Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: jellis359 on February 20, 2009, 05:42:22 PM

Title: help please- i'm building a tennisball pitching robot??
Post by: jellis359 on February 20, 2009, 05:42:22 PM

Hey, I'm currently planning the construction of a tennisball pitching machine for my dad's dog. I was going to make the barrel by using some 4'' wide pvc pipe(i say wide because i'll be cutting it in half). On the back end will be a continuous rotation servo w/ a pushwheel attachment like on many soccerbots. this will push the tennisball into the spinning wheels when the ball hits the pressure sensor (1 wheel spins, the other is freely rotating to provide the ball stability).
  Now, heres my dilemma. I've only ever used servos which only require a connection to the mcu. I know this is not the case with a brushless d/c motor, which is what i must use in order to launch tha ball properly. I know there are extras like and encoder, H-bridge, and motor controller needed in the assembly. from what i understand it goes-

MCU--> motor controller(w/ motor battery)--> H-bridge-->d/c motor

MCU------------------------------------------------------------>encoder---> D/C motor

I believe that this MCU should do the job w/ WinAVR-->(i need a cheap controller w/ some good features)
http://www.trossenrobotics.com/roboduino.aspx?a=blog (http://www.trossenrobotics.com/roboduino.aspx?a=blog)
and i figured this d/c motor should do the trick-->


i'll put up a googlesketch image soon

right now i'd just like some direction and to see what you fine, experienced engineers think.

THANK-YOU-THANK-YOU-THANK-YOU-THANK-YOU  ;)

Title: Re: building a tennisball pitching robot
Post by: Razor Concepts on February 20, 2009, 06:46:36 PM

Should be this:

MCU--> motor controller(w/ motor battery)-->d/c motor

An H-bridge is a kind of motor controller.

Title: Re: building a tennisball pitching robot
Post by: jellis359 on February 20, 2009, 06:55:30 PM

do you think an H-bridge could be as efficient, its kinda cheaper (like 30 dolla) ::)

Title: Re: building a tennisball pitching robot
Post by: MaltiK on February 20, 2009, 08:50:15 PM

Quote from: jellis359 on February 20, 2009, 06:55:30 PM

do you think an H-bridge could be as efficient, its kinda cheaper (like 30 dolla) ::)

As Razor said, an H-Bridge is a type of motor controllers.

Also, why do you need an encoder?
BTW, that motor doesn't have enough RPM to make the ball go anywhere more than 2 ft at a 45 degree angle with the horizon. What you can do is use a Trajectory Motion chart to figure out the required velocity for your specified distance, which is directly proportional to the RPM. And your MCU is kind of an overkill.

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: Jdog on February 21, 2009, 01:38:31 AM

You could probably use the $50 robot mcu if you feel like putting in the time/effort. H-bridges would also probably be cheaper than other types of motor controllers and are all you really need for this type of project. Check out this http://www.societyofrobots.com/schematics_h-bridgedes.shtml.

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: HDL_CinC_Dragon on February 21, 2009, 02:04:16 AM

Heres a question:
Why bother using an MCU and servo at all?
Why not just have a switch directly connected to the motor(s) and just set it up so that the ball rolls itself into the spinning wheels since theyre constantly spinning anyway? Youll save yourself lots of time and money.


BTW, an encoder is used to help a microcontroller to figure out the RPM and/or distance of travel for a wheel/robot

-EDIT-
Also, if you still want to use your servo idea, you can just use a pair of 555 timers(or one 556 timer) to generate the frequency needed to tell the servo to move forward and back since it will be following the same path every single time.

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: MaltiK on February 21, 2009, 06:24:59 AM

Quote from: HDL_CinC_Dragon on February 21, 2009, 02:04:16 AM

Heres a question:
Why bother using an MCU and servo at all?
Why not just have a switch directly connected to the motor(s) and just set it up so that the ball rolls itself into the spinning wheels since theyre constantly spinning anyway? Youll save yourself lots of time and money.


BTW, an encoder is used to help a microcontroller to figure out the RPM and/or distance of travel for a wheel/robot

-EDIT-
Also, if you still want to use your servo idea, you can just use a pair of 555 timers(or one 556 timer) to generate the frequency needed to tell the servo to move forward and back since it will be following the same path every single time.

