Newsgroups: comp.robotics
Path: brunix!sgiblab!sgigate.sgi.com!odin!sgi!wdl1!mail!cps233!dombrows
From: dombrows@lds.loral.com (Brian Dombrowski, 5424)
Subject: tennis robot
Message-ID: <1994Apr19.182413.10317@lds.loral.com>
Sender: news@lds.loral.com
Reply-To: dombrows@lds.loral.com
Organization: Loral Data Systems
Date: Tue, 19 Apr 1994 18:24:13 GMT
Lines: 95


Hi Y'all:

I recently saw a 'robot' tennis ball server that oscillates side to side
lauching balls into both service courts.  The operator can set the
ball speed, ball feed rate, and launch attitude angle via ultrasonic
remote control.  It's very portable and uses a special sealed lead
acid battery for power.  It's an awesome tool for praticing 
and... it cost $1500!!!! 

The deal is that I think I can build one of these things with more
'brains' for much less.  The heart of the mechanical part of the
ball server is shown in figure 1.  I plan to use a 68HC11 to control
the thing.  However, being a computer engineer and not a mechanical
engineer, I need some advice on chosing the right type of motors to
meet my expectations.  Any help would be greatly appreciated.


Abstract:
---------

Tennis Ball Server:

Two electric motors each drive a 5" diameter wheel in opposite directions
( the left one clockwise, the right one counter clockwise.)  The wheels
are placed so that the distance between them is about 1/2" less than the
diameter of a tennis ball.  A ball feed ramp directs a tennis ball to
roll between the two wheels.  The ball is pinched between the wheels
and is shot out the other side at 60 MPH.  ( see figure 1 below )

The 5" diameter wheels need to turn at 4035 RPM to fire the ball
at 60 MPH ( math omitted in this post.)

The wheels that launch the ball act as fly wheels.  When the ball
gets pinched between the wheels, kinetic energy is transfered from the
fly wheels into the tennis ball.  This will cause the rotational
velocity of the wheels to drop.  It will take some time for the
motors to wind the wheels back up to 4035 RPM.  The time required to
do this is dependent on the amount of torque that the motors can
deliver ( I think.)

This is my problem.  Suppose I have a maximum time I want to wait
for the RPMs to recover to the rate needed to serve a ball at 60 MPH
( Trecover ).  The wheels each have the same mass ( Mwheel ) and
each motor is rated to deliver torque ( TQmotor ).  I want to
ignore friction in my calculations.

Can someone with a fairly fresh memory of dynamics tell me 
how to figure what TQmotor needs to be based on fixed values 
of Trecover and Mwheel ??

What would help greatly would be function that gives fly-wheel RPM 
versus time for a constant torque and fly-wheel mass.  


I know it would be ideal to use huge powerful motors but I want
to keep the cost down.  For this reason I'm willing to sacrifice
ball delivery rate in favor of saving lots of $$$$.

Thanks in advance

Brian D.
Sarasota FL


			FIGURE 1
		--------------------------


			Tennis Ball
			  Feed

			|	|
			|   |	|
			|   |	|
			|   v	|
			|	|
		   .>.		    .<.	
		."     ". 	 ."     ".
	       .         .      .         .
	       .    O    .      .    O    .  <---------  Two motor driven wheels
		.       .        .       .		 each rotating opposite
		 " .<. "          " .>. "	         directions.
		 
	          <---              --->	


			   |  |
			   |  |
		           v  v
				   Ejected Tennis Ball  ( 60 MPH )
			   ."".
			  (    )
			   '__'

