Regarding the first statement in your question:
"I know that RC servo motors are designed for precise movement, rather than a D.C. motors' continual rotation. Are most RC servo motors limited to movement within one rotation or can they actually be made to continually rotate?"
A Continuous Rotation RC Servo is NOT a Servo
Here is why
What is a Servo
A Servo (Servomotor) is a motor with a position sensor and a closed-loop controller that adjusts the motor power to assure the motor is accurately held at the commanded position.
What is an RC Servo
An RC Servo is a small DC motor geared down to a drive shaft that has a potentiometer (rotary resistor) for its position sensor and is controlled by a pulse train. The width of the pulse (PW) determines the drive shaft position. The controller compares the PW to the potentiometer position and the motor is driven to compensate for the error. In a typical RC Servo, a 1.5 ms pulse is center position. For an example, in this case, the pulse is compared to the center resistance value of potentiometer. If the potentiometer is at its center value, no power is applied to the motor. If, however, the servo is clockwise (CW) of center then the potentiometer value will be lower and the servo controller will apply power to turn the motor counter-clockwise (CCW) to bring it back to center. The bigger the error, the more power will be applied to the motor. If the error is in the opposite direction, the motor will be driven CW.
The advantage of this design is that you can produce a very light weight servo out of small inexpensive components. The limitation of this design is that the travel of the drive shaft is limited by the rotational travel of the potentiometer. For RC that is not generally an issue as RC servos are usually used to drive control services that have very limited travel.
Why a "Continuous Rotation" RC Servo is not a Servo
As @Greenonline mentioned, you can modify an RC Servo for continuous rotation.
Note is the video what they guy does
- He removes the end stop that protects the potentiometer
- He cuts the wires from the controller to the potentiometer and connects a fixed resistor
So, going back to my description of an RC Servo, what does that result in?
- Since the position sensor (potentiometer) is gone, there is no longer a control loop so it is no longer a Servo.
- If you were to send in a PW of 1.5 ms, the controller would see the resistance at center point and would not apply power to the motor.
- If you were to send in a PW of <1.5 ms (commanding a position CCW of center), the controller would see the resistance at center point and drive the motor CCW to get there (which it never will) and hence it will rotate continuously CCW.
- Because there is no feedback control you will not be able to rely on the speed or accuracy of the motor; but, you will have a small DC motor that you can command using a RC Servo controller (or a using a PW)