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PICList Thread
'Futaba servo pulse widths?'
1997\03\13@122140 by Brian Scearce

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This matter is not PIC-specific, but I've seen so many people working
with servos on this list that I thought it would not be unwelcome.

The Futaba S3003 servos I bought didn't have any pulse width
specifications with them.  The generic servo docs I've been able to
track down suggest that the pulse should be 1-2ms wide.  Sure enough,
the "long" pulse to turn the S3003 all the way to one side is about
2.3ms.  But the "short" pulse to turn the servo the other way is quite
a bit shorter than 1ms, it's something like 270us.

The guy at the R/C store who sold them to me didn't know the pulse
width bounds, but he told me that the neutral position is "always"
1520us; I'm finding it's closer to 1290us.

Does anyone have real specs for Futaba S3003 servos?

Brian

1997\03\13@143819 by Andy Kunz

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face
>The Futaba S3003 servos I bought didn't have any pulse width
>specifications with them.  The generic servo docs I've been able to
>track down suggest that the pulse should be 1-2ms wide.  Sure enough,
>the "long" pulse to turn the S3003 all the way to one side is about
>2.3ms.  But the "short" pulse to turn the servo the other way is quite
>a bit shorter than 1ms, it's something like 270us.

Just because it works doesn't mean it's right to do it.

>The guy at the R/C store who sold them to me didn't know the pulse
>width bounds, but he told me that the neutral position is "always"
>1520us; I'm finding it's closer to 1290us.
>
>Does anyone have real specs for Futaba S3003 servos?

I make a living driving these servos, believe it or not.

Center on "new" Futaba servos (connector on servo is a female) is 1500uS,
with other limits at 1000uS and 2000uS.  Don't know where he came up with
1520.

Most transmitters allow you to transmit out-of-range values (typically
900uS to 2200uS), and the servos (because they're analog <G>) respond just
fine.

If you play around with a carefully calibrated, oversized protractor and a
long arm on the servo output, you will find that the range from 1000 to
2000 uS is rather linear.  Outside of that, things get less linear (one
side will be much more responsive per uS than the other).

If you servo does not look centered at 1500 uS, you have the output arm on
wrong.  It is NOT symmetrical due to the splines on the shaft.  They did
that on purpose.  Just remove the output arm, rotate about 90 degrees, and
replace.  If that doesn't make it centered, go 90 degrees more, etc.

For a better (ie, better quality and cheaper) visit http://www.fmadirect.com

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
==================================================================

1997\03\14@043141 by efoc

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face
Brian Scearce wrote:
>
> This matter is not PIC-specific, but I've seen so many people working
> with servos on this list that I thought it would not be unwelcome.
>
> The Futaba S3003 servos I bought didn't have any pulse width
> specifications with them.  The generic servo docs I've been able to
> track down suggest that the pulse should be 1-2ms wide.  Sure enough,
> the "long" pulse to turn the S3003 all the way to one side is about
> 2.3ms.  But the "short" pulse to turn the servo the other way is quite
> a bit shorter than 1ms, it's something like 270us.
>
> The guy at the R/C store who sold them to me didn't know the pulse
> width bounds, but he told me that the neutral position is "always"
> 1520us; I'm finding it's closer to 1290us.
>
> Does anyone have real specs for Futaba S3003 servos?
>
> Brian


Hi Brian,
        I recently posted some code up here for servo control using the futaba
servos and a 1-2 mSec pulse width.

       What you have to remember is that servos are only supposed to have a 90
Deg rotation. My Futaba servos rotate 90 Deg with the 1-2 mSec pulse
width variation.

       What you say about the neutral position is also true and this is why
most R/C control equipment has a trim option on it. I hope this helps a
little it frustrated me for a while also.

--

Cheers Peter ..........

==================================
New Ideas come from those who
didn't know it wasn't possible
==================================

1997\03\16@224756 by Brian Scearce

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Many thanks to Andy Kunz, Peter Bains, and Keith Kotay for the
information about servos.  My one missing piece of information was that
servos are only supposed to turn 90 degrees.  The Futaba S3003's have
the physical stops a little over 180 degrees apart, so I thought that
180 degrees was how they were supposed to work.

I did some measurements -- sure enough, the servos are pretty linear in
the 1-2mS range, and lose linearity outside of that range[*].  However,
since this is just a personal project, not anything for production (and
there doesn't seem to have been any damage to the servos from my fooling
around so far), I think I'll continue to violate the specification
rather than complicate the mechanism of my camera platform.

Thanks,
Brian

[*] I hesitate to post these figures at all.  I attached a stick to the
servo output shaft, and plotted its position (by hand) for a few pulse
widths.  I couldn't find a protractor in my junk drawer, so I figured
out the angles trigonometrically.  The pulse widths are accurate, but
there are probably only two significant figures on the angle
measurements.

Pulse width (uS)        Theta (degrees)
2062                    189
1742                    157
1614                    148
1422                    130
1166                    104
1102                     94
974                     81
782                     63
718                     55
462                     32
398                     27

The physical stops on the servo get hit at about 180uS and 2200uS.

Brian

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