Abstract:
A device and method to provide backup electrical power to a vehicle data acquisition and recording system. The device operates to control its charging state, discharge state, and interval of discharge to ensure reliable operation of the recording system. The method delineates the process by which continuous electrical power can be supplied to the recording system.

Description:
This application claims priority from U.S. Provisional Application Serial No. 60/149,008 filed on Aug. 13, 1999 and is a continuation-in-part of and claims priority from Ser. No. 09/054,059 filed Apr. 2, 1998, now U.S. Pat. No. 6,153,720, titled “Data and Cockpit Voice Recorder Enclosure”, the entire specifications of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed toward a backup power supply for a crash-survivable data memory unit that is capable of providing continued operation of the device upon loss of electrical power. 
     The Federal Aviation Administration requires that most commercial aircraft be equipped with a crash-survivable flight data recorder (FDR) and cockpit voice recorder (CVR). Such systems collect vital flight information that provides evidence for the reconstruction and analysis of a subsequent accident. The FDR and CVR is usually powered from a vital electrical bus to ensure operation in all foreseeable operating configurations. 
     There have been several aircraft accidents in which FDR and CVR recording was discontinued prior to the crash, due to a loss of some or all electrical systems in the aircraft. In these cases, the National Transportation and Safety Board (NTSB) noted that continued operation of either recorder during the power outage could have provided accident investigators with valuable information regarding the cause of the crash. 
     In its Mar. 9, 1999 letter to the FAA, the NTSB has recommended that, starting in 2005, all aircraft that are required to carry CVRs and FDRs be fitted with an independent power source that is located with the CVR, and automatically engages to provide ten minutes of operation whenever aircraft power to the recorder ceases, either by normal shutdown or through an electrical bus failure/loss. In addition, the NTSB has recommended that all aircraft manufactured after Jan. 1, 2003 be equipped with dual combination (CVR/FDR) recording systems, each powered from a separate, reliable aircraft power supply. Each requirement is an attempt to ensure that FDR and CVR data is recorded during aircraft power outages. There are no systems in the present art, however, that meet the first recommendation, and the second recommendation would be ineffective in collection of data in the event of a total aircraft loss of power. 
     Backup power sources for FDRs and CVRs have not been previously developed for several reasons. First, older FDRs and CVRs, many of which are still in the field, store data on a continuously-driven magnetic tape. The electrical power required to operate such tape drives, even for a short period of time, requires large and heavy power storage devices that are infeasible for aircraft. Second, a power backup to the CVR and/or FDR could have negative consequences if power to the recorder continues after a crash, allowing it to overwrite vital data in doing so. Finally, the FDR can continue to receive useful data during a power loss only if the sensors supplying it also retain power. The complexity and variation in modern aircraft electrical systems prevents a feasible supply of power to the sensors. Because the CVR and FDR power problems are linked, a division of the backup power supplies for them was deemed an incomplete solution to the overall problem. For at least these reasons, backup power supplies for the FDR and CVR have not been mandated by the NTSB, nor have they been installed on aircraft. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and utilizes recent developments in several fields of invention to make possible a backup power supply for vehicle data acquisition and recording systems. For example, traditional magnetic tape storage units have been largely replaced by low-power solid state memory devices. Thus, the size of a backup power storage device that will supply the required amount of power is much smaller, making it feasible for installation in aircraft. In addition, the development of lithium batteries has reduced the size and weight of batteries for a given energy storage requirement. These aspects combined with new smaller and lighter FDR/CVR designs, as described in U.S. Pat. No. 6,153,720 and incorporated herein, make it feasible to incorporate an additional backup power supply into an FDR or CVR as taught by the present invention. 
     According to one aspect of the present invention, the problems of supplying backup power to FDRs and CVRs are identified by analyzing the specific and different power needs of FDRs and CVRs. Aspects of FDR and CVR operation, during startup, operating, shutdown, accident phase, and post-crash phase are defined. According to another aspect of the invention, a system that ensures reliability of power and continuity of important data acquisition throughout each of the aforementioned phases is described, as summarized in the following paragraphs. 
     According to yet another aspect of the present invention, a device is disclosed that provides backup electrical power for a vehicle data acquisition and recording system. The device includes a charging control circuit, a discharge control circuit, and a power storage element. The invention receives charging power from an external vehicle electrical bus, and maintains a continuity of power to the recording system in the event of a loss of said bus. 
     According to still another aspect of the present invention, a method is disclosed for providing backup electrical power for a vehicle data acquisition and recording system, by delineating a process in which a vehicle electrical bus is monitored for power availability, and the system operates either in charging or discharging mode depending on the status of the bus. In addition, a process step includes monitoring the duration of a discharge cycle, and causes a discontinuation of power to the recording system after a pre-determined interval, to ensure that vital data is not over-written after an accident. 
