Abstract:
A thermal management system includes an airflow sensor that measures airflow in an air path inside an enclosure to be cooled, wherein the air path includes a filter and a fan. A controller is responsive to the airflow sensor for monitoring the rate of airflow inside the enclosure and determining the condition of the filter. In one embodiment, the airflow sensor is an active type sensor, such as a thermistor, disposed on circuit boards. In further embodiments, the airflow sensor is disposed proximate a tube through the filter.

Description:
BACKGROUND 
       [0001]    Thermal management systems generally include an enclosure such as a housing or cabinet for heat generating electronic component circuit cards or boards which must be cooled to prevent damage to the electronic components. A variable speed fan may be used to force air over the components and a temperature sensor and/or an air flow sensor monitors the temperature and/or air flow rate of the air or other cooling gas inside the cabinet. 
         [0002]    A filter is used to reduce particulate matter in order to eliminate contaminants which could otherwise foul the circuit cards. A controller, connected to the fan and responsive to the temperature sensor and/or the air flow sensor, continuously monitors the temperature of the electronic circuit cards and may adjust the speed of the fan to keep the electronic components cool. 
         [0003]    Such prior art systems, however, may detect temperature increases too late to gracefully shut down electronics being cooled. A graceful shut down would allow data to be saved prior to removing power to the electronics. 
         [0004]    As circuitry has decreased in size, it is more sensitive to increases in temperature. Further, the resistance to airflow that such smaller circuitry presents is now comparable to that provided by filters. Thus, any blockage of a filter more quickly lowers airflow and adversely affects the circuitry. Common practice today is to schedule filter replacement on a set schedule, such as every six months. This practice does not take into account the environment where the filter may be located. In a dusty environment, the filter may need to be replaced more quickly. In a clean room environment, filter may last much longer than six months. 
         [0005]    Further, prior art systems may utilize pressure sensor based airflow sensing in an effort to detect filter blockage. For airflow ranges seen in electronics enclosures, a pressure sensor with low measurement range is required. These sensors are large and susceptible to shock and vibration. They are also sensitive to mounting orientation. With smaller circuitry, the larger sensors become unwieldy and inaccurate. They may also fail to properly detect whether the filter is clogged to the point where cooling is no longer effective. Again, when the filter is clogged, the air flow rate through the cabinet may be less than optimal even at the highest fan speed resulting in component failure, thermal stress, or degradation which may not be detectable. 
       SUMMARY  
       [0006]    A thermal management system utilizes an airflow sensor that measures airflow in an air path inside an enclosure to be cooled. The air path includes a filter and a fan. A controller is responsive to the active airflow sensor and monitors the rate of airflow inside the enclosure which is representative of the condition of the filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIG. 1  is a simplified perspective view of an electronics enclosure having filter blockage detection according to an example embodiment. 
           [0008]      FIG. 2  is a perspective view of a filter cartridge having an integrated airflow sensor according to an example embodiment. 
           [0009]      FIG. 3  is a flow chart illustrating a process for detecting a blocked filter in an airflow path according to an example embodiment. 
           [0010]      FIG. 4  is a block diagram of a controller according to an example embodiment. 
           [0011]      FIG. 5  is a cross section view of a filter cartridge having a differential pressure sensor for measuring a difference in pressure across the filter according to an example embodiment. 
       
    
    
     DETAILED DESCRIPTION  
       [0012]    In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
         [0013]    The functions or algorithms described herein are implemented in software or a combination of software and human implemented procedures in one embodiment. The software may consist of computer executable instructions stored on computer readable media such as memory or other type of storage devices. The term “computer readable media” is also used to represent any means by which the computer readable instructions may be received by the computer, such as by different forms of wireless transmissions. Further, such functions correspond to modules, which are software, hardware, firmware or any combination thereof. Multiple functions are performed in one or more modules as desired, and the embodiments described are merely examples. The software is executed on a digital signal processor, ASIC, microprocessor, or other type of processor operating on a computer system, such as a personal computer, server or other computer system. 
         [0014]      FIG. 1  is a simplified perspective view of an electronics enclosure  100  having filter blockage detection according to an example embodiment. The electronics enclosure  100  has multiple devices which generate heat, such as circuit cards  110 ,  112  and  114 . In one embodiment, at least one of the circuit cards, such as card  112  has an active airflow sensor  120  disposed thereon with suitable connection  124  to a controller  130 . In one embodiment, the airflow sensor  120  is a pulsed thermistor, that may be fairly small and easily implementable on a circuit board or card. The airflow sensor  120  may be positioned proximate circuitry on the circuit card that is most sensitive to heat, or otherwise positioned as desired to measure airflow across the circuit card. In some embodiments, the sensor may be integrated directly on the card, and formed during manufacture of the card with other circuitry on the card. 
         [0015]    Airflow sensor  120  is said to be active in that it uses energy in order to sense airflow. In one form, a pulsed thermistor (or other temperature dependent resistive device) is heated and the cooling of the thermistor is measured to determine airflow. Such thermistors may be very small, and easily implemented on circuit cards at one or more desired locations. 
