Abstract:
A method of controlling a drive system of an aircraft. The drive system comprised a drive motor arranged to drive at least one wheel of an aircraft landing gear. A power signal indicative of a power level for the drive motor and a speed signal indicative of a rotation speed of the drive motor are received. A torque level for the drive motor is determined using the power signal and the speed signal. The drive motor is then driven such that the torque generated by the drive motor is at the determined torque level.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention methods and control systems for controlling a drive system of an aircraft. More particularly, but not exclusively, the invention concerns the control of an electrical drive system installed on an aircraft landing gear, using a desired power level for the drive system. 
       BACKGROUND OF THE INVENTION 
       [0002]    The taxiing of aircraft while on the ground is conventionally performed using thrust provided by the engines of the aircraft. However, it can be desirable to provide thrust through a wheel drive taxi system; in other words, an aircraft with landing gear incorporating drive motors, which drive the wheels to move the aircraft while it is on the ground. 
         [0003]    Control of movement of an aircraft while on the ground using the aircraft&#39;s engines is achieved by use of the engine thrust controller. In known wheel drive taxi systems, the equivalent control is achieved using a torque controller, with which the pilot (or other person controlling the aircraft) commands a percentage of available torque (i.e. turning force) to the drive motors. 
         [0004]    However, when stationary, the torque required for movement of the aircraft to begin can vary dependent upon the operating conditions. This is because the “breakaway resistance” of the aircraft will vary dependent upon various factors, such as the aircraft mass (which varies depending for different loads), centre of gravity, weather conditions, runway conditions (e.g. dry, wet, icy, sandy, rough, smooth), and any slope on which the aircraft is positioned, for example. When the torque is not sufficient to initiation movement of the aircraft, damage to the drive motors may be caused. If on the other hand a large torque is commanded in order to overcome a potentially large breakaway resistance, this can lead to undesirable “jerking” of the aircraft as movement is initiated. 
         [0005]    The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved methods and control systems for controlling a drive system of an aircraft. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with a first aspect of the invention, there is provided a method of controlling a drive system of an aircraft, wherein the drive system comprises a drive motor arranged to drive at least one wheel of the aircraft landing gear, the method comprising the steps of: 
         [0007]    receiving a power signal indicative of a power level for the drive motor; 
         [0008]    receiving a speed signal indicative of a rotation speed of the drive motor; 
         [0009]    determining a torque level for the drive motor using the power signal and the speed signal; 
         [0010]    driving the drive motor such that the torque generated by the drive motor is at the determined torque level. 
         [0011]    By using a power level to determine how the motor is driven, this causes a higher level of torque to be used at lower rotation speeds for any given power level, as torque=power/rotation speed. This means that a sufficiently high level of torque should always be provided to overcome the aircraft&#39;s breakaway resistance (assuming it is within the operating capabilities of the drive motor), as at very low rotation speeds a very high torque level will be required in order to provide the power level indicated. However, as the aircraft begins to move the rotation speed will increase, meaning that a lower level of torque is provided to give the same power level. This avoids “jerking” of the aircraft, as the torque level will reduce as the aircraft begins to move even if the power level indicated is unchanged. This is in contrast to the situation when the drive motors are driven at a fixed torque level, in which case jerking will often occur. 
         [0012]    The torque level may be determined by dividing the power level indicated by the power signal by the rotation speed indicated by the speed signal. Alternatively, the torque level may be determined using a calculation based on dividing the power level indicated by the power signal by the rotation speed indicated by the speed signal, but with further additional refinements. 
         [0013]    Preferably, the torque level is at most a predefined maximum torque level. This may be the maximum torque level specified for the drive motor, i.e. the maximum torque level the drive motor is able to deliver. 
         [0014]    Preferably, the torque level is, if non-zero, at least a predefined minimum torque level. This may be a minimum torque level specified for the drive motor. 
         [0015]    Advantageously, the torque level is increased over time by at most a predefined maximum torque rate. This further helps to provide smooth movement of the aircraft when there are large changes in the power level indicated for the drive motor. It also helps to provide smooth movement when the rotation speed of the drive motor changes, particularly at very low rotation speeds. 
         [0016]    Preferably, if the rotation speed indicated by the speed signal is below a predefined minimum rotation speed, the torque level is determined using the predefined minimum rotation speed. This prevents “spikes” occurring in the determined torque level when the rotation speed is very low. 
         [0017]    Preferably, the power level indicated by the power signal is a percentage of a predefined maximum power level. 
         [0018]    The rotation speed indicated by the speed signal may be determined from sensors on the drive motor. Alternatively and/or additionally, the rotation speed indicated by the speed signal may be determined from the operating properties of the drive motor. 
