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Speed control with continuous rotation servo?

Hi everyone, I have a quick question regarding servos.

I've seen many similar posts around the internet but remain a bit confused. I have 2 standard, unmodified Hitec servos. I am using a Picaxe 28x1 to control these servos. I plan on using these servos as the motors that turn the wheels on my robot, and plan on modifying them for continuous rotation the most non-destructive way, without removing any circuitry. After modifications, I plan on simply sticking wheels to the servo horns and connecting my servos, coupled with appropriate resistors, to the digital output pins on my Picaxe.

The question however, is: Pretend I successfully modify and calibrate my servos, and after connecting them, if I write a line such as SERVO 2, 175, instead of turning to that position, it will instead start to spin in a certain direction at a certain speed. And then if I were to put SERVO 2, 160, will it continue to spin in the same direction but at a slightly slower speed? And then with a SERVO 2, 125, it will spin in the opposite direction at a speed faster than if I put SERVO 2, 140? So can I speed up and slow down my individual servos by simply changing the degrees of rotation? My final goal is to make the robot smoothly turn using the speed of the wheels, instead of turning them opposite directions to rotate my robot.

Sorry if my theory's way off, but don't worry about terminology and such. This isn't my first bot, just the first with this little twist... 

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So when I want to turn the robot, I could do something like SERVO 1, 170 and SERVO 2, 160 to make it turn towards the side Servo 2 is on since it will go slower than Servo 1?

Ok, thanks a lot... one more thing though... does this apply to all standard servos, and would you happen to know whether in particular the Hitec 311 servo would be able to function like this?

Cant you simply desolder motor on circuit board and wire motors direct. You could leave the circuit in the same place and later reconnect if needed, then pwmout/duty command, although you may have to mess with the servo stop on the gearing. Just a thought as thats what i have done but in my case i have removed pcb all together.

Well, my original post was pretty old, and I ended up not even using servos. But, my purpose for using servos instead of just plain motors was so that I could use the servo's circuitry to control motor speed without having to use PWM or a dedicated motor driver. But in the robot I needed this for, I ended up just using plain motors and using PWM to control them.