I'm designing a compact tennis robot (ball launcher). I built a very decent prototype that is drastically lighter and smaller than what most others have built, is it's built using two brushless direct current motor (BLDC) motors which max out at 18000 RPM and provide ample torque on contact to allow for any type of tennis stroke/ball to be simulated. Each motors shaft has a single plastic (with rubber outline) wheel mounted directly on its shaft. The diameter of the wheel is 65mm.

However, being BLDC motors they are very loud and also more expensive/complicated due to also needing two Electric Speed Controllers (ESC's). This makes it very distracting on the field during a training session.

I noticed few others building traditional ball launchers two very similar plain DC motors that are actually very cheap:

or they use:

RS775 is louder (tho much quieter compared to BLDC) and not as reliable, XD63100 is very reliable and near silent but has lower RPM, but has a wider and longer shaft making it easier to mount things.

Test platform

I made the above test platform from old 3D printed parts I had available using the same wheels as on my BLDS prototype with diameter of 65. The wheels are close enough that ball can just barely pass with both wheels applying pressure on it.

This is what I noticed from the BLDC project its not the RPM that gives you distance but the torque, RPM gives you spin (rotation on the ball, faster RPM faster spin) but for distance you need to squeeze the ball and push it out.

However, with this platform I barely get 2-3m of distance and the ball is not really that powerful. The only difference however, that I can see between my test platform and other DIY projects is that I have much smaller and lighter wheels.

I assumed that since the wheel is directly mounted to the shaft of the motor the torque/rpm will be that at the shaft or near that but I'm not getting the performance I expected.

How does increasing the wheels diameter to something like in the linked project 100-250mm affect the contact performance (distance, speed, spin).

Also could it be I'm experiencing lower performance because I might have a too small contact surface ?

  • $\begingroup$ Bigger wheels tend to store more energy (radius is a big factor). Assuming your contact is good, that translates to more energy into the ball. $\endgroup$
    – Abel
    Sep 1 at 17:37
  • $\begingroup$ I suggest that contact area, contact time, and rotational inertia are all variables to consider. Your system has to accelerate the ball, so the lower the energy you have to take out of the system compared to the inertia in the system prior to ball induction, the easier it will be to implement. $\endgroup$
    – Tiger Guy
    Sep 1 at 17:37
  • $\begingroup$ @Abel is there a way to calculate the output speed/power of various wheel diameters? $\endgroup$ Sep 1 at 17:39
  • $\begingroup$ You'll need the torque vs speed curves of your motors, some approximation of friction and aerodynamic losses to get a max rpm; moment of inertia is pretty straight forward though from wheel alone. While this gets you energy, actual energy transferred to ball will need to model the compression of the ball and wheel as the ball gets squeezed out. en.m.wikipedia.org/wiki/… $\endgroup$
    – Abel
    Sep 1 at 17:47


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