Assignment: Draw the state machine (using the circles and arrows notation) that specifies the following requirements for the mode switching logic in the HETE Attitude Control System (ACS):
General Description of the HETE Spacecraft
The purpose of the High-Energy Transient Explorer (HETE) mission is to study gamma ray bursts. Gamma ray bursts are high-energy transients in the gamma ray portion of the elctromagnetic spectrum that seem to be isotropically distributd in the sky. These transient energy bursts range in duration anywhere from a millisecond to a few hundreds of seconds, and they involve a huge amount of energy. In addition to capturing spectral information about these bursts, the craft also relays position data about bursts as they occur to ground stations and other satellites so that they can also study the phenomena.
The satellite consists of three basic components: the body of the craft, the control system, and the scientific payload. The control system consists of some controlling hardware, a set of sensors, and a set of actuators for controlling the spacecraft.
The control hardware for the satellite consists of four compute nodes arranged around a central bus. Each node has three processors (two Motorola DSP 56001 programmable digital signal processing units, and one INMOS T805 transputer) as well as some memory and auxiliary support logic.
Each node of the controller operates independently of the others and is responsible for operating a number of the many satellite subsystems. The primary node is responsible for running the communications subsystems and the attitude control system code. The other three nodes operate the payload instruments.
The Attitude Control Software
The Attitude Control System (ACS) software is responsible for maintaining the attitude of the spacecraft. In addition, it also performs several deployment operations as the spacecraft moves into its nominal attitude.
In nominal operation, the control system will progress through the initial modes and stabilize in modes 5, 7, and 8. These are the normal operating modes, and they correspond to minor attitude correction, daytime operations, and nighttime operations. All other modes are either one-shot modes with the purpose of performing specific actions, or gross attitude control to adjust the attitude of the spacecraft into something approximating its nominal attitude. Mode 4 (deploy wheel) and Mode 6 (deploy paddles) are one-offs, mode 4 having the purpose of spinning up the wheel to shift the stabilizing momentum spin from the body of the craft to the wheel, and mode 6 having the purpose of deploying the solar paddles. Mode 0 is a delay mode, to allow the craft to clear the launch vehicle, and performs no action. Mode 1 is for gross attitude correction, and is the closest thing the craft has to a safe mode. Mode 2 applies spin on the y axis to the craft to help stabilize its attitude in the xz plane. Mode 3 performs gross adjustment to the elevation and azimuth of the craft with respect to the sun to place those vlaues within tolerances for mode 5.
At any point, if the sun elevation or sun azimuth errors or the momentum vector error exceeds tolerances, the ACS will return to an appropriate mode in order to correct the problem. Also, in the absence of good data about the attitude, the ACS will revert to mode 2.
Which of the 10 modes the ACS should be in at any time may depend on any or all of the following:
- The current mode.
- How long the software has been in the current mode, in terms of how many times it has run a complete sense-command-control loop;
- Whether or not the paddles have been deployed
- The current rotational velocity of the wheel
- The current estimated sun unit vector
- The current estimated normalized magnetic field direction, corrected for the fields generated by the torque coils.
- The current estimated body rate vector
- Whether or not the sun sensors are detecting the sun's presence
Mode Switching Requirements for HETE Attitude Control System (ACS)
The nominal sequence from the release of the satellite by the rocket to the orbit night/day cycle is as follows: The first mode to be called is mode 0 (wait), so that the sensors have time to acquire some data and the filters can initialize. The ACS exits mode 0 when a certain delay has elapsed. When leaving mode 0, the satellite is still tumbling because no actuators have been activated since the release. The ACS switches from mode 0 to mode 1 (detumble). Mode 1 will cancel most of the rotation about all three axes. The ACS exits mode 1 when the rotation has been dampened enough, that is, when the x and z components of the momentum vector drop below a certain threshold. Then the plan is to give the spacecraft some rotational stiffness about the Y axis so the elevation angle can be controlled easily. This is the role of mode 2, which comes after mode 1. The mode 2 exit criteria also concerns the momentum vector: when the satellite momentum vector is close enough to the target momentum vector, the spacecraft is ready to have its elevation angle error corrected and it switches to mode 3. Mode 3 precesses the momentum vector. Once the elevation error is stabilized under a certain threshold, the ACS enters mode 4 to cancel the rotation about Y. Mode 4 deploys the momentum wheel: the wheel spins up to a speed such that it completely dampens the rotation about Y. The wheel needs a certain delay to reach its speed. When this delay has elapsed, the ACS exists mode 4. Then the system may have to return to mode 3 if the wheel deployment has induced a disturbance strong enough for the elevation error to grow past the threshold. When the elevation is stabilized again, the ACS goes to mode 6 to have the satellite deploy its solar paddles. The solar paddle release must not be activated for too long, so the ACS leaves mode 6 either after the paddles are released or after the delay has elapsed. At this point, the satellite is in its final configuration and it will reach its final attitude by correcting the azimuth error as well as the elevation error. Modes 5, 7, and 8 all accomplish this, but they are used in different situations. Mode 5 has the higher gains, so it is used only at the end of the acquisition sequence, when the azimuth error can be big, or when the spacecraft has been subjected to disturbances that cause the azimuth error to go over its threshold. Mode 7 is the default mode during the mission at orbit-day. Mode 8 is the default mode when the optical cameras operate, that is during orbit-night.
The above specification describes the nominal behavior, but there are some additional requirements that correspond to anomalous situations. The ACS switches from mode 7 to mode 3 when mode 7 is not able to maintain a small elevation error while dealing with the azimuth error at the same time. It switches from mode 7 to mode 2 when it is orbit night and no tracking data is provided (there is a problem with the optical cameras). In this case, the ACS cannot use the sun sensors and no drift rate is available, so the best compromise is to go back to mode 2 (detumble) where only the magnetometers are used to keep the satellite roughly stable. When the sun rises, the ACS will go again through modes 3, 5, and 7 to resume normal operations. The same process occurs when the satellite is in mode 8 (orbit-night) and the data from the cameras drops out or when the rotation of the satellite as determined by the magnetometers is too big for the ACS to cope with while in the tracking mode (in mode 8, the rotation rate must be small enough for the cameras not to blur). As in the previous case, the satellite will then enter mode 2 and wait for the sun to rise.