I am working on robot and i want to ask How can i use IR sensor as obstacle detection and cliff detector?
Your question is very broad. There are a lot of different ways your might do something like that with a variety of IR sensors. You might look over the IR sensors we carry, and post specific questions about what you are using in your system (e.g. I have an Arduino Uno I am using with a Sharp GP2Y0A41SK0F Analog Distance Sensor; how can I read the values from this sensor?). More tips for making good opening posts can be found in this forum post.