ME176: (Space!) Machine Design

ME176: (Space!) Machine Design
ADCS Design & Hardware February 20th, 2003 Aaron Rogers ADCS Design & Hardware ME176: Lecture 5

Introductions and Overview
ADCS Design & Hardware ME176: Lecture 5

Review of Last Section: Orbits

Review of Last Section: Orbits Cont.
d e s c e n d i n g n o d e s a t e l l i t e ' s p o s i t i o n a t e p o c h w = a r g u m e n t o f p e r i g e e  = true anomaly at epoch line of nodes r = p e r i g e e r a d i u s p r = a p o g e e r a d i u s a r a = s e m i - m a j o r a x i s = ( r + r ) / 2  o a p e = e c c e n t r i c i t y = ( r - r ) / ( r + r ) a p a p r p r a w E a r t h a s c e n d i n g n o d e G N C - 7 9 ADCS Design & Hardware ME176: Lecture 5

Review of Last Section: Hohmann Transfer
At T0=0 min, R' = RD + DR where RD=1.5km (nom. baseline), DR > 300km (20% baseline) T0: Initiate Transfer Orbit TF: Circularize Into Target Orbit T ~ 24 min T ~ 71 min Transfer Orbit: Apogee Alt=500km – dAlt At TF~95 min, R' = RD ADCS Design & Hardware ME176: Lecture 5

Commercial Satellites at GEO

(Re) Orientation 1 - Introduction 2 - Propulsion & ∆V
3 - Launch Vehicles 4 - Orbits & Orbit Determination 5 – Attitude Determination and Control Sys. Design & Hardware Coordinate Frames Attitude Determination Environmental Disturbances Vehicle Stabilization Methods Attitude Control Control System Design Assembly, Integration & Test Simplifying ACS 6 - Power & Mechanisms 7 - Radio & Comms 8 - Thermal / Mechanical Design. FEA 9 - Reliability 10 - Digital & Software 11 - Project Management Cost / Schedule 12 - Getting Designs Done 13 - Design Presentations Another night of F=MA? Education is the process of realizing that you don’t know what you thought you knew... Sporadic Events: •Mixers •Guest Speakers •Working on Designs •Teleconferencing ADCS Design & Hardware ME176: Lecture 5

Homework Questions for 2/27
For your selected Mission: Pick two attitude control approaches that might work List the sensors and actuators necessary to implement each of them. How accurate / sensitive would each have to be? Any other special requirements ($, mass, volume, power, bandwidth etc.) Locate them on a “generic” spacecraft (e.g. a cube or faceted sphere Make a block diagram of the feedback control system you envision Pick your favorite of the two, and tell me why it’s your favorite (compare $, mass, complexity, performance…) Comments: Eg. Gravity Gradient, Thompson Spin & TS with momentum storage are options for an earth pointer. Also search Web to locate actual components that might be candidates - not selections, but possibilities. ACS thrusters want to be in pairs and far from the CG. Torque coils don’t care. Sensors have to have a clear view out etc. What is the plant, what are the sensors, what is the actuator suite? What are the “set points?” Make a “trade table” listing specs / attributes of each to justify your selection. ADCS Design & Hardware ME176: Lecture 5

Homework Questions Cont. for 2/27
For your Favorite of the two: List the ACS modes and the triggers to proceed from one to another. Diagram with a flow chart. Suggest a simple algorithm for your mission mode. Model it on Excel and show that it has a prayer of working List requirements your selected ACS imposes on the spacecraft List a candidate component suite and estimate the cost and labor to design, build & test. Comments: Modes might include sleep, initial rate killer, sun or earth finder, rough point, tight point, and safe/hold. For instance, measure an error angle or rate and actuate something to reduce that error. For example: mass distribution, symmetry, power, siting, computation, magnetic / electromagnetic cleanliness Assume an engineering year costs $200,000 including the tools and toys necessary to play with ACS in the lab. ADCS Design & Hardware ME176: Lecture 5

Reading Suggestions for 2/27
Power: SMAD Chapter 11.4 TLOM Chapter 13, 14 Mechanisms (SMAD): Chapter 11.6 Extra ADCS Review (SMAD): Chapt. 6.2: Orbit Perturbations Chapt. 6.2: Orbit Maintenance Chapt. 11.1: ADCS Extra ADCS Review (TLOM): Chapters 6, 11 ADCS Design & Hardware ME176: Lecture 5

