Abstract:
A device for detecting and/or measuring air flow in a cooling air flow passageway of an appliance using solid-state flow detectors. Heat generating components of the appliance are controlled in response to detected air flow. More specifically, the disclosure provides a device for detecting the presence of airflow and/or the approximate rate of airflow in a cooling channel of an appliance chassis. Based on this information, the appliance can perform safety-related tasks, such as de-energizing associated heating elements if there is too-low (or no) airflow detected. The solid-state flow detectors are easily fabricated, installed, and calibrated and avoids the calibration, fabrication and/or installation difficulties associated with sail switches and other such approaches such as thermal limit switches.

Description:
BACKGROUND OF THE DISCLOSURE 
     The present disclosure generally relates to appliances, and more particularly, to detecting airflow, and/or measuring airflow velocity, in a cooling pathway of an appliance. 
     Appliances such as cooking ranges are widely used. A cooking range typically includes an oven. The oven typically has a front-opening access door, and at least one heating element for heating up the inside of the oven cavity. As is known in the art, when energized, the heating element can heat up the inside of the oven cavity to a relatively high cooking temperature chosen by a user. Also as is known in the art, the cooking range often has a fan which is used to cool a component of the oven, such as the front-opening access door, or a heat sensitive component of the oven such as an electrical element (e.g., electronic controller, display unit, etc.), to a temperature which is lower than the chosen cooking temperature. Some certification institutions, such as Underwriters Laboratories Inc. (UL), require that a Fan Apparency Device (FAD) be employed in the cooking range. The FAD is used to determine or detect whether the fan is working properly, that is, whether or not airflow is being created by the cooling fan. 
     As is known in the art, when a user selects or chooses a heating operation of the oven and turns on the oven, the turning-on supposedly activates the fan. The FAD then determines or detects whether the fan is working properly. If the fan is working properly, the FAD enables the selected heating operation of the oven to proceed. On the other hand, if the fan is not working properly, the FAD prevents the selected heating operation of the oven from proceeding. Various types of FADs are used to determine or detect whether the fan is working properly. 
     The most widely used FADs are thermal switches and sail switches. A thermal switch uses the heat from the oven to heat up a bimetal member of the switch to turn off the switch when the airflow from the fan is no longer present to cool off the bimetal member. Since the thermal switch usually is not disposed adjacent the intake end or the discharge end of the fan, it does not negatively affect the airflow passing through the fan. However, the thermal switch has a relatively slow reaction time. In addition, the thermal switch needs to be installed in an area of the oven where the temperature can raise quickly when the oven is turned on. Moreover, the thermal switch requires a significant amount of calibration and testing to prove that it will work as intended in all working conditions of the oven. 
     A typical sail switch uses the airflow generated by the fan to move a sail, typically a large, flat piece of sheet metal, to turn on or off the switch, Sail switches often include a spring return for returning the sail. Compared with the thermal switch, the sail switch has a very fast reaction time, and requires significantly less time/effort to implement. However, the sail switch has its own limitations. In particular, when it is desirable to direct or point the exhaust end or discharge end of the fan toward the floor on which the appliance is placed, it can be difficult to satisfactorily employ the sail switch in this configuration. This is because in this configuration, the positive pressure side of the sail, which is positioned downstream of the fan, has to face up. As a result, the weight of the sail constantly urges the sail to move from its upper position where the sail opens the switch to its lower position where the sail closes the switch. To make matters worse, in this configuration the positive pressure side of the sail tends to collect a noticeable amount of dust particulates, grease, etc. All of these contribute to the problem that the sail may move to and stay in its lower position even when the fan is not activated, thus creating a false indication of the working condition of the fan. Sail switches also tend to be constructed of thin, flimsy metal and are easily damaged or knocked out of calibration during repair operations and abusive shipping disturbances (drops). Sail switches have also been prone to producing rattling/chattering noises, thus disturbing/annoying the operator of the appliance (also known as the consumer). 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides for detecting air flow in a cooling air flow passageway of an appliance using solid-state flow detectors and controlling heat generating components of an appliance in response to detected air flow. More specifically, the disclosure provides a method and device for detecting the presence of airflow and/or the approximate rate of airflow in a cooling channel of an appliance chassis. Based on this information, the appliance can perform safety-related tasks, such as de-energizing associated heating elements if there is too-low (or no) airflow detected. The disclosure provides a solid state flow detector that is easily installed and calibrated and avoids the calibration, fabrication and/or installation difficulties associated with prior art approaches. 
