Components of ADCS
Attitude Determination and Control Subsystem can be thought of as comprising of following parts.
Sensors and Estimators:
Sensors are instruments which measure various quantities during the flight of the satellite. These sensors are like eyes and ears of the satellite. In this section, we will only consider sensors concerning attitude determination and control.
As stated earlier, the purpose of ADCS is to determine the orientation of satellite with respect to some reference frame. This requires measurement of various vector quantities in the body frame of the satellite. This task is performed by sensors. For example, sun-sensor measures the unit vector in the direction of sun from satellite. The magnetometer measures local magnetic field due to earth. The more detailed information about different sensors will be given in subsequent sections.
Although, we want to determine the attitude, there is no direct sensor to measure it. Thus we have to use measurement from different sensors and estimate attitude using them. There are number of algorithms to achieve this.
Actuators:
One of the goals of ADCS is to bring the satellite in desired orientation and maintain it. This requires some component which will exert force or torque on the satellite body. It is called as actuation and is performed by actuator. Magnetorquer, reaction wheels are some of the actuators commonly used in small satellites. These will be discussed thoroughly in subsequent sections.
Control Law:
Control law is a mathematical framework which takes sensor inputs, reference signal and decides required actuation for controlling the plant (which is the satellite in this case). There are lots of control laws in literature. In subsequent sections some commonly used laws are described (eg. PID).
Propagator:
The small satellite faces very stringent power and mass constraints. Thus, sensors which consume lot of power for operation can not be used continuously. In such scenarios, computational models are used to simulate sensor measurement using actual measurement as a initial condition. These are called as propagator models.
On Ground Simulations:
The control system comprising of above parts is designed to achieve the required control goal. Now, it is essential to verify the performance of this control system before launching the satellite to ensure its success. Thus ADCS has to undergo following tests:
- Software simulation: The designed control law is thoroughly tested in softwares or programming languages like Matlab/SImulink, Python etc. The required environmental models are generated in software as per need. Some existing models can be used and are mentioned later. This process is iterative and occurs along with design itself.
- On-Board In Loop Simulation (OILS): In this testing, control law is implemented on actual microprocessor unlike fully software testing. Environmental models, sensor models are still simulated in software. This testing can be done with Simulink, Labview etc.
- Hardware In Loop Simulation (HILS): In this testing, control law is coded on microprocessor along with actual sensor data. This means environment experienced in space has to be simulated physically, which will bring actual sensor in the control loop test.
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