Spacecraft Bus Block Diagram
Electrical Power Arcjets Thermal Control Structure Solar Arrays AJTs LAE Tanks Valves Lines - Heat Pipes - Heaters - OSRs Deployment Mechanisms Batteries REAs Mechanisms 70 VDC Power Regulation Unit (PRU) Propulsion Fuse Box Attitude Control Pyro Power Bus RIUs Power Bus Pyro Relays Wheels Pyro Fire Pyro Control Wheel Control MIL-STD 1553 Data Bus ESA To Payload Accepted Commands TLM Words P/L RIUs SSA OBCs UDU TT&C Baseband IMU TT&C Antennas Baseband CMD/TLM RF, CMD, TLM Ranging RF Couplers Receiver/ Transmitter TT&C RF ADCS Design & Hardware ME176: Lecture 5

GN&C Coordinate Frames
Body frame Fixed in & rotates with the spacecraft Reference for sensor & actuator alignments Reference for control torque calculations Earth-Centered Inertial (ECI) frame Constant orientation in inertial space Used to define spacecraft & sun ephemeris for attitude determination Orbital frame Earth-oriented coordinate frame defines nominal attitude Orientation in space depends on spacecraft’s orbital location Target frame The frame control system aligns the body frame with Defined with commanded offsets relative to orbital frame ADCS Design & Hardware ME176: Lecture 5

Body Frame Coordinate system origin Xb-axis (yaw) Yb-axis (roll)
Geometric center of separation plane Xb-axis (yaw) Perpendicular to the separation plane Points away from the center of the spacecraft Yb-axis (roll) Perpendicular to the E & W panels Points toward the east panel Zb-axis (pitch) Perpendicular to the north & south panels Points toward the north panel ADCS Design & Hardware ME176: Lecture 5

Earth-Centered Inertial (ECI) Frame
XECI Parallel to the intersection of Earth’s equatorial plane and the ecliptic plane Positive axis points toward the sun at the vernal equinox ZECI Parallel to Earth’s polar axis Positive axis points north YECI Completes the right-handed triad ADCS Design & Hardware ME176: Lecture 5

ECI Frame (Continued) ADCS Design & Hardware ME176: Lecture 5

ECI Frame & Orbital Frame
Yaw is toward zenith (straight up from Earth) Pitch is perpendicular to the orbit plane Roll is perpendicular to yaw and pitch and points in the direction that the satellite is moving ADCS Design & Hardware ME176: Lecture 5

Target Frame During normal operations, the spacecraft body axes are controlled to the target coordinate frame The orientation of this frame relative to the orbital frame is defined by the enabled pointing offsets Constant Earth-target Fourier ADCS Design & Hardware ME176: Lecture 5

Attitude Determination: The Problem
Where am I looking in space?! ADCS Design & Hardware ME176: Lecture 5

Attitude Determination: Magnetometers
Magnetometer measures applied magnetic field, outputs two or three magnitudes: B= [X, Y, Z]. With known orbit model (IGRF2000) and ephemeris, can calculate attitude by comparing measured vs. expected field direction. Low cost and low power, though does require some EMI isolation. ADCS Design & Hardware ME176: Lecture 5

Attitude Determination: Sun Sensors

SSA Mounting & Field of View
+ P i t c h Sunline A z m u n g l e E v a o r j f s y w - p SSA boresight ADCS Design & Hardware ME176: Lecture 5

SSA Problems: Earth Albedo
Problem: Earth Albedo at Low Altitude The SSA sun detection threshold is 20% of the nominal solar intensity At low altitude, Earth albedo can be 40% as bright as the sun Albedo can trigger a false sun-presence indication and cause erroneous sun azimuth and elevation readings Conditions that can cause the problem: Transfer orbit perigee altitude below 5000 km Perigee on Earth’s sunlight side Solution (GEO spacecraft) Suspend use of sun sensor data when the spacecraft altitude is below 5000 km. ADCS Design & Hardware ME176: Lecture 5

