Servos and Potentiometer

[BorisS]

Hi,

i want to start looking at different servos for my not yet fixed but coming to live again PH AV 200. I went to a couple shops looking for 7.4 volt servos but no one could guide me in the right direction when it comes to the potentiometer.

I really dont know a lot about servos so sorry for the basics. But as far a i understand the Potentiometer is part of every servo and in the case of some camera mounts the potentiometer is taken out of the servos casing and placed on the camera gimbal were needed. Am i right about that ?

So any servos should work as long as the potentiometer can be taken out and place somewhere on the gears were ever needed ? Would be thankful for any guidance on this or wiki or blogs were this i explained. Or does it have to be a special servo or potentiometer ?

What are state of the art servos at the moment strong motor high percision, high resolution etc. but low weight ? Hitech Spektrum, Savöx ? Price dosent matter, if I dont crash in the next couple weeks again

Thanks

Boris

 

[Droider]

Boris why do you want to change the servos? What voltage are you running them at..

Dave

 

[BorisS]

Hi Dave,

running them on 6v. But i heard from some that they change to 7.4 volt servos or analoge instead of digital. Just want to start testing arround. But since i dont understand the servos potentiometer and how they work together i want to go sure to get the right one.

Boris

 

[Droider]

Mactadpole is running his a 7.4 volts with no issue.. do you have a CC bec you can increase the voltage you are giving the servos?

 

[BorisS]

nah i would have to get a new bec that would be the smallest issue. I am just not sure if I am on the right track with my thinking all i have to do is buy some good 7.4 volt servos, take them apart take out the potentiometer and place the components on the AV 200. Is it that simple ?

 

[jes1111]

Servocity.com always lists (for Hitec and Futaba servos) whether or not they are 360°-modifiable (which mean the pot can be taken out and the rotation stop-pin removed). Google for "servo 360 modification" for several guides how to do it. Or you can buy them already modified from servocity for an extra cost.

All indicators tell me that Hitec is the best choice. But it is a surprisingly deep subject - power and speed are obviously factors, but so is resolution, accuracy and repeatability. Choosing 7.4v models over 6v is only the start of it. I can't understand why some people recommend analogue servos - one of the beneficial characteristics of digital types is their quicker reaction time - i.e. how long they take to start moving - an important consideration! Once they have reacted, their ability to accelerate up to speed quickly is determined by their torque (i.e. more is better). Their maximum speed is what's quoted in their spec. Resolution on all digital servos (except one) is 8-bit, i.e. 2048 possible commanded positions in 360°. The only exception (off the shelf, that I know about) is the Hitec 7990, which has a magnetic encoder instead of a potentiometer and a 12-bit circuit to yield 4096 possible commanded positions in 360° - twice the resolution. Run that through 4:1 gearing and you have some pretty accurate positioning.

Of course all these benefits of better servos can be negated by a sloppy mechanism between them and the camera. Backlash - the "gap" that occurs when gears change direction - can spoil everything. Gear drives are inherently prone to backlash, and so too are belt drives (contrary to some popular belief). You can probably feel this on your mount - a certain amount of "play", meaning you can move the camera back and forth slightly without actually turning the gears/belt. That same play will occur (on each axis) every time the servos change direction, i.e. all the damn time! All MR gimbals I've seen use MXL-type timing belts or plastic gears, both of which are poor choices in terms of backlash. The trick of doubling up servos (as Denny has described) helps to control that backlash, if not eliminate it altogether, but it requires special construction. There are also better timing belt choices (like 2GT and 3GT) but you have to jump through hoops to find a way to mount the pulleys on a hobby servo.

Accuracy (the ability to go to a commanded position accurately) and repeatability (the ability to go back to a commanded position consistently) are variable on low-end servos but should be good on high-end Hitecs.

btw - interesting product here: http://www.01mech.com/supermodified - and the feature list makes interesting reading in terms of alerting you to what's missing in normal hobby servos.

EDIT: forgot to say - 360-modding a servo with the existing pot will probably limit you to 360° maximum - to turn multiple times (as you need to for a geared system) you need a multi-turn pot, i.e. a separate purchase, unless the pot is mounted directly to the rotation axis. Your mount may have these fitted already.

 

[Bartman]

boris,
many new servos are being marketed as HV servos due to the giant scale community demanding them basically and they're good for 7.4 volts. Hitec's a good servo, I bought two of the 7985 but it's too soon to say if they're necessarily better than other options.
bart

 

[Pano-Dirk]

I'm using modified Hitec servos with 7.4 Volt. They are working fine in the AV 200.
Sorry, can't give you the product number yet, sitting at the Beach.
Savöx are also Fine servos.
... and don't use analog servos, they are not "analog", they are only slower. Full ack to Jes111.

 

[BorisS]

okay cool thanks guys !

I will try the Hitec 7.4 volt. One more question. Can I just mechanically and connection wise unlink the existing potentiometers in the new servos and use the already mounted potentiometers in the PH AV 200, could that work ?

Thanks Boris

 

[Macsgrafs]

Boris, yes the pots can be mounted externally from the servo body & will work, but you will need a lot of experimentation to get it spot on!

Ross

 

[BorisS]

okay cool thanks project for next week