You can get any gear motor to be an RC servo by using a controller board like this one:
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https://www.ebay.com/itm/172264924286
(there are more types)
With this setup, you don't even need to have the pot on the motor output shaft, so for example, you could have a screw drive to lift the bed, but then mount the pot on the bed pivot point. The controller would run the motor to get the right angle on the bed, regardless of the mechanism used to get it there. Also, you could use a linear pot. Or even a multi-turn pot, e.g. with 10 turns to give the full range. Lots of options.
You can get them pre-mounted on gear motors so that it's actually like a normal servo (except that they require 12v / 24v additional power input).
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https://www.ebay.com/itm/131683084318
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https://www.ebay.com/itm/131683049823
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https://www.ebay.com/itm/141859318550
Lots of gear ratios and motor types to choose from.
I think the benefit of these servos, is that they are quite cheep and also that the gear output ratio match much better what you want for the steering servo than a normal RC servo does. They are mostly much too quick and not so strong.
You could also build this from scratch very cheaply using an arduino uno, a monster motor shield, the gear motor of your choice and a 10k pot. I think you could build the full controller then for less than $15.
There is a guy that has build a servo with this approach and described it here:
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http://www.instructables.com/id/DIY-Servo-Motor/
I wasn't actually thinking of having a auto-centering steering in my build. But when you mention it, it's a bit of a nuisance to continuously heard the truck to just go straight. So maybe I'll go that route myself.
I was also thinking a lot on the steering and how it plays with having one gear motor on each wheel. If your gear motors on the wheels have a high reduction, they will not turn when you steer. Rather they will just stay put and make it very difficult for the steering motor to do the steering. This is no problem on the original of course, since it has a differential between the left and right wheels and would allow the wheels to turn in the opposite directions when steering.
So I was thinking to combine a steering gear motor with the approach I used for the Bruder loader conversion:
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http://rctruckandconstruction.com/sh...ad.php?t=12500
It has tank-style driving for the left and right wheels. So when you steer left, the right motors are run faster and the left motors are run slower on the front and the reverse on the back.
I'm thinking about combining these two for the ADT, so that the tank-style steering will help the steering gear motor turn the wheels, especially when standing still or going slowly. I'l thinking on ramping the tank-style steering on the wheels so that when going fast forward or backward, there is no tank-style steering but only normal full throttle on both the left and the right.
Best regards,
Stein :-)