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Forum topic by Jeff Mazur posted 12-03-2014 04:41 PM 1325 views 1 time favorited 30 replies Add to Favorites Watch
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Jeff Mazur

69 posts in 768 days


12-03-2014 04:41 PM

Topic tags/keywords: actuator control electronic motor platform question

Yes, more engineering than woodworking, truly, but hey…the thing being lifted IS wooden (and home-made).

So here’s the scenario: I am designing a rectangular platform with four supporting posts that needs to be lifted about 30”, and the intention is to keep the area beneath the platform clear of obstruction. Thus, I cannot have center support or lifting of the platform but need to furnish all applied force at the four posts. The load is non-trivial (can be up to about 400 lb.) Factoring in cost, desire to keep the mechanism quiet, relatively unobtrusive, and independent of the strength of the user (motor driven), I have decided to use linear actuators. Unfortunately I cannot find any with a 30” or greater stroke that have potentiometers or any other positional feedback built in, and I want to keep the platform fairly level as it raises and lowers.

My notion is to use a programmable controller such as an Arduino or Raspberry Pi in conjunction with a three-axes accelerometer mounted on the underside-center of the platform, to keep the actuators in sync; but I am open to suggestions. Anybody done anything like this before, or have any ideas?

-- Woodworking is a beautiful, physical, cerebral, and noble art.


30 replies so far

View timbertailor's profile

timbertailor

1592 posts in 889 days


#1 posted 12-03-2014 04:47 PM

Check out the CNC section in the router forums.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

View Jeff Mazur's profile

Jeff Mazur

69 posts in 768 days


#2 posted 12-03-2014 04:53 PM



Check out the CNC section in the router forums.

- timbertailor

That wouldn’t have occurred to me, Brad, thanks for the suggestion. Have you seen such discussions there? Aside from the computer element I don’t see the analogy, maybe I’m being dense.

-- Woodworking is a beautiful, physical, cerebral, and noble art.

View Jeff Mazur's profile

Jeff Mazur

69 posts in 768 days


#3 posted 12-03-2014 04:57 PM

To clarify, by the way, as I was somewhat incomplete in describing: the linear actuators in question are electric linear actuators and are of the acme screw type (versus roller screw or track); not hydraulic or pneumatic.

-- Woodworking is a beautiful, physical, cerebral, and noble art.

View ChuckC's profile

ChuckC

821 posts in 2399 days


#4 posted 12-03-2014 05:02 PM

Can you use steppers and screws? An Arduino can handle that and you wouldn’t need a gyro.

You probably won’t hear much about actuators in a CNC forum (cnczone.com, for example) but you will find a lot of information about servos, steppers, and rack and pinion drives.

Is this for a motorized/raising work bench?

View timbertailor's profile

timbertailor

1592 posts in 889 days


#5 posted 12-03-2014 05:17 PM

Position feedback signals are expensive to acquire, most use limit switches to save money.

I have not spent a lot of time in the CNC section, but I have seen several different approaches to linearly controlled movement in their discussions. Thought it would be worth while for you to check out. Nice guys can probably point you in the right direction.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

View mramseyISU's profile

mramseyISU

419 posts in 1010 days


#6 posted 12-03-2014 05:34 PM

Did you check out Misumi? They have a pretty big selection of electric linear actuators that should work for you.

-- Trust me I'm an engineer.

View bigblockyeti's profile

bigblockyeti

3668 posts in 1185 days


#7 posted 12-03-2014 05:39 PM

+1 to limit switches. You can get a circuit to ramp speed up and down to avoid sudden starts and stops, plus it could be done with a regular DC motor, no stepper or servo required. Is it mandatory to use two actuators or would one on only one side suffice, this could eliminate the need to synchronize two?

View Rob's profile

Rob

704 posts in 2535 days


#8 posted 12-03-2014 05:55 PM

Rather than using linear actuators that climb up and down the threaded rods, what about turning one threaded rod and linking the others to it via chains or belts, similar to a twin-screw vise?