     In accordance with one aspect, an appliance comprises a chassis, a blower, an air passageway for circulating air from the blower within and/or around at least a part of the chassis, and an air flow measuring device supported within the air passageway for measuring a rate of flow of air flowing through the air passageway, wherein the air flow measuring device includes a differential pressure unit, and a controller that monitors the air flow measuring device to calculate the air flow rate within the air passageway and which performs certain actions based on the measured air flow rate. In one embodiment, the air flow measuring device includes a differential pressure unit and a comparator for calculating flow rate based on the sensed pressures. The differential pressure unit can include a venturi and at least one pressure sensor for measuring pressure differential between a first location of the venturi having a first cross-sectional area and a second location of the venturi having a second cross-sectional area. The venturi can include an outer cylindrical housing, a radially inner wall of the housing forming a flow passageway having a constriction. In another embodiment, the differential pressure unit includes a pressure cup having an opening positionable within the passageway orthogonal to a direction of flow, a first pressure sensor for sensing pressure at a location within the pressure cup, and a second pressure sensor for sensing static pressure immediately downstream of the pressure cup. 
     A controller can be adapted to receive input from the comparator and, in response thereto, control the heat source. The appliance can further include a relay for controlling power supplied to the heat source, and the flow measuring device can be configured to open the relay when a flow of air through the passageway is less than a threshold value. The air flow measuring device can be installed as a unit in the appliance. The comparator can include an analog comparator circuit (including, for example, any of an operational amplifier, an instrumentation amplifier, a multiplier, a subtractor, a comparator, a square-root amplifier, a logarithmic amplifier, etc.), and/or a digital microprocessor (including, for example, a circuit comprising a microprocessor with analog-to-digital (aka ADC) conversion ports and software to perform needed calculations). 
     In accordance with another aspect, an airflow detection module for detecting airflow in a passageway of an appliance comprises a differential pressure unit for measuring pressure in two locations, and a comparator for calculating flow rate based on the sensed pressures, wherein the module is installable as a unit in an appliance. In one embodiment the differential pressure sensor unit includes a venturi and at least one pressure sensor for measuring differential pressure between a first location of the venturi having a first cross-sectional area, and a second location of the venturi having a second cross-sectional area. The venturi can include an outer cylindrical housing, a radially inner wall of the housing forming a flow passageway having a constriction. In another embodiment, the differential pressure sensor unit includes a pressure cup having an opening positionable within the passageway orthogonal to a direction of flow, a first pressure sensor for sensing pressure at a location within the pressure cup, and a second pressure sensor for sensing static pressure immediately downstream of the pressure cup. The comparator can include an analog comparator circuit and/or a digital microprocessor. 
     In accordance with another aspect, a method of controlling a heat source of an appliance comprises providing a differential pressure sensor in a cooling passageway of the appliance, sensing a flow condition in the cooling passageway based on an observed pressure differential, and controlling the heat source when the sensed flow condition is below a threshold value. In one embodiment, controlling includes deactivating the heat source. The deactivating can include communicating a signal to a main controller operatively connected to the heat source for controlling operation thereof, the signal commanding the main controller to deactivate the heat source. In another embodiment, the deactivating includes opening a relay switch to disconnect a power supply operatively connected to the heat source. The heat source can be any heat generating component, such as an electrical component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an exemplary appliance in the form of a double wall oven. 
         FIG. 2  is a top view of an electronics bay of the oven of  FIG. 1 . 
         FIG. 3  is a schematic illustration of a venturi-based solid-state flow detector in accordance with an aspect of the disclosure. 
         FIG. 4  is a perspective view of an exemplary venturi in accordance with the disclosure. 
         FIG. 5  is another view of the venturi of  FIG. 4 , illustrating fluid (air) flow through the same. 
         FIG. 6  is a schematic illustration of another exemplary flow detector in accordance with the disclosure. 
         FIG. 7  is a perspective view of an exemplary pressure cup flow detector in accordance with the disclosure. 
         FIG. 8  is another view of the pressure cup flow detector of  FIG. 7 , illustrating fluid (air) flow around the same. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, and initially to  FIG. 1 , an exemplary double wall oven  10  is illustrated. The double wall oven  10  generally includes an outer housing  12  defining an interior space  13  in which food or other items to be heated can be placed, and upper and lower doors  14  and  16  for providing access to respective portions of said interior space  13  inside of which one or more heating elements  17  are located. An electronics bay  18  is positioned on an upper side of the oven  10  and contains various electronic controls for controlling operation of the oven  10 . It will be appreciated that the features of the present disclosure can be implemented in a wide variety of appliances and, thus, the specific type of oven or appliance is merely exemplary. Accordingly, only the basic features of the oven  10  are described. 
