If to be able to sense the Force you want your robot, an excellent choice this HiTechnic Force Sensor is. Hmmmmm. .. Due to come out in early October, this little bad boy can measure the amount of force applied to an axle that presses into the axle receptacle (image below left), which you see in the front of the housing. It is very easy to program for as it works just like a regular analogue sensor, and gives a very linear response to the amount of force, for values of greater than about 50 grams (image below right). It is super sensitive and will register weights, starting at about 1 gram, although there may be a small amount of variance between sensors due to the analogue nature of it.
I should be receiving one of these to play with in about a week or so and I am really looking forward to taking a close look. I’ll keep you posted!