Robotic Arm 7DOF

Posted by Homosapien on January 31, 2010, 7:24 am

I am currently studying Robotics and I’ve been lurking around reading up
on Robotics. I am working on an industrial Robotic Arm using 7DOF and I
have been out of my wits trying to get some help on this. Locally, it’s
been hard for me to get some help and somehow, I thought I would check out
if there were any websites/forums that might be able to help me out so
here I am wondering if anyone of you would be kind enough to provide with
me some designs in terms of mechanical components?

The motion that I am using is almost the same as a human arm and can hold
up 3 kg of things. I want to use electrical actuators and gears,
preferably at a low cost and light weight.

As I stated above, I need help on the mechanical design (mechanism)
movement of the arm. I would really appreciate it if anyone can help me on
this. I seriously have tried getting help elsewhere and Robotics isn’t
something that is big here – not yet, anyway! So here I am – trying on a
forum!

Many thanks in advance!

-------------------------------------

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Posted by Rafael Deliano on January 31, 2010, 9:33 am

> help on the mechanical design (mechanism)
> movement of the arm.

There are ( always and on almost anything ) books:
Rivin "Mechanical Design of Robots" McGrawHill 1988
( although didn´t find that an easy read, but i am an EE )

> I want to use electrical actuators and gears,
> preferably at a low cost and light weight.

There are several old, "educational" robots like
Microbot / Teachmover
Scorbot
Sekuria ( only here in Germany )
Often they turn up reasonably cheap at ebay.
It would be probably worthwhile to have a look how
they have done it.

MfG JRD

Posted by Homosapien on January 31, 2010, 11:19 pm

Homosapien had written this in response to
http://www.www.roboticscommunity.com/robotics/Re-Robotic-Arm-7DOF-28429-.htm
:

-------------------------------------
Rafael Deliano wrote:

> > help on the mechanical design (mechanism)
>> movement of the arm.
>> There are ( always and on almost anything ) books:

> Rivin "Mechanical Design of Robots" McGrawHill 1988
> ( although didn´t find that an easy read, but i am an EE )

> > I want to use electrical actuators and gears,
> > preferably at a low cost and light weight.
>> There are several old, "educational" robots like

> Microbot / Teachmover
> Scorbot
> Sekuria ( only here in Germany )
> Often they turn up reasonably cheap at ebay.
> It would be probably worthwhile to have a look how
> they have done it.

> MfG JRD

Thank you for your reply.

Will I be able to find mechanical designs on the robotic arm in the book
mentioned?

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Posted by Rafael Deliano on February 1, 2010, 7:21 am

>> Rivin "Mechanical Design of Robots" McGrawHill 1988

> Will I be able to find mechanical designs on the robotic arm in the book
> mentioned?

Its not an easy read, but at least its focused on the mechanical stuff,
not too much literature ( apart from conference proceedings ) on that.
Victor Scheinman
http://en.wikipedia.org/wiki/Victor_Scheinman
liked it, wrote a glowing review about it in some journal. So i got it.
Note that the experimental Stanford Arm ( which perhaps got some ideas
from the older Unimates )
http://images.google.de/images?q=Stanford+Arm&oe=utf-8&rls=org.mozilla:de:official&client=firefox-a&um=1&ie=UTF-8&sa=N&hl=de&tab=wi
evolved to the industrial PUMA about 1978 (?)
http://images.google.de/images?hl=de&client=firefox-a&rls=org.mozilla%3Ade%3Aofficial&um=1&sa=1&q=Unimate+Puma&btnG=Suche
That one is electrical and was explicitly specified to be
"almost the same as a human arm".
That size hasn´t been commercially too successfull. People are cheaper,
faster, better. There have been some later simple small systems
like the old red Mitsubishi Movemaster from the 80ies:
http://srt-aut.mv.fh-duesseldorf.de/handhabung/index.php/Mitsubishi_RM-501
These are not experimental/educational but have/had applications
in industry and are still somewhat expensive if found at ebay.

The big stuff is often hydraulic:
http://business.shop.ebay.com/Robotics-/50924/i.html?_armrs=1&_mdo=Business-Industrial&_mspp=&_pcats=42892%2C92074%2C12576
http://business.shop.ebay.de/Roboter-/40517/i.html?_npmv=3

Another good book would be:
Lhote "Robot components and Systems" Kogan 1984
That covers electrical/pneumatic/hydraulic motors
and has some chapter "transmission systems" on the mechanical
side.

MfG JRD

Posted by on January 31, 2010, 5:57 pm

Homosapien wrote:

> I am currently studying Robotics and IÂ’ve been lurking around reading up
> on Robotics. I am working on an industrial Robotic Arm using 7DOF and I
> have been out of my wits trying to get some help on this. Locally, itÂ’s
> been hard for me to get some help and somehow, I thought I would check out
> if there were any websites/forums that might be able to help me out so
> here I am wondering if anyone of you would be kind enough to provide with
> me some designs in terms of mechanical components?
>
> The motion that I am using is almost the same as a human arm and can hold
> up 3 kg of things. I want to use electrical actuators and gears,
> preferably at a low cost and light weight.
>
> As I stated above, I need help on the mechanical design (mechanism)
> movement of the arm. I would really appreciate it if anyone can help me on
> this. I seriously have tried getting help elsewhere and Robotics isnÂ’t
> something that is big here Â– not yet, anyway! So here I am Â– trying on a
> forum!
>

You don't state what you want the arm to do. If it is just to position
something within free space, you might get away with open loop stepper
motors, but if you want to push on things in a controlled way, you'll need
encoders on each joint to track where it is. I would think harmonic drives
should be small enough and cheap enough to do for the gear reducer.

You also need smaller motors as you move towards the wrist, although you
still need good torque at the end to hold up your payload at maximum
offset.

So, at a minimum, you need seven varying-sized joints, probably with stepper
or DC motors, brakes, encoders, gearboxes, motor controllers and joining
structure. Robotics Research makes a cool looking robot using roll and
pitch joints only. The 7 jointed manipulator on space station has joints
in a roll-yaw-pitch-pitch-pitch-yaw-roll sequence, but that configuration
was largely driven by the need to fold the whole thing up into a small
space for launch.

You also need a bunch of good sensors if you want to feel your environment,
and a good computer to act as overall controller that can handle 7-joint
inverse kinematics. Oh, and a good gripper, which is the whole reason for
moving those joints around.

If you can accept a little slop in the motion, you could keep the motors in
the base and use cables to transmit motion to the joints, making the arm
lighter and reducing the required torque for the shoulder joints.

Google around for some plans for a small plywood robot, I remember seeing
something like that. I think it had just 5 or 6 joints, however.

You might just be cheaper off looking through the surplus equipment sites
for a used model. What you want to do has been done many times before, so
there's no need to reinvent the wheel, unless it's for self-education.

Mike Ross

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