Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to vehicle recording systems, such as aircraft cockpit voice recorders; and more particularly to backup power supplies which enable such recording systems to operate in the absence of primary power in the vehicle. 
         [0005]    2. Description of the Related Art 
         [0006]    The United States Federal Aviation Administration (FAA) requires that most commercial aircraft be equipped with a crash-survivable cockpit voice recorder (CVR). This recorder collects vital flight information that provides evidence for the reconstruction and analysis in the event of an accident. The CVR is usually powered from an electrical bus of the aircraft. 
         [0007]    During an accident, the aircraft may lose electrical power that normally supplies the CVR, thereby causing this device to terminate recording critical information long before the culmination of the accident. As a result, the FAA regulations now mandate that all aircraft, that are required to carry CVRs, be fitted with an independent power source that is located with the recorder and that activates automatically to power the recorder for ten minutes plus or minus one minute of operation, whenever the primary aircraft power is unavailable, either due to manual shutdown or electrical system failure. 
         [0008]    Existing backup power supplies store electrical energy in either a battery or a capacitor bank. Capacitors tend to be preferred as being more able to be repeatedly cycled through charge and deep discharge operations without significantly affecting the life of that storage device. 
         [0009]    Nevertheless, the capacitance of large capacitor banks decreases over time due to the applied voltage level, as well as temperature to which they are subjected. The voltage level has a significant impact on the rate at which the capacitance decreases. For example, an individual capacitor that is rated for 2.5 volts can experience a 20% capacitance reduction, if continuously charged to 2.5 volts for two years. Then again, that same capacitor, if only charged to 2.1 volts can last twenty years at that same temperature. Therefore, it is desirable to maintain the voltage on the capacitor at as low a level as possible to provide sufficient backup power to meet the FAA ten minute requirement. 
       SUMMARY OF THE INVENTION 
       [0010]    A backup power supply has a capacitor and provides electrical power to a consuming device, such as an aircraft audio recorder, when power from a primary source is unavailable. It often is desired that the backup power supply be capable of powering the consuming device for a given period of time after loss of power from the primary source. 
         [0011]    The capacitor is charged to a given level. Thereafter, the capacitor is discharged into the consuming device while measuring the amount of time during which the discharging occurs. Upon completion of discharging the capacitor, a target energy level is defined in response to the amount of time. 
         [0012]    When charging of the capacitor commences again, an amount of charge energy that is stored in the capacitor is measured, such as periodically for example. The capacitor charging terminates when the measured amount of charge energy reaches the target energy level. Because the target energy level is defined based on the length of time that the capacitor discharged, the target energy level preferably was established so that the capacitor is sufficiently charged to ensure that the backup power supply is capable of powering the consuming device for the given period of time. 
         [0013]    In a preferred implementation of this method, the amount of energy that is discharged from the capacitor also is measured. Here, the target energy level also is defined as a function of the amount of that discharged energy. For example, the target energy is defined as the amount of discharge energy adjusted based on deviation of the discharge time from a reference time value. The reference value may be the given period of time desired that the backup power supply be able to furnish power to the consuming device. This ensures that the capacitor will be sufficiently charged to power the consuming device for the given period of time, without overcharging the capacitor which may decrease its useful life. 
         [0014]    An aspect of the present invention is that the energy level to which the capacitor is charged is adjusted based on the temperature proximate to the backup power supply. Preferably the energy level is decreases in proportion to an amount that the temperature exceeds a threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic block diagram of a backup power supply connected to an aircraft cockpit voice recorder; 
           [0016]      FIG. 2  is a flowchart of a software interrupt routine executed by a processor in the backup power supply to sample current and voltage levels; 
           [0017]      FIGS. 3A , B, C, D, E and F form a flowchart of a software program for controlling the charging and discharging of the backup power supply; 
           [0018]      FIG. 4  is a flowchart of a subroutine called by the software program at the start of a capacitor charge mode; and 
           [0019]      FIG. 5  is a flowchart of a subroutine called by the software program at the start of a capacitor discharge mode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    With initial reference to  FIG. 1 , a backup power supply  10  is provided to furnish electrical power to a cockpit voice recorder (CVR)  12  when current from the aircraft electrical system on DC supply bus  14  is unavailable. Unavailability of the supply bus current can result from an electrical failure during an emergency or the electrical circuits in the aircraft being shut down by the flight crew, as normally occurs after the final flight of a day. Normally the aircraft electrical current is applied from aircraft power supply bus  14  through a diode  16  to the CVR  12 , and specifically to an internal power supply circuit  18  therein. Although the backup power supply  10  is being described in the context of powering a cockpit voice recorder, the inventive concepts may be applied to backup power supplies for powering other types of equipment, such as fight data recorders. 
