Apogee Detection

Discussion about deployment systems including altimeters, timers, air speed flaps, servo systems, and chemical reactions.
T86157
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Apogee Detection

Post by T86157 »

So, now I am having doubts about just using a gyroscope. What is your guys' opinions on detecting apogee for parachute deployment? I read a gyroscope and an accelerometer is good for detecting angular position. I thought that either a gyroscope or an accelerometer by itself would provide enough information to a microcontroller (Arduino) to tell when the rocket reaches apogee or starts to tilt with nose of rocket starting to face the ground.
T86157
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Re: Apogee Detection

Post by T86157 »

This is what I am currently working on. This is my first mockup of using electronics to deploy a parachute. It is a modification to AirCommandRocket's "The Shadow's" deployment mechanism ( The parachute is locked radially rather than axially. The servo will turn and allow the pins to be "unlocked". The black cap on the far right of the picture still needs to be attached to the rod which will seat the parachute. The wood pieces have screw holes because my thought behind this were if I wanted to remove the components and place it on a different rocket, I would have to unscrew everything and be good to go. The only issue, which I am not sure if it is going to be a significant issue, are the screws sticking out causing a huge change in drag force. The reason I did this modification is because I am using FTC; space being very constrained in the cross sectional area. The mass is about 29 grams so far. I will still need LiPo and a "working" gyroscope (and/or accelerometer). My gyroscope is reading a huge variation in raw data while stationary, really sensitive. I'm trying to figure out how to filter out a lot of the noise to make this workable.
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air.command
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Re: Apogee Detection

Post by air.command »

T86157 wrote:So, now I am having doubts about just using a gyroscope. What is your guys' opinions on detecting apogee for parachute deployment? I read a gyroscope and an accelerometer is good for detecting angular position. I thought that either a gyroscope or an accelerometer by itself would provide enough information to a microcontroller (Arduino) to tell when the rocket reaches apogee or starts to tilt with nose of rocket starting to face the ground.
Detecting apogee for parachute deployment is usually the ideal choice since it should work regardless of how high the rocket goes or how far it arcs over. Detecting it is a little more difficult. What complicates things on a rocket is rocket spin. Virtually all rockets will have a certain amount of spin on the way up and you have to factor this into your gyro and accelerometer filtering. Are you using a multi axis accelerometer? This is why simple barometric or magnetic apogee detectors are commonly used.

The parachute ejection mechanism is looking good. Where will you be attaching the shock cord? This was one of our bigger problems so that as the parachute opens and exerts a large force on shock cord, it doesn't damage the piston and piston guide. Look forward to seeing it assembled.

- George
http://www.AirCommandRockets.com
T86157
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Re: Apogee Detection

Post by T86157 »

I was going to originally use a three axis accelerometer without a gyroscope. Then I was under the impression that a gyroscope was really wanted I needed and not an accelerometer. Now I am beginning to think that I will need both sensors in order to detect angular position aka apogee rocket arcing.

To answer your questions:
Are you using a multi axis accelerometer?
No, not yet. I may have to get a gyroscope + accelerometer package on the same breakout board.

In regards to barometric or magnetic apogee detectors, can non-ferrous metals affect the "internal needle" of that type of sensor? My piston is made of aluminum and the other electronic components (servo, wires, lipo battery, and possibly a key camera/wireless camera transmitter in the next prototype) are going to be below or next to the sensor.
Where will you be attaching the shock cord?
I have not put much thought into that part yet. Most likely on the FTC or one of the screw heads that are holding the deployment mechanism. How did you end up solving this issue? The current parachute I have is made of a modified garbage bag and fishing line.
air.command
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Re: Apogee Detection

Post by air.command »

The magnetic apogee detectors can indeed be influenced by local magnetic fields. Some of them recalibrate themselves on power up so that if those fields around them are constant they should not influence them too much. Even a guide rail can potentially affect it. How well the MAD filters the interference is dependent on the device. From our experience RC servo motors located in close proximity really haven't had an effect. (on the uMAD at least) It would make sense though to keep them away as far as practical however.

We had to attach the shock cord as close to the end of the body tube as possible to make sure the cord did not pull accross the piston. We ran the cord through a tube that was glued to the inside of the body tube. The piston stops about 5mm from the end of the tube, so it is not possible for the shock cord to apply any force to the piston. There are other arrangements possible though.

A knot at the bottom of the tube stops the shock cord from pulling out. The knot sits below the lowest point of the piston.

- George
http://www.AirCommandRockets.com
T86157
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Re: Apogee Detection

Post by T86157 »

George, do you know of other sites that have a reliable apogee detector like the one from Wooshtronics?

They have been sold out for more than a month or so, not sure if they discontinued that product or not...
T86157
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Re: Apogee Detection

Post by T86157 »

I found this product (http://www.sparkfun.com/products/10121), an IMU 6 DOF might work but is a bit pricey for a mockup/prototype. Though it sounds reliable as far as sensing. Impact and G-force resistance on the other hand...
T86157
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Re: Apogee Detection

Post by T86157 »

I found a much cheaper product (http://www.sparkfun.com/products/10621), though it's just sensing when it tilts; not how much it tilts. This is done by a metal ball touching one of four different sides. I suppose if I have it orientated to where the "+" plane is faced perpendicular to the earth, the ball will be touching one side. If it touches any of the three other sides (apogee), it would trigger the servo. Again, not sure how reliable this is, but it may work...
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U.S. Water Rockets1
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Re: Apogee Detection

Post by U.S. Water Rockets1 »

T86157 wrote:I found this product (http://www.sparkfun.com/products/10121), an IMU 6 DOF might work but is a bit pricey for a mockup/prototype. Though it sounds reliable as far as sensing. Impact and G-force resistance on the other hand...
The original Magnetic Apogee Detector based on the idea by Professor Robert Galejs of MIT is available here:
http://www.aeroconsystems.com/electronics/mad.htm

They provide the circuit if you wish to DIY.