         [0019]    In accordance with a second aspect of the invention, there is provided a control system for controlling an aircraft wheel drive system, wherein the drive system comprises a drive motor arranged to drive at least one wheel of the aircraft landing gear, and wherein the control system is arranged to: 
         [0020]    receive a power signal indicative of a power level for the drive motor; 
         [0021]    receive a speed signal indicative of a rotation speed of the drive motor; 
         [0022]    determine a torque level for the drive motor using the power signal and the speed signal; 
         [0023]    drive the drive motor such that the torque generated by the drive motor is at the determined torque level. 
         [0024]    The control system may be arranged to determine the torque level by dividing the power level indicated by the power signal by the rotation speed indicated by the speed signal. 
         [0025]    Preferably, the control system is arranged to determine the torque level to be at most a predefined maximum torque level. 
         [0026]    Preferably, the control system is arranged to determine the torque level to be, if non-zero, at most a predefined minimum torque level. 
         [0027]    Advantageously, the control system is arranged to increase the torque level over time by at most a predefined maximum torque rate. 
         [0028]    Preferably, the control system is arranged, if the rotation speed indicated by the speed signal is below a predefined minimum rotation speed, to determine the torque level using the predefined minimum rotation speed. 
         [0029]    Preferably, the power level indicated by the power signal is a percentage of a predefined maximum power level. 
         [0030]    The control system may be arranged to determine the rotation speed indicated by the speed signal using sensors on the drive motor. 
         [0031]    In accordance with a third aspect of the invention, there is provided a computer program product arranged, when executed on a computing device of an aircraft, to provide a control system that performs any of the methods described above, or any of the control systems described above. 
         [0032]    In accordance with a fourth aspect of the invention, there is provided an aircraft comprising a control system that performs any of the methods described above, or any of the control systems described above. 
         [0033]    It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0034]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
           [0035]      FIG. 1  is a schematic diagram of a control system for controlling an aircraft landing gear wheel drive system in accordance with an embodiment of the invention; 
           [0036]      FIG. 2  is a graph showing the commanded power level for the control system of  FIG. 1 ; 
           [0037]      FIG. 3  is a graph showing the wheel rotation speed of the drive motor controlled by the control system of  FIG. 1 ; 
           [0038]      FIG. 4  is a graph showing the determined torque level for the drive motor controlled by the control system of  FIG. 1 ; and 
           [0039]      FIG. 5  is an aircraft incorporating the control system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    A control system for controlling an aircraft landing gear wheel drive system in accordance with an embodiment of the invention is now described with reference to  FIGS. 1 to 4 .  FIG. 1  is a schematic diagram of the control system. 
         [0041]    The control system  1  takes as input a desired power level  11 , which indicates the level of power that the pilot wishes the drive motors driving the wheels of the aircraft landing gear to use. The desired power level  11  is commanded by the pilot using a power indicator, which may be a pedal, handle or the like. The desired power level  11  can range between 0 and 100, and is scaled by a first gain block  12  with a factor of 0.01 to give a value between 0 and 1. A multiplier block  14  takes as input the output of the first gain block  12  and a maximum power level  14  for the drive motor. (The maximum power level  14  is predefined as a design characteristic of the control system  1 .) Consequently, the output of the multiplier block  13  will be a power level ranging from 0 to the maximum power level, as commanded by the pilot. 
         [0042]    The control system  1  also takes as input the wheel rotation speed  15  of the wheels of the aircraft landing gear, as supplied by a tachometer or other rotation speed sensing device. A first max block  17  takes as input the wheel rotation speed  15 , and also a minimum rotation speed  16 . (The minimum rotation speed  16  is again a predefined value, used as discussed below to prevent “divide by zero” errors when the wheels are stationary.) Consequently, the output of the first max block  17  is the wheel rotation speed  15 , or the minimum rotation speed  16  if the wheel rotation speed  15  is below the minimum rotation speed  16  (for example if the wheels are stationary so the wheel rotation speed  15  is zero). 
         [0043]    A first divider block  18  takes as input the output of the multiplier block  13  and the first max block  17 , in such a way that the output of the first divider block  18  is that of the multiplier block  13  divided by that of the first max block  17 . Consequently, the output of the first divider block  18  is a desired level of torque corresponding to the desired power level  11 , as torque=power/rotation speed. As the rotation speed output by the first max block  17  is at minimum the minimum rotation speed  16 , this prevents “divide by zero” errors when the wheels are stationary, and ensures the desired level of torque output by the first divider block  18  stays within a suitable range at low wheel rotation speeds. 