     As is conventional, the oven  10  includes one or more cooling air-flow passageways  22  for circulating air around the oven chassis and the electronics bay  18 . The passageway  22  generally defines a flowpath F for the flow of air from an inlet I near the bottom front of the oven  10 , up the front and through the electronics bay  18 , including the blower  30  which in this embodiment creates the air flow through the passageway, and then down the rear of the oven  10  and forward to an outlet O. It will be appreciated that a variety of different air flow passageway topologies could be implemented in an oven, and the exemplary passageway is but one example. 
     With reference to  FIG. 2 , the interior of the electronics hay  18  is illustrated. As mentioned, the electronics bay  18  includes electronic controls  26  (e.g., a main PCB) for controlling the operation of the oven  10 . A user interface  28  is provided on a front surface of the electronics hay  18  for allowing a user to control the operation of the oven. The user interface can include one or more buttons and/or a display for displaying information such as set temperature, timer information, etc. 
     Supported within the electronics bay  18  are one or more electric blowers  30  for providing positive ventilation to the electronics bay  18  and/or circulating air around the oven chassis and/or housing  12  ( FIG. 1 ). These blowers  30  are activated during operation of the oven  10  to circulate air through the flow passageway  22  for cooling the oven chassis and/or electronics bay  18 . As will be appreciated, in the illustrated embodiment, the blowers  30  are configured to draw air across the electronic controls  26  and exhaust air to the rear of the electronic module  18 . Referring back to  FIG. 1 , the flowpath F of air through the oven  10  can be seen as air is drawn in the inlet I and circulated through the electronics bay  18  and then exhausted via outlet O. In some double ovens, more than one blower  30  may be provided (e.g., one blower for the upper oven and one blower for the lower oven). Again, the illustrated oven  10  and/or configuration thereof is exemplary, and thus other appliances and/or configurations thereof can be used in accordance with the disclosure. 
     In a conventional appliance, a sail switch or other device would typically be located in the flow path F near the exhaust of the blower. When the blower is activated, the sail switch would be urged to a closed position and allow the heating elements to operate as along as adequate air flow maintained the sail switch in the closed position. If air flow were to decrease to a certain level, the sail switch would return to its open position thereby disabling the heating elements. As noted, sail switches (as well as other prior art approaches) are difficult to calibrate and can be unreliable. 
     In accordance with the present disclosure, a Venturi-Based Solid-State Flow Detector (VBSSFD) is supported within the post blower area (downstream of the blower&#39;s exhaust port) towards the rear of the oven, for detecting flow of air therethrough. The VBSSFD  40  is illustrated schematically in  FIG. 3  and includes a venturi  42 , a differential pressure sensor unit  43  including first pressure port P 1  for sensing pressure at an inlet to the venturi  42  and a second pressure port P 2  for sensing pressure at a reduced diameter section (restriction) of the venturi  42 , and a circuit board  44  for processing information received from the pressure sensors P 1  and P 2  to determine the flow rate of air. It will be appreciated that instead of a differential pressure sensor (with 2 pressure ports) one could also use two single-ended (1-port, either gage or absolute) pressure sensors and electronically subtract their outputs to create a difference signal. The circuit board  44  may include one or more processors adapted to calculate airflow velocity and/or volumetric flow rate based on the sensed pressure differential between the pressure ports P 1  and P 2  and/or an analog calculation circuit for performing the same function. In the illustrated embodiment, the main PCB  46  is connected to the circuit board  44  and configured to deactivate the heating element  17  when insufficient air flow is detected. It will be appreciated that the functionality of the circuit board  44  can be up-integrated into the main PCB  46 , rather than having two separate circuit boards. Alternatively, circuit board  44  could operate a relay (or other power switching device) placed between the main PCB  46  and the heating element  17  so as to disrupt activation of the heating element when the circuit board  44  deems that insufficient airflow is present (as will be illustrated in  FIG. 6 ). Other switching mechanisms can be associated with the circuit board  44  for switching on or off various components of the oven based on detected air flow or lack thereof, such as lamps, buzzers, or other “user alert” indicators 
     The basic operation of the VBSSFD  40  is based on the concept of a “Venturi Meter” (or “Venturi Anemometer”), wherein the pressure differential generated within the venturi is directly proportional to the rate of airflow through the venturi  42 . In the present disclosure, an electronic differential pressure sensor unit  43  is employed to measure the pressure difference between the entrance (large cross-sectional area) and the constriction (small cross-sectional area) of the venturi  42 . The voltage of the differential pressure sensor unit  43  increases as the square of the airflow rate through the venturi  42 . This voltage is then measured and used to calculate the airflow rate of the airstream in which the venturi is immersed, in this case the flow passageway  12 . An analog comparator circuit, or a digital microprocessor/microcontroller, is then used to determine if the airflow rate is insufficient and thus take remedial action, if necessary (such as turning-off the oven&#39;s heating elements, for example). 
