Lecture Notes for the SSD Technical Series
How HST Works
Rodger Doxsey
2-3pm, Wednesday
Septebmer 16, 1998
Auditorium
Pointing Control System (PCS)
- Basic Control Law
- gyros
- fixed pointing using gyros
- fixed pointing using guide stars
- FGSs
- RWAs
- Slews
- large slews
- small angle maneuvers
- Use of FHSTs
- Various complications
- aberration, parallax, etc.
- safemodes
Figure: Basic Pointing Process
Gyros
- HST has 6 gyros, 2 per package in 3 packages (RGAs).
- Gyros are mounted in a skew configuration relative to the
vehicle coordinate frame.
- ANY combination of 3 gyros will adequately measure rates
in the vehicle frame
- We typically operate either 3 or 4 gyros at a time.
- Each gyro measures rotation rate about a single, fixed
axis:
Gi = Si,m Ii /0.025s
- Individual gyro rates are converted to a vehicle frame
rate vector:
R = ( 3x4 alignment matrix) (Gi)
- system automatically shifts to three gyros when it decides
one is bad.
- Gyros drift at a nearly constant rate. Drift is in the
gyro frame, but the correction is made in the vehicle frame via an uplinked bias
vector.
R = ( 3x4 alignment matrix) (Gi) - D
- There are variations in the drift rate, the instantaneous
drift rate correction is determined on-board when guiding with FGSs.
R = ( 3x4 alignment matrix) (Gi) - D - d
Control Law Basics
- 3 sources of input:
- "command generator": planned attitude change
vector
- measured attitude change vector from gyros
- measured position data from FGSs or FHSTs ("observer
data")
- Computes a correction torque vector to be applied by the
RWAs
- Computation loop runs at 40 Hz.
- Some input data is averaged over longer periods
- Fixed pointing using gyros
- planned attitude change vector is 0
- no "observer data" is available
- error (measured - planned) is processed through
- rate path
- no ability to measure residual gyro drifts (d)
- during Earth occultations, the system can apply the residual
drift (d) measured in the previous orbit
Figure: Vehicle Control Law (Simplified)
Fine Guidance Sensors (FGSs)
- HST has three FGSs
- All three FGSs are left on and ready to use all the time
- At most, two are used for guiding at any time
- FGSs can also be used for astrometry (FGS3, FGS1R)
- Three FGSs provide redundancy and a larger area for selection
of guide stars
- FGS control is partly internal to the FGS, partly done
by the DF224 flight software
- Each FGS in use provides a very accurate measurement of
the location of a known guide star in spacecraft coordinates.
- In single FGS guiding mode, the FGS provides v2-v3 position
data to the vehicle control law. Roll is determined only with the gyros.
- In the normal situation, with two FGSs in the guiding loop,
the FGSs provide v2-v3 position data. They also provide roll stability data.
- FHSTs provide better roll accuracy than FGSs
- FGSs provide better roll stability than FHSTs
Fixed pointing using gyros + FGSs
Angular Momentum Management
- Pointing control is carried out by modifying the direction
and magnitude of the rotation of HST
- Reaction wheels provide the primary mechanism for altering
the rotation of HST
- RWAs provide fine rotation control for pointing adjustments
- RWAs also provide higher rotation rates for large slews
- HST has 4 RWAs, mounted in a skewed configuration relative
to the vehicle axes.
- Normal operations have all 4 RWAs in use
- It is possible to operate with 3 RWAs, with some restrictions
on slew rates
- A reaction wheel is essentially a fly wheel with a set
of motor coils that allow its speed to be adjusted
- Wheels operate in both directions, routinely changing direction
of spin
- Wheels have operational limits on rotation speed, primarily
to minimize induced vibrations in the HST structure
- RWAs are not ideal systems, corrections are needed for
friction effects and for zero crossing effects
- Angular momentum is conserved in the absence of external
torques:
Hhst = I Wv + (3 x 4 alignment matrix) Hi
- Magnetic Torquer System (MTS) consists of 4 solenoids mounted
in a skew configuration around the light shield
- Magnetic Sensing System (MSS) consists of two sets of three
axis magnetometers mounted near the aperture door
- The current through the magnetic torquing bars is adjusted
to reduce the total angular momentum as much as possible at that instant.
- Additional torques are acting on the vehicle, and are considered
in determining the actual commands sent to the RWAs.
- Kinematic disturbance torque : Wv x Hhst
- Gravity gradient torque: 3Worb2 (rv x I rv)
- Magnetic field torque
- Command torque from control law
- Feed forward torques from command generator
- The final desired torque is divided among the RWAs
- Minimize maximum wheel speed
- Correct for friction effects
- Correct for zero-crossing effects
- SAGA (Solar Array Gain Augmentation) filter is applied
to damp SA vibrations (0.1 - 0.6 Hz).
