Abstract:
An incubator is described which uses a thermoelectric cooler (TEC) as a heat pump in order to provide a stable temperature environment in the infant chamber. Use of a TEC allows a preselected temperature to be maintained in a large range of ambient temperatures during transport, with improved operating efficiency.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to incubators. More particularly, it relates to improvements in the operating efficiency of incubators which improvements derive from the use of a thermoelectric cooler as a heat pump for providing a stable thermal environment for an infant in an infant chamber of the incubator.  
           [0003]    2. Background of the Invention  
           [0004]    Infant incubators are designed to provide a stable thermal environment for neonatal infants whose own thermoegulation systems are too immature to maintain their body temperature. Neonatal infants may be required to be in an incubator from a few days to several weeks depending on the maturity of the infant. The air temperature in an incubator is typically maintained near body temperature of 37° C. Some incubators control the air temperature based upon the skin temperature of the infant. Visibility of the infant is an important concern and most incubator infant chambers are constructed of a clear plastic such as Plexiglas, offering clear views of the infant from five sides. Unfortunately, the Plexiglas is not a good thermal insulator. There have been studies indicating the benefit using a double wall infant chamber, which helps reduce the thermal loss due to the air layer between the two walls of the chamber. More importantly the double wall chamber maintains a higher inner wall temperature, which reduces the radiative heat loss of the infant.  
           [0005]    Transport incubators, which have been in existence for over 30 years, are designed to provide a stable thermal environment for the neonatal infant during transport, such as transport to a hospital with the appropriate critical care facilities or possibly transport within the hospital itself. Transport can last from a few minutes, such as transport within the hospital, or can be several hours when the infant must be moved large distances. Incubators designed for use within the hospital operate in a controlled thermal environment (20° C. to 30° C.) and have access to virtually unlimited electrical, since they plug into wall outlets power from the mains outlet. On the other hand, incubators operate in a wider possible range of temperatures (−20° C. to 40° C.) and often have no access to external power sources. Internal batteries must be used to provide a power source. Lead-acid batters are most commonly used because of their reliability and their ability to provide high rates of discharge when large amounts of heat are required in cold environments. Unfortunately, lead-acid batteries are heavy and reducing weight is an important concern both for the transport personnel who at times must lift the incubator and for those people wanting to reduce payload in weight sensitive aircraft.  
           [0006]    Heat loss through natural convection and radiation of the infant chamber must be replaced. The heat to be replaced determines the size of battery required for a given operating time. Efforts to reduce heat loss can be made, but are usually limited by the need for visibility of the infant and the use of practical materials and construction. Typical heat loss from the infant chamber with an air temperature of 37° C. and ambient temperature of 23° C. can range from 30 watts to 100 watts, depending on the size and construction of the infant chamber. Air flow patterns in the infant chamber can also have a significant effect of the amount of heat loss.  
           [0007]    Typically resistive heaters have been used in all incubators to provide the source of heat. A resistive heater requires a battery to provide the heat loss by the infant chamber. Heat loss is used here to mean loss of energy while power loss represents the rate of heat loss. The theoretical minimum power required to raise 30 joules of heat per second from 23° C. to 37° C. is 1.4 watts, as determined from the second law of thermodynamics. In practice the power required is considerably greater due to practical considerations. Compressors may be used to accomplish the heat transfer, but the increased complexity and expense associated therewith makes use in incubators not practical.  
         DESCRIPTION OF THE RELATED ART  
         [0008]    U.S. Pat. No. 3,918,432 to Franz et al. relates to a method of heating an incubator during transport without the use of an external supply of electrical power. The reference describes the known electrically heated incubators.  
           [0009]    The prior art teaches various mechanisms for heating/keeping air in the infant chamber of incubators. Yet the problem of maintaining a predetermined infant chamber temperature remains when the temperature of the ambient air is above that desired as well as when the ambient air temperature is below that desired. Further, it is also desirable to decrease the power required while prolonging the operating time.  
           [0010]    U.S. Pat. No. 5,240,857 to Lahetkangas relates to a temperature gradient incubator for studying temperature dependent phenomena. This reference describes cooling as well as heating. An incubator as described, is not suitable for transport of human infants.  
           [0011]    Representative prior art techniques for maintaining the interior of an incubator at an appropriate temperature are found in the following U.S. patents: U.S. Pat. No. 3,876,859 to Franz; U.S. Pat. No. 3,919,999 to Gluck et al.; U.S. Pat. No. 4,458,674 to Lemburg et al.; U.S. Pat. No. 4,681,090 to Koch; U.S. Pat. No. 4,846,783 to Koch et al.; U.S. Pat. No. 5,018,931 to Uttley; U.S. Pat. No. 5,100,375 to Koch; U.S. Pat. No. 5,186,710 to Koch et al.; U.S. Pat. No. 5,534,669 to Schroeder et al.; U.S. Pat. No. 5,707,337to Franz; and U.S. Pat. No. 5,773,287 to Binder.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The present invention uses a thermoelectric cooler (TEC) to pump the heat from ambient temperature to the chamber temperature in order to reduce the power required from internal batteries in an incubator. The use of a heat pump has the added advantage of providing the ability to remove heat from the infant chamber as required in hotter ambient temperatures or when a significant source of radiative heat, such as the sun, is present.  
