Servo as a Position Sensor

[Mycroft2152]

I came across this recently. It's actually kind of obvious, but I have never seen it mentioned.

By tapping into the internal wiper in a servo, you can use it as a position sensor.

http://vimeo.com/2223262

Not PICAXE based, but easily PICAXE-able.

Myc

 

[InvaderZim]

That's a good idea, if you can get to the right wires in the servo.

On the down side, you don't want to corrupt the feedback signal. Attaching long leads near the servo power wires is guaranteed to inject noise into the servo feedback signal (the pot/wiper), which could prevent normal operation of the servo. Or it could cause the servo to "jitter" which both wears it out and makes your robot do a little dance.

It looks like they're doing all that while the servo is unpowered, which is pretty safe. It gets trickier when you power it up. Perhaps an op-amp on the servo to buffer the feedback signal? It's probably a tight fit...

 

[boriz]

"This means that one can turn the servo by hand and record the position"

Bad idea. Can damage the gears.

 

[westaust55]

Servo Position feedback

I have read about such feedback methods before but have not played with servos.

In terms of the interference, thinking about instrumentation work on plant sites, we use screened cable with the screen only earthed at one end (to prevent earth loop currents also causing problems).

Bringing this back to the hobby level, a solution might be to use the screened audio cable such as that used in computers from CD-ROM drives to the audio cards.

 

[jppizhere]

It is just a feedback pot inside the servo, could easily be set up for position information by plotting resistance to degrees. If you take apart your average servo you will easily find the pot and it's three wires. I regulary use one brand of servo for another brand of radio gear, the servos run backwards from the originals so have to rewire them to run in reverse, have to switch the two motor wires as well as th two outside wires on the pot.