Patent Publication Number: US-8541720-B2

Title: Apparatus for remotely measuring surface temperature using embedded components

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under N00024-05-C-5346 awarded by DDG 1000. The Government may have certain rights in this invention. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     This disclosure relates generally to temperature and heat flux sensors, and more particularly, to an apparatus for remotely measuring surface temperature using embedded components. 
     BACKGROUND OF THE DISCLOSURE 
     Temperature sensors have been developed to provide electrical sensing of temperature at virtually any point of interest. Common types of temperature sensors include thermocouples or resistance temperature detectors (RTDs) that utilize known variations in thermal gradients or electrical resistance, respectively, in order to generate an electrical signal representative of the temperature sensor&#39;s ambient temperature. Known manufacturing techniques have enabled the creation of temperature sensors that are relatively small in size to facilitate measurement of temperatures at correspondingly small regions of interest. 
     In one example, a temperature sensor may be used to determine the temperature of a surface. In certain circumstances, however, mechanical, environmental, and/or aesthetic considerations may prevent or discourage the placement of a temperature sensor on the surface being measured. As a result, two or more temperature sensors may be positioned at different depths with respect to the surface being measured. The temperature of the surface may be extrapolated from the temperatures measured by the multiple temperature sensors at the differing depths. Because extrapolation of heat flow is used to determine an estimate of the surface temperature, however, such techniques are not precise. Additionally, requiring the placement of temperature sensors at multiple depths presents fabrication challenges and other inefficiencies. 
     SUMMARY OF THE DISCLOSURE 
     In one embodiment, a temperature sensing apparatus includes a temperature sensor disposed in a structure at a first depth from a first surface of the structure. A heat flux sensor is also disposed in the structure at substantially the same depth as the first depth. A measurement circuit is coupled to the temperature sensor and the heat flux sensor. The measurement circuit calculates a surface temperature of the first surface based on a temperature of the temperature sensor and a heat flow of the heat flux sensor. 
     Embodiments of the disclosure may provide numerous technical advantages. Some, none, or all embodiments may benefit from the below described advantages. According to one embodiment, measurement of a surface of a structure may be obtained without placement of temperature sensors directly on the surface. This feature may be particularly beneficial for systems where direct placement of a temperature sensor on a particular surface is not practical or may hamper the performance of other associated mechanisms that may use this surface. For example, there are known radome designs that incorporate environmental coatings which are not well suited for placement of temperature sensors directly on their surface. Placement under the surface of the environmental coating may protect the temperature sensors from potentially harsh environments, such as radiation, reactive chemicals, extreme temperatures, physical impact, and/or severe weather. Additionally, embedding the sensors in the wall of a piping structure or tank or placement on the outer surface isolates the sensors from the wear of fluid flow or damage by hazardous or caustic chemicals contained therein yet allows accurate measurement of the fluid temperature. 
     Because the heat flux is directly measured, rather than extrapolated from temperature sensors at differing depths, a more accurate indication of the surface temperature may be obtained. Additionally, eliminating the need to embed sensors at varying depths within the structure enables the construction of the structure and the electrical connections to the sensors to be simplified. 
     Other technical advantages will be apparent to one of skill in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of embodiments of the disclosure will be apparent from the detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a partial perspective view of one embodiment of a temperature sensing apparatus that is configured on a structure; 
         FIG. 2  is a perspective view of another embodiment of a temperature sensing apparatus that is configured on a radome of an antenna; and 
         FIG. 3  is a flowchart showing several actions that may be taken by the temperature sensing apparatus of  FIG. 1  or  2  to measure the temperature of the desired surface. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE DISCLOSURE 
     As previously described, the relatively small thickness of temperature sensors has enabled measurement of temperatures at relatively small regions of interest. There are some devices, however, for which temperature measurement using these temperature sensors are still generally impractical. For example, placement of a temperature sensor directly on an outer surface of a radome may be generally impractical due to environmental coatings on its outer surface. A radome is a type of covering that may be placed over an antenna for shielding the various elements of the antenna from the environment. It may be desired in some cases, however, to measure the outer surface of the radome. During inclement weather conditions, a layer of ice may form on the outer surface of the radome that may hamper proper operation of the antenna. A temperature sensor may be used to monitor the outer surface for icing conditions; however, this sensor is unprotected and, therefore, at risk of damage due to external hazards. 
