debe dárseles, y de qué manera
Capítulo 19. Del trasiego de las colmenas
2.5.4.1
Attitude control
The Table 19 shows the main attitude control methods and summerizes their features.
Type Pointing options Maneuverability Typical Accuracy Lifetime limits Gravity gradient Earth local vertical
only
Very very limited ±10 degree (2 axes) no
Gravity gradient and Momentum wheel
Earth local vertical only
Very limited ±10 degree (3 axes) Life of the wheel
Passive magnetic North/south only Very very limited ±10 degree (2 axes) no
Pure Spin stabilization
Inertially fixed any direction
Repoint with precession
maneuvers
High propellant usage to move stiff momentum vector
±0.1 to ±1 degrees (proportional to the spin rate), 2 axes
Thrusters propellant (if applied) Dual spin stabilization Limited only by articulation on despun platform
High propellant usage to move stiff momentum vector; Despun platform constrained by its geometry ±0.1 to ±1 degrees (proportional to the spin rate), 2 axes
and de-spin Thrusters propellant (if applied) bearings Bias momentum (1 wheel)
Best solution for local horizontal pointing
Momentum vector of the wheel prefers to stay normal to orbit plane, constraining yaw maneuver
±0.1 to ±1 degrees Propellant and life of avionics and wheel bearings
Zero momentum (thrusters only)
No No constraints ±0.1 to ±5 degrees Propellant
Zero momentum (three wheels)
No No constraints ±0.001 to ±1 degrees
Propellant (if applied) and life of avionics and wheel bearings Zero momentum CMG No No constraints ±0.001 to ±1 degrees Propellant,. life of avionics and wheel bearings
Active magnetic No One axis pointing is lost in the some singularity points
±0.1 to ±5 degrees Life of avionics
97
The complete description of the cited control methods and functions is in [1], [2], [16], [8], and [17].
2.5.4.2
Orbit and trajectory control
The trajectory control refers to the types of orbital maneuvers performed in open or closed loop, generally using small or large thrusters as actuators:
Free drift motion: no thruster are involved – “natural orbiting” from initial condition
o Motion on a coplanar orbit at different altitude
o Release from a station along R-bar (for RV missions only) o Release from a station along H-bar (for RV missions only)
Impulsive manoeuvres: they foresee instant change of velocity, similar to boost manoeuvres
o Thrust in an orbital direction Hohmann transfers o Thrust in a radial direction
Radial impulse transfer along V-bar (for RV missions only) Radial impulsive fly-around (for RV missions only)
Thrust in out-of-plane direction (orbit plane correction) Lambert transfer
Continuous thrust: continuous application of control forces (open or closed loop) along the trajectory
o Straight line along the velocity vector approach with constant velocity
with a velocity profile
o Straight line along the radial vector (R-bar, mainly for RV mission) with constant velocity
with a velocity profile
Station- keeping on a position outside the target orbit
o below /above the target orbit o out-of-plane position
Transfer by continuous x-thrust
o quasi-impulsive x-thrust
o continuous x-thrust transfer to a different altitude
Transfer by continuous z-thrust
o quasi impulsive z-thrust
o continuous thrust transfer along velocity vector o continuous thrust in y-direction
98
2.5.4.3
Satellite
Orbital phase Control methods
Detumbling / damping phase
Attitude:
Rate damping + desired attitude acquisition
Gravity gradient + damper
Three-axis stabilization through thrusters, reaction wheels and/or magnetic torque.
Orbit: N/A Transfer to final
orbit
Attitude: is related to the control of the trajectory
Orbit and trajectory: moderate orbit change or major orbit change Pointing phase Attitude:
Gravity gradient (2 axis only) for a low accuracy, very limited manoeuvrability, no life time problem
Passive magnetic
Pure spin stabilization
Dual spin
Bias momentum (1 wheel)
Three axes stabilization – active magnetic
Zero momentum (thruster only)
Zero momentum (three wheels)
Zero momentum (CGM) Orbit/trajectory: N/A
Station-keeping phase
Attitude: can be disturbed by the not perfect actuation of the thruster to maintain the orbit. So it could be useful to maintain the attitude using one of the methods presented for the pointing phase.
