brake. This observation eliminated two of the three branches of the fault tree, leaving only the failures  listed under “Brake in Open Condition”. No tilting of the friction plate was apparent and nothing unusual  was observed in the brake assembly. The only fa ilure remaining that could be determined without  disassembly was “Residual Magnetic Field Holding Brake Open”. While still under X-ray examination, a  reverse polarity voltage was slowly applied to the brak e starting at 0 volts, a demagnetizing action for the  friction plate. At negative 0.3 volts, the friction plate moved to the engaged position against the motor  rotor indicating the failure was caused by residual magnetism holding the friction plate against the  solenoid even though a non-magnetic washer was in the a ssembly to prevent this particular failure. The  motor stalled when operated again with the brake unpowered proving that the demagnetized brake was  now fully mechanically engaged. Residual magnetism was the source of the failure in this LPA! 

Assessment of Other LPAs 

A survey of the acceptance test data for all L PA brake assemblies was performed as a consequence of  the SN 007 LPA brake failure. Particular attention wa s given to the pull-in and drop-out voltages for the  brake assemblies since these measurements are an i ndication of the electromechanical performance of  the units. With no voltage applied to the brake, the fric tion plate is pressed against the brake rotor by the  compression springs. Pull-in voltage is measured by slow ly raising the brake voltage from zero volts until  the friction plate is pulled in to the solenoid, me chanically disengaging from the brake rotor and permitting  the motor to turn when the motor is powered with its drive electronics. Figure 7 illustrates the force  balance for pull-in. Increasing the voltage across the br ake coil causes the current in the solenoid to  increase. As the current increases, the magnitude of the magnetic field increases thereby increasing the  magnetic force, F M, on the friction plate.  The two fo rces that act in opposition to F M are F S, the total forc e  from the 6 compression springs, and F f, the friction force between the 3 guide pins and the friction plate.  When the magnetic force exceeds the sum of the spring and friction forces, or 

  (1) f S M FF F+>

the friction plate moves away from the brake roto r and toward the solenoid, mechanically disengaging the  brake. Once motion starts, the brake plate moves quickly open since FM increases much faster than F S as  the air gap decreases. F M is a squared function of air gap while F S is a linear function. 

Drop-out Voltage  Pull-in Voltage 

 Friction Plate mov es w hen  FM > F S + F f Friction Plate mov es w hen  FS > F M + F f  FM = magnetic force   FS = total s pring forc e Ff = friction forc e  Brake Rotor  Solenoid i V Ff FS FM Brake Rotor  Solenoid i V Ff FS FM  Figure 7.  Force balance for brake pull-in and drop-out v oltages  

Drop-out voltage is then measured by slowly lowering the voltage until the friction plate releases from the  solenoid, mechanically engaging the brake rotor again. Figure 7 illustrates the force balance for drop-out.  Decreasing the voltage across the solenoid reduces its current and therefor e the magnitude of the  226