     These and other aspects of the present invention are described further in the detailed description of the preferred embodiments of the invention which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a prior art flight data recording system. 
     FIG. 2 illustrates a cockpit voice recording system, with portions to be supplied by backup electrical power supply enclosed within the dashed line, according to one preferred embodiment of the present invention. 
     FIG. 3 illustrates an embodiment of the backup electrical power supply in which the backup source of power is stored in a battery. 
     FIG. 4 illustrates an embodiment of the backup electrical power supply in which the backup source of power is stored in a capacitor. 
     FIG. 5 is a graphical representation of the time versus supply voltage curves for determining the required electrical energy storage capacity of a capacitor, based on the power requirements of a typical cockpit voice recorder system for a period of ten minutes, as determined according to the teachings of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The backup power supply invention enables continued operation of a FDR or CVR following an event which otherwise would prevent operation of the recording system. In the past, there have occurred several accidents in which continued operation of either recorder would possibly have provided post-crash investigators with valuable additional information. 
     Events which pose threats to the power supply for the FDR or CVR also threaten the aircraft wiring which connects the recorder to the signal sources providing the requisite information to be recorded. Therefore, any attempt to provide for continued operation of a flight recorder in a situation which jeopardizes the aircraft must consider both the power and the signal source interconnections. 
     Feasibility Considerations For the FDR 
     FIG. 1 provides an illustration of a typical prior art aircraft flight data recording system. The flight data recorder  10  is a single component of a data acquisition and recording system that is widely distributed in an overall aircraft system. The recording system includes individual sensors that provide voltage signals  12  representing a wide variety of activities and digital links  18  to other aircraft systems such as a digital air data computer (not shown). The information is gathered and formatted into a predetermined digital representation by a digital flight data acquisition unit (DFDAU)  16  which is usually a separate line replaceable unit (LRU) centrally located for ease of interconnection. 
     In the most modern digital aircraft systems, the function of DFDAU  16  may be performed within some of the individual elements of the larger system outside of DFDAU  16 . Formatted digital signals  18  from those elements can be collected by DFDAU  16  for further consolidation and processing. The formatted digital representation  14  of the aircraft information is then sent to the digital flight data recorder  10 , which serves as the crash protected memory element of the entire data recording system. The recorded data  20  is then re-transmitted to DFDAU  16  for integrity checking. 
     For an uninterruptible power scheme to be effective for the data recording system, then, it is necessary to ensure continuity of power to a number of sensors and other aircraft avionics in addition to the FDR. The elements of the flight recording system are inter-connected with an extensive, complex network of aircraft wiring. For the system to continue useful operation during a serious casualty situation, it may be unlikely that the system wiring integrity can be maintained, either because of system damage or as a result of tripping of circuit breakers by the flight crew during emergency troubleshooting. Providing continuous power to an FDR may cause more harm than good by overwriting useful data with null values from sensors in-activated by the emergency. Thus, since backup power for all FDR-related systems is infeasible, it is also not desirable to provide backup power solely to the FDR. 
     Feasibility Considerations For The CVR 
     Compared to the flight data recording system, the cockpit voice recorder (CVR) system is more self-contained. As illustrated in FIG. 2, the CVR system is comprised of a cockpit voice recorder  30 , a subsystem for the cockpit area microphone(CAM) consisting of a CAM preamplifier/control panel  32  and a remote CAM  34 , and the aircraft audio control system  36  for pilot, co-pilot, and public address/flight engineer audio signals  38 . The CAM subsystem receives power  40  directly from the CVR  30 . A backup power supply for CVR  30  thus also provides the CAM subsystem a high probability of continued operation during an extensive aircraft electrical system failure. In addition, if the CVR is located in the forward section of the aircraft close to the cockpit, the length of wire necessary to connect the CAM subsystem to the CVR is minimized, improving electrical system robustness to the CAM/CVR combination during emergencies. For the other components of the system, namely aircraft audio control system  36 , a design complication arises similar to that discussed in relation to FDR. Namely, the involvement of other aircraft systems whose function is not specifically related to the CVR adds sufficient complexity to reduce the likelihood of successful continued operation. 
     Many modern control systems and flight parameter indicators are powered electrically, and therefore a flight crew may experience a complete loss of aircraft control in a serious electrical system failure in-flight. Hence, the most important data to be recorded during such an emergency may be a record of the crew&#39;s cockpit voice conversation as it troubleshoots the system and attempts to restore electrical power. A preferred embodiment of the present invention provides a backup power supply for the CVR and CAM systems. 
