FIELD OF THE INVENTION  
       [0001]     The present invention relates to a projection display, and more particularly, to a control method for cooling a projection display.  
       BACKGROUND OF THE INVENTION  
       [0002]     Please refer to  FIG. 1 . A conventional projection display  10  mainly includes a case  11 , and a lamp  12 , an optical engine  13 , a control unit  14  and a set of fan  151  and fan  152  are disposed inside the case  11 . Because the lamp  12 , the optical engine  13 , the control unit  14  generate heat, the set of fan  151  and fan  152  is needed for cooling so as to maintain the normal operation of the projection display  10 .  
         [0003]     However, the environment of the projection display  10  changes according to a user&#39;s need, such as: a high elevation above sea level or a low elevation above sea level. The heat conduction of the projection display  10  is according to the following formula: 
 
 Q=ρ*C   p   *V*□T  
 
 wherein, Q is the total heat, ρ is the density of the air, C p  is the specific heat of the air, V is the airflow rate of the fan and □T is the temperature difference of the air flowing out of the projection display after being heated and the air flowing in the projection display. The number of ρ decreases with the increasing of the elevation above sea level, thus, air of same volume can take away more heat on a high elevation above sea level than a low elevation above sea level. Consequently, the set of fan  151  and fan  151  that provides constant airflow cannot take away the heat inside the projection display completely, and then heat is accumulated in the projection display device  10  when at a high elevation above sea level, resulting the temperature of each component rises and each optical component in the projection display is damaged, and thereafter the lifetime of each component in the projection display is shortened. In addition, if the influence of the elevation above sea level is considered in the beginning of design, the noise will become louder and a user need to put up with a noisy operation environment even at a normally low elevation above sea level. 
 
