Mechanics and Construction > Mechanics and Construction

Unable to decide motor spec for Quadruped

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saba_rish91:
@jwatte Thanks a lot.

I was browsing through internet and I found the following site on hexabot's mechanics.

http://www.robotshop.com/robot-leg-tutorial.html

In that site, it says that the servomotor near to the ground requires more torque compared to the middle servomotor!

saba_rish91:
Also, is there ay software which lets us to test robots virtually?

jwatte:

--- Quote from: saba_rish91 on January 13, 2013, 09:46:18 AM [email protected] Thanks a lot.

I was browsing through internet and I found the following site on hexabot's mechanics.

http://www.robotshop.com/robot-leg-tutorial.html

In that site, it says that the servomotor near to the ground requires more torque compared to the middle servomotor!

--- End quote ---

I suppose by "near to the ground" you mean servo A in this diagram. The question is one of statics/dynamics. Most of the force of the servo A is taken by the bearings of the servo, rather than the servo motor itself, because the foot is pointing straight down. This is not the case for the servo B -- it is close to pessimal in this regard.

waltr:

--- Quote from: jwatte on January 13, 2013, 12:19:01 PM ---
--- Quote from: saba_rish91 on January 13, 2013, 09:46:18 AM [email protected] Thanks a lot.

I was browsing through internet and I found the following site on hexabot's mechanics.

http://www.robotshop.com/robot-leg-tutorial.html

In that site, it says that the servomotor near to the ground requires more torque compared to the middle servomotor!

--- End quote ---

I suppose by "near to the ground" you mean servo A in this diagram. The question is one of statics/dynamics. Most of the force of the servo A is taken by the bearings of the servo, rather than the servo motor itself, because the foot is pointing straight down. This is not the case for the servo B -- it is close to pessimal in this regard.

--- End quote ---

If the joint at A has a horizontal pivot then the level arm is the distance from the pivot at A the the body of the Bot at point C. Therefore this servo would require the greatest amount of torque on hold the bot's body off the ground.

jwatte:

--- Quote ---If the joint at A has a horizontal pivot then the level arm is the distance from the pivot at A the the body of the Bot at point C. Therefore this servo would require the greatest amount of torque on hold the bot's body off the ground.
--- End quote ---

I thought so too for a long time! But, it turns out, if the rest of the robot is rigid, then that's not actually the case. Assuming servos B and C can keep the body rigid, A will not see much torque at all.

Consider putting four flat sticks on the ground. Now, put a flat plate on each of those sticks in the corners. (Like a table on table legs.) The table/plate will sit still just fine. If there were any sideways torque on the sticks, they would collapse inwards, but BECAUSE THE PLATE IS RIGID, this does not happen; the sticks simply see downward weight without any torque.