Abstract:
In an aircraft windshield defogging/deicing system, a pair of transparent sheets maintained in spaced facing relation by a transparent interlayer have a resistive coating positioned between the transparent sheets. An inverter is provided for applying to the resistive coating a square wave or quasi-square wave AC signal. In response to receiving this AC signal, the resistive coating generates heat, which reduces or avoids the accumulation of moisture or ice on the aircraft windshield.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to aircraft windshields and, more particularly, to defogging/deicing aircraft windshields.  
         [0003]     2. Description of Related Art  
         [0004]     Today&#39;s aircraft windshield defogging/deicing systems generally include a resistive coating deposited on an inner surface of one of a plurality of transparent layers, which are laminated and held together via transparent interlayers. Many of the resistive coatings in use today are optimally powered with DC electrical power that is available on the aircraft for powering other systems, such as avionic systems. However, certain resistive films are optimally powered by AC electrical power. Heretofore, this AC electrical power was supplied by an inverter, which inverted the DC electrical power into AC electrical power having a sinusoidal waveform.  
         [0005]     A drawback of utilizing an inverter to invert DC electrical power into AC electrical power having a sinusoidal waveform is that the inverter must be configured to perform this function. In order to output a sinusoidal AC waveform, however, the inverter needs components, such as filter capacitors and/or filter inductors, which add weight and cost to the inverter.  
         [0006]     What is, therefore, needed, and not disclosed in the prior art, is an aircraft windshield defogging/deicing system that overcomes the above drawbacks and others. Still other drawbacks that the present invention overcomes will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides an aircraft windshield defogging/deicing system having a pair of transparent sheets maintained in spaced facing relation by a transparent interlayer, a resistive coating positioned between the transparent sheets, for example between the interlayer and one of the transparent sheets, and an arrangement for applying a square wave or quasi-square wave AC signal to the resistive coating.  
         [0008]     The resistive coating is desirably a transparent film that has been deposited on the one transparent sheet. In one non-limiting embodiment, the AC signal can have a duty cycle that varies in a range from 0% and 100% over a time period of 1 second, e.g. ranging from 25% and 75% and/or a frequency ranging from 25 and 1000 Hertz.  
         [0009]     The arrangement for applying the AC signal can include an inverter for inverting an input DC signal into the AC signal and a controller responsive to the input DC signal for outputting to the inverter one or more control signals, which cause the inverter to invert the input DC signal to the AC signal. The AC signal can be an AC voltage and the DC signal can be a DC voltage. In one non-limiting embodiment of the invention, the DC voltage can have a value ranging from 24 and 32 volts DC and the AC voltage can have a value ranging from 115 and 230 volts AC.  
         [0010]     The system can further includes a switch, such as a relay, a transistor, etc., for selectively connecting the means for applying to a DC power source.  
         [0011]     The DC power source can be a DC generator that is driven by an internal combustion engine of the aircraft. The internal combustion engine can be a jet engine and/or a reciprocating engine.  
         [0012]     The resistive coating can be comprised of indium tin oxide (ITO) having a resistivity no greater than one of 50 ohms/square, for example no greater than 25 ohms/square or no greater than 10 ohms/square.  
         [0013]     Each transparent sheet can be comprised of glass or polycarbonate. The interlayer can be comprised of polyvinyl butyral.  
         [0014]     The invention also provides an aircraft windshield defogging/deicing system that includes a glass deicing coating formed on a first transparent sheet comprising the aircraft windshield, inverter circuitry coupled to the glass deicing coating, and a controller for causing the inverter circuitry to invert DC power into an AC power having a square wave or quasi square waveform.  
         [0015]     The AC power can have a square wave or a quasi-square wave waveform, a duty cycle that varies in a range from 0% and 100%, for example ranging from 25%-75%, and a fixed frequency, for example ranging from 25-1000 Hertz.  
         [0016]     The aircraft windshield can further include a second transparent sheet coupled to the first transparent sheet by a transparent interlayer. The glass deicing coating can be sandwiched between the first transparent sheet and the transparent interlayer.  
         [0017]     Each transparent sheet can be comprised of a glass sheet or a polycarbonate sheet. The transparent interlayer can be comprised of polyvinyl butyral.  
         [0018]     The source of the DC power can be a DC generator that is driven by an engine of the aircraft. The glass deicing coating can be comprised of indium tin oxide (ITO). The glass deicing coating desirably has a resistivity no greater than 50 ohms/square, for example no greater than 25 ohms/square or no greater than 10 ohms/square.  