     As will be appreciated, flow velocity can be calculated using the equation derived from Bernoulli&#39;s principle and the continuity equation: 
                     ⁢       v   1     =         2   ·     A   2   2     ·     (       p   1     -     p   2       )             A   1   2     ·   ρ     -     A   2   2                   
wherein p 1  and p 2  are pressures at different locations in the venturi, and A 1  and A 2  are cross-sectional areas of the venturi at the respective location in the venturi where the pressures are measured.
 
     Turning to  FIGS. 4 and 5 , the details of the venturi  42  are shown. The venturi  42  has a generally cylindrical body  51  having an inlet  50  and an outlet  52 . Air is designed to flow from the inlet  50  to the outlet  52  through a central bore  54  that extends through the length of the body  51 . The central bore varies in diameter and is greatest at the axially outer ends of the body near the inlet  50  and outlet  52 . A constriction  56  ( FIG. 5 ) is defined by a reduced cross-sectional area in the bore  54  defined by a radially inner wall of the generally cylindrical outer housing of the venturi  42 . It will be appreciated that while venturis are generally created in a cylindrical shape, there is no requirement for them to be made as such; any generally-linear flow path with any variable-area cross-sectional geometry may be used, such as ovals, ellipses, and polygons. 
     Given the relatively small size of the VBSSFD  40 , it is easily implemented within an oven&#39;s cooling airflow path, such as the oven  10  in  FIG. 1 . Moreover, the modular nature of the VBSSFD  40  allows its installation in a wide variety of locations within a given appliance, and easy installation in a wide variety of appliances as well. The design also lends itself to either an “analog” solution (discrete op-amps and comparators) or by means of a “digital” solution (microprocessor/microcontroller). 
     Turning now to  FIG. 6 , another exemplary solid-state flow detector  60  is illustrated that can be supported within the electronics hay  80  of an appliance  82  in the same manner as VBSSFD  40  described above to detect air flow generated by a blower  78 . The solid-state flow detector  60  is illustrated schematically in  FIG. 6  and includes a pressure cup  62 , a differential pressure sensor unit  64  including first pressure port P 1  for sensing pressure within the pressure cup  62  and a second pressure port P 2  for sensing ambient pressure outside of the pitot tube  62 , for example directly downstream from (i.e. behind) the pressure cup  62 , and a circuit board  66  for processing information received from the pressure ports P 1  and P 2  to determine flow rate of air. The circuit board  66  may include one or more processors adapted to calculate airflow velocity and/or volumetric flow rate based on the sensed pressure differential between the pressure ports P 1  and P 2 . A main PCB  84  controls heating element  86  in response to user input (e.g., bake, 350 degrees) to regulate a cooking compartment (in the case of an oven). 
     In contrast to the previous embodiment shown in  FIG. 3 , the circuit board  66  of this flow detector  60  communicates a signal to a relay switch  90  (or some other power switching device) rather than the main PCB  84 . The relay switch  90  is configured to interrupt the power provided to the heating element  86  when insufficient air flow is detected. In other words, the solid-state flow detector  60  can operate to detect a deficient air flow condition and, in response thereto, deactivate the heating element  86  regardless of the commanded state by the main PCB  84 . In this configuration, the solid-state flow detector  60  functions completely independent from the main heating element control. 
     It will be appreciated, however, that the solid-state flow detector  60  could be configured to communicate with the main PCB  84  in the manner shown in the embodiment of  FIG. 3 . The communication between the flow detector  60  and the main PCB  84  can be any variety of electronic/electrical signals, including, but not limited to, a discrete on/off (i.e. binary) signal, or a serial data (aka packet) messaging scheme such as SCI, SPI, CAN, LIN, etc. Likewise, the VBSSD  40  could be configured to operate in conjunction with a relay to deactivate a heat source in the manner shown in  FIG. 6 . 
