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'[PIC] Moving Faders'
2012\05\29@131044 by Josh Koffman

face picon face
Hi all.

Second email! As generally happens when I'm about to travel, I'm
struck with all sorts of ideas I want to try out, and no time to do
them. For now this will remain a thought experiment, but I hope to do
some of the design while traveling, then build a prototype when I
return.

I've had an idea for a project that would use motorized faders. These
are linear (slide) potentiometers with a DC motor attached. I've
started reading about these, and apparently driving them can be a bit
tricky. They have two resistance tracks, originally one was used to
control your signal (audio or otherwise), and the second was used as
positional feedback for your servo control. In essence these were
linear servos.

I'd like to drive mine digitally, as I'm going to be connecting the
unit to an ADC input anyways. What's tricky is that there's a
mechanical component to the design that needs to be taken into
consideration, otherwise you stand the chance of having the fader
yo-yo around as it "homes in" on the correct value it should be at.
This is due to inertia and friction. In addition, you need to be
careful about not slamming the unit into the end stops repeatedly, or
trying to drive the fader if it's stuck (lest you burn out the motor).

I'm thinking about an H bridge to drive the motor, and software to
help with the positioning. I will also need to come up with some sort
of capacitive touch sensor, as the unit has a touch path from the
slider, and I'd like to know when it's being touched so as to disable
the motor drive.

I've never done any PID work, and I'm wondering if that's the correct
route to go for the control. I've done some searching, but haven't
come up with much for this particular application. Does anyone have
any tips, or links to other documentation? I've been sent an
application note from the North American rep for Penny and Giles, one
of the most respected fader manufacturers. I'm working through it, but
it's mainly based on analog control schemes. Still, useful stuff.

Thanks!

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2012\05\29@141352 by Denny Esterline

picon face
I've not used _linear_ motorized pots, but the rotary ones I have used were
quite slow. Inertia and friction were basically non-issues.
The other detail is they had slip clutches so the knob could be moved
without the motor, this also acted to slip when the motor "hit" the end
stop.

Yes, an H-bridge is a necessary driver. You can get these as a driver chip
(TI's DRVxxx series comes to mind) or do it with discrete transistors.

For my money, PID is complete overkill for this application. i drive lots
of resistive feedback linear actuators with full ON-OFF control and a
deadband of +-2 ADC counts, never seen a hunting problem with that.

Good luck.
-Denny


On Tue, May 29, 2012 at 10:10 AM, Josh Koffman <spam_OUTjoshybearTakeThisOuTspamgmail.com> wrote:

{Quote hidden}

>

2012\05\29@151537 by Vicent Colomar Prats

picon face
As a driver for small motors you can use mcp14e4 or similars. And a small
pic to communicate with.
And if you don't need external mechanical activation, you can even use a
simple digital potenciometer:
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=11026&mid=10&lang=en&pageId=79


El martes, 29 de mayo de 2012, Denny Esterline <.....desterlineKILLspamspam@spam@gmail.com>
escribió:
> I've not used _linear_ motorized pots, but the rotary ones I have used
were
{Quote hidden}

wrote:
{Quote hidden}

>> -

2012\05\29@200826 by IVP

face picon face
part 1 820 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

Josh,

I don't think you'll have any problem with the pot yo-yoing or
hunting. It will be very slow compared with the processing speed
of the PIC and has no significant mechanical momentum to carry
it on past a set point. The reduction gearbox means a pretty much
instant stop

Attached is what I use for a rotary pot controlled manually or
with IR. The wiper goes to an 18F, but could also go to the 12F
for feedback. It's a little slower than spec because of the voltage
drops across the transistors, but that's not too important in my
application

Might not apply to your pot, but I notice in the Penny And Giles
tech notes for the PGFM9000 series that the motor should NOT
be driven against the end stops, so you'll need to address that.
Rotary pots have a clutch

Joe

part 2 3367 bytes content-type:image/gif; name="ir_h-bridge_675.gif" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2012\05\30@124229 by Josh Koffman

face picon face
On Tue, May 29, 2012 at 2:13 PM, Denny Esterline <.....desterlineKILLspamspam.....gmail.com> wrote:
> I've not used _linear_ motorized pots, but the rotary ones I have used were
> quite slow. Inertia and friction were basically non-issues.
> The other detail is they had slip clutches so the knob could be moved
> without the motor, this also acted to slip when the motor "hit" the end
> stop.

