Why Do Torques Use a Negative Sign in Inverted Pendulum Equations?

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In summary, The person is having a problem with the equation in the figure and is questioning why torques are being taken in the negative direction. They suggest that it should be -VLsin+HLcos=I(theeta)'', but explain that theta is defined to increase in the negative direction (clockwise). The person also mentions that the body is rotating along its center of gravity and provides an explanation for the direction of rotation. However, this is irrelevant as it is just a matter of sign convention.
  • #1
Arslan
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i have problem with the equation encircled in red in the figure. I(theeta)''=sum of all torqes acting on the body. but they are taking torques in the negative direction why.
It should be -VLsin+HLcos=I(theeta)''
 

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  • #2
Arslan said:
i have problem with the equation encircled in red in the figure. I(theeta)''=sum of all torqes acting on the body. but they are taking torques in the negative direction why.
It should be -VLsin+HLcos=I(theeta)''
Because the way theta is defined (with respect to the vertical) it increases in the 'negative' (clockwise) direction. If they defined theta with respect to the horizontal then you'd be correct.
 
  • #3
but the body is rotating along its center of gravity. If the force is applied on cart, the cart will move in the forward direction and transfer the force to the rod in the direction of red arrow, and the body will rotate along its center of gravity. So body is rotating in counter clockwise direction.
 
  • #4
Arslan said:
but the body is rotating along its center of gravity. If the force is applied on cart, the cart will move in the forward direction and transfer the force to the rod in the direction of red arrow, and the body will rotate along its center of gravity. So body is rotating in counter clockwise direction.
All true, but irrelevant. It's just a matter of sign convention. Since theta is defined in the clockwise direction, torques in the clockwise direction will be positive.
 
  • #5


The equation you are referring to is the equation of motion for an inverted pendulum, which describes the angular acceleration of the pendulum as a function of the torques acting on it. The negative sign in front of the sum of torques indicates that the torques are acting in the opposite direction of the positive direction, which is typically defined as the direction of increasing angle. This convention is commonly used in physics and engineering, and it allows for a more intuitive understanding of the system's behavior. In other words, the negative sign is simply a matter of convention and does not affect the validity of the equation. If you are having trouble understanding the equation, I suggest seeking further clarification from your instructor or referring to additional resources on the topic.
 

1. What is an inverted pendulum problem?

The inverted pendulum problem is a classic control theory problem that involves balancing an upright pendulum on a moving base. The goal is to design a control system that can keep the pendulum balanced despite the constant disturbance from the base.

2. Why is the inverted pendulum problem important?

The inverted pendulum problem is important because it serves as a benchmark for testing and developing control theories and algorithms. It also has practical applications in areas such as robotics, aerospace, and transportation.

3. What are the main challenges in solving the inverted pendulum problem?

The main challenges in solving the inverted pendulum problem include the highly nonlinear dynamics of the system, the constant disturbance from the base, and the limited information available for feedback control.

4. How is the inverted pendulum problem typically solved?

The inverted pendulum problem is typically solved using various control strategies, such as proportional-integral-derivative (PID) control, model predictive control, and reinforcement learning. These strategies rely on mathematical models and algorithms to calculate the appropriate control actions to keep the pendulum balanced.

5. What are some real-world applications of the inverted pendulum problem?

The inverted pendulum problem has several real-world applications, including balancing robots, self-balancing scooters, and aircraft control systems. It can also be used to study and improve human balance and stability control, as well as for developing advanced control techniques for complex systems.

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