         [0021]    An aircraft electrical current also is applied to a terminal  20  of the backup power supply  10  and charges a storage device within the backup power supply  10 , as will be described. In the event that power from the aircraft power supply bus  14  is unavailable, the backup power supply  10  furnishes current via terminal  20  to power the CVR  12 . 
         [0022]    The backup power supply  10  includes a capacitor  22  that stores electrical energy for subsequent use in powering the CVR  12  when current from the aircraft power supply bus is unavailable. The storage device is represented by a single capacitor  22  with the understanding that a bank of capacitors typically will be employed to provide sufficient electrical storage capacity. The term “capacitor”, as used herein, covers not only a single capacitor, but a plurality of capacitors connected to function together in storing and thereafter supplying electrical energy. 
         [0023]    The backup power supply  10  includes a charge/discharge control circuit  24  which controls the application of electrical voltage from the aircraft power supply bus   14  to the capacitor  22  and also controls the discharge of energy from that capacitor through terminal  20  of the backup power supply  10 . The charge/discharge control circuit  24  includes a converter that employs conventional pulse width modulation to convert the voltage level on aircraft power supply bus  14  to a desired voltage level for charging the capacitor and, during a power bus failure, to convert the capacitor voltage to a desired level for powering the CVR  12 . The supply voltage V S  and current Is flowing between the capacitor  22  and the backup power supply terminal  20  are measured by a current and voltage monitor  26 . Operation of the backup power supply  10  and specifically the charge/discharge control circuit  24  is governed by a microcontroller  28  in response to that measured current and voltage. In particular, the microcontroller has an output that is connected to a charge and discharge circuit to command that latter component to connect the capacitor  22  to the terminal  20  and to define operation of the PWM converter. The microcontroller  28  executes a software control program that governs operation of the charge/discharge control circuit  24  in response to signals received at various inputs of the microcontroller. The microcontroller receives the supply voltage V S  and current I S  measurements provided by the current and voltage monitor  26  and also directly senses the voltage V C  across the capacitor  22  via input  23  to an internal analog to digital converter. A temperature sensor  29  provides the microcontroller  28  with a signal that indicates the temperature of the capacitor  22 . 
         [0024]    The microcontroller  28  is a microcomputer based device that includes a memory for storing a software program, that governs the operation of the backup power supply  10 , and for storing data used and generated by that program. 
         [0025]    In addition to executing the control program that governs the overall operation of the backup power supply  10 , the microcontroller  28  therein also executes an interrupt routine that samples data for use by the control program. With reference to  FIG. 2 , the interrupt routine  30  is executed periodically based on a software interrupt timer executed by the microcontroller  28 . For example, the interrupt routine  30  waits at step  32  for ten milliseconds, at the expiration of which, the levels of the supply voltage V S  and the supply current I S  at the power supply terminal  20 , as measured by the current and voltage monitor  26 , are read and stored at step  34 . At that time, the voltage V C  across the capacitor  22  is also read and stored by the microcontroller. 
         [0026]    Interrupt routine then advances to step  36 , at which a determination is made whether the microcontroller  28  is commanding the charge/discharge control circuit  24  to operate in the Discharge mode in which voltage from the capacitor  22  is applied to the power supply terminal  20 . If that is the case, the interrupt routine  30  returns immediately to step  32  to delay for another sampling period. Otherwise the execution branches to step  38  where the microcontroller examines the power supply voltage V S , which should be greater than the minimum aircraft voltage, e.g., 18.5 volts, of the aircraft power supply bus  14 . If substantially the nominal aircraft power supply bus voltage is found, the interrupt routine returns to step  32 . If significantly less that that nominal voltage exists, a power failure is declared at step  38  and at step  39 , the backup power supply operating mode is set to Discharge and a subroutine entitled “Start Discharge” is called. That subroutine will be described in detail subsequently. Thus, the interrupt route  30  periodically reads the supply voltage and current and the capacitor voltage and store those readings for use by the control program. 