SSA Problems: Moon Interference
Problem: Moon Interference (Partial Solar Eclipse) The SSA’s detection threshold is 20% of the nominal solar intensity The SSA will detect the sun during a partial solar eclipse During a partial eclipse, the centroid of the visible solar crescent is offset from the sun’s true centroid This produces a 0.1° to 0.2 ° error in the measured sun angle Note The sun's visible surface has an angular diameter of 0.53 deg. as seen from Earth ADCS Design & Hardware ME176: Lecture 5

Attitude Determination: Earth Sensors
Earth sensor assembly (ESA) provides Roll & Pitch attitude data Used to update inertial attitude reference Data used indirectly in a highly filtered form during normal operations Data used directly with little filtering during Earth acquisition ADCS Design & Hardware ME176: Lecture 5

ESA Scans With a Pitch Offset
Pitch is determined from the offset between the center reference pulse and the center of the Earth. ADCS Design & Hardware ME176: Lecture 5

ESA Scans With a Roll Offset
Roll is determined from the difference between the lengths of the north and south scans across Earth. ADCS Design & Hardware ME176: Lecture 5

ESA Problems: Multiple Targets
ESA response to this condition: Detects two targets in the south scan Automatically inhibits the south scan Uses north scan for pitch angle calculations Uses north scan and standard chord for roll angle calculations Outputs sun presence bit = 1 (multiple targets detected) ADCS Design & Hardware ME176: Lecture 5

ESA Problems: Non-Distinct Targets
ESA response to this condition: Detects only one target in the south scan Continues to use the south scan (scan is not inhibited) Outputs erroneous pitch and roll angles Outputs sun presence bit = 0 (only one target detected) ADCS Design & Hardware ME176: Lecture 5

Attitude Determination: Star Trackers
Roll, Pitch, and Yaw Attitude (x, y, z) Processor Image Star Catalog Pattern Recognition Software Pinhole Lens Active Pixel CMOS Imager 30° Field of View Utilizes a light sensitive medium (CMOS, CCD) Pattern recognition of detected images against internal star catalog Acquisition, track modes Extremely high precision (typically high cost) Sensitive to stray light (baffles) ADCS Design & Hardware ME176: Lecture 5

Attitude Determination: Propagation
Data output from the IMU CPU: The sampled angular outputs developed by each gyro The sampled acceleration outputs developed by each accelerometer Data is typically only available during significant orbit adjust maneuvers! Integrates linear and angular rates in order to propagate state vectors Typically operates much faster than sensor measurements are taken Important when attitude update is not available (e.g. no sun). ADCS Design & Hardware ME176: Lecture 5

IMU Functional Block Diagram

Gyro & Accelerometer Alignments
Positive sense axis is in the Y-Z plane, offset 125.3 from the +Z axis Gyros B, and C Positive sense axes are offset 125.3 from the +Z axis Projections of the sense axes on the X-Y plane are 60 from the +Y axis (the positive roll axis) Gyro D Senses rotation about the +Z axis (the positive pitch axis) Accelerometers Positive sense axes point in the +X direction ADCS Design & Hardware ME176: Lecture 5

Attitude Determination Options
Accuracy Operational Flexibility Recurring Cost ADCS Complexity Design Impact Development Risk GPS + INS < 1 deg Low Medium Sun Sensor + Magnetometer < 5 deg Medium (Software) Sun Sensor + Horizon Sensor < 0.5 deg Star Tracker < 0.1 deg High AeroAstro Mini ADCS Design & Hardware ME176: Lecture 5

Environmental Disturbances
Atmospheric Drag (LEO) Function of Ballistic Coefficient, Altitude Solar radiation Function of Surface Area, |CG – CSP| Produces torque about all three axes Varies with season and time of day Payload transmissions (recoil effect) Primarily a pitch torque Total Environmental Disturbance Torque ADCS Design & Hardware ME176: Lecture 5

Environmental Disturbances Cont.
Geomagnetic field (compass needle effect) Due to residual uncompensated dipole, varies with R-3 Primarily a yaw and roll disturbance Pitch torque produced only when solar storms temporarily distort the geomagnetic field Gravity Gradient (LEO) Due to asymmetric mass distribution Torques about pitch and roll axes Function off-nadir angle (theta), R-3 Thermal radiation (recoil effect) A function of the heat radiated from various spacecraft surfaces Geomagnetic Torque Gravity Gradient Torque ADCS Design & Hardware ME176: Lecture 5

