balancing of an air-bearing-based acs test bed
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Balancing of air-bearing-based ACS Test Bed
Facoltà di Ingegneria Civile e IndustrialeCorso di Laurea in Ingegneria Spaziale e
Astronautica
Candidato:Cesare Pepponi
Relatore:Prof. Luciano IessCorrelatore:Ing. Mirco Junior Mariani
A.A. 2015/2016
ACS TEST BED GENERAL DESCRIPTION
• It is a test bed for satellite ACS testing, with the goal of reproducing the space environment.
• It is composed by:– HELMHOLTZ COILS: to reproduce the Earth magnetic field the
satellite will meet along its orbit.
– MOVING SOLAR LAMP: to reproduce the Sun position WRT the satellite during its orbit.
– PLATFORM: to reproduce a frictionless environment with no external torques
This thesis focuses on the platformmass balancing
MOTIVATIONS:My thesis aims at determine a mass balancing technique for an ACS test Bed with the following features: • The platform shall host satellites up to 50 kg.• Maximum tilt angle allowed: 40°.
GOALS:• Reduce, by a suitable balancing technique, the residual gravitational torque to a value lower than 10-4 Nm.
The residual gravitational torque is due to the offset between the CM and CR:
• Estimate the inertia (platform + S/C) matrix elements with an accuracy lower than 10-2 kgm2.
• Validate the model through Monte Carlo simulations.
PLATFORM MASS DISTRIBUTIONThe elements composing the platform have been modeled as discrete, point-shaped, masses.
mass [kg] X [m] Y[m] Z[m]
Platform 20 0 0 0
Mx 20 XMx -0.75 0
My 20 -0.75 YMy 0
Mz 20 0.75 0.75 ZMz
mx 0.2 Xmx 0.75 0
my 0.2 0.75 Ymy 0
mz 0.2 -0.75 -0.75 Zmz
DUT 50 XDUT YDUT ZDUT
EQUATIONS OF MOTION
Quaternions are not affected by trigonometric singularities.
Mz
My
Mx
mx
mz
my
DUT
Platform
SENSORSSensors that have to be implemented on the platform are:• 2 inclinometers;
• 1 triaxial gyroscope.
Resolution Noise Output data rate3.125·10-5 [rad] 10-4 [rad] RMS Up to 125 [Hz]
Resolution Random walk, σu White noise, σv
3.125·10-3 [rad/s] 10-4 [rad/s] 10-5 [rad/s2]
Farrenkopf model
ACTUATORSActuators that have to be implemented on the platform are:• 3 Step motors, reduced, and connected to a 1mm pitch (p) threaded rod;
The mass displacement resolution is:
• 3 Reaction wheels.
Angular step size, αst
Max rotational speed Reduction, Red
1.8 [°] 2000 [rpm] 100
Max stored momentum Max torque4 [Nms] 0.06 [Nm]
MASS BALANCING PROCEDUREGROSS MASS BALANCING
• Made by a manual adjustment of 20 kg masses• Masses adjustments are made upon a spacecraft CAD model and
platform properties• It aims at reducing the CM-CR distance to allow a correct fine
balancing
FINE MASS BALANCING
• It is driven by a PD control law fed by inclinometers readings• The mass displacement actuation is made by stepper motors
INITIAL CONDITIONS• ωx = ωy = ωz = 0• αx = αx0
• αy = αy0
• Unbalanced• Stable equilibrium
TARGET αx=αY=0
PD SYSTEM
INCLINOMETER
αx , αy
FINAL MASS DISPLACEMENTXmass_x = A Ymass_y = B
EVALUATION OF Zmass_z DISPLACEMENT
BALANCETres < 10-4 Nm
FINE BALANCING PROCEDURE
STEPPER
NO
END
YES
PROPORTIONAL CONTROL DERIVATIVE CONTROL
Kyp= kxp = 0.02 Kyd =Kxd= 4
BALANCING PLOTS
No balancing mass displacement overrun, max. 0.75 m
Tilt angle tends to 0°
No reaction wheel saturation, max. 4 Nms
MONTECARLO SIMULATION FOR BALANCING METHOD VALIDATION
Two Monte Carlo simulations have been made to validate the method:
• MC simulation for overall method characterization, different initial conditions for every sample.
120 samples Mean Standard deviation
Residual torque [Nm] 2.91E-05 2.81E-05
Total balancing time [s] 1476 203
• MC simulation for method repeatability characterization, same initial conditions for every sample.
200 samples Mean Standard deviation
Residual torque [Nm] 7.52E-05 7.56E-06
Total balancing time [s] 1856 3.5
LSE FOR INERTIA MATRIX DETERMINATION
The solution was obtained by a rearrangment of the system equations
• Π is the state vector:• Ψ is a function of gyroscopes’ readings• W is the weight matrix• P is a function of the torque applied
The system is observed for 30 s, no need for a gyroscope correction.
Problems arose:• Define a suitable torque waveform• Define a suitable weight matrix
SIMULATION AND RESULTS
• The method was validated by a Monte Carlo simulation.• Monte Carlo results have been compared to those obtained by the covariance matrix corresponding to a singular simulation.
Monte Carlo 200 samplesReal Mean Std
Jxx [kgm2] 38.600 38.600 3.07E-03Jyy [kgm2] 38.571 38.571 4.28E-03Jzz [kgm2] 45.489 45.489 1.29E-03Jxy [kgm2] -11.436 -11.436 2.93E-03Jxz [kgm2] 11.212 11.212 1.52E-03Jyz [kgm2] 11.382 11.382 2.13E-03
Correlation matrix
1.00E+00 1.87E-01 1.20E-01 -5.02E-01 4.11E-01 -1.95E-01
1.87E-01 1.00E+00 1.33E-01 -5.01E-01 -2.07E-01 4.29E-01
1.20E-01 1.33E-01 1.00E+00 1.64E-01 4.29E-01 4.49E-01
-5.02E-01 -5.01E-01 1.64E-01 1.00E+00 1.51E-01 1.28E-01
4.11E-01 -2.07E-01 4.29E-01 1.51E-01 1.00E+00 -2.45E-01
-1.95E-01 4.29E-01 4.49E-01 1.28E-01 -2.45E-01 1.00E+00
Std from covariance matrix
Jxx [kgm2] 4.31E-03
Jyy [kgm2] 4.25E-03
Jzz [kgm2] 5.10E-03
Jxy [kgm2] 3.29E-03
Jxz [kgm2] 3.50E-03
Jyz [kgm2] 3.50E-03
• True value inside ±1σ• Std from LSE compliant to Std from Monte Carlo simulation• No correlation between estimated values
CONCLUSIONSBy the balancing algorithm and the inertia matrix determination procedure have been obtained the following results:
• Residual torque lower than 10-4 Nm over 90% of the times.• Balancing time of 1450s ± 600s(3σ)• Inertia matrix determination accuracy lower than 1.5·10-2
kgm2 (3σ)
FUTURE WORK• Test the balancing procedure and the LSE technique on a
real ACS Test Bed
THANK YOU FOR YOUR ATTENTION
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