Abstract:
A warming chamber defining a heated internal cavity is presented having a door that may be opened or closed to provide access to the cavity for the storage of, for example, food products. The chamber includes a temperature sensor that sends signals to a control regarding the sensed temperature within the cavity. The control then activates heating elements when necessary to maintain the cavity at a predetermined temperature. The control further senses when the door has been opened, and supplies a power boost to the heating elements in anticipation of a cooling effect even though a temperature drop within the cavity has not yet been sensed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to warming chambers, and in particular, relates to a method and apparatus for anticipating and correcting for temperature fluctuations due to, for example, opening the door of the chamber. 
     2. Description of the Related Art 
     In order to enable restaurants and other food establishments to provide fresh food in bulk, it is necessary to precook the food items and subsequently place them in a warming chamber for preservation before being served. Conventional warming chambers are in widespread use and include an enclosed cavity having a heating element that is operable to direct heat into the chamber. The heating element receives signals from control circuitry operating in conjunction with a temperature sensor, such as a thermostat, to maintain the temperature of the cavity at a desired level. The warming chamber includes a door that is placed at the front end of the warming cavity and that may be opened and closed to provide access to the food products within the chamber. Many such warming chambers include humidity control to maintain the moisture content within the cavity in relation to the equilibrium water activity of the food product, and to further achieve the desired final characteristics of the food product. 
     Most such warming chambers contain racks within the cavity that support the food products to be heated. However, when the door is opened to either remove or insert the products into the cavity, heated air escapes into the ambient environment and is typically replaced by the cool ambient air. The actual temperature of the chamber is typically measured by a thermostat, which is located toward the back of the chamber so as to avoid being damaged by the products held on these racks or the racks themselves. As a result of the thermostat placement, there is considerable delay before the thermostat senses the heat loss within the warming chamber. The resulting delay in actuating the heating element produces a rapid cooling within the chamber, even after the door is closed, which adversely affects the quality of the food products that are stored therein. 
     What is therefore needed is a method and apparatus for anticipating and correcting for rapid heat loss in advance that would otherwise occur due to opening the door of a warming chamber. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a method for anticipating heat loss in a warming cavity having 1) a door that is disposed at an open end of the cavity and that is movable from a closed position, thereby enclosing the cavity, to an open position, thereby exposing the open end of the cavity, 2) at least one heating element operable to supply heat to the warming cavity so as to maintain the cavity substantially at a set temperature, and 3) a temperature sensor disposed within the cavity at a location remote from the open end, wherein the temperature sensor is operable to measure an actual temperature within the cavity. The method includes automatically determining that the door is open prior to the temperature sensor detecting a drop in the sensed temperature within the cavity, and activating the at least one heating element to heat the cavity towards a target temperature greater than the set temperature. 
     This and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not limitation, a preferred embodiment of the invention. Such embodiment does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is hereby made to the following figures in which like reference numerals correspond to like elements throughout, and in which: 
     FIG. 1 is a perspective view of a warming cabinet including a warming chamber constructed in accordance with the preferred embodiment; 
     FIG. 2 is a perspective view of the warming chamber illustrated in FIG. 1 having a portion cut away to reveal the heating elements; 
     FIG. 3 is a perspective view of a heating element illustrated in FIG. 3; 
     FIG. 4 is a schematic view of the anticipator circuit of the warming chamber illustrated in FIG. 1; 
     FIG. 5A is a flowchart of anticipator circuit logic performed by the microprocessor of the anticipator circuit illustrated in FIG. 4 in accordance with the preferred embodiment; and 
     FIG. 5B is a flowchart of the *** steps performed by the microprocessor of the anticipator circuit illustrated in FIG. 2 in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIGS. 1 and 2, a warming cabinet  10  includes a generally rectangular housing that encases a warming cavity  14 . Warming chamber  12  is defined by oppositely disposed internal side walls  16  and  18 , upper and lower walls  20  and  22 , and rear wall  24 . Thus, the warming chamber  12  is configured to receive food product therein via, for example, a plurality of food racks (not shown) that maybe supported, for example, by the side walls  16  and  18 . If desired, the cabinet  10  may be mounted on appropriate rollers to facilitate its portability. 
