A Robot's sensor has to differentiate  between ball and a wall, which
economical sensor can i use in my robot?

An array or ultrasonic sensors would probably be a good start. Your
robot would have to determine from the data coming from the array if
the surface it is seeing is a straight edge or a curve. The real magic
is in the logic needed to do this!

An array of ultrasonic sensors would probably be a good start. Your
robot would have to determine from the data coming from the array if
the surface it is seeing is a straight edge or a curve. The real magic
is in the logic needed to do this!

hi 
u dont explain which type of rebort it is either it has camera on it
or not if yes then u should add differenciate program in it then your
work will be done ,if not then i think em waves is the best becaue it
will tell u better the quality of the substance like it is hard or
soft and of which thing it is made of ,and it will also tell u
distance and size and fezeqe more easily then ultra sound
 ultra sound waves are sound so it some time get absorb in some
substance so can damage your result