       FIG. 1  shows one embodiment of a temperature sensing apparatus  10  for measuring surface temperature using embedded components according to the teachings of the present disclosure. Temperature sensing apparatus  10  generally includes a temperature sensor  12  and a heat flux sensor  14  that are disposed in a structure  16  at substantially the same depth D from an outer surface  18  of the structure  16 . A measurement circuit  20  is coupled to the temperature sensor  12  and the heat flux sensor  14 . Measurement circuit  20  is operable to calculate the temperature of the outer surface  18  based on a measured temperature of the temperature sensor  12  and a calculated temperature difference between the temperature sensor  12  and the surface  18  at any point in time. The calculated temperature difference is determined based on the heat flux measured by the heat flux sensor. 
     In certain embodiments, structure  16  may be formed of a number of layers  22  that are disposed adjacent to one another. The layers  22   a ,  22   b ,  22   c , and  22   d  may be formed of any suitable material. In one embodiment, layers  22   a ,  22   b ,  22   c , and  22   d  may be made of a similar material. In another embodiment, layers  22   a ,  22   b ,  22   c , and  22   d  may each be made of differing materials. For example, each of layers  22  may be formed of one or a combination of quartz laminate, fiberglass, RAYDEL™, KAPTON™, or other material that may provide beneficial electro-magnetic and/or structural characteristics. 
     In a particular exemplary example, temperature sensor  12  may be disposed between layer  22   b  and  22   c  such that the thermal resistance between the outer surface  18  and temperature sensor  12  is a function of the material(s) from which layers  22   a  and  22   b  are formed. Alternatively, temperature sensor  12  could be disposed between layers  22   a  and  22   b  or between  22   c  and  22   d . Further, although temperature sensor  12  is depicted as being disposed along the boundary between two layers of  22 , it is recognized that temperature sensor  12  may be disposed at any location within  22 . Heat flux sensor  14  is nominally placed at the same depth as temperature sensor  12 . 
     Temperature sensor  12  may be any suitable type that is operable to create an electrical signal representative of an ambient temperature. In one embodiment, temperature sensor  12  may be a thermocouple that is configured to generate an electrical voltage that is a temperature dependent function of the dissimilar metals from which it is made. In another embodiment, temperature sensor  12  may be a resistance temperature detector (RTD). A resistance temperature detector provides relatively accurate temperature measurement using materials with a known resistance that varies predictably according to its ambient temperature. Materials commonly used for this purpose may include platinum or palladium, which are relatively stable over a wide temperature range. In another embodiment, temperature sensor  12  may be a 2-wire, 3-wire, or 4-wire resistance temperature detector. However, it may be recognized that a 2-wire resistance temperature detector may not be as accurate as a 3-wire or 4-wire resistance temperature detector. 
     In order to provide relatively accurate measurement of the heat flow (Q), heat flux sensor  14  may be disposed at substantially the same depth as and in relatively close proximity to temperature sensor  12 . Accordingly, where temperature sensor  12  is disposed between the two layers  22   b  and  22   c , heat flux sensor  14  may also be disposed between the two layers  22   b  and  22   c  at a location that is substantially the same depth D from outer surface  18  as the location of temperature sensor  12 . Specifically, heat flux sensor  14  is a transducer that generates an electrical signal proportional to the heat flowing toward surface  18 . The measured heat flux is multiplied by the surface area of the heat flux sensor  14  to determine the heat flow (Q). In one embodiment the heat flow may be measured in Watts and the surface area may be measured in square inches, and the heat flux is measured in Watts per square inch. Though described as including a heat flux transducer, heat flux sensor  14  may alternatively include heat flux gauges or heat flux plates. 
     For calculating the temperature of outer surface  18 , system  10  includes a measurement circuit  20 . Measurement circuit  20  may be any type of circuit operable to calculate the temperature of the outer surface  18  using signals received from temperature sensor  12  and heat flux sensor  14 . In one embodiment, measurement circuit  20  may be a digital circuit, such as a processor-based computer circuit in which calculation of the temperature of the outer surface  18  is performed using digital signals. In another embodiment, measurement circuit  20  may be an analog circuit such that calculation of the outer surface temperature is accomplished using known analog circuit techniques. 
     According to the teachings of the present disclosure, calculation of the temperature of the outer surface  18  may be provided using known thicknesses and thermal resistance values of materials from which the structure  16  is made along with known heat flow values existing in the structure and an internal reference temperature. As described above, embedded temperature sensor  12  and heat flux sensor  14  are located at the substantially the same level or depth within structure  16 . Based on the known thermal conductivity to the outer surface  18  and the thickness to the surface along with measured heat flux provided by heat flux sensor  14 , measurement circuit  20  may operate to calculate a temperature difference between the outer surface  18  and temperature sensor  12 . The temperature difference can be combined with the measured temperature of temperature sensor  12  to derive the temperature of outer surface  18 . 
