Instrument Science Report NICMOS 2005-04
NICMOS Small ∆T Dewar / NCS PID
Model for Orbit Night Power Reduction
T. Wheeler & M. Robinson
October 05, 2005
ABSTRACT
Without SM4 and the installation of new Ni-H2 batteries, HST is facing a future with
decreasing battery capacity. This ISR explores one option for reducing HST’s battery
load during orbit night while maintaining the science capability of the NICMOS.
Modulating the NCS setpoint temperature, i.e., increasing it during orbit night and
decreasing it during orbit day, can accomplish this. A linear NICMOS small ∆T thermal
model was developed based on flight data serendipitously obtained during Proposal
10097, NICMOS Temperature Setpoint Darks, to predict camera one’s temperature, NCS
compressor speed, and total NCS power for modulations of +/-1, +/-2, and +/-3K. The
results are presented.
B
B
Introduction
Ni-H2 batteries power HST during orbit night and during spacecraft emergencies. Battery
capacity tests performed during 2004 revealed an increasing decline in system state of
charge (SOC) since servicing mission SM3B with a total average decline of 6.55 Ahr/year (Hollandsworth et al). Battery capacity will fall to 45 A-hr by late 2005-mid 2006
(Hollandsworth et al). (The current safe-mode entry SOC value is 37.5 A-hr.) During
orbit day, the solar arrays provide power in excess to recharge batteries and to power all
subsystems including science instruments and the NCS. If the batteries are not replaced
or SM4 is cancelled, the NCS and NICMOS are eligible to be reconfigured to a lowerpower, safe state albeit no official decision has been made by the Project thus far. The
NCS temperature modulation scheme proposed in this ISR provides one option of
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reducing the batteries’ load during orbit night while utilizing the solar arrays’ excess
power during orbit day to extend the scientific lifetime of the NICMOS.
The Model
A linear model was developed using 5Spice – a Spice-based electronic circuit simulation
and circuit analysis software package available from www.5spice.com. The model’s
parameters were adjusted to empirically match the step function response obtained from
Proposal 10097 while keeping the model minimally complex.
Proposal 10097 changed the setpoint temperature in steps from 72.4 K, to 72.9 K (+0.5
K), to 71.9 K (-1.0 K), to 71.4 K (-0.5 K) and back to 72.4 K (+1.0K) over approximately
7.5 days while taking darks on all cameras. Thermal, compressor speed, and NCS total
power data were collected throughout the test via normal engineering telemetry. The
model’s step response was based on this telemetry.
With the step response established, the model was then used to predict the response of a
setpoint periodic pulse train with the temperature increasing during orbit night followed
by the temperature decreasing during orbit day of 1, 2, and 3 K. The width of the
setpoint temperature increase was determined by using the average sun-time and eclipsetime per day/night cycle for CY 2005 (Kroll).
TP
PT
The model’s characteristics follow.
Model Limitations
The model is only valid for small ∆Ts around 77.15K as measured at camera one’s
detector. This is the current nominal operating temperature for this camera. The model
does not incorporate thermal affects due to orbital day/night changes, pointing, seasonal
variations, or activities caused by adjacent SIs (∆Ts in aft shroud), nor does it model ∆Ts
cause by detector auto-reset activities.
Model Topography
The model is divided into two sections: the NCS PID control law simulation and the
Dewar thermal model. The NCS simulation, for small ∆Ts, matches the PID control law
behavior. The Dewar thermal model was developed to match the thermal step response
from proposal 10097 and to match the quiescent heat flow inside the Dewar to within a
few milliwatts of predictions by the HST thermal group at GSFC.
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Model Verification
Flight step function and model-simulated data are presented in Figures 1-12. As an aid to
understanding, all mnemonics present in the figures are presented with brief descriptions
below:
Mnemonic
MNPDSTPT
MNCONTRL
NDWTMP11
NDWTMP14
NDWTMP16
MNCONTRL Sim
NDWTMP11 Sim
NDWTMP14 Sim
NDWTMP16 Sim
MNCORSPD
MNCORSPD Sim
MNTOTPWR
MNTOTPWR Sim
Description
PID control law temperature setpoint – the input
The control temperature – feedback to the control law
Camera #1 temperature
Bottom of cold well temperature
Aft tank head near cooling coil temperature
Simulated control temperature
Simulated camera #1 temperature
Simulated bottom of cold well temperature
Simulated aft tank cooling coil temperature
Compressor speed
Simulated compressor speed
Total NCS power
Simulated NCS total power
Notes:
1. In Figures 1 and 4, NDWTMP11 (dark blue line) saturates (flat lines) at 77.43 K.
2. MNCONTRL in Figure 7 appears as a wavy pink line between –1 and +9 hours.
This occurred because the NCS was operating at maximum cooling and minimum
temperature.
