How would i make a turret oscilate up and down at say 0 - 70 degrees without having to reverse the polarities to go either way?
I will only be using one motor.
Simplistic is the best answer.

servo or a cam
_________________
Mechanical engineers build weapons, civil engineers build targets

I would say a really low RPM gear motor and a simple machine called a cam.
http://en.wikipedia.org/wiki/Cam

_________________
( •_•)

( •_•)>⌐■-■

(⌐■_■)

YEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH

Something along the lines or that?

Another question: Whats a good capacitance for a drive motor in order for steady acceleration. (I have tracks, not wheels) NO ESC.

Servo - possibly modified for endless rotation. or a motor with a crank linkage such as used in windscreen wipers or steam engine drives.

EDIT: a crank and cam are very simlar, but the crank is easier to make and doesn't rely on gravity for the turret to follow the cam.
_________________
Australian 2015 Featherweight champion
UK 2016 Gladiator champion

yeah my suggestion would be a crank like the windscreen wipers.. least you have a positive force to bring it back down.

and making two pieces of flat steel for the cranks is easier than grinding out a cam anyway
_________________
www.demon50s.com - Minimoto parts
http://www.youtube.com/user/HyzerGlen - Videoooozzz

you aren't going to be able to use a cap to give you a slow start on a motor.
not unless its measured in farads.
_________________
Mechanical engineers build weapons, civil engineers build targets

Assuming the Tank is RC-Controlled - Why not just use a servo to directly control the turret elevation through a linkage ?

That way it doesnt just go up and down in a cycle, you can actually control it (if you have a spare RC Channel) - This place has some interesting high power servos and linkage and stuff - http://servocity.com/

If battery life/efficiency isnt as critical as it is in a combat robot (where carrying more battery power than you need is a waste of weight), the cheapest way to do soft-start without an ESC, is something like the Tamiya 3-speed resistive speed controller - provided your drive motors dont suck too many amps.

You could even cobble a more variable one up out of a big wirewound potentiometer (maybe a dashboard dimmer out of a 70's car ?) and a servo. Probably be ok for 10 - 20 amps if you dont get into a pushing/stalled motor situation.

How big is this tank and its motors ?
_________________
Great minds discuss ideas. Average minds discuss events. Small minds discuss people

Maybe use one of those $5 Jaycar solinoids to raise the turret? on/off switch is all you need then
_________________
https://www.halfdonethings.com/