Model Boat Mayhem - Forum

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: steve pickstock on July 27, 2009, 01:07:01 PM

Title: Stripped down servos and potentiometers
Post by: steve pickstock on July 27, 2009, 01:07:01 PM
I'm running my LCT with two stripped servos. These provide sufficient power for what I need - so far, but I am having difficulties setting them up because of the potentiometers on the servos.


So - question! Can I remove them without impeding the control and operation of the motors? If so, how is it done?
Title: Re: Stripped down servos and potentiometers
Post by: Umi_Ryuzuki on July 27, 2009, 02:22:57 PM
You can replace the pot with two resisitors equal to the value of potentiometer when centered.
Thhe pot is typically 5k so use two or more resistors equal to 2.5k from the center wire to each side.

http://www.microrccenter.com/forum/index.php?showtopic=15681&hl

Title: Re: Stripped down servos and potentiometers
Post by: Rex Hunt on July 27, 2009, 03:24:12 PM
carefully...strip down the servo,
there will be a gear cog that turns the potentiometer shaft.
Remove this cog and reassemble.
When rebuilt set the Transmitter sticks to the mid (neutral) point.
If the motors still run, manually adjust the potentiometer (by hand now)_ until they stop.

You should now have forward and backward movement available on the motors as normal.

The trimmer control gives a small variation in the pot setting so set them to midway before doing any of the above!

It is the system I use on my HMS Bulldog.

 ok.-?


Rex
Title: Re: Stripped down servos and potentiometers
Post by: andrewh on July 27, 2009, 03:32:53 PM
Steve,

I replace the pot with a trim pot liberated from dismantled equipment - preferably 5K.  This holds its setting much more securely that the servo pot

andrew
Title: Re: Stripped down servos and potentiometers
Post by: steve pickstock on July 27, 2009, 04:35:44 PM
carefully...strip down the servo,
there will be a gear cog that turns the potentiometer shaft.
Remove this cog and reassemble.
When rebuilt set the Transmitter sticks to the mid (neutral) point.
If the motors still run, manually adjust the potentiometer (by hand now)_ until they stop.

You should now have forward and backward movement available on the motors as normal.

The trimmer control gives a small variation in the pot setting so set them to midway before doing any of the above!

It is the system I use on my HMS Bulldog.

 ok.-?


Rex

I've done all of that - stripped the whole case back (initially it was about weight and where they were going, now it is about clearance, so all of the gears and casing have gone. The response of the pots is so sensitive I have found it difficult to keep them at the mid setting. I was wondering about using a dab of superglue to hold them at the mid point ...?

Title: Re: Stripped down servos and potentiometers
Post by: andrewh on July 27, 2009, 05:02:54 PM
steve -
blutack, acrlyic paint even balsa cement are much less permanent - I also find you may possibly need to reset neutrals with voltage changes, etc.
andrew
Title: Re: Stripped down servos and potentiometers
Post by: Rex Hunt on July 27, 2009, 05:04:36 PM
Try a bit of candle wax , sealing wax or her indoor's nail varnish , Tippex works too, to seal the pot.....you may need to readjust later.

And yes...they are a tad 'touchy'!

Rex
Title: Re: Stripped down servos and potentiometers
Post by: riggers24 on July 27, 2009, 05:09:52 PM
Try here (http://www.seattlerobotics.org/guide/servohack.html[here) it may be a little help in the modifying of the servo
Title: Re: Stripped down servos and potentiometers
Post by: barriew on July 27, 2009, 05:43:13 PM
The other option is to replace the pots with multi-turn versions which will give you a wider neutral band. If you do this you can also extend them from the PC board to give easier access.

Barrie
Title: Re: Stripped down servos and potentiometers
Post by: steve pickstock on July 27, 2009, 08:08:41 PM
I will try the blu tack variation first - I like low-tech.

Thanks for the advice, guys.
Title: Re: Stripped down servos and potentiometers
Post by: amdaylight on July 27, 2009, 09:11:06 PM
Steve,

This is the method that I used, when you are done you can adjust the pot through the screw hole.
( http://www.kronosrobotics.com/an116/GAN116.shtml )

Andre
Over yonder in Portland Oregon
Title: Re: Stripped down servos and potentiometers
Post by: malcolmfrary on July 27, 2009, 10:47:25 PM
All of the above is true.  In addition, I control my conversions using the trim tabs as engine room telegraphs.
Without doing the extra circuit mods as described in the robotics sites, the dead band is very narrow, and due to the way the circuit works, the distance you have to move the sticks to go from stop to full speed is small, handily enough about what you get moving the trim tab.
Title: Re: Stripped down servos and potentiometers
Post by: Martin [Admin] on September 08, 2009, 04:39:56 AM

 Has anyone use a big servo as a main drive, say a quarter scale servo?
Title: Re: Stripped down servos and potentiometers
Post by: andrewh on September 08, 2009, 12:31:57 PM
The motor in my Thames barge is a 1/4scale  servo without the gears - the motor is a nice 380 size and mounted directly on the electronics board and drives the prop direct. 
Usual problem of narrow neutral band, but I can live with that for the price  :}
andrew
Title: Re: Stripped down servos and potentiometers
Post by: malcolmfrary on September 08, 2009, 02:22:47 PM
Back in the elder days (about 8 years ago) I used to use the board from a servo to offer PWM drive pulses to a dual opto isolator.  The servo thought that this was the motor.  The output transistors were hooked up to drive an H-bridge of TIP141 and TIP142 transistors.  This arrangement allowed control of anything up to a 545 or 385 on up to a 12volt SLA (done) or up to 18volts (in theory).  I stopped bothering when the price of the real thing came down to about the same as the components, and the likes of Mtronics, Action etc were physically smaller and better performing.
The lack of a deadband I never found to be a problem, I preferred to use the trim tab as an "engine room telegraph".