Abstract:
A sensor for detecting icing conditions in an airstream includes a flow housing mounted on an aircraft and in which one or more probes are mounted. At least one of the probes subjected to impingement of the airstream and liquid moisture droplets in such airstream. The heat removal, or cooling effect on the probe in the airstream carrying liquid droplets is determined. A temperature signal indicating the airstream temperature is combined with signals from the at least one probe for determining whether or not icing conditions are present.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a divisional of and claims priority of divisional application Ser. No. 12/174,957 filed Jul. 17, 2008, which is a divisional of and claims priority of divisional U.S. patent application Ser. No. 11/316,187 filed Dec. 22, 2005 (now U.S. Pat. No. 7,416,329), which is a divisional of parent U.S. patent application Ser. No. 10/299,207 filed Nov. 19, 2002 (now U.S. Pat. No. 7,014,357). 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to icing conditions detectors or sensors that use temperature sensitive probes, which are connected to sensing circuitry and positioned such that measuring voltage or power characteristics provides information for detecting moisture in a fluid flow, and when combined with an ambient temperature measurement serve to detect icing conditions in the atmosphere. 
         [0003]    Emerging regulatory requirements for operating aircraft in icing conditions are being interpreted to require more conservative estimates of sensing icing situations than may be provided with existing ice detectors. Application specific Conditions that conventional accretion based detectors may not be able to detect due to Ludlam Limit effects need to be addressed to meet the new requirements. The ability to detect the existence of icing conditions, rather then actual ice accretion, may therefore be required. “Icing conditions” require the presence of liquid moisture in the airflow, and an air temperature below some selected threshold temperature usually specified to be slightly above freezing. 
       SUMMARY OF THE INVENTION 
       [0004]    In its broadest form, a single temperature sensitive probe is deployed in the airstream, and is a heated sensor. The sensor can be self-heated from the power used to excite the sensing element, or a separate heater integral to the probe. Air data information, from other sources which are sufficient to calculate area normalized mass flow rate are needed. The power consumed by the probe to maintain itself at a selected temperature above ambient in dry air is known to have a fixed relationship to mass flow rate calculated from the other air data sources. The air data information is independent of the presence of moisture, but, moisture in the air will increase the power drawn by the heated probe relative to the dry condition to maintain the selected temperature. Thus, if the power drawn by the probe deviates from the expected dry air relationship, the presence of moisture is indicated. A measurement of temperature of the ambient air is also needed to determine whether icing conditions are present. 
         [0005]    This ability to obtain information relating to the power drawn to provide heat to maintain the probe temperature to indicate the presence of icing conditions is also achieved by providing two identical heated temperature sensors or probes at different locations in substantially the same mass airflow, but where liquid water is removed from the airflow at one location. As shown in  FIG. 1 , a bifurcated flow channel is provided. One branch channel is essentially free of liquid moisture due to inertial separation, and the other branch channel carries the liquid moisture in the airflow. 
         [0006]    As shown, a flow housing similar to that used with some total air temperature sensors may be used to provide inertial separation between flow channel branches. A heated or self-heated temperature probe that is in the moisture carrying channel branch will respond differently from a similarly heated temperature probe in the channel that is free of moisture, assuming there is moisture present in the free stream airflow. Assuming the probes are maintained at a fixed temperature, in non-moisture or dry air flow there will be increasing amounts of heat removed from each probe as flow rate increases, but the amount of heat removed from each will be substantially the same. 
         [0007]    By connecting the two resistance type probes into a bridge, the bridge output voltage will remain near zero and independent of flow rate or air speed in dry air, but if there is liquid moisture present the heat removed from one of the probes, where removal of heat is enhanced by evaporation and/or blow off of warmed water, will cause a temperature change at that probe and therefore a resistance change if the probe is a resistance type temperature sensor. The offset in voltage would be expected to increase with increasing liquid water content. When an ambient air temperature measurement, that is the temperature of the freestream airflow, is provided from a separate source, a determination of icing conditions can be made. Alternately, a temperature probe may be located in one of the flow channels, preferably that from which moisture has been removed, to approximate the freestream air temperature. 
         [0008]    If a resistance temperature probe is used, this approach can be modified by including this probe in a bridge circuit. In this modified approach it is not necessary that mass airflow through each channel be substantially equal. By measuring suitable combinations of voltages, the presence of moisture in the branch carrying liquid moisture from the freestream airflow can be determined because the relationships between the measurements will differ compared to conditions when the free stream flow is dry. Temperature can be determined by measuring the voltage drop across the temperature sensor. 
