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University of Leicester
PLUME
Ref: PLM-PAY-SoftwareInter-024-1
Date: 12/02/2009
Software Interface Document
P. Peterson
Date
Updated Reference Number
change
12/01/2009
PLM-PAY-SoftwareInter-024-1
first version issued
Italic words refer to pin and function names.
Camera software interface to be provided later.
Pin descriptions
All pins use 3.3V TTL except the analog output pins, which carry a voltage that can be
anywhere between 0 and 5V. Logic pin ‘High’ refers to 3.3V, and ‘Low’ to 0V. Most of the
circuitry in the detector side of the payload is analog and does not require a constant clock
frequency or carrier to operate. For reading the Analog out pins, set the computer’s ADC to
5V operating range. Mode 00 (single reading) should be adequate.
Input pins:
- X Signal reset,
- Y Signal reset: Send a short high signal (at least 100ns) to this pin to reset the respective
payload signal buffers to zero volts after you have read the signal voltage from it. Hold it low
the rest of the time, or the buffer will not work.
- X Support on,
- Y Support on: Hold either of these pins high to power up the electronics chain for the X or Y
detector. Obviously, the chain must be powered up for any of the other pins to do any good.
Never power up the HV output of a detector without powering on the respective signal chain,
as the signal chain also supplies power to the HV supply's voltage monitors.
- X HV on,
- Y HV on: The HV on the plates must be powered up separately to the electronics chain.
Drop these pins low to interrupt power to the HV supply.
- HV set x1,
- HV set x2,
- HV set x4: These three pins control a 3-bit digital-analog converter circuit that varies the
voltage across both the detector plates between 1000V and 1150V. Set a pin high to set its
respective bit.
HV set HV set HV set Expected HV
x4
x2
x1
input reading
Low Low
Low
4.41
Low Low
High
4.49
Low High Low
4.57
Low High High
4.65
High Low
Low
4.73
High Low
High
4.81
High High Low
4.88
High High High
4.96
Expected HV
output reading
4
4.07
4.14
4.29
4.73
4.36
4.43
4.50
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Approximate
plate voltage
1000V
1021V
1042V
1064V
1086V
1107V
1129V
1150V
University of Leicester
PLUME
Ref: PLM-PAY-SoftwareInter-024-1
Date: 12/02/2009
Figure 1: HV control pin truth table. Voltages listed here are approximate and will likely be
change as a result of the Heidelberg test campaign. HV input reading is to ±0.61mV, plate
voltage is to ±0.366V.
Output pins:
The first six pins listed here are analog output pins and carry various analog signals from
the payload to the OBDH. They should be tied to the OBDH ADC on port 6. The signal pins
carry an analog voltage range of zero to five volts referenced from common ground.
- X HV input voltage,
- Y HV input voltage: The voltage on this analog output pin is the same as the voltage going
into the HV supply that biases the MCP.
- X HV output voltage,
- Y HV output voltage: The voltage on this analog output pin is equal to one three hundredth
of the voltage output of the HV supply, and approximately two fifths of the plate voltage. See
figure 1 for voltage information.
- X Detector signal,
- Y Detector signal: The buffered analog output of the signal processing chain waits on this
pin for the computer to read and then reset it.
- X Signal received,
- Y Signal received: This pin will go high if there is an event in the specified detector since
the last time the signal was reset, and low if not.
Operating the detectors
Inactive state
All HV set, and both HV on and Support on pins should be low when the payload is not in
use. The other input pins do not matter. In this mode, current will only be drawn by some of
the level shifting circuits.
Power up
Operating the detectors in an atmosphere will damage them, and even after launch
outgassing and residual atmosphere could potentially cause problems. The first MCP
powerup should take place after about four[1] days in orbit. Power up the detectors one at a
time.
• For most of the following instructions, you’ll be working with specifically one detector or the
other. I’ll use ‘the selected detector’ from here on out to represent this, and represent
detector-specific pins without their X or Y prefix.
1. Before anything else, send a Signal reset pulse (set Signal reset to high for at least 100ns
and then low). This will clear any charge on the signal buffer’s capacitor so that its voltage
does not damage the op-amp that drives the Signal received pin. This procedure should be
repeated every time the signal processing electronics are turned on or off.
2. Hold the support on pin for the selected detector high and wait 100ms for the circuitry to
warm up.
