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Author Topic: My first "Robot"  (Read 3436 times)

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Offline gamefreakTopic starter

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My first "Robot"
« on: November 05, 2007, 06:49:31 PM »
Hey i've finally done it, i've finally started on my first "robot", it is RC and will hopefully get me a full ride to florida this summer.

It is an RC hover car for a robotics club competition and a TSA event, it will be made out of styrofoam and balsa  wood. I will post the items and price list later.

This is a 3D model i've made using solidworks, there are a few things missing such as wires and a way to stop the motor from floating in the air. I intend to hook strings to the servo and have them go to either side to steer the rudder.

it is basically a 18,by 9, by 8

Do you think that this duct is large enough, or should i go about half? Also, should i just use some stryofoam from a hobby store or is there a better one out there?

« Last Edit: November 06, 2007, 08:17:00 PM by gamefreak »
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Re: My first "Robot"
« Reply #1 on: November 07, 2007, 08:13:33 AM »
be careful to make sure the weight is distributed . . . a big fan motor in the back will make your robot butt-heavy :P

I'd probably make the duct longer, but its just based on my intuition of fluid flow . . .

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #2 on: November 07, 2007, 08:33:10 AM »
by longer what do you mean? do you mean go further back?

Also does the duct need to be bigger? I took a old computer power supply ripped the fan blade out and attached it to my motor, the fan i sprobably 5 inches in diamter and has 5 or 6 blades(sorry i cant be more descriptive, i dont have it in front of me now) So far im up to 10-12 cuts on my finger from a lack of clamps and having to hold it by hand to test it, those fans are sharp.
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Re: My first "Robot"
« Reply #3 on: November 07, 2007, 09:01:12 AM »
Quote
by longer what do you mean? do you mean go further back?
The point of a duct is to be a 'flow straightener' - meaning it directs flow in a single direction.

You want the duct to be just slightly larger than your fan diameter in width, and not any more so the flow doesn't 'escape'.

A longer tunnel can straighten flow better . . . but too long creates too much air resistance . . . too short and your flow isn't straightened.

So what I meant was, make it longer (not diameter but the other dimension).

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #4 on: November 07, 2007, 11:28:57 AM »
alright but what about the duct on the bottom that takes thrust and turns it into lift.
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Re: My first "Robot"
« Reply #5 on: November 07, 2007, 12:48:38 PM »
The reason you want to straighten the flow is to direct the air flow in a single direction - thereby directing thrust in one single direction.

However with the airbag under your craft you aren't creating a vectored thrust, just keeping the air pressure high enough to lift the weight of your bot. In this case you want the diameter to again be the same as your prop (to reduce air leakage), but also air does not need to be straightened. A straightener would just interfere with airflow in this case.

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #6 on: November 07, 2007, 08:10:12 PM »
but do you think it needs to be taler then it is now? also, I'm planning to go from a seven inch airplane prop, to hopefully a 10 inchish computer fan(where do you acquire these? a techie at my school said he might have one i could have...) now due to the fact that I've sliced my fingers open 8 times now in 1 day with a 7 inch(which i shattered) any safety ideas for testing purposes?
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Re: My first "Robot"
« Reply #7 on: November 07, 2007, 08:18:07 PM »
Quote
but do you think it needs to be taler then it is now?
if I were to do this project, I'd use a seperate fan laid horizontal in the smack middle of the vehicle to fill the air bag.

Quote
due to the fact that I've sliced my fingers open 8 times now in 1 day with a 7 inch(which i shattered) any safety ideas for testing purposes?
lol . . . get a bendable wire mesh to put over the blades . . . you can get some at any craft store . . . or bend and solder a frame out of hanger wire . . .

Quote
a 10 inchish computer fan
think it will have the thrust you need?

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #8 on: November 07, 2007, 08:45:39 PM »
according to my physics teacher(hes the one that shatter the fan and had shrapnel shoot across the room) when we ran a 3ish inch blade on the motor on a hinge it raised to about 30 degrees, i don't remember what all math he did(something with sin) but he came to a thrust of approximately 4 newtons, 1 less then weight of the expected finished project. 3 times 4 gives me 12 newtons(probably wont be this high) so if i take 50 percent of the thrust and turn it into lift, then i should be able to make this work...


also, i cant seem to find any info about ducted fans and how to make them correctly for optimum thrust
« Last Edit: November 07, 2007, 08:46:19 PM by gamefreak »
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Re: My first "Robot"
« Reply #9 on: November 08, 2007, 06:05:31 AM »
Quote
also, i cant seem to find any info about ducted fans and how to make them correctly for optimum thrust
Im not surprised, this stuff isn't easy to calculate and most people fudge it (if they don't want to run experiments) . . . thats why I was using intuition. More recently though people have been using CFD. You can get away with fudging it, it will be good enough.

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #10 on: November 08, 2007, 06:41:50 AM »
right, so just take some styrofom and sand it out so there is a small gap(around 1/8 inch) between the blades and the duct?

but should i make the duct into a cone or have a wder opening or anything like that?
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Re: My first "Robot"
« Reply #11 on: November 08, 2007, 08:03:11 AM »
Quote
but should i make the duct into a cone or have a wder opening or anything like that?
you want the air to be straight as it exits the fan . . . so I would say straight walls but thats only based on intuition.

Your rudders can also be a part of the duct to save space and reduce drag caused by the straighteners.

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #12 on: November 08, 2007, 06:43:26 PM »
alright, so do you have any ideas on the stryofoam part?
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Re: My first "Robot"
« Reply #13 on: November 08, 2007, 08:31:21 PM »

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #14 on: November 09, 2007, 11:55:35 AM »
yea i read that, i also went to the mcmaster.com, but i get multiple products with each search, and im not sure which one to pick, ex. Melamine Foam, semi-rigid, or flexible? i dont want it to be totally flexible, but how rigid is semi-rigid?

The one i think might work is: Quick-Recovery Super-Resilient Polyurethane Foam(part number:86375K234), at 1/4th inch at 22.61 $ for a 2' by 2' block with a density of 15 lbs/cu.ft.  But I'm confused on how i can get one with a much lower tensile strength for the same price, it seems odd.




« Last Edit: November 10, 2007, 05:34:35 PM by gamefreak »
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Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #15 on: November 12, 2007, 11:25:23 AM »
so any ideas on a cheap foam for this project? The lightest and most rigid is preferable.
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Re: My first "Robot"
« Reply #16 on: November 12, 2007, 11:31:38 AM »
Just go get some pink foam core at homedepot or lowes.

You can get a sheet the size of your door for like $10 . . . or if you are lucky, some free scraps in the back.

This will give you a good idea of what foam can do, and tons of it to mess around with.

This is the stuff I made my robot boat out of, painting it red to make it look nice.

You could always lay a thin sheet of plastic over it for reinforcement, as I did to hold my servos onto it.

« Last Edit: November 12, 2007, 11:34:42 AM by Admin »

Offline gamefreakTopic starter

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Re: My first "Robot"
« Reply #17 on: November 13, 2007, 06:38:57 AM »
alright thanks, i'll post an update on my project when i get the foam and have something built.
All hail Rodney, the holy 555 timer
And Steve said: "Let there be lead!"

 


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