         [0027]    The control program  40  executed by the microcontroller  28  commences at step  42  on the flow chart of  FIG. 3A , at which different variables used by the control program are initialized. As will be described, some of those variables have values set and stored in a non-volatile memory of the microcontroller by a previous execution of the control program, however, during the first execution of that program after installation, a set of default values are read from the non-volatile memory. Thereafter at step  44 , a variable that indicates the operating mode of the backup power supply  10  is set to the Initial mode. 
         [0028]    At step  46 , further execution of the control program is delayed for twenty sampling periods of the timed interrupt routine  30  in order to acquire twenty sets of voltage and current samples from the backup power supply. Next at step  48 , the twenty sets of samples of each of the supply current I S , supply voltage V S , and capacitor voltage V C  are individually averaged to produce average values that then are used as representing those electrical parameters. In addition, each pair of supply voltage and supply current samples are multiplied to produce a power measurement and then the resultant twenty power measurements are averaged to produce an average power value Ps for subsequent use. Next at step  50 , a charge energy ratio (EFR) is calculated according to the expression: 
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         [0000]    where V MIN  is the minimum capacitor voltage, e.g. 0.7 volts, at which satisfactory operation of the CVR  12  can occur, and V C  FULL is a previously defined target charge voltage for a fully charged capacitor  22 , which value was read from memory at step  34  when the control program  40  commenced. The capacitor energy ratio EFR indicates the amount that the capacitor  22  is presently charged as a proportion of its full charge. 
         [0029]    At step  52 , the microcontroller  28  reads the temperature value produced by the temperature sensor  29 . The FAA regulations requires that the backup power supply  10  only has to be able to power the CVR  12  at temperatures less than 70° C. Above that temperature, the backup power supply is not required to be functional. Nevertheless, the present backup power supply  10  provides short term backup power capability for operating temperatures above 70° C. Assume that the maximum temperature to which the capacitor  22  can be exposed and function satisfactorily is 85° C., which serves as an upper limit for the backup power supply operation. The amount of the backup power supply&#39;s capability is determined at step  54  by a desired value (EFRD) for the capacitor charge energy ratio. Specifically, if the present temperature is less than 70° C., then the value of EFRD is set equal to one, however, when the temperature is in excess of 85° C., the value of EFRD is set equal to zero. In between those two temperatures, the desired charge energy ratio EFRD is set to a proportion that the temperature is above the 70° C. level by the expression: 
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         [0000]    where TEMP is the temperature of the capacitor as sensed at step  52 . Then at step  56 , the control program  40  branches to a program section for the present operating mode of the backup power supply  10  as indicated by the MODE variable. 
         [0030]    Operation commences in the Initial mode  57  which is shown in  FIG. 3B . The Initial mode occurs only until the voltage on the aircraft power supply bus  14  reaches its minimum operating level, which in this exemplary system is 18.5 volts, as sensed at step  58 . As long as the aircraft power supply bus is below that voltage level, the Initial mode  57  returns to step  46  on  FIG. 3A . The looping through steps  46 - 58  continues until the aircraft power supply bus  14  reaches the minimum voltage level, at which time, the mode variable is set to the Charge mode and the Start Charge subroutine is called at step  59 . 
         [0031]    The Start Charge subroutine  60 , depicted by the flowchart in  FIG. 4 , initializes the charge/discharge control circuit  24  to charge the capacitor  22  with the aircraft power supply bus voltage applied to the power supply terminal  20 . At step  61 , a determination is made whether the capacitor voltage V C  is less than 0.75 volts, which is the minimum capacitor voltage at which the CVR  12  can operate. If so, a variable designated “Charge_Full is set true and the capacitor energy level variable E C  is set equal to zero at step  62 . Otherwise, when the capacitor voltage is greater than or equal to 0.75 volts, the Charge_Full variable is set true at step  64  and at step  65 , the energy E C  stored in the capacitor is calculated according to the expression: 
         [0000]        E   C =0.559* C*V   C   2   *V   MIN   2    (3) 
         [0000]    where C is the capacitance of the capacitor  22 , V C  is the capacitor voltage, and V MIN  is the minimum capacitor voltage, e.g. 0.7 volts, at which the CVR  12  can operate. 
