ENdless Rotation Potentiometers?

[dknguyen]

Hello. I was staring at a servo (not exactly an RC sevo, but similar with serial protocals and a butt load of other features) and it had the ability to be software configured for 300 degree limit or continuous rotation. I was wondering how they managed to do this with a potentiometer (as I have never seen one that did not have end limits). I had assumed they used some fancy magnetic absolute encoder or something since that is the cheapest smallest thing I have seen that allows for absolute position sensing with no end limits. But the manual says something a potentiometer....so I'm just wondering...are there potentiomters with no end limits?

Thanks!

 

[blueroomelectronics]

The pot is disconnected, usually replaced with a pair of 4.7K resistors.
http://www.junun.org/MarkIII/Manual/kevin/servohack.html
**broken link removed**

 

[dknguyen]

I know that's what you do to physically modify them. BUt this servo was software configurable. Like I could program it for continuous rotation OR program it for 300 degree limited rotation.

 

[blueroomelectronics]

Do you have a link to said servo?

 

[dknguyen]

**broken link removed**

 

[freeskier89]

I haven't heard of one but I don't see why there couldn't be one considering the wiper contact could easily slide continuously along the graphite annular resistive element. I would imagine most potentiometer mfgs just stick some mechanical stop in there so it won't do it.

*consults google*
here's a patent on it:
https://www.freepatentsonline.com/4117386.html

 

[dknguyen]

I would have preferred them to use one of these
https://www.usdigital.com/products/ma3/

No silly pot calibration for the center zero point.

 

[blueroomelectronics]

At those prices they're probably digital servos, not sure how they encode could be optical.

 

[dknguyen]

Yeah, they're definately digital. I'm just curious about the encoding *scratches head*

 

[blueroomelectronics]

Are you trying to duplicate it or just figure it out?
A grey encoder wheel would be one way, the other is a stepper motor with a hall effects sensor for the middle position. I'm sure there are quite a few ways to do it.

 

[dknguyen]

JUst figure out. If they did it one way it would convince me more to buy them than if they did it another way. I would replicate it using that magnetic encoder, except I have no motors, servo case, or gears. It's not a stepper I Know that. If they did it any way other than a potentiometer it would convince me to buy it more.

 

[blueroomelectronics]

What is the motor for?

 

[dknguyen]

Steering motors on my rock crawling robot. THese have a serial protocal and temperature, current, torque, etc feedback so I wouldnt need to build my own (it looks like it would be realy hard to fit everything I want to into a standard servo case). THey also run off 12V so I Can use 12V for my drive motors and not need to step down the voltage for the servos (or drive everything with 7V since I don't want to use a regulator for the servo motors).

 

[JimB]

....so I'm just wondering...are there potentiomters with no end limits?

Yes.
I have one somewhere, it came out of some ex-military piece of electro-mechanical equipment.
It has full 360 degree rotation and jumps from one end of the track to another over a range of a couple of degrees.

The question just occured to me, does it short the ends to the track together as is travels through the end point?
I dont know.

JimB

 

[ecerfoglio]

Yes.
I have one somewhere, it came out of some ex-military piece of electro-mechanical equipment.
It has full 360 degree rotation and jumps from one end of the track to another over a range of a couple of degrees.

The question just occured to me, does it short the ends to the track together as is travels through the end point?
I dont know.

JimB

"Once upon a time" .... (Before the cable / satelite era), it was a common practice to use a TV antenna mounted on a rotator to point it to the different stations.

The rotator was a kind of servo mounted between the mast and the antenna, with a "set top" control box.

Some of those rotators used a 360º potentiometer for feedback.. I don't remember if it had just 360º (ie 0º to 360º between stops) or allowed continuous rotation.

As radio amateurs also used (on some bands) directional antennas mounted on a rotator, you may try the ARRL handbook.

 

[mvs sarma]

I know that's what you do to physically modify them. BUt this servo was software configurable. Like I could program it for continuous rotation OR program it for 300 degree limited rotation.

How about Helical pots. thay have 10 turns of 360 deg each
Multiturn pots are common.- ofcourse they are not endless.....

 

[mvs sarma]

The pot is disconnected, usually replaced with a pair of 4.7K resistors.
http://www.junun.org/MarkIII/Manual/kevin/servohack.html
**broken link removed**

Thank you for such magnificient link and to Kevin Ross the author of the mod. Really nice hack as he put it.

 

[ericgibbs]

hi,
Vishay manufacture the '357' series of 360deg and continous rotation pots.

 

[JimB]

"I don't remember if it had just 360º (ie 0º to 360º between stops) or allowed continuous rotation.

As radio amateurs also used (on some bands) directional antennas mounted on a rotator, you may try the ARRL handbook.

All the antenna rotators I know of are limited to 360 degree rotation (or just a bit over), If they could go round and around for ever, it would be rather hard on the coax feeder cable.

Also, I think you will find that the average antenna rotator uses a standard 270(?) degree potentiometer for feedback but with some cheap plastic gears to convert 360deg to less than 270deg for the pot.

JimB

 

[mvs sarma]

All the antenna rotators I know of are limited to 360 degree rotation (or just a bit over), If they could go round and around for ever, it would be rather hard on the coax feeder cable.

Also, I think you will find that the average antenna rotator uses a standard 270(?) degree potentiometer for feedback but with some cheap plastic gears to convert 360deg to less than 270deg for the pot.

JimB

I hope even a 90 deg actual rotation sufficient for ANTENNA As such normal pot may serve well