         [0044]    A second divider block  20  takes as input the output of the first divider block  18  and a maximum motor torque  19 , in such a way that the output of the second divider block  20  is that of the first divider block  18  divided by the maximum motor torque  19 . (The maximum motor torque  19  is an operating property of the drive motors of the aircraft landing gear, and is again predefined as a design characteristic of the control system  1 .) In particular, the minimum rotation speed  16  and maximum power level  14  are preferably predefined at suitable levels so that the desired level of torque output by the first divider block  18  is at most the maximum motor torque  19 , and so the output of the second divider block  20  is between 0 and 1. The output of the second divider block  20  is then scaled by a second gain block  21  with a factor of 100, to give a torque demand percentage value that should range between 0 and 100. The output of the second gain block  21  is passed through a rate-limiter filter block  22 , which limits the rate at which the torque demand percentage value can change. The output of the second gain block  21  may also be passed through a saturation filter block (not shown), which limits the torque demand percentage value to be in the range of 0 to 100 should the minimum rotation speed  16  or maximum power level  14  be defined at values which allow that to occur. 
         [0045]    The torque demand percentage value output by the saturation filter block  22  is passed as a selectable input to a first switch  23 , which also takes an alternative selectable input a zero value  24 . A non-negative check block  25  takes as input the wheel rotation speed  15 , and outputs a 1 if the wheel rotation speed  15  is zero or above (i.e. the aircraft landing gear wheels are not moving backwards), and 0 if it is below zero. The output of the non-negative check block  25  is passed as the selecting input to the first switch  23 , so that torque demand percentage value is selected if the wheel rotation speed  15  is zero or above, and otherwise zero. 
         [0046]    A second max block  25  takes as input the output of the first switch  23 , and a minimum torque percentage value  26 . (The minimum torque percentage value  26  is again predefined as a design characteristic of the control system  1 .) Consequently, the output of the second max block  25  is the torque demand percentage value output by the saturation filter block  22 , or the minimum torque percentage value  26  if the torque demand percentage value is below the minimum torque percentage value  26 , or if the wheel rotation speed  15  is below zero (and so the output of the first switch  23  is zero). 
         [0047]    The output of the second max block  25  is passed as a selectable input to a second switch  27 , which also takes an alternative selectable input a zero value  28 . A positive check block  29  takes as input the desired power level  11 , and outputs a 1 if the desired power level  11  is above zero, and 0 if it is zero or below. The output of the positive check block  29  is passed as the selecting input to the second switch  27 , so that output of the second max block  25  is selected if the desired power level  11  is above zero, and otherwise zero. The output of the second switch  27  is the torque demand percentage  30  with which the control system  1  drives the drive motors of the aircraft landing gear. 
         [0048]    An example of the control system  1  in operation is now described with reference to  FIGS. 2 to 4 .  FIG. 2  shows the desired power level  11  for control system  1  over time. Initially, the desired power level  11  is P=0%. At time t=20 seconds, the desired power level  11  increases to P=4%. At time t=50, the desired power level  11  increases again to P=50%. At time t=76, the desired power level  11  drops to P=10%, and then at time t=100 increases to P=100%. 
         [0049]      FIG. 3  shows the corresponding wheel rotation speed of the drive motor of the aircraft landing gear, and  FIG. 4  shows the corresponding torque level determined by the control system  1 . Initially, the rotation speed s=0 and the torque level is T=0. At time t= 20 , when the desired power level  11  is increased to P=4%, the torque level increases to T=7000 at around t=24 (the rate of increase in the torque level being limited by the control system  1 ). At the same time, the rotation speed remains at s=0, until at around t=24 when the torque level is sufficiently high the breakaway resistance of the aircraft is overcome, and so the it begins to increase until around t=27 when it is at around s=1.5, at which point it continues to increase at a much slower rate. As can be seen in  FIG. 3 , once the rotation speed has increased, the torque level drops in order to maintain the same power level, until at t=27 it has dropped to the minimum torque level. At time t=45, when the desired power level 11 is increased to P=50%, the torque level increases to around T=3600, and the speed increases more quickly. At t=60 the torque level starts to drop, to maintain the power level as the speed continues to increase, to around s=8 at t=76. At t=76, when the desired power level  11  is reduced to P=10%, the toque level drops to the minimum torque level again, and the speed the increases very slowly. At t=100, when the desired power level  11  is increased to P=100%, the torque level increases rapidly and then drops slowly, to maintain the desired power level  11  as the speed increases. 
         [0050]      FIG. 5  shows an aircraft  100  incorporating the control system described above. 
         [0051]    Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. 
         [0052]    Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.