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Final External Report 01-045
Revision 01-045
WFPC-2 Shutter-A
Position Sensing Error
Fault Isolation
Chris A. Long
February 13, 2001
Wide Field and Planetary Camera-2
I. Problem Overview....................................................................................................................................... 1
II. Fault Isolation............................................................................................................................................. 1
A. System Level Isolation........................................................................................................................... 2
1. External to Spacecraft ......................................................................................................................... 2
2. Spacecraft (excluding payload)........................................................................................................... 2
3. Science Instruments ............................................................................................................................ 3
B. WFPC2 System Level Isolation ............................................................................................................. 3
1. Clock I/F circuits................................................................................................................................. 3
2. Mechanism Power Supply .................................................................................................................. 3
3. Engineering Telemetry........................................................................................................................ 3
4. Microprocessor ................................................................................................................................... 3
5. LED Drive Electronics........................................................................................................................ 3
6. Sensor-side Components..................................................................................................................... 3
C. Possible Faults on the Shutter Assembly................................................................................................ 4
1. Description.......................................................................................................................................... 4
2. List of Faults ....................................................................................................................................... 4
III. Targeted Fault Isolation ............................................................................................................................ 5
A. Mechanical Degradation/ Detent Translation......................................................................................... 5
1. Method of Elimination ........................................................................................................................ 5
2. Measuring Sensor-to-Encoder Alignment........................................................................................... 5
3. Sensor Alignment Evaluation ............................................................................................................. 7
B. Mechanical Degradation/ Detent Play.................................................................................................... 8
1. Method of Elimination ........................................................................................................................ 8
2. Detecting Mechanical Play ................................................................................................................. 8
C. Hard Mechanical Failures ...................................................................................................................... 9
1. Types of Failure .................................................................................................................................. 9
2. Alignment Offset ................................................................................................................................ 9
3. Blade Not in Detent .......................................................................................................................... 10
D. Evaluate Electrical Degradation........................................................................................................... 11
1. LED Degraded Output ...................................................................................................................... 11
2. Phototransistor Degraded Sensitivity................................................................................................ 12
3. Combined Effects in Sensor Package ............................................................................................... 12
E. Evaluate Hard Electrical Failure........................................................................................................... 12
1. LED Drive Resistors ......................................................................................................................... 12
IV. Remaining Faults.................................................................................................................................... 12
A. Evaluate Remaining Faults .................................................................................................................. 12
1. Sensor Package Degradation............................................................................................................. 12
2. Open Resistor in LED Drive Path..................................................................................................... 13
3. Sticktion near Open Detent ............................................................................................................... 13
V. Monitoring & Risk Reduction.................................................................................................................. 14
VI. Future Efforts.......................................................................................................................................... 14
VII. Conclusions ........................................................................................................................................... 15
VIII. Acknowledgements.............................................................................................................................. 15
IX. Appendix A: Interpolated Time Variance.................................................................................................ii
A. Description.............................................................................................................................................ii
1. Data Manipulation ..............................................................................................................................ii
2. Plots ....................................................................................................................................................ii
X. Appendix B: Interpolated and Speed Flag Timing..................................................................................... v
A. Description............................................................................................................................................. v
1. Data Manipulation .............................................................................................................................. v
2. Plots .................................................................................................................................................... v
XI. References .............................................................................................................................................xiii
WFPC-2 Shutter-A
Position Sensing Error
Fault Isolation
Chris A. Long
Space Telescope Science Institute
Baltimore, Maryland
February 13, 2001
The Wide Field Planetary Camera II science instrument reported shutter errors in the
summer and fall of 2000. No further errors have occurred since the RAM patch
installation to advance the sensor turn-on time. Reduced photon coupling in the shutter
position sensor has been identified as the cause of the errors. Fault analysis has limited
the list of possible problems to the degradation of two electrical position-sensing
components, an open resistor, or an occasional offset of the shutter blade position. No
health and safety risks have been identified.
I. Problem Overview
The Wide Field Planetary Camera 2 (WFPC-2) issued an error message on August 31, 2000 (day 244).
The message content of 0688066816 indicated that both shutter blades reported their position as "closed"
during the pre-move check at Real-Time Interrupt (RTI) 1640i. The shutter's mechanical design makes it
impossible for both blades to actually be in the closed position. The instrument recognized that the shutter
condition was unknown and aborted commanding to open the shutter.
The science data shows that the shutter was closed during the exposure. When the exposure reached it's
scheduled end time the microprocessor commanded blade-A to close so that the shutter would enter a
known condition. During this move the blade-A time-of-flight was higher than normal showing that the
blade did close and that it was pushing blade-B open in the process.
The Goddard Space Flight Center (GSFC) investigation revealed that the errors began earlier than the
first one received at the console. Telemetry recorded previous errors on August 21 (234) and August 30
(243). Subsequent errors occurred on October 2 (276), 11 (285), 25 (299), 26 (300), 29 (303), 30 (304),
and November 1 (306). WFPC-2 was safed on November 1 and a change to the flight software was
installed before recovering the instrument on November 7. The software change (RAM patch) turns on the
blade position sensors earlier in the process thereby increasing the time between energizing the
LED/photodiode package and reading the blade position. Since this change there has been no further
errors.
An early evaluation of the problem led to the conclusion that the errors could be the result of
mechanical degradation or failure. Certain failures in this category could lead to instrument damage unless
identified and precluded by operational changes so the STScI Engineering Team deemed an in-depth
analysis necessary.
II. Fault Isolation
This report describes the effort to answer the following questions:
• What caused the error on August 31, 2000 (day 244)?
• Is the cause of this error a health and safety concern?
