So what do people want from a "flight computer" ?

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strud
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So what do people want from a "flight computer" ?

Postby strud » Sat Jan 02, 2010 9:55 am

Since I've never bought a commercial altimeter or flight computer in my life, I'm keen to hear from those who have &/or have done some study and put some thought into what is it they wan't from a flight computer.

Keen to hear about every aspect including dimensions, functionality blah blah...

I won't bias this by putting my thoughts forward at this stage.

CS

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Postby cha05cat » Sat Jan 02, 2010 11:48 am

I have been looking to buy the Featherweight Raven since matches all of my requirements.

My requirements for an altimeter are as follows:
  1. Barometer plus accelerometer, for mach immunity.
  2. Recording function for download to a PC. Preferably with an open specification for writing my own interface.
  3. At least 2x pyro outputs (3x or more is my preference for later usage when I advance further into rocketry).
  4. Small form factor to fit inside a 38mm airframe.
  5. Compatibility with hybrids.
  6. User programmable output (not fixed to apogee, x feet from the ground, etc).
  7. Cheap as possible still considering the above mentioned requirements (cheap = US$100~US$200 --- ei, not US$200+).
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Postby air.command » Sat Jan 02, 2010 11:50 am

- Ability to drive RC servo motors directly.
- The ability to run from low voltage single cells. Say a single 1.5V AAA battery or a 3V CR123A when extra current is needed. (perhaps provide for power out to drive peripherals at higher voltage)
- Data logging capability for 2 - 4 channels (analog/digital).
- USB connectivity for programming / downloading data
- Self checks and diagnostics, and safety interlocks to prevent accidental firing of ejection charges.
- Ability to continue uninterrupted operation even with brief loss of power.
- Built in launch detection
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Postby Scoop1261 » Sat Jan 02, 2010 12:16 pm

Accelerometer based
Recording function
2x Pyro outputs as a minimum (4x being ideal with 2 being programmable for cluster/staging ignition)
USB connectivity
Small form factor (38mm AF)
Onboard LiPo battery
Self diagnostics would also be desirable but not essential
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Postby PK » Sat Jan 02, 2010 12:44 pm

if we are talking about an "all in one" device (which I personally think isn't the way to go), then Apogee detection by Hall efect would be right up there on my list. It's mach, cato, and bullet proof..

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Postby strud » Sat Jan 02, 2010 3:28 pm

I agree that something that 'does everything' in the end will be quite complicated in every aspect ie development, setup, configuration use.

My intention is that it would include the following core functionality :

- Acceleration sensing
- Barometric pressure sensing
- 4MB flash memory storage (onboard not removable)
- >= 2 digital inputs for launch or other detection (separation etc)
- min 4 pyro/high current outputs
- audible buzer
- additional analog inputs for logging purposes
- USB interface
- Attempt to make it fit within 38mm tube
- open data format for post flight analysis (eg CSV text or similar)
- expansion header with SPI or similar + additional digital and analog I/O

If space permits, a single axis magnetometer would be a valuable addition, since as PK has pointed out they can be a robust method for determining 'apogee'. An alternative approach would be that this is provided for on an additional board attached to the expansion header.

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Postby air.command » Sat Jan 02, 2010 4:27 pm

What if it came in two sections? The basic unit would only have the bare essentials such as the processor, USB connectivity, perhaps barometric sensor and a single/dual pyro port and some IO pins.

This would cover perhaps 60-70% of applications most people would put it to while keeping the size and cost down. It would also have the expansion port that you could plug in the other section with all the additional sensors such as accelerometers, magnetometers, additional pyro outputs, logging capability etc. This would be used on the bigger projects where more capability is necessary. This would also allow people to make their own expansion modules for specific needs.

Would it have any kind of display?
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Postby PK » Sat Jan 02, 2010 4:40 pm

strud wrote:I agree that something that 'does everything' in the end will be quite complicated in every aspect ie development, setup, configuration use.

Since you're intimating that you're building it then:

GPS, gotta have a serial port that can just log everything that gets sent to it and the option of initialising a GPS connected to said port.

Telemetry, Add a socket for the Max stream modules. There are half a dozen radio modules of varying frequency, modulation, and power that you can add for bidirectional comms, whether it's to arm the unit on the pad, or telemeter the GPS stats from the edge of space, you gotta have a radio link..

Must have the option of a separate power supply for pyro outputs.



It strikes me that, as you go through this process, you end up with something a lot like the RDAS unit, which you can just buy. It already already has GPS, magnetometer, video overlay etc.. addons. The software is cool, can output to google earth etc...

IMHO there's not a lot of point of putting a project of this spec out as a circuit diagram and code on a website. There just aren't enough people with the assembly and software skills to put it together. (not saying that no one could do it, just that it'd be of limited net benefit to the rocketry community). I had a basic altimeter design on the web for 5 or 6 years, I reckon about 10 people made them.

So you end up making something and selling it. Fair enough, most rocketry people seem to prefer to buy their electronics.

I recently did some pricing on the parts for the most basic of basic altimeters. In small quantities (ie <100 units) it worked out that they'd need to retail at $100. That's with one sensor, no ram, no PC interface and two pyro chanels, but including an on board lithium polymer battery.
Labour and amortised development costs on a full blown unit like this would drive that retail price to well over $200.


I also looked at what was already out there (in terms of simple altimeters), and got the distinct impression that most commercial hobby rocket electronics seems to be done on very low margins.


