Physics Mouse trap project

[BANE]

Hello, I have to construct a mouse trap powered car that has to go 5m and stop there.  I already have a few designs but if any one would like to post some it would be greatly appreciated .  So far in my design there will be an encoder on one of its wheels and a servo that will act as a brake caliper.  What I need help on is making a simple circuit that can activate the standard servo when it approaches the 5m mark.  I have two micro controllers that would work if they didn't weigh so much .  I have an Atmega8 chip at my disposal and get everything else needed. 
So I was thinking of powering everything with a small 9v bat (because of weight), and of course using a 5v regulator of IC and resistors for servo.  The encoder would be a parallax QTI sensor or if anyone else has any ideas.  I've ever built the $50 robot, but i'm sure the programmable circuit would be similar .  Please post pics or links to schematics of similar circuits. 
Thanks,
Bane

[Razor Concepts]

Tie a string a little longer than 5m long unto one of the axles, and the other end of the string unto a spool on the chassis. Once the spool runs out of string the car will stop. KISS 

[BANE]

Some other kids have already tried that and it seems kind of hard to land it exactly on the 5m mark (We're being graded on accuracy ).

Bane

[Admin]

I attached a pic of my mouse trap car for fun.

It was never off by more than a centimeter. The problem we had was wheel slip, so whether you use a string or encoder it wouldn't matter.

To reduce wheel slip we used a variable gearing system, keeping the wheel speed slow during the acceleration/deceleration phases.

[HDL_CinC_Dragon]

Keep gear ratios in mind. Big wheels will mean more speed but less torque = less wheel slip.

lets say your axle has a circumference of 1cm and your wheels are 10cm in circumference (just for reference). For every one rotation of the axle which would also equal to 1cm of your mouse trap your vehicle will travel 10cm.

I had to build a spring powered vehicle designed to go as far as possible. We had a 6" spring that we were able to stretch to about 13" I think it was before we got uncomfortable with the potential of damaging the spring. Then we had large wooden wheels about 10" in diameter and about 3/16" in thickness being driven by a small axle, about 1/4-3/8" in diameter. Out of that 7 inches of spring our vehicle was able to travel 80+ feet after coasting to a stop. I think we were able to go like 30' before the string let go of the axle allowing it to coast along. The design worked amazingly because of the fact that we were converting everything to speed instead of torque which meant zero wheel slip, a nice steady start and a very good speed when the string let go of the axle.

Hope this helped!

-Dragon

[BANE]

Thanks guys .  So from what i've seen it probably won't be nessary to use an encoder or servo as a brake.  ( :-[darn it; was hoping to include them)  Admin, I can only use one mouse trap that has a torque of .568 N M 90 degrees from the base and it has to go 5 meters.  We are being graded on how fast it gets there and how close it is to 5 meters.  After finding the springs torque im thinking i need to make it as light as possible.  To avoid wheel slip, I'm going to use some LEGO motorcycle tires (rear traction) in the back and one plane CD in the front.  The problem that i'm seeing is getting 5m of string (would need to make an extention arm from spring, thus decreasing torque) to work and still getting enough speed.  I had been looking for a design that would accelerate, then coast, and then brake.  Having a larger spring would give me more opitions .   Any other suggestions??

Bane

[HDL_CinC_Dragon]

As I said in my previous post. I was able to get my vehicle to drive 30 feet(9.14400 meters) while powered. The only reason it went 80 feet is because of it was designed to coast really far after the string let go. If I wanted to, I could have fixed the string to the axle so that when it hit the 30 foot mark, it would be stopped and then go backwards and forewards for a few seconds until all the energy was expended and it would come to rest at 30 feet. You can do the same thing with your car except make it so that when the string is about to leave the axle, it pulls on some kind of braking device. Even a little wooden dowel in the axle that it just pulls out a little bit so that it hits something and cant spin anymore. That would stop it in its tracks on your desired distance mark. Experiment on paper with wheel diameters, axle diameters, and swing arm(The part that kills the mouse) travel. I am certain that you can do this.

[BANE]

Thanks; I'll take your word for it.  I'll draw up some designs, do some math and post my results .

