### Author Topic: building a bi-ped - tension calculator  (Read 2841 times)

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#### TrickyNekro

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##### building a bi-ped - tension calculator
« on: November 22, 2007, 04:31:52 AM »
Greetings to everybody!!!

Lately I'm trying to materialize my new idea to build a biped robot (walker)
I know programming will be tough and building the case even more difficult but I have somethings that I need to work out...
First of all I'll be using some (10 by number) HS - 311 from hitec at 49 oz/in @ 6V
Considering I'll apply 7.2Volts to the servos (which is a max rated value) and building a extremely lightweight robot is this torque enough???
Probably my source of power will be some cell phone Li-po so same samples I've got will do, from Dallas!!!
And the case will be made out of plastic (or steel sheets at worst occasions)...
So the weight probably won't be over 1kilo or 2.2 libs (if I'm right!!!)
I came to the conclusion I need a tension calculator and any suggestions available for building a case (not the right forum I know)
But most important to me is the calculator!!!!
So if anyone knows the math or a Java program I 'd like it very very much!!!!

With best regards,
Tricky Nekro, Greece
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

• Supreme Robot
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##### Re: building a bi-ped - tension calculator
« Reply #1 on: November 22, 2007, 10:37:51 AM »
Tension is calculating the stretching of something. Like if you pull a rubber band, you'd use a tension calculator to determine how far it stretches.

What you want is a statics calculator, more specifically a moment arm (torque) calculation of the knees.

Here is what you do. Draw your robot as a stick figure, with the robot is a sitting position, knees bent at 90 degrees. This position requires more servo torque (in the knee servo) than any other position.

The torque is simply robot_weight * thigh_length. Thigh_length being the distance from the knee to the hip of the robot, and robot_weight being the weight of everything on the robot above the knee.

You might want to consider using lighter/smaller micro servos (that also cost ~\$10) for the arms and head to reduce robot_weight.

Oh and if you use a double knee design (makes legs faster), then both servos need the same torque you calculated earlier (they dont add unfortunately).

One day I'll make a biped calculator for SoR . . .

#### TrickyNekro

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##### Re: building a bi-ped - tension calculator
« Reply #2 on: November 24, 2007, 03:39:29 AM »
Thanks for the reply!!! I'll be able work with that!!!
I'm not planning to use any head or hands! I'm olny planning building a walking platform!!!

Thanks again,
Best Regards, Tricky Nekro
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

• Supreme Robot
• Posts: 11,680
##### Re: building a bi-ped - tension calculator
« Reply #3 on: November 24, 2007, 08:45:14 AM »
just an FYI, some people put arms on their bipeds solely to improve weight shifting and balancing

(for example, run around your room and notice what your arms do)

#### TrickyNekro

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##### Re: building a bi-ped - tension calculator
« Reply #4 on: November 24, 2007, 06:11:55 PM »
I know, but I just don't have the money to buy more servos...
I have been trying to walk as a robot days now to understand the weight balancing and the movements needed to be done!!!

BTW do you know , or anybody else, how to build the joints for the legs. I mean that in order to make a stable joint you need at least two points grabbed together... I one is the servo horn, how can I build the other... (It's tough finding any links in google since I don't know what to type!!!)

Best regards,
Tricky Nekro, Greece
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

• Supreme Robot
• Posts: 11,680
##### Re: building a bi-ped - tension calculator
« Reply #5 on: November 24, 2007, 10:06:56 PM »
Have a browse at other biped designs for ideas (and steal them!!! hehe):

http://www.societyofrobots.com/robotforum/index.php?topic=196.0

http://www.gizmag.com/go/7555/picture/35490/

#### TrickyNekro

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##### Re: building a bi-ped - tension calculator
« Reply #6 on: November 26, 2007, 06:06:38 AM »
So we have the same interests eeehhh!!!
OK I'll try to figure out how to build the joints and reduce the weight to minimum!!! Plastic is a good choice and I'm thinking of vacuum forming, but metal can be much more stable... The mechanical part can be tough some times!!!

Anyways...
Thanks for the answer and if I came up to anything I'll post here...
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

#### Robotboy86

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##### Re: building a bi-ped - tension calculator
« Reply #7 on: November 27, 2007, 11:07:15 AM »
Also.. another article of intrest:

http://en.wikipedia.org/wiki/Zero_Moment_Point

Although to be honest most of the modern bi-pedal work is moving away from ZMP and towards dynamic balancing..  either way.. you will have to refrence ZMP at one time as most of the dynamic balancing articles I have seen refrence ZMP as a starting point.

As far as the joint question(which would probably be better answered in the structure forum) try and use other discplines solutions to that.. look at cranes, or hinges for inspiration.  I personally either A buy the joint metal from a store(a lot of places offer them) or B just build it from metal at homedepot..  I prefer to keep the stress of holding the joint together off of the servo..  so I build a joint that will move on its on, and support the weight alone without a servo..  and then mount the servo inside to provide torque.

Anyways, best of luck!  You might want to look into a 2 wheeled balance bot(ala segway) also.. lots of good info in there