         [0032]    Then, the Start Charge subroutine  60  advances to step  66  where a decision is made whether the capacitor voltage is greater than the maximum voltage rating of the capacitor, e.g. 2.5 volts. If so, the capacitor energy E C  is stored as a value E C SAVE, a variable designated V C SAVE is equal to 2.5 volts, and the operating mode is set to “Sustain” at step  67 . If the capacitor voltage is not greater than 2.5 volts at step  66 , a branch occurs to step  68  at which the charge/discharge control circuit  24  is initialized. This is accomplished by computing a pulse width modulation (PWM) duty cycle and a timer period for the control circuit converter in the charging state. The Start Charge subroutine  60  then returns to step  59  of the Initial mode  57  ( FIG. 3B ) from which the subroutine was called. 
         [0033]    Thereafter, the program execution returns to step  46  in  FIG. 3A  after which steps  46 - 54  are executed again. Now, upon reaching the branch to mode step  56 , the control program enters the Charge mode as that mode was previously set at step  59  in the Initial mode  57 . 
         [0034]    The Charge mode  70  commences at step  71  on  FIG. 3C  where a variable designated DONE is set false to indicate that charging has not been completed. Then at step  72 , a determination is made whether the present capacitor voltage V C  is greater than the minimum capacitor level V MIN  required for operation of the CVR  12 . If so, a value corresponding to the incremental amount of energy applied to the capacitor  22  during the last sample period is added to a value E C  denoting the total energy on that capacitor and the updated total capacitor energy value E C  is stored at step  74 . 
         [0035]    Thereafter at step  76 , a decision is made whether the present capacitor voltage V C  is greater than the maximum voltage level to which the capacitor  22  can be charged (e.g. 2.5 volts). If so, at step  77  the DONE variable is set true, the Charge_Full variable is set false, and a variable V C FULL, designating the target charge voltage for a fully charged capacitor  22 , is set equal to that maximum voltage level before advancing to step  78 . Otherwise if at step  76 , the capacitor voltage V C  is found to be less than the maximum allowable voltage, the program execution branches to step  80  where is determination is made whether the desired energy ratio EFRD is equal to one, which occurs when there is not an over temperature condition (i.e. the capacitor is at less than 70° C.). 
         [0036]    If an over temperature occurred, the program branches to step  82  where a decision is made whether the present charge energy ratio EFR is greater than the desired ratio EFRD. When that condition exists, the DONE variable is set true at step  83  before the program advances to step  78 . 
         [0037]    On the other hand, when at step  80  the desired energy ratio EFRD is found equal to one, the program branches to step  84 . At that point, a determination is made whether the newly updated capacitor energy level E C  is greater than a variable E C FULL, that designates the target charge energy level to which to charge the capacitor  22 . That target charge energy level is set during the Discharge mode of the backup power supply, as will be described, or by a stored default value if the discharge mode has never been used. If the newly calculated capacitor energy level E C  is not greater than the target charge energy level E C FULL, further charging is required and the program jumps to step  78 . Otherwise, the capacitor  22  is fully charged and at step  85  the DONE variable is set to true and the present voltage level V C  across the capacitor  22  is stored as the target charge voltage level V C FULL for a fully charged capacitor. The execution then advances to step  78 . 
         [0038]    At step  78 , a decision is made whether the DONE variable is true. In that event, the program execution advances to step  86  at which the converter within the charge/discharge control circuit  24  is turned off, a variable designated V C SAVE is set equal to the present capacitor voltage V C , the present capacitor energy level E C  is stored as a variable E C SAVE, and the operating mode is set to Sustain. Otherwise if at step  78  the charging is not completed, the program execution branches to step  88  where a new PWM duty cycle and timer period for the charge/discharge control circuit  24  in the Charge mode are computed and sent by the microcontroller  28  to that circuit. Then from either step  86  or  88 , the control program returns to step  46  on  FIG. 3A . 