In a nutshell the health and safety boils down to a question of whether the cause is electrical or
mechanical. If mechanical there may be a health and safety concern but if electrical, probably not. The
analysis will largely be driven by the effort to eliminate one or the other.
1
A. System Level Isolation
Each spacecraft system and outside systems that are related will be considered for the possibility
that they could cause the error message on day 244.
1. External to Spacecraft
a) Environment (non-thermal): The anomaly with 13 consecutive exposures reporting an error
rules out a SEU problem. Had there been a SEU the blade position would have been correct on
subsequent exposures. No other environmental possibilities have been identified.
b) TDRSS: The only TDRSS function is passing data. Ruled out as with SIC+DH.
c) White Sands: The only White Sands function is passing data. Ruled out as with SIC+DH.
d) Ground-system (PASS, CCS, TRANS, RPS2): Data in and out has been eliminated.
2. Spacecraft (excluding payload)
a) Transmitter: Data in and out has been eliminated.
b) Safing: SMAC-20 is the only connection and is not relevant in this case.
c) Thermal: Thermal cannot cause the error messages directly.
d) PCS: WFPC-2 and PCS have no direct connection.
e) EPS (PCU fault): Power is doing it's job, providing power
f) 486: The TDF is not polled at the time that the shutters are checked and an error generated.
There is nothing else in the PIT that WFPC2 checks and since this is the only communication
between the two the 486 is not the problem.
g) OTA: Photons are the only link.
h) SIC+DH/NSSC-1: Commanding to the WFPC2 must be correct because the command
sequences are occurring on time. Data coming out is not being corrupted because the science
data is showing real problems (blank exposures). Timing errors would be generating their own
problems so timing isn't the issue. Science data readout through the SIC+DH can be discounted.
i) DMS: Ruled out because the only communications path is through the 486 and it has been
eliminated.
Cal Flip
Mirror
Mechanism
HST WFPC-2
OTA
Image
SDF
Bus A
SDF
Bus B
F/24
Pickoff Mirror
Mechanism
UV Cal Lamps
Pyr. Lamps
SOFA
Pyramid
Mirror
Shutter
Assy.
DSP/ADC
(Bay 4)
CH Elec.
(Ch 1)
CCD
(Ch 1)
CCD Window
(Ch 1)
PC Relay
Optics
(F/28.3)
PC Fold
Mirror
(F/28.3)
#1 AFM
DSP/ADC
(Bay 3)
CH Elec.
(Ch 2-4)
CCD
(Ch 2-4)
CCD Window
(Ch 2-4)
WFC Relay
Optics
(F/12.9))
WFC Fold
Mirrors
(F/12.9)
#2 Fixed
#3/4 AFMS
SDF Select
SDF A
Interface
Electronics
Clock A
Clock B
Clock I/F
Select
RIU C
Clock I/F
Circuit
AFM
Electronics
Microprocessor
PROM/RAM
TEC
(Ch 1-4)
Heat Pipes
(Ch 1-4)
Radiated
Heat to
Space
Radiator
Sup/Rpl
Bus B
RIU Pw r B
Sup/Rpl
Bus A
LEGEND
Optical
Thermal
Electrical
VIS Cal Lamps
OR
SDF A
Interface
Electronics
Cal Lamps
OR
RIU B
Logic
Temperature
Control
TEC PS
On/Off
Bay 1-5 &
Ch 1-4
RIU A
Eng Data
Mechanism
Drivers
Mechanism
PS
LVPS
Bay 5
Heaters
Repl. Htrs.
On/Off
Bay 1-4 &
Ch 1-4
Heaters
HP Htrs.
On/Off
HP 1-8
Heaters
RIU Pw r A
Pwr Bus
A
B
Relays/
Selects
Sensors
Mechanisms
& Lamps
EMI Filter
Box
Power
Select
Mech
PS Htrs
OR
On
Power
On/Off
Off
cal
Figure 1: WFPC-2 functional block diagram with suspected systems shown in red.
2
RIU
Serial
Cmds
Clock A
Figure 2: WFPC-2 systems not yet
eliminated from the list of possible
faults.
Clock B
Power
On/Off
Microprocessor
PROM/RAM
Clock I/F
Select
Clock I/F
Circuit
Logic
Shutter Enable
LVPS
Mechanism
PS
OR
Eng Data
A open/close
B open/close
Mechanism
Drivers
Solenoid drive
Failsafe drive
Mech PS Htrs
Shutter Assy.
+5V
Sensors
Sensor A
Sensor B
cal
3. Science Instruments
a) COSTAR: Only shares power with WFPC2 that would be seen as an EPS problem.
b) FOC: No direct connection to WFPC2.
c) NICMOS: Only shares power with WFPC2 that would be seen as an EPS problem.
d) STIS: No direct connection to WFPC2.
e) WFPC2
The conclusion of the system level analysis is that the only possible location for the cause of this
error is within the WFPC-2 instrument.
B. WFPC2 System Level Isolation
Most WFPC-2 systems can be eliminated on the basis that there is no interaction with those systems
generating the shutter error message. When the obvious sub-systems have been eliminated the
functional block diagram reduces as shown in figure 2. Those systems shown in red remain
candidates for harboring the fault.
1. Clock I/F circuits
a) The clock interface circuitii provides the timing from which microprocessor clocks are
derived. It also supplies the clocking for CCD readout. Since the clamp-to-sample time and
synchronization of WFPC2 electronics with the readout are confirmed to be correct by proper
imaging the clock interface circuits and upstream components are not the source of the problem.
