using a servo/encoder for motion recording and playback

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grindlay's Avatar (by Gravatar)grindlay posted on
January 16, 2007, 11:17 pm
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Hi all,

I'm working on a project which requires that I be able to 'record'
externally driven rotations of a driveshaft and then 'play' them back
using a servo/stepper motor.  It'd be nice if I could just use the
encoder in the servo/stepper for recording, but I'm not sure about
backdriving servos.  Its also very important that during recording, the
external driver experience as little friction as possible.  This makes
me think that I need some kind of clutch, but I haven't been able to
find anything suitable despite my best googleing efforts.

Are there any off-the-shelf parts (i.e. servos with a clutch) that
could be used to do this?  If I were to use a sufficiently powerful
motor such that it didnt need to be geared down at all (or very little)
would it be okay to just backdrive it?

If its useful, the motions that I need to record will vary, but will
likely not exceed oscillations of about 5 degrees at 15Hz.  My torque
requirements are likely to be modest, but I'd imagine than what typical
RC servos can deliver.

thanks in advance,
Graham


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Joe Strout's Avatar (by Gravatar)Joe Strout posted on
January 16, 2007, 11:50 pm
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Re: using a servo/encoder for motion recording and playback



Some servos will indeed be unhappy if you backdrive them, but others are
made for it.  Many of the newer robo-one kits use this as the primary
means for building new animations: you simply pose the robot, and then
click "capture" in the software to record all the servo positions.

I've been looking at a lot of servos, and the Robotis AX-12 seems to me
to be by far the best value for the price.  It's $45 from CrustCrawler,
offers 16.5 kg cm of torque (which is quite a lot!), and is a "smart"
servo -- that is, it speaks a two-way serial protocol, and can report
various things like its current position, current draw, etc.

Here's a community wiki page about it:
  http://www.bioloid.info/tiki/tiki-index.php?page=AX-12%20page

And here's a demo of one AX-12 being manually driven as the input to
control another AX-12:
  http://www.bioloid.info/tiki/tiki-download_file.php?fileIdH

So, this certainly should be doable.

Best,
- Joe

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RMDumse's Avatar (by Gravatar)RMDumse posted on
January 17, 2007, 12:22 am
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Re: using a servo/encoder for motion recording and playback



I looked into some digital Hitec's recently for someone on the forum. I
searched for the post and haven't found it though. Anyway...

I looked into the spec and found they (Hitek iirc) had a mode (very
short pulse) where the servo would answer back with a pulse indicating
where they were positioned. When they were in this mode, they stopped
driving the gear train as well. So they are ready made to be positioned
and read. Then with regular pulse lengths, you can play back the pulse
which was read and it will cause a positioning.

This sounds exactly like what you were asking for, however, the new
digital servos are not the cheapest possible way to acheive this, and
you'll have to either have a special driver to position/read them, or
do some fancy programming.

--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear


grindlay's Avatar (by Gravatar)grindlay posted on
January 17, 2007, 12:29 pm
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Re: using a servo/encoder for motion recording and playback

Thanks to both of you; this info is very helpful.

Randy - Any luck locating that post?  I'm quite interested in the
posibility of using the HSR-5995TG as I think that it is powerful and
quick enough for my purposes.  I did, however, find the following
discussion on getting position info out of Hitec servos:

http://www.lynxmotion.net/viewtopic.php?t21&postdays=0&postorder=asc&start &sid=f2e4f6b40a5ada92c6ebbb36b6e3f4ae

The first post links to an interested pdf containing analysis of
dissected HS-5475HB, HS-5245MG, HS-5645MG, and HSR-5995TG servos.  In
that document the author states that these servos use an H-bridge
configuration that only allows the motor to be in drive or brake mode
(not freewheel) when the power is applied.  But perhaps the mode you
mentioned removes power from the bridge entirely?

In anycase, any other information about the position readout mode you
mentioned would be greatly appreciated.

thanks again,
Graham

RMDumse wrote:


RMDumse's Avatar (by Gravatar)RMDumse posted on
January 17, 2007, 1:11 pm
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Re: using a servo/encoder for motion recording and playback



I was begging to wonder if I was dreaming. But then I searched on my
posts without any qualifiers and 17 pages of scanning later...

http://groups.google.com/group/comp.robotics.misc/browse_frm/thread/28fe26c=
6cbf76f3e/5de8488b6610feb5?lnk=st&q=group%3Acomp.robotics.misc+author%3=
Armd%40newmicros.com&rnum=170#5de8488b6610feb5

which refered to this page:

http://web.ukonline.co.uk/r.ibbotson/files/HSR8498HB%20Servo.pdf

which linked to this spec Hitec Multi-protocol Interface

http://www.hitecrobotics.com/Tony%20information/HMI%20Protocol.pdf

and says:

"The first thing to note is that the 50·S pulse which initiates the
position feedback also
disables the motor drive."

Sounds like they only have 10% accuracy though.

HTH's

--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear


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