     The cabinet  10  further includes a door  46  that is connected to the cabinet via hinges  48  that allow the door to swivel between an open position to render the cavity  14  accessible to the user, and a closed position to completely enclose the cavity. Door  46  further includes a latch  50  having a magnet  52  disposed on an inner contact surface that mates with a strikeplate  54  to form a door switch  55 . The switch forms part of a sensor  56  that is mounted into the front surface  13  of the cabinet  10 . Accordingly, when the door is closed, the magnet  52  and strikeplate  54  are in direct contact, which is disrupted upon opening the door  46 . As will be described in more detail below, the sensor  56  is connected to control  44  and provides a signal indicating whether the door  46  is open or shut. The door  46  may include an insulating material, or any other suitable material that is capable of minimizing heat loss from the warming cavity  14  when the door is closed. Additionally, the door  46  may include a transparent section so as to reveal the warming chamber  12  to the user when closed. 
     The warming chamber  12  further includes a temperature sensor  42  that is mounted onto the inner surface of rear wall  24  so as to avoid being damaged by either the food product or accessories disposed within the cavity  14 . The temperature sensor comprises a thermostat in accordance with the preferred embodiment. Cabinet  10  further includes a control  44  that is installed having a faceplate  45  generally flush with a front surface  13  of the cabinet  10  above the warming chamber  12  so as to be accessible to a user. For example, several operating parameters for the warming chamber  12  may be selected via knobs and or buttons on the control, such as power, temperature control, and humidity control. As will be described in more detail below, the control  44  will provide a current supply to a heating element  26  when it is determined that the temperature of the warming cavity  14 , as indicated by the thermostat  42 , is less than the desired temperature indicated by the user on the control  44 . 
     It should be appreciated that conventional warming cavities such as that in accordance with the preferred embodiment are typically configured to introduce moisture into the cavity so as to control the humidity therein. The present invention contemplates such a configuration, and accordingly a reservoir may be attached to the cavity  14  via a conduit having an opening into the cavity (not shown) that is configured to introduce a fluid, such as water, into the cavity at predetermined intervals as is understood by those having ordinary skill in the art. 
     Referring still to FIG. 2, a plurality of heating elements  26  extend through side and rear walls  16 ,  18 , and  24 , respectively, and optionally through bottom wall  22 , and are connected to the control  44  via a bus  25 . Alternate embodiments may be produced having only one heating element that may comprise a single long heating wire that wraps around the cavity numerous times. The heating elements  26  comprise elongated wires in accordance with the preferred embodiment having a high resistance that produce a significant amount of heat in response to current, as is well known in the art. Accordingly, when elements  26  receive current from the control  44  via a power source (not shown), heat is supplied uniformly to the interior cavity  14 . Each wall of the warming chamber  12  comprises a laminate having an inner and outer shell  28  and  30 , respectively, that surround an insulation layer  32 . The insulation layer  32  may comprise fiberglass sheets, or a suitable alternative insulator, so as to maintain the generated heat within the warming chamber  12 . Outer shell  30  encloses warming chamber  12  and provides protection from the ambient environment. 
     Referring also to FIG. 3, each heating element  26 , in accordance with the preferred embodiment, comprises a high resistance wire  34  surrounded by a silicone rubber sheath  36 . The heating element  26  further includes a metallic braid  38  for strength and grounding purposes that is surrounded by an outer silicone rubber jacket  40 . A similar warming chamber is described in U.S. Pat. Nos. 3,521,030 and 3,800,123, the disclosures of which are incorporated by reference as if set forth in full herein. 