2. Mechanism Power Supply
a) Providing the shutter drive pulse is the only mechanism power supplyiii function related to
shutter operation. Mechanism power is applied to the shutter assemblyiv through connector
M3A1/P1. There is a connection to the encoder circuit board but only as a place to mount the
anti-kickback diodes. The error under consideration occurs during shutter movement setup
before the move is commanded and a drive pulse is needed so the mechanism power supply and
upstream OR circuit cannot be the source of the problem. The Low Voltage Power Supply
cannot yet be eliminated as it has a separate connection to the sensors.
3. Engineering Telemetry
a) The shutter position error occurs prior to any related engineering telemetry collection. As
such engineering telemetry cannot be the source of the problem.
4. Microprocessor
a) All physical constructs within the microprocessor are shared by many functions that would
independently generate errors should a failure occur. Unique to blade-A are small code
segments and RAM space. The code is verified to be good by ongoing checksum testing and the
RAM by memory tests so the microprocessor is not the source of the problem.
5. LED Drive Electronics
a) The sensor electronics schematic is shown in figure 3. Except for two resistors for each LED
the drive electronicsiii power the LEDs for both blades in parallel. The blade-B sensor package
has been working properly and therefore all components may be eliminated as a problem except
for the resistors unique to the A-side, R5 and R105. This also eliminates the logic circuits,
mechanism drivers, and power supplies associated with commanding the LED circuit.
6. Sensor-side Components
a) Failure of the sensor noise reduction componentsiii R8 or C2 will affect both phototransistors.
R9 failure will result in the output fixed at either the high or low rail. The U1 buffering device
3
Figure 3: WFPC-2 shutter encoder electronics. Excerpt from dwg10093548.
must be working properly as it is too fast to "slow" the output signal and intermittent operation
would show up in blade position telemetry. The register G002v (sheet 4) signals and the
electronics to support them have already been eliminated since this is the same signal that drives
each LED. Both G002 and J008 must be working properly as good telemetry is passing through
this same path. This eliminates all receiver-side components and the supporting logic circuits
except for the A-side phototransistor itself.
The fault is now isolated to the shutter assembly.
C. Possible Faults on the Shutter Assembly
1. Description
a) The previous step concluded that only the shutter assembly remains a harbor for the fault.
The possible problems on the shutter assembly that could cause the errors are shown in figure 4
and listed here:
2. List of Faults
a) Mechanical
(1) Blade-A stop integrity.
(2) Encoder-to-Sensor alignment.
(3) Sensor-A contamination.
(4) Shutter-A drive mount integrity.
(5) Magnetic detent integrity on drive-A.
(6) Shutter-A drive arm integrity.
(7) Shutter-A encoder arm integrity.
(8) Shutter-A keel integrity.
b) Electrical
(1) LED-A degradation.
(2) Phototransistor-A integrity.
(3) Sensor-A wiring.
(4) R5 or R105 open.
4
Electrical
(E1) LED-A degradation.
(E2) Phototransistor-A integrity.
(E3) Sensor-A wiring.
(E4) R5 or R105 open.
SHUTTER-A OPEN
(M1)
(M8)
(E4)
(E1) LED
Sensor
(E3) Package
(M7)
(E2) PT
(M6)
Mechanical
(M1) Blade-A stop integrity.
(M2) Encoder-to-Sensor alignment.
(M3) Sensor-A contamination.
(M4) Shutter-A drive mount integrity.
(M5) Magnetic detent integrity on drive-A.
(M6) Shutter-A drive arm integrity.
(M7) Shutter-A encoder arm integrity.
(M8) Shutter-A keel integrity.
N
S
(M4)
(M2)
(M3)
0
1
2
N
(M5) S
3
Blade
Open
4
5
6
cal
Figure 4: Possible faults on the shutter assembly.
III. Targeted Fault Isolation
Mechanical degradation can cause the error messages only by changing the encoder disk to sensor
alignment thereby interfering with the light passing through slot zero. As the system ages there will be
wear in various components that could cause the open detent position to slowly translate away from the
original rest position or the detent could become larger with the actual rest position become less well
defined.
A. Mechanical Degradation/ Detent Translation
1. Method of Elimination
a) The key to eliminating degradation that causes mechanical translation from the list of possible
failures is to prove that the encoder disk slot 0 to sensor package alignment remains unchanged.
To prove this:
(1) It will be shown that alignment changes can be measured with high precision.
(2) Such measurements will show that the alignment is unchanged.
b) Important note: The following argument is not commutative. If the position of the detent
changes it must cause a change in the interpolated time. However, it is not true that a change in
the interpolated time necessarily reflects a change in position!
2. Measuring Sensor-to-Encoder Alignment
a) WFPC2 telemetry provides timing measurements that allow the relative position of the sensor
package to encoder disk to be measured with significant precision. "Shutter A Interpolation"
measures the time from the start of the current Major Frame to the encoder disk slot 1 passing by
the LED. The only variable is encoder disk time-of-flight.
b) When opening the shutter the interpolated time includes the encoder time-of-flight from the
closed position to the slot 6 leading edge passing by the LED. Figure 6 shows this as the
5
Figure 5: Shutter-A encoder timeof-flight in milliseconds during
blade closing on day 244, 2000.
distance between the red (middle) and black (upper) lines. The closing interpolated time is the
time-of-flight from the open (red, middle line) to the slot 1 trailing edge (blue, lower).
c) Angular resolution of the encoder between slots 0 and 1 while opening.
(1) Figure 5 plots the angular position of the blade-A encoder versus elapsed time in
milliseconds for a blade opening shortly before the day 244, 2000 error message. The data
denoted by squares is from the speed flags and is well defined. The ∆t between zero and the
start of flag 1 is not directly known. This is the region of interpolated time that does not
have a defined start time. To complete the curve the data is fitted to the diamonds then the
interpolated ∆t is selected to meet the boundary condition that the starting point must be at
the origin.
