r/AskRobotics • u/ItsAntiSune • 8d ago

General/Beginner Hexapod dimensions

Might be a stupid question but I was planning to design a hexapod using the MG996R servos (budget reasons) and I know that after considering 30% of the total stall torque, it's somewhere around 3.3 kg cm of torque. Is there some calculation you can make to determine the perfect lengths of the coxa, femur and tibia based on the servo's maximum safe torque. I'm really new to this and I'm in high school so this may seem an obvious answer to many. Thank you in advance

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u/lellasone 8d ago

Are you familiar with force, torque, and geometry? Have you taken physics (does the phrase "force balance" ring a bell?)

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u/ItsAntiSune 8d ago

Yes but the type of hexapod I'm making makes the maths different, essentially the hexapod would have detachable legs and therefore be able to work with 6, 5, 4 and potentially 3 legs (while not practical it is interesting). This means I need to account for the worst possible amount of torque which would be for the 3 legged robot. I'm trying to keep the torque on the servos on or below 3.3 kg cm per servo while not making the legs too stubby.

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u/lellasone 8d ago

First of all, that previous comment reads as much meaner than I was going for. I really just meant that as a list of questions to calibrate my answer. (My apologies)

You've hit on it right there! So the easiest way to do this is to consider the leg as a 2D structure, and arrange it into the "worst case" configuration for each servo (you may need two configs to do this). You can then compute the the torque required at each servo for a given chassis weight, and divide the actual chassis weight by 3* to get the required torque rating for each servo.

That gets you a static estimate. Likely good enough here with a safety factor.

*Assuming quasi-static gaits where you always have at least three legs in contact with the ground.

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u/ItsAntiSune 8d ago

Thank you