Society of Robots  Robot Forum
Mechanics and Construction => Mechanics and Construction => Topic started by: gamefreak on January 18, 2007, 09:58:35 PM

Well i have started on the design of a robot just to see if i can go through with it. I have a basic stamp board of educattion made by Parallax. I also have 2 hitec continuous rotation servos. I plan to use these with my new design.
Now for velocity the site says: diamter*PI *rps
I know the torque of my motors is equal to 60 rpm when unloaded. So i am going to assume that the rpm will be roughly 30 to 40 when i construct the bot.
The wheels are going to be 3 inches in diamter so: 3*PI*2/3=6.283185307, what does this mean? 6 feet per second?
Also the torque is equal to 47 OZin, so 47/3= 15 2/3 multiply that by the RPS then my RMF = 10.444
so i need my RMF to be equal to or greater then my Mass times my Acceleration times 6.28/(2*PI).
Is all of this right? and how do i figure out ym acceleration?
So far i have a rough idea of my robot and here is its design so far(the drill hole is for my castor and the two side boxes are the servos):
(http://i9.photobucket.com/albums/a90/monkey4sale/robot1.jpg)

The velocity of the robot will be in inches per second ( you have 3*PI inches/round * 2/3 rounds per second = 2*PI inches per second)
If one OZ is about 0.03 kg (what do i know, i'm european) and one kg is 10ish Newton then the torque is about 47*0.03*10 Nin = 14.1 Nin.
With one inch being 0.0254m the torque becomes 14.1*0.0254 Nm = 0.36Nm.
If you divide the torque by the diameter of your wheels (3*0.0254 m) you get the force each motor is pushing your robot with: 0.36/(3*0.0254) = 4.7 N. Since you have 2 motors, you double this number and get a total force of 9.4N. We then apply Newtons 2nd law:
F=m*a => a=F/m, in other words, divide 9.4 with the mass of your robot (in kg) and you will get the acceleration in m/s^2. Divide this by 0.0254 and you get it in inch per second square.
This is the maximum possible acceleration (with these parameters), and the actual acceleration will be reduced due to friction, variable motor torque and evil little gnomes.
Warning
Physics ws never my strong side, so feel free to verify/falsify. An sorry for using the Metric system.

im confused ???
why did you chnage the OZ to Kg?
the page on dynamics said nothing about this sortof math and headaches involved.

Not to worry, the headache math will probably only give approximate results anyway, so i don't think you really need them to build the robot. It's just nice stuff if you want to impress teachers/nonroboticists/girls with all the hard work you can pretend to put into your really cool bot ;D At least you know that the velocity will be 6ish inches per second.
I changed OZ to Kg because i know what Kg is (I have to many of them) and not really what OZ is, unless theres a wizard there.

For unit conversions, use google  just type '1 ounce in kg'. For example:
http://www.google.com/search?q=1+ounce+in+kg&ie=utf8&oe=utf8&rls=org.mozilla:enUS:official&client=firefoxa
As for calculations, yea they can be a pain and confusing. So I wrote up an excel sheet that does it for you:
http://www.societyofrobots.com/downloads/RMF_calculator.xls
Just make up and enter your desired specs of your robot in the first section. The second section will then calculate how powerful your motor needs to be to achieve those specs. For the third section, find motors you like online and enter in the torque and rps. If the RMF in the third section is greater than the calculated RMF in the second section, then you can use that motor.
headache math will probably only give approximate results anyway, so i don't think you really need them to build the robot. It's just nice stuff if you want to impress teachers/nonroboticists/girls with all the hard work you can pretend to put into your really cool bot
For small cheap robots that use servos, you dont really need to do any calculations. But what if you were going to spend say $300 on motors, wouldnt you like to know they would actually work on your robot first (as opposed to guessing and wasting that money)? :P
Calculations let you optimize for peak performance, instead of using guess work and 10x the amount of time in tweaking and redoing components. You can actually know how well your robot will perform before its even built! :P

sorry if im sounding dumb, but it doesnt say what the units are, ex. OZ,Kg,Pd,In,M
It makes me more confused because i dont know what the units i should be typing in is.
So far i figured that to keep my wheels at the 3 inces in diameter then my velocity is 6, so i go 6 feet per second with a 3 inch diamter wheel going at an RPS of 2/3?

hey gamefreak,
so for working out your speed, the first thing you want to do is convert every thing to the same units.
so for example, if your motor turns at 40 RPM, that is 40 Revolutions Per Minute.
and the diameter of your wheel is 3 inches so the circumference (ie, the distance moved if your wheel goes one complete rotation) is: 3*PI=9.424719 inches.
so 40 Revolutions Per Minute * 9.424719 inches = 377 inches per minute.
377 inches per minute. / 60 = 6.28 inches per second
6.28 inches per second / 12 = 0.5 feet per second.
hmm, i'm starting to confuse myself.
dunk.

dunk, the circumference of a circle is PI times the diameter, period...
 Jon

hehe. yes. sorry. tired.
i'm editing my original post now in the hope no one will ever know...
dunk.

i missed dunks mistake? DARN, i wanted to take a picture ;D,
And .5 feet per second sounds pretty fast, course i would love to double that (thinks of evil uses) to help humanity.
Also this is all imaginary, i dont know the RPM, or how to find it for that matter, once i actually build the bot. I know im going to use HDPE and it will be about an 8*6.6 and 3/16 inches thick, Not to mention breadboard,servos, maybe radio controll and im clueless on the weight.
Withen 2 months i want to either have an animatronic or an RC robot for TSA, but im unsure of which to do.