(2) A 110ms drive pulse to the solenoid provides acceleration to the shutter blade throughout
the flight shown in figure 5. The angular velocity therefore increases throughout the flight.
The average angular velocity between slots 0 and 1 may be found using telemetry point
USHRAINT, "Shutter A Interpolation."
(3) The angular distance between slots 0 and 1 is 4° as shown in figure 7. When the encoder
has traveled this distance the LED will be aligned with the middle of slot 1. The exact
alignment between the LED and the slot when the sensor reads a "0" is unknown but will be
assumed to be 1° further, or 5° from the starting point. At this point the LED will only have
a small fraction of its area still exposed. A similar end point will be used for the speed flags.
(4) From figure 5 the time-of-flight is 11.7ms. The average angular velocity is then
ϖ 0→1 = ∆θ ∆t = 5°11.7ms = 0.43° / ms.
(5) The resolution of the "Shutter A Interpolation"
telemetry point USHRAINT is 40µs. At an angular
velocity of 0.43°/ms the distance traveled during 40µs is
∆θ 40 µs = 0.43° / ms × 40 µs = 0.017° .
(6) 0.017° is the angular motion that will occur during a
single increment in interpolated time. Figure 7 provides
that the slots travel on a radius of 0.8″ and each slot is
0.02″ widevi. The slot width in degrees is then
wslot =
w × 360° 0.02"×360°
=
= 1.43°
π × 2r
π × 2 × 0.8"
(7) Dividing the 1.43° slot width by 0.017° of angular
resolution yields 1/80th the distance across a single slot.
This is the minimum shift in encoder position that will
change the interpolated time.
Figure 6: Shutter-A encoder
in the open (blue) and close
(black) positions.
6
Figure 7: Mechanical layout of the
shutter-A encoder blade.
d) Encoder alignment resolution
(1) Interpolated time is able to detect positional changes as small as 0.017°.
(a) Note that interpolated time does not reflect the actual time-of-flight between slots 0
and 1 as the timer is started at the last Major Frame boundary. The 10ms timer will have
rolled over many times prior to capturing shutter motion, but since the motion is the only
variable in timing the data will report time-of-flight changes with great precision.
3. Sensor Alignment Evaluation
a) The mechanical issue in question is whether or not a change in the shutter-A encoder blade to
sensor alignment is blocking the LED light resulting in a closed reading when the blade is
actually open. The only speed flag data that is directly pertinent to this question is the shutter-A
interpolated time while the blade is closing. Since minute shifts in encoder to sensor alignment
will cause a change in interpolated time, interpolated time being stable will rule out all
mechanical degradation causing drift as the source of the problem.
b) Figure 8 shows the shutter-A interpolated time for microprocessor-controlled blade closing
between the installation of WFPC-2 and the first occurrence of a shutter error. Mechanical
degradation, or wear, that causes the open detent position to shift will result in a change to this
data. If the detent position shifts to the left (figure 4) the encoder blade will rotate
counterclockwise. For every 0.017° of rotation the interpolated time will increment by a 40µs
LSB. Similarly the interpolated time will decrement if the detent position shifts to the right.
Figure 8 clearly shows that the interpolated time has been stable over the span of the mission
Figure 8: Shutter-A interpolated
time for a microprocessor
controlled closing. The black line is
a 30-sample moving average filter.
The data spans from launch to the
1st shutter error.
7
Figure 9: Shutter-A closing
interpolated time variance from
launch to the first shutter error.
Data is biased to 11.7ms,
normalized, and filtered with a 30sample sliding average. Appendix
A contains information on shutterB and on the opening times.
indicating that the open detent position is unchanged within 1/80th of a slot width.
c) The stability of the interpolated time is sufficient to show that mechanical wear resulting in a
shift of the open detent is not the cause of the shutter errors. There is a related failure mode that
has not been ruled out. That is mechanical degradation that does not shift the position of the
detent but instead results in excessive play of the detent position. This possibility will be
evaluated in the next section.
B. Mechanical Degradation/ Detent Play
1. Method of Elimination
a) The key to eliminating degradation that causes mechanical play from the list of possible
failures is to prove that the encoder disk slot 0 to sensor package alignment is repeatable. To
prove this:
(1) It will be shown that alignment changes can be measured with high precision.
(2) Such measurements will show that the alignment repeatability is unchanged.
b) The argument for monitoring play in the detent is similar to using the interpolated time mean
to show that the detent position is unchanged. In this case the interpolated time variance will be
used to show that the detent position is repeatable. The use of variance in this manner is
meaningful but less rigorous than the connection between mean and position. As such the
conclusions drawn are more open to interpretation.
2. Detecting Mechanical Play
a) The telemetry used to evaluate the amount of play in the shutter-A interpolated time is
described in the previous section. The interpolated time measures the time-of-flight from the
open detent to the slot 1 trailing-edge (angular distance between the figure 6 red and blue lines).
b) If the detent position changes by 0.017°, or 1/80th of the slot width, the interpolated time will
increment so small uncertainties in the detent position will cause spreading of the interpolated
time. To say it another way, if the exact positioning of the blade in the open detent is not always
the same then the interpolated time will take on more values. Therefore if the detent is less well
defined the interpolated time variance will become greater.
c) The variance of the blade-A interpolated time has been evaluated for AP-17 commanded
openings throughout the mission. Among other things this measurement will indicate if blade-A
always rests in the same position when open. Figure 9 shows the normalized variance. To
normalize the data the interpolated time (shutter-A AP-17 closing) has been biased upward so
that the average value equals 11.7ms, the detent to slot 1 time-of-flight. The 20k points of
interpolated time are then divided by a 30-point moving average to take out the slope of the line.
