Subject:
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Re: Gripper and lift arm with single motor
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 9 Aug 1999 21:28:11 GMT
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Viewed:
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959 times
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In lugnet.robotics, lego-robotics@crynwr.com (Paul Speed) writes:
> Hello,
>
> Ok, I haven't actually done this but I have given it some thought.
>
> First, imagine that you just want to lift the arm. A classic
> way would be to put a gear into the pivot point that was fixed to the
> arm. Another gear in the platform could turn this gear to make it go
> up and down.
>
> Now, imagine that instead of being fixed to the arm that the
> gear in the pivot is free to move. Connect it to drive the gripper.
>
> So, when the arm is hanging limp then turning the gear will
> open and close the gripper. However, when the gripper is completely
> closed, ie: can no longer move, the pivot gear now acts as in the
> first example and lifts the arm.
>
> Hopefully, this will be enough of a hint.
> -Paul (pspeed@progeeks.com, http://www.progeeks.com/)
>
>
> David Walton wrote:
> >
> > I hope someone can help me with this
> >
> > I tried most of the weekend to try and build a mechanism to grab an item
> > (49.6x28 VR wheel) and lift it using one new type motor (the ones in the
> > RIS). All my attempts seem to either self destruct or just didn't work.
> >
> > Basically my designs were based upon fixing an axle at the lift pivot point
> > and using a pair of bevel gears to produce rotation to drive a gripper.
> >
> > What I would like to be able to do is drive the jaw once it is closed the
> > lift mechanism would engage. I just couldn't get it right and its getting
> > me a bit frustrated.
> >
> > NO I can't just use another motor and some touch sensors, I need the motors
> > for drive adder/subtractor based tracked vehicle and the sensor inputs for
> > navigation and object detection
> >
> > Anybody give me any pointers to designs on the web which actually work - I
> > know Ben Willamson had a design but that doesn't give enough movement of
> > the gripper jaws for my project
> >
> > TIA
> >
> > David Walton
> >
> > Email: d.walton@ndirect.co.uk
I have built a one-motor, grasp-then-lift arm as part of my (incomplete)
solution to Joel Shafer's soda can challenge. I will try to post some
pictures of it soon.
The idea is based on the "directional transmission" idea that I first saw at
Michael Powell's web site: http://www.sonic.net/~rci/transmission.htm
I use the sliding worm gear to engage a gripper gear at first, then slide onto
a pivot gear (which is fixed relative to the robot frame) to raise the arm.
The whole arm assembly, motor and all, pivot as the worm gear makes direct
contact with the pivot gear. I've even arranged it so that the motor behaves
as a counter-balance to help lift the object.
It took quite a while to get everything laid out just right, and my design
definitely works the best with something the general size, shape, and weight
of a soda can. But you should be able to use the same concepts to create an
arm that is specialized for other kinds of lifting.
I have found that the directional transmission relies on a couple of subtle
mechanical realities if it is to work properly. For one, you must have a way
to hold one part still while the other drive gear is engaged. For me, this
meant using Lego rubber bands to hold my gripper hand closed unless the motor
was actively working to hold it open. This also helps when you are going in
the other direction, because the back-pressure ensures that the motor finishes
task A (closing the claw) before the worm gear slides over to gear B (to raise
the arm.) I use good-old-gravity on the other end to ensure that the arm is
lowered completely before the gear slips down to open the claw.
I have found it necessary to "bolt" cross-beams to build a cage around any
motor/transmission combination to prevent it from exploding under stress.
I've found that you will almost always stall the motor before you will break a
gear made out of ABS, but your Lego will pop apart long before that will
happen.
I'm sorry that this description is so useless. As I said, I'll try to post
some pictures to make this clearer.
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Message is in Reply To:
 | | Re: Gripper and lift arm with single motor
|
| Hello, Ok, I haven't actually done this but I have given it some thought. First, imagine that you just want to lift the arm. A classic way would be to put a gear into the pivot point that was fixed to the arm. Another gear in the platform could turn (...) (25 years ago, 9-Aug-99, to lugnet.robotics)
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