I have a Robotic platform controlled by Arduino .
since I am new to Arduino , how do I make my platform turn exactly 90* left or right??
To answer that "a Robotic platform" is just not enough info about your hardware...
how do I make my platform turn exactly 90* left or right?
You initiate a turn. Periodically, you check how far you have turned. If you have turned far enough, stop turning.
So, what are you checking?
how do I check how far it has turned?? I do not know
how do I check how far it has turned?? I do not know
You need something that can provide feedback - a compass, a GPS, or ?.
You can NOT guess how far you have turned and expect to turn exactly 90 degrees. Without feedback, you can expect to turn 90 +/- 10000 degrees. (No, the decimal point was NOT inadvertently omitted.)
I totally forgot..... there are 4 ultrasonic sensors on each side which periodically takes values .
BrainStormer0906:
I totally forgot..... there are 4 ultrasonic sensors on each side which periodically takes values .
How about telling us all about your Robot so we don't just occasionally get little facts when you suddenly remember.
How do you expect ultrasonic sensors to measure rotation?
...R
BrainStormer0906:
I totally forgot..... there are 4 ultrasonic sensors on each side which periodically takes values
I don't think those are going to be sufficient to determine an exact 90° turn. Perhaps a LIDAR device would be sufficient. They are only about $400.
You could use a compass chip to get fairly close to 90°.
With a more capable platform you could use visual flow in a 360° camera to turn 90°.
Options are limitless, but please do as told you in reply #1 
If you know how big the wheels are, and they are on steppers or you have a quadrature encoder on the shaft so that you can measure how far they turn, then turn the wheels and stop turning them when you hit the angle you want.
PaulMurrayCbr:
If you know how big the wheels are, and they are on steppers or you have a quadrature encoder on the shaft so that you can measure how far they turn, then turn the wheels and stop turning them when you hit the angle you want.
That, incorrectly, assumes that the wheels don't slip.
Hi,
Can you post a picture of your RoboticPlatform.
Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html
Thanks.. Tom.. 
PaulS:
That, incorrectly, assumes that the wheels don't slip.
Ok: ", and you can assume that the wheels don't slip (or that slipping is negligible)"
aka: "assume a spherical chicken …".