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X-Ray Calorimeter Mission. Attitude Control Philip Calhoun, Dave Olney, Joe Garrick Attitude Control Systems Engineering Branch Code 591 2 – 6 April 2012. ACS Overview. Sensors Coarse Sun Sensor (CSS) 8 units aligned to provide 4 π steradian coverage of sky

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x ray calorimeter mission

X-Ray Calorimeter Mission

Attitude Control

Philip Calhoun, Dave Olney, Joe Garrick

Attitude Control Systems Engineering Branch

Code 591

2 – 6 April 2012

acs overview
ACS Overview
  • Sensors
    • Coarse Sun Sensor (CSS)
      • 8 units aligned to provide 4πsteradian coverage of sky
      • Used for Safe mode sun acquisition
    • Gyro
      • 1 internally redundant unit
      • Sense the attitude rate of change
      • Used in Kalman Filter to propagate and smooth state
    • Star Trackers
      • 1 electronics and four heads
      • A star tracker head is used for attitude determination (inertial frame)
      • Second star tracker head is used for alignment between science instrument components
      • Two pairs of star trackers provide fully redundant capability
  • Actuators
    • Thrusters
      • 12 – 4.5 N (1 lbf) thrusters for attitude control and orbit maneuvers
        • Thrusters full on for orbit maneuvers, off-pulsed for attitude control
        • Thruster on-pulsed for attitude control during momentum unloading
    • Reaction Wheels
      • 4 – 75 Nms, 0.2 Nm reaction wheels
      • Used to attitude actuation
acs functional block diagram
ACS Functional Block Diagram

Actuators

Sensors

Reaction

Wheels

4 total

(pyramid about Z)

ACS FSW

CSS

Tot.8

Safe Mode Att.

Determination

Gyro

Attitude

Control

Mission Attitude

Determination

Star

Tracker

Thrusters

12Total

(See slide #18)

Momentum

Management

Orbit

Maintenance

observatory coordinate system and key terms
Observatory Coordinate System and Key Terms

Observatory Coordinate System

Origin is at the Mirror Node

+YOBS

points from the Mirror Node,

forming a right handed orthogonal frame with X and Z

Y is the axis for PITCH

(side S/A’s are aligned w/ Y)

Target

+ZOBS

points from the Mirror Node

to the Target

Z is the axis for ROLL

(the Boresight is aligned w/ Z)

“FORE” is the +Z (FMA) end of IXO

“AFT” is the –Z (Instruments) end of IXO

+XOBS

points from the Mirror Node

towards the Sun,

X is the axis for YAW

Sun

operations requirements
Operations Requirements
  • Launch
    • Direct insertion into transfer orbit in full sun with continuous ground contact (have TDRSS capability)
    • Indefinite duration safe mode available immediately after LV separation
    • Deployments start right after LV separation
  • Cruise to L2
    • Start with one month commissioning phase for checkouts, calibrations
    • Continuous DSN contacts during commissioning, then twice daily for 30 minutes for OD during cruise
    • Correction burns as required
    • Mirror cover deployed after observatory outgassing
      • No exposure of aperture to sun light allowed for remainder of mission
    • Science observations may start during cruise
  • L2 Insertion
    • Performed in Operational configuration (10-3 g level forces only)
  • Observations
    • Pointing at a target for < 106 seconds
    • 1 – 20 observations per week, re-pointing accomplished in less than an hour
    • Observing efficiency 85%
  • EOL disposal
    • Passivate observatory, impart 1 m/s towards deep space
  • Mission Ops
    • Highly autonomous observatory, 8 x 7 ground staffing
    • Data latency 2 weeks required, 72 hours goal from completion of observation to product delivery, excludes bright source observations
requirements and considerations
Requirements and Considerations
  • Requirements
    • 3 year (5 year goal) mission lifetime at L2
    • Attitude Requirements
        • Pointing: 10 arcsec, Pitch/Yaw (3-sigma); Roll number not provided by customer
        • Knowledge: 3 arcsec, Pitch/Yaw (3-sigma); Roll number not provided by customer
        • Jitter: 1 arcsec, all axes (3-sigma) over 1 seconds

Slew Requirement

        • Complete a 60 degree (yaw) slew in 60 minutes (including settling)
  • Considerations
    • Impact of changing pitch field of regard to +/- 45 degrees
    • Impact of Solar force center of pressure (CP) to Center of Mass (CM) offsets
  • Assumptions
    • HGA not slewing during science observations
    • Adequate calibration slews and observation time for sensor alignments
requirements and considerations cont
Requirements and Considerations (cont)

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Target

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Pitch: +/-25, -25 

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Sun

Earth

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Moon

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Yaw:

+/-180 

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Roll:

+/-10 

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acs modes sensors and actuators
ACS Modes, Sensors and Actuators
  • Orbit Adjust Maneuvers (Delta-V Mode)
    • Gyros and thrusters
    • Full-on thrusters for orbit maneuvers
    • Off-pulse thrusters for attitude control
  • Momentum Unloading (Delta-H Mode)
    • Gyros, thrusters and wheels (spin down)
    • Attitude control on thrusters as wheels spin down to commanded momentum level
  • Science/Mission (Science Mode)
    • Gyros, star tracker, wheels, Kalmanfilter
    • Inertial Pointing (point at sun during coast phase; target pointing during science)
    • Calibration slews (science and ACS sensor alignments)
    • Re-pointing slews (slewing to next science target)
  • Safe (Safehold Mode)
    • CSS, gyros and wheels
    • Point solar array at the sun
instrument pointing star tracker specifications
Instrument Pointing Star Tracker Specifications

