Awesome servo mod! Closed loop feedback, all that is needed is a wire...

rwgast_logicdesign

Ok I might be a little late to the game on this but I figured I would share my finding just in case.
[FONT=arial, sans-serif]So I have been looking in to various ways to position a laser very accurately at a few different angles. My first idea was to just use a servo, as it can move in 1 degree incraments, but apparently an open loop servo can have real issues with repeatability and jitters. Voltage regulation can help this out quite a bit, but it still does not account for gears wearing out and other external phenomenon.[/FONT]
Well check this out


http://www.youtube.com/watch?v=ygX7sQM2gq0


the how to can be found here, with easily portable Arduino code


http://forums.trossenrobotics.com/tutorials/how-to-diy-128/get-position-feedback-from-a-standard-hobby-servo-3279/

[FONT=arial, sans-serif]basically someone had a the idea to tap an extra wire on the the wiper of the feedback pot in the servo, then connect it to an ADC for feedback. It seems like one of those "Why didn't I think of that?" ideas, so simple it almost seems as though it should be built in! I can only imagine how helpful this may be for something like Martin_H's figure 8 drawing robotic arm. Pair this, steady regulated voltage and a Hi Quality servo and I see no reason to buy an expensive open servo, or other super servo mod board.

For those of you that did not watch the you tube video, he demonstrated using the feedback turning the servo by hand while running some code to save the motions he twist the servo horn in, then the micro controller play it back. That is only the tip of the ice berg as far as usefulness but cool non the less.[/FONT]

Martin_H

Thanks for the pointer to that video. It's not a bad idea because the servo already has position feedback, it's just not exposing it to the consumer. With robot arm you could use that for teaching via positioning the arm and recording a motion script. Beau Schwabe developed a technique to probe the servo's location by controlling the amount of current it gets:

http://forums.parallax.com/showthread.php/84991-Propeller-Application-Proportional-feedback-from-a-Standard-Hobby-Servo-(Upda

I think taping the feedback pot is a little more approachable.

rwgast_logicdesign

Ya that is why I said it is one of those "Why didn't I think of that?" Ideas, the feedback device has been there the whole time just waiting to be tapped in to an read. I have never seen Beau's post on this before, he always has all kind of interesting little circuits! But not only is tapping the pot a lot easier it could also achieve much better speeds. Think about this you run a calibration program which sweeps the servo slowly and records the pot's value at every degree then you could fill a look up table with each feed back value obtained in the calibration.

My idea is to put a WiiCam in a fixed position, then next to it mount a laser on a servo using this mod. This way I can use a protractor to set the laser to precise angles relative to the WiiCam and then record the feedback value of the pot. This way when I move my laser to different positions I can use the feedback values to make sure Im at an exact 45 degree angle or 20 degree angle, or whatever angle may give the best resolution for whatever I am trying to measure.

There are all sorts of applications for this mod and im surprised it isn't more popular.

dgately

rwgast_logicdesign wrote: »

My idea is to put a WiiCam in a fixed position, then next to it mount a laser on a servo using this mod.

Have you seen this Instructable?

http://www.instructables.com/id/Satellite-laser-cannon/

Uses a stepper motor for the base and a servo to point a laser at specific satellite coordinate... Kinda cool!

dgately

mindrobots

dgately wrote: »

Have you seen this Instructable?

http://www.instructables.com/id/Satellite-laser-cannon/

Uses a stepper motor for the base and a servo to point a laser at specific satellite coordinate... Kinda cool!

dgately

Great! Another cool project that will go on my project list! Thanks!! :0(

rwgast_logicdesign

Wow that is impressive thank you for the link!! I have a feeling it will come in handy as as far as the needed math goes. I had actually planned to use the servo in order to angle my laser and rotate the whole thing 360 degrees on a stepper! so this project is right up my ally!