accelerometer mounting

jacqueskleynhans · Posted: Wed Aug 27, 2008 12:36 pm

So i have successfully written the the code build the board now for the maths. How can i do a vector compensation so that the reading on the accelerometer is not influenced by the natural force of the earth??

My accelerometer is reading accurate till 4mg best i can do for 10bit. Just need to solve this very last problem. To simple subtract the value is not going to work!! So has anybody have any ideas regarding my problem. Help will be greatly appreciated

Kind regards
Jacques Kleynhans
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Guest · Posted: Wed Aug 27, 2008 12:45 pm

Hi,

If you AC couple the output you loose the effect of static G.

Not sure if this is what you are looking for, give more info. if not.

Regards

jacqueskleynhans · Posted: Wed Aug 27, 2008 1:46 pm

The thought of an ac couple has crossed my mind, but dont you lose resolution? I need to monitor slight vibrations from almost 20 hz to vibrations in the range of 1200 hz. Im doing a remote monitoring system for high voltage transformers.

Measuring their vibration at fundamental and at the harmonic intervals of 300, 500, 700, and so on.

Kind Regards
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newguy · Joined: 24 Jun 2004 Posts: 1903

Some thoughts.

It's reasonable to assume that the vibration of a transformer will be small compared to the force of gravity. In other words, you can very likely expect the vibration to be much much less than 1g.

Under this assumption, your signal of interest (the vibration) will be very difficult to see because of the effect of gravity. I also assume you don't care about DC acceleration (gravity) because you want to analyze vibration. Basically, without AC coupling, your signal will be like a very small AC signal added to a very large DC bias. Sort of like trying to determine the mass of a fly by measuring the change in mass of an elephant with and without the fly on its arse. Depending on your accelerometer IC and the A/D, you really won't see a large variation in the accelerometer's values. Good luck extracting useful spectral information from that.

Your best option is to AC couple the accelerometer's outputs, amplify & filter the signals, then feed them into an A/D. Designing a HP or BP filter circuit with a low frequency cutoff of ~20Hz is actually very trivial. Implement a programmable gain amplifier into the signal chain, and you can then implement an automatic gain control function in your product that can automatically tailor the gain to obtain the best resolution without clipping.

RLScott · Joined: 10 Jul 2007 Posts: 465

jacqueskleynhans · Posted: Wed Aug 27, 2008 2:48 pm

well firstly I'm not using any amplification circuitry on my sensor just the basic LPF described in the data sheet. My sensor has a g range of +-2g and i am utilizing it fully. We did some tests in the lab on diy vibration system running a 100hz sine signal through a amp into a speaker with a makeshift table and connecting the accelerometer to that and it actually shows its vibrating a -+500mg.

RLScott · Joined: 10 Jul 2007 Posts: 465

jacqueskleynhans · Posted: Wed Aug 27, 2008 4:34 pm

Thx for the information.
There must be a way of doing it by means of some sort of vector analysis then compensating for it. I don't know all the maths but surely I can figure it out?
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RLScott · Joined: 10 Jul 2007 Posts: 465

jacqueskleynhans · Posted: Thu Aug 28, 2008 4:38 am

Is there an sort of easy way doing this, or is it going to be hard you whould not now of an tut or something descibing this procedure this. This method is cosine interpolation right.
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SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

The accelerometer will have an offset due to gravity. But it will have other offsets too, due to sensor imbalance, temperature drift, aging, etc. The result will be the sum of all these offsets. It does not pay to work too hard on one offset while others are still unknown. I would just output accelerometer readings, and let the Matlab guys identify and remove offsets.
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FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

I basically agree with the opinion, that AC coupling doesn't give much additional sensitivity with an 10 Bit ADC. You didn't yet tell, which device you are using, is it an Analog ADXL? The DC bias can be removed in software as well, and you don't need to know about the exact gravitation component, I think. So there's nothing to calculate particularly.

For a maximum sensitive vibration detector, usually a seismic acceleration sensor (either piezoelectric or moving coil) is better suited. But I guess, you have checked that your device has sufficient sensitivity respectively low noise for your application.

jacqueskleynhans · Posted: Thu Aug 28, 2008 8:09 am

Im using a LIS3L02AL 3 axis -+2g analog accelerometer. with an sensitivity of 660mv/g. This device has very small sensitivity change wrt temperature change. You said this can be done in software, otherwise the matlab oukes are going to have to remove it.

I know the device does not have military precision but i suppose for my application its alright.
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