     Independent Power Sources 
     There are at least two alternative embodiments for a backup power source capable of supplying sufficient electrical energy to enable the CVR to continue operation for 10 minutes following the loss of main aircraft power. FIGS. 3 and 4 illustrate each approach. 
     The first embodiment, a battery-powered system, is illustrated in FIG.  3 . This embodiment includes a battery  50  for energy storage, preferably a 28-volt lithium cell. Available batteries typically supply considerably more energy, of the order of 10 Amp-Hours, than is required for this application, so an element to disconnect the battery after the specified operational duration must be included. Thus a timer element  54  is included within the discharge control circuit  52 . A charging control circuit  55  functions to sense the state of the battery charge and to control battery recharging. Diodes  56 ,  57  and  58  permit the interconnection of the aircraft DC power source  60 , the battery backup invention, and the CVR system. 
     The battery backup system functions as follows. When the aircraft electrical system is initialized, aircraft DC power source  60  provides power to the CVR and to the battery backup system. Power source  60  charges battery  50  through diode  56  and charging control circuit  55 . Charging control circuit  55  is configured to bring the battery to a charge state sufficient to operate the CVR system for a specified period of time, which, in a preferred embodiment, is 10 minutes. AC preferred charging time to bring the battery to the sufficient charge state is 30 minutes, which is the shortest time expected between aircraft startup and departure from the gate area. After battery  50  is fully charged, charging control circuit  55  maintains the battery charge state, and may provide additional functionality to improve battery life. During normal operation, CVR power is supplied by aircraft DC power source  60  through diode  57 . 
     In the event of an in-flight electrical power loss, battery  50  automatically provides backup power to the CVR system through discharge control circuit  52  and diode  58 . Diodes  56  and  57  prevent battery power back-flow to the remainder of the aircraft power bus. Discharge control circuit  52  regulates current flow for protection and also includes a timing element  54  which automatically discontinues backup power to the CVR after a pre-determined period of about ten minutes. This feature prevents inadvertent overwriting of null data into the CVR after an accident. 
     When aircraft power is secured under normal circumstances, discharge control circuit  52  activates in the same manner as described for emergency situations. The backup power system is then ready for re-initiation. 
     The second embodiment of the invention is shown in FIG.  4 . This embodiment employs a high-capacity capacitor  60  which stores electrical energy during normal operation. When main power is removed, the capacitor discharges to the CVR to provide backup power. 
     The desired capacitance can easily be determined, and is shown for nominal conditions in FIG.  5 . As illustrated in curve  101  of FIG. 5, a 28 Farad capacitor can supply a required 10 watt power for about 10 minutes before voltage drops below the 16 volt minimum threshold for a typical CVR. The capacitor can be more easily sized such that the CVR will shut off on undervoltage near the specified backup period, so no timing circuit is required within discharge control circuit  62 . A charging control circuit  65  senses the state of the capacitor charge and controls capacitor recharging. Diodes  66 ,  67  and  68  permit the interconnection of the aircraft DC power source  60 , the capacitor backup invention, and the CVR system. 
     The capacitor backup system functions as follows. When the aircraft electrical system is initialized, aircraft DC power source  60  provides power to the CVR and to the capacitor backup system. Power source  60  charges the capacitor  61  through diode  66  and charging control circuit  65 . Charging control circuit  65  is configured to bring capacitor  61  to a charge state sufficient to operate the CVR system for a specified period of time, which, in a preferred embodiment, is 10 minutes. A preferred charging time to bring capacitor  61  to the sufficient charge state is 30 minutes, which is the shortest time expected between aircraft startup and departure from the gate area. After capacitor  61  is fully charged, charging control circuit  65  will maintain the capacitor charge state. During normal operation, CVR power will be supplied by aircraft DC power source  60  through diode  67 . 
     In the event of an in-flight electrical power loss, capacitor  61  automatically provides backup power to the CVR system through discharge control circuit  62  and diode  68 . Diodes  66  and  67  prevent capacitor power back-flow to the remainder of the aircraft power bus. Discharge control circuit  62  regulates current flow for protection until the capacitor voltage falls below the minimum CVR operating voltage, after a pre-determined period of about ten minutes. This feature prevents inadvertent overwriting of null data into the CVR after an accident. 
     When aircraft power is secured under normal circumstances, discharge control circuit  62  activates in the same manner as described for emergency situations. The backup power system is then ready for re-initiation. 
     Typical operating requirements for an independent power source for the CVR according to one embodiment of the invention, are shown in Table 1. For some entries, suggested tolerance ranges are indicated. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical Specifications of the Preferred Embodiment 
               