A 30-point sliding variance is then applied to the result.
d) The variance does not show degrading performance throughout the mission. This is not
unexpected for the detent positioning. The detent position is most likely established by the
blade resting against the stop or the physical carbon steel detent resting against the drive
8
Figure 10: Shutter-A closing interpolated time including shutter errors with a 20-point sliding
average overlaid.
solenoid. In each case the permanent magnet on the drive arm should hold the shutter against a
well-defined stopping point so the detent position would remain repeatable and fixed.
e) The result of this analysis is that the detent position is not showing signs of increased
sloppiness or play. Combined with the detent position analysis of the previous section it can be
concluded that the shutter-A encoder alignment is unchanged from launch up to the start of the
shutter errors and therefore general mechanical degradation or wear can be ruled out as the
source of the problem.
f) Appendix B provides further details on the derivation of the normalized variance and shows
the results for all 4 detents: Shutters A and B both open and close.
C. Hard Mechanical Failures
The analyses in the previous sections eliminate slow mechanical degradation as a source of the
shutter errors. A mechanical fault coinciding with the onset of the errors is considered here.
Whatever the source, a mechanical issue can only cause the shutter errors in question by changing
the encoder-to-sensor alignment thereby imposing on the sensor light path. This type of problem
can be either:
1. Types of Failure
a) An alignment offset that partially blocks the light path reducing the effectiveness of the
position sensors resulting in a system that works most of the time with occasional failures.
b) A condition where blade-A does not always come to rest in the open detent.
2. Alignment Offset
a) An offset in the encoder alignment brought on by a hard, or rapid, failure of the shutter-A
assembly would be characterized by a step in the interpolated time at or near the first error. The
step would be a result of the change in angular distance between the open detent and the slot 1
trailing edge. Figure 10 is the same as figure 8 but is extended to include the interpolated time
data following the onset of shutter errors. The figure shows that there is a marked increase in
the interpolated time coinciding with the shutter errors. However, the magnitude of the shift is
very small in absolute terms.
9
b) The interpolated time while opening blade-A has
incremented by approximately 3 least significant bits. This
corresponds to a 120µs, or a 1%, increase in the time-offlight preceding slot 1. If the increase were due to the blade
starting farther to the left (figure 4) this shift would only
translate to an encoder rotation of 0.051°. Figure 11 depicts
the magnitude of this shift relative to the slot and sensor
dimensions (assuming the increase in interpolated time is
due to a change in encoder alignment). The errors were
occurring while the interpolated time had only changed by a
small fraction of that represented in figure 11. The small
physical shifts represented by the interpolated time change
Figure 11: Lost LED area
must raise doubt that a misalignment is responsible for the
corresponding to a 120µs
errors.
change in interpolated time.
c) For encoder flight following the interpolated time
measurement a series of point-to-point timing measurements are made and reported as speed
flags. The speed flags for blade-A closing also show changes that correspond to the changes in
interpolated time and can help to determine what is happening. Table 1 compares the average
time-of-flight for each speed flag before the onset of errors—July 2000—and then during
January 2001. Flags 1, 2, and 4 show a small increase in the average time-of-flight while flags 3
and 5 a small decrease. Careful examination reveals that the actual time-of-flights between each
slot is not changing but the varying outputs reflect a shift in the clock pulses that are intercepted
for each measurement. This data confirms that the time to travel to slot 1 has actually increased
as indicated by the interpolated time. Furthermore the data shows that the time to travel between
slots 1 and 6 is unchanged and therefore the angular velocity has not changed (within the 625µs
resolution of the speed flags). This may imply that the detent to slot 1 distance is increasing.
However, it must be noted that these are very small numbers and therefore the conclusions be
considered with some skepticism.
(1) One other interpretation of the delayed arrival at slot 1 is that the blade is actually
moving slower early in flight then recovers. If so then the slowing must be due to
phenomena localized to beginning of flight such as increased friction just in this region.
3. Blade Not in Detent
a) The final possibility for mechanical fault is that the shutter blade does not always end up in
the open detent. When the blade rests in the detent the position is reported as open and the
exposure proceeds normally. On the occasion when the blade is out of the detent the encoder is
partially blocking the sensor light path making the blade report closed and an error is generated.
The blockage would have to be partial, not full, since the RAM patch has eliminated the errors.
b) A pulse is used to drive the blade to the open position. The pulse remains on even after the
stop is reached thereby "sticking" the blade to the end of travel. Even if the blade bounces
against the stop the drive pulse will simply push it back to the end. Therefore the blade will be
sitting still against the stop when the drive pulse is removed. The blade will stay in this position
due to the attraction of a permanent magnet in the drive arm assembly. The position of the blade
will not change in the absence of a disturbing force.
c) Between the times that blade-A is established in the open detent and when the error in
Table 1: Shutter-A closing speed flag
averages during July 2000 and January
2001.
