Abstract:
A method and apparatus for measuring the cooling efficacy of air comprising coupling a resistive element to a temperature sensing element, applying a voltage across the resistive element, adjusting the voltage across the resistive element so that the temperature indicated by the temperature sensing element is nearly proportional to the temperature of a locally disposed heat sensitive component for a change in environmental parameters, and coupling the output of the temperature sensing element to a control circuit wherein the control circuit controls a space cooling element.

Description:
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     Contained herein is material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the field of thermal management. In particular, the present invention is related to a method and apparatus for measuring cooling efficacy of a fluid medium. 
     2. Description of the Related Art 
     Power supplied to electronic equipment produces heat. Too much heat causes electronic components to deteriorate and eventually fail. Hence, some means of cooling the electronic components is necessary. Many methods and devices are employed to cool electronic components. Movement of a fluid medium, such as air, over the electronic components is one of the most common methods. Heat sinks, fans, heat exchangers, and coolant pumps are some of the most commonly used devices for facilitating the removal of heat. In a system that uses air as the cooling fluid, heat from an electronic component is transferred to the air surrounding the component. The heated air is moved away from the electronic component and is replaced by fresh air, resulting in the cooling of the electronic component. 
     The cooling efficacy of a fluid is a measure of how well a fluid cools a particular mass. The efficacy of a fluid depends on several parameters. The most significant of these parameters are the temperature of the fluid (air), and the quantity of the air moved over the mass. The temperature of the air commonly depends on the ambient environment. The quantity of air moved is controlled by any of several devices. Typically, these devices are fans and blowers. Thus, the quantity of air moved depends upon how fast a fan turns. 
     The quantity of air moved by a fan or blower also depends on the density of the air. At lower altitudes air is more dense, and therefore for a given fan speed more air is moved at lower altitudes that at higher altitudes. This means that the cooling efficacy of air depends, in part, on altitude. The simplest solution to guarantee sufficient cooling of electronic components under any conditions is to move enough air by simply running the fans at full speed. However, this mode is not efficient since more cooling effort is expended than is necessary. Also, moving air generates acoustic noise. Acoustic noise generation is undesirable especially when generated by a fan in a projector such as a movie-projector or a data-projector. 
     In some instances when a fan is used to cool heat sensitive electronic elements, such as to cool a lamp in a projector, using a fan controlled by a temperature sensor mounted on a circuit board is not the optimum solution. The fan speed, when controlled by a temperature sensor mounted on a circuit board, could be variable. This means that for various modes of operation, as different components on the circuit board are turned on, the variation in heat generated causes the fan speed to fluctuate. This variation in fan speed is undesirable, as a high-speed fan with continually changing speeds may generate disagreeable noise. Moreover, in the situation described above, the speed of the fan changes in response to the heat generated by components on the circuit board rather than in response to the changes in cooling efficacy of the air. 
     A heat sensitive element such as a lamp in a movie or data projector may be located some distance away from the circuit board. In other instances, due to the variability in the design of the circuit boards for different models of the same product, the location of the temperature sensor on the circuit board may be at varying distances away from the heat sensitive element. This variability complicates engineering predictions and increases development time. 
     Moreover, since air density changes with altitude, any product used to cool electronics at random altitudes most cope with this variability. This means that a cooling system that uses a temperature transducer on a circuit board for its input may not operate efficiently enough in order to cool a heat sensitive element operating at random altitudes. What is needed, therefore, is a method and apparatus to directly measure the cooling efficacy of air. 
    
    
     BRIEF SUMMARY OF THE DRAWINGS 
     Examples of the present invention are illustrated in the accompanying drawings. The accompanying drawings, however, do not limit the scope of the present invention. Similar references in the drawings indicate similar elements. 
     FIG. 1 illustrates an embodiment of the invention. 
     FIG. 2 is a graph that illustrates an embodiment of the invention measuring fluid efficacy for changes in altitude. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Described is a method and apparatus to measure the cooling efficacy of a fluid (air) medium. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known architectures, steps, and techniques have not been shown to avoid unnecessarily obscuring the present invention. 
     Parts of the description will be presented using terminology commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. Various operations will be described as multiple discrete steps performed in turn in a manner that is helpful in understanding the present invention. However, the order of description should not be construed as to imply that these operations are necessarily performed in the order they are presented, or even order dependent. Lastly, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may. 
     FIG. 1 illustrates an embodiment of the invention in apparatus  160 . As shown in FIG. 1, a heat source (hereafter resistive element)  110  is coupled to a temperature sensing element  120  to form a sensor  170 . It is appreciated by one skilled in the art that resistive element  110  can be any one of a number of resistive elements including but not limited to resistive elements such as carbon film resistors, metallic alloy resistors, metal oxide resistors, thick film resistors, and any active device. Likewise, temperature sensing element  120  can be any one of a number of temperature sensing elements. For example, temperature sensing element  120  may be a temperature sensing transducer such as an integrated circuit temperature sensor, thermocouple, thermistor, or platinum resistance thermometer. 
