Patent Publication Number: US-6341238-B1

Title: Robust engine variable vane monitor logic

Description:
TECHNICAL FIELD 
     This invention is directed to a control system for verifying the proper operation of aircraft engine/surface actuator controls, and more particularly, to a control system for verifying the proper operation of a control associated with the inlet guide vanes (IGVCS) of an aircraft engine. 
     BACKGROUND ART 
     Increased demands for improved aircraft performance and reliability have resulted in the development of electronic controllers having multiple levels of redundant channels. These redundant channels can consist of redundant electronic channels and/or mechanical channels. In order to utilize the reliability benefits of multi-channel systems, various hardware and software built-in test (BIT) methodologies have been developed to provide high levels of fault coverage. 
     More specifically, for some modern aircraft, the inlet guide vane control system (IGVCS) has a dual lane architecture comprised of an electronic primary lane and a hydro-mechanical secondary lane. The IGVCS controls the inlet guide vane position as a function of engine speed adjusted for air inlet temperature. The electronic primary lane consists of two redundant channels, an active and a standby channel. The hydro-mechanical secondary lane serves as a back-up in the event of a failure of the two electronic channels. 
     In order to ensure proper operation of the IGVCS there are several levels of fault coverage provided by the BIT diagnostics. The first level is referred to as “In-Line Built-In Test ” (ILBIT). The ILBIT of the electronic primary lane includes signal range checks, processor checks, memory checks, output wrap around checks, etc., to isolate any faults within a specific channel. 
     The second level of fault coverage is provided by a cross channel comparison of the processed inputs and outputs of the active and standby channels to detect failures not detected by the ILBIT (XCHBIT). The second level is not capable of isolating the failure to one of the electronic channels and therefore must switch control from the electronic primary lane to the hydro-mechanical secondary lane. 
     A third level of fault coverage serves as a “last line of defense” against primary lane failure by utilizing the difference between simulated vane position, predicted in real time, and actual vane position to detect system failures which are not detectable by the ILBIT and XCHBIT. This difference is then evaluated using a Pass/Fail criteria to determine whether the actuator is tracking properly. This logic is present in both channels of the primary control lane so that if a failure occurs which causes the actuator position to track incorrectly, either channel has the capability to detect this and initiate a switch over to the secondary control lane. 
     The problem with the present art is that any tolerance which meets the above described criteria is not necessarily robust in the presence of external vane aerodynamic disturbances. In particular, this third level of coverage cannot distinguish between an actual fault and a naturally occurring aerodynamic disturbance such as surge or stall, resulting in erroneous transfer to the secondary/backup, reducing the reliability of the system by incorrectly concluding a system fault existed. For some engines, the probability of occurrence of a surge condition is much higher than the probability of failure of the vane hardware. 
     When transfer occurs a fault is indicated and maintenance action is required. Nuisance faults caused by false transfers result in wasted time and monetary resources and limits aircraft availability. 
     Therefore, there exists a need, for a fault detection system that can distinguish between actual system failures and temporary system disturbances, thus reducing system false alarm rates. 
     DISCLOSURE OF INVENTION 
     The primary object of this invention is to provide an improved control system for verifying the proper operation of aircraft engine/surface actuator controls. 
     Another object of this invention is to provide an improved methodology which can distinguish between actual system faults and temporary system disturbances. 
     A further object of this invention is to reduce aircraft maintenance time associated with nuisance faults. 
     Still another object of this invention is to provide BIT diagnostics for an IGVCS utilizing filtering of a comparison between a simulated inlet guide vane position and an actual inlet guide vane position to prevent transfer of control from the primary to secondary lane in the event of a false alarm, without compromising the ability to detect, in a timely manner, system faults and the appropriate corrective action. 
     The foregoing objects and following advantages are achieved by the test method of the present invention for distinguishing between actual system failures and temporary system disturbance not caused by failures. 