         [0019]     The present invention further provides a method of defogging/deicing an aircraft windshield. The method includes providing an aircraft having a windshield including a resistive coating on a surface thereof other than the surface of the windshield that is exposed to the exterior of the aircraft, coupling inverter having a controller for controlling the operation of the circuitry between the resistive coating and a source of DC electrical power, causing the inverter circuitry to invert the DC electrical power into an AC electrical power having a square wave or quasi-square wave waveform, and supplying the AC electrical power to the resistive coating. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a perspective view of an aircraft including windshields that can utilize the windshield defogging/deicing system of the present invention;  
         [0021]      FIG. 2  is an isolated enlarged view of the windshields of  FIG. 1 ;  
         [0022]      FIG. 3  is a cross-section taken along lines III-III in  FIG. 2  showing a resistive coating coupled to an inverter for supplying AC electrical power thereto;  
         [0023]      FIG. 4  is an block diagram of the internal components of the inverter shown in  FIG. 3  coupled to an aircraft engine driven DC generator via a DC buss; and  
         [0024]      FIG. 5  shows an exemplary waveform output by the inverter of  FIG. 3  in operation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The present invention will be described with reference to the accompanying figures where like reference numbers correspond to like elements.  
         [0026]     All numbers expressing dimensions, physical characteristics, performance criteria and so forth, used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical values set forth in the following specification and claims can vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Moreover, all ranges disclosed herein are to be understood to encompass any and all subranges subsumed therein. For example, a stated range of “1 to 10” should be considered to include any and all subranges between (and inclusive of) the minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a minimum value of 1 or more, e.g. 1 to 6.1, and ending with a maximum value of 10 or less, e.g., 5.5 to 10.  
         [0027]     With reference to  FIGS. 1-3 , an aircraft  2 , such as a winged aircraft as shown or a helicopter (not shown), typically includes one or more windshields  4  positioned adjacent the fore or front end thereof.  
         [0028]     Each windshield  4  desirably has a form that conforms to the shape of the corresponding aircraft  2  where each windshield  4  is installed. To facilitate attachment to aircraft  2 , each windshield  4  includes a support frame  6  that surrounds the windshield and provides a mechanical interface between the windshield  4  and the body of aircraft  2  for connecting windshield  4  to aircraft  2 .  
         [0029]     A typical windshield  4  includes at least two transparent sheets joined together by a transparent interlayer. In the non-limiting embodiment of windshield  4  shown in  FIG. 3 , the transparent sheets include outer glass layer  8 , inner glass layer  10 , and intermediate glass layer  12 . Glass layers  8 ,  10  and  12  are typically heated and bent to a desired curved configuration. Outer glass layer  8  and intermediate glass layer  12  are joined together in facing relationship by a first transparent interlayer  14 . Inner glass layer  10  and intermediate glass layer  12  are joined together in facing relationship by a second transparent interlayer  16 . Although not required, each interlayer can be polyvinyl butyral. Outer glass layer  8 , first interlayer  14 , intermediate glass layer  12 , second interlayer  16  and inner glass layer  10  are bonded together in a manner well known in the art. Accordingly, a detailed description of how this bonding occurs will not be included herein for simplicity of description.  
         [0030]     In use, it is not uncommon for windshield  4  to accumulate moisture or ice on the exposed surface of outer glass layer  8  due to climatic conditions. In order to overcome this accumulation, whereupon the operator of aircraft  2  retains an unimpeded view through windshield  4 , a system is provided for the defogging/deicing windshield  4 . This system includes a resistive coating  20  coupled to an inverter  22  and a signal ground  24 . Inverter  22  is coupled to a source of DC electrical power, such as a DC buss  26 , via a switch  28 . Switch  28  can be any suitable and/or desirable switch, such as a mechanical switch, a power transistor, and the like.  
         [0031]     With reference to  FIG. 4  and with continuing reference to  FIG. 3 , inverter  22  of the present invention includes a single phase DC-to-AC inverter circuitry  30  coupled to a controller  32 . Since conventional inverter circuitry  30  is well known in the art, and since the internal components of inverter circuitry  30  are not relevant to the present invention, details regarding the components of inverter circuitry  30  have not been included herein for simplicity of description. In the nonlimiting embodiment of the present invention shown in  FIG. 4 , controller  32  is shown as being positioned within the inverter  22  housing. However, it should be appreciated that the controller can be a separate element positioned external to and remote from the inverter  22  housing.  
         [0032]     In operation, in response to closure of switch means  28 , controller  32  outputs to inverter circuitry  30  suitable control signals that cause inverter circuitry  30  to invert DC electrical power received from DC buss  26  into single phase AC electrical power which is output to resistive coating  20 . In response to being supplied with the AC electrical power output by inverter circuitry  30 , resistive coating  20  produces sufficient heat to either avoid the formation of moisture or ice on the outer surface of outer glass layer  8  and/or to reverse the accumulation of moisture or ice on the exposed surface of outer glass layer  8 .  
         [0033]     In one non-limiting embodiment, resistive coating  20  is a transparent film that has been deposited, e.g., sputtered, on the inner surface of outer glass layer  8 . One embodiment of resistive coating  20  is a NESATRON® indium tin oxide (ITO) coating available from PPG Industries, Inc. or a NESA® tin oxide coating also available from PPG Industries, Inc. However, the recital of these particular coatings is not to be construed as limiting the invention since the use of any suitable resistive conductive coating is envisioned.  