These ones can zip along fairly quick, but from what I'm now hearing,
inertia and friction might not be as big a problem as I was initially
thinking. I think I'll try to whip up a quick circuit at some point
and just try it to see what happens.

> Yes, an H-bridge is a necessary driver. You can get these as a driver chip
> (TI's DRVxxx series comes to mind) or do it with discrete transistors.

I wasn't able to locate much that was relevant in the DRVxxx series.
Admittedly TI's website seemed to be acting a bit weird so perhaps I
missed something. Do you have any more exact reference that I can
search for on there?

> For my money, PID is complete overkill for this application. i drive lots
> of resistive feedback linear actuators with full ON-OFF control and a
> deadband of +-2 ADC counts, never seen a hunting problem with that.
>
> Good luck.
> -Denny

Thanks!

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2012\05\30@124659 by Josh Koffman

face picon face
On Tue, May 29, 2012 at 3:15 PM, Vicent Colomar Prats
<EraseMEvicentecolomarspam_OUTspamTakeThisOuTgmail.com> wrote:
> As a driver for small motors you can use mcp14e4 or similars. And a small
> pic to communicate with.

Hm, this part looks quite nice for this application. The MCP14E5 is
even a dual driver, one inverting, one non inverting. Seems that I
could tie the two inputs and two enables together, and have a two
control line solution without any external signal inverting. The only
area I'm a little hazy on is if I'm able to run the motors on a higher
voltage than the PIC this way. According to the MCP14E5 datasheet,
logic level 1 on the input is minimum 2.4V, typical 1.5V, which
doesn't make any sense. It does seem as though I could run the chip
off say 9V, and the PIC off 5V and not require level translation, but
I'm not entirely sure.

Thanks!

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2012\05\30@125125 by Josh Koffman
face picon face
On Tue, May 29, 2012 at 8:08 PM, IVP <joecolquittspamspam_OUTclear.net.nz> wrote:
> Josh,
>
> I don't think you'll have any problem with the pot yo-yoing or
> hunting. It will be very slow compared with the processing speed
> of the PIC and has no significant mechanical momentum to carry
> it on past a set point. The reduction gearbox means a pretty much
> instant stop

Sadly these guys don't have gearboxes, at least, I'm pretty sure they
don't. They can move rather rapidly, but I could just limit the speed
all the time rather than feeding it full juice at every go. We'll see.

> Attached is what I use for a rotary pot controlled manually or
> with IR. The wiper goes to an 18F, but could also go to the 12F
> for feedback. It's a little slower than spec because of the voltage
> drops across the transistors, but that's not too important in my
> application

Thank you!

> Might not apply to your pot, but I notice in the Penny And Giles
> tech notes for the PGFM9000 series that the motor should NOT
> be driven against the end stops, so you'll need to address that.
> Rotary pots have a clutch

Yep, and while I'm a bit worried about that, I think I should be ok.
The servo track of the linear pot means that I'm always getting
feedback, so unless something gets stuck on, I shouldn't be in a state
of trying to drive it too far. One of the application notes also
suggests having a timeout on the action equal to double the time to
travel the total length. This means that if the fader is stuck, you
aren't trying to dump current in the motor indefinitely. And coupled
with the feedback by the linear pot, it means that errors can be
recognized by the PIC and indicated with an LED or something.

Thanks!

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2012\05\30@132804 by Vicent Colomar Prats

picon face
I tried it with a two cell LiPo (8,4V at max charge) directly for the motor
and 5V for dspic, and it works fine. So, I think it can also do the job
with 9V, but look for absolute max. in datasheet, I do not remember what
the value it was.

2012/5/30 Josh Koffman <@spam@joshybearKILLspamspamgmail.com>

{Quote hidden}

>

2012\05\30@134924 by Denny Esterline

picon face
www.mouser.com/tidrv884x/?cm_sp=homepage-_-newproducts-_-Texas+Instruments+DRV8840+DRV8842+DRV8843+Motor+Driver+ICs

Was where I first seen them.
Note, I've not used them, just thought they were interesting little
beasties.

I used to use the TI TPIC107b parts, but TI obsoleted them and caused me
much grief. Now unless there's a compelling reason not to, I try to design
with discrete transistors/fets instead - many, many more second source
options.

