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

Wed Jan 8 15:27:29 CET 2025

The width of the paper is around 12inches.
And because of the VERY FRAGILE state of many of these rolls
I could'nt consider the pinch roller way to restrict the paper pace
It would rip the paper apart..
So far the only way I could consider is to 'gently' press a rubber wheel
on the advancing paper and read the optical pulses to re-adjust the pace.

But.. the pinch roller idea would have been an excellent one
if using an other thing that a stepper motor for speed.

> -----Message d'origine-----
> De : rburnett at richieburnett.co.uk <rburnett at richieburnett.co.uk>
> Envoyé : 7 janvier 2025 17:57
> À : Jean-Pierre Desrochers <jpdesroc at oricom.ca>
> Cc : 'Spiros Makris' <spirosmakris92 at gmail.com>; synth-diy at synth-diy.org
> Objet : Re: [sdiy] Looking for a mini rotary encoder with very low torque (servo
> type)
> 
> JP would it be practical to use a capstan and pinch-roller arrangement to keep
> the velocity of the paper constant, despite the increase in the effective
> diameter of the take-up roller? Like how tape was driven through an old reel-
> to-reel tape machine.
> 
> Apologies if someone already suggested this and I missed it.
> 
> -Richie.
> 
> 
> On 2025-01-07 21:19, Jean-Pierre Desrochers via Synth-diy wrote:
> > Hi Spiros,
> >
> > Thanks for your kind suggestions..
> > But let me point out some lines heres :
> >
> > _>  is that to achieve an 1:1 translation of linear movement of paper
> > to rotation of encoder you need a way to ensure that the two are
> > always in optimal contact, which basically translates to keeping the
> > tension > controlled within some window, so that the paper presses on
> > it properly at all times. If that was the case you wouldn't need this
> > method at all,_ …
> >> since a rotation of your motor would already translate directly > to
> > linear motion of the paper,
> >
> > That’s true but the acumulating thickness of the take up spool
> > accelerates the paper pace..
> > So checking the motor speed to adjust the pape has never been the
> > solution.
> >
> >> As a final thought, you mentioned that you don't care about encoder
> > resolution, but my experience with controlling slow moving motors is
> > that as speed goes down,
> >> worse. The reason is that you produce "corrections" in the form of
> > pulses as the shaft turns;
> >> keeping the resolution constant and reducing the speed means you get
> > to wait more time between those corrections,
> >> essentially creating temporal "gaps" in your control loop.
> >
> > I think youd didn’t notice I’m using step motors so adjusting the
> > steps timing is much precise than do ON/OFF stuff on DC motors which
> > I’m nort using here.
> >
> > _>Doing a search in aliexpress with the keywords "motor encoder" could
> > yield some results worth trying out._
> >
> > I don’t want to check the motor shaft speed using an internal
> > encoder..
> > It’s the paper pace I need to have an eye on.
> >
> > _> Quite the head scratcher you have here. As you can probably tell I
> > love these sorts of control puzzles!_
> >
> > That’s the fun of it !!
> >
> > De : Spiros Makris <spirosmakris92 at gmail.com> Envoyé : 7 janvier 2025
> > 05:53 À : Jean-Pierre Desrochers <jpdesroc at oricom.ca> Cc :
> > synth-diy at synth-diy.org Objet : Re: [sdiy] Looking for a mini rotary
> > encoder with very low torque (servo type)
> >
> > yes, I did after posting my message.
> >
> > It does seem fiddly, but if it works it works, I guess. 😅
> >
> > As is common in robotics, the exact thing you need to know and the
> > method you observe it can be a very convoluted topic. My understanding
> > of the problem is that you need the paper to move with a constant pace
> > under some sort of reader (I vaguely remember your past emails about
> > how to detect the hole patterns on the piano roll). In theory,
> > directly measuring that rotation by making the paper rotate the
> > encoder as it moves would suffice. What I'm thinking (and maybe
> > playing devil's advocate here), is that to achieve an 1:1 translation
> > of linear movement of paper to rotation of encoder you need a way to
> > ensure that the two are always in optimal contact, which basically
> > translates to keeping the tension controlled within some window, so
> > that the paper presses on it properly at all times. If that was the
> > case you wouldn't need this method at all, since a rotation of your
> > motor would already translate directly to linear motion of the paper,
> > not to mention that dodgy sensor measurements will definitely manifest
> > as weirdness in your control behaviour. As an example, think what
> > would a PID do if suddenly the measured speed was dropping to zero
> > because of slippage; it would accelerate thinking it needs to
> > compensate, increase the tension again and produce measurements, try
> > to slow down, then possibly lose tension again etc and enter a kind of
> > oscillation that might be hard to tame.
> >
> > Using a second passive wheel to sandwich the paper and force it to be
> > in good contact with your encoder wheel at all times could be
> > something to try out if you stumble on that problem.
> >
> > As a final thought, you mentioned that you don't care about encoder
> > resolution, but my experience with controlling slow moving motors is
> > that as speed goes down, the speed measurements become worse. The
> > reason is that you produce "corrections" in the form of pulses as the
> > shaft turns; keeping the resolution constant and reducing the speed
> > means you get to wait more time between those corrections, essentially
> > creating temporal "gaps" in your control loop. To solve it you will be
> > forced to increase the integration time (which converts position to
> > speed), which in turn will make your control loop slower. If this
> > speed is not enough orders of magnitude above the process you are
> > trying to control, you are in for some fun times trying to tune this
> > (more or less) untunable loop. In practice, spending more money for a
> > much more accurate encoder could solve this, but those buggers can get
> > pretty expensive when you go into the 100s of pulses per revolution
> > region. Doing a search in aliexpress with the keywords "motor encoder"
> > could yield some results worth trying out. Using gears to multiply the
> > revolutions measured by the sensors would be a cheap way out, but this
> > will proportionally increase the torque required to turn the wheel,
> > possibly making the paper slip.
> >
> > If this approach fails, it could be worth investigating ways to
> > control the tension of the paper, rather than its speed.
> >
> > Quite the head scratcher you have here. As you can probably tell I
> > love these sorts of control puzzles!
> >
> > On Mon, 6 Jan 2025 at 17:26, Jean-Pierre Desrochers
> > <jpdesroc at oricom.ca> wrote:
> >
> >> Thank You Spiros for your nice message.
> >>
> >> Did you see my last post about the final setup I will use ?
> >>
> >> The small wheel with tire is making a good steady contact with the
> >> scrolling paper I want to measure the speed of..
> >>
> >> So far so good.
> >>
> >> De : Spiros Makris <spirosmakris92 at gmail.com> Envoyé : 6 janvier 2025
> >> 07:29 À : Jean-Pierre Desrochers <jpdesroc at oricom.ca> Cc : Gerry
> >> Murray <synthimuse at gmail.com>; synth-diy at synth-diy.org Objet : Re:
> >> [sdiy] Looking for a mini rotary encoder with very low torque (servo
> >> type)
> >>
> >> 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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