Autonomous Foosball Table

Step 2 Servo Motor Assembly

Two different types of servos are used. First, the lateral motion is controlled by a group of four high-torque servos: Robotis Dynamixel Tribotix AX-12. These four run on a single serial line and provide amazing functionality. The high torque enables these servos to be geared in such a way that provides a high tangential velocity for lateral movement. We were able to find a set of 3.5 inch gears and tracks to go with them from Grainger at a cost of about $10 for each. The servos provide torque overload protection, an individual servo addressing scheme, quick communications, internal temperature monitoring, two-way communications, etc. The downside of these servos is that they're expensive and not very fast (though gearing helps them). So in order to get faster motion for kicking, Hitec HS-81s are used. The HS-81s are relatively inexpensive, have a decently fast angular velocity, and are easy to interface (standard PWM). The HS-81s only rotate 90 degrees, however (though it is possible -- and not recommended -- to attempt to modify them to 180 degrees). Additionally, they have internal nylon gears that strip easily if you attempt to modify the servo. It would be worth the money to find a 180 degree rotating servo that has this type of angular velocity.

The entire system is tied together with pieces of medium-density fiberboard (MDF) and high-density fiberboard (HDF). This was chosen for its low cost (~$5 for a 6'x4' sheet), ease of cutting, and ability to interface with virtually any surface. A more permanent solution would be to machine aluminum brackets to hold everything together. The screws holding the PWM servos in place are standard machine screws (#10s) with hex nuts holding them from the other side. 1mm metric machine screws, about 3/4" in length, hold the AX-12 into the MDF that connects the two servos together. A double-action drawer track holds the entire assembly down and in-line with the track.