How many ounce inch is needed? What's the magic formula

Lynxmotion Legged Robots
1

I am sure there is some mathmatical formula to calculate how many oz. inch of torque are needed at the servo to life number of pounds number of inches away, right? What is it? :slight_smile:

We’ll use an arm for example (but I assume it applies to nearly everything, including BiPeds - I plan to build both).

Let’s say the forearm is 12" and the shoulder is 12" and we want to life 1 pound from a fully extended position. How many oz. inch of torque would we need at the shoulder? at the forearm?

This all stems from my love of BiPeds and how cool they look walking. It branches off to the fact that everything in this world was made for humans (and therefor BiPeds), so it’s natural that if you want to create a robot that can maneuver in our world that you build a BiPed.

I also have come to the conclusion that in order for that BiPed to be truely useful, it needs to be the size of a small child. Stair heights are approximately 175mm (~7-8 inches) tall, and my guess would be that you would want your BiPed to “step” onto the stairs at no more than 135 degree angle which would place your BiPed hips at approximately 18-20 inches off the ground, which in turn makes the robot approximately 3-3.5 feet tall.

Now I’m pretty sure servo’s wouldn’t be able to handle this, I was thinking stepper motors but I’m not sure that you can measure feedback on those so I might go with the whole super-servo concept and try to build my own.

I’ve looked at those linear actuators someone else posted about, but I don’t know anything about them. Can you position them exactly where you want (like a servo)? Do they operate strictly on electricity, or do you need some sort of pneumatic/hydraulic system as well? Can they measure feedback?

P.S. If this question has been asked already, I apologize, I’ve tried looking thru the forums but it’s fairly overwhelming. If you just want to point me to the answers that’s fine as well.

Thanks for your help, it’s much appreciated!

2

Torque is the vector cross product of the force vector and the distance between the point of rotation and the the point where the force is being applied to the object.

Math Formula: T = R (cross) F

Now if you don’t want to deal with vectors, then you can look at the worse case situation where the torque is equal to the radius multiplied by the force.

Simplified Math Formula: T = R x F

So for your elbow servo, T = 12" x 1 lb = 12 in-lb = 192 in-oz.

For the shoulder servo, T = 24" x 1 lb = 24 in-lb = 384 in-oz.

The height of the biped really depends on what you what it to do. If you want it to climb stairs then it will need to be able to lift its legs to clear the steps. If you only want it to walk across a table, then it can be less than a foot tall. The bigger you make it, the stronger the actuators will need to be. Stepper motors is one choice, so are linear actuators, pneumatics and hydraulics. Not to mention good ol’ fashion cordless screw driver motors.

Pete

3

Don’t for get to include the weight of the arm components themselves in the total weight calculations.

4

Hey guys dont forget when you have something this long and heavy u have to consider momenteum and enertia andleverage , bad coding on something like this will strip gears and falls are magnified ,you need something like servo savers or shear pins,

PS car window motors are geared and very strong thats what i,m tring to use for my giant arm