In our design, ping pong balls first drop down a chute and slide into two spinning wheels which launches the ball towards the goal. To initiate this dropping action we use a servo operated trap door described below.
Servos are controlled by sending a PWM signal. The parameters for this pulse are minimum pulse, maximum pulse, and period. Generally minimum pulse = 1ms, maximum pulse = 2ms, and period = 18-20ms. The angle of the servo arm is determined by the duration of the pulse that is applied to the control wire. For example, a 1.5ms pulse will make the motor turn to the neutral position (90 degrees). This was true for our servo. For example code see the Shooting files in the github repository.