### Author Topic: Exoskeleton design and motor sizing help.  (Read 4184 times)

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#### JAm1

• Beginner
• Posts: 2
##### Exoskeleton design and motor sizing help.
« on: May 07, 2011, 08:44:52 AM »
Hi I'm designing an exoskeleton that would be fitted to a persons leg. I've been doing some calculations to solve for the minimum required torque to carry a persons leg at the hip joint. The equation I'm using is..

Mhip = Mweight + Mexo + (Ileg + Iexo)*alpha

where:
Mhip = torque required at hip joint
Mweight = resisting torque produced by the weight of the leg
Mexo = resisting torque produced by the weight of the exoskeleton
Ileg  = moment of inertia of the leg
Iexo = moment of inertia of the exoskeleton
alpha = angular acceleration at hip joint

I've been doing some rough calculations based on some anthropometric data and I'm getting values around 40-50Nm(I used 30 degrees as the maximum angle for the leg) for Mhip.

My problem is this. Am I doing the mechanics correctly? is 40-50Nm a reasonable torque value? How/Where can i look for DC motors + gears that would accommodate this torque? Can i use relatively small motors then just gear it up to get my req torque?

Thanks to anyone who can help. I'm kinda new to this designing thing. But i'm really willing to learn.

#### JAm1

• Beginner
• Posts: 2
##### Re: Exoskeleton design and motor sizing help.
« Reply #1 on: May 08, 2011, 08:16:20 AM »
Anyone?

• Supreme Robot
• Posts: 611
##### Re: Exoskeleton design and motor sizing help.
« Reply #2 on: May 08, 2011, 03:50:18 PM »
There are tons of threads concerning pretty much exactly what you're talking about.

Building an exoskeleton is pretty difficult and shouldn't be taken as a "project" for someone who doesn't know much about robotics.  I don't mean to sound mean but that's just how it is.

You probably won't be able to use "small" motors (unless you're willing to spend an arm and a let).  Even with proper gearing.  Although it depends on your definition of "small".  You have to remember, not only do these motors have to be pretty powerful, they also have to move quickly.  How fast do you move your legs?  Sure you could probably gear up a small motor, but it'll move REALLY slow.

I don't claim to be a robotics expert (or even beginner for that matter), but searching often provides answers to most of your questions.  Just go back to the "Mechanics and Construction" section and use the search box in the upper right corner.  That should give you plenty of info to look over.  There are also motor calculators somewhere on this site.

Here. http://www.societyofrobots.com/RMF_calculator.shtml

I hope that helps.

#### nickc

• Jr. Member
• Posts: 33
##### Re: Exoskeleton design and motor sizing help.
« Reply #3 on: May 17, 2011, 02:02:07 PM »
what about the weight of the person attached to the leg?

#### garrettg84

• Robot Overlord
• Posts: 187
• Armchair Roboticist Extraordinaire
##### Re: Exoskeleton design and motor sizing help.
« Reply #4 on: May 17, 2011, 02:14:55 PM »
what about the weight of the person attached to the leg?

DONT CALL ME FAT!!! I'm just big boned.
-garrett

#### Gertlex

• Supreme Robot
• Posts: 763
• Nuclear Engineer ยท Roboticist
##### Re: Exoskeleton design and motor sizing help.
« Reply #5 on: May 19, 2011, 07:15:58 AM »
what about the weight of the person attached to the leg?

He's calculating moment at the hip.  Non-leg parts (roughly) contribute no moment about the hip.
I

#### nickc

• Jr. Member
• Posts: 33
##### Re: Exoskeleton design and motor sizing help.
« Reply #6 on: May 19, 2011, 12:50:58 PM »
what about the weight of the person attached to the leg?

He's calculating moment at the hip.  Non-leg parts (roughly) contribute no moment about the hip.

http://www.asbweb.org/conferences/2007/214.pdf

Hip flexion would be the same but hip extension under load is greater than with no load.   It's obvious that there is a difference between hanging from a pull-up bar and lifting your legs versus standing from a seated position.  Both rotate the hip but one includes body weight and one doesn't. He's calculating just the torque required to lift the leg and exoskeleton which is hip flexion.  The load to extend the hip includes the body weight. If you look at the graph in the paper you will see the extension torque under load is 2x than no load.