     The method includes the steps of initializing the primary system upon engine start; reading system inputs, calculating an ideal system output, reading the actual aircraft engine/surface actuation control position set by the primary control, calculating the difference between the ideal system output and actual control position setting, comparing the difference to a severe disturbance threshold, switching system control to the secondary/backup system if the difference exceeds the severe disturbance threshold, if the difference is less than the severe disturbance threshold, comparing the difference to a mild disturbance threshold and switching control to the secondary/backup system if the difference exceeds the mild disturbance threshold for greater than a predetermined time period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of one system with which the method of the present invention is used ; 
     FIG. 2 is a flowchart of the subject method; and 
     FIG. 3 is a graph of IGV angle difference over time. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to the drawings in detail, there is shown in FIG. 1 a schematic diagram of one system with which the method of the present invention is used, designated generally as  10 . System  10  includes a primary electronic lane  12 , hereinafter primary lane, and a secondary hydro-mechanical lane  15 , hereinafter secondary lane, used for controlling the position of inlet guide vanes (IGV)  16  of an engine (not shown). 
     The primary lane  12  includes the IGV digital electronic control (IDEC)  18 , which consists of redundant channels A  19  and B  21 , and the hydro-mechanical unit (HMU)  14  exclusive of the secondary lane  15 . The HMU  14  provides hydraulic force actuation for IGV  16  positioning during primary lane  12  operation and both control and actuation during secondary lane  15  operation. The method of the present invention prevents a false error determination from transferring control from the primary lane  12  to the secondary lane  15 . 
     The primary lane  12  consists of an inlet guide vane digital electronic control (IDEC)  18 , electro-hydraulic servo valve  20  to command IGV  16  position via IGV position actuator  27 . The primary lane  12  schedules IGV  16  position as a function of engine speed (NH) corrected for engine inlet air temperature (T 1 ). Temperature at the engine inlet is provided by RTD probe  23  while engine speed is provided by magnetic speed sensor  24 , sensing the speed of shaft  30  which is driven by the engine. 
     The transfer valve  25  is responsive to a command from the primary lane  12  for selecting whether the primary lane  12  or secondary lane  15  controls IGV  16  position. In addition, an IGV position resolver  22  provides IGV  16  position feedback to the primary lane  12  via bus  26 . Finally, a permanent magnet alternator (PMA)  28 , located in the HMU  16 , provides power to the IDEC  18 . The PMA  28  is driven by shaft  30 . 
     The secondary lane  15  comprises a hydro-mechanical control  32  which in combination with IGV actuator  27  provides hydraulic force actuation for positioning of the inlet guide vane  16  during the secondary lane  15  operation. Secondary lane  15  senses engine air inlet temperature via a liquid bulb temperature device  34  and engine speed via a flyball actuator  36 , which is driven by shaft  30 . The secondary lane  15  continually schedules inlet guide vane position, even during operation and control by the primary electronic  12  lane, so as to maintain readiness in the event of primary lane  12  failure. However, secondary lane  15  remains isolated from IGV actuator  27  until power is removed from the transfer valve  25  due to a failure in the primary lane  12  or the primary lane  12  commands a switch to the secondary lane  15 . 
     The IDEC  18  of the primary electronic lane  12  has two operational channels and starts up automatically upon rotation of shaft  30  which causes PMA  28  to provide power to the IDEC  18 . The IDEC  18  takes control of system  10  by powering up either of the redundant channels  19  or  21  and providing an electrical signal to the transfer control valve  25 . The dual channel architecture operates in an active/standby mode such that one channel is active and the other is in standby. If a failure occurs in one of the channels, an automatic transfer to the standby channel is made. In the case where both channels fail, or if a fault is detected which cannot be isolated to either channel, control is automatically transferred to the secondary lane  15  by removal of power or by command to the transfer control valve  25 . 
     In the preferred embodiment the BIT software is resident in the IDEC  18  but may be located in other portions of the control. The method of the present invention is performed as part of the normal operation of the control. The method  100  is illustrated in the flow chart of FIG.  2 . As the engine starts shaft  30 , shown in FIG. 1 rotates in response to engine rotation. At an engine speed of approximately 23.75% of full speed the PMA  28  will provide power to the IDEC  18  and the IDEC  18  will initialize. Channel A  19  or channel B  21  will begin to control IGV  16 . For the discussions here it is assumed that channel A  19  is controlling. Therefore, channel A will energize the transfer valve  25  and electro-hydraulic valve  20  to place the primary lane in control of the IGV  16  and set the IGV  16  to an initial angle. At a speed equal to 31.25%, IDEC  18  initialization is complete. 
