There are really two possibilities I can think of:
- mount the sensor at the back of your robot, looking forwards over the top of your robot - if your robot is longer than 20 cm, then the minimum distance is at the front of your robot
- use a close-IR sensor to detect when the object is inside the min range. The Sharp sensors have an interesting property in that they start measuring backwards once an object is inside the min range, so you can possibly use a combination of a close IR sensor that is tuned to max at 20 cm, and the Sharp sensor.
- Jon