Orbit/Trajectory: orbit maintenance through vernier thrusters
Disposal phase Attitude: three-axes active control to reach and maintain an attitude that helps the orbit/trajectory manoeuvre(s)
Table 20: Control strategies and related techniques for satellites
2.5.4.4
RVD/B
In mission of RVD/B, the performance requirements for the reduction of the trajectory errors increase with decreasing range to the target. For example, for manoeuvre at 30-50 Km from the target the necessary increments ΔV per boost are smaller w.r.t. the orbit boost during the phasing but they are higher w.r.t. the manoeuvre in mating or close approach. The last consideration highlights that different actuators have to be used according to the phases: large thrusters are needed for phasing and far RV manoeuvre, vernier thruster as well as CGM or reaction wheels for the attitude control shall be used in the latter phases before the mating and, immediately after the departure. The final manoeuvres to re-entry or change the orbit still require the larger thrusters.
Orbital phase Control features
Phasing and far RV
Open loop boost manoeuvres: large thrusters are fired for a duration calculated from the expected acceleration and the required ΔV. Sometimes middle-course open loop manoeuvres should be performed in order to correct too high error due to the previous open loop manoeuvres that can carry out a too low accuracy
Rarely, trajectory that heavily changes the orbit of the spacecraft can be performed with a low but continuous acceleration during the transfer. Closed loop with high bandwidth maneuver are performed using small
99
thrusters for long time. The high bandwidth allows controlling “continuously” both the trajectory and the attitude that can deviate for long firings of the thrusters, uncertainties, difference in the thruster levels, etc…
Close RV Control is sufficiently accurate to satisfy constraints due to optical sensors range and the reaching of the final corridor.
Closed loop manoeuvre are surely indicated taking into account that the main issue is to follow a pre-defined trajectory and correct the deviation from the desired state.
A high bandwidth is required to track the profile of velocities, relative position and attitude, maximizing the performances in terms of consumption, time to approach, accuracy.
Good controllers can be LQR and LQG and a well trained Neural Network
Final to approach Closed loop control with the higher possible bandwidth has to be considered because high capability to react wrt. disturbances and changing reference is required.
Small vernier thrusters can be used but they influence both the translational and the rotational mode, so they should be coupled with CGM or high-speed reaction wheels to guarantee the optimal alignment with the docking port or the berthing element.
Elegant design can require H∞ (in general, robust controllers) because they have high capability to recover the desired state in presence of larger uncertainties and disturbances.
Mating Very small actuations to finalize the rendezvous can be made but, in general, thrusters cannot fire in the last meters before the docking/berthing.
Electric thruster for trajectory control in this phase should be evaluated as well as RW or CGM for the attitude control.
In some case chaser shall be completely switched off before the mating: it means that no attitude and trajectory control are permitted in the last 2- 3 meters.
Departure and de-orbiting
Closed-loop control is made to damp the angular velocity after the release (as if it was a detumbling manoeuvre).
When the attitude is stabilized, the thrusters fire in order to perform the computed escape manoeuvre and to reach a new orbit, in general for the re-entry on the Earth.
LQR is a good controller for this space as well as the robust controls
100
2.5.4.5
Launcher and Re-entry vehicle
Orbital phase Control function
Boost To maintain attitude and attitude rates within safe limits during region of high dynamic pressure
To direct thrust vector to provide velocity changes commanded by guidance
Normally, robust and/or optimal control techniques are chosen in order to maximize the system capability to track a reference (the pre-determined trajectory) and react to the disturbance.
Orbit To change/stabilize attitude via thrusters or other actuators
Thrusters to move to a new orbit
Re-entry To change/stabilize attitude via thrusters or other actuators
Thrusters to move to a new orbit
Table 22: Control strategies and related techniques for launcher and re-entry vehicle