         [0016]    Airflow is provided by a fan  135  in one embodiment, which may be a variable speed fan. Fan  135  is coupled to controller  130  in one embodiment to control the speed of the fan to increase airflow and correspondingly decrease temperature of the circuit cards. A filter  140  may also be provided in an airflow path that begins at openings  142  in enclosure  100 , traveling through the fan  135 , filter  140 , past the circuit cards  110 ,  112 ,  114  and other heat producing components as desired, exiting the enclosure  100  at openings  144 . 
         [0017]    Controller  130  in one embodiment, monitors the airflow, and detects when airflow has changed and is indicative of a filter that is becoming or is blocked. The term blocked as used herein means that the filter has become clogged with particulate matter, and the resistance it provides to airflow has increased. While still allowing air to flow through it, the airflow may not be adequate to sufficiently cool the components in the enclosure  100 . This may be determined by the airflow rate reaching or passing a predetermined threshold, or by more complex algorithms. It may also be a function of air temperature in various embodiments. For instance, a lower air temperature may result in less flow being needed to adequate cool the components. Once the controller determines that the airflow is insufficient, it generates a signal to an output device  150 , which may sound an audible alarm or provide some visual alarm or indication of the need to replace the filter. The alarm may be in the form of a message or blinking icon in various embodiments that are designed to call an operator&#39;s attention to the need for filter replacement. 
         [0018]    In a further embodiment, an airflow sensor may be combined or integrated into the filter  140  as illustrated in perspective view in  FIG. 2 . In one embodiment, the filter  140 , which may be a common filter material optionally encased in frame, has a tube  210  disposed through it that allows air to flow. The tube may be positioned anywhere through the filter material or adjacent to it, such as through a frame, but is positioned such that airflow through the tube  210  is representative of airflow through the filter  140 . In one embodiment, the tube  210  is very small in diameter, such as approximately 1 mm in diameter, such that very little air bypasses the filter material and air quality within enclosure  100  is not significantly adversely affected. In further embodiments, the shape of the tube may be varied from circular to other shapes, such as rectangular or other shape. In still further embodiments, the tube simply comprises an opening through the filter. The filter material may be fused to provide a wall that forms the tube, or any type of opening designed to stand up to environmental conditions and maintain its shape may be used. 
         [0019]    An airflow sensor  215  is positioned proximate the tube, such as at or near either end, or within the tube, to provide an airflow indication representative of airflow through the tube. In one embodiment, the airflow sensor is a thermistor type airflow sensor, where an element is heated, and cooling of the element is representative of the flow of fluid such as air, past the element. Such thermistors are commercially available, and may be made small enough to fit within fairly small tubes. Power to the thermistor, as well as signals from the thermistor may be provided by line  220 , which may be one or more conductive lines to provide current to heat the thermistor, as well as carry signals from the thermistor back to the controller  130  for analysis. In further embodiments, the thermistor may be battery powered, or powered by some other means, such as solar or by the airflow it is designed to measure. Small turbines may be used to power the sensor in further embodiments. The sensor may also have wireless capabilities for transmitting sensed airflow information. In still further embodiments, other types of sensors may be used to measure flow through the tube, such as non-active types of sensors, including turbines or other mechanically based sensors that rotate or bend (piezoelectric type) in response to airflow. 
         [0020]    In one embodiment, the airflow through the filter is inversely proportional to the amount of blockage accumulated on the filter. The flow through the sensor is a linear function of pressure across the filter which is proportional to blockage. In other words, as the filter becomes more clogged, it allows less air to flow through it, building up pressure upstream of the filter. This increase in pressure results in an increase in the airflow through the tube, which is detected by the sensor  210 . The controller  130  receives signals from the sensor  210  indicating the airflow levels, and may compare such levels to predetermined thresholds to determine whether or not the filter is blocked and should be replaced. In further embodiments, various algorithms in the controller may be used to track the rates at which a filter is becoming blocked, and provide predictions of when the filter should be replaced. This may be valuable in dusty environments, since a filter may become quickly blocked by dust. If an operator can see this happening, plans may be made to ensure filters are available when needed. The pressure developed across the filter depends on filter blockage and the speed of the fan that is generating airflow. If the fan speed is kept constant, the pressure across the filter directly represents its blockage. In an application where the fan speed is variable, the fan speed information may be used to normalize the pressure reading to determine filter blockage. In further embodiments, other algorithms, such as a table indexed by fan speed and pressure reading, may be used to determine filter blockage. 
         [0021]    In a further embodiment, where sensor  120  proximate the circuit cards  110 ,  112 ,  114  is used, the airflow is inversely proportional to the amount of blockage of the filter. In other words, as the filter becomes more blocked with dust or other particles, the airflow through it, and thus past the sensor  120  decreases. If the fan speed is kept constant, the airflow reading directly represents the blockage level. If the fan speed is variable an additional compensation for fan speed is used in the controller  130 . The controller  130  may again use a threshold to determine when the filter should be replaced. Historical temperature and or airflow data may be used to establish the threshold. The airflow can be associated with circuit card temperatures to more effectively estimate when the temperature of a circuit will exceed safe operating levels. Airflow is a leading indicator of such temperature excursions, and therefore may provide more advanced warnings of the need for replacement. Such advanced warnings may help ensure the ability to replace filters efficiently, and further allow for graceful shut down of electronics if the filter is not replaced prior to potentially damaging heat buildup. 