You don't really "calibrate" these sensos. You essentially need to degauss them before use. The sensors have a built in circuit for doing this that you need to trigger on power up or by a pushbutton.

We bought one of these in 2004 and it still works.
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T86157
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Re: Apogee Detection

Post by T86157 »

Thanks, I might check them out. Wooshtronics got back with me and they said they have an alternate MAD sensor. Looks like either way I'll be spending $30 for a prototype.
kgrayson
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Re: Apogee Detection

Post by kgrayson »

Hi all,

While this is only my first post on this forum, I have been observing it for many years and have done my own fair share of tinkering with water rockets. I thought I might just add my own 2 cents to this discussion, since I have also been toying with the idea of using gyros in water rockets, with an arduino.

From what I understand with my own messing around, the combination of sensors used in this kind of deployment system really depends on what you are looking for. If you want to have a fully recorded model of a rocket’s orientation AND rate of change in orientation then you probably need to be looking at some of the 6DOF sensor systems, which are essentially a 3-axis accelerometer and a 3-axis gyro working together.

I am yet to test it in an actual rocket, but I have been thinking that a slightly cruder approach using just a 3-axis gyroscope (or potentially one with fewer axes) could be used to achieve similar results. A gyroscope measures the rate of change of the sensor’s orientation (in units like degrees per second). This does not then actually give the exact orientation of the sensor but it can show how quickly any orientation change may be occurring and in which axis. My thought was then to monitor this rate of angular change data in a rocket. As the rocket pitches over at apogee, this may not necessarily happen that quickly (depending on the rocket design and balance) but it should be a reasonably large non-zero value for a few fractions of a second – which should stand out from sensor noise even if you don’t have that much filtering. Essentially you would pick some value above the noise, and given the sensor data rate (equivalent to some time period), the pseudo-code would be:

Code: Select all

[Looped]
counter=0
N=chosen threshold value
If x>N
	add 1 to counter
	If counter > 100 (arbitrary number of consecutive counts above threshold desired to define a rocket pitchover)
		Pitchover detected! - activate parachute deployment
        end
end
This threshold value could be adjusted to suit a particular rocket, but fancier code could also be used for filtering and smarter detection of the pitchover could also be done using an integrator-type analysis where the area underneath a rate of angular change plot could be found – so that faster pitchover and slower pitchovers could both be detected by the same code as they would add to the same net change in rocket angle.

As I mentioned before I have been looking at a 3-axis gyro for flexibility but one axis would have be assigned as being parallel to the rocket axis, and thus ignored for the purposes of detecting the pitchover (as it would only record the axial spinning of the rocket).

I have tested this rough concept on the ground very roughly using an Arduino Uno (and so it was tethered to the desk) and it seems to work, but I am yet to adapt it across to an Arduino Pro Mini for more portable and realistic tests. The gyro I have been using here is a 3-axis gyro taken out of a Wii Motion Plus, an addon for the Nintendo Wiimote. They are pretty affordable ways of sourcing decent gyros, the onboard gyro measures at up to 1500 degrees per second which should be plenty quick enough for this application. Compared with the breakout boards from sparkfun you can probably get a Motion Plus for more like $15-25. It communicates with the arduino via I2C and code for Kalman filtering the sensor data is also around on the net. Plus, if you are wanting to upgrade to a 6DOF system then adding in the sensor from a Wii nunchuck adds the 3-axis accelerometers.

Some relevant links:
http://randomhacksofboredom.blogspot.co ... -love.html
http://www.arduino.cc/cgi-bin/yabb2/YaB ... 1253202298

That tilt board you found on sparkfun may work also, but it might depend on how violent the pitchover is - because everything should technically be in freefall should it? Only one way to find out though, and you might pick up the ‘wobble’ as the rocket tilts over…

Anyway just thought I would share some thoughts and I look forward to seeing how things go!

Cheers,
Kristian
T86157
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Re: Apogee Detection

Post by T86157 »

Kristian, what I have learned from 3 axis gyros with no 3 acceleromers is that the possibility of Jerk or Acceleration will prematurely trigger the parachute on initial take off. That is, if you have a shock absorber mechanism than you should be fine. I may go a much crude way, which is the tilt sensor; basically a ball inside a cylinder chamber that detects tilt in four different directions or two axis. In theory this method will trigger after apogee or before it due to intertial momentum of the ball inside the chamber.
kgrayson
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Re: Apogee Detection

Post by kgrayson »

Yes, you are right in that the unavoidable motion of the rocket during launch and ascent could prematurely trigger the gyro to deploy a parachute. However, my hunch is that those readings should be able to be filtered out fairly easily to differentiate between them and the actual pitchover at apogee. It of course has to be proven first though...
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Re: Apogee Detection

Post by Willem »

If you want to keep things simple I would recommend you to buy a uMAD Apogee detector from whooshtronics.com :)