Abstract:
A method and apparatus for operating an air cycle machine is disclosed. The apparatus includes a detector for measuring a rotational speed of a component of the air cycle machine. A processor estimates a rate of change of rotational speed of the component from the measured rotational speed and shuts down the air cycle machine when the estimated rate of change of the rotational speed of the component is greater than a selected rate threshold.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Air cycle machines are often used in aircraft systems, such as in aircraft air conditioning systems, aircraft pressurization systems, etc. An air cycle machine includes an air bearing that uses a film of pressurized air to provide a low-friction load-bearing interface between two surfaces. At least one of the two surfaces may be rotating at high speed. The pressurized air prevents the two surfaces from touching each other thereby allowing the rotating surface to rotate freely. However, when an air bearing becomes overloaded, the surfaces can touch, leading to abnormal speed fluctuations of the rotating surface. Failure of the air bearing can lead to substantially constant contact between the two surfaces, which can cause materials from the surfaces to rub off and be emitted into the air. Such air emissions can be transmitted into a cabin of the airplane, requiring the aircraft to land. Thus, there is a need to be able to monitor the air cycle machine in order to be able to shut down the air cycle machine before such an event occurs. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0002]    According to one aspect of the invention, a method of operating an air cycle machine includes: measuring a rotational speed of a component of the air cycle machine; estimating a rate of change of the rotational speed of the component; comparing the rate of change of the rotational speed to a selected rate threshold; and shutting down the air cycle machine when the rate of change of the rotational speed is greater than the selected rate threshold. 
         [0003]    According to another aspect of the invention, an apparatus for operating an air cycle machine includes: a detector configured to measure a rotational speed of a component of the air cycle machine; and a processor configured to estimate a rate of change of rotational speed of the component from the measured rotational speed, and shut down the air cycle machine when the estimated rate of change of the rotational speed of the component is greater than a selected rate threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0005]      FIG. 1  shows a schematic diagram of a system for operating an air cycle machine in one embodiment; 
           [0006]      FIG. 2  shows speeds and deceleration rates for a rotating component of a properly operating air cycle machine that is not expected to experience a failure in a near future; 
           [0007]      FIG. 3  shows speeds and deceleration rates of the rotational component for a failing air cycle machine; and 
           [0008]      FIG. 4  shows the speed curve and acceleration curve of  FIG. 3  along with a pack flow curve associated with the air cycle machine. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]      FIG. 1  shows a schematic diagram of a system  100  for operating an air cycle machine  102  in one embodiment. The air cycle machine  102  includes at least one rotating part allowed to rotate via an air bearing. A speed sensor  104  measures a rotational speed of the air cycle machine  102 , i.e., a rotational speed of the at least one rotating part or component. In one embodiment, the speed sensor  104  measures the rotational speed at a rate of about 20 times per second (20 Hertz). A processor  106  receives the measurements of the rotational speed and estimates or calculates a rate of change of the rotational speed from the received measurements. The rate of change of the rotational speed may be an acceleration or a deceleration of the at least one rotating part. In one embodiment, the calculated rate of change of the rotational speed is compared to a selected rate threshold. When the calculated rate of change of the rotational speed is greater than the selected rate threshold, the processor  106  may shut down or turn off the air cycle machine  102  in order to prevent further wear to the air cycle machine  102 . The processor may provide a message at display  108  indicate that the air cycle machine  108  has been shut down. In various embodiments, the rate of change of the rotational speed is a deceleration of the component, and comparing the deceleration to the selected rate threshold refers to comparing a magnitude of the deceleration to a magnitude of the selected rate threshold. In one embodiment, the selected rate threshold may be about 1000 rpm/sec. 
         [0010]      FIG. 2  shows speeds and deceleration rates for a rotating component of a properly operating air cycle machine  102  that is not expected to experience a failure in a near future. The component speed is indicated by speed curve  202  and is in units of revolutions per minute (rpm). The rate of change of the rotational rate of the component is indicated by acceleration curve  204  and is in units or revolutions per minute per second (rpm/sec). Time is shown along the abscissa. The speed curve  202  shows normal (i.e., non-erratic) variation consistent with proper operation of the air cycle machine  102 . The acceleration curve  204  shows that the acceleration/deceleration of the component is generally small, i.e., less than the selected rate threshold. In a few instances, the acceleration curve  204  exhibits spikes, as shown at spikes  206 ,  208  and  210 , for example. The spikes face downward, indicating a deceleration of the component. Acceleration is present in operation of the air cycle machine  102  but is not shown in  FIG. 2 . 