         [0039]    From there, steps  46 - 56  are executed again. If the backup power supply  10  is still in the Charge mode, the program execution will then branch again from step  56  to step  70  in  FIG. 3C , thereby repeating the charging loop over and over again until the capacitor is fully charged. When that occurs, the operating mode will be changed to Sustain at step  86  which causes the program execution to subsequently branch from step  56  in  FIG. 3A  to the Sustain mode. 
         [0040]    With reference to  FIG. 3D , the Sustain mode  90  maintains the capacitor  22  at the fully charged state during operation of the aircraft. To determine whether the charge remains at the desired level, the most recently computed charge energy ratio EFR is compared to the desired ratio EFRD at step  91 . If that comparison indicates that the capacitor  22  is not overcharged, the execution advances to step  92  at which another comparison occurs to learn whether the capacitor charge is below the desired level, and if so, the program advances to step  94  where a check is made whether the converter within the charge/discharge control circuit  24  is already turned on. If not, step  95  is executed to set the proper PWM duty cycle and time period for recharging the capacitor to the desired level and then activate the converter. Alternatively, if the converter is already found to be on at step  94 , the program branches to step  96  where the PWM duty cycle and timer period are recalculated and fed to the control circuit  24 . Upon completion of the steps of the Sustain mode  90 , the execution jumps back to step  46 . 
         [0041]    Returning to step  92  in the Sustain mode, if the present capacitor charge energy ratio (EFR) is not found less than the desired ratio, in essence equal to that desired ratio because of the prior decision at step  90 , the program branches to step  97 . There a check is made whether the converter is on, in which case it is turned off at step  98  and the Sustain mode routine ends by returning to step  46 . 
         [0042]    The control program  40  remains in the Sustain mode until power on the aircraft bus  14  is lost, at which time the interrupt routine  30  changes the operation to the Discharge mode. In another situation the mode changes when the capacitor is determined to be overcharged at step  91 . In that event, the Sustain mode branches to step  99  where the operating mode is changed to the Bleed mode before returning to step  46  in  FIG. 3A . At that time, steps  46 - 54  are executed again, but upon reaching step  56 , a branch occurs to the Bleed mode to cure the overcharge condition. 
         [0043]    With reference to  FIG. 3E , the Bleed mode  100  commences at step  102  where based on the capacitor energy charge ratios, a determination is made whether the capacitor remains in an overcharged state. If so, the converter within the charge/discharge control circuit  24  is turned on at step  104  to discharge the capacitor  22  into the CVR  12  even though power still is present on the aircraft power supply bus  14 . When the capacitor returns to the desired charge level, a branch in the bleed mode occurs from step  102  to step  106  where the converter is turned off, terminating the capacitor discharge, before returning to step  42  in the main part of the control program shown on  FIG. 3A . 
         [0044]    If the interrupt routine  30  in  FIG. 2  detects a failure of power on the aircraft power supply bus  14 , the operating mode to Discharge and the Start Discharge subroutine is called to initialize the charge/discharge control circuit  24  to discharge the capacitor  22 . The Start Discharge subroutine  110 , depicted in  FIG. 5 , commences at step  112  where a timer, implemented in software by the microcontroller  28 , is set to zero and started to measure the length of time that the backup power supply powers the CVR  12 . Then at step  114 , the charge/discharge control circuit  24  is configured for the discharge operation by computing the appropriate PWM duty cycle to provide 6.8 volts at the power supply terminal  20 . That voltage level is the nominal voltage required to operate the cockpit voice recorder  12 . Then at step  116 , the operating mode of the backup power supply  10  is set to Discharge before returning to step  32  in the sampling interrupt routine of  FIG. 2 . This not only initiates the backup power supply  10  for discharge operation, but also configures the control program  40  so that the next time the mode branch step  56  is executed, a transition to the Discharge mode will occur. 
         [0045]    With reference to  FIG. 3F , when the control program  40  branches to the Discharge mode  120 , the operation commences at step  121  by the microcontroller  28  comparing the recently sensed supply voltage V S  to the nominal voltage on the aircraft power supply bus  14 , e.g., 18.5 volts. It should be understood that in the Discharge mode, the backup power supply  10  only applies the minimum voltage level (e.g., 6.5 volts) to the power supply terminal  20  that is required to power the CVR  12 . Therefore, at this time, if the voltage level V S  at the power supply terminal  20  approximately 18.5 volts, the microcontroller  28  knows that voltage on the aircraft power supply bus  14  has been restored. If that is the case, the backup power supply  10  is placed into the Charge mode to recharge the storage capacitor  22 . That is accomplished by branching to step  122  at which the converter within the charge/discharge control circuit  24  is turned off to terminate the Discharge operation. Then at step  124 , the operating mode is set to Charge and the Start Charge subroutine is called. Thereafter the program jumps to step  46  in the main section from step  56  of which a branch then will occur to the Charge mode so that the capacitor can be recharged in the manner previously described. 
