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Adjustable robotic links
5

Adjustable robotic links

Adjustable robotic links

(OP)
Simple question guys.

Anyone come across legs or links that have an adjustable length?
For example, there exists synthesis methods for 4-bar linkages in which adjustability of a link provides several advantages.
Has anyone seen a practical version of an adjustable link? For mechanisms or robots.

Thanks and cheers!  

peace
Fe

RE: Adjustable robotic links

See "Johnson rod".  Once ubiquitous on British motorcycles.

Practical?  No.  They were always falling off, which is part of the reason there are now so few British motorcycles.



 

Mike Halloran
Pembroke Pines, FL, USA

RE: Adjustable robotic links

(OP)
Thanks Mike!
Is there another name for it? I can't get a decent description of one online it seems.  

peace
Fe

RE: Adjustable robotic links

(OP)
I forgot to mention in my original post that I meant for things other than the obvious like double side threaded rods ect.
I never knew about the Johnson rod, so that's a good start. smile

Basically, I want to know if something I can use exists or not. Otherwise I wil simply design a custom part. I may be forced to modify a linear actuator of some sort.
cheers

peace
Fe

RE: Adjustable robotic links

(OP)
You guessed exactly right Greg.
Adjustable displacement. Basically, the robot I am working on right now needs one of its links to be adjustable in length. But not actively (as the robot is being used). It can be adjusted offline for different duties ect. It is a custom robot, but I thought I might be lucky to get some parts that might exist.
So it will have to be rigid and reliable.
Custom ballscrew setup is very feasible.
So I guess not much exists in this domain of mechanisms in practicality.

cheers guys

peace
Fe

RE: Adjustable robotic links

Well, you could have a look at what racing yachts use to fiddle with their standing rigging. I'm guessing it is more likely to be hydraulic than electrical.

The usual manual method I've seen is a locknutted turnbuckle with left and right hand threads, like a tie rod in a car.

 

Cheers

Greg Locock


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RE: Adjustable robotic links

(OP)
Thanks guys.

Quote:

like a tie rod in a car.

I am leaning towards something like this in the preliminary design stage. Thanks Greg.

Mint,
Very interesting links you provided. I never knew those existed. I will look into their feasibility into my design. Thanks.

 

peace
Fe

RE: Adjustable robotic links

Mint, that is a pretty cool device.

FeX32, everything you've specified would seem to indicate that a turnbuckle or tie-rod type of device would be ideal for your application, yet you say you want something else. Why? Turnbuckles can use left-hand/right-hand threads for "coarse" adjustment, or differential thread pitches of the same handedness for very fine "micro" adjustments.

RE: Adjustable robotic links

Race car suspensions often use Rod Ends with left and right hand threads [one each end of the attaching rod] to make the link length adjustable.

RE: Adjustable robotic links

you say "other than double side threaded rods" ... why ? what's wrong with a tube with an internal thread, two threaded rod ends, and two jam nuts ?

RE: Adjustable robotic links

Telescoping tubing with snap spring buttons.  Downfall is strictly manual manipulation, coarse adjustment, but is very robust and inexpensive.  Linear electric actuators with built in braking mechanism make wonderful mechanical linkages (the actuator itself is the linkage).  These are used extensively in the medical equipment industry for precisely that purpose (of adjusting linkage length).

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Adjustable robotic links

(OP)
Thanks for the responses!

I was just wondering if there was some other sort of link or mechanism that could get the job done.
The tie-rod now mentioned by many of you suggests to me that this is likely the best way to go.
One of the reasons I was wondering if I could get something else is due to the large difference in link adjustability. For example, lets say I want the link to go from 500mm to 1200mm (or 100mm to 200mm). The male thread may end up protruding too far into the female part making the lesser length not practical. Maybe there is a way to work around this. I will do have some simple modeling to find out.

Quote:

Telescoping tubing with snap spring buttons.
Interesting. I have never seen this used.

Thanks again guys for the responses!

peace
Fe

RE: Adjustable robotic links

Very common now to adjust the height of crutches maybe in one inch increments.