Bane

[HDL_CinC_Dragon]

You wont be disappointed im sure

[Metal Slug 2]

i have an idea that might work. 
firstly attach a string to the mouse trap.  Attach the string to an Axel with a wormgear attached (http://lpmpjogja.diknas.go.id/kc/g/gear/gear-worm.jpg), have the worm gear connect to a Spur gear.  attach the spur gear onto the rear Axel of your vehicle.  use the Lego wheels you mentioned earlier on the rear Axel.  add some weights 1/3 of the distance from the rear Axel to the front Axel (most weight will be on the rear end of the car)  this will allow for more friction with the floor = less wheel "slippage".
once the car is launched as the mouse trap pulls the string it rotates the worm gear, once the mouse trap stops pulling the string, the worm gear will NOT spin anymore.  this is because the worm gear can spin the spur gear, but the spur gear cannot spin the worm gear!  therefore when the momentum of the car tries to force itself to continue on its path, the spur gear will not be able to spin the worm gear, thus restricting the rear Axel to move. since the wheels are attached to the rear Axel and the axe attached to the spur gear and the spur gear attached to the worm gear, the previously gained momentum will be the brake of the vehicle.  its like running and than hitting a brick wall.

It is only a matter of calculating/observing how far the mouse trap moves until the car is moved 5m.  placing a stopping device on the mouse trap at that point is what will need to be done then.

The only thing bad with this idea is that the rear Axel will stop almost immediately, and unless those Lego wheels of yours have amazing traction, the car will slide a bit.  secondly the worm-to-spur gear thing acts as quite a big gear reducer, just fix this problem with the reverse of a gear reducer, than connect it to the worm gear Axel.

[HDL_CinC_Dragon]

well you should be losing a lot of torque in the system which is actually a good thing because your wheels wont spin. Because of the fact that the little bit of travel of the mouse traps arm has to do so much work in moving the vehicle, it can do it but it wont have much strength at all(a near empty soda can could probably stop it). Your idea is actually pretty similar to the original design for my spring powered car. The idea was to use a rack and pinion. The pinion gear was going to be on the axle and the rack was going to be pulled past it via the spring. The major flaw was that we couldnt get the right materials to do it where we didnt lose like half the efficiency. Then we switched to the other design I described above and were very very pleased with the results.


One concern i would have with your design with a worm and spur is the angles. because of the way the worm gear needs to turn, your mousetrap would have to be mounted parallel to your wheels rather than perpendicular. Heres a quick pic I made up in Paint: http://img101.imageshack.us/img101/88/wormspurexamplexb5.jpg

[Metal Slug 2]

One concern i would have with your design with a worm and spur is the angles. because of the way the worm gear needs to turn, your mousetrap would have to be mounted parallel to your wheels rather than perpendicular. Heres a quick pic I made up in Paint: http://img101.imageshack.us/img101/88/wormspurexamplexb5.jpg

, you must be telepathic, your design was exactly what i had in mind.
Would the mouse trap on the side be a big problem though?  If you add a bevel gear, you could potentially get the mouse trap to be perpendicular with the chassis of the car, and nearly centered, ...maybe?

[Metal Slug 2]

switch "perpendicular" with "parallel"..ooops

[Metal Slug 2]

heres a modification of your drawing, which i changed (i hope you don't mind) showing the bevel gear idea.




oh and thanks i guess, though your image link, i discovered the "imageshack" thing, it could come in quite handy, like now.

[HDL_CinC_Dragon]

Image shack is awesome I downloaded their tool bar which is also awesome

That could work but youd be losing some energy in the bevel gears. Youd be better off making the rack and pinion idea work. Tons less energy conversion/transfer. It wouldnt really be difficult to do either, I was only material restricted.

And technically you would be the telepathic one, I had the idea first

[Metal Slug 2]

And technically you would be the telepathic one, I had the idea first

sorry.

just out of curiosity...
Is there any kinda simple-ish formula(s) to calculate how much energy would be wasted/transfered with all the gears?