Environmental Disturbances Cont.
Solar wind (flow of charged particles) Very small effect Earth’s magnetosphere deflects the solar wind before it strikes the spacecraft Force can increase temporarily (for a few hours) during strong solar storms that distort the magnetosphere Worst-case is still a small effect Micrometeoroid impact Occasional small events (several times a year) Most impacting particles are so small that the effects are barely noticeable Angular impulse almost always <0.5 in.lb.sec (<0.06 Nms) ADCS Design & Hardware ME176: Lecture 5

Anarchy Happens: Unstabilized
Pros Cons Cheap, Simple, Reliable Can still determine attitude Complicates Radio Antennas Many missions impossible (eg imaging) How to ensure thermal balance? How to guard against spin-up? ADCS Design & Hardware ME176: Lecture 5

Passive Stabilization: Gravity Gradient
Pros Cons Cheap, Simple, Reliable Can still determine attitude Typical pointing performance: ±5° Complicates Radio Antennas Many missions impossible (eg imaging) How to ensure thermal balance? Major deployable How simple & cheap is it? Very weak GG Torque = 3w2∆I = 3 x (2π/6000s) 2 x 1 kg-m2 = 3x10-6 N-m = 2 millionths of a foot pound ADCS Design & Hardware ME176: Lecture 5

Passive Stabilization: Permanent Magnet
Pros Cons Cheap, Simple, Reliable Pointing sideways often handy Passive yet strong No yaw control Flip 2x per orbit at poles Damping? Pointing typically ±5° ADCS Design & Hardware ME176: Lecture 5

Passive Stabilization: Aerodynamic
Pros Cons Only simple way to detect & point straight ahead Pointing sideways often handy Passive yet strong Pointing typically ±3° Yaw damped but not controlled Narrow altitude range => short lifetime May require deployables Damping may be necessary Aero Torque = 1/2rAV2(cp-cg) = 1/2 x 10-10kg/m3 x 1m2 x m2/s2 x 1 m 300 km) = 2.5 x 10-3 N-m = 1.9 thousandths of a foot- pound ADCS Design & Hardware ME176: Lecture 5

Spin-Stabilization Pros Cons
A non-spinning body subjected to a torque impulse will begin to tumble and continue doing so. Disturbances cancel / avg out Easy attitude determination Thermal rotisserie Typical pointing performance ±2° Sensor deconvolution Only one locale nadir pointing CG Control A spinning body subjected to a torque impulse will precess its spin axis and otherwise will be unaffected ADCS Design & Hardware ME176: Lecture 5

Spin Stabilization and Mom. of Inertia
Neutrally Stable Minimum Axis IX < IY, IZ Unstable Intermediate Axis IY < IX < IZ For stable, minimum ACS design, SS prefers rotation about the principle axis aligned along the thrust vector, PX, where in general: IX > 2*(IY, IZ). Rotation is possible, with nutation damping, about minimum axis, where: PX, where in general: IX < IY, IZ. Absolutely Stable Principal Axis IX > IY, IZ ADCS Design & Hardware ME176: Lecture 5

Thompson (non) Spinner
Momentum Wheel (non-spinner) Pros Cons Disturbances & thermal loads cancel / avg out Inherently stabile Antennas broadside to earth (+ 3 dB) No moving parts Scan pattern for sensors Whole earth nadir pointing spinner Solar panel usage (1/π) Non-spinner requires single mo. wheel CG Control Non-spinner can stare and track subsatellite and lateral to subsatellite points ADCS Design & Hardware ME176: Lecture 5

Sun (non) Spinner Pros Cons X-coil Z-coil Y-coil
Coarse Sun Sensor (12) Huge electric power gen. Stabile thermal / illumination environment High performance at low cost Pointing accuracy 0.2° Pointing knowledge 0.05° Roll angle hard to determine Attitude solution in umbra requires filter CG critical - difficult with deployables Magnetometer Coil Driver X-coil Flight Computer Horizon Z-coil Crossing Indicator (HCI) Fine Sun Sensors (2) Y-coil Non-Spinner: add just one wheel. Q: On which axis? ADCS Design & Hardware ME176: Lecture 5