I understand that, however, what is the use of one where the motor spins for a couple of seconds? Encoders are usually used on drive motors, not something like this

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: HDL_CinC_Dragon on February 21, 2009, 10:36:56 AM

Quote from: MaltiK on February 21, 2009, 06:24:59 AM

I understand that, however, what is the use of one where the motor spins for a couple of seconds? Encoders are usually used on drive motors, not something like this

That wasnt for you, that was for Jellis ;)
That was so hed know why one isnt needed here

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: jellis359 on February 21, 2009, 12:53:46 PM

thanks guys
it is supposed to be automatic, so it can't spin constantly (for power consumption and dog safety reasons)
i would like it to spin only for a set time after the ball hits the touch sensor so that if for some reason the dumb dog puts sumtin else in it, it won't break anything
sorry i put the wrong motor in last time:http://www.trossenrobotics.com/store/p/5142-FIRST-CIM-Motor.aspx (http://www.trossenrobotics.com/store/p/5142-FIRST-CIM-Motor.aspx)
this one has the highest rpm to torgue ratio i could find for 30.00 ( by my calcs- it should be able to throw the ball over my expected 11ft)
it has ~88r/s

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: ArcMan on February 21, 2009, 01:32:15 PM

That motor is a bit of a butt-kicker, so I agree - It should launch a tennis ball nicely.  But it's not just about the motor.  You're going to need some momentum from your launching wheel or it will tend to stall when the tennis ball hits it, giving you poor distance.  You will need a decent amount of mass on that wheel, but not too much or it will take too long to get up to speed.  What are you planning on using as a launch wheel?

Also, when that ball hits the wheel, your motor current is going to spike (the less massive your wheel is, the more the current will spike).  It could spike to > 100A.  I hope you plan on dropping a few hundred dollars for a motor drive that can handle that.

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: MaltiK on February 21, 2009, 04:06:04 PM

Quote from: jellis359 on February 21, 2009, 12:53:46 PM

thanks guys
it is supposed to be automatic, so it can't spin constantly (for power consumption and dog safety reasons)
i would like it to spin only for a set time after the ball hits the touch sensor so that if for some reason the dumb dog puts sumtin else in it, it won't break anything
sorry i put the wrong motor in last time:http://www.trossenrobotics.com/store/p/5142-FIRST-CIM-Motor.aspx (http://www.trossenrobotics.com/store/p/5142-FIRST-CIM-Motor.aspx)
this one has the highest rpm to torgue ratio i could find for 30.00 ( by my calcs- it should be able to throw the ball over my expected 11ft)
it has ~88r/s

Now thats a hell of a motor, and very cheap!

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: HDL_CinC_Dragon on February 21, 2009, 11:04:02 PM

These motors are AWESOME! Ive been looking for these very motors for like 3 years now....... we used them as the drive motors for our robot in the FIRST robotics competition at my old high school. Very powerful especially with the right gear box :)

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: HyperNerd on February 22, 2009, 02:07:47 AM

the stall torque on that motor is 133 amps :o ! That is gonna take one hell of a motor driver to not spontaneously combust when the ball touches the wheel!

Instead, you could try using some kind of air pressure cannon, like my spud gun I made a few years ago:
(http://i584.photobucket.com/albums/ss286/HyperNerd/IM000025.jpg)

(http://i584.photobucket.com/albums/ss286/HyperNerd/IM000026.jpg)

Easy to build and pretty fun shooting pencils at cereal boxes ;D

 -HyperNerd

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: MaltiK on February 22, 2009, 06:58:18 AM

Quote from: HDL_CinC_Dragon on February 21, 2009, 11:04:02 PM

These motors are AWESOME! Ive been looking for these very motors for like 3 years now....... we used them as the drive motors for our robot in the FIRST robotics competition at my old high school. Very powerful especially with the right gear box :)

I think your thinking of MagMotors, they look the same but are not, they have soo much higher torque, and will run you about +$100

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: HDL_CinC_Dragon on February 22, 2009, 05:23:34 PM

nah they were definitely these CIM motors. 100% sure of it. I remember the name of them but for w/e reason when I did a google search for "CIM motor" nother ever came up O.o

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: Admin on March 01, 2009, 07:08:08 AM

A motor driver is an H-bridge plus additional electronics, like thermometer, servo PWM to motor output converter, etc.

Just like a servo is a motor but with additional electronics.

Title: Re: help please- i'm building a tennisball pitching robot??
Post by: jellis359 on March 04, 2009, 08:02:58 PM

I was planning on using a solid black urethane rubber wheel
3.22oz.
2.5"diameter

i want to use solid urethane so i can mold it later (maybe to better fit the ball)