Telemetry
Flag 1
Flag 2
Flag 3
Flag 4
Flag 5
Resolution
625µs
625µs
625µs
625µs
625µs
TInterpolated
39µs
Average ∆t (ms)
Jul '00 Jan '01
15.686
15.693
11.294
11.407
9.412
9.379
8.157
8.600
7.529
6.970
52.078
52.049
0.21183
0.35409
10
measured position occurs a disturbance
force is always applied to the blade. This
occurs when blade-B is closed and hits
blade-A at the end of travel. As long as the
blade-B closed detent position is resting
against blade-A then the earlier logic
applies here and the B drive pulse will force
both blades against the stop. The blade-B
closing pulse will remain on while both
blades settle against the stops and blade-A
will again be in the open detent. However,
A. H. Johnston, JPL
if the blade-B springs back after impact and
comes to rest separated from blade-A then
Figure 12: LED light output as a function of
blade-A would have to rely on the
proton fluence. Reference (viii).
permanent magnet to be returned to the
open detent. The latter is the case that occurs with the WFPC-1 shutter as recorded during
testing at GSFC in fall of 2000. The video shows that the strength of the permanent magnet is
well in excess of that needed to restore blade-A to the detent with a normal system.
d) The force provided by the permanent magnet to pull the shutter into detent is much less than
the solenoid drive force. It is possible that sticktion1 in blade-A near the detent could prevent
the permanent magnet from restoring the detent position.
e) Following an error the blades are moved in a default sequence that involves one blade
pushing the other. Due to this the timing data that would indicate the blade position during an
error is altered by the unusual shutter operation.
f) The WFPC-1 shutter-A does get knocked out of the open detent when blade-B closes and it
must rely on the permanent magnet to restore proper positioning. It is likely that the WFPC-2
shutter experiences the same situation and conceivable that frictional changes near the detent
could cause shutter-A to stick outside of the detent. Coupled with the lack of position
information this possibility has yet to be ruled out.
D. Evaluate Electrical Degradation
Performance degradation of two components on the shutter assembly could cause the shutter errors,
the shutter-A position sensor Light Emitting Diode (LED) and phototransistor. Shutter errors will
be generated if the coupling between the two devices is reduced to a level that the phototransistor
does not "see" the light from the LED through encoder slot 0. This condition would occur if the
LED light output power drops, the ability of the phototransistor to convert light to electrical signals
is reduced, or a combination of the two.
1. LED Degraded Output
a) There are numerous publications that
show that exposure to ionizing radiation
will cause a decrease in luminance from
gallium arsenide (GaAs) infrared-emitting
(IR) diodes. When discussing IR LEDs
Siemensvii states that "A high degree of
crystal perfection is a precondition for the
creation of effectively radiant
recombination as crystal defects act as
centers for non-radiating recombination."
Since ionizing radiation damage will
generate such crystal defects this provides
an explanation for the loss in light output.
b) Figure 12, taken from a JPL reportviii on
Figure 13: Trapped proton fluence at the
radiation effects, shows LED output
surface of HST. Reference (x).
1
Sticktion: A contemporary term that has many definitions in technical circles. In this report sticktion will
be defined as a localized region of high friction in a mechanism.
11
degradation as a function of proton fluence for an OD880 GaAlAs device. The output drops
sharply for the first 5x1010 neutrons/cm2 and then continues to degrade at a slower rate with
continued exposure. This plot is useful in that the effects are shown for dosing with the part
deenergized, as is the case for WFPC-2. According to the data the light output will halve for a
proton dose of 2x1010 protons/cm2. This behavior is consistent with the results of radiation
effects testingix undertaken by GSFC that show the output drops to half for 1x1011 neutrons/cm2
for an operating LED.
c) The proton fluencex external to the spacecraft has been estimated for the MLI degradation
investigation and is given in figure 13. From this graph the WFPC-2 dose, at the onset of shutter
errors, was approximately 2x1010 protons/cm2 divided by the spacecraft shielding effect. Until
shielding effects are calculated a dose of 1x1010 protons/cm2 will be assumed. At this dose the
LED output can now be expected to be 75% of the launch luminance.
2. Phototransistor Degraded Sensitivity
a) Phototransistor response to radiation damage has not been analyzed but can be assumed to be
a reduction in sensitivity.
3. Combined Effects in Sensor Package
a) Although the on-orbit response of the phototransistor has not been demonstrated, the ageing
effect of the sensor pair will be a loss of coupling greater than the loss of LED output alone. For
that reason the degradation of either component will be considered in the context of a loss of
sensitivity of the sensor pair. Sensor package degradation is largely consistent with the
symptoms of the shutter errors and has not been ruled out as the cause.
E. Evaluate Hard Electrical Failure
In the "Fault Isolation" section it was shown that the only electrical components that could fail in a hard
manner and still cause the shutter errors are the two parallel resistors in the current path to the side-A
Light Emitting Diode (LED), R105 and R5.
1. LED Drive Resistors
a) The two resistors form a 79Ω resistance in series with a 2N2907A transistor and the TIL252
LED. The series circuit is across 0 to +5V with a 0.18V drop across the saturated transistorxi
and 1.5V across the forward biased LED. Therefore the current is set by the remaining 3.32V
through the 79Ω, or 42mA. The TIL25 datasheetxii provides that for this current the LED will be
operating at 80% of the rated 0.75mW output power. If either resistor were to fail open the
circuit resistance would double to 158Ω thereby reducing the current to 21mA and the output
power to 34%. This value would be further reduced by the loss of output due to on-orbit
radiation damage.
b) There is insufficient information available to confirm if the system would operate with the
LED these output levels. This problem could be ruled out with ground testing. Neutral density
filters placed as interference in the WFPC-1 sensor assembly would indicate the reduction in
photon coupling at which the sensor package would no longer work.
IV. Remaining Faults
A single cause of the errors has not been identified but the field of choices has been limited to the
degradation of the two position-sensing components, a failed resistor, or an occasional offset in the
blade-A position when open. Table 2 summarizes these findings and provides arguments for and
against each possibility. It is known that reduced coupling in the sensor package is the source of the
errors. The question that remains open is whether the reduced coupling is due to a less sensitive sensor
package or is the encoder blade interfering with the light path.
A. Evaluate Remaining Faults
1. Sensor Package Degradation
a) The LED and phototransistor are known to be degrading and that the coupling between them
is decreasing even if the light path is unchanged. It is not known if the degradation has reached
2
JPL drawing 10093548, "Shutter Optical Encoder," specifies a TIL24 LED. This is superceded by the
WFPCII shutter encoder quality assurance document CL-SH-13-0011, "Assembly and Inspection Data
Sheet," that reports TIL25 devices were used.
