how reaction wheel can effect gyroscope ?

[JimboJay]

hey guys !

i don't if i am posting this thread in the right place

The thing for sure is that reaction wheel can cause errors on gyro specially when using on a cube sat

my question was first ; How ?

then : What to do ?

any paper or journal that have covered this issue is appreciated .

thanks

[munlander1]

While I don't think I can provide an answer, but you're asking why and how a reaction wheel can interfere with a gyroscope?.

[Kyle Worden]

Not quite sure what you are asking, because gyroscopes and reaction wheels work together to orient a spacecraft.

 

Think of the gyro as a compass, and the reaction wheel as the steering mechanism. The gyroscope detects the changes in rotation and sends that data to the reaction wheels. The wheels can then spin faster or slow down to compensate. This all works because of the conservation of angular momentum.

 

These systems do have their weaknesses, namely, you must have at least 3 reaction wheels to maintain full control over the pitch, yaw, and roll axis'. Reaction wheels are also prone to saturation, which is where they spin up to their max allowed speed, and so they can not compensate any further (this can be prevented by having 2 wheels per axis that spin in opposite directions).  And lastly you have the issue of gimbal lock, where if the gyroscopes become in-line with each other, the system can no longer detect changes in its position.

Also since you have moving parts, the wheels and gyros are prone to wear and mechanical failure.

There is a whole host of engineering problems that had to be overcome to make the current hardware as reliable as it is, but nonetheless, having moving parts does have its weaknesses.

 

Hope this helps.

[Shpaget]

 

12 hours ago, JimboJay said:

The thing for sure is that reaction wheel can cause errors on gyro specially when using on a cube sat

What specific issue are you talking about?

12 hours ago, JimboJay said:

my question was first ; How ?

Reaction wheels are a possible source of vibration, which could conceivably induce errors in gyro (if for example, the vibration frequency is in some resonance with the gyro sampling frequency).

As for why it would be more noticeable on cubesats, I'd say because cubesats are much smaller than "regular" satellites, meaning much smaller masses involved, leading to lower robustness of the system. 

12 hours ago, JimboJay said:

then : What to do ?

Don't use reaction wheels. For cubesats (specifically the ones in Earth orbit, due to the Earth's magnetic field) magnetorquers are enough. They are simple, (relatively) cheap and small.

All that being said, reaction wheels are not practical for cubesats, since you still need RCS to dump the momentum of the spinning wheel and that introduces a whole lot of other issues.

[Snark]

20 minutes ago, Kyle Worden said:

Reaction wheels are also prone to saturation, which is where they spin up to their max allowed speed, and so they can not compensate any further

This is true, however,

21 minutes ago, Kyle Worden said:

(this can be prevented by having 2 wheels per axis that spin in opposite directions)

^ this is not the case.  Having a wheel that spins in the opposite direction on the same axis would cancel out the torque and make nothing happen.

Reaction wheels can't impart a net angular momentum to the craft, due to conservation of angular momentum.  The wheel spins fast in one direction, causing the craft to rotate slowly in the other direction.  The fact that the wheel saturates simply sets an upper limit on the rotational speed of the craft it can achieve.  Once the craft rotates to the desired position, the wheel just brakes to a halt, which stops the rotation of both the craft and the wheel.

That works fine, until/unless you have some external force that starts the craft rotating by imparting a net angular momentum.  If it's a slow enough rotation, the reaction wheels may be able to counter by spinning up, but if the external torque keeps getting applied, pretty soon the wheels will saturate and then there's nothing they can do to fix the situation.

That's why pretty much every satellite has some form of RCS thrusters, even if they have reaction wheels.  RCS thrusters, since they operate externally, can affect the net angular momentum of a craft.  They can do more than the reaction wheels can-- at the cost of requiring them to use up consumable reaction mass, of which the craft has a finite supply.

The reaction wheels basically give you the ability to change the orientation of the craft at will, but not (much) the net rotation.

[Kyle Worden]

Just now, Snark said:

This is true, however,

^ this is not the case.  Having a wheel that spins in the opposite direction on the same axis would cancel out the torque and make nothing happen.

Reaction wheels can't impart a net angular momentum to the craft, due to conservation of angular momentum.  The wheel spins fast in one direction, causing the craft to rotate slowly in the other direction.  The fact that the wheel saturates simply sets an upper limit on the rotational speed of the craft it can achieve.  Once the craft rotates to the desired position, the wheel just brakes to a halt, which stops the rotation of both the craft and the wheel.

That works fine, until/unless you have some external force that starts the craft rotating by imparting a net angular momentum.  If it's a slow enough rotation, the reaction wheels may be able to counter by spinning up, but if the external torque keeps getting applied, pretty soon the wheels will saturate and then there's nothing they can do to fix the situation.

That's why pretty much every satellite has some form of RCS thrusters, even if they have reaction wheels.  RCS thrusters, since they operate externally, can affect the net angular momentum of a craft.  They can do more than the reaction wheels can-- at the cost of requiring them to use up consumable reaction mass, of which the craft has a finite supply.

The reaction wheels basically give you the ability to change the orientation of the craft at will, but not (much) the net rotation.

You sir, are correct, apologies!

[WinkAllKerb'']

well i guess there many many way to say how and then as well as there many way to say sort of thks ; ) but not sure if this is a proper answer ^^ welcome around btw ^^

Edited July 11, 2017 by WinkAllKerb''