than Tsc, the setpoint temperature for cooling, plus Ttc, the temperature tolerance for cooling. A negative result in either step  420  or step  422  results in the flow returning to step  302 . A positive result in both will result in the thermostat switching  424  to the cooling mode and recording the new mode in EEPROM. In addition, the thermostat will turn the fan on  426  and demand cooling, after which the flow will return to step  302 .  
         [0023]     If at step  428  a positive result is produced, the flow skips immediately to step  422 . This advantageously allows a user to force the thermostat to switch from the heating mode to the cooling mode by lowering both the heating and cooling setpoint temperatures by about two degrees F. below their current settings. Perhaps more importantly, step  428  acts as a “safety net” for forcing a switch to the cooling mode when no peak is found in step  412  and Tr has moved higher than expected, e.g., 1.25 degrees F. above the setpoint temperature. This anomaly can occur when normal daytime heating follows closely after a demand for heat. Under such conditions the overshoot following the demand can blend seamlessly with an upward trend in Tr produced by the normal daytime heating, leaving no detectable peak in the Tr sequence.  
         [0024]     As described herein above, the combination of an early inactivation of the demand for heat and judicious operation of the fan thereafter advantageously reduces the amount of overshoot occurring after the demand for heat. In one embodiment before these techniques were incorporated, the observed overshoot was about two degrees F. beyond the setpoint temperature for heat. After incorporating these techniques, the observed overshoot has been reduced to a much more desirable limit of about 0.8 degree F. above the setpoint temperature for heat.  
         [0025]     It is important to note that, while the foregoing disclosure has described separate heating and cooling setpoint temperatures, it is possible to utilize the same identical temperature value for both setpoints. In other words, the thermostat in accordance with the present invention can be manufactured as a single-setpoint thermostat, advantageously making the thermostat easier for the user to understand and operate. All the user has to do is set the desired temperature, and the thermostat will demand heating or cooling, as needed, to maintain the desired temperature.  
         [0026]     Referring to  FIG. 7 , a flow diagram depicts a startup operation of the thermostat in accordance with the present invention. The flow begins with a processor restart  702 , which can happen, for example, after power is removed from the thermostat and then restored. After the processor restart, the processor reads  704  the cooling and heating setpoint temperatures and the mode (heating or cooling) from EEPROM. The processor then checks  706  whether the mode is heating or cooling. If the mode is heating, the processor initializes  708  Te to a big value, e.g., 600 degrees F. If the mode is cooling, the processor initializes  710  Te to a small value, e.g., 10 degrees F. The flow then moves to step  302  ( FIG. 3 ) to measure the temperature.  
         [0027]     Referring to  FIG. 8 , an electrical block diagram  800  of the thermostat in accordance with the present invention comprises a temperature sensor  802 , e.g., the SHT11 sensor manufactured by Sensirion AG of Zurich, Switzerland, for sensing a room temperature, and a user interface  804 , e.g., a conventional liquid crystal display and pushbuttons for interfacing with a user. It will be appreciated that, alternatively, other similar types of sensors and displays can be utilized as well. The temperature sensor  802  and the user interface  804  are coupled to a conventional processor  806 , e.g., the BS2p processor available from Parallax, Inc. of Rocklin, Calif., for controlling the thermostat in accordance with the present invention. It will be appreciated that, alternatively, other similar processors can be utilized for the processor  806 . In addition, the processor  806  is coupled to a heating, ventilation, and air conditioning (HVAC) interface  812  for controlling the HVAC system. The HVAC interface preferably includes three conventional relays (not shown) for independently controlling the heating, cooling, and fan portions of the HVAC system. The processor  806  is also coupled to a conventional memory  808 , e.g., RAM, ROM, EEPROM, for programming the processor  806  in accordance with the present invention, and for storing operating variables and constants. It will be appreciated that the processor  806  and the memory  808  can be manufactured in combination as a module  810  for use in a thermostat in accordance with the present invention. It will be appreciated that additional conventional elements (not shown), such as a battery or an external power source can be utilized to provide operating power for the thermostat.  
         [0028]     Referring to  FIG. 9 , a graphical depiction of the performance measured on a working model of the thermostat in accordance with the present invention includes a plot  902  of the sensed room temperature versus time. Three additional plots  904 ,  906 , and  908  are included depicting, respectively, the state of the fan (on or off), the demand (active or inactive), and the mode (heating or cooling). For the duration of this test, both the heating and cooling setpoint temperatures were kept at 75.0 degrees F. The window of time depicted is a period of just under seventeen hours, during which the thermostat began in the cooling mode, switched to the heating mode overnight, and then returned to the cooling mode during the next day. Throughout the period, the thermostat smoothly maintained the sensed room temperature between 74.3 and 75.8 degrees F. Note that even with the heating and cooling setpoint temperatures set to identical values, there advantageously is no oscillation between heating and cooling. Note also that, in the heating mode, operation of the fan is extended past the end of the demand, advantageously reducing the overshoot.  
         [0029]     It should be clear from the preceding disclosure that the present invention provides an automatic changeover thermostat in which the first and second setpoint temperatures advantageously can be set independently of each other, without concern for excessive cycling between heating and cooling. Such a thermostat beneficially allows the use of a single setpoint temperature for both heating and cooling, if desired, without requiring manual user intervention to select between the heating and cooling modes.  
         [0030]     This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.