A servo and its Internal POT

Hi
I got a Servocity panning Gearbox and they have taken out the internal pot
and replaced it with a 5K 3 Turn multiturn precision pot.

Can some expert comment on the following scenario:

I am guessing that if I replace it with a 5K 10 turn pot, the pan will go around 360 degrees more times
and that the precision will improve because it takes 10 turns to reach 5K?

I was wondering , what will happen if the pot is changed to 2K 3Turn ?
I am guessing that the Servo will turn very fast. but the precision will
drop? or maybe at 2K the servo controller will still be seeking a new position
because it is expecting a higher resistance?

Thanks

Eagleeye

Servos don’t directly measure the resistance of a potentiometer, they use the potentiometers as voltage dividers. Basically what they’re measuring is the ratio of the position of the wiper, or turning part of the potentiometer, to the total turning range. The overall resistance of your potentiometer needs to be in the same ballpark (1Kohm to 10Kohm should be fine) to limit the current going through the potentiometer, but the precise value isn’t important.

A servo can only measure the potentiometer position to a given resolution, lets say a particular servo can divide up the potentiometer range into 128 discrete regions. When you use a potentiometer with more turns, you’re increasing the turning range, and the area of each measurable region, so with more turns you’ll actually have less positioning precision.

The extra down-gearing of the servo gearbox is going to reduce the output speed of your servo, and you’ll notice the servo will slow down a lot as it approaches a commanded position (for an example check out the video here of Phil Warner’s ‘Panobot’). The potentiometer won’t affect the maximum speed, but the fewer turns your potentiometer has, the closer the gearbox will get to it’s final position before it begins to slowing down.

In general you want to the pot with the fewest number of turns that give you the motion range you need. I hope this helps, I was totally confused by this when I started working with servos too! What are you building with your gearbox?

-Adam

Hi
I am making Phils Pano bot but I wanted to stop exactly every 20 degrees.
It seems I found that the servocity servo gearbox cannot do this… I still
have not fully experimented with the precision pan servo product…
so I am looking for more information.

I also want to pan the servo at the highest speed and take photos
as the pan is in motion so that moving people around the camera
so that they wont affect the pano so much.

Eagleeye

Cool, the panobot is becoming quite popular!

Is there a reason you need the system to stop precisely every 20 degrees? So long as there’s some overlap in the photos, a good stitching program will be able to figure out how they fit together.

I can understand your desire for speed though. I have a webcam and some other sensors mounted to one of the Servo City SPT200 direct drive pan/tilt platforms, and it is quite speedy (i.e. as fast as the unmodified servos can turn under the load). The specs say it’s for cameras up to 2lbs, but I bet you could even push that a little, especially with some beefy servos like the HS-5995 TG! Actually if speed is the most important thing to you, and your camera isn’t too heavy, they’re having a sale on the high-speed HS-965 MG. Come to think of it, I’m not really sure what the 180 degree modification is that Servo City wants to charge 10 bucks for, you should be able to drive an unmodified servo to it’s stops with the Pololu controller anyway.

The downside to this sort of direct-drive table is you’ll be limited by the range of the servos and mechanical interference of the platform with itself. With 180 degree servos you can flip the camera around and upside down to look up and backwards, but you won’t be able to look down much. Still, setting one of these on the floor and taking pictures of the upper-hemisphere of a room would be pretty cool.

-Adam

P.S. I totally agree with you about a Pololu stepper motor controller, that would be quite sweet!