     Referring now to FIG. 4, the anticipator circuit  58  constructed in accordance with the preferred embodiment includes the control  44  having a microprocessor  60  disposed therein that receives signals from the door switch  55 , via an input  63 , that indicate whether the door  46  is open or closed. In response to the status of the door switch  55 , the processor  60 , under operation of anticipator circuit logic  62 , as will be described below, is configured to send control signals via output terminal  59  to a relay  61  which controls the current supply to heating elements  26 . It should be appreciated that the anticipator circuit  58  operates concurrently with a closed loop temperature control circuit whereby a temperature sensor  42  sends temperature signals to the microprocessor, which determines whether to activate the heating elements  26  based on a desired temperature, which is entered on the control  44  as a user input. Closed loop temperature circuits of this type are well known and understood to those having ordinary skill in the art. In addition, a door timer is incremented while the door  46  is open, and decremented to a value not less than zero once the door has been closed, as will become more apparent from the description below. 
     Because the temperature sensor  42  is located in the rear of the chamber  12 , the cold ambient air that enters the front of the cavity  14  will not be sensed immediately, and the temperature control circuit will therefore not respond immediately to the temperature drop within the chamber  12 . As a result, activation of the heating elements by the control circuit lags behind the cooling of the chamber  12  by an amount of time sufficient to potentially decrease the quality of the food product being stored. Accordingly, the anticipator circuit  58  is configured to activate the heating elements  26  for a predetermined length of time once the door  46  has been opened, as indicated by the door switch  55 . Accordingly, when a substantial temperature drop within the warming chamber cavity  14  is expected to occur, the heating elements will be activated before the temperature within the cavity decreases substantially. 
     Referring now to FIG. 5A, the control operates an anticipator circuitry logic sequence  62 , which is performed in conjunction with the closed loop temperature control process, as will now be described. In particular, at step  64  the control  44  is activated, such as by depressing a power switch on the faceplate  45 . Typically, an operator will enter a temperature at which to maintain the cavity  14  under normal operating conditions, which as referred to herein as the set temperature. Conventional warming chambers activate the heating elements when it is sensed that the actual temperature within the chamber has fallen below the set temperature. Such conventional warming chambers do not take into account an anticipated heat loss that is not sensed until the temperature within the chamber has dropped to potentially unacceptably low levels. Advantageously, sequence  62  prevents significant temperature drops from occurring within the cavity, as will now be described. 
     Once the sequence  62  has been activated, it begins at step  64  by initializing the parameters that will be used during subsequent steps, such as a door timer, an INHIBIT flag, and a RECOVER flag, whose respective functions will become apparent from the description below. At decision block  66 , it is determined whether the door  46  is open, as indicated by door switch  55 . If so, the door timer is examined, and if found to be below a minimum threshold amount, the timer is set to that threshold. Otherwise, the timer is not adjusted. In accordance with the preferred embodiment, the minimum threshold is set to 15 seconds such that the timer will begin accumulating time starting at 15 seconds and not at 0 seconds. In other words, the timer is preloaded with 15 seconds and begins accumulating time immediately starting at 15 seconds. It will become apparent from the description below that, barring a timeout condition, the heating elements will be activated for the length of time indicated by the timer. Accordingly, setting the door timer to 15 seconds at step  70  ensures that the heating elements  26  will be activated for at least 15 seconds once the door is closed. It should be easily appreciated, however, that this minimum threshold may vary depending on the location of temperature sensor  42 , and further depending on the difference between the temperature within the cavity  14  and the ambient temperature. 
     Next, at decision block  72 , the measured temperature within the cavity  14  is compared to a desired temperature. In accordance with the preferred embodiment, the desired temperature is selected to equal a predetermined amount greater than the set temperature (Set_Temp+Δ). More particularly, Δ is set to equal 25° F. such that the temperature sensor will indicate that the temperature within the cavity  14  is 25° F. is greater than the set temperature, it being appreciated that the temperature within the cavity is likely less than the temperature indicated by the temperature sensor  42 . However, it should be appreciated by those having ordinary skill in the art that Δ could vary based on the threshold amount of time selected for decision block  68 , the length of time that the door  46  has been open, the location of the temperature sensor  42  within the cavity  14 , and the difference between Set_Temp and the temperature of the ambient environment. This invention further contemplates that the minimum threshold length of time as well as Δ could be determined by the control  44  on a real-time basis taking into account the parameters mentioned above, as would be appreciated by those having ordinary skill in the art. 
