Pezo vibra tab

Mike15

I need a way of detecting the launch of a rocket. I was thinking the pezo vibra tab might be the soloution.

I do also have an amature radio on board the rocket could this combanation cause problems for this sensor?

Sarten-X

I don't know much about the dynamics of rocketry, but what about a thermistor?

EDIT: Drastic changes in value are fairly easy to detect, I do believe.

I think something like:
if oldvalue * 3 / 2 < newvalue then <launch happened>
oldvalue = oldvalue * 9 / 10 + newvalue

but I'm not sure...

Post Edited (Sarten-X) : 3/25/2007 3:17:51 AM GMT

T Chap

Clarify what you mean, you want a way to determine if the rocket launched, as if you need to start some timer started at launch or what?

Mike15

The payload consists of two parts:

1. An APRS (automatic position reporting system) using amature radio.

2. A Stamp sampling an accelrometer, pressure sensor, and a RTC. Then stores these samples to an EEPROM.

The·APRS data is sent back to the ground station which is received with a handheld radio which displayes the the GPS sentence strings on an LCD.
This is for rocket retreval.

The flight data from the Stamp board is then downloaded using a Palm PDA.


· I have flown the Stamp board in a preveous payload wich was in a much smaller rocket so I just had the Stamp wait a short period of time and then give a visual countdown using a LED.
·This worked well with the smaller rocket, but now the rocket is alot larger.· The distance needed between me and the rocket makes this inaffective.
·To solve this problem I need a way for the Stamp to detect launch so it can start storeing samples to the eeprom.

Any·ideas on a "Launch detection" system?


·····

Franklin

How about using the accelerometer as an indication that the rocket is moving? Or you could place a small wire across the nozzle or to the gantry that would break at launch.

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- Stephen

PJAllen

An I-R emitter [noparse][[/noparse]38kHz] on the lauch pad, shining on/through a small window on the rocket·to an I-R detector [noparse][[/noparse]38kHz] -- maybe the detector could be right·on or flush with the rocket body itself.· Beam = On Pad, No Beam = start accumulating data.

T Chap

The thing is how do you intend to arm it to be ready for launch?

What about you arm the rocket's Stamp data logger at anytime, whether it is in position or not, by some method like for example the launch detect is based on a hall sensor mounted near the base of the rocket. The Stamp will not start the sequence until the hall sensor detects a magnetic field. After arming and placing the rocket on the pad, you move a very strong electromagnet(solenoid) next to the body of the rocket where the hall sensor is. On launch, you simultaneously trigger the electromagnet off the same lines as the launch system(ignitor or whatever).

Post Edited (TChapman) : 3/26/2007 4:39:55 AM GMT

metron9

All good ideas. How about this Acceleration switch? http://www.findtheneedle.co.uk/products/163-acceleration-switches.asp
I love these little buggers. Put 4 on a circuit board at 90 degree angles to each other for a 360 degree tilt switch.

You would put it on a wire tilted a few degrees up from horizontal on launch the wire bends the switch turns on, on zero gravity the switch turns off perhaps not 0 gravity but a second switch angled down could trigger when the rocket reaches apogee and starts to fall back I think if it falls tail first on it's initial fall back to earth. And they only cost $1.00 each. But I bought 100 of them through my distributor. I don't know who sells them in small quantities but being a rocket man myself in years past, I would be happy to send you a few from my stash if you PM me with your address.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Capt. Quirk

I need a way of detecting the launch of a rocket. I was thinking the pezo vibra tab might be the soloution.

Why not?,

Mounted inside the rocket, it should act like an accelerometer. Simple and clean

LoopyByteloose

I agree, I think you will save a lot of $$$ by using the piezo sensor and some EEPROM space. Essentially, you would program a waiting loop to wait for it to spike.

When it spikes, the program would jump into a functional mode.

You will have to fool a bit with the threshold of sensitivty, but it is essentially a one-shot use. You can restrain part of the device to make it less sensitive.

Additonally, the device is light as a feather. Good luck.

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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

T Chap

For more sensitivity, also incorporating some Low Pass, what about adding some slight amount of weight to the vibe sensor?

Post Edited (TChapman) : 3/26/2007 7:01:29 PM GMT

metron9

That was initially what I was going to post,I have a piezo kit from MSI and they recommend exactly that , they have switches that have a small spring steel tab connected to a circuit board with the electronics to filter the input to give a solid on signal. I have a few of thoes too but they are about 3 inches long and that would be a big rocket so I went with the switch because of it's dual function. You could also put one vertical normally closed and when the rocket reaches apogee and flips over it would go on.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Mike15

Capt. Quirk said...