         [0009]    Again, the presence of liquid moisture and an air temperature below a threshold, usually slightly above freezing, is required for icing conditions, and these parameters can be provided by the instrument of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic side sectional view through a flow housing that can be mounted onto an aircraft and provides an inlet leading to two branch flow channels, one of which has liquid moisture removed by inertial separation, and illustrating a first form of the invention; 
           [0011]      FIG. 2  is a schematic bridge circuit illustrating the operation of the first form of the invention; 
           [0012]      FIG. 3  is a sectional view through a flow housing having branched flow channels showing probes of a second alternative form of the invention; 
           [0013]      FIG. 4  is a schematic bridge circuit utilizing the probe arrangement shown in  FIG. 3 ; 
           [0014]      FIG. 5  is a sectional view of a flow housing mounting a single probe directly in the liquid moisture carrying airflow. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    In  FIG. 1 , an aircraft skin  10  supports a flow housing  12  that as shown, has a hollow strut  14  and a fore and aft facing flow tube  16  mounted onto the strut  14 . The flow tube  16  can have any desired cross-sectional shape, and is generally rectilinear or shaped like a flattened circle, and has an inlet end flow channel  18  through which freestream air low indicated by the arrow  20  is introduced. The flow through the flow tube  16  is controlled by having an outlet orifice  22  at the aft end of the flow tube. There is an opening  24  between the flow channel  18  and an aft branch flow channel  27  which opens to the hollow strut, which forms a branch flow channel  26 . 
         [0016]    Liquid water is represented by the dashed lines  28 , and the flow housing  12  provides inertial separation of the liquid moisture so that little of the liquid water passes into the branch flow channel  26 . The branch flow channel  26  has an exhaust opening  30  at its rear or downstream side. This type of a flow housing is used in various temperature sensors, and for example is of the type shown in U.S. Pat. No. 2,970,475 for a gas temperature probe. 
         [0017]    In the present invention, the flow housing  12  mounts temperature sensing probes for determining presence of icing conditions, and in this form of the invention, a probe indicated at  34  is mounted in the branch flow channel  27  of flow tube  16 , so that the freestream liquid moisture laden air impinges on the probe  34 . Any liquid moisture impinging on the probe  34  will affect the power needed for heating or self-heating the probe, assuming it is desired to maintain the probe at constant temperature. 
         [0018]    A second temperature sensing probe  38  is mounted in the branch flow channel  26 , the flow in which branch channel is essentially free of liquid water, so the airflow across probe  38  is and remains substantially the same as dry, non-liquid water carrying air. 
         [0019]    Since the detector must operate in icing environments the detector housing is provided with heaters  35 , preferably electrical, to prevent ice build-up. Heaters  35 , for example, may be routed internally within the walls of the housing  12  or applied as a mat in a fashion similar to that currently done with many devices that must be ice protected such as temperature probes, pressure probes and antennae. 
         [0020]    To prevent deicing heat from significantly influencing the probes within the housing  12 , the flow tube,  16 , is provided with a number of small holes or perforations  36 , to bleed off the heated boundary layer that forms at the inside walls. This technique is currently used in some aircraft total temperature sensors for the same purpose. 
         [0021]    A baffle or heat shield  37 , is positioned in flow channel  26 , to further minimize the influence of deicing heaters located in the forward walls of strut  14 , or probe(s) located within flow channel  26 . An orifice,  39 , provides venting between the baffle  37  and the inner surface of the forward wall of the strut to prevent excessive temperature rise of the baffle wall. 
         [0022]    As shown in  FIG. 2 , where in the schematic diagram the resistances of probe  34  indicated as P 1 , and probe  38 , indicated as P 2 , are coupled into legs of a bridge circuit  40 . Resistors R 1 , also indicated at  42 , and R 2 , also indicated at  43 , are coupled into the bridge and when the air in both of the branch flow channels  26  and  27  is essentially dry, and balanced to be substantially equal flow rates, the resistances of probes P 1  and P 2  ( 34  and  38 ) will react substantially the same and the bridge will remain balanced. This is indicated by the ratio P 1 /P 2 =R 1 /R 2 . 
         [0023]    The voltage source  46 , designated V supply , excites the bridge. The output of the bridge is across the opposite terminals from the input, and is designated V b  in  FIG. 2 . This output signal is provided to an air data computer  50 . It should also be noted that bridge resistors R 1  and R 2  are selected to be substantially greater than the resistances of P 1  and P 2  to minimize heating of R 1  and R 2 . The computer  50  is provided with an air temperature signal from a temperature sensor or source indicated at  54  and this air temperature signal source can be a separate sensor mounted on the aircraft, or as will be explained in connection with  FIGS. 3 and 4 , can be an additional probe mounted in the flow housing  12 . The sensor or source  54  provides freestream of ambient temperature. 
