Maximum linear speed
Maximum rotation speed
Yaw angle range
Pitch angle range
Maximum error in angle positions
Maximum error in spin rate
For many diehard baseball fans, the coming of spring heralds trips to the batting cages and a few sessions in front of a pitching machine. Since the first organized baseball game in 1846, the game has evolved into the moneymaking machine it is today. High-tech equipment is the name of the game from liquid-metal alloy bats that increase the "sweet spot" to advanced pitching machines. But tackling tough pitches, such as the spiral, is a constant challenge that not even the most high-tech pitching machines address.
Enter the Sports Biomechanics Lab at the University of California, Davis. This lab consists of undergraduate and graduate mechanical and biomedical engineering students led by Mont Hubbard, professor of mechanical and aeronautical engineering. The lab studies the mechanics of sports, including the motion of athletes and sports equipment, using mathematical and computational models.
One such research project focused on different spins on pitched baseballs, specifically slider and screwball pitches. A slider pitch falls between a fastball and curveball. The pitcher holds his palm in and twists his wrist when releasing, similar-to turning a doorknob. The ball breaks downward and to the left. Essentially a reverse slider pitch, the screwball pitch has the pitcher holding his palm outward, twisting, and releasing, forcing the ball to break downward and to the right.
Spiral spin is not possible with today's pitching machines. An automated full degree-of-freedom pitching machine, developed by graduate student Sean Mish and Hubbard, carries separate spin and propulsion mechanisms. By having complete control of the spin axis, a variety of spins can be studied including topspin, pure spiral, right or left spins, or any combination.
Here's how it works. There's a propulsion system to shoot the ball forward, spin mechanism to impart ball spin, spin-axis motors that control ball spin-axis alignment, and a support frame. The propulsion system is an air gun with a tank, a release valve and gate, and an outlet barrel. A release pin on an air cylinder holds the gate. When the air cylinder retracts, the gate swings open and pressure thrusts the ball down the barrel and propels it out of the tank.
The spin mechanism consists of a retractable suction cup, a dc motor with a tachometer and drive belts, and pneumatic controls. The suction cup is an aluminum cylinder fitted with a rubber O-ring. The ball is held onto the suction cup under vacuum and the cup extends a quarter inch when the ball attaches. Two air chambers deliver air pressure to control the ball release. A timing-belt drive system attaches to a dc-motor that maintains spins between 0 and 45 rev/sec. A tachometer provides closed-loop control of motor speed. Two control arms with bushings align the shaft and allow rotation. The spin mechanism grips the ball, spins it up to speed as well as controls the speed, releases the ball, and retracts out of the ball's path.
The spin-axis mechanism consists of two separate stepper motors that set and maintain the position of the spin axis. The Z-axis stepper motor provides 90° of adjustment of the spin axis. The X-axis motor rotates the barrel and spin mechanism on two bushings. Limit switches and encoders close the feedback loop on each motor for accuracy.
A pulse of pressurized air propels the ball out of the tank at speeds ranging between 60 and 100 mph. A thermodynamic model of the airflow to and around the ball correlates the set pressure in the tank to the estimated release speed of the ball. The air tank fills to the programmed level and a control loop consisting of a pressure transducer, analog/ digital converter board, and software maintains it. A horizontal gate valve seals the tank's 2-in. outlet pipe. The valve releases by an air-cylinder-controlled pin and the ball is on its way. Pitch and yaw frames align the support frame and barrel through dc servomotor-driven leadscrews. Each positioning system also has two limit switches and encoder feedback.
A controller from Z-World Inc., Davis, Calif., manages the system. The PK2200 is a C-programmable controller with an LCD keypad interface to input commands. The system uses eight expansion boards on a backplane: one each digital/analog converter and an a/d board; four stepper-motor controller/quadrature encoder counter boards; and two SPDT relay boards with six relays each. There are also five DPDT relays managed by the controller's digital out lines, three dc motors, two stepper motors, associated power supplies, and three fans to cool the stepper-motor power supplies.
Currently there are no plans for commercial use of the automated full degree-of-freedom baseball pitching machine.
University of California, Davis, Sports