         [0011]      FIG. 3  shows speeds and deceleration rates of the rotational component for a failing air cycle machine  102 . The component speed is indicated by speed curve  302  (in rpm) and the rate of change of the component is indicated by acceleration curve  304  (in rpm/sec). Time is measured along the abscissa. The speed curve  302  exhibits erratic fluctuations  306  in a time period indicate by interval  320 . In the same time interval  320 , the acceleration curve  304  displays multiple spikes  308 ,  310  indicating acceleration/deceleration. The spikes  308 ,  310  indicate sudden deceleration of the component of the air cycle machine  102  as a result of frictional contact between components of the air cycle machine  102  due to momentary overload of the air bearing. In addition, the spikes  308 ,  310  are generally of greater magnitude than the spikes  206 ,  208  and  210  shown in  FIG. 2 . An impending failure of the component may be indicated by the deceleration spikes  308 ,  310  crossing the rate threshold. In one embodiment, the magnitude of the rate threshold may be about 1000 mm/sec. For illustrative purposes, the rate threshold is indicated by line  315 . Deceleration spikes  206 ,  208  exceed the rate threshold (i.e., drop below line  315 ) several times during time interval  320 . 
         [0012]    In one embodiment, the processor ( 106 ,  FIG. 1 ) determines a crossing count by counting a number of times the acceleration curve  304  crosses the rate threshold  315  (i.e., a “crossing”) in a downward direction as shown in  FIG. 3 . When the crossing count exceeds a crossing count threshold, the processor  106  may shut down the air cycle machine  102 . The crossing count threshold may be a preselected value. In another embodiment, the processor  106  determines the crossing count related to a selected time interval, such as time interval  320 . When the crossing count for the selected time interval exceeds the crossing count threshold, the air cycle machine  102  may be shut down. The selected time interval may be a moving time interval so that a crossing is dropped from the crossing count when the time since the crossing is greater than the time duration of the time interval. For example, for a time interval of 10 minutes, the crossing count includes all of the crossings in the previous 10 minutes, but not any crossings than occurred more than 10 minutes from the present time. 
         [0013]    Additionally, the processor ( 106 ,  FIG. 1 ) may maintain a count of a number of times that the rate of change of the rotational speed changes its sign (i.e., a “sign change count”) such as when the rate of change of the rotational speed switches from an acceleration to a deceleration or switches from deceleration to acceleration. The sign change count may be obtained over a selected interval such as time interval  320 . A sign change may be dropped from the sign change count when the time since the sign change is greater than the duration of the time interval  320 . The sign change count may be used along with the crossing count in order to shut down the air cycle machine  102 . In one embodiment, the air cycle machine  102  may be shut down when the crossing count exceeds the crossing count threshold. Alternatively, the air cycle machine  102  may be shut down when both the crossing count exceeds the crossing count threshold and the sign change count exceeds a sign change count threshold. In the example of  FIG. 3 , the air cycle machine  102  may be shut down at a time of about 12:26 in order to prevent experiencing a failure of the air cycle machine  102  that otherwise occurs at about 12:48. 
         [0014]      FIG. 4  shows the speed curve  302  and acceleration curve  304  of  FIG. 3  along with a pack flow curve  402  associated with the air cycle machine  102 . In one embodiment, the speed curve  302  can be compared to the pack flow  402  to determine whether the component speed changes as expected with the pack flow. For example, a faster than normal decrease in speed can be an indication of the air bearing damage. At times  404 ,  406  and  408 , the pack flow is constant while the component speed is changing, indicating that the air cycle machine  102  is not affecting the pack flow as expected. Such comparison between pack flow  402  and speed curve  302  may be used to indicate a failing air cycle machine  102 , which in turn may be used to shut down the air cycle machine  102  or at least to determine an operational state of the air cycle machine  102 . 
         [0015]    In addition to using the acceleration/deceleration curves, magnitudes and sign changes, one can also use a rate of change of the acceleration and/or deceleration to control the air cycle machine  102 . The processor  106  can compare the rate of change of the acceleration/deceleration to a selected threshold and shut down or turn off the air cycle machine  102  when the rate of change of the acceleration and/or deceleration exceeds the selected threshold. 
         [0016]    In various embodiments, the air cycle machine  102  may be shut down when the appropriate threshold or thresholds are exceeded. Alternately, a warning signal may be presented to an operator such as a pilot in order to allow the pilot to shut down the air cycle machine  102  manually. Once the air cycle machine  102  has been shut down, the pilot may be able to restart the air cycle machine  102 . 
         [0017]    While the invention has been described with respect to air cycle machines, the same bearing system and speed signatures can be seen with systems that use other gases or gas mixtures as bearing components. Such gases may include, but are not limited to, refrigerant gases such as Freon, for example. 
         [0018]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.