[HDL_CinC_Dragon]

probably but I cant think of anything off the top of my head. I have to go to class soon also so I cant quickly think of a ball park equation. Same thing as tomorrow too >_< and Thursday im busy as hell also... and then Friday is my birthday so I wont be doing anything but sitting around doing nothing but fun stuff after work Perhaps if no one can help out before then, Ill see what I can do

[Metal Slug 2]

Happy Birthday!

[BANE]

I've got a  design .  It has a speed issue though; i'm going to look into Admins system on this.  I'm also including some LEGO elements in the design , some will never return to my box though  .  The braking system is really cool (all in LEGOs)

Will post pic soon

Bane

[HDL_CinC_Dragon]

ok its been almost 3 hours. You should have a finished product with plenty of documentation and pictures by now! SLACKER!
lol j/k

Define speed issue? too fast or too slow? Im assuming too slow? Figure out if you can take some of the energy out of the distance and put it to speed, AKA change up the size of a gear or axle or your wheels to get more speed. Maybe a little bit more torque is needed but less top speed since your going to be breaking hard?

[BANE]

ok its been almost 3 hours. You should have a finished product with plenty of documentation and pictures by now! SLACKER!
lol j/k

Ouch, the truth hurts!  

I've got some pics on my camera but can't find my downloading cable!  (probably somewhere under all the clothes in my room )   

I will post pics and finish this blasted project during ThanksGiving, since it's due when i get back to school

Just a thought, if the pully attached to the main axle that the string goes on was to be a cone like shape; assuming that when winding the car that the string ravels right beside each other, that this would produce low torque starting off and increase as it goes .  BTW my english teacher would kill me if she read that sentence ^

Bane

[HDL_CinC_Dragon]

I do agree with you on the notion that your english teacher would kill you after reading that sentence. But luckily, this is the internet where no one cares about grammar! YAY!  ....  ok well people care but not THAT much lol

And yes, a cone on the axle would in fact be able to vary the speed vs torque ratio as it unraveled. Assuming the string is the source of input power, starting it on the larger end of the cone would in fact generate less torque at the start. Generally you want this to be opposite though as you want more torque to get it going to break its inertia (Newtons first law)

[BANE]

Generally you want this to be opposite though as you want more torque to get it going to break its inertia (Newtons first law)

  Even with rubber lego wheels theres quite a bit of torque (enough for it to spin, which i'm tring to avoid )  I think if i make the cone big enough i will have good torque and speed.

And i can always reposition the sring so i can try both ways


bane

[HDL_CinC_Dragon]

very true. You also dont even have to bother with a cone as much either. You could just make a cylinder with 2 different diameters so that it starts off with only a little torque for like 5 revs and then suddenly the torque jumps as if the driver hit the NOS button

[Admin]

Is there any kinda simple-ish formula(s) to calculate how much energy would be wasted/transfered with all the gears?

http://www.societyofrobots.com/mechanics_gears.shtml


Just an FYI, you can upload images to the forum too (click 'Additional Options')
Oh and don't delete your older images because then your forum post will become dead-link useless.

[BANE]

Ha, found my cable .

[BANE]

Ok, that one ^ didn't go fast enough, but this one does!  It goes 5m in 3.2 seconds and stops within 3 cm of the mark.  I think the inaccuracy is because of the braking system skids before it stops and what type of floor it is.  But overall this is the fastest and most accurate one i've made .

[HDL_CinC_Dragon]

Excellent! Good work Bane
Id say 3cm is pretty darned accurate for a lego car :-P Ill bet you can compensate for that skidding though. If your going to be doing the experiment on a school floor, the coefficient of friction is going to be much lower because those floors are really slippery especially when they get all that dust and crap on them. You should surprise your teacher by asking "Whats the average coefficient of friction between these school floors and rubber?" Then calculate that out. Also youll want to find it out for the surface youre testing it on as well so that you know how much you need to compensate.

Good work on the car

[BANE]

I think what I'll do is come to school early and test it out.  I have a bolt with two washers that clamp the encoder-like string to adjust how far it will go.  So adjustments will be easy .

Thanks for everyone's support and I'll be sure to notify you guys then i get first place .

Bane

[HDL_CinC_Dragon]

ah good man, made it easily adjustable
Lets us now how badly you beat everyone else




hmmm its not cheating that we helped you is it? lol