3. Check that the HV input voltage, HV output voltage and event signal pins are all close to
zero volts. If the HV input voltage is above zero, it is either because some of the HV set pins
are high, or there is a fault with the HV supply’s power transistor/amplifier. If the output
voltage is high, then the inductance of the HV supply’s transformer is probably to blame if it
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University of Leicester
PLUME
Ref: PLM-PAY-SoftwareInter-024-1
Date: 12/02/2009
has been working recently. If the event signal pin is high, then it indicates a problem with the
discharge FET or buffer circuit.
4. Hold the HV on pin for the selected detector high. The HV supply’s voltage is capacitance
driven so that it rises slowly the higher the output voltage with a time constant of roughly 20
seconds. This is deliberate.
5. Immediately after the HV on pin is high, start checking the HV input voltage and HV output
voltage as often as you can, at least ten times per second. If the ratio of HV input voltage to
HV output voltage fluctuates by more than ±1% [1] from its initial value during powerup, then
immediately set the HV on pin low. If the ratio changes so that HV input indicates a plate
resistance below 30Mohms[1], then immediately set the HV on pin low. Low plate resistance
manifests itself as the HV output voltage falling while the HV input voltage remains steady.
The former condition is a sign of instability due to atmospheric pollution, and the prognosis is
to wait a further day before attempting another powerup. The latter is a sign that the plate is
going into thermal runaway - where the plate dissipates more and more heat from the
current through it until it melts. If the HV output is reading more than 10% less than its
expected level for a given HV input reading when HV input is anywhere above a volt, then it
is a sign that one of the components in the high voltage circuit has shorted out. The rest of
the satellite power bus is protected from such a catastrophe by a transient voltage
suppressor that should dissipate the thousand volt spike before it does too much damage,
but it has a limited ability to dissipate heat in space. Immediately set the HV on pin low and
keep it there for the remainder of the mission.
6. Provided that nothing goes amiss in step five, after ten minutes or so the voltage readings
from the detector should be steady. You are now free to set the plate voltage to the desired
level using the HV set pins and begin taking measurements.
Taking measurements
When a micrometeorite impact is detected in either of the detectors, then that detector stores
the voltage amplitude it generated on the detector signal pin and pushes it’s signal received
pin high. Use the OBDH ADC’s 00 mode for recording signals with a 5V range.
Immediately record which detector the event occurred in, satellite attitude, time, HV input
voltage and HV output voltage. After the pin signal recieved pin has gone high, wait one
second for the signal to stabilise before taking reading the detector signal pin.
After all the information on the event has been safely stored, set the signal reset pin high for
100ns and then low again. This will clear the buffer on the detector, ready to receive a new
signal. Buffer overflows on the detector will cause the previous impact value to be
overwritten, but we predict only about ten impacts a day through both detectors so this
should not be a serious problem.
Housekeeping
Housekeeping data should be taken at least once per second in normal operation. Make a
note of the values of HV input voltage and HV output voltage, detector signal, and signal
received, for both detectors. Remember that when the signal received pin goes high, retrieve
the detector signal a second later and before you send a reset pulse to the signal reset pin.
Shutting down a detector
To turn off a detector, hold both the HV on and support on pins for that detector low. When
powering down the HV supply, leave at least a ten minute interlude before powering it back
up.
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Ref: PLM-PAY-SoftwareInter-024-1
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Failure modes
While there is a high voltage on the plate, the computer must actively monitor the detector at
least once per second for various failure modes and conditions. While diagnosing and
dealing with them should be an automated process, since many of them involve taking a
detector out of action it is important that a manual override of specific failure conditions is
available. When recording an error, the computer should report the last ten seconds worth of
high voltage readings (input and output) before the error, and the latest values of any of the
other pins as well as the time at which the error occurred and what kind of problem it was.
• Thermal breakdown - Manifests itself as a fall in plate resistance (and hence a drop in HV
output voltage. Once the plate resistance which is nominally 34Mohms starts to drop
precipitously the detector must be immediately shut down. An operating resistance of below
30Mohms[1] is dangerous. Thermal runaway is characterised as a fall in resistance of more
than 500kiloohms in one second[1] will result in a melted plate if or low plate resistance
occur and the plate is shut down in time, it can be powered up again once it has cooled off give it a day or so. A melted plate looks like a plate with infinite resistance, ie a bad plate
contact. It’s an incurable condition.
• Gas instability - Outgassing of the glass plates can cause a buildup of atmosphere inside
the channels even in the vacuum of space. The large electric field can cause gas breakdown
and conduction to occur, which at high voltage will damage the plate. If the ratio of HV input
voltage to HV output varies by more than ±1% [1] during one minute, then shut down the
plate and wait a day before powering it up again.