         [0022]      FIG. 3  is a flow chart illustrating a process for detecting a blocked filter in an airflow path according to an example embodiment. Airflow is detected at  310  from either sensor  210  or one or more sensors  120 . In some embodiments both types of sensors may be used. At  320 , the detected airflow or airflows is compared to one or more thresholds. If a threshold is exceeded or passed at  330 , a signal indicating blockage of the filter, or indicating that the filter needs to be replaced is provided at  330 . These different indications may each have an associated threshold, such that the system first provides a filter replacement indication, then as the second threshold is passed, a shut down of electronics may be initiated at  340 . The second threshold may be set at a level that provides sufficient time to allow a graceful shutdown of the electronics. A graceful shutdown allows the electronics to save current information and status to allow the electronics to be turned back on without loss of data, and such that they may be turned back on in the same state they were in prior to shutdown. 
         [0023]      FIG. 4  is a block diagram of a controller that may execute programming for performing the above algorithms. A general computing device in the form of a computer  410 , may include a processing unit  402 , memory  404 , removable storage  412 , and non-removable storage  414 . Memory  404  may include volatile memory  406  and non-volatile memory  408 . Computer  410  may include—or have access to a computing environment that includes—a variety of computer-readable media, such as volatile memory  406  and non-volatile memory  408 , removable storage  412  and non-removable storage  414 . Computer storage includes random access memory (RAM), read only memory (ROM), erasable programmable read-only memory (EPROM) &amp; electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, compact disc read-only memory (CD ROM), Digital Versatile Disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium capable of storing computer-readable instructions. Computer  410  may include or have access to a computing environment that includes input  416 , output  418 , and a communication connection  420 . The computer may operate in a networked environment using a communication connection to connect to one or more remote computers. The remote computer may include a personal computer (PC), server, router, network PC, a peer device or other common network node, or the like. The communication connection may include a Local Area Network (LAN), a Wide Area Network (WAN) or other wired or wireless networks. 
         [0024]    Computer-readable instructions stored on a computer-readable medium are executable by the processing unit  402  of the computer  410 . A hard drive, CD-ROM, and RAM are some examples of articles including a computer-readable medium. 
         [0025]    In a further embodiment illustrated at  500  in  FIG. 5 , a filter  510  has a pressure sensor  515  associated with it to measure a difference in pressure across the filter  510 . Many different types of differential pressure sensors may be used, such as capacitive type pressure sensors with the difference in pressure applied across a diaphragm that bends in response to the pressure difference. In one embodiment, pressure sensor  515  is simply an airflow sensor and tube combination for measuring airflow through the tube. As indicated above, the airflow through the tube is a function of the pressure difference across the filter. Information from the pressure sensor, such as a differential pressure sensor is combined with information from an airflow sensor  520  in the airflow path. Airflow sensor  520  and pressure sensor  515  may be formed with or integrated with the filter and/or filter frame as a package. In one embodiment, an arm  525  is attached to the filter, and positions the airflow sensor in a path of air flowing through the filter. Arm  525  may be attached to the tube of the pressure sensor  515 , optionally forming a single integrated piece that may be supported by the tube through the filter. Airflow sensor  520  may optionally be located proximate the electronics to be cooled. 
         [0026]    Both sensors provide information to the controller to generate a signal indicative of filter blockage. This filter blockage signal may be derived from the pressure differential and airflow indication without regard to the fan speed. The ratio of pressure to airflow represents the filter blockage (Ohm&#39;s law). The combination of pressure and airflow sensors may be provided in an assembly, and can read filter blockage independent of fans speed or system resistance. It may also be used to detect a damaged filter, such as one that is ripped or torn, or otherwise allows significant airflow around the filter material. In such cases, the resistance will be below a selected threshold. 
         [0027]    In further embodiments, the controller may be integrated or otherwise attached to the filter, providing a single replaceable unit. A battery or other source of power for the controller and sensors, such as a connector for external power may be included with the filter. The controller may include a light, such as an LED or other type of display device that is controlled by the controller to indicate that the filter is blocked or becoming blocked and should be replaced. In one embodiment, the controller may be supported by the tube of sensor  515 . In further embodiments, it may be supported by the arm  525 . The controller is electrically coupled to one or more of the sensors as desired. The same structure as shown in  FIG. 5  may be used with one or both of the sensors. For instance, just using sensor  520 , with the tube of sensor  515  closed off or even left open, allows one to measure airflow through the filter. This is useful at least in embodiments with a single fan speed, or if multiple fan speeds, providing a fan speed indication to the controller. Similarly, just sensor  515  may be used to determine the difference in pressure across the filter. These embodiments provide a self contained filter assembly that is easily replaced. 
         [0028]    The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.