         [0046]    If, however, at step  121 , the inspection of the voltage V S  at the power supply terminal  20  indicates that the aircraft bus power has not been restored, the Discharge mode branches to step  126 . At this junction, a determination is made whether the power supply current I S  is greater than a maximum safe level, which indicates that either a short circuit is present or the CVR  12  has placed an unusually large load on the backup power supply  10 . In either situation, the execution branches to step  128  at which the converter within the charge/discharge control circuit  24  is turned off and the operating mode is set to the Initial mode before jumping back to step  46 . 
         [0047]    Assuming that an overload condition does not exist at step  126 , the control program branches to step  130  at which a decision is made whether the capacitor voltage V C  has dropped below the minimum level, e.g. 0.7 volts, at which the backup power supply can satisfactorily operate. Dropping below that voltage level indicates that the backup power supply can no longer satisfactorily power the recorder. If that is not the case, a branch occurs to step  132  at which the PWM duty cycle and timer period for the converter within the charge/discharge control circuit  24  are updated based on the sensed voltages and current. The Discharge mode then returns to step  46  to perform another loop through its operation. 
         [0048]    Alternatively, if at step  130  the voltage V C  being supplied by the backup power supply  10  is found to be below the minimum voltage level, step  134  is executed to terminate the Discharge mode because the capacitor  22  is too depleted to power the cockpit voice recorder  12  adequately. At this junction, a determination is made whether a valid discharge operation occurred. In other words, a decision is made whether the capacitor was fully charged at the commencement of the Discharge mode, the discharge proceeded in a normal manner, and that an over temperature condition did not occurred. When a valid discharge transpired, the program executes step  136  to compute the amount of energy that was furnished by the capacitor  22  in the Discharge mode, otherwise the program jumps to step  139 . To compute the amount of energy discharged from the capacitor, the microcontroller obtains the value from the software timer that indicates how long the capacitor charge was able keep the CVR  12  operating. That amount of time in seconds is divided into the target amount of time (e.g. 600 seconds) that the backup power supply  10  is desired to power the CVR  12 . This calculates the proportion of the desired period that the capacitor actually supplied power. That proportional value then is multiplied by the target charge energy level E C FULL to which the capacitor had been charged previously, that is before the Discharge mode commenced. The arithmetic result produced by step  136  is a new value for the target charge energy level E C FULL to which the capacitor should be charged. This energy level is employed at step  84  in the Charge mode to determine when the charge on the capacitor has reached a level that can power the CVR  12  for the desired length of time during a failure of the aircraft power supply bus  14 . Then at step  138 , the new target charge energy level E C FULL and the target charge voltage level V C FULL, desired for a fully charged capacitor, are stored in the non-volatile memory of the microcontroller  28 . Thereafter, the converter within the charge/discharge control circuit  24  is turned off at step  139  and the operating mode is set to the Initial mode before returning to step  46  of the control program. 
         [0049]    Therefore every time the aircraft is shut down at the end of operation, the backup power supply capacitor  22  is discharged into the CVR  12  and the amount of time that the CVR continues to operate in that Discharge mode is measured along with the discharge energy that is supplied. If the backup power supply  10  does not continue to operate the CVR for approximately the FAA mandated time period, the target charge energy E C FULL and target charge voltage V C FULL are adjusted accordingly for use in charging the capacitor  22  the next time the aircraft is powered up. This ensures that the backup power supply  10  will be sufficiently charged to power the CVR  12  for at least the FAA mandated operating time without excessively exceeding that time interval which over time adversely affects the life of the capacitor. This dynamic adjustment of the target energy level to which the capacitor  22  is to be charged compensates for changes in the capacitor due to age and other effects. 
         [0050]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.

Technology Category: y