The robot developed by Swiss Institute of Technology is able to juggle a ball without sensors, just by the mechanical feedback from the bouncing ball, and mathematical calibration of the machine, without using any sensors.
The two variables, the force that the ball exerts on the tray (therefore the height of ball bounce) and the location which the ball hits the tray are constantly measured. The tray is also slightly concave, which helps to keep the ball on it in instances where it bouncess off the center line.
The height of ball bounce is controlled by the tray accelerating or decelerating at the top point at correct instances, such as hitting the ball at a higher point than previous bounce, to give it a boost, or lower to decrease the bouncing height. The constant mechanical feedback from the force that is applied by the ball to the tray help the machine adjust the force accordingly.
Measuring these two variables required the team to make precise mathematical calibrations to the machine. Making instant corrections helps the machine to keep the ball at the desired height and location, even when the variables change, as can be seen from the video below.