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Modding Servos for Continuous Rotation.....

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LAN_403

Patrick

Wed Sept 05 2012, 08:26PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

When modding the usuall servos that have mechanical stops and seperate potetiometers, is there a deliberate position to leave the pot in?

should it be cenetered? it seems like fixing the pot in one position, and sending PWM signals should not just control continous CW and CCW rotation, but the speed of rotation as well.

any hints, i plan to try it tonight but i dont want to CA glue the pot till im totally sure.

Steve Conner

Wed Sept 05 2012, 10:06PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Seems to me like the servo would just spin endlessly if the pot was disconnected. I guess it depends if the internal control loop is P or PI.

Patrick

Wed Sept 05 2012, 10:54PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Steve Conner wrote ...

Seems to me like the servo would just spin endlessly if the pot was disconnected. I guess it depends if the internal control loop is P or PI.

I dont mean disconnecting the pot, you dont do that for fear of what you mention. you turn the pot up or down then fix it in place permenttely, i think. Then you can send less than or more than the center 1500uS pulse for speed and direction... i just dont know where to turn the pot and leave it.

...

Thu Sept 06 2012, 01:08AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You usually want to set it in the center, so that sending a command for 'center' results in no rotation, and longer/shorter pulse widths give cw/ccw rotation. I am not exactly sure what you mean with the speed part, with all of the servos I have played with (pot centered) the speed has been proportional to the input pulse length.

Patrick

Thu Sept 06 2012, 02:50AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

... wrote ...

You usually want to set it in the center, so that sending a command for 'center' results in no rotation, and longer/shorter pulse widths give cw/ccw rotation. I am not exactly sure what you mean with the speed part, with all of the servos I have played with (pot centered) the speed has been proportional to the input pulse length.

yes, yes yes !!! thats what i thought, keep it centered, then send the center command (1500uS) for stop.

but does it have to be perfect or can the pot be off a little, then compensate by nudging the 1500uS pulse up or down a tiny bit for zero rotation?

Sulaiman

Thu Sept 06 2012, 12:18PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I believe that the speed of rc servos is determined by the supply voltage and the motor type,
i.e. the controller just switches the + and - supples to the motor.

There is almost always an offset in practice, that's why rc controllers have an offset adjuster.
You can have any offset you wish, then compensate at the controller end.

Patrick

Thu Sept 06 2012, 04:30PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ill have to try it then tonight.

Carbon_Rod

Fri Sept 07 2012, 08:10AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Measure the larger axial variable resister value, and replace with two identical matched 1% resisters or a good 20-turn variable resister brick. Cut off the internals of the variable resister and the stop-tab to allow continuous rotation. Thus, one can have a more mechanically reliable and predictable control calibration using this method.

You may also wish to wrap the servo body in grounded copper Foil tape as some brands may be sensitive to RF noise affecting positions.

Every hobby servo may still require timing calibration even with precise mcu timing, but note sending no signal usually stops rotation. This is due to the back-EMF from the motor being used in the control loop of the servo.

Cheers,
Rod

Patrick

Sat Sept 08 2012, 03:31AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ive removed the pot labeled 5k, yet it reads 4.8k end to end, so the nearest standard value is...im thinking of using 2.2k in series, otherwise ill have to add 200 ohms more in each branch.

...

Sun Sept 09 2012, 04:14PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

A pair of 2.2k should work fine, although frankly just leaving the old pot in place works fine too as long as you can find a spot to put it. Like you said, you can null out any small error by adjusting the pulse width slightly, hobby servos usually aren't super precise devices to start with.

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