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Data Acquisition Hardware Setup

Old 07-27-2008, 02:23 PM
  #1  
Golani51
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Default Data Acquisition Hardware Setup

Hello All:

I am looking to set up a data acquisition system to measure the 6DOFs (x,y,z displacement and rotations), in addition to control surface(3) deflections of a small RC model. Control surface deflection is done by taking the signal output from the servos.

I am looking for an easy to assemble design that is light and compact. Data should be sampled at a fairly high frequency. Noise minimization is a concern.

If anyone can help me out I would be most appreciative. Until now I have failed miserably as my knowledge of circuitry is pretty low.

All assistance gratefully accepted.

R
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Old 08-01-2008, 12:54 PM
  #2  
CamLight
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Are you looking to buy or build yourself?

What are you measuring for the 6 DOF's? That is, what sensors are measuring the movements?

A low-noise (i.e., high-resolution), nine-channel data acquisition system is certainly possible. But, we need a lot more info regarding the minimum data capture rate, how much data you want to store, how you want to retrieve the data, the voltages and frequencies of the signals, available voltages to power the system, max size and weight you can fit, etc.

Are you up to speed on laying out a surface-mount PCB and soldering the componnts to it? I'm not sure this will all fit in a small model if you use thru-hole components.

Which microprocessor would you prefer to use? Which language and development tools do you have?

How are you going to retrieve and view the data?

LOTS of more questions but that's a good start.
John

[Edit] LOL...sorry for the barrage of questions. I'd be happy to help if I can but wanted to get a better idea of what the requirements were and what resources you already had.

Last edited by CamLight; 08-01-2008 at 09:46 PM.
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Old 08-01-2008, 02:43 PM
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pd1
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This might sound silly and not work for your application.

If you had a second receiver and servos set on a frame and a moving paper sheet,
like an EKG machine, you can attach pens to the servo arms.

The servo arm movement will mirror the movement of the servos in the plane.
The moving paper will give you a time frame.

This way you won't have to carry all that equipment in the plane.
Might be too simplistic.
Won't show actual aircraft displacement, but it would be a good approximation of servo or control movement.
Paul
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Old 08-03-2008, 02:39 AM
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Golani51
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Hi Guys:

I would prefer keeping it as simple and 'clean' as possible.....and light of course due to the nature of the platforms I will be testing. If possible, and within reason due to the financial limitations (any sponsors out there with old stock to get rid of??), I would obviously prefer COTS equipment.

I will need to fly MAVs in wind tunnels and record the control surface deflections (already wired the HS-55 servos to allow for feedback from the signal wire), and the x,y,z displacements of the aircraft in addition to rotations around the x,y,z axis. For this we have already sourced a couple of 5DOF(cannot have everything!!) gyro accelerometers from sparkfun (http://www.sparkfun.com/commerce/pro...roducts_id=741).

Ultimately, what I wish to to is record, onboard or continous download, are the rotations and displacements measured by the gyro-accel, and the signal feedbacks of 3 or 4 servos. Although the option for other sensors in the future would be great, these listed above are what I am hoping for at this point ideally. The nine-channel system would be more than enough.

Weight and bulk are big limiters. To keep them down, I have no issue with an onboard memory( probably a 4 MB flash chip). Data can be downloaded at the end of the flight. Flights will be up to a minute so I am not looking for huge memory.
The data should be sampled at 50 to 100Hz and resolved using a 12-bit analog-digital converter.
My soldering skills aren't brilliant but I will have the time to ensure I can do what needs to be done properly.

I don't have much going in the way of microprocessors and so forth but will have to find a solution to that too.

Too Easy!

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Old 08-03-2008, 09:29 AM
  #5  
CamLight
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I'm still not sure if you want to build it yourself (your mention of microprocessors) or buy off-the-shelf (your mention of COTS equipment).

What's your weight limit for the onboard equipment?

What's your time frame for completing development and testing of the system?

What's your budget?

Sounds like your MAV will be free-flying, not static. That probably rules out running wires down the a DAQ system from sensors in the MAV. May not need to do wireless as you don't mind downloading data after the flight.

Assuming 9 channels, sampled at 75Hz, you'll need to store 675 two-byte readings every second. That's a sample about every 1.5mS (one byte every 750uS). Any data storage setup will have to handle that. You can always split the data storage duties among multiple micros to reduce the speed requirements.

What's the signal format coming out of the servos? A standard PPM pulse train or analog voltage equivalent?

A big problem will be the high sampling rate and amount of data vs. the high noise environment the data will be gathered from. Not much of a chance to add heavy low-pass filtering or to grab multiple samples and average them for each reading.

Do you have a requirements document written up with all the specs that needed to build the system? Or are you in the exploratory phase, just defining the rough requirements?

John
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Old 08-03-2008, 12:40 PM
  #6  
Golani51
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Hi John:

What I was trying to say in the previous post was that I would prefer to purchase as much as possible from a COTS source. That being said, I am limited in the financing I have available and will have to keep spending to a minimum if at all possible. I do not have a set budget, but it will mostly come from my pocket, which as a PhD student with a new baby, is not too deep to be honest.

As far as weight is concerned, I will be testing small aircraft up to and including the FLASH 3D. Weight must obviously be kept to a minimum. Bulk is less of an issue, although important nevertheless. The killer, however, will be the weight issue. I am realistic that small, light components will come at a price but need to keep this down as much as I can.

I have the wind tunnel booked in for mid-December so have 4 1/2 months to get this together. I am prepared to do what I must to ensure it is completed on time.

The MAV will be free-flying. Data does not need to be downloaded continuously. The lightest and simplest option is where I wish to head.