     In one embodiment, a heater element  24  may be provided that is disposed on a surface of the structure  16 . The measurement circuit  20  may be coupled to heater element  24  and operable to selectively apply electrical power to the heater element  24  such that the temperature of outer surface  18  may be controlled. Measurement circuit  20  may selectively apply heat to the heater element  24  using any suitable control loop. In one embodiment, measurement circuit  20  may be implemented with a cascading control loop for controlling the temperature of the outer surface  18 . In another embodiment, measurement circuit  20  may be implemented with a proportional/integral/derivative (PID) control loop for controlling the temperature of the outer surface  18 . In another embodiment, measurement circuit  20  may be implemented with a combination of a cascading control loop and a proportional/integral/derivative (PID) control loop for controlling the temperature of the outer surface  18 . 
       FIG. 2  shows one particular embodiment of a temperature sensing apparatus  10  that may be implemented on radome  28  in which several layers  22  have been peeled away to reveal several components of the temperature sensing apparatus  10 . As described previously, radome  28  may be configured to cover the opening of an antenna  30  for shielding various elements (not specifically shown) of the antenna  30  from the environment. In one embodiment, the radome  28  may be formed of a number of layers  22   a ,  22   b , and  22   c  such that temperature sensor  12  is disposed between layers  22   b  and  22   c . It is recognized, however, that temperature sensor  12  may be disposed between layers  22   a  and  22   b  or any other layers within radome  28 . 
     Where temperature sensor  12  and heat flux sensor  14  are disposed between layers  22   b  and  22   c , heater element  24  may be disposed on a surface of the layer  22   c . In one embodiment, heater element  24  may be substantially flat and extend over the surface of layer  22   c  for heating the outer surface of  22  of the radome  28  in a relatively even manner. In the particular embodiment shown, one temperature sensing apparatus  10  is implemented for determining the temperature of the outer surface  18 ; however, a number of temperature sensing apparatuses  10  may be disposed at various locations on the radome  28 . 
     An outer ring  32  may be included for mounting the edge of the radome  28  to the antenna  30  and/or controlling the radiation pattern of the antenna  30 . A field region of the radome  28  generally refers to a portion of the radome  28  that is surrounded by the outer ring  32 . It is through this field region that electro-magnetic radiation may pass. In one embodiment, temperature sensor  12  may be disposed within this field region for providing a relatively accurate measurement of the outer surface  18  where electro-magnetic radiation may be undesirably affected by the presence of ice. 
       FIG. 3  shows a series of actions that may be performed to measure the temperature of the outer surface  18  of a structure  16 , such as a radome  28  or, alternatively, the inner surface of a tank or pipe. In act  100 , the process is initiated. The process may be initiated by applying electrical power to the measurement circuit  20  such that the measurement circuit  20  may process signals from the temperature sensor  12  and heat flux sensor  14  and perform other functions as described below. 
     In act  102 , the thermal resistance between temperature sensor  12  and outer surface  18  may be determined. The thermal resistance generally refers to a resistance to the movement of thermal energy through a material, which in this particular case is the material from which the structure, such as a radome  28 , is made. In one embodiment, thermal resistance values may be estimated as a function of the intrinsic thermal resistivity of the material(s) and the thickness(es) (totaling D) between temperature sensor  12  and the outer surface  18 . 
     In another embodiment, thermal resistance values may be determined by calibrating the temperature sensing apparatus  10  in which the thermal resistance of layers  22   a  and  22   b  are measured. Calibration of the temperature sensing apparatus  10  may be performed following manufacture and/or periodically throughout its serviceable life. The temperature measurement apparatus  10  may be calibrated by measuring various temperature values of temperatures sensor  12  while the outer surface  18  is subjected to a range of temperatures. In this particular embodiment, a non-permanent temperature sensor may be temporarily attached to the outer surface  18  for temperature measurement of the outer surface  18 . While the structure is subjected to different steady state heat flow conditions, measured values may be obtained from temperature sensor  12  and heat flux sensor  14 . These measured values may then be used to derive apparent thermal resistance values that may then be used as calibration factors for calculating the outer surface temperature during operation of the temperature sensing apparatus  10 . Certain embodiments incorporating a calibration process may provide an advantage in that apparent thermal resistance values may be determined for each structure  16  manufactured in order to cancel distribution error that may occur during the manufacturing process. 
     Acts  104  through  108  describe one embodiment of a method of operation of the temperature sensing apparatus  10 . In act  104 , the temperature sensing apparatus  10  may measure a first temperature of structure  16  at a first depth from outer surface  18  using temperature sensor  12 . In act  106 , heat flux sensor  14  may measure the heat flow (Q) through the heat flux sensor  14  based on the measured heat flux multiplied by the sensor area of the heat flux sensor  14 . Using the measured temperature value from step  104  and the measured heat flow (Q) from step  106 , the measurement circuit  20  may then calculate the outer surface temperature of structure  16  at step  106 . Specifically, measurement circuit  20  may calculate the outer surface temperature (Ts) of structure  16  according to the formula:
 