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Figure 1: Thermal response for +0.5K step in setpoint temperature
Figure 2: Compressor speed response for +0.5K step in setpoint temperature
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Figure 3: NCS total power response for +0.5K step in setpoint temperature
Figure 4: Thermal response for -1.0K step in setpoint temperature
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Figure 5: Compressor speed response for -1.0K step in setpoint temperature
Figure 6: NCS total power response for -1.0K step in setpoint temperature
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Figure 7: Thermal response for -0.5 degree step from 71.9 to 71.4K setpoint temperature
Figure 8: Compressor speed response for -0.5K step in setpoint temperature
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Figure 9: NCS total power response for -0.5K step in setpoint temperature
Figure 10: Thermal response for +1.0K step in setpoint temperature
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Figure 11: Compressor speed response for +1.0K step in setpoint temperature
Figure 12: NCS total power response for +1.0K step in setpoint temperature
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Model Predictions
Because the model closely matches the +/- 0.5 and +/- 1.0K step functions from proposal
10097, the model is now used to predict NICMOS Dewar thermal behavior, compressor
speed, and NCS total power for setpoint pulse trains of 1.0, 2.0, and 3.0 Kelvins
modulations. Predicted performance plots are provided in Figures 13-18.
Figure 13: Model thermal predictions for 1.0K setpoint temperature pulse train
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Figure 14: Model speed and power predictions for 1.0K setpoint temperature pulse train
Figure 15: Model thermal predictions for 2.0K setpoint temperature pulse train
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Figure 16: Model speed and power predictions for 2.0K setpoint temperature pulse train
Figure 17: Model thermal predictions for 3.0K setpoint temperature pulse train
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Figure 18: Model speed and power predictions for 3.0K setpoint temperature pulse train
Results
Table 1: The results for 1, 2, and 3 K step pulse trains modulations
Setpoint
Pulse Train
Amplitude
(K)
NDWTMP1
1
Tinitial
(Increase during
orbit night,
decrease during
orbit day)
1.0
2.0
3.0
76.538 K
76.538 K
76.538 K
NDWTMP1
1
Tfinal
(K)
NDWTMP1
1
Tincrease
(K)
NDWTMP1
1
Tp-p
(Temp ripple)
(milliK)
Avg Power
Saved During
Orbit Night
(Watts)
77.150
77.774
78.394
0.607
1.236
1.857
0.8
1.0
2.1
26.0
71.6
117.4
Compresso
r Min-Max
Speed
(rps)
(As compared to
no temp cycling
for NDWTMP11
at 77.15 K)
6800-7300
6800- 7300
6800-7300
For all three simulations, the initial temperature of detector one, telemetry mnemonic
NDWTMP11, was adjusted to 76.538 K. The purpose was to demonstrate that for a 1 K
modulation, the final and desired temperature of 77.15 K was obtainable. For the 2 and 3
K modulations, the final detector one temperature increased to 77.774 K and 78.394 K,
respectively. In all three simulations, the peak-to-peak ripple temperature at
NDWTMP11 was minimal (2.1 milliK maximum) and not deemed a problem. Column 6
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shows the estimated power saved in watts with the 3 K simulation offering the best
results. In all simulations, the NCS compressor speed ranged from 6800 to 7300 rps.
Conclusions
If SM4 does not occur, HST will need methods of reducing battery load during orbit
night to extend its lifetime. Modulating NCS’s temperature can achieve this as stated in
Table 1 while preserving the scientific usefulness of the NICMOS. For the 2 and 3 K
modulation simulations, detector #1 will operate at a higher stable temperature.
Calibrations will need to be redone (dark currents will be higher). The fidelity of the
model and analyses performed provide realistic, ballpark expectations of power savings.
Because of the risk, an on-orbit, proof-of-concept test is NOT recommended at this time.
If extreme circumstances dictate, e.g., loss of a battery or sharp increase in SOC fading,
many considerations will be included on the merits of setpoint cycling. This is not an
uncomplicated decision. One last consideration is that by incorporating setpoint cycling,
the Dewar temperature will also be a function of duration of orbit day/night. The
magnitude of this affect was not studied in this report.
References
Hollandsworth, R., et al., “Hubble Space Telescope 2004 Battery Update,” pg. 45, in
2004 NASA Battery Workshop presentations; available from World Wide Web @
http://edocs1.hst.nasa.gov/eps/eps_main.htm.
Kroll, S. “2005 Sun-time/Orbit Data&Plot,” tab Beta Data 2005; available from World
Wide Web @ http://edocs1.hst.nasa.gov/eps/eps_main.htm.
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