servos back to back to double strength

I have noticed several hobby/educational type robot arms use 2 servos back to back so that the horns are on the outside and
therefore used as a pivot point. I assume this is to more or less double the lifting power, etc. My question is are there simple splitter cable/device that one can simply plug into a pair of these that will "flip" the signal going to one of the servos so that they both rotate in the same direction when mounted back to back like this? I know that I could send commands to each one separately, but it seems like it might generate some timing latency between the two that would eventually wear on the servos.
I'll try to find an example of this and post a link later on ...
Thanks for any help ! JCD
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"pogo"

I've never seen such device, but I don't believe it would be too difficult to make one. A small PIC with CCP + two timers would be enough. VCC and GND could be drawn from the servo wires, and I believe you could use the internal oscilator to save on parts.
Another alternative, which I don't know if its possible would be to hack the servo and reverse its motor polarity (and possibly the internal pot as well), so you don't have to deal with generating two complementar pwm signals.
Cheers
Padu
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pogo wrote:

This is a Y-cable with a servo reverser. Common finds at the hobby shop.
But, it's actually better and easier to provide separate signals. No two servos are exactly alike, and it's helpful to be able to trim them in software. Ganged servos tend to fight each other, so generally it's not something you want to do unless you absolutely have to.
-- Gordon
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Cool! I knew someone on here would know! Thanks for the info, and the suggestion on separate signals is noted. Thanks again ! JCD
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On Tue, 18 Jul 2006 14:25:48 -0700, Gordon McComb

So very true. Not only are the servos not perfectly identical, but your linkage is also likely to be slightly asymmetrical.
A much better solution is to use one servo that is twice as strong.
Alternatively, you might have the space to rig up a mechanical lever system that can absorb small differences.
--
RoRo


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Robert Roland wrote:

A linkage with some compliance won't help much; the servos will draw power stressing the compliant element.
Electrically, the right solution is to run both motors from the same error signal. If you're good at rewiring the interior of servos, and you have ones whose electronics are able to handle the extra load, let one servo unit's electronics do the control and wire the other one as a dumb motor in parallel (reversed if necessary) with the first motor.
                John Nagle
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wrote:

That's not what I meant. I am talking about what is essentially a mechanical mixer, similar to what you find in the rotor head of a helicopter with a Bell/Hiller system, only less complicated.
Maybe that's what pogo meant in the first place? I did not understand what he meant by "pivot point". Maybe you could elaborate a little, pogo?

You still could not be sure the two motors share the load evenly. If the two motors are not perfectly identical, they will be unevenly loaded and maybe even fight each other.
--
RoRo


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Robert Roland wrote:

You mean a combiner gearbox or a differential? That will work, but those things are kind of bulky. You only use that for applications where you need to survive an engine failure.

If they're running off the same error signal, they'll turn in the same direction and will go to zero torque at the same time, so they won't fight each other. You're adding torque, which is valid.
This isn't perfect, because DC motors don't load-share evenly, but it will probably do what the original poster wanted.
                John Nagle
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wrote:

Yes, but it would not necessarily have to be a gearbox. A simple lever or bellcrank system like Curt describes would work in many cases.

It does not have to be. Look at model helicopters. They have two such mechanical mixers on the mast to mix input from the servo and swash plate with input from the flybar. Here's an example:
http://www.dream-models.com/eco/images/BHMzeropitch.jpg
The red lever is the mixer. The center is connected to the pitch control arm, and the two ends are connected to the swash plate and flybar, respectively.
--
RoRo


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If you just hook the two servos through a linkage to a common point on the device you are trying to move, you can easily create a mechanical load balancing effect. There are many simple ways this could be done depending on the configuration needed. For example:
s1 ----------O | | O--------- load | | s2 ----------O
So s1 and s2 are a linkage coming from the servos and they are attached to a common bar which is attached to your load.
If the application allowed it, you could use cord and pulleys (pull only).
For combining rotary motion, a standard differential configuration would work just fine. But if you only need the limited motion of a servo, then there are probably easier ways to link them together with fixed links what effectively work the same as the diagram above.
However, I can't really think of a reason it would ever make sense to try and make two servos work instead of just buying one with twice the power.
--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
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Curt Welch wrote:

...
What the original poster wanted, I think, was a setup where he had some object with a servo on each side of it, with the servos working in cooperation. At least I think that's what he wanted. There are times when that might be an easier solution than building a heavy, stiff structure and driving it from one end, or building a linkage.
                John Nagle
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