         [0024]    When moisture is such as that indicated by the lines  28  in flow channels  18  and  27  in  FIG. 1 , is present in the freestream air flow, the probe  34  (P 1 ) will experience liquid moisture impingement, whereas little or no liquid moisture will impinge on probe  38  (P 2 ). Probes  34  and  38  are electrically self-heated to a temperature in the range of 50 degrees to 100 degrees C. above ambient. 
         [0025]    As an alternative to self-heating, separate heater elements integral with, or in close proximity to, the temperature sensing elements in the probes  34  and  38  can be used. The mass flow rate of flow stream in the branch channels  26  and  27  is controlled by regulating the size of outlets  22  and  30 , as well as the size of opening  24  so that the mass flow is substantially the same over each of the probes  34  and  38 . 
         [0026]    In a non-moisture situation, there will be more heat removed from each probe as the flow rate increases, but the amount of heat removed from each will be substantially the same. The bridge  40  will remain substantially balanced. Thus, the output voltage designated V b  is independent of the flow rate or air speed. 
         [0027]    If, however, there is liquid moisture present in the freestream airflow in branch channel  27 , the heat removed from the probe  3 A (P 1 ) is enhanced by evaporation and/or blow-off of warmed water since the probes are maintained at a temperature significantly above ambient. This results in a probe temperature change at probe  34  and a resistance change in the probe, and consequently an offset or change in output signal voltage V b . The offset in V b  increases with increasing liquid water content at the same mass flow of air. There is sensitivity to frozen precipitation such as snow and ice crystals but this sensitivity will be relatively low, and will appear in the form of output voltage spikes that can be filtered by signal conditioning prior to providing the output signal to computer  50 , or filtering can be done in the computer  50 . 
         [0028]    The temperature measurement from the temperature sensor or signal source  54  is combined with the output of the bridge  40 , so that the computer provides an output that indicates icing conditions. Icing conditions are indicated when the temperature T is slightly above freezing or less, and when a voltage output from the bridge circuit  40 , is caused by liquid moisture being present in branch channel  27  and impinging on probe  34 . 
         [0029]    In  FIGS. 3 and 4 , an alternative form of the invention is shown. Probes  34  (P 1 ) and  38  (P 2 ), are positioned the same as in  FIG. 1 , but an optional temperature sensing probe  60  is provided in the branch flow channel  26 . Probe  60  preferably is positioned upstream of the probe  38  (P 2 ) to avoid heating influences from the probe  38 , which as stated is held above ambient temperature. The resistance of probe  60  (P 3 ) and a resistor  62  that is shown connected into an alternative bridge circuit  64  are chosen to be at least an order of magnitude greater than the resistances of probes  34  and  38  (P 1  and P 2 ). This selection or resistances will significantly limit self-heating effects. 
         [0030]    The resistance element in probe P 3  is in the leg of a bridge circuit  64  that is shared by heated probes P 1  and P 2 , as shown in  FIG. 4 . This bridge arrangement affords two bridge voltage outputs, designated V 1  and V 2  in  FIG. 4 . The output V 1  indicates the change in resistance that occurs in moisture laden or water laden air in branch flow channel  27 , and V 2  is an output that is indicative of the resistance of the probe in the branch flow channel  26 , where moisture has been separated. 
         [0031]    The arrangement of  FIGS. 3 and 4  reduces the dependency of determining mass flow rate, or making the mass flow rates equal over the probes  34  and  38  as shown in  FIG. 1 , because there is an independent measurement of heat loss from probes located in flow branch channels  27  and  26 . There is a known relationship between V 1  and V 2  as a function of dry airflow rate. Furthermore, dry air mass flow rate can be discerned from voltage V 4  across the heated probe  38  in the dry air channel,  26 , and V 3 , the voltage drop across the temperature sensing probe  60 , also in dry air channel  26 . With moisture laden air in the channel  18 , the relationship between V 1  and V 2  will be different because of additional heat losses at probe  34  (P 1 ) from evaporation and/or blow-off since the branch channel  27  carries the liquid moisture, while branch channel  26  carries air with little or no liquid moisture. Therefore, if the voltage relationship between V 1  and V 2  changes, from the expected relationship with dry air in both branch flow channels, the presence of liquid moisture in branch flow channel  27  is indicated. 
