Model Boat Mayhem

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: Crossie on February 15, 2016, 07:40:28 pm

Title: constant and variable rotation servo
Post by: Crossie on February 15, 2016, 07:40:28 pm
  I wanted a couple of such servos for a current build and after looking at the price (14) and anyway was bigger than I needed, I had a look though Mayhem's pages, and yes several threads on the subject, so decided to make my own. I took a few pictures of the hack for which I claim no invention but just share them here for anyone else who wants a similar, low power and importantly-cheap!!  item, especially if you buy several at once for about 4 each. It took less than 10 minutes for this clumpy fingered old man to do two of them and so is within anyone's capabilities.
       1/.  Open up the case and remove idler gears,
       2/.  pull the final drive gear off the pot shaft, trim off the 'limit' lug that projects under the gear and drill the gear to be free on the shaft
        3/. power up the Tx, set all trims to neutral, power up Rx and plug servo into one of the stick ports, the servo motor will probably  run, slowly turn the pot shaft till it stops. Twiddle the sticks a few times to get it running both ways and to ensure that at neutral the motor stops.
        4/.without moving the pot at all, apply a small dab of cyano to the wiper, allow to set, then reassemble the servo.
You'll now have a servo that can be variable speed, constant rotation or start and stop in either direction depending on whether you use  a centring or non self-centring stick or its trim lever to make it move.
Title: Re: constant and variable rotation servo
Post by: malcolmfrary on February 16, 2016, 08:51:35 am
As Mr Punch said, "Thats the way to do it". 
Things to bear in mind are that the deadband is very narrow and that variations in voltage can cause the servo to creep.  On some of my later efforts using standard size servos I have used those small cheap chinese escs (10A without brake) in place of the servo electronics.  They are too big to live inside the servo case but still small. 
There have been items on the net about modding the circuit of the servo board to get a wider deadband in robotics groups.  My eyes and fingers did not feel up to doing that with standard servos, much less with mini versions.
Title: Re: constant and variable rotation servo
Post by: Crossie on February 16, 2016, 10:17:07 am
Hi Malcolm,
                  I had read the earlier posts by knowledgeable folk such as yourself on the subject and the fact that because of the narrow deadband the servo might creep at times, though for my application this wouldn't matter too much and I can stop movement just by nudging the trim one way or the other. At the very slowest speed (about 1 rev/min ) the movement would probably not be noticed much on a boat bobbing about on the pond 50 feet away, well not by my ancient eyes anyway! I'd probably be more focused on not running into other things. Using one of these cheapo servos fills the need for an easy to achieve way of rotating say, a radar, or animating something where there is little load on the rather flimsy nylon gears- - -but then again a broken one can be replaced easily from a scrap servo.
       Thanks for your interest and I'll try your suggestion of a cheap esc some-when as I'm sure I've got one somewhere,