     IF the temperature within cavity  14  has reached the desired temperature, the RECOVER flag is set False at step  74 . Otherwise, at step  76 , the RECOVER flag is set True, which will cause the heating elements  26  to become activated at subsequent steps, as will become more apparent below. Once the RECOVER flag has been appropriately set, the door timer is incremented at step  78 . In particular, the door timer advances by one second increments in accordance with the preferred embodiment. 
     Next, at decision block  80 , the door timer is compared to an alarm threshold length of time. In accordance with the preferred embodiment, the alarm threshold is set to two minutes, such that an alarm will be activated when the door timer advances to predetermined time intervals beyond two minutes. The alarm could comprise either an audible or visible indicator that alerts the user that the door  46  has been open for an extended period of time. In accordance with the preferred embodiment, the alarm is activated every ten seconds once the door timer has exceeded two minutes at step  82 . 
     Sequence  62  then advances to decision block  84 , where it is determined whether the door timer has advanced beyond a maximum threshold door open time, which is selected to facilitate the deactivation of heating elements  26  once it has been determined that the door has been open for a maximum length of time. If the door timer exceeds the maximum door open time, the INHIBIT flag is set TRUE at step  88 , and the recover flag is set FALSE, thereby ensuring that the heating elements  26  will not activate during the present iteration of sequence  62 . Otherwise, if the door timer is less than the maximum door open time, the INHIBIT flag is set False, and the RECOVER flag remains at the state that was set at either step  74  or  76 . Sequence  62  then advances to step  106 , which in turn, advances to a flag examination sequence  107 , which will be described in more detail below. 
     As a result, once the door timer has reached the maximum door open time, the heating elements  26  will not be activated until it is determined that the door  46  has been closed at decision block  66 , as will be described in more detail below. Sequence  62  thus provides a safety feature by overriding a closed loop temperature control process to discontinue current to the heating elements at step  76  when the door has been open an unacceptable length of time. This step additionally conserves energy that would unnecessarily be lost while supplying power to the heating elements  26  unnecessarily. While the maximum door open time is chosen as 10 minutes in accordance with the preferred embodiment, this length of time could differ, as is appreciated by those having ordinary skill in the art. Because the maximum door open time is greater than the alarm threshold time, sequence  62  advances from decision block  80  directly to step  106  if the door timer is less than the alarm threshold. 
     Once the door is closed, decision block  66  advances to step  90 , where the door timer is truncated, if necessary, to a value not greater than a maximum heating time. As described above, once the door  46  is closed, the heating elements  26  are activated for as long as the door was open, as indicated by the door timer. However, it is undesirable to leave the heating elements activated for an extended period of time and, accordingly, if the door timer has exceeded the maximum heating time at step  90 , the timer is truncated to equal the maximum heating time. This limits the length of time that the heating elements are activated, once the door  46  is closed, as will become more understood from the description below. The timer is not adjusted at step  90  if it is either equal to or less than the maximum heating time, which equals two minutes in accordance with the preferred embodiment, though it is easily appreciated that this time could differ according to, for example, the desired temperature and the type of food product being heated. 
     Next, at decision block  92 , if the temperature sensor  42  indicates an actual temperature inside the cavity  14  as being greater than the desired temperature, the RECOVER flag is set False and the INHIBIT flag is set True at step  94 , thereby ensuring that the heating elements  26  will not be activated during the present iteration. Additionally, at step  94 , the timer is decremented by one second. However, the door timer is not decremented if it equals zero. Sequence then proceeds to step  102 , as will be described below. 