         [0004]     Hence, it is to be researched that how to make the projection display  10  have the best cooling efficiency in different operation environments.  
       SUMMARY OF THE INVENTION  
       [0005]     One object of the present invention is to provide a control method for cooling a projection display, wherein a pressure sensor automatically detects the elevation above sea level of a projection display for controlling the cooling unit to operate at a corresponding rotating speed so as to provide the projection display a better cooling efficiency.  
         [0006]     Another object of the present invention is to provide a control method for cooling a projection display, wherein the rotating speed of the cooling unit is adjusted simultaneously for extending the lifetime of the cooling unit and preventing noise.  
         [0007]     Still another object of the present invention is to provide a control method for cooling a projection display, wherein a plurality of cooling modes is provided for increasing the flexibility in using the projection display.  
         [0008]     For attaining the above objects, the present invention is a control method for cooling a projection display, wherein a pressure sensor automatically detects the elevation above sea level of a projection display and then transmits a signal to the control unit for controlling the cooling unit to operate in a corresponding cooling mode, thereby the cooling efficiency varies with the elevation above sea level and then the cooling unit can operate in a corresponding rotating speed in order to effectively cool the whole projection display and increase the flexibility in using the projection display, and further extend the lifetime of each component inside the projection display.  
         [0009]     Further features and advantages of the present invention, as well as the structure and operation of the embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0011]      FIG. 1  is a perspective diagram showing a conventional projection display.  
         [0012]      FIG. 2  is a perspective diagram showing a projection display in the present invention.  
         [0013]      FIG. 3  is a diagram showing the relation between the outputting voltage of the fans and the elevation above sea level of the projection display in the present invention.  
         [0014]      FIG. 4  is a flow chart showing the control method for cooling the projection display in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     A preferred embodiment of the present invention will be explained with reference to the accompanying drawings.  
         [0016]     Please refer to  FIG. 2 . The projection display  20  mainly includes a case  21 , and a lamp  22 , an optical engine  23 , a control unit  24 , a pressure sensor  25 , a control unit  26  and a cooling unit  27  are disposed inside the case  21 , an intake  281  and a outtake  282  disposed respectively on the two lateral sides of the case  21 . The pressure sensor  25  is such as the ATP  015  series produced by the company “apm”, and is made of a piezoelectric material and processed by MEMS (Microelectromechanical System). The inside of the pressure sensor  25  is vacuum and the pressure sensor  25  senses the difference in atmospheric pressures of different elevations above sea level for outputting a corresponding tiny voltage. The pressure sensor  25  is disposed near the intake  281  and connected with the control unit  26  for detecting the pressure outside the projection display  20  and outputting a signal to the control unit  26 , wherein the signal varies with the pressure outside the projection display  20 . The control unit  26  is disposed inside the projection display  20  and connected with the pressure sensor  25  and the cooling unit  27  respectively for receiving the signal transmitted from the pressure sensor  25  and simultaneously controlling the cooling unit  27 . The control unit  26  controls the rotating speed of the cooling unit  27  according to different signals. The cooling unit  27  can be a set of fan  271  and fan  272  that can change the rotating speed in a stepless way by changing the voltage of the fan  271  and the fan  272 , or can be set to a plurality of cooling modes with constant rotating speed (the first cooling mode, the second cooling mode, the third cooling mode or the fourth cooling mode).  
         [0017]     Please refer to  FIG. 3 . According to the effect on cooling efficiency at each elevation above sea level, the set of fan  271  and fan  272  is set to be in the first cooling mode, the second cooling mode, the third cooling mode and the fourth cooling mode. When the elevation above sea level of the projection display  20  is 0 feet, the atmospheric pressure is 760 mm-Hg and the density of the air is 0.0765 (lb/ft 3 ). When the elevation above sea level of the projection display  20  is 2500 feet, the atmospheric pressure is 694 mm-Hg and the density of the air is 0.07105 (lb/ft 3 ), and the cooling efficiency is 92.8% of that at 0 feet. For maintaining constant cooling efficiency of the projection display  20 , when the elevation above sea level of the projection display  20  is 0˜2500 feet, the set of fan  271  and fan  272  outputs a outputting voltage of 8 voltage for making the rotating speed 1.0767 times of that at 0 feet, and this is the first cooling mode. Similarly, when the elevation above sea level of the projection display  20  is 2501˜5000 feet, the set of fan  271  and fan  272  outputs a outputting voltage of 9 voltage for making the rotating speed 1.1608 times of that at 0 feet, and this is the second cooling mode. When the elevation above sea level of the projection display  20  is 7000 feet, the atmospheric pressure is 586 mm-Hg and the density of the air is 0.062 (lb/ft 3 ), and the cooling efficiency is 81% of that at 0 feet. For maintaining constant cooling efficiency of the projection display  20 , when the elevation above sea level of the projection display  20  is 5001˜7000 feet, the set of fan  271  and fan  272  outputs a outputting voltage of 10 voltage for making the rotating speed 1.2338 times of that at 0 feet, and this is the third cooling mode. When the elevation above sea level of the projection display  20  is 10000 feet, the atmospheric pressure is 532 mm-Hg and the density of the air is 0.0565 (lb/ft 3 ), and the cooling efficiency is 73.8% of that at 0 feet. For maintaining constant cooling efficiency of the projection display  20 , when the elevation above sea level of the projection display  20  is 7001˜10000 feet, the set of fan  271  and fan  272  outputs a outputting voltage of 12 voltage for making the rotating speed 1.3539 times of that at 0 feet, and this is the fourth cooling mode. Thereby, the relationship between the elevation above sea level and the cooling efficiency is used to set the voltage of the fans, and the set of fan  271  and  272  can provide the best cooling efficiency for the projection display  20 , besides, a plurality of cooling modes is set to maintain a constant voltage in an elevation region above sea level for simplifying the control of fans.  
         [0018]     Please refer to  FIG. 4 . The control method for cooling the projection display  20  is at first detecting the atmospheric pressure of the projection display  20  by the pressure sensor  25  for obtaining the elevation above sea level thereof, and then outputting a signal corresponding to the elevation above sea level to the control unit  26 , and thereafter the control unit  26  controls the cooling unit  27  to operate at different rotating speeds according to the signal received, so that the cooling unit  27  starts to cool the projection display  20  effectively. The pressure sensor  25  can automatically detect the pressure changes (that is, changes of the elevation above sea level) and control the cooling unit  27  to operate in different cooling modes (or rotating speeds) so that the cooling unit  27  can provide enough airflow for cooling the projection display  20 . Thus, the projection display  20  can obtain the best cooling efficiency in different environments and operate normally, and the rotating speed of the cooling unit  27  is adjusted simultaneously for extending the lifetime of the cooling unit  27  and preventing noise.  
         [0019]     While this invention has been described in the context of preferred embodiments, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.
A control method for cooling a projection display, wherein a pressure sensor automatically detects the elevation above sea level of a projection display and then transmits a signal to the control unit for controlling the cooling unit to operate in a corresponding cooling mode, thereby the cooling efficiency varies with the elevation above sea level and then the cooling unit can operate in a corresponding rotating speed in order to effectively cool the whole projection display and increase the flexibility in using the projection display, and further extend the lifetime of each component inside the projection display.