360 degrees continuous rotation potentiometer

[Ro-Bot-X]

Does anyone know of a rotary potentiometer with the size and value of a servo pot?

I am thinking of using it instead of an encoder inside a servo, with different electronics of course. I have looked around on the net but couldn't find the appropriate size, only bigger ones. I'll keep searching, but if you know where I can find it, please let me know.

Thanks!

[airman00]

I think a non-contact optical encoder would be best

[Ro-Bot-X]

I know, I was just exploring ways to reduce the servo mod to minimum.

Thanks.

[izaktj]

Are you designing a continuous rotation servo?

[Ro-Bot-X]

I am thinking to replace the electronics inside the servo with a motor controller and replace the pot with a cont. rotation pot instead of an encoder. At first it seemed a good idea, but after some more thought it may be harder to use, since it needs analog to digital conversion to read the sensor. Then at every complete rotation a counter has to be incremented (or decremented). It simplifies the electronics though...

[fuzzyt]

I've actually been contemplating a similar type of system, but primarily for sensors to determine what direction the sensor is facing in relation to the bot.  The only solution I've really come up with is to use conductive paper and create my own sensor mount.   

[want2learn]

How you getting on with this?

Are you going with the encoder route? If so will you be able to make one that fits inside the gearbox or something. Probably need to be external?

[Ro-Bot-X]

There is a way to make an internal quadrature optical sensor that will be placed instead of the pot and it will read 2 stripes (1 black and 1 white) made on the bottom of one of the intermediary gears. Openings for the sensors need to be created, but with a Dremel it's easy. The sensors are Hamamatsu IR sensors like these: http://www.acroname.com/robotics/parts/R64-P5587.html, they are going to be mounted on a separate board that will be plugged in some extension pins to the main board inside the servo. For the prototype I'll use double layer perfboard, but I'm going to redesign it after no modifications are needed.

More info about using optical encoders inside servos you can get here: http://homepage.mac.com/sigfpe/Robotics/encoder.html and here:
http://www.portlandrobotics.org/howto/quadrature.php?link_id=14

[klims]

I just had a very similar idea and was wondering why no one else had thought of it... lol.

My idea is not to entirely rip out the internals, but place a continuous rotation pot in place of the 270deg one and leave the 270deg pot sitting inside. This would mean I can modify the servo for continuous rotation while still having some kind of feedback.

My only snag is finding a continuous rotating pot that will fit inside a standard servo. Has anyone seen one around??? I will keep hunting.

[Ro-Bot-X]

There is a tutorial in another thread that shows how you can mod the pot inside the servo to add a magnet to the pot's shaft and a sensor underneath it that can sense 360 degrees of rotation. But you will also need to replace the electronics with your own, to include a microcontroller and an H-Bridge. I still find the continuous rotation servos too noisy, and for a competition robot they're too slow most times. I have replaced the CR servos in my robot with Faulhaber motors (surplus, see the Electronic Goldmine) that are faster, silent enough, have internal encoders and cost the same as a servo. But you also need to buy a hub to mount your wheels.

Now I'm still looking out to find miniature DC motors (think 1 inch square robot) that are cheap (under 10 usd / piece).

[klims]

Yeah I know there can be a number of different solutions to this, but I think a continuous rotation is the easiest and cheapest solution.
The biggest benefit of a pot would be no extra electronics required to run it! Just an extra analogue pin on my micro to read the position.