     The basic operation of this embodiment ( FIG. 6 ) is similar to the concept of a “Pitot Tube”, wherein the pressure generated at a static surface (and obstruction) is proportional to the rate of airflow past the obstruction. In this embodiment, an electronic differential pressure sensor  64  is employed to measure the pressure difference between the front and back ends of a pressure cup (in one embodiment, essentially a short, plugged-off venturi)  62  placed within the airflow path. The voltage of the differential pressure sensor  64  increases with the airflow rate past the pressure cup  62 . This voltage is measured and used to calculate the airflow rate of the airstream in which the pressure cup  62  is immersed. An analog comparator circuit, or a digital microprocessor/microcontroller, is then used to determine if the airflow rate is insufficient and whether to take remedial action (such as turning-off the oven&#39;s heating elements). 
     Unlike prior art pitot tube arrangements wherein the “Low Pressure” is simply a measurement of “ambient” pressure taken from a port placed in a surface which is parallel with the airflow, in the present embodiment the pressure differential is amplified by placing the low pressure port on the low-pressure side (back-end) of the pressure cup  62 . This location is in partial vacuum, as is the zone immediately behind a semi-truck or race car (e.g. the “Drafting” zone). Thus, as the airflow increases, not only does the pressure rise within the pressure cup  62 , but also so does the vacuum (negative pressure) behind the pressure cup  62 . This provides a greater “signal” (larger pressure differential) for the sensor to measure, thus allowing the use of less-sensitive (and typical less expensive) pressure sensors. 
     Turning to  FIGS. 7 and 8 , an exemplary pressure cup flow detector is generally identified by reference numeral  100 . The pressure cup flow detector includes an elongated cylindrical body  102  having an upstream cavity  104  and a downstream cavity  106  in each end. The cavities  104  and  106  are blind and are not in fluid communication with each other. To this end, a central portion  108  of the body  102  physically separates the cavities  104  and  106 . Pressure ports  110  and  112  provide access to the axially inner end of each cavity  102  and  104  for measuring/detecting pressure. 
     When the pressure cup  100  is placed in a flowpath of air, or any other fluid) the air pressure in the upstream cavity  104  increases while the air pressure in the downstream cavity  106  decreases. To this end, it should be appreciated that fluid dead ends, or stagnates, at  114  as it cannot flow through the cup but must flow around the cup. Accordingly, the difference in pressure between the upstream cavity  104  and the downstream cavity is greater than simply measuring pressure in the upstream cavity  102  and at a location within the flowpath, as would be the case for a pitot tube or the like. The increased pressure differential can make it easier to measure small flow rates, and/or can allow the use of less sensitive, and therefore less expensive, hardware for measuring the pressure differential while maintaining performance. 
     This disclosure thus provides a more robust manner with which to detect airflow within, for example, an oven&#39;s cooling path. It measures airflow directly, like a “Sail Switch”, but using solid-state technology. Unlike “thermal switches” this disclosed airflow detectors are “resettable” (i.e. sensing a stalled-fan condition is non-destructive to the sensing system). 
     Being based on solid-state devices (i.e. no moving parts) this design is more robust than the commonly-used “Sail Switch” for measuring airflow and determining its sufficiency. Since it is directly measuring the airflow in the cooling path of the appliance, it is more robust than schemes which simply measure the RPM of the fan impeller (RPM is not a 100% sure indication of actual air flow). 
     Moreover, the present disclosure provides air flow detection devices and methods that can be immersed in a flowpath of liquid to detect flow within a conduit or the like. Unlike prior art systems, the present disclosure only needs to be exposed to a portion of the flowpath and does not need to be exposed to the entire flow stream, such as a sail switch or the like. 
     It will be appreciated that aspects of the disclosure are applicable to a wide variety of appliances. Although the illustrated embodiments are directed to ovens, aspects of the disclosure are applicable to other appliances such as refrigerators, washers, dryers, hot water heaters, air conditioners etc. Thus, it will be appreciated that, depending on the appliance, a heat source can comprise a wide variety of heat producing components including a circuit board, a magnetron, a light, resistive and inductive heating elements, a gas burner, a combustion engine, etc. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.