Development of Active Dribbling Mechanism for RoboCup SSL Robots
Ball control and handling, also known as dribbling, is one of the fundamental design problems of any soccer playing robot. The dribbling mechanism can be categorised into active and passive dribbling. Passive dribbling is simply dragging the ball with the bot and it can easily lose contact if proper enclosure for the ball is not provided, while in case of active dribbling it can dribble the ball in any required direction and it gives greater ball handling capability.
According to rules of RoboCup SSL the robot cannot cover more than 20% of the ball’s projected area as seen from the top. Neither can it bound the ball such that the opponent robot is unable to take it away. We brainstormed on various possible mechanisms such that the rules are not violated. Well, it turned out that rollers imparting backspin to the ball would be the best and time-tested mechanism for the purpose.
Physics Behind the Scenes of a Horizontal Roller Dribbler
A roller mechanism consists of a horizontal or angular cylindrical bar which is driven by a motor and gives backspin to the ball. The concept is very simple, the cylindrical dribbler rod continuously rotates at a high rpm and when the ball comes in contact with the roller it is imparted a backspin so that in the case of momentary loss of contact with the ball – for instance during deceleration – the ball will move back towards the robot, allowing the robot to regain control of the ball.
A free-body diagram of a horizontal dribbler mechanism is shown on the right. In order to impart the greatest amount of reverse spin to the ball, the friction force at the dribbler should be maximized. This means maximizing the coefficient of friction and also maximizing the normal force applied by the dribbler. Also, it can be noted that in this state of motion the ball is not rolling, but rather slipping on the floor.
The coefficient of friction is determined by choice of dribbler material, and various foams and rubbers have been tried out for this purpose.
The normal force applied by the dribbler bar is a function of the geometry of the bar and the friction at the floor-ball interface. Thus it can be easily seen that maximum normal force is obtained when the dribbler rod is vertically above the ball. But the Robocup SSL rules restrict that no more than 20% of the ball’s area as seen from the top can be enclosed by the robot.
Other Design Considerations
If we dig deeper into more efficient dribbling then we can design a mechanism that can enable a robot to dribble the whole length of a field and score a goal. This can be achieved using two dribbler rods inclined at an angle. But new SSL rules restrict the use of any dribbler except for the horizontal ones. Thus in order to increase dribbling capabilities, there has been major lead towards self-centring mechanisms for horizontal dribbler. Figure on the left illustrates two such mechanisms – this dribbler has a corkscrew-shaped surface, which tends to centre the ball when moving straight ahead, and a gap in the middle of the bar, which provides a better ball handling ability when turning.
Dribbling mechanism is a basic yet an important design challenge. Over the years the mechanism has undergone many changes and we thrive to optimise the design to achieve the maximum capabilities in the months to come.
Ankush Roy 