Friday, March 14, 2008

ROBOT Automation-Moment Arms concept

ROBOTIC concepts on Moment Arms
Moment arms will probably be the most useful for you. The basic equation is moment equals force times the distance of the beam the force is being applied perpendicularly at.

Moment = Force * distance

Here is the first example. What you see is an object of some length. It is fixed rigidly at one end. And the other has some force being applied to it. This force can be something hanging on it, something pushing it, a hammer hitting it, a gear moving it, gravity/weight, etc. Does not really matter.

All you do is measure the distance and multiply that by the force that is being applied. You should always know the expected force being applied to your robot, or you are taking a risk of buying an actuator that is too weak or too big and strong. If the robot is lifting a beer can, know the weight of the can. If the robot is climbing, know the estimated weight of the robot. Even rudimentary calculations can help you better understand the force requirements of your robot.
Now suppose your robot is lifting a beer with an arm. A moment about the shoulder is being created by both the weight of the can, but also the weight of the robot arm itself. How do you calculate this? You would add the moments created by each together.

Moment = can_weight * arm_length + arm_weight * 1/2 * arm_length

Notice that for the arm length we only use half the value. Why? Because weight is distributed throughout the entire arm. Theoretically all you are doing is adding up all of the force across the arm, and applying it to the center of mass of the robot arm. The center of mass is the exact point where an object can be perfectly balanced. I estimated the center of mass to be the midpoint (1/2 length) of your robot arm. However it may not be. You can easily find the center of mass of any object by balancing it on your finger and then measuring that distance with a ruler.

Now suppose you have calculated the moment. What do you do with this number?
This is actually the torque being applied. So when you look for a motor to power the shoulder of your robot, just reference this calculated value as your minimum required torque.

The concept of the moment arm can be applied for many different situations. Sometimes the moment arm can be hidden



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