12
Failure
LED and/or
phototransistor
degradation
Pros
Cons
1) Documentation supports this effect. 1) Sudden appearance of errors.
2) Ground testing supports this effect. 2) Would not explain timing trends.
3) The patch is working.
3) No long-term interpolated time drift.
R105 or R5 open
1) Consistent with sudden onset of
errors.
1) Unlikely failure mode.
2) Would not explain timing trends.
Sticktion near blade-A 1) A is knocked out of detent by B
1) Small offsets have not shown up in
open detent (causing an
closing on WFPC-1.
telemetry.
occasional offset)
2) Due to blade gap on WFPC-1, B
does not restore A to the detent.
3) Higher Tintpl TOF could be due to
sticktion near the open-detent.
Table 2: The remaining three problems that could generate the shutter errors with arguments for and
against each being the culprit.
a level sufficient to cause the shutter errors. Ground testing that varies the lamp intensity or
reduces photon coupling using neutral density filters would help to make this determination.
However, there remain several points that weigh in favor of this problem and some that weigh
against.
b) The best argument for this problem is that the lamp output is known to be decreasing. Also
the fact that the November 2000 patch was designed to fix this problem and is working adds to
the credibility.
c) Working against the sensor degradation argument is the timing trends that coincide with the
onset of the shutter errors. If sensor degradation is accepted as the fault then these trends cannot
be related to the problem. It is possible to explain how degraded lamp output can cause a shift
in interpolated time. However, this is a slow process that would have to cause a long-term trend
beginning well before the onset of errors and could not generate a sudden knee in the data. It is
certainly possible that the errors and the timing trends are unrelated but the coincidence shown
by figure 10 is unsettling.
d) A second argument against the lamp explanation is the suddenness with which the errors
became present. Problems that develop due to a slow decline of some parameter usually have a
slow onset then increase in frequency. To the contrary, the shutter errors developed rapidly in a
period of weeks. The lamp effect may be subject to environmental influences such a change in
temperature or a new unidentified noise source but no such correlation has been found.
2. Open Resistor in LED Drive Path
a) The arguments for this failure are the same as for sensor package degradation. In addition
this failure would be consistent with the rapid onset of errors. The previously described ground
testing involving filters on the WFPC-1 shutter would also shed light on this type of failure.
b) This type of failure does not explain the recent timing trends. One can concoct a scenario in
which the resistor opens slowly or a resistor solder joint is slowly degrading but such a situation
is somewhat farfetched and will not be considered here in any further detail.
3. Sticktion near Open Detent
a) Surprisingly, this is the one failure that is consistent with all of the data. However difficult it
is to believe that such a specific problem could be found through analysis the following
arguments can be made.
(1) Ground testing with the WFPC-1 shutter shows that blade-A bounces out of the open
detent when blade-B closes. Once out of detent blade-B does not push blade-A back to the
correct position due to a gap between the blades, the permanent magnets must apply the
restoring force.
(2) Increased friction near the start of flight would increase the time-of-flight to slot 1 but
would have a negligible effect on the blade speed thereafter. This is consistent with the
interpolated time and speed flag data.
(3) This would cause a small binary effect on the open detent. There would be the normal
detent and a second resting spot nearby that causes a partial blockage of the sensor light path.
13
Figure 14: Shutter-A and shutter-B interpolated times since launch.
This could cause both the occasional shutter errors and then be corrected by the RAM patch
the same as a dimming lamp.
V. Monitoring & Risk Reduction
Although a single fault for the shutter errors has not been identified there is enough understanding of
the issues to make a knowledgeable assessment of the current risks. Analysis of the available data
continues in an effort to assess any ongoing risk to the instrument.
The electrical possibilities pose no threat to the health of the instrument however operations could again
be impacted. If the sensors are losing sensitivity the A-blade may degrade to the point to overcome the
software patch benefits and the errors would reoccur. Furthermore, if the A-side sensors have experienced
this condition the B-side sensors can be expected to have similar problems in the future. Although there
has been speculation that LED degradation could be detected in the timing data, no such results have been
proven to date. Currently there exist no monitors of the sensor package health that have been shown to
provide early warning of a problem or predict if and when another problem will occur.
If a mechanical problem is responsible for the errors there is still a possibility of a health risk but
currently the risk appears to be small. Unlike the electrical issues there is very good telemetry in the form
of timing data that can be used to monitor the shutter's mechanical health. Regular monitoring of this and
other data has commenced and will continue while the WFPC-2 operates. The usefulness of the data to
predict problems has yet to be proven but as an analysis aid the information content is excellent. Software
has been developed to allow regular monitoring of the timing data. Following further refinement the code
it will be incorporated into the STScI Telemetry Evaluation System (TES).
VI. Future Efforts
1. Further analysis may reveal offsets in the open detent that can be measured as a result of the RAM patch.
Such offsets would reduce the interpolated time and would now be reported in the absence of errors.
2. Neutral density filter testing of the WFPC-1 shutter would provide insight into the electrical issues.
14
3. The shutter speed history could be constructed using the methods that created figure 5.
4. Improve the radiation dose calculations to better predict LED degradation.
VII. Conclusions
The source of the error is reduced coupling in the blade-A position sensor. Depicted in figure 15, the
issue is LED/phototransistor pair decreased output and sensitivity or an encoder blade offset that interferes
with the light path. Remaining are two possibilities that could cause the LED to dim and one problem that
could cause the offset.