I think Allegro micro has some suitable H-bridge chips in your current /
voltage range as well.
-Denny



On Wed, May 30, 2012 at 9:42 AM, Josh Koffman <RemoveMEjoshybearTakeThisOuTspamgmail.com> wrote:

{Quote hidden}

>

2012\05\31@195907 by IVP

face picon face
> Sadly these guys don't have gearboxes, at least, I'm pretty sure
> they don't. They can move rather rapidly, but I could just limit
> the speed all the time rather than feeding it full juice at every go

On audio boards for example they'll snap to memorised positions
pretty quickly, as you say. If you don't want them to move that
fast then you can run the motor at a lower voltage and/or PWM

If PWM, you'd have the option of full speed until within 'braking
distance' of the destination, rather like driving a car. And braking
could mean either coast to a stop or applying oppositie drive, ie
slam it in revers

2012\05\31@202416 by Lyle Hazelwood

picon face
Admittedly off-topic, but not by too much:
Audio line mixer, signal path is all analog, the audio is never digitised.
Controls are all digital (MIDI).
There are NO POTS, NO SLIDERS, no chance of getting "scratchy" with age.

And regarding how fast they change level, these can "jump" directly to the
setting without traversing the distance in between.

This uses Burr-Brown audio chips, and an 18F452 (I think) to manange the
controls.

Sounds great, I'm listening through it right now.

http://www.midibox.org/dokuwiki/doku.php?id=midiboxmixer

LyleHaze

On 6/1/12, IVP <TakeThisOuTjoecolquittEraseMEspamspam_OUTclear.net.nz> wrote:
{Quote hidden}

>


'[PIC] Moving Faders'
2012\06\06@101350 by Josh Koffman
face picon face
By the way, if anyone has seen conductive plastic or metal fader caps
for sale somewhere, I'd appreciate a steer. I have found straight
plastic ones at Mouser, but I'm looking for a supplier of ones that
will allow me to sense a finger on the fader. I know I can order them
as spare parts from audio console manufacturers, but I'd prefer to
find a farther upstream source.

As for the status of my project, I've thrown together a quick demo
board (well, an add on to an existing board), but I won't get to try
it out for at least a month. Ah well!

Thanks for the help!

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2012\06\06@101448 by Josh Koffman

face picon face
On Thu, May 31, 2012 at 8:24 PM, Lyle Hazelwood <RemoveMElylehazespamTakeThisOuTgmail.com> wrote:
> Admittedly off-topic, but not by too much:
> Audio line mixer, signal path is all analog, the audio is never digitised..
> Controls are all digital (MIDI).
> There are NO POTS, NO SLIDERS, no chance of getting "scratchy" with age.
>
> And regarding how fast they change level, these can "jump" directly to the
> setting without traversing the distance in between.

Looks cool Lyle! Unfortunately my project has nothing to do with
audio, it's more about using the faders as a control interface.

Thanks!

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2012\06\06@140919 by Josh Koffman

face picon face
On Wednesday, June 6, 2012, Josh Koffman wrote:

> By the way, if anyone has seen conductive plastic or metal fader caps
> for sale somewhere, I'd appreciate a steer. I have found straight
> plastic ones at Mouser, but I'm looking for a supplier of ones that
> will allow me to sense a finger on the fader. I know I can order them
> as spare parts from audio console manufacturers, but I'd prefer to
> find a farther upstream source.
>

Come to think of it, if anyone has any examples of using a PIC to detect
touch in this way, I'd love to see it. I was looking at cap sense but I
don't think it's quite the right solution. I believe there's an Arduino
library for doing touch sense that doesn't require too many external
components, ideally I'd be looking for something similar. I have found one
at Bytecraft (http://www.bytecraft.com/Touch_Sensitive_Switch) but I'm
always on the lookout for more!

Thanks!

Josh


-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adam

2012\06\07@123021 by Jonathan Hallameyer

picon face
> If PWM, you'd have the option of full speed until within 'braking
> distance' of the destination, rather like driving a car. And braking
> could mean either coast to a stop or applying oppositie drive, ie
> slam it in reverse
>
>
>

Braking could also mean braking the motor by grounding (or connecting to
Vcc for that matter) both leads via the H bridge, which would help eat up
any inertial energy, without needing to be precise, like reversing it to a
stop.

You could also use this to  damp/brake the sliders when they are manually
moved (I.E. it could be free moving, or damped/braked).  For a soundboard
application, you could possibly brake the slider if the channel is
clipping, etc. The user could still move it, but would have to do so
intentionally, just as an example.

-Jon

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