Mechanics and Construction > Mechanics and Construction

First non-kit robot

**gamefreak**:

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 OZ-in, 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):

**Zeol**:

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 N-in = 14.1 N-in.

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.

**gamefreak**:

im confused ???

why did you chnage the OZ to Kg?

the page on dynamics said nothing about this sortof math and headaches involved.

**Zeol**:

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/non-roboticists/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.

**Admin**:

For unit conversions, use google - just type '1 ounce in kg'. For example:

http://www.google.com/search?q=1+ounce+in+kg&ie=utf-8&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a

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.

--- Quote ---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/non-roboticists/girls with all the hard work you can pretend to put into your really cool bot

--- End quote ---

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

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