The analysis to date shows minimal risks to the WFPC-2 instrument. Close monitoring of the shutter
timing data will continue while the instrument remains on orbit. Automation of the data analysis provides a
wealth of insight into the health of the shutter and should offer warning of mechanical degradation.
This report is not the final word on the shutter errors. The effort will continue at STScI to refine the
analyses of this problem and provide more conclusive evidence. Others are encouraged to view these
findings with a critical eye and to explore any possibilities that have been overlooked.
The discussion has often returned to the trends in interpolated time. In closing I will put the amplitude
of the recent trends into perspective by comparing it to the history of the other three interpolated times:
shutter-A open, shutter-B open, and shutter-B close. The shutter-A closing is shown as the bottom (blue)
trace in figure 14. Clearly it has been the best-behaved interpolated time over the life of the mission.
Hopefully this can be used to allay fears of a major mechanical failure.
Appendix A provides further information on the interpolated time variance calculations. Appendix B
contains interpolated time and speed flag history plots.
VIII. Acknowledgements
This report reflects the work of the entire STScI Engineering Team. The progress is a credit to Colleen
Townsley's ability to find the manpower with limited staffing. The circuit analysis by Tom Wheeler has
been invaluable. The early insight provided by Tom Bickler of JPL and the Anomaly Review Board
brought the problem into focus and rapidly provided a repair to restore the integrity of the science program.
Thank you to the GSFC mechanisms group for providing video documentation of the WFPC-1 testing.
Figure 15: Source of shutter errors:
dimmed lamp or blocked light path?
15
IX. Appendix A: Interpolated Time Variance
A. Description
1. Data Manipulation
a) Data since launch is segregated into streams of interpolated time. Each interpolated time is
then separated by open or close, and AP17 or microprocessor commanding. The software
determines the type of move based upon patterns of independent telemetry. The output is then
filtered to remove unwanted data such as repeated records.
b) The interpolated times for each blade open and close are then considered. Since the starting
point of the timer is unknown the time-of-flight is not represented by interpolated time. To
correct for this the data is biased to a lifetime average flight of 11.7ms. This value represents
the blade-A closing time-of-flight in the interpolated region as found on the chart below.
c) Dividing each point by a 30-point moving average then normalizes the biased data.
d) A 30-point moving variance is then taken from the normalized data.
2. Plots
a) The following plots are provided in this appendix:
(1) Top/Left: Interpolated time.
(2) Bottom/Left: The variance of the interpolated time.
(3) Top/Right: Biased and normalized interpolated time.
(4) Bottom/Right: The variance of the biased and normalized interpolated time
ii
iii
iv
X. Appendix B: Interpolated and Speed Flag Timing
A. Description
1. Data Manipulation
a) Data since launch is segregated into streams of interpolated time. Each interpolated time is
then separated by open or close, and AP17 or microprocessor commanding. The software
determines the type of move based upon patterns of independent telemetry. The output is then
filtered to remove unwanted data such as repeated records.
2. Plots
a) The red triangle on each plot indicates the first occurrence of the shutter errors.
b) All plots encompass the period from February 1997 to February 2001.
c) All plots use equal ranges for the y-axis scale except for U170 uProc close.
v
Blade-A Microprocessor Controlled Closing
vi
Blade-A AP17 Controlled Opening
vii
Blade-A Microprocessor Controlled Opening
viii
Blade-B AP17 Controlled Closing
ix
Blade-B Microprocessor Controlled Closing
x
Blade-B AP17 Controlled Opening
xi
Blade-B Microprocessor Controlled Opening
xii
XI. References
i
Ken Stowers, "Appendix VI to the SI to SIC&DH Interface Control Document," ST ICD-08 revision A,
July 1993, table VI-3.
ii
Jet Propulsion Laboratory, "Wide Field Planetary Camera II Instrument Description and User Handbook,"
JPL D-11212, Appendix C, December 1993, California Institute of Technology.
iii
Jet Propulsion Laboratory, "Schematic Diagram, Mechanical Drivers," WFPC-II schematic number
10093516 revision H, 1980.
iv
Jet Propulsion Laboratory, "Shutter Assembly," WFPC-II schematic number 10093548 revision D, 1980.
v
Jet Propulsion Laboratory, "Schematic Diagram, Logic Circuit Boards," WFPC-II schematic number
10093506 revision D, 1983.
vi
Jet Propulsion Laboratory, "Disc, Encoder," WFPC-II schematic number 10085196, revision B, 1980.
vii
Siemens, "General IR and Photodetector Information," Appnote 37,
http://webook.fset.de/20091999PHCHO/OPTOELECTRONICA/app37_1.pdf
viii
A. Johnston, "Recent Work on Radiation Effects in Microelectronics at JPL,"
http://rd49.web.cern.ch/RD49/RD49News/Allan_Johnston.pdf
ix
R. Reed, K. LaBel, H. Kim, H. Leidecker, and J. Lohr; "Test Report of Proton and Neutron Exposures of
Devices that Utilize Optical Components and are Contained in the CIRS Instrument;"
http://radhome.gsfc.nasa.gov/radhome/papers/i090397.html
x
J. Dever, K. de Groh, B. Banks, J. Townsend, J. Barth, S. Thomson, T. Gregory, and W. Savage
"Environmental Exposure Conditions for Teflon FEP on the Hubble Space Telescope Investigated,"
http://www.grc.nasa.gov/WWW/RT1999/5000/5480dever2.html
xi
Semicona Semiconductors, "Type 2N2907A," data sheet number 2N2907A.
xii
Texas Instruments, "Types TIL23, TIL24, TIL25 P-N Gallium Arsenide Infrared-Emitting Diodes,"
bulletin No. DL-S 11312, February 1970—Revised January 1976.
xiii