             
          
           
               
                   
                 Requirement 
                 Specification 
               
               
                   
                   
               
               
                   
                 Duration of continued 
                 10 minutes (10≦ t ≦ 12 
               
               
                   
                 operation 
                 minutes) 
               
               
                   
                 Voltage 
                 Greater than the minimum 
               
               
                   
                   
                 operating voltage for the 
               
               
                   
                   
                 CVR for the duration. 
               
               
                   
                 Charging time 
                 &lt;30 minutes {The system 
               
               
                   
                   
                 is preferably capable of 
               
               
                   
                   
                 providing the required 
               
               
                   
                   
                 function from the actual 
               
               
                   
                   
                 departure from the 
               
               
                   
                   
                 originating terminal gate 
               
               
                   
                   
                 until the safe arrival at 
               
               
                   
                   
                 the destination terminal 
               
               
                   
                   
                 gate.} 
               
               
                   
                 Isolation from aircraft 
                 During charging, must not 
               
               
                   
                 systems 
                 interfere with the 
               
               
                   
                   
                 continued proper operation 
               
               
                   
                   
                 of other LRU supplied from 
               
               
                   
                   
                 the same bus; during 
               
               
                   
                   
                 discharge, preferably 
               
               
                   
                   
                 supplies only the CVR. 
               
               
                   
                 Maintenance 
                 Minimal 
               
               
                   
                 Interconnection 
                 The installation pre- 
               
               
                   
                   
                 ferably minimizes the risk 
               
               
                   
                   
                 of interconnection failure 
               
               
                   
                   
                 during hazards to the 
               
               
                   
                   
                 aircraft. 
               
               
                   
                 Prevention of accidental 
                 The system should 
               
               
                   
                 erasure 
                 preferably be installed to 
               
               
                   
                   
                 prevent continued 
               
               
                   
                   
                 recording should the CVR 
               
               
                   
                   
                 become separated from the 
               
               
                   
                   
                 aircraft. 
               
               
                   
                   
               
             
          
         
       
     
     Table 2 lists a number of design considerations and provides a relative indication of the complexity of the two independent power options discussed above. The information provided is intended only as a qualitative indication of the relative standing of the two options. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Comparison of Battery and Capacitor Sources 
               
             
          
           
               
                   
                 Item of Comparison 
                 Capacitor 
                 Battery 
               
               
                   
                   
               
               
                   
                 Cost 
                 − 
                 + 
               
               
                   
                 Charging circuit 
                 − 
                 + 
               
               
                   
                 complexity 
               
               
                   
                 Discharging circuit 
                 + 
                 − 
               
               
                   
                 complexity 
               
               
                   
                 Weight 
                 − 
                 + 
               
               
                   
                 Volume 
                 − 
                 + 
               
               
                   
                 Maintainability 
                 + 
                 − 
               
               
                   
                 Reliability 
                 − 
                 − 
               
               
                   
                 Maintenance hazard 
                 − 
                 + 
               
               
                   
                   
               
               
                   
                 + indicates a relatively favorable characteristic.  
               
             
          
         
       
     
     Vehicle Interface 
     Supplying the FDR or CVR with an independent power source capable of sustaining proper operation following the removal of the main vehicle power requires that the device be supplied from a DC power bus. For large transport aircraft, FDRs and CVRs have normally been supplied with 115 VAC/400 Hz AC power. One alternative solution to operating with an aircraft ac power source instead of a dc power source includes the addition of a dc-ac converting element to discharge control circuit  52 / 62  and eliminating diode  57 / 67 , for the battery supply embodiment and the capacitor supply embodiment respectively. Another alternative solution to operating with an aircraft ac power source is the addition of an ac-dc rectifier element between the ac power source and diodes  56 / 66  and  57 / 67 , when utilizing a dc-powered CVR or FDR. 
     To achieve the maximum probability of continued operation of the CVR during a hazardous condition, the equipment should preferably be located as close to the cockpit as possible. Thus, a third alternative solution to the ac power source utilizes a small, lightweight, DC powered CVR mounted directly in the cockpit, with a power connection to the main electrical system only. An embodiment of this embodiment of 
     CVR is described in U.S. Pat. No. 6,153,720. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Variations and modifications will be readily apparent to those of ordinary skill in the art. The invention is defined by the following claims, with equivalents of the claims to be included therein.