Complex Systems Control

1. Space Station
Complex Systems Control
Naz Bedrossian
The Charles Stark Draper Laboratory
Hubble Space Telescope
Space Shuttle
42Klb RS Functional Cargo Block Berthing
1 - 05/09/08

2. Introduction
With many years spaceflight experience, Draper has developed unique
- work processes, technologies, tools
for reliable and high confidence control of complex space systems which
- meet and exceed performance requirements
while
- reducing overall cost and risk
with demonstrated capability to
- extend lifetime, expand capability, recover from failure
System Life-Cycle Phases Where Draper Can Contribute
LIFE EXTENSION EXPAND CAPABILITY FAILURE RECOVERY
2 - 05/09/08

3. Spaceflight Experience – Space Shuttle (1980’s – Present)
Design agent for on-orbit Flight Control System (FCS) for 20+ years
- For unique Shuttle payload flights and Space Station assembly
- Payloads: Space Station, MIR, Hubble Space Telescope, Space Radar Topography Mission
Perform FCS Flight Readiness Certification for all payloads
- Formal process by which spacecraft performance is assured to meet mission objectives despite
uncertainties and failure contingencies
Perform Controller-Structure-Interaction (CSI) Certification
- To preclude dynamic interaction between control systems and spacecraft structure
Provide mission support
- Perform real-time development of contingency responses to in-flight anomalies
Develop FCS Updates
- To meet changing program objectives and reduce operational cost
- Thruster pulse-train solutions have been designed and certified to minimize structural loads
Provide simulation and modeling expertise
- Develop flight validated simulations for Orbiter & Shuttle Remote Manipulator System (SRMS)
- Draper SRMS is NASA’s highest fidelity simulation
3 - 05/09/08

4. Spaceflight Experience – Other Spacecraft
Space Station (1998 - Present)
- Perform CSI Flight Readiness Certification for each assembly stage
⋅ For both US CMG and Russian Thruster control systems
- Designed attitude control system for Orbiter Repair Maneuver contingency
⋅ 250Klb Orbiter maneuvered by flexible SRMS while attached to flexible 400Klb Station
flexible
- Implemented in flight software load minimizing thruster pulsetrain
- Provide Russian control systems modeling expertise
Large Space Structures (Classified)
- Derive mission requirements and develop control systems for flex control
Hughes Satellites
- Developed independent vehicle dynamics and control simulations
⋅ Draper derived and implemented the equations of motion for the 5 body flexible dynamics model
- Designed qualification tests
4 - 05/09/08

5. Expand Capability – Shuttle Control Of Very Large Payload
Developed new attitude control system capabilities within existing flight
software in order to control very large payloads
- MIR space station, International Space Station, Hubble Space Telescope
Challenge
- Low frequency flex can cause instability
Solution: Modify Attitude Control System
- Developed notch flex filters to attenuate flex and thruster command shaping to not excite flex
Notch Filtering
- Attenuate low frequency and large amplitude flex for stable control without adversely
affecting rigid-body performance
Thruster Command Shaping
- Used to reduce structural excitation due to control action
- Break-up command into a pulsetrain and introduce time delay between pulses
5 - 05/09/08

6. Expand Capability – Shuttle Control Of Very Large Payload
Flexure at Docking
Interface
Can Cause Instability
When Orbiter Is In
Control
amp *1, freq *0.7, no notches amp *2 freq *1: notches enabled
Without Notch With Notch
0.080 0.20
0.040 0.10
0.000 0.00
Tekplot
Tekplot
WERRY
WERRY
hall
hall
-.040 -0.10
Thu Oct 12 16:20:26 2000
Fri Oct 13 14:18:28 2000
-.080
-12.0 Unstable
-8.0 -4.0
ATTERY
0.0 4.0
-0.20
-15.0 -10.0 Stable -5.0
ATTERY
0.0 5.0 10.0
6 - 05/09/08

