Questions and musings about the Parallax Continuous Rotation Servo

Migs

Friends:

Where can I learn more about the Parallax continuous rotation servo? I'd like to know the range of pulses it can be sent, and some more specs about the servo. Also, are all servos respondent to the same set of commands? Is there some sort of standardization to this? What about Futaba vs. Hi-Tec? As I work on my BoeBot, I can't help but wonder why you would have to send numbers so different to the servos for them to go at the same rpm but in different directions. I understand the commands must be different, but the numbers themselves (P13 850, P12 650) seem so "different" and not "amicable". (Yes, I know what I'm saying sounds weird)

One of the Parallax people said the Continuous Rotation Servo was a Futaba XXX (I forgot the model and cannot find the post!). I did an online search for the model number and couldn't find anything in the Web.

Can someone shed some light on this?

Thanks amigos (and amigas if we have any!) -Migs

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"Sometimes we forget that the world itself is paradise." Douglas Coupland, 'Microserfs'
"If the doors of perception were cleansed every thing would appear to man as it is, infinite" - William Blake
"We either make ourselves miserable, or we make ourselves strong. The amount of work is the same." Carlos Castaneda
"One single grateful thought raised to heaven is the most perfect prayer. " G. E. Lessing
“How much of human life is lost in waiting.” Ralph Waldo Emerson
"Men often mistake notoriety for fame, and would rather be remembered for their vices and follies than not be noticed at all.” Harry Truman
My website: www.intoku.net my e-mail:mreznicek@pretensa.com me:Miguel Reznicek

Mike Green

Most servos (with one or two exceptions) use the same control information ... a pulse repeated roughly every 20ms with a width from roughly 1ms to 2ms. Some servos accept a wider pulse range (maybe 0.5ms to 2.5ms). Standard servos will position themselves based on this pulse width with 1.5ms representing the center of the range. Continuous motion servos are modified standard servos with the mechanical stop removed and the position feedback potentiometer disconnected from the gear train so that they move with a speed and direction controlled by the pulse width. 1.5ms is approximately the stop point. Longer pulses cause rotation in one direction and shorter pulses cause rotation in the other direction with the speed increasing the further the pulse width is from the stop point.

That's about it for standardization and you will rarely find much more in terms of specs. R/C servos in general are not designed for fine control. They were originally developed for controlling radio controlled model aircraft and boats where manual control and visual feedback are expected.

There is one line of servos that use an asynchronous serial channel for control and can provide information on position and mechanical load, but they're completely different from a "standard servo".

Migs

Thanks for the excellent repy Mike! What is the range of comands for the Parallax C R Servo? How high up or down can I go with the numbers I Pulsout? -Migs

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"Sometimes we forget that the world itself is paradise." Douglas Coupland, 'Microserfs'
"If the doors of perception were cleansed every thing would appear to man as it is, infinite" - William Blake
"We either make ourselves miserable, or we make ourselves strong. The amount of work is the same." Carlos Castaneda
"One single grateful thought raised to heaven is the most perfect prayer. " G. E. Lessing
“How much of human life is lost in waiting.” Ralph Waldo Emerson
"Men often mistake notoriety for fame, and would rather be remembered for their vices and follies than not be noticed at all.” Harry Truman
My website: www.intoku.net my e-mail:mreznicek@pretensa.com me:Miguel Reznicek

allanlane5

It's a Futaba 148. And yes, the Servo control signal is standardized. It's a pulse, repeated every 20 mSec to 50 mSec (kind of sloppy there, which is a good thing, really). The pulse itself is a 1 mSec (full left) to 2 mSec (full right) with 1.5 mSec being 'centered'.

When you 'modify' a servo for continuous rotation, there's a position feedback resistor which is 'fixed' in center position. What this does is a pulse less than 1.5 mSec 'spins' left, greater than 1.5 mSec 'spins' right, and at 1.5 mSec 'stands still'.

Now, since a Servo was designed to move smootly to a position, the vendors don't really do much for 'speed control'. When you use the servo in "continuous" mode, you get a limited form of speed control -- I think it's all used up by the time your control signal is 650 and 850.

There are hardware mods you can do to the servo to 'widen' the speed control -- google "modified Servo" to get more detail.

David H.

650 & 850 is the highest you should go. Any higher numbers will not make it turn any faster. using 650 it will turn one way, 850 will turn it the other way. Also the pulse of 750 should keep it steady, and not move.

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David


There are 10 types of people in this world,...
Those that understand binary numbers, and those that don't!!!