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

Tue Jan 7 23:57:03 CET 2025

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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