Attitude dynamics and control
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The attitude of a vehicle is its orientation with respect to a defined frame of reference.
Attitude dynamics is the modeling of the changing position and orientation of a vehicle, due to external forces acting on the body. Attitude control is the purposeful manipulation of controllable external forces (using vehicle actuators) to establish a desired attitude, whereas attitude determination is the utilization of vehicle sensors to ascertain the current vehicle attitude.
Mathematical and physical treatment of the basic aspects of these topics is well-developed, but the field is quite active with respect to advanced topics and applications.
Attitude control is the exercise of control over the orientation of an object with respect to an inertial frame of reference or another entity (the celestial sphere, certain fields, nearby objects, ...).
Controlling vehicle attitude requires sensors to measure vehicle attitude, actuators to apply the torques needed to re-orient the vehicle to a desired attitude, and algorithms to command the actuators based on (1) sensor measurements of the current attitude and (2) specification of a desired attitude. The integrated field that studies the combination of sensors, actuators and algorithms is called Guidance, Navigation and Control (GNC).
Relative attitude sensors
Many sensors generate outputs that reflect the rate of change in attitude, these require a known initial attitude, or external information to use them to determine attitude. Many of this class of sensor have some noise, leading to inaccuracies if not corrected by absolute attitude sensors.
- Devices that sense rotation in 3-space, without reliance on observation of external objects. Classically, a gyroscope consists of a spinning mass, but there are also "Laser Gyros" utilizing coherent light reflected around a closed path. Another type of "gyro" is a hemispherical resonator gyro, where a crystal cup shaped like a wine glass can be driven into oscillation as a wine glass "sings" as a finger is rubbed around its rim. The orientation of the oscillation is fixed in inertial space, so measuring the orientation of the oscillations relative to the spacecraft can be used to sense the motion of the spacecraft with respect to inertial space.
Motion Reference Units
- Motion Reference Unit
- Motion Reference Units are single or multiaxis motion sensors. They utilize Micro-Electro-Mechanical-Structure (MEMS) sensor technology. These sensors are revolutionizing inertial sensor technology by bringing together micro-electronics with micro-machining technology, to make complete systems-on-a-chip with high accuracy. Typical applications for Motion Reference Units are:
- Antenna motion compensation and stabilization
- Dynamic positioning
- Heave compensation of offshore cranes
- High speed craft motion control and damping systems
- Hydro acoustic positioning
- Motion compensation of single and multibeam echosounders
- Ocean wave measurements
- Offshore structure motion monitoring
- Orientation and attitude measurements on AUVs and ROVs
- Ship motion monitoring
Absolute attitude sensors
This class of sensors sense the position or orientation of fields, objects or other phenomena outside the spacecraft.
A horizon sensor is an optical instrument that detects light from the 'limb' of the Earth's atmosphere, i.e., at the horizon. Thermal Infrared sensing is often used, which senses the comparative warmth of the atmosphere, compared to the much colder cosmic background. This sensor provides orientation with respect to the earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation. Sometimes referred to as an Earth Sensor.
Similar to the way that a terrestrial gyrocompass uses a pendulum to sense local gravity and force its gyro into alignment with earth's spin vector, and therefore point north, an orbital gyrocompass uses a horizon sensor to sense the direction to earth's center, and a gyro to sense rotation about an axis normal to the orbit plane. Thus, the horizon sensor provides pitch and roll measurements, and the gyro provides yaw. See Tait-Bryan angles
A sun sensor is a device that senses the direction to the Sun. This can be as simple as some solar cells and shades, or as complex as a steerable telescope, depending on mission requirements.
A star tracker is an optical device that measures the position(s) of star(s) using a photocell, a (charge-coupled device) or a camera. Several types of models are currently available. Star trackers, which require high sensitivity, may become confused by sunlight reflected from the spacecraft, or by exhaust gas plumes from the spacecraft thrusters (either sunlight reflection or contamination of the star tracker window). Star trackers are also susceptible to a variety of errors (low spatial frequency, high spatial frequency, temporal, ...) in addition to a variety of optical sources of error (spherical aberration, chromatic aberration, ...). There are also many potential sources of confusion for the star identification algorithm (planets, comets, supernovae, the bimodal character of the point spread function for adjacent stars, other nearby satellites, point-source light pollution from large cities on Earth, ...). There are roughly 57 bright navigational stars in common use. However, for more complex missions, entire starfield databases are used to determine spacecraft orientation. A typical star catalog for high-fidelity attitude determination is originated from a standard base catalog (for example from the United States Naval Observatory) and then filtered to remove problems stars, for example due to apparent magnitude variability, color index uncertainty, or general "unreliability" based on location within the Hertzsprung-Russell diagram. These types of star catalogs can have thousands of stars stored in memory onboard the spacecraft, or else processed using tools at the ground station and then uploaded.
A magnetometer is a device that senses magnetic field strength and, when used in a three-axis triad, magnetic field direction. As a spacecraft navigational aid, sensed field strength and direction is compared to a map of the Earth magnetic field stored in the memory of an onboard or ground-based guidance computer. If spacecraft position is known then attitude can then be inferred.