• Bad plate contact - If the HV output reads more than 105% of it’s expected value at any
point, then it is a sign that the plate has infinite resistance. This signifies a destroyed plate,
unfortunately, and the detector should be shut down for the rest of the mission.
• Shorted plate - If for some reason the Microchannel plate becomes conductive, then the
HV output will suddenly drop by about 15%. The HV power supply is protected from this
condition by a HV resistor, but prolonged operation in overcurrent conditions will certainly
damage the power supply though overheating. If the HV output to HV input ratio is more than
10% less than expected, then it indicates this problem. Immediately shut down the detector
in question and keep it that way.
• Damaged HV sensor - if the HV output suddenly starts reading zero volts, it indicates
possible problems with either the HV supply or the sensing resistive divider and amp.
Turning the HV supply off and waiting ten minutes before powering back up again may clear
the problem. If it does not, the detector should be shut down permanently.
• Hung HV sensor - There is a transient voltage suppressor that protects the HV output
sensing amplifier in case a spark causes one of the high voltage to resistors and subjects
the amp to high voltage. It is capable of dissipating up to 1500W, but only for a very short
time. If HV output suddenly swings up to 5V regardless of HV input, immediately shut down
the detector in question permanently.
• Faulty signal buffer - The internal discriminator in the signal processing electronics does
not pass signals that have less than 0.125V amplitude. If the Signal received pin goes high
without the detector signal voltage being over this level, or if the voltage exceeds this level
without the pin going high, then it’s a sign of a possible problem. First diagnostic is to record
the detector signal voltage, and then try and use the signal reset function described above. If
this clears the signal buffer, then the problem is likely to be a single event upset. If it doesn’t,
then the problem could be unrecoverable. In either case, it is probably a good idea to turn off
the support on pin for one second and then turn it back on again.
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University of Leicester
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Ref: PLM-PAY-SoftwareInter-024-1
Date: 12/02/2009
• Signal blackout - The signals from micrometeorite impacts may suddenly stop coming in.
This could be caused by a number of things. Faults in the HV supply can damage the
amplifiers, but there could also be a problem with the signal buffer. Either way, turning off the
support on pin for a second and then turning it back on may clear the problem.
• Excessive current drain - If the PSU reports that the detectors are drawing current far
above their rated level, it is a sure sign that something has gone wrong. Power down the
entire subsystem to see if the fault is isolated in the payload, and restart it one component at
a time after a delay of a day or so, as follows.
First, power up the support electronics for the X detector. If the high current drain returns,
then the fault is either with the signal chain, or with the auxiliary power supply. In this case,
turn off the X support on pin and turn on the Y support on. If the current drain returns, then it
is definitely the fault of the auxiliary power supply. This unfortunately means that the
detectors can no longer be used. If the current drain does not return, then discontinue use of
the X detector.
If the X support electronics are fine, power up the Y support electronics. If the problem
returns, then discontinue use of the Y detector.
The problem then must lie in the HV power supply stage. One thing that causes excessive
current drain is a damaged HV power transformer or a short of the resistive divider powering
the microchannel plate. Watch for the symptoms of these conditions as the power supplies
are turned on. Switch the HV supplies on one at a time, noting when the spike in current
occurs, and stopping use of the detector with the faulty HV supply.
The expected current drain for different detector power configurations is given in figure 2.
Status
All circuits idle
One support electronics chain on
Both support electronics chains on
Each active HV supply
All circuits active
Approximate power drain (mW)
4mW
247 to 327mW
253 to 333mW
255mW
763 to 843mW
Figure 2: Power consumption table. Again, subject to modification.
Emergency HV powerdown
The high voltage transformer circuits are designed in such a way that they can be powered
down instantly in case of trouble. To do this, hold the respective HV on pin low. If the other
detector is running, you do not have to adjust the HV set pins.
Every time a HV supply is shut down, wait at least ten minutes before attempting to restart it.
A note on power consumption
There are two signal processing chains onboard the payload, but only one of the power
converters needed to run them. This converter is automatically powered up when either or
both of the signal chains are turned on. Different power drains give different efficiencies from
this converter, and so shutting down one of the support electronics chains doesn’t improve
power consumption very much. Still, if the power situation on board becomes critical, you
can save about 260mW by shutting down one of the detectors and leaving the other one
running. See figure 2 for information on power requirements.
[1] I use bolded numbers in the middle of text where I do not know the actual figure, but
reckon that it must be something like my estimate. I always tag them using this reference.
They will be updated once I’ve done the calculations.
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