Butterfly Table Tennis Question and Answer
Question: Is it true that the Amicus Prime throwing wheels can go in the opposite directions to give a higher level of spin? – Mike
This Ask the Experts question was answered by Larry Thoman. He is a Table Tennis Robot Guru. Larry’s response is below:
Yes, that is correct. The Amicus Prime allows their wheels to spin backward. All the other models do not allow the wheels to spin backward. Therefore they cannot do high spin/low speed shots like a slow loop or ghost serve.
This is actually a very important distinction for the Amicus Prime. Very few robots allow their wheels to spin backward. It’s a tricky thing to try to control. You cannot ever allow a backward spinning wheel(s) to overpower the forward spinning wheel(s). Why? Because if you do, the robot will be trying to push the balls back into the feed channel, which of course, will result in a lot of service calls about the robot not feeding. However, if done correctly with the forward force more powerful than the backward force, the backward wheel increases spin and decreases speed. If that same wheel is spun forward, then that wheel decreases spin and increases speed.
Try this experiment with the Amicus Prime to understand how the wheel direction and speed affects the spin on the ball:
Start with no spin (Spin 0) and have the robot throw several balls to FH or BH at a comfortable feed and frequency. Then increase spin to 1, and have the robot throw several more balls (you don’t have to stop ball feed to do this). Continue on by increasing spin by one unit, returning several balls at that setting until you get a feel for the spin, then advance to the next spin level. When you get to 5, that will be the most spin & speed that the robot can throw. Up to this point what has happened is that all three wheels are spinning forward. Each time you advance to a higher spin setting, the top wheels are spinning a little faster and the bottom wheel is spinning a little slower. When you reach 5 then the bottom wheel is held in a static position (not spinning at all). So there is increasing forward force applied to the top of the ball and less forward force to the bottom.
Now increase spin to 6. You will notice a big difference in the spin and feel of the ball. At 6, the bottom wheel is spun backward instead of forward. This results in significantly more spin on the ball, but also you will notice that the ball is slower. And at 7, the bottom wheel is still spinning backward, but at a faster rate. So you should again experience an increase in spin and a decrease in speed. When the bottom wheel is spun backward, both wheels are making the ball spin in the same direction. And since that force applied to the ball is in a backward direction, it is essentially producing a negative forward force, which is what is making the ball travel slower.
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