PID Control using Image processing

[torchd]

Hi Guys
This is my first post in the forum. I am building a Golf Ball Collecting Robot that uses a camera to find the location of golf balls. I have a differential drive motors. I have already implemented the image processing and I am able to determine the the (x,y) coordinates of the ball. I was wondering if it is possible to implement PID control on my robot using just the coordinates of the ball and robot.( I don't have encoders on my motors). If I can, then can you guys give me any links that will help me.
(I have a reasonable understanding of Control Theory, I just have had no experience implementing it)
Thanks

[newInRobotics]

Hi here 

[...] ( I don't have encoders on my motors).

If there are no encoders, how is Your robot going to know how fast it is going, how far it went and where in the X - Y plane it is?

Encoders are useful if You want to compensate for different RPM of each motor, as even 2 identical motors will have slightly different RPM and torque.