Three-Axis Stabilized
Pros Cons Arbitrary pointing & staring Simple propulsion for station keeping Mass distribution not critical Difficult thermal control & power generation High power required Cost, mass & complexity Spin-up Wheel control Lost wheel Torque noise 4 wheels divide three axes ADCS Design & Hardware ME176: Lecture 5

Vehicle Stabilization Methods
Option Accuracy Operational Flexibility Design Cost Complexity Development Risk Passive Stabilization Low Magnetic Atmospheric Spin-Stabilized Medium 3-Axis Control High ADCS Design & Hardware ME176: Lecture 5

Spin vs. 3-Axis Stabilization
Rank Parameter Spin Stabilization 3-Axis Stabilization (Thrusters) 1 Disturbance Rejection Directly proportional to stack MOI and spin-rate. High rpm might constrain AD, OD, and ConOps. Will require propellant for spin/de-spin. Requires appropriate sizing of thrusters and propellant. 2 Sensitivity to Stack Moments of Inertia Large dependency on payload mass properties (MP). Will likely require trim mass, frequent measurements of stack MP, and update of spin rate. ACS can accommodate spacecraft plant through modification of software. 3 Thrust Misalignment See (1). Commanded thrust will precess and nutate spacecraft attitude. See (1). 4 Slewing Requires “turning” stiff rotation vector. Easily accomplished; see (1). 5 Pointing Inertial pointing only; accuracy highly dependent upon MOI and spin rate (2). Capable of high accuracy, tracking, offset commanding. 6 C.G. Management CG migration measurement must be extracted from off-axis rotation rates and may require analysis/update of spin rate (2). CG migration can be determined from system performance and is easily accommodated; see (1). 7 Propellant Management Inherently settles propellant for primary orbit adjust, but may inhibit fuel flow near end of life. May require a short settling burn before start of primary ignition. 8 Slosh High spin rates can reduce slosh effects. Slosh effects handled through software and filter design. ADCS Design & Hardware ME176: Lecture 5

Attitude Control H/W: Torque Rods/Coils
A torquer consists of a coil (or two redundant coils) around a soft iron core. Coil magnetizes the iron core Long, slender core magnetizes easier and more uniformly than a short, “fat” bar. Longer bar uses less power and coil mass. Subject to hysteresis saturation effects. Typically with wheels and/or gravity gradient booms. ADCS Design & Hardware ME176: Lecture 5

Attitude Control H/W: Reaction Wheels
The positive momentum/torque axes are all 45 from the pitch axis The projections of the momentum/torque axes onto the yaw/roll plane are all 45 from the yaw and roll axes ADCS Design & Hardware ME176: Lecture 5

Reaction Wheel vs. Momentum Wheel
Bi-directional Operates over a large speed range (positive & negative) Generates torque by controlling motor current Momentum Wheel Unidirectional Operates in a narrow range about a high nominal speed Torque depends on difference between commanded speed & current speed Mechanically, there is no difference between a reaction wheel and a momentum wheel. ADCS Design & Hardware ME176: Lecture 5

Typical Pitch Momentum Profile
Daily oscillation is due mainly to solar radiation pressure Long-term slope is due mainly to payload transmissions ADCS Design & Hardware ME176: Lecture 5

Yaw/Roll Momentum Interchange
If there are no disturbances The angular momentum vector has a constant magnitude and a constant direction in space The spacecraft rotates once per orbit Necessary for the payload to continuously face Earth As viewed from the spacecraft coordinate system Angular momentum is exchanged between the yaw and roll axes The wheel speeds vary once per orbit, with the yaw and roll momentum 90 out of phase ADCS Design & Hardware ME176: Lecture 5

Momentum Interchange (Continued)
If there are no disturbances, the angular momentum vector has a constant magnitude and a constant direction in space. ADCS Design & Hardware ME176: Lecture 5

Typical Roll & Yaw Momentum Profiles

Attitude Control H/W: Thrusters
Want 6-DOF Control ADCS Design & Hardware ME176: Lecture 5

Attitude Control Options
Torque Authority Pointing Accuracy Recurring Cost Disturbance Rejection Development Risk Gravity Gradient Low Momentum Bias, 1 Reaction Wheel Assembly (RWA) High, Single-Axis Medium RWA (3 or 4) High, All-Axes High Magnetic Torquers Low, All-Axes Thrusters Thrusters + RWAs ADCS Design & Hardware ME176: Lecture 5