I need a way of detecting the launch of a rocket. I was thinking the pezo vibra tab might be the soloution.

Why not?,

Mounted inside the rocket, it should act like an accelerometer. Simple and clean

·Kinda what I thought.· In the example code on the web page uses on and off, how can I adjust the sensitivity of the vibra tab?

Also any suggestions on a power source? I would like to run everything off of one battery




And the stamp board. I have the circut attached. I'm looking for something lightweight and small.



Post Edited (Mike15) : 3/26/2007 11:06:48 PM GMT

metron9

The piezo creates its own voltage. By connecting a comparator + and - to the tab you can snap your fingers 20 feet away and get a response as the voltage is AC. Use a 10K pot (trimpot) across the vibratab to adjust its sensitivity by shorting it with 0 ohms it wont trigger the comparator, the more resistance you put on the pot the more sensitive it becomes. A standard buzzer cep-1110 for example would pick up the rocket engine noise as well and give a constant high signal if the sensitivity was set to allow it. So with that you can know the engine time , launch time, flight to apogee if you include a switch I talked about above.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Mike15

Can someone post a schematic of the vibra sensor with the zener diode?

T Chap

www.parallax.com/dl/docs/prod/compshop/PiezoInfo.pdf

Mike15

So one end of the vibra tab goes to the stamp pin and the other to 5v or is it NC?

T Chap

One end of the sensor goes to GND, the other goes to the Stamp through a 200ohm resistor. A 5.1v Zener diode goes across the sensor to limit the transient voltages above 5 volts. The vibra sensor is capable of outputting higher transient voltages than the Stamp can handle, thats why the Zener is used for protection. Where you see 5.1V next to the Zener diode, tha simply is the voltage value at which the zener starts shunting the excess above 5.1 to GND.

Chris Savage

It is true we list the Zener for many reasons, however because the current on this device is so low it is possible to use it without the Zener and increase sensitivity. The internal clamping diodes within the BASIC Stamp Interpreter should be able to handle the voltage spikes at the low current put out by the Piezo Vibra Tab. It will increase sensitivity since the diode essentially clamps the Piezo. The Zener is required conceptually in that we didn’t want customers thinking any device that outputs the voltage levels this device does could be safely connected to the BASIC Stamp. Take care.

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Chris Savage
Parallax Tech Support

Mike15

I have ordered a couple of the tabs. I might have a question or two after the have arrived.
Thanks for the help.

Capt. Quirk

Your next problem is the battery?. I have heard of people using a charged capacitor in rockets and F1C Free Flight airplanes, but I don't know how they do it.

Mike15

Yes I'm trying to find a smaller power source. I have two diffrent systems and I want to run them both off the same battery.

The two systems are:

·an APRS Tracker with a supply range of 9-12 VDC and a current consumption of 180mA.

And the stamp board containing all the sensors.

I have alotted a little bit of weight for the battery but lighter is better. Also this rocket is a little bigger so I do have a little bit of room.

I have attached two pictures of the payload. On one side is the APRS traker and GPS on the otherside is the Stamp board and the antenna for the APRS tracker. Next to the antenna is where I plan to put the power.

And on another note I just got the Vibra Tab in the mail today so I'm going to hook it up and check it out.

Mike15

I hooked up the vibra tab and ran the example code.

A few things I notice:

Some times when triggered it wont pop back to 0 more so in one direcction then the other.

Another is the sensitivity. It seems to be pretty finicky. I tried adding the 10K pot, but I'm not quite sure on how this should be wired in. Do I connect A and B of the pot and connect the pot's wiper to the stamp pin?

metron9

"Some times when triggered it wont pop back to 0 more so in one direcction then the other."

Sounds like you have some capacitance in the circuit. Use a resistor to ground on both sides of the piezo tab perhaps 100k to bleed off any capacitance. Sensitivity can be adjusted by shorting across the piezo tab. If the tab is shorted with 0ohms it can't give any signal. The higher the resistance between the two sides through a pot the higher the voltage can be.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Mike15

I'm not quite sure how to connect the pot. There are three terminals A, B, and wiper.

I connect A and B to the Tab and Wiper to ground or positive?