     In one embodiment, resistive element  110  is coupled to temperature sensing element  120  via a thermal conductor placed between the resistive element and the temperature sensing element. However, in another embodiment, the resistive element  110  is affixed to the temperature sensing element. There are various means well known in the art exist to affix the resistive element to the temperature sensing element, and are therefore not described further herein. Power-supply  100  applied across resistive element  110  can be any one of a number of power sources including but not limited to a variable voltage source, a constant voltage source, a variable current source, or a constant current source. 
     The output of temperature sensing element  120  of sensor  170  is coupled to a control circuit  130 . Control circuit  130  is further coupled to space cooling element  140 . Space cooling element  140  can be any device that removes heat from the heat sensitive element  150 . In one embodiment, heat sensitive element  150  can be any heat sensitive element, such as a lamp in a movie or data projector, a motor hub, motor windings, power semiconductor device, signal processing semiconductor device, digital semiconductor device, semiconductor imaging device, optical device and touchable surface. In effect, a heat sensitive element can be any element that not only generates heat, but also, those elements that are adversely affected by elevated temperatures. In another embodiment, space cooling element  140  can be any cooling element including but not limited to a fan, blower, heat pump, coolant pump, heat exchanger, and thermoelectric cooler. 
     Fluid mass flow rate is defined as the amount of fluid (air) mass that passes a certain point in a certain interval of time, for example, the amount of air that crosses a particular section in an apparatus in a certain interval of time. Since the amount of air that crosses a particular section in an apparatus depends in part on altitude, an alignment procedure is performed at the time of manufacture on sensor  170  to set the sensitivity of the sensor to account for fluid mass flow rate changes that occur as a result of changes in altitude. 
     For the alignment procedure, the power that is applied to resistive element  110  is adjusted to set the sensitivity of the temperature sensing element  120  to track fluid mass flow rate changes within apparatus  160 . In particular, the alignment procedure requires adjusting the voltage  100  such that the temperature of the resistive element  110  proportionately tracks the surface temperature of the heat sensitive element  150  as the altitude changes, keeping all parameters such as ambient temperature, air pressure, fan speed etc. constant. In one embodiment, the surface temperature of the heat sensitive element  150  is tracked by placing a temperature sensor on or near heat sensitive element  150 , and by monitoring the temperature indicated by the temperature sensor. The temperature indicated by the temperature sensor placed on or near the heat sensitive element is compared with the temperature indicated by sensor  170 . Once the two temperatures are nearly equal to each other, the altitude is changed, and once again the two temperatures i.e. that indicated by the temperature sensor on or near the heat sensitive element  150 , and the temperature indicated by sensor  150  are compared to each other. If the two temperatures are nearly equal to each other at the new altitude, the alignment procedure is complete. If they are not, a different voltage is applied across resistive element  110 , and process is repeated. Once this alignment procedure is complete, voltage  100  is fixed to the value obtained in the alignment procedure. In one embodiment, the temperature indicated by sensor  170 , is equal in magnitude to the temperature of the heat sensitive element. In another embodiment, the temperature indicated by sensor  170 , is proportional to the temperature of the heat sensitive element. In another embodiment, the time taken by the sensor  170  to equal the critical temperature of the heat sensitive element  150  is quicker than the time taken by the heat sensitive element to equal its critical temperature. The critical temperature of the heat sensitive element is the temperature at which the heat sensitive element is destroyed. Thus, for a constant air (fan) speed, the temperature reading output by sensor  170  increases at higher altitudes by an amount nearly proportional to the temperature increase in the heat sensitive element  150 . Thus, sensor  170  simulates the thermal behavior of the heat sensitive element  150 , and is adjusted to account for fluid mass flow rate changes. In other words, sensor  170  represents a thermal analog of the heat sensitive element  150 . 
     The graph illustrated in FIG. 2 is for illustrative purposes only and does not represent actual data. The graph illustrated in FIG. 2 is not to be construed as limiting the scope of the invention. FIG. 2 illustrates a graph of the temperature of heat sensitive element  150 , and that of sensor  170 , as a function of altitude and ambient temperature, for a fixed input voltage  100 , and for a constant fan speed. The graph in FIG. 2 is plotted for the embodiment described in FIG.  1 . The resistance value of resistive element  110  is fixed, and the input voltage  100  is adjusted in accordance with the alignment procedure disclosed above. The x-axis of the graph in FIG. 2 is time. The y-axis on the graph is temperature in degrees Celsius. Trace  280  is the ambient temperature, trace  210  is the surface temperature of the heat sensitive element, trace  290  is the altitude in feet, and trace  240  is the output providing the temperature of sensor  170 . 
