designing a DAQ

Hi all, I have to design a data acquisition system for two different signals. One is within the frequency range 300 hz (+ or - 25 Hz) and the other one is less than 5 Hz. I do not know the amplitude ranges of these frequencies(I guess it may depend on the sensors used). I do not know how to select the sensors and how to design the other hardware. If you can either give a pointer to me or if you could tell me what are the dos and dont's and the important things to consider while designing. Can I design a single daq card that can capture the 300Hz and 5 Hz signals. I just know that I need to use a D/A converter and a signal conditioning (amplifier and filter) circuit. How to proceed. Are there any proper commercial techniques related to DAQ design? I have collected some data from some micrcontroller project sites but I am seeing some expert comments or suggestions being passed in this group and I want to get some suggestions.

Thanks in advance.

Reply to
kkrish
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The design depends on your exact requirements. There are many processors/controllers that have support for multilpe DACs. Else, you can use a single DAC that can have multiple channels. There are many opensource tools like COMEDI are very famous in DAQ world. Further, RTlinux also supports COMEDI.

Karthik Balaguru

Reply to
karthikbalaguru

The first thing you should research is the difference between a D/A converter (DAC) and an A/D converter (ADC). If you are acquiring data, you need an A/D converter.

The next thing you need to do is get firm specififications on the input signals and sensors. Without those specifications you will have a tough time designing a good system.

A good place to start would be the National Instruments web sit,

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They've been in the PC data acquisition business about as long as there have been PCs. I'm sure they have notes and white papers on the general process of data acquisition.

Mark Borgerson

Reply to
Mark Borgerson

Well, maybe, but he said DAQ - Data AcQuisition, not DAC.

Reply to
larwe

What he said was:

" I just know that I need to use a D/A converter and a signal conditioning (amplifier and filter) circuit. "

Mark Borgerson

Reply to
Mark Borgerson

While I hate to be blunt, you seem to not understand what you're doing. Now that's far from a crime, but that being the case, and in a world saturated to the gills with commercially available data acquisition cards in every combination of form factor, speed, resolution, and range, do you really want to be building this yourself?

I'm an instrumentation guy, I've got a bias towards selling my stuff (though it sounds like what you're looking for would be lower end than anything I could offer you). But it sounds like you're spotty on the theory side of things and have no experience designing this kind of hardware.

And so before you get too deep into planning your "design", you want to: A) Figure out what your actual system requirements are. So far, you know that you've got "signals", and their approximate frequencies, but nothing more about them. You'll never get anywhere from there. What are you actually trying to sense and why? Then you figure out the transducers to turn that information into an electrical signal, and THEN you'll know what you need in terms of data acquisition hardware.

Then B), take a step back to the build/buy decision, and try to get real pricing on something that will accomplish your task. I second someone's recommendation to look at National Instruments. They're a competitor of mine, but there's no denying that their webpage is a cornucopia of information for someone trying to get a feel for what sorts of DAQ hardware is out there. Then once you've got an idea of what you're looking for, go to Google and see who else can do it for you. It very well may be that a couple hundred dollars will get your problem solved.

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Rob Gaddi, Highland Technology
Email address is currently out of order
Reply to
Rob Gaddi

LOL! I pointed out National Instruments, because they're NOT a competitor of mine---at least not until they can put together a system that runs on 15mA and will fit inside a 3" ID pressure case.

Thanks for seconding my opinion. The OP does have some basic research to do. If he wants more help, we'll need more information on the system requirements.

Mark Borgerson

Reply to
Mark Borgerson

Hi all, Thanks for your replies.

you are right, I wrote it as DAC but actually I meant A/D and thanks for pointing out the mistake.

What are the other information you need? I do not know if the sensor and sensor connectors I am going to use is a low capacitance or high capacitance because I do not know what type of sensor I need to choose. I am going to measure rotational force. Specifically I am going to measure the amplitude and phase difference of two rotating forces with a pair of sensors. As of now I assume noise level of the surroundings is negligible(Of course I have a filter) and I do not worry about the bandwidth now. I saw some application notes from IC manufacturers and NI website(after you said) and I understand something about what is DAQ but I still need help related to component selection and hardware.

Thanks in advance.

Reply to
kkrish

What do you mean by "rotational force"? Are you discussing torque on a shaft? What, specifically, are you trying to accomplish?

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Rob Gaddi, Highland Technology
Email address is currently out of order
Reply to
Rob Gaddi

What frequencies? How do you know that you need analog sensors? It depends on how you pickup the signals. Your first task is to figure out the sensors.

Reply to
linnix

Hi all, Thanks for your replies.

Yes, I am trying to measure torque on a shaft, and I also have find to the amount of force or vibration on the body containing the shaft when the shaft rotates.

Reply to
kkrish

There are probably better sensor guys here than me, but just offhand, the answer that comes to my mind is some kind of pickup monitoring the=20 gear teeth, either an optical or variable reluctance. Those two signals together can tell you the phase relationship between the two gears, from which you should be able to infer the torsional flex on the shaft, and from there the torque.

--=20 Rob Gaddi, Highland Technology Email address is currently out of order

Reply to
Rob Gaddi

Couple of 3D accelerometers should do it. One stationary for vibrations and one for rotations.

Reply to
linnix

Though getting the data out of the rotating one is left as an exercise for the particularly apt student.

--=20 Rob Gaddi, Highland Technology Email address is currently out of order

Reply to
Rob Gaddi

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