Position feedback on a modified servo?

I am modifying a few Traxxas 2055 servos for continuous rotation, because I am gearing them down further and will need several rotations
to get enough movement from my output arm.
I still want to know the position of that output arm, of course, so I'm planning to put a potentiometer right at that joint to monitor it. What I'm wondering is this: do I really need to build a whole new circuit to monitor this added pot, or can I just use the same type of pot as came in the servo, and wire it in place of the servo's original pot? That would let me use my exisiting serial servo controller to control the servo just as if it were unmodified, albeit with slower movement.
It seems too easy though - am I missing some dealbreaker here? Any thoughts on the matter would be appreciated!
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It should work, depending on the control alrorithm in the servo. Use a good pot, and one that matches the specs of the pot internal to the servo.
(I've not used this approach for a robotic application, and not for a modified servo. What I did was create a remote-control where turning a pot moved a small camera that was mounted on the servo.)
The approach must have some merit because someone is trying to patent this concept, or at least one very close to it. It seems obvious to me, but I'm not a patent examiner. I make no comment regarding the validity of the concept as a patentable idea. You can look it up on the uspto.gov Web site, under patent applications (patents not yet granted). It's number 20060003865.
In any case, you may find better results by connecting the external pot to an ADC input, assuming you're using an MCU or something that has a readily available ADC port. This may provide more overall control.
-- Gordon
Eriswerks wrote:

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Gordon McComb wrote:

On rereading, a bit of clarification on this. I guess a servo would be "modified" if the pot were removed. What I meant was not modified with the intention of continuous rotation, which my servo didn't do. It needed only about a 30 degree movement.
My system was a bit more involved, in that the voltage from the pot also was read by a microcontroller, so that a new setpoint could be made by manually rotating the camera on the servo (yes, I know it's bad for the gears!). The idea was to enable someone to manually adjust the position of the camera, while still allowing the microcontroller to sweep the camera. In the end we decided it was just as good to use a standard servo and an external control near the camera.
BTW, is there a reason to use the Traxxas servos? I always thought they were a bit pricey compared to Hitec and Futaba. Are they better?
-- Gordon
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Thanks Gordon and dpa for the helpful posts. I'll try to find a good match for the servo's 5K pot, or maybe just pull the thing out of the sevo and mount it externally.
I'm using the Traxxas servos because they have pretty good torque (80 oz-in) even before I gear them down, and because I've been able to find some good deals on them. People are often selling used ones on ebay, and I've found them new for US $11 at this site: http://www.koltenw.com/products.asp?cat "
My application is a smallish robot arm which, with any luck, will be coming to Burning Man with me this year to autonomously serve drinks. I'm still debating whether I want to control this thing with a PIC, or hook it up to a junk laptop and write some control software in Python. I'm new to both those things, so I'm not sure which would end up being easier.
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Eriswerks wrote:

Thinking Burning Man, how about an automatic body painter robot arm? Wait a minute... It's more fun to do it manually!
Good luck with the project.
-- Gordon
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Gordon McComb wrote:

...
Yes, RobotZone. You can get such gearboxes (complete with external pot) at http://www.servocity.com/ (go under the link for Hitech servos).
And yes, it was obvious to me also.

I like that idea also. -- D. Jay Newman http://enerd.ws/robots /
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dpa wrote:

I don't know the date of the patent application, but I would say that this is prior art. -- D. Jay Newman http://enerd.ws/robots /
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D. Jay Newman wrote:

This requires Dave to have posted the page publicly. The date of the page is 2000, but this doesn't mean it was publicly available then.
DPA could file an interference if he were so inclined, if he feels this was originally his idea, and he publicly disclosed it any time before the patent filing.
However, I will say the patent appears to a combination of two of the approaches Dave has on the page; that is, the page doesn't explicitly detail what's covered in the patent. However, it could be argued that combining the two, as the patent does, is trivial, obvious, and/or non-novel. Again, could be argued. Not that I am...
-- Gordon
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Hi,
Gordon McComb wrote: ...