7. Expand Capability – Space Station Control Of Very Large Payload
Developed new attitude control system capabilities within existing flight software in order
to control Shuttle attached to robotic arm
- Required capability after Columbia accident to repair Shuttle Tile Protective Services
Tile
Challenge
- Damp large rate errors (up to 0.2deg/sec) while avoiding large Orbiter motion or contact with Station
Orbiter
- Low frequency (in 0.01Hz range) and 10X larger amplitude flex than any other robotic operation
than
- Controller rigid body BW and flex close to each other
- Achieve adequate stability margins while maintaining vehicle control
control
- Design controller not requiring updates even though mass properties and flex different at each waypoint
properties
- Russian thruster controller could not be used as it could not limit thruster on times resulting in large loads
limit
Solution: Modify CMG Attitude Controller For Thruster Control
- Developed new Pulse-Width-Modulation mode for US CMG attitude hold PID controller with new flex filters
Pulse- Width-
to command thrusters instead of CMGs (CMG-RCS in figure)
(CMG-
- This US Thruster Only (USTO) mode has been in use since 2005 for nominal operations
Flex Filters
- Active control of very low frequency flex modes, < 0.04Hz
- Attenuate higher frequency flex modes, > 0.04Hz
- Designed using novel Computational Optimization approach
7 - 05/09/08

8. Expand Capability – Space Station Control Of Very Large Payload
Reposition Orbiter Using Robotic Arm
8 - 05/09/08

9. Expand Capability – Space Station Large Angle Rotation w/ CMGs
Developed new CMG attitude control system capability within existing flight
software to perform large angle rotations without saturating CMGs
- Does not use any thrusters/propellant
Challenge
- Avoid CMG momentum and torque saturation during large angle rotation
- Do not impose restrictions on Station operation, e.g. array articulation
Solution: Develop New ZPM Guidance Method To Command CMG Controller
- Exploit knowledge about environmental dynamics to optimally plan commanded attitude trajectory
in order to extract angular momentum from environment
- Solved using Computational Optimization without any modifications to flight software
Flight Demonstration
- Commanded CMGs to rotate Space Station 90deg (11/5/2006), and 180deg (3/3/2007) without
need to desaturate CMGs with thrusters or change flight software
- On 1/2/2007 identical 180deg rotation with thrusters used ~110lbs of propellant @ ~$1,100,000
9 - 05/09/08

10. Expand Capability – Space Station Large Angle Rotation w/ CMGs
90o ZPM - Flight Telemetry Animation
10 - 05/09/08

11. Recover From Failure – Catch Tumbling Station With CMGs
Applied ZPM guidance to command CMGs and recover attitude control of
Space Station from a tumbling state without saturating momentum
Challenge
- Russian computers failed on June 11, 2007 during STS-117 Shuttle mission
- Closed-loop Station attitude control capability with thrusters was lost
- Station would tumble out-of-control after Shuttle undocking
- Astronauts would have to abandon Station
Solution: Recover Station Attitude Control With ZPM Guidance
- First rate damp without controlling attitude
- Then rotate Station to desired long-term attitude hold orientation
11 - 05/09/08

12. Recover From Failure – Catch Tumbling Station With CMGs
0.1deg/sec Initial Rate Simulation Animation
12 - 05/09/08

13. Unique Technologies To Reduce Cost/Schedule/Risk
Nonlinear Control
- Phase Plane for thruster actuated systems
Forcing Functions
- Used to rapidly evaluate Controller-Structure-Interaction and Loads
Computational Optimization
- Used to design linear and nonlinear control systems for
⋅ Space Station Orbiter Repair PWM thruster control PID gains and flex filters
⋅ Space Station Momentum Manager with peak momentum constraint
- Used for nonlinear trajectory generation
⋅ Space Station ZPM guidance for CMG large angle propellant-free rotations
Command Shaping
- Used to reduce structural excitation due to control action
⋅ Break-up command into a pulsetrain and introduce time delay between pulses
Active Structure Control
13 - 05/09/08