I have wired up the servos by piggy- backing off the signal wire and measured a voltage output that is consistent among the four HS-55 servos we will be using (0.92-1.6V for the extreme ranges of motion).

The sampling rate is one of the big issues I have had. Noise is obviously one of the others. The combination of the two gives me a headache!!

I don't know enough to define the requirements much more at this point. I haven't enough experience in this area to do so but if you have any queries, I can put it past some of the people involved in different facets of the program and pass on the responses.

I appreciate any advice you can give, and thank you for everything so far.

Reuven
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Old 08-03-2008, 01:02 PM
  #7  
cbatters
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What commercial solutions have you considered?

http://www.eagletreesystems.com/

http://milehighwings.com/index.php?cPath=30

http://www.rcgroups.com/forums/showthread.php?t=881344

http://www.magtechinc.net/SkyTraceGPS.htm

http://home.arcor.de/d_meissner/d_logg2_engl.htm

http://www.wattflyer.com/forums/showthread.php?t=26382
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Old 08-03-2008, 01:13 PM
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CamLight
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Hi Reuven,
Thanks for being so patient with all of the questions.
COTS equipment, as you know, will give you an almost plug-and-play solution very quickly, but will cost you....the classic tradeoff. If your budget is US$5000, we're OK. If it's US$500, that's a big problem.

I'm not familiar with any COTS equipment that will do the job for a relatively low price.

If you've already done the calculations and 12-bit resolution will work for you there are a couple of options to try...
-I was going to mention the National Instrument's 6008 DAQ Module (or the 6009 for 14-bits): http://sine.ni.com/nips/cds/view/p/lang/en/nid/14604
You can remove the PCB from the case or order the OEM version that comes without the case and uses a bare header for interfacing. These are incredible units and are very reliable. We use them for all sorts of stuff. But, no onboard logging capability. <deep sigh> Would have been perfect.

- Ask around on the forums (here, and I'd recommend RC Groups too) to see if anyone has a lead on small multi-channel data loggers. But, most of them are single channel and read the voltage across a current shunt. You'd have to hack them pretty heavily to get them to fit your application.

- http://www.omega.com has quite a few multichannel dataloggers. Don't kjnow if any are small enough for your use though.

- Find another student who needs to design a multi-channel logger for their thesis. No...really, this can work very well.

- Find a tiny single-channel logger, rip it apart to shrink the size and install several of them in your MAV. I've seen some great USB-interface loggers that are about 1" in diameter and 4" long for about $79 or so. A lot smaller without the case.

Not much more I recommend without knowing the size and weight limitations you have and the max budget limit for the project. Those three things will really help to focus the project and determine whether you can go COTS, a mix of COTS and custom, or if you need an all-custom solution.

John

[Edit] I forgot another method. Take something like cbatter's link to the "Self-made Data Logger" (http://home.arcor.de/d_meissner/d_logg2_engl.htm) and modify as necessary....half COTS, half custom. But, unless you have access to the source code, you're very limited in what functionality you can change.

More and more I'm thinking that you need to go fully custom with this project. It'll be the least expensive option, will do exactly what you need, and you may be able to use stuff from Parallax or PICAXE to make development incredibly fast and easy. In fact, check out http://www.parallax.com and http://www.rev-ed.co.uk/picaxe/. They might already have some sort of datalogger development boards. Otherwise, using their micros and Basic makes for quick work of a datalogger. Check how fast you can log the data though....not sure of their throughput.
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Old 08-03-2008, 01:42 PM
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cbatters
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I suggest you add additional paramaters to the project including budget, time, software expertise (embedded programming and system level programming) as well as analog and digital electronics experience to bound the possible options.

Lots of tools out there to build a one off loggger but it would be much more efficient to modify a commercial solution that is close to your requirements.


Note: Microchip has a full line of components and development tools if you have lots of time.

http://www.mouser.com/catalog/634/199.pdf

http://forum.microchip.com/tm.aspx?m=123533

http://search.microchip.com/searchap...sperpage=10&id=

http://www.greta.dhs.org/UBW/

http://www.uelectronics.info/primarn.../microchip-pic
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Old 08-03-2008, 01:54 PM
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A little more searching

http://library.ncrtm.org/AT/RESNA_20...es/Simone.html

http://www.sparkfun.com/datasheets/W...matic_V1_0.PDF
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Old 08-03-2008, 02:01 PM
  #11  
CamLight
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At a quick glance, that looks like one heck of a place to start for this project!

John
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Old 08-03-2008, 03:13 PM
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cbatters
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Originally Posted by CamLight View Post
At a quick glance, that looks like one heck of a place to start for this project!

John
And you don't even need to build a PCB to add a 3 axis accelerometer..

http://www.sparkfun.com/commerce/pro...roducts_id=692

http://www.analog.com/en/mems-and-se...s/product.html



Or step up the pace and add additional gyro and compass sensors.

http://www.sparkfun.com/commerce/pro...oducts_id=8726





Disclaimer: I used to work for Analog Devices (Great Company!) and I have also done some work over the years with the folks at SparkFun.
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Old 08-03-2008, 03:18 PM
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CamLight
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Analog Devices, eh?
We're using their products all thru a new line we're working on....very nice stuff!

MEMS sensors are just incredible. I can't believe how small and inexpensive they are compared to the way this kind of stuff was done just a few years ago.

John
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Old 08-03-2008, 03:27 PM
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Originally Posted by CamLight View Post
Analog Devices, eh?
We're using their products all thru a new line we're working on....very nice stuff!

MEMS sensors are just incredible. I can't believe how small and inexpensive they are compared to the way this kind of stuff was done just a few years ago.

John
Product Manager Data Acquisition and Control Group....
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