# Society of Robots - Robot Forum

## Mechanics and Construction => Mechanics and Construction => Topic started by: benji on October 19, 2007, 01:07:26 PM

Title: hexapod legs
Post by: benji on October 19, 2007, 01:07:26 PM
hllo folks
im doin a hexapod robot ,i want to make the length 50 cm and the width from leg to leg about 40 cm
problem is i dont know what should be the dimentions of the legs
anyone can help with defining the legs dimentions?
thank you
Title: Re: hexapod legs
Post by: Admin on October 19, 2007, 07:53:37 PM
Try CADing your design and see what looks right.

If you want to calculate it to be optimal, read this tutorial:
http://www.societyofrobots.com/mechanics_statics.shtml

You also need to decide what the legs will do (climb over stuff? how fast robot will move, etc)
Title: Re: hexapod legs
Post by: benji on October 20, 2007, 02:29:23 AM
i did read that, nice ideas
is there some software for calculating the weighs and forces on the parts of the bot?
the most important thing is to define dimentions that makes the robot stable when moving standing ..etc
,, how can i define em before cuttin up alminum?
Title: Re: hexapod legs
Post by: Tsukubadaisei on October 20, 2007, 05:53:16 AM
No, there is no software because each robot is different, so each robot will require diferent equations. But as long as you have the equations you can peform simulations using MATlab(high level but more difficult) or excel(low level but easier) or by programming.
In order to find the equations get a book of Mechanics(I also recommend MITOCW) and you will get the physics background. For example how to calculate force, torque and momentum. However, physics books usually study systems with only one or two bodies. You are building a hexapod so you have (6 legs) x (at least 3 bones per leg) +(main body)=  at least 19 bodies. In order to calculate the precise optimal position of the system(robot) you will need a set of matrices(one for each leg). Hence you need to know Linear Algebra(Specially to undertand the concept of linear tranformation and eigen values/vectors for solving diff. equations). You can also try a mechanical design book(in my university the class is called Dynamics of Mechanisms for Mechatronics).
By doing that you will get the perfect robot. You can use trial and error or copy someone's else design, but then I cannot garantee 100% efficiency. Articulated robots (like robotic arms, legs, insects, humanoids) require way more mathematical background than the usual 2 wheeled robot that anyone can make. If want an hexapod, you must do some maths. If you want a good eficient hexapod, you need heavy maths. That is why you dont see a many of them, people dont like maths (not to mention they are expensive).

EDIT: I forgot to mention that I might partially help you with the maths, like I always do in this board.
Title: Re: hexapod legs
Post by: Admin on October 20, 2007, 06:47:06 AM
My CAD software calculates weight for me . . . but you can just use excel to add it up yourself.

The math really isnt that hard, its just highschool physics mostly. A little trig too.

The math isnt any harder than whats in my tutorial . . . its all moment arm balancing equations.

Just assume your hexapod is one big rigid structure and that should be good enough.
Title: Re: hexapod legs
Post by: benji on October 20, 2007, 02:46:22 PM
thanks guys for the explanition , its not a matter of complex maths its just that i want to define the best dimentions for this robot's leg
i checked a design in the crustcrawler website ,,a hexapod thats 50 cm long and 40 cm weidth leg to leg
,,but the site doesnt give info about the dimention of its legs

anyways ill try to do some math n see