guided rpckets

[bldavis]

So...I'm new to this, but very i have taken on a very demanding project....the first of many. I first being a Guided Rocket; I am working on the MCU and control systems right now. I have already designed the airframe, and started working out the weights to determin the Center of pressure and the Center of Gravity to confirm that this thig will be stable. But.......

Here is the Problem.....

I was wondering if there is a way to find out what information is required to determin how many times per second a IC at say 20mhz can read about 15 inputs calculate and output to 4 control surfaces per second.

So is there any one out there that thinks that they can help, or is this going to be a trail and error project?

[airman00]

well it depends on what kind of inputs  and what kind of outputs

please provide us with all the details possible.  Also why can't you go higher than 20 mhz?  ( even though I feel that 20mhz should be enough)
http://www.microcontroller.com/news/microchip_pic32.asp    - those run at 72 mhz

[bldavis]

well right now it going to be a mix of inputs from the following sensors
Pressure, accel.,2 axis gyro, camera, transciever, tempature, humidty, and i'm still debating on a couple of others.

the outputs will be PWM for 4 servos and 1 serial for TX/RX and 2 other outputs for deployment of recovery devices, maybe two additional output lines for a beacon and/or buzzer to aid in location.

i don't have much more than that, as i have not completely descided on which sensors, therfore idon't know if they are going to be digital or analog devices. does that help any?

BTW thanks for the 72Mhz chip i think that should be a good starting point.

[airman00]

well here is what I would do if processing speed was an issue ( which it probably isnt in your case)

Use external hardware that control many outputs or inputs and have a master microcontroller control (or receive data) this external hardware through serial.  This sharing of processing will make your system much faster.

What is the application for these rockets neways?

[bldavis]

The application, just personal for now, although i would like to build a UAV in the near future!

I don't know if speed will be a problem, but I don't want it to be, i need the controls to be on top of everything from the start. right now i'm looking at about 20 pounds of thrust on a 4 pound rocket, then an additional 15 pounds of thrust on a 3 pound second stage. that is supposed to be in a horizontal flight path.

But, back to the point to think that 72Mhz is going to suffice? last thing that i need is a rocket hurdeling towards the earth with all my hard work attached, heck uncontrolled in any fashion could lead to a very eventful ending.

Do you have any sugections on the sensors that should be used?
I'm open for suggestions!

THX

[Ro-Bot-X]

What is the speed of the rocket? How many meters will it travel in 20ms (a servo pulse period)? For some reason I think your system will be fast enough to process all the data but the servos will respond slow compared to this and the rocket speed. You will be able to read the sensors a few times (and do the processing) untill you will be able to give a new servo pulse...

[RobD]

You could practice smashing and burning things up with the dsPIC33's from microchip - runs around 40Mhz and up to 21 IO pins at anly a couple of bucks a pop.  Plus they supposedly migrate easy to the 32bit pic's.    I imagine trimming weight might also be an issue with all those items on board     This would be a fun project to work on!

Things happen really fast with amatuer rockets so you'll probably want to get as much done as quickly as possible during the flight phase.  I helped a guy put an accelerometer and 2ghz transmitter on a water propelled rocket a few summers ago.  I think the design is on my work computer.  I'll have to look and see what models he used.            

[bldavis]

The speed at burn out and after coast will be approx. 30 meters/sec

This is low as the weights used in the sim were high and the propellent selected was two classes low than the class i intend to use.

20ms can be a long time to have no contol, servos have a 4 microsecond deadband so the need to be feed continual data. does that help?

[Ro-Bot-X]

A regular servo receives pulses at 50Hs, that means a 20ms period. So when you are using a RC transmitter, it will send pulses 50 times a second. So you can't change the servo position faster than that (well, maybe you can go as low as 15 ms). If you don't send the pulses in time (you wait longer than 20ms) the servo will get soft and lose it's position.