Interesting.
Oldest reference I can find on the wayback machine internet archives is August 2003:
<http://web.archive.org/web/20030818211150/www.geology.smu.edu/~dpa-www/robo/servo/servohac.htm
regards, dpa
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dpa wrote:

<http://web.archive.org/web/20030818211150/www.geology.smu.edu/~dpa-www/robo/servo/servohac.htm
The earliest filing of the application is June 18, 2004. The patent I cited is a later application, based on the 2004 app.
Now, I have no problem with entrepreneurs who innovate, but I do get concerned if someone obtains a patent for what we've been doing all along. Rather than innovation it leads to a stiffling of progress. I know that I have built exactly what is described in the patent as early as 1991, but I did not publicly disclose it.
Bear in mind the above is not a legal analysis, is not legal advice, and it is not a professional (or otherwise) judgement on the merits of the patent claim. It's merely a layman observation.
-- Gordon
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Hi
I found an older reference to the same article on the Seattle Robotics Society Encoder web page, from 2000:
<http://www.seattlerobotics.org/encoder/200010/servohac.htm>
I tend to agree, and the fact that you and I (and who knows who else) came up with this independently is highyl suggestive that it is a common practice.
So, who do we send the link to?
regards, dpa
Gordon McComb wrote:

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wayack machine internet archive has the SRS Encoder page archived on January 26, 2001:
<http://web.archive.org/web/20010126202100/http://www.seattlerobotics.org/encoder/200010/servohac.htm
dpa
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dpa wrote:

Well, one way to do it is do nothing. The patent may not be granted. The official procedure is to file an interference, but my non-lawyer understanding is that this is usually done by people with conflicting interests in an invention. I'm not sure what the process is for if you're just a concerned citizen. I'd think it's more of a concern for Hitec, Futaba, and the other servo makers. Let them make the effort.
Even if the patent is granted, if these previous disclosures are indeed cogent, then the patent is unenforceable anyway (NON lawyer opinion), and whoever the inventor is wasted a lot of time and money. The Wayback Machine is often used for this sort of thing, and I'm surprised a search of prior art didn't turn up these pages. Even Google would have found something.
It's not like this field is wildly lucrative, and frankly I'm perplexed anyone would bother with the expense of a patent. You have to be prepared to lititagate it, and that can costs hundreds of thousands.
-- Gordon
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Gordon McComb wrote:

Yeah, October 2000 in the Encoder seems to be a pretty good hallmark.
Last week, I happened to pick up a magazine from an old stack sitting around. Forbes ASAP [now defunct, I think] mag from Summer 2002. Most of the issue was devoted to the mess that is the USPTO. In 2001, 344,717 patents were applied for and 187,822 were granted, almost "double" on both counts from the year before. An examiner typically devoted about 20 hours to each patent, including searches on prior art.

be resolved in court, not by the USPTO. I think the system is setup for
the benefit of the lawyers. They make money on both ends, writing up the patents and litigating them afterwards. The article termed patent litigation as a "growth industry".
I'm not sure what the process is for if

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From what I gather, filing can be really cheap. This gives them the "patent pending" marketing tool at little expense.
On an off-side, do you recall the debalce with servo city, aka Brian Thomas robotics, regarding anonamously posting in forums about how great servo city was?
Mike

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No expense at all. Patent Pending only means you intend to seek a patent. You have a year from first commericalization to file then protected.

Yes, I remember. Faux Pas extraordinaire as I recall.
I (and some of my attendies) openned a servo and picked off the wiper signal and measured it in a public class in 2001 during our "Arm in a day" project. Does that mean I have former art?
--
Randy M. Dumse
www.newmicros.com
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blueeyedpop wrote:

In this case they did file, and through an attorney, who I assume charged the usual fees. So I'm sure it was at least a couple of grand.
Is the market big enough to even bother with "patent pending" marketing? Is this a product people are falling over themselves to use? I think a couple of people have asked about the idea here last year. For folks who need such a thing it's great to have, but you'd think if there was a big enough market to patent the idea that servo makers would already be providing ready-made products with these specs. Servo makers have for years made units with 3-4 turn rotations, and high outputs for things like sail winches.

I don't remember that one. I'm sure I was here, but probably shilling for my book, and too busy to notice! <g>
-- Gordon
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