Inverted pendulum -- What force is applied at the center of mass?

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Discussion Overview

The discussion revolves around the forces acting on the center of mass of an inverted pendulum, particularly focusing on the balance of torque due to gravity and the forces required to maintain equilibrium. The scope includes theoretical considerations and mathematical reasoning related to the mechanics of the inverted pendulum.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that to keep the inverted pendulum balanced when it leans forward, the bottom must be accelerated, questioning what force acts on the center of mass to balance the torque due to gravity.
  • Another participant suggests that if one considers the torque due to gravity, it is necessary to define an axis about which this torque applies, indicating that moving the attachment point alters the torque's effect.
  • A further contribution discusses the concept of fictitious inertial forces (D'Alembert force) when viewing the system from an accelerated coordinate system, contrasting this with real forces such as compression or tension in the pendulum arm when viewed from an inertial frame.
  • One participant requests a visual representation of the forces and torques involved in the second case described, indicating a desire for clarification through illustration.
  • Another participant provides a description of a diagram illustrating the forces acting on the pendulum, noting the clockwise torque from the compression force and the counter-clockwise torque from gravity.
  • A participant shares a link to a paper that may provide helpful equations and design insights related to the inverted pendulum, mentioning its historical significance in fuzzy logic applications.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the nature of the forces acting on the center of mass and the interpretation of torque in relation to different reference frames. No consensus is reached on the specific forces involved or the best way to conceptualize the problem.

Contextual Notes

Participants express uncertainty about the definitions of forces and torques in different reference frames, and there are unresolved aspects regarding the mathematical treatment of the forces acting on the pendulum.

fizzyfiz
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Summary: I know that in order to keep inverted pendulum balanced when it leans forward, I should accelerate the bottom of it, but what is the force which is applied to center of mass of the pendulum with balances torque due to gravity? Is it inertial force? We assume that our inverted pendulum is massles rod with a center of mass at its top. As a Radius of rotation we denote "L"as angle between vertical and pendulum "a"

M*g*sin(a)*L= F*L
 
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fizzyfiz said:
what is the force which is applied to center of mass of the pendulum with balances torque due to gravity? Is it inertial force?
If you imagine that there is a torque due to gravity, you must imagine that there is an axis about which the torque applies. Let us choose the point where the pendulum is attached to the ground.

If you move the point where the pendulum is attached to the ground, you adjust the torque due to gravity by redefining your coordinate system. If you insist on viewing this as an applied force then yes, it would be a fictitious inertial force (the D'Alembert force) associated with the acceleration of the coordinate system.

If you prefer to stick to a single inertial frame then the relevant force is real and is, instead, the compression or tension force on the pendulum arm. When the pendulum's bottom end moves, this compression or tension force no longer produces zero torque about a reference axis at the original attachment point.
 
Thank you for reply! Could you draw the second case?
 
fizzyfiz said:
Thank you for reply! Could you draw the second case?
The star is the axis of rotation, the thin line is the downward force from gravity, the rectangle is where the support point was moved, the pendulum arm is the bold line and the circle is the pendulum bob. You can see that the compression force of arm on bob amounts to a clockwise torque while gravity amounts to a counter-clockwise torque.

My skills with mspaint are of questionable value.
inverted.png
 
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Check out this paper. It should be helpful with the equations, and the design.

https://link.springer.com/content/pdf/10.1007/0-387-29295-0_7.pdf
This thread triggered a memory. The inverted pendulum was the earliest and only? clear victory for fuzzy logic. So I searched for inverted pendulum fuzzy logic, and found many hits.
 

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