Of course if a group gets together and can amass the collective skills to assemble a batch of open source controllers that changes things a little.

PK
Last edited by PK on Sat Jan 02, 2010 4:41 pm, edited 1 time in total.

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Postby Sumo310 » Sat Jan 02, 2010 4:40 pm

Programable outputs would be a must I think.

Most of what I'd like to see has been covered, BUT, what I find most 'annoying' about my perfectflite altimeters is the long startup time before ejection charge continuity is reported.

An option to arm and immediately report continuity only would be great.

The other improvement I'd like to see is that once continuity is reported, its often quite hard to distinguish how many charges have continuity when there is more than one altimeter beeping away inside the airframe. More prominent, and probably 'slower' beeped continuity would be nice.

IE, Perfectflite report 2 active charges as "bip-bip-bip" *pause* "bip-bip-bip".
If it could be done like: "BEEP-BEEP-BEEP" *ppaauussee* "BEEP-BEEP-BEEP", life would certainly be easier.

Lastly, I assume its not hard to program some code that would ensure that if either the baro or accelerometer disagree, ie say you're going through mach and the baro sensor gets confused, that the charge won't fire until a set of criteria are met?
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Postby Avachovy » Sat Jan 02, 2010 6:01 pm

http://www.thefintels.com/aer/hla2.html

i landed 2 of those at my door for at total of $75AUD, they do everything i need them to do at the moment
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Postby PK » Sun Jan 03, 2010 7:22 am

Avachovy wrote:http://www.thefintels.com/aer/hla2.html

i landed 2 of those at my door for at total of $75AUD, they do everything i need them to do at the moment

Yeah, but that's a design, not a product, you're not paying for assembly, inventory, ordering time, marketing, retail margin or taxes..
$30 is about right for parts for a basic alt. If you were buying them as a completed item, they'd be $100.

I agree with you that deployment devices should be separate from payload systems. Mainly to reduce the risk of failure of the deployment device and make adding redundancy easier and cheaper..
PK

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Postby strud » Sun Jan 03, 2010 7:35 am

What if it came in two sections? The basic unit would only have the bare essentials such as the processor, USB connectivity, perhaps barometric sensor and a single/dual pyro port and some IO pins.


Exaclty the intention I had, ie not built a single board with everything on it.

http://picasaweb.google.com/lh/photo/1xL6OJKZbVZ4XcVC1I1rzw?authkey=Gv1sRgCPy34NLE_K3wqgE&feat=directlink

http://picasaweb.google.com/lh/photo/9D ... directlink

The expansion header has 3 analog inputs, 4 digital I/O and connection to the UART. Hence attaching 'daugher' boards to this would be no problem.

The analog inputs are 10bit resolution (same as used for pressure and accel input). This micro is reasonably fast (16MIPS) and has plenty of room (32kb flash, 1.5kb of RAM) to implement pretty much any algortihms and other functions we could desire.

PK, I agree with all you are saying. I don't know what approach is best to take for this.

I want to build this for myself as a minimum anyway, but how to proceed past that point is unclear.

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Postby PK » Sun Jan 03, 2010 7:44 am

strud wrote:PK, I agree with all you are saying. I don't know what approach is best to take for this.

I want to build this for myself as a minimum anyway, but how to proceed past that point is unclear.

Sorry if I sounded a bit negative, 1.5 hours sleep will do that to you. 14 hours of catch up sleep later....
It's a conundrum. If you can build these things for yourself then it's a no brainer way to go. If you can't then you have to buy them and it ends up costing a fair bit of cash.
The challenge is to find a way for those who can build them to help those who can't without it being a huge drain on anyone.

The closest thing to solutions I've come up with are:
Get a group together where the guy who can do the building does it, and others do as much as they can, ordering, cleaning up, wiring jobs, pizza.

and

Find someone who can build them and set him up to run a hobby business making very little money from the things. It's a bit like the rocket kit thread we had. I've met people who would be quite happy to dissapear into the shed for an evening to put kits in bags if it made them $100 cash.

PK

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Postby Avachovy » Sun Jan 03, 2010 9:13 am

PK wrote:
Avachovy wrote:http://www.thefintels.com/aer/hla2.html

i landed 2 of those at my door for at total of $75AUD, they do everything i need them to do at the moment

Yeah, but that's a design, not a product, you're not paying for assembly, inventory, ordering time, marketing, retail margin or taxes..
$30 is about right for parts for a basic alt. If you were buying them as a completed item, they'd be $100.

PK


nope, fully built, programed and ready to chuck into a rocket and fly. Just needs a power souce
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Postby PK » Sun Jan 03, 2010 9:37 am

Avachovy wrote:
PK wrote:
Avachovy wrote:http://www.thefintels.com/aer/hla2.html

i landed 2 of those at my door for at total of $75AUD, they do everything i need them to do at the moment

Yeah, but that's a design, not a product, you're not paying for assembly, inventory, ordering time, marketing, retail margin or taxes..
$30 is about right for parts for a basic alt. If you were buying them as a completed item, they'd be $100.

PK


nope, fully built, programed and ready to chuck into a rocket and fly. Just needs a power souce

Parts cost on his 50,000' variant (using qty 10 pricing) is about AU$30 + board + shipping.
So someone is assembling them for nothing and making no profit on them. A good deal for everyone else but unfair on one person.

PK


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