It's very difficult or next to impossible to precisely control wheeled robot motion using open loop motor control. Many applications require accurately setting the pose or travel distance of a wheeled robot. Small continuous rotation micro servo motors are a great low cost solution to drive small robots but they lack the feedback control of larger servo motors.
Converting these little servos to use closed loop position feedback control is actually very easy once you add a Tamiya analog Encoder sensor and a simple closed loop feedback algorithm to an Arduino or Raspbery Pi controller.
This is a companion discussion topic for the original entry at https://community.robotshop.com/tutorials/show/add-an-encoder-to-feetech-micro-360-degree-continuous-rotation-servo-fs90r