Reaction Wheels
Contents
Internal Momentum Exchange devices
Unlike aircrafts, spacecrafts can't rely on aerodynamic forces to provide stability.So they have to rely on reaction type devices as actuators for their attitude control in space. These actuators utilise the concept of exchange of momentum. The exchange of momentum happens either between satellite and earth (in case of Magnetorquers), spacecraft and ejected mass from craft (in case of reaction thrusters) or within satellite body.
The devices in which momentum redistribution takes place within the satellite body falls under the category of internal momentum exchange devices. The advantage of these devices is that they do not need any fuel as thrusters do. They require electric motors which can be powered by batteries and the battery can be charged by solar panels. So this proves to be an attractive alternative option.
Types:
- Reaction Wheels
- Momentum Wheels
- Control Moment Gyroscopes (CMGs)
Reaction Wheels
A reaction wheel is a device consisting of a spinning wheel attached to an electric motor (brushless DC), whose speed can be controlled by onboard computer, producing the required torque for attitude control.
It works on the principle of angular momentum conservation of flywheel or disc and satellite body. Each wheel can produce torque only along its axis of rotation. So by mounting three reaction wheels along mutually perpendicular axes, we can get the desired magnitude and direction of torque. However there is a limit to the torque they can produce because the wheels can’t be accelerated and decelerated limitlessly.
Mechanisms for Control/Working
Since reaction wheels are internal momentum exchange devices they can’t alter the total angular momentum of the satellite. But the angular momentum of the satellite can be redistributed between the wheels and satellite body. Therefore the required amount of angular momentum can be transferred to satellite body in order to achieve the desired attitude rates.
Dynamics of the satellite using reaction wheels
In space there are disturbances that provide external torques (Ne) to the satellite which tend to destabilize the satellite. Therefore it is necessary to understand the dynamics of the satellite and model the same to actively control the attitude of satellite. The total angular momentum of satellite (h_{tot}) can be described by the equation below
While dealing with satellite dynamics we need rotating reference frame. The rate of change of a vector in rotating and stationary frames can be related by the equation:
Here subscript s is for observation in stationary frame and r is for observation in rotating frame.
Substituting h_{tot} in the above equation,
Now we can treat rate of rate of change of angular momentum of wheels as the control torque to the satellite in satellite body coordinate system.
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