14. Unique Tools To Reduce Cost/Schedule/Risk
Modular, dynamically scalable, and automated tools using COTS SW
- Extensively used to support Shuttle & Space Station Flight Readiness Certification
SiVAT - Singular Value Analysis Tool
- Provides Shuttle stability margins for nonlinear reaction control systems
NoFDAP - Notch Filter Design Analysis Package
- Design Shuttle notch filters to ensure flex stability, stability envelope for robotic motion, and
worst case structural loading
DRS - Draper RMS Simulation
- Flight validated super hi-fi Shuttle arm model
DSS - Draper Space Station Simulation
- Family of modular, hi-fi flex on-orbit simulations including
⋅ Russian Service Module, US CMG, Interim Control Module (NRL) flight control systems, Solar and
flight
Radiator Array Controllers, Robotic Systems
DSAT - Draper Station Analysis Tool
- Automated simulation configuration/analysis process/analysis documentation
⋅ Stability, CSI and robust performance evaluation
14 - 05/09/08

15. Capabilities
Modeling
- Multi-Body Rigid + Flex Dynamics
Analysis
- Frequency Response
- Robust Stability
- Monte Carlo
Control
- Vibration Isolation
- Active Structure Control
- Command Pre-Shaping
- Controller-Structure-Interaction
- Precision Pointing & Tracking
15 - 05/09/08

16. Backup Material
16 - 05/09/08

17. Expand Capability – Space Station Control Of Very Large Payload
Orbiter Position wrt Station Space Station Attitude wrt LVLH
Existing CMG controller design
Orbiter CM Rotation [deg]
CMG controller interacts with SRMS dynamics
Orbiter CM Translation [in]
!
iter
y Orb Space Station Holding Attitude!
na wa
Ru
Large Orbiter motion with respect to Station
A really bad day at the office!
17 - 05/09/08

18. Unique Work Processes – Draper Control Framework
Key Design Criteria
- Controllability, Stability, Loads, Performance
Key System Functions
- Dynamics, Control, Filtering, Command Shaping
18 - 05/09/08

19. Space Station GN&C Systems Certification
Integrated platform for design and verification of aerospace systems
Developed by Draper and in use since 1998 for flight readiness certification of integrated
performance of ISS flexible structure with Russian and US control systems
19 - 05/09/08

20. Unique Work Processes – Flight Readiness Certification I
Quantify achievable performance and risk exposure for available resources
Quantify achievable performance and risk exposure for available resources
Start with simple system models and gradually add fidelity
- Capture most significant dynamics first
⋅ Nonlinear time-invariant rigid dynamics, linear modal form time-invariant flex dynamics
time- time-
⋅ Provides qualitative understanding and confidence to proceed
- Allows low cost and rapid turnaround
- Risk reduced due to rapid evaluation capability for wide range of conditions and parameters
Reduce analysis scope where possible
- All steps for all flights/operations => enormous scope, large $!
- Use analytic and frequency domain methods to rule-out and identify worst-cases
⋅ Classify and prioritize analysis
- Reduces cost and schedule while identifying high risk conditions
High-fidelity complex dynamic models are expensive and time consuming to
develop and do not provide insight and qualitative understanding
- Only gives the illusion of understanding and confidence
20 - 05/09/08

21. Unique Work Processes – Flight Readiness Certification II
Confidence in System Performance is Exponentially Faster & Cheaper To
Confidence in System Performance is Exponentially Faster & Cheaper To
Achieve with Simpler Models Than Accuracy with Complex Models
Achieve with Simpler Models Than Accuracy with Complex Models
Draper uses a two-stage process
- Screening & High-Fidelity Simulation Analysis
Screening Analysis: ~90% of resources
- Wide scope evaluation with high speed tools to ID high risk conditions
- More important to assess performance for a large number of system parameters/conditions
than perform detailed analysis of a few operating points
High-Fidelity Simulation Analysis: ~10% of resources
- For spot-checks and detailed narrow scope evaluation
- Using nonlinear time-varying articulating multi-body rigid/flex dynamics
⋅ Insures time varying effects do not adversely impact performance
Risk vs Cost vs Schedule Tradeoffs
21 - 05/09/08

22. Shuttle – Flexible SRMS – Space Station Simulation
22 - 05/09/08