Hello guys,
I would use the IR sensor fc-51 as in the picture to determine the distance from an obstacle.
I don't find on the web its datasheet for the characteristic voltage-distance.
Have you this datasheet?
Alternatively, if you have used this sensor for this purpose, I'd like to see your c code. 
Thank you all.
Sadly for you I suspect your sensor has only a digital output- looks similar to Obstacle Avoidance Infrared Sensor Module Reflection Photoelectric Sensor Board for Tracking Smart Car. You wont be able to get an analog signal to estimate distance unless you tap into the internal wiring of your module. Even then its likely to be difficult- the IR sensor is designed for an ON/OFF signal.