-- Ask an expert or be the expert - http://woodworking.stackexchange.com

View dschlic1's profile

dschlic1

330 posts in 1434 days


#9 posted 12-03-2014 05:59 PM

How about cables and a drum? Just count the turns. Gravity will pull the platform down, roll the cables up to lift the platform. Should always be flat and level. One actuator for all four posts.

A slightly different way is to use one linear actuator, and use cables, both going up and down to the other three posts to keep the platform level. Many four post car lifts are like that.

View Jeff Mazur's profile

Jeff Mazur

69 posts in 768 days


#10 posted 12-03-2014 06:24 PM

@timbertailor I see your point – it’s probably worth a shot, they may not be doing the same sort of synchronization exactly but they’re a crew who would be familiar with the tech, thanks!

@mramseyISU Thanks, will look at their offerings, I’ve been looking at Firgelli and Progressive Automation so far.

@bigblockyeti I haven’t seen any affordable electric linear actuators that are designed as variable speed, but I’ll check again, thanks!

Thank you all for the ideas – I have, as many have suggested, considered some sort of mechanically linked transmission from a single power source, as it is a simple and elegant solution to the synchronization issue, but since this is for a loft bed in a family home any pulleys or sprockets bearing significant weight would for safety and appearance have to be enclosed. The actuators on the other hand are rather elegant and can sit happily in full view, and pose no pinching or amputation threat like a pulley or gear under load might.

It has occurred to me that for the most part the load and friction on all four corners will be pretty even and sync might not even be a problem, but that’s not guaranteed and I’d rather be robust in my assumptions. I also can’t have someone riding the bed up since it’s designed to go almost up to the ceiling, so I’m going to monitor current draw from the power source and have an adjustable threshold built into the control circuit to cut off if the current drawn indicates a person is on the bed when it’s being raised.

-- Woodworking is a beautiful, physical, cerebral, and noble art.

View timbertailor's profile

timbertailor

1592 posts in 889 days


#11 posted 12-03-2014 06:36 PM

Something like this would make it a quick and fairly inexpensive solution.

Not saying this solution exactly but you get the idea. $300 and little wood work and you are done.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

View ChrisK's profile

ChrisK

1809 posts in 2546 days


#12 posted 12-03-2014 06:45 PM

If the screw is of good quality you can gang all 4 with a chain or timing belt drive. A little tricky to get adjusted, but once done it is done. The accelerometers may be fun to play with , but will be tricky to program to get 4 drives working together. If you can use threes legs, it will a lot easier to get the top level.

-- Chris K

View Jeff Mazur's profile

Jeff Mazur

69 posts in 768 days


#13 posted 12-03-2014 07:33 PM

@timbertailor You and I think alike, this is pretty much what I would do for an adjustable workbench or assembly table. This bed needs to be open under the center platform to allow a desk or small sofa to sit there (and to be able to access them.)

@ChrisK Can’t do it, would need to enclose the drive elements, see my post above, but thanks!

-- Woodworking is a beautiful, physical, cerebral, and noble art.

View Rob's profile

Rob

704 posts in 2535 days


#14 posted 12-03-2014 07:54 PM

Another solution would be to have a single motor drive a chain, which in turn drives sprockets with ACME-threaded center holes.

-- Ask an expert or be the expert - http://woodworking.stackexchange.com

View ChrisK's profile

ChrisK

1809 posts in 2546 days


#15 posted 12-03-2014 08:51 PM

I have built systems that the screw does not rotate, the nut does. That way the rotating parts stay in place so they can be covered. Use a timing belt so every thing stays quiet and cleaner. You can then build a base which covers all of the moving bits. This is easier if you have a machine shop that can make the parts to hold the bearings, though MDF and a router may be all you need.

Another though is to use on or two screws on the back side and cantilever the bed frame off these. This will require some way to keep the moment load off the screw. I have used linear bearings on round and square shafts.

Look at what www.igus.com has to offer for some ideas. Linear actuators are pricey. Any self locking are mostly likely an acme screw. Look at www.roton.com, they have screws off all types and are price competitive.

http://www.nookindustries.com/Product/ProductCategory/2102

Poke around Nook as well.

Good Luck.

-- Chris K

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