     Alternatively, if the temperature within the cavity  14  is not greater than the desired temperature, the door timer is examined at decision block  96  to determine whether it is greater than zero. As stated above, sequence  62  will attempt to heat the cavity  14  to the desired temperature only while the door timer is greater than zero. Once the door  46  has been closed as long as it was open, sequence  62  will maintain the temperature within the cavity  14  at the set temperature. Accordingly, if the door timer is not greater than zero at decision block  96 , thereby indicating that the timer equals zero, both the RECOVER and INHIBIT flags are set False at step  98  before proceeding to step  106 . If, however, the door timer is greater than zero at decision block  96 , the RECOVER flag is set True and the INHIBIT flag is set false at step  100 . Additionally, the door timer is decremented by one second, but not less than zero, as described above, before proceeding to decision block  102 . 
     Alternatively, if the temperature within the cavity  14  has not reached the desired temperature, and the door timer is greater than zero, the RECOVER Flag is set True, the INHIBIT Flag is set false, and the door timer is decremented by one second. Next, at decision block  102 , if the door timer equals to one second, a burst of water is applied to the cavity  14  from the water reservoir at step  106  to control the humidity within the cavity, as is understood by those having ordinary skill in the art. Otherwise, if the timer has not yet reached one second, sequence advances from decision block  102  to step  106 . 
     Because the temperature within the cavity  14  is only compared to the set temperature only when both the RECOVER and INHIBIT Flags are set False, sequence  62  effectively overrides a conventional control temperature loop by independently activating or deactivating the heating elements  26  whenever the door is either open, or closed less than a predetermined length of time, without comparing the temperature within the cavity  14  to the set temperature entered by the user. 
     Referring now to FIG. 5B, the flag examination sequence  107  determines whether to activate the heating elements  26  based, at least in part, on the status of the INHIBIT and RECOVER flags. For instance, if the INHIBIT flag is True at decision block  108 , the heating elements  26  are deactivated at step  116  regardless of the status of the RECOVER flag, and regardless of the temperature within the cavity  14 . As discussed above, the INHIBIT flag is set True when the door  46  is open, and the door timer has exceeded the maximum door open time. Additionally, the INHIBIT flag is set True when the door  46  is closed, and the temperature within the cavity has reached the desired temperature. 
     Otherwise, if the INHIBIT Flag is false at decision block  108 , sequence  107  proceeds to decision block  110 , where the RECOVER Flag is examined. In particular, the heating elements  26  are activated if the RECOVER Flag is True. As discussed above, the RECOVER Flag is set True when the door  46  is open and the temperature within cavity  14  has not reached the desired temperature, assuming that the door timer has not reached the maximum door open time. Additionally, the RECOVER Flag is set true when the door  46  is closed, and the temperature within cavity  14  has not reached the desired temperature, and the door timer is greater than zero. As a result, when the door  46  is opened, the heating elements are activated until the temperature sensor  42  indicates an actual temperature within the cavity  14  equal to the desired temperature, assuming that the door is not open longer than the maximum permissible amount of time. Once the door is closed, the heating elements will be activated until either the temperature within the cavity  14  is sensed to equal the desired temperature, or the door timer expires. 
     If the RECOVER flag is False at decision block  110 , the temperature indicated by temperature sensor  42  is compared to the set temperature at decision block  112 . For example, when the door  46  is closed, and the door timer has expired, both the INHIBIT and RECOVER flags are set False, which causes sequence  107  to only activate the heating elements when the indicated temperature within the cavity  114  has fallen below the set temperature. Once the heating elements  26  have either been activated or deactivated at step  114  or  116 , respectively, sequence  107  reverts to decision block  66 , as described above. 
     The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. For example, while the invention has been illustrated as being implemented in combination with a warming chamber that operates at approximately 150° Fahrenheit in accordance with the preferred embodiment, it should be easily appreciated that the principle of controlling heat supplied to a heated cavity based on an anticipated heat loss could apply to any type of cavity, such as a conventional oven that reaches significantly higher temperatures, for example approximately 500° Fahrenheit or greater. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, as set forth by the appended claims.