How to build an electronically-height-adjustable desk?

[Dan Friedrichs]

I realized I spend way too much time sitting, so when I'm working at my desk at home (at least), I'd like to stand. That said, I'm not sure I want to commit to a standing desk, just yet, as I might want to alternate between standing/sitting/etc. So I'm building an electronically-height-adjustable desk. I bought a couple of linear actuators (18" stroke, 600lb rating), and built up the attached (still missing a few pieces - like the foot rail, etc)


A few thoughts:
1) I wish I had gone with a simple mechanical height adjustment, rather than electronic. A few acme screws with a chain linkage and hand crank would have worked much better.

2) I wish I had gone with 4 legs, for stability.



A few problems I have left to solve:
1) If you just apply power to the linear actuators, they don't necessarily raise at exactly the same rate (due to different friction in each leg, etc), so the desk can become un-level. How shall I fix this? At this point, my thought is to incorporate some sort of circuit that will continue to apply power to just one side (once the user up/down button is released), until the desk has re-leveled. I'm assuming I can buy some sort of level-sensing integrated circuit (or, what I'm really thinking of is a mercury switch...). Any other ideas? There are hall sensors in the linear actuators...

2) When at full height, the "outer" portion of the leg overlaps the "inner" portion of legs by about 3". This is apparently not enough to keep the desk from rocking front-to-back. I'm thinking I'll put some UHMW tape on the inside of the "outer" leg to tighten up the fit between the pieces. Would this be sufficient? Or should I put in some nylon screws so I can adjust it as wear occurs?

[Dan Hintz]

Our desks at work raise up and down... it's like $3k for the things, and everyone gets one. It's a nice option to have when you're trying to get it to the perfect position, but once there, it became little more than a novelty. I never bothered to look at how it levels itself, but I'll check when I get back into work on Monday... it may be one motor in the central bar and screw leads going to the sides, but I'll check.

[Bill Huber]

To start I am not a digital person just old analog stuff and this is just food for thought to get the brain going. I am sure some of the guys on here can come up with a digital way to do the same thing or much better but here goes.

Mercury switch, double pole.
2 relays
Main switch.
The mercury switch would control the relays and though normally closed contacts would control the motors. As the table comes up and gets out of level the mercury switch would active one of the relays and open the contact which would stop that motor until it was level again.

Again, this is just food for thought.

[Dan Friedrichs]

Wow - thanks for drawing that out, Bill! That is exactly what I was thinking (well, I had an idea that something like that would work, but hadn't sat down and figured out the connections). I had thought about a PID controller tied to the hall effect position sensors in each device, but that gets complicated in a hurry :) I think the DP mercury switch is a fantastically simple idea.

Dan -yeah, they're expensive. Even the base (without a top) is $1k. I bought the linear actuators for $45/ea :) I'm curious how the commercial ones work. In pictures I've seen, I've never noticed any chain linkages between the sides, etc.

[Kevin Groenke]

Hey Dan,

I designed a similar desk (http://www.design.umn.edu/current_students/leo/hall/UMod.html) for our students. Ours have telescoping pedestals on each side of formed sheet metal with UHMW tape to take up the slack and keep things moving over time, the manufacturer aims for .015" tolerance on the formed parts. Everything is laser cut, CNC formed and robotically welded. My first prototype had telescoping square tubes similar to yours, we found that movement of the top was minimized by increasing the depth of the pedestals, there is also a bracket that slides on the rear riser to minimize rocking. We use SUSPA MOVOTEC (http://www.suspamovotec.com/) lift components in ours which are basically a closed hydraulic system: we use integrated pistons, but they also make bolt-on's (and pedestals made of telescoping aluminum extrusions (http://www.suspa.com/us/products/table-lift-systems/hydraulisch-movotec/atu-system/)). The lift system can be either electro-mechanically or manually actuated. Since each side or leg is a closed system, the lifting is consistent, I think this will be much tougher to achieve with linear actuators. To my knowledge we have not had any issues with the tape wearing out over time, BUT, the last batch had a different powder-coat with significantly different coefficient of friction which really screwed things up. There are now over 1000 of these things in service in a number of schools, there have been a few failures of lift system components, but most have been assembly issues or user error.


