Self Adjusting Rocket

[Ostrianiel]

Hello guys, I am completely new to much of this but a group of friends and I are working on a model rocket.

The idea is to have sensors to adjust the rockets trajectory, by moving its fins, in order to keep it going straight up. At first i thought wiimote but those things are huge and u pretty much need a computer to get its data, something that obviously cant be placed on a rocket's payload.

Doing some research i found that the wiimotes use accelerometers without any gyros.

Accelerometers used:
http://www.analog.com/en/prod/0%2C2877%2CADXL330%2C00.html

I know i may be able to use these but I'm not sure were to go from here. I know ill need a micro processor of some sort to do the calculations and control the small motors that will move the fins.

I don't know how to program the little processors or what language to use. (I know some VB.Net)


This is a bit of an ambitious project since I know little about the subject so ANY help or advice (even ideas) will be GREATLY appreciated.
Thanks!

-Ostrianiel

[AndrewM]

Accelerometers are prone to vibrations, which I would imaging a model rocket would be producing in abundance.  Gyros on the other hand are less prone to vibration but will rezero themselves (there is probably a technical term, but I don't know it), meaning that after a gyro is tilted at a certain angle for a period of time, that angle becomes the new level mark.  So you basically need both a two axis accelerometer and a two axis gyroscope, using both readings-the accelerometer to know where level really is and the gyro to know when the rocket has tilted.

Now we get into the other problem.  G-forces.  Hobby level devices like these are designed to handle only a certain level of pressure before they are useless and/or decide to break.  The lower the g-force tolerance the more accurate the device is, but the less rapid of change it can handle and I am pretty certain a rocket taking off is going to be producing quite a few Gs.  So you will need to first figure out the amount of thrust your rocket is going to be producing so you can determine what level of accelerometer you will need (5G is the highest I have seen at the hobby level).

You can read more on accelerometers here:  http://www.societyofrobots.com/sensors_accelerometer.shtml

How big is the rocket overall (length, diameter and fin sizes)?  That, combined with the force of thrust will be your determining factor for what you use to move the fins.  Most likely some type of micro or mini servo, but that is a complete guess at this point.

For microcontrollers (mcu) parallax makes a line called BasicStamp, which oddly enough is programmed in basic, and can probably handle what you are trying to accomplish.  I believe there is a Basic compiler for the pic mcus as well.  Or you can take a crack at programming in C and a whole world of mcus open up to you.

Now if I hear on the news about a homemade guided missile being launched at any point and time, I will be very put out and will require large sums of money for any further assistance.   

[Ostrianiel]

Is programing in C much more difficult?


We are going to use class C, D, & possibly E which equals out to be from 10.79 to about 30(D and E class) newtons, mind you these are the max thrust. So Ii can't quite give you the G's that it will experience because, since the rocket isn't built, I don't know its mass, etc.

[bukowski]

I had my suspicions on why an accelerometer wouldnt work, and I found this:
http://www.lunar.org/docs/LUNARclips/v5/v5n1/Accelerometers.html

[AndrewM]

I've always believed that if you can program in one language, you can program in any language (except maybe lisp).  Start looking over some of the sample code available in the tutorials and see if it makes any sense to you, most of it is written in C.

The more I think on this, the more an accelerometer is not needed anyway, as the point is to keep the rocket flying straight up, 0deg.  Even with a gyro autocentering, you will be aware of how much each change in angle is and be attempting to compensate (hopefully successfully) for the change in the rocket immediately via the fins, thus allowing the gyro to stay centered on 0deg.

[Ostrianiel]

Ill get to that C researching

any suggestion on what gyros and microprocessors to use?

[Admin]

The military missiles today use expensive IMU's and GPS, with additional targeting sensors all thrown into a complex Kalmen filter.

Its definitely not simple . . .

Look into the german V-2 rockets - no microcontroller yet still highly stable:
http://www.google.com/search?hl=en&safe=off&rls=DGUS%2CDGUS%3A2006-11%2CDGUS%3Aen&q=german+V-2+rocket

[Ostrianiel]

Well it doesn't have to have any type of GPS. The only thing it needs to do is sense how far off a vertical it is, and adjust to once again be completely vertical.

So no guidance system at all.

What would be cool would be to have a guidance system on the parachute so it steers away from trees.

[JesseWelling]

10g dual axis accelerometer
18g triple axis accelerometer

but since they have such wide ranges, it will be hard to dig the resolution you need out of the analog noise and vibrational noise... My advise would be to wire it up with a basic setup and drive around in your car and log the data and see what it looks like.

[Centaur]

I don't know what level of experience you have with control surfaces, but have you put any thought into how you will move the fins to put an out of level rocket back to vertical?  Some questions that you will want to know the answer to would be: do you need 3 servos, 4 servos, 8 servos?  How much torque will they be under?  You have to have a power supply able to put out enough current for your servos to have enough torque to hold in a given position, which could be a lot with a very fast moving rocket.  Something that I don't know which could be a factor, how often do servo positions update?  A rocket moving at 100 mph, if the servo updates it's position to slowly, could radically change directions very quickly with a small change fin position.

You may want to consider fin movement first because this will determine what kind of motors/servos/batteries you will need to use which in conjunction with the required sensor input will determine what kind of circuitry you need to use.

Keep us updated.  This could be pretty neat if it works.  You could do a launch at an angle and watch it turn vertically (please be careful if you do this).

[Ostrianiel]

Thanks for all of the support and help guys!

Ill keep you guys updated on the progress of this thing. At the moment we are experimenting with rocket designs and trying to make one light and large enough to hold onto all of the electronics.