         [0032]    Voltage source V supply  and voltage V 3  shown in  FIG. 4 , can be measured and provided to a computer  70 , to determine the ambient air temperature. Temperature and moisture information is thus available to determine the presence of icing conditions as an output  72  from the computer  70 . The computer is provided with a set point signal so that when liquid moisture is sensed to be present and the measured air temperature is below the set point, icing conditions are indicated. 
         [0033]    It is to be noted that any type of inertial separation flow path can be utilized, and the structure shown herein is merely an example of the type that could be used. The change in direction of a flow can be caused by baffles, obstructions such as posts that cause diversion of particles, and various other shapes and forms of channels that have flow paths branching at a sufficient angle such that the heavier particles will continue in their flow direction under inertial forces and the branch path or bleed path will carry airflow that is substantially free of any liquid moisture particles. 
         [0034]    The ability to provide orifices or other flow controls such as the outlets  22  and  30  in  FIG. 1  for exhaust of fluids, is a way of ensuring that the mass flow rates in the separated channels are substantially the same, and yet inertial separation will keep the liquid particles moving in the same direction along in the straight flow path through branch channel  27 . 
         [0035]    In  FIG. 4 , the quantity R 1 /P 1  approximately equal to R 3 /P 3  and is approximately equal to R 2 /P 2 . Also R 3  is substantially greater than R 1 , and R 1  is substantially equal to R 2  in order to have the bridge perform satisfactorily. 
         [0036]    In  FIG. 5 , a flow housing  80  is illustrated, and is of substantially the same form as the flow housing  12  and strut  14 , but in this instance, the flow tube  81  forms a flow channel directly from a flow inlet  84  and through a control orifice  86  at the outlet. A strut  88  is used for supporting the flow housing  80  relative to an aircraft skin  90 , and in this instance, the strut opening does not carry flow and is a hollow pipe that has no flow outlet for exiting air. The strut could be solid, in other words, in this form of the invention. 
         [0037]    A probe indicated at  92  and which can be represented as P 4  is a heated, or self-heated temperature sensitive probe that is deployed in the airstream, and there is no special ducting required. A flow housing for providing ducting is preferred particularly to control airflow over the probe, to minimize probe operating power and to protect the probe, although the probe can protrude directly into an airstream so long as the liquid water is not separated from the airstream in which the probe is mounted. 
         [0038]    In this form of icing conditions detector, the operation of the probe  92  is based upon the well known fact that power consumed by a heated body maintained at a constant temperature above ambient of the airstream is a function of the mass flow rate. It is desired to maintain the body, in this case the probe  92 , at a fixed temperature above ambient. Power consumed in a dry environment will have a fixed relationship to the mass flow rate calculated from the air data information available from another source. 
         [0039]    Probe  92 , which again is self-heated or with a separate heater that is shown schematically at  92 H in  FIG. 5 , is connected to a computer  96  with a controlled power source, and the computer provides power to the heater or the self heating resistor along a line  98 , and through a “power consumed” indicator  100 , which essentially is the power input to the probe  92 . The computer will measure the power that is drawn to maintain the temperature of the probe  92 . This power consumed signal provided along a line  102  back to the computer  96  and is maintained at a desired level. 
         [0040]    In order to provide the data or information necessary to determine the mass flow rate, a pitot (total) pressure input  104 , a total air temperature input  106 , and a static pressure input  108  can be used to calculate the mass flow rate, and provide the known parameters to the computer  96  for determining the power that would be consumed at the existing mass flow rate if the probe  92  is in dry air. Then, using the actual power consumed from the indicator  100 , the computer provides an indication when moisture is present in the airflow. The amount of moisture can also be determined by empirical tests, or calculations that are related to the particular probe  92 , and what power this probe consumes or requires to maintain a selected temperature in airflow having liquid moisture conditions at different mass flow measurement rates. 
         [0041]    Again, to assess icing conditions, a measurement of temperature from a temperature sensor  106 , providing a temperature parameter to a computer system is necessary. Temperature sensing is well known in aircraft, and air data sensors. 
         [0042]    The effect of water vapor, that is, humidity, in the airflow will have little influence on performance of the detector of the present invention. The detectors are very sensitive, however, to the presence of water droplets, that is liquid water in the air. It is also recognized that the heat transfer capability of air is not only a function of mass flow rate, but also temperature. Compensating for temperature, if necessary, can be done by suitable analytical techniques that would provide information to a control computer, or direct compensation in the circuitry by having temperature dependent circuit elements. The ability to provide these compensation techniques are presently done in existing mass flow measurement products. 
         [0043]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.