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Author Topic: Comment on hexapod design?  (Read 1576 times)

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Offline ZondartulTopic starter

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Comment on hexapod design?
« on: December 06, 2010, 08:52:15 AM »
Hello, I am planning, after some trial and error, to build a perfect hexapod handyman robot. I was wondering if you could give me some suggestions as I am really amateur at this stuff. I am well aware that I would not be able to complete something as complex from the first try so I definetely deserve being shouted at to do something simpler instead and I shall. But in the mean time, please say something about this design.

The design:
-A hexapod robot with 3 walking legs and 3 hands capable of using tools or grippers.
Why:
When traveling off-road, with lots of obstacles, or climbing, legs are better than wheels. Three legs are enough to be stable on a surface. One more leg is needed to walk because I am not a god of unbalanced walking. Two more legs (hands) are needed to hold two objects. The fourth walking leg is not needed when standing still so it could be used to perform some action between these two objects - cutting, welding, soldering, duck-taping, whatever.

- Preferably somewhere the size of a laptop if you look from above.
- The group of 3 legs should alone be able to lift the body plus 3kg of payload, and so should be the group of 3 hands.
- Hands should have 6 DOF and be able to use interchangable tools (custom saw, gripper, sensor, and other kinds of modules)
- Should have enough battery power to perform work for one hour without recharging.

Programming tasks ( at least I can handle those =/ )
- Uses machine vision to detect shape of terrain and obstacles to adjust it's gait for either fastest travel, or most energy-efficient.
- Also uses machine vision for orienting in space, and to make sure the tools accurately follow the working plan;
- Radio-controllable
- Limited autonomy (returning to where a radio signal was lost, returning to recharge station when power is low, doing work in a defined space without supervision)

Kind of work it could perform:
- Getting coke from the fridge.
- Mawing the lawn.
- Woodwork, cutting planks to certain length, sanding them etc.
- Assembly-disassembly of random stuff held together with screws.
- Soldering and assembly of electronic things from components and PCB's.
- Washing the floor.
- Finding salvageble electronics in junkyards.
- Scouting (mapping of areas).


Okay now the question time:
1) 3*3 DOF + 3*6 DOF = 27 DOF. If I use, say, 4.8-6 V servos like this one, would that mean I need 168 Volts from my power source or should I go back to school and learn Ohm's law? (also where would I get such voltage without a wall plug?)

2) Should I use DC motors, stepper motors, servos or pneumatics for this?

3) Should it look like a spider or like centaur? Question of hands easily reaching the object while legs not stretching to far for balance.

4) Where should a camera go for a good field of view and actually seeing what the hands are doing?


Thanks in advance.

Offline Graynomad

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Re: Comment on hexapod design?
« Reply #1 on: December 06, 2010, 09:27:09 AM »
Quote
would that mean I need 168 Volts from my power source
Where do you get 168v from? The servos will be connected in parallel so you only need 5-6v, it's the amps that will get a bit on the large side.

Honestly if you have to ask the above I feel this project is out of range for the time being.

______
Rob

Scattered showers my arse -- Noah, 2348BC.

Offline ZondartulTopic starter

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Re: Comment on hexapod design?
« Reply #2 on: December 07, 2010, 05:22:56 AM »
Okay, got it. What about the servo vs. stepper vs. pneumatic question though? Does it really matter? How would a pneumatic limb compare to a motor one?

Edit: Why is everyone suddenly so silent?
« Last Edit: December 08, 2010, 12:01:55 PM by Zondartul »

Offline beautifulsmall

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Re: Comment on hexapod design?
« Reply #3 on: December 09, 2010, 07:32:10 PM »
I built a hexapod as a uni project many years ago,
the concept of insect locomotion is that the front two legs are used for climbing, the middle two for balance and the aft two for thrust, I wouldnt err two far from nature.

Offline Graynomad

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Re: Comment on hexapod design?
« Reply #4 on: December 09, 2010, 08:01:26 PM »
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How would a pneumatic limb compare to a motor one?
I don't know a lot about robotics as such but in general air is difficult to control with precision, and as most (all?) hexapods seem to use servos I'd say that's the way to go.

______
Rob

Scattered showers my arse -- Noah, 2348BC.

Offline ZondartulTopic starter

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Re: Comment on hexapod design?
« Reply #5 on: December 10, 2010, 03:24:44 PM »
I guess I'll just post my progress so you can check in every few weeks and facepalm.

Random thoughts:
Torque = force / distance.
Weight = force.
Physically, servos being able to lift a robot does not depend on their power alone, but on length of the lever that they are mounted on (leg length).
Torque = 1 newton / meter, length = 1 cm => force = 100 newtons ~= weight of a 10 kg object.
Therefore, if there is a set weight one wants liftable, then servo power only determines leg length (also a power source but nvm that). Longer legs = faster movement and better coping with uneven terrain.

So I'll just order 27 servos rated 14 kg/cm and a 6V12AH battery. It IS an overkill, but adjusting leg length means taking full advantage of it. If my calculations are right and each leg must cope with 1 kg in worst case, 14 cm is max length between two servos on parallel axes (not max length of the leg... right?....) In that case, the absolute maximum mass it can lift is 12 kg (itself included).

Offline Soeren

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Re: Comment on hexapod design?
« Reply #6 on: December 12, 2010, 03:59:39 PM »
Hi,

So I'll just order 27 servos rated 14 kg/cm and a 6V12AH battery. It IS an overkill, but adjusting leg length means taking full advantage of it. If my calculations are right and each leg must cope with 1 kg in worst case, 14 cm is max length between two servos on parallel axes (not max length of the leg... right?....) In that case, the absolute maximum mass it can lift is 12 kg (itself included).
You'd do wisely by not ordering anything until you have a much better idea of what you need.
A servo rated 14kg-cm (not kg/cm) can move the equivalent of 14kg when 1cm from the center of the axle (7kg at 2cm, or 1kg at 14cm).

However... Placing a servo on a leg run by another servo, the entire load is still what the first servo can handle, so you cannot use a "shoulder" and an "elbow 14 cm apart and expect the shoulder to handle the load you calculate for each.


Best advice I can give is to forget all of this for now and start with something like the $50 'bot, keep on with still more complicated constructions and perhaps in 5..10 years, you'll be ready to start ordering stuff for your screw and duct tape applying robot.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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