Attitude Control System

Attitude Control System (ACS) controls the dynamic behavior of satellite like orientation and motion of satellite. In Swayam, the unique technique of Passive Magnetic Attitude Control (PMACS) is implemented. The uniqueness of the technique is that it controls the orientation of satellite without any power consumption with reasonable accuracy for communication. It is the scientific mission of the Satellite to prove that PMACS is a robust and cost effective solution for attitude control.

System Objectives

The primary objectives of the Attitude Control subsystem are

1)     Damp the initial random angular motion of the satellite.

2)     Attain a orientation favorable for the communication payload

Orbit of the Swayam :

Swayam will be launched in sun-synchronous, low Earth orbit.

Mission Requirement :

Communication is the payload of the satellite. To attain a configuration favorable for communication, the ground station antenna should lie within the cone of radiation of satellite antenna.  The beam width of satellite antenna is 900.

(a) Cone of radiation of satellite antenna (b) Configuration of antenna while communicating with ground station

System Design : PMACS utilizes the Earth’s magnetic field for attitude control. Components of PMACS are :

1)  Permanent Magnet : Permanent magnet provides pointing direction to the antenna of the satellite i.e. along the Earth’s magnetic field. It is placed along the antenna direction (X axis) as shown in figure.

2)  Hysteresis rods : Hysteresis rods being soft magnetic, damp the angular velocity and angular oscillations of the satellite. They are placed perpendicular to magnet in mutually perpendicular directions (Y and Z) . The size and number of rods of the permanent magnet and hysteresis rods are decided for optimum performance of the satellite. The placement of the rods inside the satellite is as shown in figure below.

Placement of permanent magnet and hysteresis rods

Governing parameters of  PMACS :

1)    Earth’s magnetic field

2)    Magnetic properties of permanent magnet and hysteresis material

3)    Inertia of the satellite

4) Disturbance Torques

Simulation :

To evaluate the attitude behavior of the satellite in the orbit, the space conditions and dynamic behavior of the satellite have been simulated through a C Code. The simulation provides orientation, angular velocity and communication feasibility of the satellite in an orbit and over a time.

The graphs of residual angle (angle between magnet and Earth’s magnetic field) angular velocity Vs. Time are as shown below.

Residual angle vs. time

Residual angle vs. time

Angular Velocity vs. Time

Angular Velocity vs. Time

 

 

 

 

 

 

 

 

From the simulation results it has been observed that, the stabilization of the satellite to the prescribed orientation i.e. antenna along Earth’s magnetic field, takes around 15 to 20 days.

Stabilization Detection Gyroscope :

On-board MEMS Gyroscope has been implemented in the system for stabilization detection. The gyroscope measures the on-orbit angular velocity of the Satellite. Using these values, the On-Board computer will detect the stabilization of the Satellite.

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