In the quantities that these have been produced (20-300) the lift system parts cost $200-300. Depending upon quantity and configuration (tables vs drawers, # of shelves, etc.) complete units have been $900 - $1600. All orders to date have been directly from the manufacturer with whom I work closely re:design and iterative improvements, obviously the cost would be significantly higher if available from stock via wholesaler/retailer (and I would be closer to retirement).

Ergotron uses constant force springs in it's adjustable height tables and workstations, so there is another means to the end.

-kg



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[Dan Hintz]

That mercury switch would have to be pretty angle sensitive... I don't know what accuracy the sensor market currently offers, but anything less than a few degrees might be easily noticeable to the person sitting at the desk.

[Bill Huber]

Well you could always us a pendulum type leveler and you could make it as sensitive as you want.

[Dan Friedrichs]

Looking into this further, I'm not sure it's a great idea. I think Kevin's design is fantastic, but I think he also pointed out a key consideration: the telescoping pieces have to be very tightly machined to eliminate sway. I don't think I can get (and keep) the wooden pieces that closely matched.

I'm reconsidering just buying a base and building my own top. For $500, this looks like a much simpler solution...
http://www.thehumansolution.com/uplift-hover-adjustable-height-desk-446-base.html

[Bill Huber]

Looking into this further, I'm not sure it's a great idea. I think Kevin's design is fantastic, but I think he also pointed out a key consideration: the telescoping pieces have to be very tightly machined to eliminate sway. I don't think I can get (and keep) the wooden pieces that closely matched.

I'm reconsidering just buying a base and building my own top. For $500, this looks like a much simpler solution...
http://www.thehumansolution.com/uplift-hover-adjustable-height-desk-446-base.html

Dan, you may want to hit some used office furniture places, I know there are a lot of them in the Dallas and you may be able to find one you could redo or use the parts to make it the way you want.

[Dan Friedrichs]

Just to follow-up on this thread, in case anyone in the future is curious: I gave up on the DIY version. The tolerances required to get the "wiggle" down to acceptable levels are just too tight to be accomplished in wood. I ended up buying a manually-adjustable table base (from MultiTable), which is fantastically rigid and widely adjustable (I'm building my own top, at least!). For only $500-ish, I don't think there's any way I could have built one for less materials cost.

[Tom Jones III]

Its a little late now, but if anyone is interested you really need to see the GeekDesk http://www.geekdesk.com/default.asp?contentID=634 They sell it as frame only for around $550.

[Alan Schaffter]

Dan, Also a little late, but you could have motorized a lighter version of the legs I designed for my adjustable height woodworking bench (http://www.sawmillcreek.org/showthread.php?120032-New-Woodworking-Bench-%28Parts-I-amp-II-of-II%29) or just left it manually operated. Once the height has been adjusted the weight of the table causes the mechanism to lock the legs together very tightly (V-shaped mating surfaces eliminate slop). The more weight the tighter- no wobble in my bench because of the thick maple top. It may look complex but was simple and inexpensive to make. The ratchets were made on the tablesaw with the simplest of indexing jigs- a 1 X 4 with a wooden peg attached to my miter gauge.

http://www.ncwoodworker.net/pp/data/1463/medium/IMG_2829.jpg

[Dan Friedrichs]

Its a little late now, but if anyone is interested you really need to see the GeekDesk http://www.geekdesk.com/default.asp?contentID=634 They sell it as frame only for around $550.

I looked at that - very cool. I ended up with this one, which is about $480 (a little cheaper)
http://heightadjustableworktable.com/Index.php/products/modtable-manual/modtable-manual-black.html