         [0034]     In one embodiment of the present invention, resistive coating  20  has a resistivity no greater than 50 ohms per square, for example no greater than 25 ohms per square or no greater than 10 ohms per square. However, this is not to be construed as limiting the invention. In the embodiment shown in FIG.  3 , layers  8 ,  10  and  12  are formed from glass. However, this is not to be construed as limiting the invention since any one or more of these layers can be formed from polycarbonate or other suitable transparent materials.  
         [0035]     With reference to  FIG. 5 , and with continuing reference to  FIGS. 3 and 4 , the AC electrical power output to resistive coating  20  is desirably one of a square wave AC signal  35  or a quasi-square wave AC signal  34 . During operation, the amount of power delivered to the coating  20  is either 0%, i.e. power off, or 100%, i.e. power fully on. The amount of time that the power can vary based on, e.g. the atmospheric conditions outside the aircraft  2 . More specifically, as used herein, the term “duty cycle” means the ratio or percentage of (a) the amount of time that power is delivered to the coating during a specified period of continuous operation versus (b) the total specified period of operation. For example, if over a 2 minute period, the power is delivered to the coating for 1 minute, the duty cycle would be ½, or 50%. In the present invention, at any given time during operation, the duty cycle can be any value ranging from 0% to 100% over a time period as short as 1 second, and can change many times during operation. In one nonlimiting embodiment of the invention, the duty cycle varies in response to the temperature of the windshield  4 . The temperature of the windshield  4  can be monitored during flight in any manner well know in the art, e.g. by a temperature sensor  40  positioned on or within the windshield. In one nonlimiting embodiment of the invention, the temperature sensor  40  is positioned on the same internal surface of the windshield as the resistive coating  20 . In operation, the temperature sensor  40  is used to monitor the temperature of the windshield  4  and based on the windshield temperature, the controller  32  establishes the duty cycle required to maintain the windshield clear of any icing and/or fogging. It should be appreciated that the duty cycle can also be preset and not controlled by the temperature sensor. In one nonlimiting embodiment, the AC signal has a duty cycle ranging from 0% and 100% over a time period of 1 second, e.g. from 10% to 90%, or 25% and 75%, and/or a frequency ranging from 25 Hertz and 1000 Hertz. However, these ranges of duty cycle and frequency are not to be construed as limiting the invention. In one non-limiting embodiment, inverter circuitry  30  inverts DC electrical power, for example ranging from 24-32 volts DC, supplied by DC buss  26  into an AC electrical power having a value of ranging from 115 and 230 volts AC RMS. However, this is not to be construed as limiting the invention since the use of other ranges of DC electrical power and/or AC electrical power are envisioned. In one non-limiting embodiment of the invention, inverter circuitry  30  inverts 28 volts DC into 115 volts AC.  
         [0036]     With reference back to  FIG. 4 , the DC electrical power supplied to DC buss  26  can originate from a DC generator  36  which is driven by an aircraft engine  38  in a manner known in the art. Aircraft engine  38  can be an internal combustion engine such as a jet engine or a reciprocating engine. However, this is not to be construed as limiting the invention.  
         [0037]     In a method of deicing an aircraft windshield  4 , aircraft  2  is provided with windshield  4  having glass defogging/deicing or resistive coating  20  on a surface of a ply thereof other than the surface of the windshield that is exposed to the exterior of the aircraft. Inverter circuitry  30 , having controller  32  for controlling the operation thereof, is coupled between resistive coating  20  and DC generator  36 . At a suitable time, switch means  28  connects DC buss  26  to controller  32  and inverter circuitry  30 . In response to switch means  28  forming this connection, controller  32  causes inverter circuitry  30  to invert the DC electrical power provided by DC buss  26  into AC electrical power having either a square wave or a quasi-square wave waveform, a fixed duty cycle, for example ranging from 25% and 75% over a 1 second time period, and a fixed frequency, for example ranging from 25 Hertz and 1000 Hertz. As discussed above, although not limiting in the present invention, the duty cycle can be linked to the temperature of the windshield. The thus produced AC electrical power is supplied to resistive coating  20 , which responds to the supply of AC electrical power by producing heat of sufficient extent to avoid or reduce the accumulation of moisture and/or ice on the exposed surface of outer glass layer  8 . In one non-limiting embodiment, the DC power is converted to AC power having a fixed duty cycle of 50% over a 1 second time period and a fixed frequency of 25 Hz.  
         [0038]     Configuring inverter circuitry  30  to output a fixed frequency square wave or quasi-square wave signal having a fixed modulation duty cycle enables inverter circuitry  30  to be constructed with a minimum number of components, e.g. large filter capacitors and/or inductors, whereupon the weight, size and cost of the inverter  22  can be less than inverters in use today that output sinusoidal waveforms. In one non-limiting embodiment, configuring inverter circuitry  30  to output the fixed frequency square wave or quasi-square wave signal enables the overall weight of inverter  22  designed for use with aircraft windshield to be reduced to no more than 8 lbs., for example no more than 6 lbs., or no more than 5 lbs.  
         [0039]     The invention has been described with reference to the preferred embodiment. Obvious 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 insofar as they come within the scope of the appended claims or the equivalents thereof.