speed control with modified servos?

I've been avoiding the use of modified servos as gearmotors because I'm under the impression that you don't have much control over their speed.
My understanding is that you give them (via an analog servo pulse) a "position" that's ahead of or behind the center, and it will spin forward or backward attempting to reach that position. So, unlike a regular gearbox driven by a motor controller, you don't have full control over how fast it spins; just whether in spins and in what direction.
But is this true, or am I laboring under a misconception?
Thanks, - Joe
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The farther the control pulsewidth gets away from the "center" [or null speed] position, positive or negative, the faster the servo turns, up to its particular max speed. Should be easy to demonstrate.

had within +/-200 usec or so of the center point, but still controllable.
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Joe Strout wrote: <snip>

Misconception, but a common one. As Dan notes there is a speed differential about center (in this case, the neutral position, or 1500) us. This is inherent in the servo position algorithm to ramp down the speed as the servo reaches its desired destination -- cuts down overshoot.
The problem is, if the servo exhibits speed changes with only +/- 200 us (I've found it a bit more limited, but it's servo brand/model specific), and the average servo has a deadband of about 8-10 us, you can see the speed control isn't very granular.
If you don't like the limited speed resolution, you can always gut out the electronics and use your own, and then you have a very nice gearmotor that's easy to mount and attach wheels to, and that probably cost less than most surplus gearmotors.
-- Gordon
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Fair enough.

Yes, that's a good point. An even easier modification than just converting it for continuous rotation, I would imagine -- just rip out the whole control board and get two leads onto the motor itself. Thanks for pointing this out.
Best, - Joe
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Joe Strout wrote:

You still need to convert it mechanically. The output gear on most servos has a stop on it that needs to be removed. You usually also need to decouple the pot from the output gear, and this may involve removing the pot, drilling out the bottom of the output gear, or removing the slotted clip placed in the bottom of the gear (GWS servos are like this).
-- Gordon
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Thanks again. It's heartening that at least some servo manufacturers have noticed that robotics hobbyists are using them this way so often. Though this seems to be mostly the new kids on the block, like Bioloid; the old R/C servo companies seem to be surprisingly slow to adapt. (Though at least GWS makes the hack easier, which is something.)
Best, - Joe
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Joe Strout wrote:

Take a look at this circuit. You can build a real speed controller for the thing, using the brush noise as an encoder.
                John Nagle
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What circuit? Did you mean to include a URL or something?
Thanks, - Joe
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wrote:

What I meant to say was that [from my recollection], a servo adjusted for zero speed at 1.5-msec will hit max speed forward and backwards at about 1.7 and 1.3 msec, give or take a bit. Also, over the range of 1.5 up to 1.7-msec the speed will be graded, although possibly not a linear relationship. It doesn't get much faster for pulses > 1.7-msec or so. But helps to check specific servos.
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Joe Strout wrote:

If you want a speed controller that understands R/C pulses, get one. Every R/C car has one. An R/C servo is the wrong tool for the job.
                John Nagle
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John Nagle wrote:

You missed the point of the post. Joe wants to use a modified RC servo and still be able to have reasonable control of speed, rather than just off, full CW, and full CCW. It's a common request.
-- Gordon
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The SSC-32 Servo Controller i have been using has a rudimentary speed control operation (http://www.lynxmotion.com/Product.aspx?productID95). You can direct the servo to reach a predefined position X in T amount of time. I think underneath it is done by splitting the total X position in small segments and pulse the servo to reach these segments every few ms creating a 'slower' impression. The fastest then would be the servos default capability.
regards
Konstantinos Dermitzakis

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Almost any servo controller has that, but it won't work with a servo hacked for continuous rotation, which is what's under discussion here.
Best, - Joe
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In fact, I don't want that; I'm generally of the opinion that R/C pulses are a very crude way for devices to communicate. A serial protocol makes a lot more sense. But I do see other advantages to using servos as gearboxes -- and they certainly are popular, so I'm trying to be sure I understand all the ramifications of using them that way.
Best, - Joe
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I have one of Gordon's servo tanks, this one has modified Parallax servos. I measured linear tread speed [inches/sec] versus servo pulsewidth [usec] as follows.
pulse speed 1500 stopped 1525 0.9 ips 1550 1.9 1575 2.2 1600 2.6 1650 2.8 1700 2.8 1750 2.8 ... etc
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