Micro-Advanced Stellar Compass (μASC)

coarse sun sensor adcole
Coarse Sun Sensor: Adcole
  • Adcole analog CSS detectors provide close to

2πsteradiancoverage (85 deg half angle)

  • 8 detectors distributed across theobservatory provide near full4πsteradiancoverage
  • 4 CSS on SA panel facing sunward
  • 4 CSS on body facing anti-sunward
solar radiation torque and its effect on wheel momentum storage
Solar Radiation Torque and its Effect on Wheel Momentum Storage
  • Solar Array size and position have been selected to make the center of mass (Cm) and center of pressure (Cp) nearly coincident along longitudinal axis for a pitch angle = 0 degrees
    • Cp / Cm offset
      • along X axis (toward Sun) ~= 1m
      • along Z axis (11.5 cm)
  • For pitch ≠ 0the Cp and Cm will not coincide
  •  increased Solar Torque

Baseline (Pitch = 25 deg)

Ref

Cm

Cp

0.43 + 0.115 m

Projected

silhouette

toward sun

25 o

four reaction wheels momentum storage
Four Reaction Wheels – Momentum Storage
  • RWA body alignment
  • RW1 RW2 RW3 RW4
  • | 0.6124 0.6124 -0.6124 -0.6124 |
  • | 0.6124 -0.6124 0.6124 -0.6124 |
  • | 0.5000 0.5000 0.5000 0.5000 |
  • (bias to X and Y axis, momentum accumulates mainly on these axis)
  • Baseline (Pitch < 25 deg)
  • Momentum per wheel = 100 Nms, (HR-16)
  • Torque per wheel = 0.2 Nm
  • Plot shows momentum capacity (min = 130 N-m-sec)
  • > 165 Nms within adequate range of +/- Y axis
  • Trade (Pitch < 45 deg)
  • Momentum per wheel = 125 Nms (HR-16)
  • Torque per wheel = 0.2 Nm
  • Impact of 125 N-m-sec wheel
    • mass & imbalance increase

Allowable Range for Momentum Buildup along Y axis

Momentum Capacity ( 1 whl = 100 N-m-sec)

Elevation (deg)

Azimuth (deg)

slew capability
Slew Capability
  • Requirement: Slew 60 deg in 60 deg (allow time to settle)
  • Design:
    • Goal: Slew should not significantly impact momentum / control authority usage
      • Use 5% of minimum Momentum Capacity (~6.5 N-m-sec)
      • Use 10% of torque capability (~0.02 N-m) during ramp
    • 5 min ramp time to minimize slew transients
    • Slew completes in 58 min, 2 min for settle
pitch yaw error budget
Pitch / Yaw Error Budget

Uncorrelated items can be RSSed but a more conservative estimate is to Sum items

SUM

SUM

SUM

SUM

Few things in our favor:

Large inertias (torque produces small angles)

Motion in star tracker filters out biases

Large observation times

Note: RWA jitter is allocation (based on experience of similar missions)

slide18
Thruster Configuration

8 Thrusters canted 10⁰ in two planes

Couple

4 Thrusters canted 45⁰ in one plane

Solar Array

Notional CG

Lines of Action

momentum unloading
Momentum Unloading
  • Thruster Isp = 210 sec
  • Wheel torque = 0.2 Nm
  • Momentum accumulation = 125 N-m-s about Y axis (worse case) every 21days (Baseline: Pitch < 25 deg)

= 180 N-m-s about Y axis (worse case) every 21 days (Pitch < 45 deg)

  • For 5 year mission, every 21 days is about 87 unloads
  • Max Thruster torque about each axis ~= +/- [1.4, 8, 8 ] N-m
  • Minimum on time thrusters = 5 m-sec
  • Thruster Pulsing Duration = 15.6 sec of thruster on-time to remove 125 Nms (Baseline: Pitch < 25 deg)
  • = 22.4 sec of thruster on-time to remove 180 Nms (Pitch < 45 deg)
  • Accuracy of unloading to < 0.03 N-m-s
  • Time to spin down wheels = 125 / 0.2 = 625 sec, about 10.5 minutes (Baseline)

180/ 0.2 = 900 sec, about 15 minutes (Pitch < 45 deg)

  • Fuel usage = Total momentum (5 yr) / ( Isp * r * 9.8) = (125*87) / (210 * 2 * 9.8) = 2.6 kg (baseline)

= (180*87) / (210 * 2 * 9.8) = 3.8 kg (baseline)

  • Assume r = 1m
summary
Summary

Potential future work:

  • Need Jitter Assessment to determine impact to Pointing Budget
    • Reaction Wheel Imbalance
    • Cyrocooler
    • Passive isolators could be used to reduce jitter if needed
      • Ex: Chandra Reaction wheels mounted on isolator
  • Evaluate launch vehicle tip-off rate damping, thrusters and/or wheels

Issues/concerns:

  • Momentum Buildup due to Solar Pressure is driving to larger wheel size for off pointing about Pitch axis  Jitter increase
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