RE: Adjustable robotic links

And it suddenly occured to me that crutches are what you use after the Johnson Rod fails on your motorcycle.

RE: Adjustable robotic links

Eccentric cam on the end of the smaller tube of a telescoping tube set.

 

RE: Adjustable robotic links

You didn't mention size or load capacity so here goes.
The first is a Push Pull type screwdriver, Yankee,  For light loads and somewhat limited reach.

Next is the Spieth  locking bushing, etc where you can use a tube for almost unlimited adjustment.  We use this arrangement for heavier loads and lengths.

The Serapid product is for very heavy loads.

Not referenced is the tube clamp nut as seen on the above mention crutches and on adjustable camera legs.  This approach comes in many flavors.  We use precision telescoping tubes with a lock nut similar to aforementioned products.

http://www.thefind.com/appliances/info-yankee-push-drill

http://www.spieth-maschinenelemente.de/cms/p@c=19&l=5.html

http://www.serapid.com/gb/new_products/electric_spindle_actuator.html




 

RE: Adjustable robotic links

Just looking and I think the adjustable metal walking cane/stick might be a good configuration if your loads will allow it's use. The walking cane has both spring detentes as posted above and a compression locking collar. It's possible to take the metal part right out of the plastic handle for a ready made arm.
I think a trip to to the drugstore might be worth the effort.

This is the type I had in mind.

http://www.physioonline.ie/catalogue/products/home-help-&-walking-aids/metal-adjustable-walking-stick#

RE: Adjustable robotic links

(OP)
unclesyd,

Thanks for those 3 first links. I think I'm going to get one of those yankee screw-drivers for my personal "screwing around" pipe.

I like the way an adjustable walking stick type mechanisms works with a locking collar. It seems stable enough for lighter applications. Easy to manufacture. The only drawback might be the precision of the adjustability.  But I am sure there is a way to get around this.
Thanks for the ideas. Much appreciated!

cheers

peace
Fe

RE: Adjustable robotic links

if you're manually adjusting the rod length, you could add segments ... any extending rod (telescoping, threaded, ...) would be limited to dubling the length.

adding segments would be easier than nesting more than two tube together.

i've presumed these rods are compression loaded; any chance these things will be loaded in tension only ?  that' d open up a whole different set of solutions.

RE: Adjustable robotic links

Dumb question:
How does the robot controller 'know' the length of the arm after you've changed it?
 

Mike Halloran
Pembroke Pines, FL, USA

RE: Adjustable robotic links

(OP)

Quote:

any chance these things will be loaded in tension only ?  that' d open up a whole different set of solutions.

The robot is to be potentially used for high speed applications. Thus, the loading will be tension and compression. From my modeling the closest loading condition is the "repeated-reversed" condition. Imagine something like a delta robot (not exactly, but it's the closest I could find): http://www.youtube.com/watch?v=foTE0Mau5a8
And add about 10kg to those ping pong balls.
I wish I could tell you guys more information about the robot, but I can't leak the designs for reasons I'm sure you guys understand. smile

Quote:

so you can make 3/4" increments where locking collar locks it in.

I agree that this variation of a telescoping tube idea is indeed a potential solution, particularly because of the large difference in lengths I require for re-configurations.
So, I have decided to model at least 2 different solutions, one of which being this.
Then later I will conduct a feasibility study on each of the designs likely be experiment.

Thanks!
cheers

peace
Fe

RE: Adjustable robotic links

(OP)

Quote:

How does the robot controller 'know' the length of the arm after you've changed it?

Not a dumb question IMO, more like fundamental but important.
Initially, it will be changed manually in the control architecture. Potentially, the robot is to be re-configured for certain "blank" due to "blank" (sorry) and may be fitted with a simple sensing mechanism (maybe something like Hall sensors) to provide a signal of link length.  

peace
Fe

RE: Adjustable robotic links

You might be able to combine the detent adjustment for corse length with a threaded adjustment for the fine length.

RE: Adjustable robotic links

(OP)
Thanks again everyone!

peace
Fe

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