DC Servo Amplifier w/ step-dirn control and encoder feedback

[Chief Geek]

Dear Reprap'ers

My plan is to use quiet, energy efficient DC motors for my repstrap-
EMC2 driven gantry mill. I'm not a big fan of steppers as they are
noisy, energy inefficient, and can't tell me when they've stalled.

So.... I've put together an arduino based servo amplifier that takes
step and direction inputs and uses encoder feedback.

Right now it is working nicely on a breadboard driving a Mabucci 540
sized motor from an ink jet printer. The circuit currently can drive
a 12V, 20A motor, but it also works with OSMC which can drive
fantastically large motors.

Currently position feedback is a hand cut encoder wheel mounted on the
lead screw and a pair of optical slot sensors. I'd like more
resolution, preferably directly reading from the moving carriage.

Does anyone know where I can get the following:
- lengths of magnetic encoder scale for use with the Austria Micro
hall effect encoder chips
- plastic optical encoder scales and sensors like the style typically
used in ink jet printers

Chief Geek

[Peter Olson]

>Does anyone know where I can get the following:
>- lengths of magnetic encoder scale for use with the Austria Micro
>hall effect encoder chips
>- plastic optical encoder scales and sensors like the style typically
>used in ink jet printers

You could try this:

http://www.goldmine-elec-products.com/prodinfo.asp?number=G15602

13 inch strip with lines at approximately 150/inch

peter

[Chief Geek]

Thanks. That is the kind of optical encoder scale I was thinking of,
but it's tough without the sensors designed for it. Do you know if
there are industry standards for such things?

[Peter Olson]

>Thanks. That is the kind of optical encoder scale I was thinking of,
>but it's tough without the sensors designed for it. Do you know if
>there are industry standards for such things?

I don't know, but you might google to find some of the optical mouse hacks
people have done. An optical mouse can probably resolve the lines on that
strip.

peter

[Gregg Vallan]

You might want to take a look at this project: openservo.com  it's an open source servo motor design. It might be able to scale the way you want.

Gregg

On Sat, Jan 9, 2010 at 1:43 PM, Peter Olson <pe...@peabo.com> wrote:

Thanks.  That is the kind of optical encoder scale I was thinking of,
but it's tough without the sensors designed for it.  Do you know if
there are industry standards for such things?

I don't know, but you might google to find some of the optical mouse hacks
people have done.  An optical mouse can probably resolve the lines on that
strip.

peter

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[Larry]

@Peter,
Thanks for posting that link. Those look handy.

@Chief Geek,
I don't know of any sensors made to work with that strip, but I'll bet
one can be made without too all much trouble. For resolving fine-
pitch, you'll need a lens (short focal length preferred) and a
contrast mask. For the latter, I suggest you buy an extra strip to
cut up. (If you can't find info on encoders/contrast plates/masks
online, let me know and I'll sketch something out.)

BTW, if you're going to order some of those strips, maybe we can
organize a group buy and split the shipping. (Or trade for other
parts.) I'd be interested in at least four of those, probably
more.

If you have photos of your machine (or subassemblies, parts, etc.) I
encourage you to post them here, or post a link.

Best Regards,

Larry

[Chief Geek]

Actually, I've been collecting old ball-style computer mice for just
this occasion. Does anyone have any experience interfacing to the
optos on them or read any good web pages?

Thanks for the link to open servo. This looks like hobby servo type
stuff rather than proper motion control servos, but I can steal good
ideas from anywhere, I'm not proud.

My preference is to read the position of the carriage directly with a
linear encoder, but I haven't been able to find any packaged sets and
making one from scratch with the 0.001 inch resolution I'd like sounds
hard. For now I'll just have to trust that hardware store 1/4-20
threaded rod is a reasonably accurate 0.050"/rev.

Austria Micro makes some cool hall effect sensor chips that read a
magnetic strip. I thought I read somewhere that one of the for profit
reprap parts companies was trying them, but I can't remember where.
According to the chip manufacturer, making magnetic scales for these
chips would be dead easy in mass production. Someone, somewhere must
have access to samples or development tools for these.

As soon as the camera is charged, I'll take some pictures of my servo
amp.

Chief Geek

[Thomas Charron]

On Sun, Jan 10, 2010 at 7:11 PM, Chief Geek <pma...@gmail.com> wrote:
> Actually, I've been collecting old ball-style computer mice for just
> this occasion.  Does anyone have any experience interfacing to the
> optos on them or read any good web pages?

Are you talking about the old fashion slot encoders? I did a quick
google search, as I'd messed with them for play in the past, managed
to find

http://hackaday.com/2008/05/16/how-to-scavenge-a-mouse-for-parts/

I know there are many other sites, but most of them are just 5V
sensors, and should be pretty easy to use, esp if you can manage to
use then with quadrature inputs on a board.

> Thanks for the link to open servo.  This looks like hobby servo type
> stuff rather than proper motion control servos, but I can steal good
> ideas from anywhere, I'm not proud.

I haven't seen the original message, but you may also want to look
at http://www.instructables.com/id/Low_Cost_Hobby_Servo_XY_Table/

Never really looked at it much, but I recall looking at that and
pondering the ideas. Could be done with
http://www.hobbypartz.com/12exiseb1.html

I fly planes with my kids, so I had nitroplanes.com bookmarked. :-D

--
-- Thomas