So the rocket will fly 0.6 meters until a change in course can be done. Is this ok?

[airman00]

perhaps there are servos that respond quicker?

[bldavis]

i belive there are servos that will respond faster, digital ones, however they are twice and much but offer more torque and faster speed, with a feedback option in many of them, but the price is even higher on micro and submicro servos. that could be an option.

i think .6 meters is okay as long and i allow enough movement in the fins for correction, i think that will work as long as the corrections correct the problem, i dont think that this rocket will be able to recover if out of ballance for more than a couple of seconds, depending on the orientation.

any sensor suggestions?

[RobD]

I found the accelerometer.  It was called a PICO. 

Basically, it sent an output pulse once the rocket was at it's highest point in flight.  The controller acted as a timer/servo controller to initiate other actions.  One was to move a weight which caused the rocket body to orient differently in freefall.  Another job was to release a latch to open parachute door.  We tried to rig up a multi-stage pin release system but he opted for a pressure release system instead.  I don't think the transmitter ever worked quite right, but I remember the accelerometer logged some data.       

[Iron Man]

just a side note, heavier rockets often go farther than lighter ones. more kinetic energy to over come the air friction.

u might want to look at more linear type actuators ... nvm probably too expensive/ too heavy.

another idea, your rocket should be stable enough to fly directly straight up without any correction from the servos... you really only need those to flip to horizontal. so you could reach a certain height, and then apply air brakes, to slow you down so that you can take the corner with a slower approach... at least maybe the first few times?

[JonHylands]

You don't mention where you live, but if you live in the US, you might want to do a little checking. The powers that be take a dim view of individuals building things like guided rockets (missiles).

- Jon

[bldavis]

You don't mention where you live, but if you live in the US, you might want to do a little checking. The powers that be take a dim view of individuals building things like guided rockets (missiles).

- Jon

thanks for the side note jon, but i have done the reasearch, FAA will be notified prior to launch, I have requested a LEUP (low explosive user permit) form the ATF and I am following all the Regulations post by the NAR. Plus this model still doesnt require a Level Certification even though i am working on that! But for the General public, we'll continue to call this one a guided rocket, not a missle! 

[bldavis]

thanks for the advise ironman the  complete rocket with the lower stage has a center of pressure 1.5 times with diameter of the rocket behind the center of gravity. so at the initial launch the rocket will be stable, but after the intial ejection charge is fired and the 1 stage booster has seperated the center of pressure moves to .25 times the Diameter and this is still a sable loaction, but not nearly as stable as before. with that said the fins will ahve to do the correcting and all this has be done serveral times a second at the bare min. that's why i need the MCU's plus i want it fly straight and level for approx 10 sec, that's the burn time on the second stage motor.

I think that it can be done, what about you?

[Iron Man]

ya definitely a good project. I wanted to do that with one of my model rockets, but i just never got around to it, because my pockets are empty being a college student and all.

[Iron Man]

on a side note, microchip sells a board that you can insert an SD card into and then communicate with your pic via the board. it's like $30.

At very least you can use it as a reference design because you can get the schematic off their website.

[bldavis]

thanks ironman, where are you from pm me if you get a min, i know that this is going to be fairly involved, and this design or project is the start of as string of projects, with the finished product being a UAV, so if you have any other advice, or sujestions please let me know.

I do have side note or question for everyone on the board, Is the a simple (schimatic) way to mount the PIC32MX360F512L, with power regulation so that i can have access to all of the I/O pins? Microchip has the starter Kit, but there are 3 leds and 3 switchs, I was thinking that i would be able to have access to all of the pins, but they say that i have to spend an additional 72 bucks for the I/O board that will give me access. is this reall required? i don't want to have to wait for this before i get to testing.

[RobD]

sparkfun also carries breakout boards for sd cards starting at around 15 bucks with app notes for interfacing. 

http://www.sparkfun.com/commerce/product_info.php?products_id=204