 Ts=T 1− Q*R  
 
     where:
         T1—measured temperature of temperature sensor  12     Q—heat flow (heat flux*sensor area of sensor  14 )   R—thermal resistance of the layers between temperature sensor  12  and outer surface  18  of structure  16         

     As stated above, thermal resistance (R) is calculated based on the distance between temperature sensor  12  from outer surface  16  and, the thermal resistivity of the material. Specifically, the measurement circuit  20  may utilize the thermal resistance (R) according to the formula:
 
 R=D /( K*A )
 
     where:
         D—depth of temperature sensor  12  from outer surface  18     K—thermal conductivity   A—sensing area of heat flux sensor  14         

     Thus, the formula for calculating the outer surface temperature (Ts) of structure  16  may be considered:
 
Ts=T1− Q*[D /( K*A )]
 
     where:
         T 1 —measured temperature of temperature sensor  12     Q—heat flow (heat flux*sensor area of sensor  14 )   D—depth of temperature sensor  12  from outer surface  18     K—thermal conductivity   A—sensing area of heat flux sensor  14         

     For example, the heat flow may be important in the case of a surface coated with ice. The latent heat of fusion of the ice will draw out more heat than water at the same freezing temperature. In this circumstance the greater heat flow will indicate a colder temperature at the surface than would physically be measured. The end result is that the control system may operate to compensate for the colder temperature by supplying more heat, which is as required in the presence of ice. 
     In a particular embodiment in which heat movement through layer  22   c  or layer  22   d  may not unduly affect the accuracy of the calculated temperature, layer  22   c  may not be needed. Thus, heater element  24  may be configured adjacent temperature sensor  12 . 
     In one embodiment, the measurement circuit  20  may selectively provide electrical power to heater element  24  for controlling the outer surface temperature. Control of the outer surface temperature may be provided using a control loop configured in measurement circuit  20 . In one embodiment, measurement circuit  20  may incorporate a cascading control loop. In another embodiment, measurement circuit  20  may incorporate a proportional-integral-derivative (PID) control loop. The proportional-integral-derivative control loop may provide an advantage in that each portion of the PID control loop may be selectively weighted for tuning the control loop. That is, various weightings of the proportional, integral, or derivative portions of the PID control loop may be individually weighted to counteract any foreseeable temperature rate changes or extremes to which the structure  14  may be subjected during operation. 
     At step  110 , measurement circuit  20  determines if the outer surface temperature of outer surface  18  (as determined at step  108 ) is greater than a desired temperature. If the outer surface temperature of outer surface  18  is not greater than the desired temperature, electrical power is applied to heater element  24  at step  112 . The measurement circuit  20  may determine how much power to apply, in certain embodiments. The application of electrical power may result in an increase in the outer surface temperature of outer surface  18 . Conversely, if the outer surface temperature of outer surface  18  is greater than the desired temperature, electrical power is removed from heater element  24  at step  114 . The removal of electrical power may result in a decrease in the outer surface temperature of outer surface  18 . The control loop determines the timing of the power application and or the amount of power applied. 
     At step  116 , a determination is made as to whether to continue the heater operation. If the heater operation should continue, the method returns to step  104 , and the temperature at a first depth may be measured. Steps  104  through  116  may repeated throughout operation of the temperature sensing apparatus  10 . When it is determined at step  116 , that measurement of the outer surface  18  is no longer needed or desired, operation of the measurement circuit  20  may be halted in which case the process is ended in act  118 . 
     A temperature sensing apparatus  10  has been described that may provide temperature sensing of a surface  18  without the need for placement of temperature sensors  12  directly on the surface  18 . Using measurements provided by a temperature sensor  12  and a heat flux sensor  14  placed at substantially the same depth (D) from the surface  18 , a relatively accurate measurement of the surface  18  may be obtained. The temperature sensing apparatus  10  may be particularly beneficial in scenarios where direct placement of a temperature sensor at a point of interest is impractical or may hamper the performance of other associated mechanisms that may use this surface  18 . Because the temperature sensor  12  and the heat flux sensor  14  are located at the same layer, construction of structure  16  and electrical connections to sensors is simplified. Furthermore, because the heat flux is directly measured, rather than extrapolated from temperature sensors at differing depths, a more accurate indication of the surface temperature may be obtained. 
     Although the present disclosure has been described in several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present disclosure encompass such changes, variations, alterations, transformations, and modifications as falling within the spirit and scope of the appended claims.