     In FIG. 2 at T 1 , which represents readings at sea level, for ambient temperature 20 C., the temperature of the heat sensitive element  150  is stable at 45 C. and the temperature detected by sensor  170  is also stable at 45 C. At T 2 , which represents readings for an altitude of 10,000 feet for the same ambient temperature 20 C., the temperature of the heat sensitive element slowly ramps up to 55 C. and the temperature detected by sensor  170  ramps up at a steeper rate to 55C. At T 3 , the temperature of sensor  170  stabilizes, while the temperature of the heat sensitive element continues to rise. At T 4 , the temperature of the heat sensitive element stabilizes. At T 5 , the altitude is at sea level one again, and at T 6  both the heat sensitive element and the sensor output are stable. At T 7 , the ambient temperature is increased to 30 C. and the temperature of the heat sensitive element ramps slowly to 55 C. However, the temperature of sensor  170 , ramps up at a steeper rate to 55 C. At T 8 , the temperature of the sensor  170  stabilizes, while the temperature of the heat sensitive element continues to increase. At T 9 , the temperature of the heat sensitive element stabilizes. At T 10 , the altitude is increased to 10,000 feet and the temperature of the heat sensitive element slowly ramps up to 65 C. The temperature output by sensor  170 , ramps up steeply to 65 C. At T 11 , the temperature detected by sensor  170  stabilizes, while the temperature of the heat sensitive element continues to rise. At T 12 , the heat sensitive element stabilizes. 
     Thus, the voltage  100  has been adjusted at the time of manufacture for sensor  170  to proportionately track the changes in the surface temperature of heat sensitive element  150  as the altitude and the ambient temperature change. For the voltage value set during manufacture, during normal operation of sensor  170 , the temperature reported by sensor  170 , is analogous to the temperature of the heat sensitive element, since the sensor  170  reports the resultant effect of both air density and ambient air temperature. Since both the factors that affect cooling efficacy i.e. air density and ambient air temperature are accounted for by a single measurement by sensor  170 , sensor  170  measures the cooling efficacy of the air within apparatus  160 . The reading obtained from sensor  170 , when input to control circuit  130 , is used to adjust the operation of space cooling element  140  in order to maintain a constant temperature value at the surface of the heat sensitive element  150 . 
     Since the time required for the sensor  170  to stabilize is less than the time required for the heat sensitive element  150  to stabilize, the heat sensitive element is adequately cooled by the space cooling element  140 . The temperature of the heat sensitive element  150  will therefore, stabilize below its critical temperature. 
     One skilled in the art will appreciate, that various means are available to adjust the resistive element  110  to proportionately track the increase in the heat generated by the heat sensitive element. Adjusting the voltage of battery  100 , adjusting the current supplied to resistive element  110 , and adjusting the value of the resistive element  110  are a few means by which the heat source/resistive element  110  may be adjusted. It is understood that the output of temperature sensing element  120  may feed a plurality of control circuits, which in turn control a plurality of cooling elements. In another embodiment, the output of the sensor  170  may be input into a microprocessor based thermal management system to control one or more thermal control elements, wherein the thermal control elements could be either a heating element or a cooling element or a combination thereof. 
     If a thermal conductor is used to separate the resistive element from the temperature sensing element, adjusting the thermal conductor&#39;s material and dimensions can also affect the ability of the temperature sensing element to track the heat generated by the heat sensitive element. One skilled in the art will realize that any number of materials may be used as the thermal conductor, including but not limited to silicon, aluminum nitride, aluminum etc. Although, the embodiment describes a single space cooling element, one skilled in the art will appreciate that any number of space cooling elements may be employed. For instance, if it is necessary to maintain the temperature of more than one heat sensitive element, then multiple space cooling elements may be employed, each being coupled to one or more sensors  170 . 
     In an alternate embodiment, resistive element  110  and temperature sensing element  120  may be designed on an application specific integrated circuit (ASIC). The ASIC may incorporate circuitry including but not limited to signal processing circuits for reading the output of the sensor  170 , and for filtering the same if necessary, control circuits for feedback control and for controlling the rate at which fluid is moved past the heat sensitive element, drive circuits for providing the power needed to operate one or more space cooling elements, programming circuits, threshold circuits for using the device as a set-point controller, time multiplex circuits for making specialized measurements such as for obtaining mass flow rates or ambient temperature readings, and switching circuits. The switching circuits being used to control the application of power to one or more elements within the ASIC. 
     Thus a method and apparatus has been described to measure the cooling efficacy of a fluid and, in particular, the cooling efficacy of air. The apparatus described is very sensitive to changes in ambient temperature and mass flow rate and can detect anomalies such as a blocked vent in a projector almost instantly. Similarly, the apparatus described can detect a stalled or inoperative fan, eliminating the need for three wire fans. By detecting a blocked vent or a stalled fan in devices wherein vents and fans are needed to expel heat to the surrounding environment, the described apparatus reduces the risk of fires, and other forms of thermal damage. The described apparatus also allows for the maintenance of a tighter thermal control loop which may allow a cooling fan in an electronic apparatus to run at lower and hence quieter speeds.