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Continuous Rotation servo specs

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Posted: 2009-04-15 15:46 
EddieB wrote:
Bear in mind the real speed adjustment range is probably only about +/-100uS from the "off" value so there will not be any difference in speed between 1300, 1000, or even 500us.


I hooked up one of the 1425CR servos to LynxTerm and came up with the following results.

Stopped from 1513 to 1535.

Full CW at 1735

Full CCW at 1315

I had to basically put the servo to my ear to hear the tone change, as trying to "see" a difference in output rotation speed was impossible. So it looks like the following specs apply.

Stop Value = ~1500
Deadband = 20uS
Full CW = Stop + 200uS
Full CCW = Stop - 200uS

I'll make this a sticky for future inquires.

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Posted: 2010-12-01 16:56 
OK, I'm part of a team working to come up with and teach a basic robot class. Idea is to buy or create a "kit" to build a simple wheeled 'bot (I have a simple sheet-metal chassis made). Goal is to navigate table top without falling off, maybe follow walls in a MicroMouse maze.

I had GWS S03N 2BB Standard-torque Ball-bearing Servo motors on hand from some Mark III experiments:

http://www.junun.org/MarkIII/Info.jsp?item=18

Simple to modify. Setting the pot to deadband at 1500 is not accurate. I have to take the R/C servo apart to adjust. I'm considering replacing pot with a pair of resistors.

I'm open to the 1425CR or other servos. would you say they're better? (Don't need CR mod?)

I was disappointed with the "matching" of the two servos on a little 'bot. Yes, I can create a pair of offsets for better dead-band, but what about the scaling? Does that need an adjustment as well? Goal is to get the pair to track reasonably close across a table top.

Maybe I can qualify them better with an encoder and light load selected. I saw a fixture somewhere...

Other thought is to just use the DC motors in the R/C servos, and drive them with PWM. BasicMicro Nano 18 chip (not much room!) running Basic.

Edit:
I take it the 1425CR has the HiTec splined shafts?

Thoughts?

Thanks!

Alan KM6VV

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Posted: 2010-12-02 19:42 
Another question, which "builds" use these continuous rotation servos?

Thanks!

Alan KM6VV

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Posted: 2010-12-02 20:56 
I've tinkered with the below servos and find they have a ~10us deadband, and are easy to modify. Using the installed pots in a continous rotation mode can result in some drift. Possibly getting the pot adjusted to the center 1500us position and then using a small dab of glue on the pot shaft might help things. Adjusting a pot for zero motion is an interesting exercise in itself. The below servos have dual ball bearings on the output shaft which might help with loading when used to directly connect to wheels.

viewtopic.php?f=31&t=6388

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Posted: 2010-12-02 23:59 
Thanks for the links.

They are inexpensive! I've got four of the GWS servos modified. I haven't determined the dead band or linearity yet. I need an adjustment slot cut, then I can adjust them assembled.

Did you do any offset/gain calcs for your servos? The zero-drift seems excessive on mine. I was wondering if the HSR-1425CR servos did any better (similar price).

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Posted: 2010-12-03 16:27 
Quote:
Did you do any offset/gain calcs for your servos? The zero-drift seems excessive on mine. I was wondering if the HSR-1425CR servos did any better (similar price).


I just did some testing to determine the +-5us dead band. From neutral to full foward/reverse seems to be in the range of +-100us. You can use the up/down keyboard arrows when using lynxterm to increase/decrease the servo timing in 1us increments to see your servo dead band and determine the current middle neutral value. Securing the pot shaft with a tiny dab of hot glue may prevent drift once the pot is set for ~1500us.

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Posted: 2010-12-03 17:09 
The deadband / center position will drift with temperature as will all analog servos. The only way to get accurate response is to adjust the deadband after the servo has warmed up. Back in the day I would go to ham fasts and set up a booth for demonstrations and sales. Sometimes we were outside where the temperature would be 60's in the morning and 80's later in the day. I had a two arm display where they would pass a small object back and forth. I would have to adjust the positioning several times during the event. The deadband on the HSR-422CR is a little broader to accommodate for this drift.

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Posted: 2010-12-03 17:12 
KM6VV wrote:
Edit: I take it the 1425CR has the HiTec splined shafts?


Yes...

KM6VV wrote:
Another question, which "builds" use these continuous rotation servos?


Currently, none.

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Posted: 2010-12-03 19:19 
I can try "warming them up" a bit before checking the zero. I don't have any extremes of temperature to deal with, but I've observed something like a "warm up" drift in mine.

And I think I'll order some of the new ones also. I need some stocking stuffers!

Alan KM6VV

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