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It's Raining Americans (Better Take an Umbrella)

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Max The Magnificent
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Re: Simplifying the mechanism
Max The Magnificent   3/24/2017 3:38:57 PM
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@perl_geek: ...Simple Beats Baroque...

That has a nice ring to it -- I'll be snaffling this and using it myself one day...

perl_geek
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Re: Simplifying the mechanism
perl_geek   3/24/2017 11:13:38 AM
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I had in mind a spring-loaded switch between the arms and the grippers, to give a sort of finger-tip sensitivity. Mounting on a bending-beam arm, (like a reversed torque wrench) would be another possibility, but it sounds as though you have an even more robust mechanical solution.

Either way, something that works directly has to be better than measuring some internal attribute like current, and then inferring results through a chain of questionable assumptions. Simple beats baroque when it comes to design.

 

David Ashton
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Re: Jack the Gripper
David Ashton   3/23/2017 5:25:32 PM
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@Elizabeth...that'll just be one more line of code for Max to write :-)

elizabethsimon
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Re: Jack the Gripper
elizabethsimon   3/23/2017 5:19:05 PM
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@David  Make sure it only chases politicians!

I think I'd rather have one that chases lawyers. This would have the advantage of also chasing most of the politicions

eetcowie
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Re: Jack the Gripper
eetcowie   3/23/2017 4:20:31 PM
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Max - if you are planning to take Jack the Gripper out into "the field" as it is optimistically called, and especially if it is going to be wrapping its fangs around trees, get rid of those protruding wires.  They won't survive a week like that.  If you can't run them inside, run them on the outside of the joints and put them in flexible conduit:

The red and black wires you see at the left are not in the final version. There are steel cables under the gripper-arms that are not wires.

There are no exposed wires in the final version.

Good observation, though!

eetcowie
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Re: Simplifying the mechanism
eetcowie   3/23/2017 4:16:15 PM
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Maybe I'm misunderstanding the problem, but wouldn't an opening pressure switch on the ends of the gripper be sufficient to stop the motors when they've gripped the tree? When it touches, it opens the circuit and disconnects the power.


Answer: As the teeth bite into the variable bark and shape of the various trees, a switch closure doesn't insure the teeth have bitten sufficiently (or as much as possible given the variations). If you go with this approach, you need a force sensor, because this says we grip enough to be sucure around the trunk. But then, where does the force sensor go? At the base of the gripper's moving arm. Now there is a cost issue. Additionally, as brush-type motors age, the commutator gets less efficient, as brush particles get between the slits in the copper ring -- beginning to shunt current around the coils which nets less force for the same current. This means that new motors produce higher torque which translates to more force than older motors. One might argue that measuring the force could provide a measure of the age of a motor, by monitoring the profile of any given closure over time. In short, yes, but supporting this function is for a SKU that has more features comes at higher cost. The 'ratchet' that is pre-built into the motor's attached gear head does the minimum of what is required (for free) -- begins ratcheting at a pre-defined force, which is known to have sufficient grip for safety. As the motor ages, the customer knows something is wrong, because it begins gripping, and never gets to the ratchet stage. Cheap, dirty -- but gets the job done.

David Ashton
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Re: Jack the Gripper
David Ashton   3/23/2017 3:19:47 PM
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@Max...well that's where your programming skills come in.  Make sure it only chases politicians!

Max The Magnificent
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Re: Jack the Gripper
Max The Magnificent   3/23/2017 3:16:11 PM
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@David: ...If you produced a version called the Politician Cruncher, I bet you'd sell more of them...

It's hard to fault your logic, but then I start to think -- do you remember my recent column One Metallic Step Closer to the Robot Apocalypse? Can you imagine one of those Boston Dynamic Beasts equipped with a "Politician Cruncher" chasing us down the street? (Or course, I wouldn't have to run faster than the robot... I'd just have to run faster than you, as the old joke goes :-)

David Ashton
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Re: Jack the Gripper
David Ashton   3/23/2017 2:53:37 PM
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@Max....Exactly.  If you produced a version called the Politician Cruncher, I bet you'd sell more of them than the Jack the Grippers :-)

Max The Magnificent
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Re: Jack the Gripper
Max The Magnificent   3/23/2017 10:26:04 AM
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@David: ...Not for trees, for politicians...

You jest, but when you see this thing in action you realize that you don;t want to be anywhere near it when it's closing -- it truly is a powerful beast (much like myself :-)

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