Calculating torque for precessing gyroscope

In summary, the conversation discusses the motion of a gimbal mounted spinning gyroscope wheel and calculates the torque required to prevent pitching. The calculation involves integrating the motion quadrant by quadrant and using the swivel and rolling rates, as well as the mass and velocity components. The final result matches the expression given in textbooks and involves the angular momentum around the rolling axis.
  • #1
Cleonis
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I'm presenting a computation. I'm posting here to ask if anyone spots an error either in the reasoning or in the math.

The image shows a gimbal mounted spinning gyroscope wheel.
The brown cilinder represents a weight, exerting a torque.

Naming convention:
Spinning of the wheel: rolling
rotation of the red housing: pitching
rotation of the yellow housing: swiveling

attachment.php?attachmentid=24771&d=1270068885.png


For computation I visualize the motion quadrant by quadrant.

attachment.php?attachmentid=27073&d=1279725740.png


All parts of that quadrant are moving towards the swivel axis.
The wheel is swiveling at a constant rate. Whenever circumnavigating mass is pulled closer to the central axis of rotation it tends to pull ahead of the circumnavigating motion. The green arrow represents that tendency. Conversely, when mass moves away from an axis of rotation it tends to lag behind.

Combined over all four quadrants the wheel will have a strong tendency to pitch. If the gimbal mounted gyroscope wheel is free to pitch then the rolling axis will become aligned with the swivel axis. What I want to calculate: given a particular swivel rate and rolling rate, how much torque is required to prevent that pitching?
This calculation is for the simplest case: a thin-walled cylinder with radius R.

Method: integration along the circumference of the wheel.
[tex] \int\limits_{0}^{2\pi} f(\theta) \, d\theta [/tex]

Naming:
Rolling rate: [tex]\omega_r[/tex]
Swiveling rate: [tex]\omega_s[/tex]

Mass per unit of arc : M/2*pi
Velocity component towards/away from central axis: [tex] v_r = \omega_r R \sin(\theta)[/tex]
Distance to pitch axis: [tex] \sin(\theta) R[/tex]
Tendency to pull ahead/lag behind overall swiveling: [tex]F = 2 m \omega_s v_r[/tex]

This gives the following integral:
[tex]
\int\limits_{0}^{2\pi} \big( 2 \frac{M}{2\pi} \omega_s \omega_r R \sin(\theta) \big) \big( \sin(\theta) R \big) \, d\theta
[/tex]

Rearranging:
[tex]
\int\limits_{0}^{2\pi} \frac{M}{\pi} \omega_s \omega_r R^2 \sin^2(\theta) \, d\theta
[/tex]

sin2(x) is substituted with (1/2 - 1/2cos(2x))

[tex] \int\limits_{0}^{2\pi} \sin^2(\theta) \, d\theta = \int\limits_{0}^{2\pi} \big( \tfrac{1}{2} - \tfrac{1}{2}cos(2\theta) \big) \, d\theta = \pi [/tex]

The question was: given a particular swivel rate and rolling rate, how much torque is required to prevent pitching? This calculation yields:

[tex] M \omega_s \omega_r R^2 [/tex]

I haven't yet compared this result with the usual calculation that is presented in physics textbooks.
 

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  • #2
Cleonis said:
[...] given a particular swivel rate and rolling rate, how much torque is required to prevent pitching? This calculation yields:

[tex] M \omega_s \omega_r R^2 [/tex]

I haven't yet compared this result with the usual calculation that is presented in physics textbooks.

Rearranging (and using the greek letter 'tau' for 'torque'):

[tex] \tau = \omega_s \omega_r M R^2 [/tex]

The angular momentum of the rotor is [tex] \omega_r M R^2 [/tex]

[tex] \tau = \omega_s L_r [/tex]

Gratifyingly, this matches the expression given in textbooks, where it is usually derived in the following form:

[tex] \omega_s = \frac {\tau}{L_r} [/tex]

What I find interesting is that the calculation strategy of integrating the rotation-effect around the circle did not use the angular momentum around the rolling axis, but it did arrive at an expression that contains that angular momentum.
 

FAQ: Calculating torque for precessing gyroscope

What is torque and how does it relate to a precessing gyroscope?

Torque is a measure of the force that causes an object to rotate around an axis. In the case of a precessing gyroscope, torque is the force that causes the gyroscope to spin around its axis of rotation.

How is torque calculated for a precessing gyroscope?

To calculate torque for a precessing gyroscope, you need to know the angular velocity of the gyroscope, the moment of inertia of the gyroscope, and the angle of precession. The formula for torque is torque = moment of inertia * angular velocity * sin(angle of precession).

What factors can affect the torque of a precessing gyroscope?

The torque of a precessing gyroscope can be affected by changes in angular velocity, moment of inertia, and the angle of precession. Other factors that can affect torque include the shape and mass distribution of the gyroscope and any external forces acting on it.

How can torque be controlled in a precessing gyroscope?

To control the torque of a precessing gyroscope, you can adjust the angular velocity, moment of inertia, or angle of precession. Additionally, you can apply external forces to the gyroscope to counteract or enhance the torque.

What are some real-world applications of calculating torque for precessing gyroscopes?

Precessing gyroscopes are commonly used in navigation systems, such as in airplanes and spacecraft. They are also used in gyrocompasses, which are used for navigation at sea. Other applications include stabilizing platforms for cameras and surveying equipment, and in artificial satellites for attitude control.

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