     As shown in S 1 , the primary control reads in T 1 , NH. The actual IGV angle commanded by the primary lane  12  is also read from IGV position resolver  22 . The BIT software then calculates an ideal IGV angle as shown in S 2 . The difference between IGV actual and IGV ideal is then calculated as shown in S 3 . S 4  illustrates application of a first filter. The IGV difference is compared to a severe disturbance threshold  38 . This threshold represents a disturbance that cannot be safely tolerated by the system. If IGV difference exceeds the severe disturbance threshold  38  the IDEC  18  switches control of the IGV  16  from the primary lane  12  to the secondary lane  15  as shown in step S 5 . 
     If IGV difference is less than the severe disturbance threshold  38 , then IGV difference is compared to the mild disturbance threshold  40  as shown in S 6  of FIG. 2 . The mild disturbance threshold  40  is an out of tolerance condition that can be tolerated by the system for a finite period of time. Such a disturbance might be a surge condition due to pilot action or sudden change in the environment. If IGV difference is greater than the mild disturbance threshold  40  then the duration of the disturbance is tracked. In S 7  the counter is queried to determine if it has already been set indicating an ongoing disturbance. If the counter is not set, as in a newly detected disturbance, the counter is set as shown in S 8 , and control returns to S 1 . If the counter is already set, the duration of the disturbance is determined as in S 9 . 
     If the value of the counter exceeds the acceptable duration for a mild system disturbance condition then the IDEC  18  switches command from the primary lane  12  to the secondary lane  15  as illustrated in S 10  and the test is complete. If the counter is less than the maximum acceptable duration, then the program returns to S 1  and the process is repeated. 
     If at any time prior to switching of control from the primary lane  12  to the secondary lane  15 , the IGV difference is less than the mild disturbance threshold  40  of S 6 , the counter is reset as shown in S 11  and the process continues at S 1 . 
     FIG. 3 illustrates several possible scenarios for system disturbances. Curve  1  illustrates a real system fault wherein the IGV difference is greater than the severe system disturbance threshold  38 . IGV  16  control is transferred from the primary lane  12  to the secondary lane  15 . Curves  2  and  3  show an IGV difference that is greater than the mild disturbance threshold  40  but less than the severe disturbance threshold  38 . The system will track the time duration of the disturbance. If the IGV difference falls below the mild disturbance threshold  40  within a predetermined time, as in curve  2 , then control remains with the primary lane  12 . If however, IGV difference continues to exceed the mild disturbance threshold  40  as in curve  3 , then control is transferred from the primary lane  12  to the secondary lane  15 . By proper choice of the maximum duration the majority of the failures are represented by curve  2  and hence do not result in a false alarm. Maximizing the time allowed without causing hazardous or undesirable operation is the key to obtaining the benefit of the invention. Curve  3  represent some limited number of false alarms. However, the majority of false alarms is eliminated by selecting the proper maximum time duration. 
     Curve  4  represents a system disturbance that exceeds the severe disturbance threshold  38 . This disturbance is not a real fault as indicated by the recovery of the system. The system cannot distinguish between this failure and a real system failure and therefore control is transferred from the primary lane  12  to the secondary lane  15 . By proper adjustment of the mild and severe disturbance thresholds the occurrence of these false alarms is minimized. Curve  5  represents a failure detected and accommodated by the ILBIT and XCHBIT. The vanes remain within the mild disturbance threshold  40 . 
     The primary advantage of this invention is an improved method which can distinguish between actual system faults and temporary system disturbances. 
     Another advantage of this invention is that an improved system is provided for an IGVCS which utilizes filtering of a comparison between a simulated inlet guide vane position and an actual inlet guide vane position to improve system reliability. 
     A further advantage of this invention is that it reduces maintenance associated with nuisance faults and therefore increases aircraft availability. 
     Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions in the form and detail thereof may be made without departing from the spirit and scope of the invention.