Fast motorised potentiometers, or alternative design ideas - Page 1
Forum section: electronics / projects

author: huub8
author_rank: Contributor
timestamp: September 05, 2018, 02:20:12 pm
content: Hi everybody,I'm currently working on a project (it's a research project on a comparison between tangible interaction and touchscreens, that includes a big design aspect), for which I can't really find a particular type of part.I would like to have the lever shown below to be automated. This lever will be one of many and will protrude from a cover plate (so they can move almost 180 degrees). Its used to control part of the lights in a room and shows its effect on the lights through its own light as well. Right now this lever is only connected to a rotary potentiometer trough some gears. It should both be able to be actuated by the user and by my system.The most obvious way to automate this would be to simply replace the potentiometer with a motorized version. The problem is, however, that I cannot find any fast motorized rotary potentiometers. I need something that can turn 180 degrees in no more than a few seconds. The ones I found, like the ones sold by alps, take 12 seconds to rotate 300 degrees; way to slow.An alternative would be to use sliding potentiometers and translate the sliding movement through a rack and pinion, but this doesn't seem very elegant (and would be rather big). I could also use expensive servo's that can easily be backdriven, but because of the quantity I need this would become too expensive.Does any of you know of faster rotary potentiometers, or can you think of an alternative way of doing this?P.S. I do have acces to 3d printers and laser cutters if needed.

author: Benta
author_rank: Super Contributor
timestamp: September 05, 2018, 02:23:38 pm
content: Buy an RC servo and take it apart (there's a potentiometer inside). You won't find an easier or cheaper solution.

author: edavid
author_rank: Super Contributor
timestamp: September 05, 2018, 02:59:32 pm
content: Since it's just for lighting, you don't need super high resolution. Use just a portion of the ALPS pots motion, which will speed up the transit time. You can also increase the motor voltage to speed them up.

author: ajb
author_rank: Super Contributor
timestamp: September 05, 2018, 03:06:26 pm
content: Standard RC servos are not easily backdriven, and it would take kind of a messy hack to make them work in a situation where you want to be able to manually drive the mechanism as an input device and at other times use the servo to drive the mechanism. You also have to consider the situation where the user and the controller are trying to move the lever at the same time. Most motorized pots are designed with fairly weak motors just for this reason.I think there are some digital hobby servos that allow you freewheel the motor and read the position feedback, but I'm not sure how much they cost or, again, how well they'd contend against a human who's also trying to move the lever. I'd be wary of stripping gears or burning out the servo.I would look into what things like digital audio desks use, they have lots of motorized linear pots. I'm not sure if there are dedicated ICs available for the purpose, but the other option is building something up from scratch, using a low power (cassette deck style?) motor, and a small MCU doing PID control based on the feedback from the pot, with an input to switch between freewheeling and driven modes.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2018, 04:40:13 pm
content: If you are up to it, you could use a linear motor or voice coil actuator.

author: m3vuv
author_rank: Super Contributor
timestamp: September 05, 2018, 11:43:28 pm
content: i dont know what you would call it,but could you use a toothed rack with a solenoid coil around it and have that drive a gear on the spindle/shaft of a normal pot?,cheers m3vuv.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 06, 2018, 01:27:16 am
content: Quote from: ajb on September 05, 2018, 03:06:26 pmExactly the correct answer. This problem has been addressed and solved for decades. Unless you must have a different approach, why not benefit from all of that experience?

author: RobK_NL
author_rank: Frequent Contributor
timestamp: September 06, 2018, 11:05:12 am
content: Quote from: IDEngineer on September 06, 2018, 01:27:16 amThe OP is aware of the availability of commercial motor driven potsHis problem is their (lack of) speed. Alps used to make a very fast one; 300° in 1 second, but it seems that part is no longer made.

author: huub8
author_rank: Contributor
timestamp: September 06, 2018, 12:29:32 pm
content: Thanks for the reactions!I am indeed aware of the existence of motorized potentiometers, but as stated I can't find any fast ones. I also came across that one fast version alps used to make, but I too can't find it for sale anywhere.I can indeed just use part of the range of the alps motorized potentiometer, and increase the voltage a bit. I would however rather not reduce the used potentiometer range to less than about half, because fine adjustments do need to be made and any slack between gears would become more noticeable then. Using half would mean about 6 seconds, increasing the voltage can bring that to about 4 seconds without frying the motor in the long run. Sadly this still isn't fast enough.@David, How would you use a voice coil for rotary movement? And how do u suggest using a linear motor?@m3vuv, Could you elaborate, I don't really understand what you mean.Internally the alps have a small gearbox of sorts, ending in a worm wheel driving a friction clutch (making up the last gear too) connected to the shaft, with the potentiometer around it. So you don't actually turn the motor at all when operating it manually. I could make a similar system myself, but I can't find any small friction clutch parts for sale.

author: CM800
author_rank: Frequent Contributor
timestamp: September 06, 2018, 12:53:51 pm
content: depending on the force and angle needed, rotary voice coil actuators could be a good idea indeed.Either buy them, or DIY them. You can also get them out of HDDs.

author: ajb
author_rank: Super Contributor
timestamp: September 06, 2018, 01:08:19 pm
content: Quote from: RobK_NL on September 06, 2018, 11:05:12 amThe motorized linear pots I've used (on digital audio and lighting desks) are plenty fast. I don't know what make or model they are, but they'll move 75-100mm about as fast as a human reasonably can. I wasn't necessarily thinking that the OP would want to use them directly, since the mechanical coupling would be tricky, I don't know if they can provide the required force, and they might be too expensive ($10-20+/ea without any driving electronics).I was actually suggesting that if the OP wanted to build their own motorized pots that digital mixing desks would be a good thing to look into in terms of how the motorized pots are controlled.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 06, 2018, 01:47:09 pm
content: Quote from: ajb on September 06, 2018, 01:08:19 pmThat's what I meant too. I wish some folks didn't always dogpile when they misinterpret people's attempts to help.Someone mentioned RC servos, and worrying about backdriving them. Must the mechanically active element maintain registration with the position? Those RC servos can be difficult to move if powered, but most can be moved easily if unpowered. Is it acceptable in this application to only power them when autonomous movement is desired? Can you leave the existing pot in place for measurement, and just add a selectively powered rotary servo? My unknown here is whether you would ever want to manually reposition things at the same time as the system would be trying to do it too... I don't know the usage case, or indeed how your system is supposed to handle that scenario. But a selectively powered RC servo would be pretty inexpensive and would retrofit directly into what you already have.

author: EugenioN
author_rank: Contributor
timestamp: September 06, 2018, 02:01:30 pm
content: Quote from: huub8 on September 05, 2018, 02:20:12 pmQuite fast:https://www.bourns.com/docs/Product-Datasheets/psm.pdfTravel speed is 20mm/100ms!Almost every professional motorized sliding pot has a stunning speed (and force, and precision).Maybe it'll be more elegant too: no gears and no shafts, just the (fancy) lever acting *directly* to the sliding pot; adjust overall angle by calculating the arm length after the pivot point.