           [0013]    A TEC presents a hot or cold side to a heatsink in the base of the infant chamber as a function of the direction of current supplied thereto.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a more complete understanding of the present invention and the advantages thereof, reference should be made to the following Detailed Description taken in connection with the accompanying drawings wherein like reference numerals are used throughout to denote the same elements and in which:  
         [0015]    [0015]FIG. 1 is a schematic illustration of an incubator;  
         [0016]    [0016]FIG. 2 shows a prior art mechanism for heating the interior of the incubator of FIG. 1;  
         [0017]    [0017]FIG. 3 shows the heating mechanism of the present invention; and  
         [0018]    [0018]FIG. 4 shows the heating mechanism of FIG. 3 installed in an incubator.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    [0019]FIG. 1 shows the primary external characteristics of an incubator. Infant chamber  10  is defined by side walls  12  and  14 , a base  16 , rear wall  18 , front wall  20  and a top  22 . Care giver access to infant chamber  10  occurs through appropriate openings in door  24  and infant wall  20 . Those skilled in the art will understand that other conventional access ports, playing no essential role in the present invention, are not shown.  
         [0020]    Infant chamber  10  rests in and is operably connected to control unit  30 . Schematically illustrated are a ventilator  32 , temperature control panel  34  and patient monitor device  36 . Control unit  30  includes within its housing a mechanism for heating the interior of infant chamber  10 .  
         [0021]    [0021]FIG. 2 illustrates a prior art heating mechanism used with an incubator such as that shown in FIG. 1. Control electronics unit  40  is interconnected to temperature control panel  34 . Electrical power is supplied from wall current via line  42  or from a battery  44 .  
         [0022]    A temperature sensor  46  provides data over line  48  to control electronics unit  40 . Temperature sensor  46  is located in the ambient air at a point in air flow path  50  upstream of a fan  52  which draws ambient air through a channel defined by the bottom of infant chamber  10  and base wall  54 . A resistance heater  58  is located in air flow path  50 . The operation of resistance heater  58  is controlled by control electronics unit  40  as a function of the temperature sensed by sensor  46 . Air flow path  50  continues counter clockwise through an opening  60  formed by base wall  54  and mattress tray  61 , through infant chamber  10 . Air is exhausted through an opening  62  formed by base wall  54  and mattress tray  61 .  
         [0023]    Refer now to FIG. 3 for a better understanding of the improved technique of the present invention for providing and maintaining a stable temperature environment within the infant chamber  10  of an incubator. Two TECs,  70  and  72 , are shown. TECs  70 ,  72  are solid state devices which use the Peltier effect to pump heat. They are commonly made from bismuth telluride and the direction of current flow can be reversed to change the direction of heat flow. The efficiency of the TEC is reduced by resistive heat generation and by thermal conductivity between the cold and hot junctions. TECs have an optimal operating point based upon the cold and hot junction temperatures and the geometry of their individual elements. There are a variety of the TECs available with differing geometries and number of elements. Generally performance is increased by reducing the thermal resistance of the heatsinks used for the cold and hot junctions.  
         [0024]    In FIG. 3, the temperature maintenance apparatus of the present invention is shown. Heatsink  74  is provided in the ambient airflow path  76 . As opposed to the prior art implementation illustrated in FIG. 2 which uses only one heatsink ( 58 ), the present invention requires two heatsinks. A second heatsink  78  is provided for the hot junction when heating infant chamber  10  or the cold junction, when cooling infant chamber  10 . The thermal resistance of both heatsinks  74  and  78  must be kept to a minimum for best TEC performance. When using a resistive heater of the prior art, thermal resistance of the heater to air is relatively unimportant. An ambient airflow fan  80  is provided for drawing ambient air in to follow airflow path  76  through the chamber temperature maintenance apparatus. Airflow outlet  82  and exhaust duct  84 .  
         [0025]    TECs  70 ,  72  are connected via split line  86  to control electronics unit  90 . Control electronics unit  90  is also operatively connected to ambient airflow fan  80 .  
         [0026]    [0026]FIG. 4 is a perspective view of an incubator with the improved temperature maintenance scheme of the present invention, shown in FIG. 3, installed. For ease of understanding a mattress tray comparable to mattress tray  61  in FIG. 1 has been omitted from the figure. As shown, there are two airflow paths  76  and  92 , each with its own fan,  80  and  52 , respectively. Infant chamber  10  surrounds airflow path  92  which airflow is used to transfer heat to or from TECs  70  and  72 , which are in planar contact with infant chamber heatsink  78 .  
         [0027]    Ambient airflow path  76  transfers heat to or from TECs  70 ,  72  to the ambient air.  
         [0028]    The operation of an incubator embodying the present invention is as follows. TECs  70 ,  72  draw heat from the ambient air to raise the temperature of infant chamber  10 . Ambient air is pumped to heatsink  78 . When the temperature inside infant chamber  10  needs to be lowered, the direction of current supplied to TECs  70 , 72  is reversed and ambient airflow heatsink  74  is used to exhaust heat from the infant chamber  10 . To further improve the operating efficiency, the ambient airflow path includes and draws heat from the heat generating control electronics interior of the electronics control housing  30 .  
         [0029]    Current to the TECs  70 ,  72  is controlled by use of a digital implementation of a PID feedback loop, based upon the error temperature between the desired operating temperature and the temperature sensed by sensor  46  for infant chamber  10 .  
         [0030]    While the present invention has been described having reference to a particular preferred embodiment, those having skill in the art will understand that various changes in form and detail without departing from the scope of the following claims.