How can i sense weight? i'm looking to sense upto 20 kgs which will be placed on top of a platform on my bot??
If you need to know how much mass, look at load cells. If you find a cheap one, let me know. 
If you just need to know if something is up there, a simple switch, possibly combined with a spring to set a threshold, may do the trick.
-j
If it doesn't need to be too accurate perhaps you could re-purpose a cheap electronics bathroom scale.
I always favour a pile of conductive foam with a suitable material to give some resiliency, like a spring or some more dense foam.
I have to find the exact mass... kind of an intelligent weighing scale.
Then as suggested a load cell would be a appropriate method. Load cells use embedded strain gage(s) and are measured in a resistance bridge circuit as a analog voltage.