Need help with a school project

[alex643]

Alright so I'm new and I need help with a few things. I have been interested in robotics for a long time and I'm exited that I finally get to start building real robots. Right now I'm waiting for my parts to come for the $50 robot, back order sucks. Another project I'm working on is a school project which is to build a simple combat robot for my tech class. So far I have just about everything figured out for this project but I need some help with number crunching. My Bot is going to be a shell spinner and I need some help with how much I should gear the provided motors down to get good RPM and torque here are the specs for the motors I have, I have an unlimited supply of them so adding more than one is a definite possibility but I can't use anything stronger then them sadly these are those cheap 75cent motors so I  will need more than one. The shell of the robot is going to weigh about a kilo, so I'm wondering how much I need to gear down the motors to where they will be useful and still maintain a decent RPM.

6 volts .25 amps 17,000rpm 20.72 gr/cm

I know that if you have two batteries hooked up in series it increases voltage and in Parallel it increases the amps per hour but how does this work if I'm to hook up a 9.6 volt battery with 700miliimps to say, 4 motors? If i hook them up in parrellel would it just take more amps then meaning i would get about 42 minutes of power at 9.6 volts to the motors? or does it decrease the voltage?

Another thing I'm wondering about is CAD. What would you suggest to someone who is new in it and doesn't want to spend a boatload on the program?

 Any help is appreciated, thanks!

[szhang]

The power consumption of a motor depend greatly on the load, so is it .25A with zero load, or .25A stalled?  What is a "decent RPM" anyways?

Running a 6V motor at 9.4V will make it go much faster (but alot hotter too) and might burn it out.  In any case, wiring two 9V batteries in parallel would give you 9V, but it would last 2 times as long.

By the way, it'll probably be more useful to you to add more angular momentum to the shell than trying for a high RPM.

[gamefreak]

Does your school not have a cad program? My school has had autocad for forever and had recently bought Inventor for the engineering curriculum(woot).

[sotu]

You can allways use Google Sketchup.
A free software where the quality on your CAD result depends on your skill. You can make all from basic stuff, with easy colors, to advanced stuff with details.

[alex643]

Thanks for all the replies guys. No, my school doesn't have any CAD programs, they are just starting to get their tech program up after getting a new teacher last year but I asked and they will be getting something soon.

What I was asking with the voltage question is if you have multiple motors hooked up to one motor controller will the voltage drop or will it just take more amps from the battery as needed per motor?

[szhang]

What I was asking with the voltage question is if you have multiple motors hooked up to one motor controller will the voltage drop or will it just take more amps from the battery as needed per motor?

Yes, and no.

Chances are, as your current consumption goes up, unless you have a good regulated power supply the voltage will drop, how much depend on the current and you voltage source.  This happens because all voltage sources have a tiny internal resistance, and because V=IR, the higher the current, the higher the voltage drop across this resistor, and lower the output voltage of the battery.

But, as long as you can supply enough current, the voltage drop would not be insignificant.

For example, if your battery is designed to put out a max of 2A, and you try to use around 2A of current, the voltage will take a plunge.  However if you only use 500mA, there still will be a voltage drop, but not enough to matter.