GN&C Block Diagram Sensors ACS Algorithms (resides in OBC) Actuators
0.2 lb REA (12) ODDS LOGIC Attitude Command Processing I M U (1) B u s S c h e d l r Sensor Processing Logic B u s S c h e d l r R I U Control Mode Specific Logic Timed Pulse 5 lb REA (6) Thruster SSA (2) P I D Controller RWA R I U ESA (2) LAE 1 Redundancy Mgt. Logic Automatic Switching Logic Momentum Mgt. Logic RWA Tach (4) AJT (4) Attitude Determination and Ephemeris Propagation Logic RWAs (4) 1553 Data Bus 1553 Data Bus ADCS Design & Hardware ME176: Lecture 5

Spacecraft Feedback Controller
+/- Thruster Torque a time d2Q/dt2 = Torque/I “point at sun” Q = 0 => V=0 V = Volts Error Angle = Sun Sense - 0 T = -k(Q) Actuator Set point Error Control Algorithm Plant (satellite) Disturbances Sensor Amazingly complicated - every controller has all these parts Simple example - keeping a car between the white lines: - set point is middle of the lane - error is veering toward one of the white lines (dx from centerline) - Control algorithm - turn the wheel proportional to the error or rate of change of the error - actuator is steering system (via your arms) - Plant is the car which has dynamics - ie how it “responds” to steering. Every car is different, and the algorithm has to adapt. - disturbances: wind gust, banked roads, curves, construction zones (white lines zigzag) Sensor - your eyes - plus kinesthetic sensations Sun Sensor V a Q ADCS Design & Hardware ME176: Lecture 5

0th Order Angle Controller
The rotational analogue of the spring / mass system where the spring is a controller proportional to position - here our controller is proportional to angle of rotation. The spring produced a force which accelerates the mass linearly. Applying a torque accelerates the angle (constant rate of change of omega) ADCS Design & Hardware ME176: Lecture 5

1st Order Angle Controller
In the Spring / Mass / Damper system, the damper does not know or care about position - it provides a force proportional to, and opposite, any velocity. In space there’s no “friction” so the control must be active, not like a passive damper since there is no rotational sink (ie the non-rotating room your rotating object is within). ADCS Design & Hardware ME176: Lecture 5

Assembly, Integration & Test

ACS Test Setups Level 1 (sometimes all in 1 computer) Level 2
Sensor Signals • Actuator model Control • Actuator commands Algorithm --> Controller Plant • Dynamic model • Orbit / Universe model • Sensor Model Level 2 => Sensor Signals Actuator Commands Simulated Environment Real Control software • Actuator model Real sensors Plant • Dynamic model Spacecraft Real output to • Orbit / Universe model actuators --> Controller • Sensor simulation Actuator => simulated environment Commands ADCS Design & Hardware ME176: Lecture 5

More ACS Test Setups • Level 3 • Level 4 (Big $) (on-orbit tweaking)
Spacecraft in vacuum on zero-mass, zero-friction simulation mount • Level 4 (on-orbit tweaking) ADCS Design & Hardware ME176: Lecture 5

Simplifying ACS Sensors Development and Test Managing the Payload
Use simple ones: sun, magnetometers Use payload instruments as sensors but beware accuracy limits and loop time Design using low-cost vendors, flight spares etc. Development and Test Build testability into design Use Matlab or equivalent Simulate dynamics and sensors with external PC Use safe modes and assume final tweeks on orbit Managing the Payload Can it search a little bit? Scanning vs. staring Larger apertures = shorter integration duration Duty cycling to avoid interference Self registration and non-real-time attitude reconstruction System Design Choose simple modes - spinners, gg Avoid deployables Relax pointing / determination accuracy Use switching antennas and other techniques to eliminate some pointing requirements Basic autonomy / safing on board - Handle anomalies on the ground Actuators Air core torque coils where possible single wheel vs. 4-wheel momentum storage avoid propulsion (toxic, leaks, fluid handling, safety, lifetime limits) Alternative Approaches Attitude Determination vs. Control Wide FOV instruments / multiple instruments Unstabilized and passive stabilization ADCS Design & Hardware ME176: Lecture 5

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