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Author Topic: Calcualting Torque on a Bipedal Robot  (Read 1292 times)

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Offline BunjaiTopic starter

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Calcualting Torque on a Bipedal Robot
« on: September 28, 2011, 07:08:24 PM »
Right now I am working on a project on a skating robot and I need to calculate if the servo I have has enough torque for my design. What it looks like at the moment is this:
http://eyesofwall.blogspot.com/2011/09/my-robot.html

Using solidworks i've found the mass to be about 2kg and I'm using Hitec HS-422 servos which have a torque at 3.3kg.cm
I'm a bit stumped at finding out how to calculate the torque required from the servo with my legs looking the way they are.

Offline georgeecollins

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Re: Calcualting Torque on a Bipedal Robot
« Reply #1 on: October 07, 2011, 11:12:00 AM »
That is a really cool design. 

Just off the top of my head I would guess that an HS-422 does not have enough torque for your design.  To know for sure I would to know the measurements of your robot and the weight distribution. 

But here is why I would guess a HS-422 is not strong enough.  At 6v an HS-422 has 57 ounce/inch f torque or 4.1 Kg/cm.  If 80% of your robot's weight is above those ankle servos, then those ankle servos will have to lift 1.6kg.  With a 4.1Kg/cm of torque and 1.6kg of weight the servos will only work with an action arm of 4.1/1.6 = 2.56 cm.  Joints are levers and the longer they get the stronger they need to be to carry the weight.  Those ankles seem to carry most of the weight of the robot with an action arm of about half the robot.  The "about" comes from not knowing the weight distribution of the robot.  If most of the weight is in the top cross member you need more torque, if most of the weight is in the feet you need less. 

But assuming your robot lifts at the ankles, stands on one foot, and has an even weight distribution above the ankle totaling 1.6kg, it could only 5 cm tall.  It looks a lot taller then that, perhaps 20cm tall?  So you would need a lot more torque than that.  You have to think about how much weight the servo has to carry, and how long the lever is to decide how much torque you need.  In your design it looks like your ankle servos have to be the strongest because they carry the weight of almost the whole robot when it is on one leg. 

These are very inexact estimates based on looking at your cool design. 

Offline BunjaiTopic starter

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Re: Calcualting Torque on a Bipedal Robot
« Reply #2 on: October 13, 2011, 08:43:01 PM »
Hey thanks for the help, it really does help shed some light on the calculations that I need for the servo. For calculating the ankle servo is it the length of the whole leg or just the length of the "feet" since thats the length of the member which the servo is attached to? Some rough dimensions of the robot is that it is 30cm tall at the moment and I plan on having a body and arms being added to it later. From the quick calcs you have said it does seem like that the HS-422 won't be enough for this design.

 


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