[sdiy] Looking for a mini rotary encoder with very low torque (servo type)

Mon Jan 6 13:28:52 CET 2025

Did you have a look at encoders built for motor control (and robotics in
general)? They can in many flavors (and prices), and are usually mount on
the shaft of the motor; in your case you might need to devise a different
mounting method. Using rubber bands is probably going to slip, but if space
allows you could use a belt, the way 3D printers do - if properly designed
for the application it will not slip.
In my experience, trying to make your own using hall sensors and magnets
will be hard to nail down, unless you have a 3D printer (or other CNC type
machine) that will take care of tolerances for you. If your goal is to keep
speed constant, or measure amount of rotation, this can have a very big
impact on your application.
Finally, you can get rotary encoders aimed for UI/controls that have a
smooth rotation. They specify the torque needed to turn them, so you can
pick the lowest. If what you measure is actively moved by a motor then it
will definitely have no problem turning it, and will be unlikely to slip,
as long as it is mated to the rotating axis properly.
You might also consider tensioning your moving paper in a different way, so
that the slip will become negligible.

On Thu, 12 Dec 2024 at 18:55, Jean-Pierre Desrochers via Synth-diy <
synth-diy at synth-diy.org> wrote:

> *> Is it more important to measure the surface of the paper itself due to
> torque/slippage reasons like on a tape machine, *
>
> Yes.
>
> *> or will measuring the speed of the take up spool get you the same info?*
>
> No.
>
>
>
>
>
> *>  If the latter is an option, what about a magnet and a hall sensor on
> the take up spool, > disk the same diameter as the take up reel in the
> drive mechanism with some other sort of sensor on it like cheater
> mentioned, > or even a small notch in the take up spool (or equivalent disk
> in the drive mechanism) that has a limit switch with roller against it.*
> Measuring the take up spool rotation speed will hide the actual problem I
> have
> which is paper thickness accumulation on the take up spool (with fixed
> speed)
> *that speeds up the overall moving paper speed !*
>
> So far my software only put 25 fixed parameters (by means of a
> potentiometer) to control
> the PLAY stepup motor steps and would not check for any speedup in a song.
> That’s what I discovered some days ago and am trying to fix.
>
>
>
> The solution is to read the paper moving surface (regardless of the take
> up spool speed )
> But in a very small space to do it.
> So no place for any encoder disc (it would need to be smaller than 18mm in
> diameter.. see why below),
> nor any ‘normal’ size Opto-Interrupter..
>
> Because of the very small and limited space I could mount this thing,
>
> I will use a model car wheel with rubber tire (overall outside diameter
> 18mm),
>
> mounted on a small 1/16in. shaft that will turn a small glued piece of
> opaque plastic
> in the slot of a very small SMD opto-interupter like this one :
>
>
>
> The below picture shows the actual tiny wheel with rubber tire I will use
> (to put against the moving paper)
> and part of the setup I used in a past project with a small rotating piece
> of plastic
> moving inside a Opto-Interrupter.
> Mine will be much smaller.
>
>
>
>
>
> I’m now waiting my Digikey order for the Opto-Interrupters + nylon
> hardware.
>
>
>
> And thanks Gerry for your nice suggestions !
>
>
>
>
>
> *De :* Synth-diy <synth-diy-bounces at synth-diy.org> *De la part de* Nathan
> Trites
> *Envoyé :* 12 décembre 2024 10:17
> *À :* Gerry Murray <synthimuse at gmail.com>
> *Cc :* synth-diy at synth-diy.org
> *Objet :* Re: [sdiy] Looking for a mini rotary encoder with very low
> torque (servo type)
>
>
>
> This is the direction I was just thinking, and was also on the Adafruit
> site looking at plug and play sensor options.
>
>
>
> Jean-Pierre - I'm assuming this is to make the speed on your piano roll
> player programmable/repeatable? Is it more important to measure the surface
> of the paper itself due to torque/slippage reasons like on a tape machine,
> or will measuring the speed of the take up spool get you the same info? If
> the latter is an option, what about a magnet and a hall sensor on the take
> up spool, disk the same diameter as the take up reel in the drive mechanism
> with some other sort of sensor on it like cheater mentioned, or even a
> small notch in the take up spool (or equivalent disk in the drive
> mechanism) that has a limit switch with roller against it.
>
>
>
> If it must be against the paper, opto-interruptors and disks are available
> on their own, and then you could rig something up so that assembly rests
> lightly against the paper:
>
> https://bc-robotics.com/shop/opto-interrupter-with-mounting-tabs/
>
> https://bc-robotics.com/shop/encoder-disc/
>
>
>
> Nathan
>
>
>
> On Thu, Dec 12, 2024 at 9:22 AM Gerry Murray via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
> Hi JP
>
> You could maybe try a resolver. The resolution of them can be very high
> : up to 16 bit.
> https://www.mouser.co.uk/applications/resolver-encoder-motor-control/
> https://www.instructables.com/Arduino-Resolver-Module/
>
> I always like it when you put an inquiry on the list because, usually 6
> months later on, a really cool project emerges. :-)
>
> Best wishes
>
> Gerry
>
>
> On 11/12/2024 15:09, Jean-Pierre Desrochers via Synth-diy wrote:
> > I'm looking for a very small (20mm or less body diameter)
> > rotary encoder with very low torque (servo type).
> > The low torque shaft is needed because it will turn by means
> > of a small wheel with rubber sides that will be put against a moving
> surface, turn and 'follow'
> > the moving surface.. no slipping allowed !
> >
> > The precision of steps/turn is not important.
> > But the higher will be best..
> >
> > I was thinking of using a dismantled PC mouse (rubber ball type)
> > and use one of the two  X/Y optical rotary encoders, but these PC mice
> > nowadays are hard to find.. So.
> >
> > I checked Digikey and Mouser but they have encoder too big
> > for my uses..
> > The smaller I found has 20mm in diameter..
> >
> https://www.digikey.ca/en/products/detail/nidec-components-corporation/RES20D-50-201-1/6469509
> >
> > Any suggestions ?
> >
> >
> > ________________________________________________________
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>
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