Control Algorithms are computer programs that receive data from vehicle sensors and derive the appropriate commands to the actuators to rotate the vehicle to the desired attitude. The algorithms range from very simple, e.g. proportional control, to complex nonlinear estimators or many in-between types, depending on mission requirements. Typically, the attitude control algorithms are part of the software running on the hardware which receives commands from the ground and formats vehicle data Telemetry for transmission to a ground station.
Attitude control can be obtained in several ways, of which use of
- These are the most common, as thrusters may be used for station keeping as well. Thrusters (often monopropellant rockets), must be organized as a Reaction control system to provide triaxial stabilization. Their limitation is fuel usage, engine wear, and cycles of the control valves. The fuel-efficiency of an attitude control system is determined by its ISP (essentially, the rocket's exhaust velocity) and the smallest torque impulse it can provide. In practice, vehicle spin is reduced to a rate equivalent to this amount. Typically there is a tiny blip of thrust in one direction, and a few tens of seconds later, an opposing blip of thrust is needed to keep orientation errors within limits. To minimize this fuel limitation on mission duration, auxiliary attitude control systems are used to reduce vehicle rotation to lower levels, notably smaller, lower thrust vernier thrusters that accelerate ionized gases to extreme velocities electrically, using power from solar cells.
- Spin stabilisation
- The entire space vehicle itself can be spun up to stabilize the orientation of a single vehicle axis. This method is widely used to stabilize the final stage of a launch vehicle. The entire spacecraft and an attached solid rocket motor are spun up about the rocket's thrust axis, on a "spin table" oriented by the attitude control system of the lower stage on which the spin table is mounted. When final orbit is achieved, the satellite may be de-spun by various means, or left spinning. Spin stabilization of satellites is only applicable to those missions with a primary axis of orientation that need not change dramatically over the lifetime of the satellite and no need for extremely high precision pointing. It is also useful for missions with instruments that must scan the starfield or the Earth's surface or atmosphere. See spin-stabilized satellite.
- Momentum wheels
- These are electric motor driven rotors made to spin in the direction opposite to that required to re-orient the vehicle. Since momentum wheels make up a small fraction of the spacecraft's mass and are computer controlled, they give precise control. Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. To maintain orientation in three dimensional space a minimum of two must be used, with additional units providing single failure protection.
- Control moment gyros
- These are rotors spun at constant speed, mounted on gimbals to provide attitude control. While a CMG provides control about the two axes orthogonal to the gyro spin axis, triaxial control still requires two units. A CMG is a bit more expensive in terms of cost and mass, since gimbals and their drive motors must be provided. The maximum torque (but not the maximum angular momentum change) exerted by a CMG is greater than for a momentum wheel, making it better suited to large spacecraft. A major drawback is the additional complexity, which increases the number of failure points. For this reason, the International Space Station uses a set of four CMGs to provide dual failure tolerance.
- Solar sails
- Small solar sails, (devices that produce thrust as a reaction force induced by reflecting incident light) may be used to make small attitude control and velocity adjustments. This application can save large amounts of fuel on a long-duration mission by producing control moments without fuel expenditure. For example, Mariner 10 adjusted its attitude using its solar cells and antennas as small solar sails.
- Gravity-gradient stabilization
- In orbit, a spacecraft with one axis much longer than the other two will spontaneously orient so that its long axis points at the planet's center of mass. This system has the virtue of needing no active control system or expenditure of fuel. The effect is caused by a tidal force. The upper end of the vehicle feels less gravitational pull than the lower end This provides a restoring torque whenever the long axis is not co-linear with the direction of gravity. Unless some means of damping is provided, the spacecraft will oscillate about the local vertical. Sometimes tethers are used to connect two parts of a satellite, to increase the stabilizing torque. A problem with such tethers is that meteoroids as small as a grain of sand can part them.
- Magnetic torquers
- Coils or (on very small satellites) permanent magnets exert a moment against the local magnetic field. This method works only where there is a magnetic field to react against. One classic field "coil" is actually in the form of a conductive tether in a planetary magnetic field. Such a conductive tether can also generate electrical power, at the expense of orbital decay. Conversely, by inducing a counter-current, using solar cell power, the orbit may be raised. Due to massive variability in Earth magnetic field from an ideal radial field, control laws based on torques coupling to this field will be highly non-linear. Moreover, only two-axis control is available at any given time meaning that a vehicle reorient may be necessary to null all rates.
- Pure passive attitude control
- This method of gravity gradient and magnetic field pointing can be combined to form a completely passive attitude control system. Such a simple system has limited pointing accuracy, because the spacecraft will oscillate around energy minima. This drawback is overcome by adding a viscous damper, a small can or tank of fluid mounted in the spacecraft, possibly with internal baffles to increase internal friction. Friction within the damper will gradually convert oscillation energy into heat dissipated within the viscous damper. As this system has two stable states, if the satellite has a preferred orientation, e.g.,a camera pointed at the planet, some way to flip the satellite and its tether end-for-end is needed.
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Published in July 2009.
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