Figure: Magnetic Torque vs. Time
Figure: Magnetic Torque vs. Time
Figure: RWA RPM vs Time
Command Generator processing
- The command generator is the portion of the DF224 flight
software which establishes and maintains the on-board knowledge of where HST should
be pointing as a function of time.
- Slews, large and small
- velocity aberration
- parallax for solar system targets
- There is a command generator for the vehicle and one for
the FGSs.
Figure: Jerk vs. Time
Large Slews
- Determining slew parameters:
- Starting point is current on-board quaternion
- Slew command from ground indicates desired final quaternion
- Software computes eigenaxis and angle for slew
- Maneuver sizing is the set of computations which determine
the angular acceleration, velocity, and timing profile for the slew. These are done
in a manner to minimize the mechanical impulse to the HST to avoid exciting vibration
modes. Four parameters are determined:
- Jerk magnitude (rad/sec3)
- Jerk pulse width (sec)
- Constant acceleration interval (sec)
- Constant rate interval (sec)
- Maneuver sizing parameters are used throughout the slew
to compute the instantaneous rates and accelerations at a 40 Hz rate
- The rates and accelerations are used to compute inputs
to the control law.
- command frame increment
- command acceleration vector
- the control law ensures that the slew path is followed
- Slew errors come from gyro drift, gyro scale factor errors,
and gyro alignment errors
- A similar process is used to move the FGS star selectors
through large angles
Small Angle Maneuvers
- The maneuvers are in two axes, not three
- The maneuvers can be executed either under gyro control
or with FGSs active
- Several types of maneuvers are available:
- Basic small maneuver for offsets or tracking (#51)
- Continuous and dwell scans (obsolete)
- Step and peakup (obsolete)
- SI error nulling (#59, from NSSC-I)
- Maneuver sizing
- #51 parameters are calculated on ground
- #59 parameters are calculated on board (NSSC-I sends v2,v3
deltas)
- Sizing parameters are limited to avoid FGS loss of lock
- Feed forward information is provided to the FGSs to compensate
for the motion of the HST
- The only error source is the residual field distortions
in the FGSs. When FGSs are not used, then gyro drift rate is the only source of error.
Use of Fixed Head Star Trackers (FHSTs)
- HST has 3 FHSTs
- mounted in a skew configuration on the anti-sun side of
the spacecraft
- skew configuration increases instantaneous sky coverage
and provides good lever arm for attitude determination
- 8x8 degree field of view
- sensitive to about 6 mv
- After corrections for distortion, temperature, and star
brightness effects, accuracy is about 10 arc-seconds
- Three operational modes
- Map Mode maps 8x8 field of view for ground determination
of vehicle attitude
- Delayed mode roll updates use one FHST to update the roll
orientation prior to a large slew
- 3 axis updates use 2 FHSTs to determine and correct pointing
orientation after a large slew
- FHSTs are not (yet) used for guiding
- FGS/FHST mode was tested in August
- When implemented, will use one FGS for v2-v3 control and
a combination of FHST and gyro data for roll control
- Project goal is to have mode available after SM3
Aberration and Parallax
- Corrections for velocity aberration and parallax are applied
as small deltas to the planned attitude change vector in the control law.
- Aberration due to Earth’s motion about sun is 20 arcseconds
- Aberration due to HST motion about Earth is 5 arcseconds
- Parallax correction is based on distance to the target
(uplinked with command load)
- Differential velocity aberration is computed and applied
to FGS control law
- Computed to maintain target in aperture as HST velocity
vector changes relative to pointing direction
- effect is of order 10s of milli-arcseconds
Pointing Related Safemode States
- Software inertial hold
- complete current command
- Timeline is stopped, no further slews
- FGSs, FHSTs are not used
- On-board gyro drift value (d) zeroed
- Pointing data is provided only by gyros
- Pointing is maintained at current attitude
- Magnetometers used to measure B field
- Software sun Point
- Terminate current command
- Take other inertial hold actions
- Use Coarse Sun Sensors (CSS) to point +v3 axis at sun
- Will carry out predefined slews to get sun in proper CSS,
if it is not there.
- Adjust Solar Arrays to proper orientation
- Gyro hold after Sun is found
- Zero Gyro Sun Point
- Does not use gyros
- Uses CSS and magnetometers to develop rate and position
data to point +v3 to sun
- Drifts during night portion of orbit
- Close aperture door
- Spin up RWAs to "stiffen" system against drift
- Spin Stabilized Sun Point
- Uses CSS to point -V1 at sun
- Does not use RWAs
- Uses magnetic torquers to spin HST to about 0.8 degree/second
to provide stability
- PSEA (Hardware safemode)
- Used if DF224 is suspect
- Control laws executed by PSEA microprocessor
- Similar to software sunpoint mode
- Will use RGAs, if they are "good"
- Will power up Retrieval Mode Gyros (RMGAs) if needed
- RMGAs are low quality gyros intended only to provide sufficient
pointing to maintain HST health. They are normally left off.
