battery powered low voltage amplifier

[mark.arkinstall]

I would like to build a battery powered low voltage amplifier but don't know where to find a suitable circuit.

The purpose of the amp is as follows. As a structural engineer i need to measure building vibration. I have an accelerometer and a analog to digital converter which has a +/-10V range, that powers up and transfers data to my laptop via USB. Sometimes the signial from the accelerometer is so low that it needs to be amplified so i can take advantage of the full +/- 10V range . The battery is for mobility on site. I am envisaging a rechargeable battery.

As building vibration occurs at low frequency the amp needs to operate down at frequencies as low as 0.2Hz.

Ideally the amp would have low noise, and multiple preset gain settings to chose from.

I would appreciate any help at all. I did electronics at school for 3 years so i have some familiarity with the jargon.

thanks in advance
Mark

 

[Nigel Goodwin]

Probably the easiest way to power it is two 12V batteries, giving a +/-12V supply rail. You can then use a simple opamp circuit, DC coupled (so no problems with low frequencies) and switchable gain.

What sort of gain do you require?.

 

[mark.arkinstall]

the accelerometer is 100mV

 

[mark.arkinstall]

the a/d converter max out at +/- 10V

 

[Nigel Goodwin]

So you only need a maximum gain of 100 - it's very easy to do with an opamp.

 

[mark.arkinstall]

Thank you for your help so far.

I am a novice at this stuff. Do you have an opamp circuit i could see?

 

[Nigel Goodwin]

Have a look at for some help with opamps, however DON'T use a 741 - it's a seriously old lo-spec antique, use a slightly more modern device like the Texas TL071 or TL081.

 

[mark.arkinstall]

Thank you Nigel I shall take a look.

Can you please explain what you mean by DC coupled?

and also how would i achieve "switchable gain"? Is there any way that I can calculate the gain that i would get?

 

[mark.arkinstall]

ok i have read up on op amps... it seems that its justs a transistor with two resistors, with feedback.. is that correct.

Voltage Out = (R1+R2)/(R1) * Voltage in

So the resistors are sized to get the gain your after...

So how does the two batteries and accelerometer fit into the circuit???

 

[Nigel Goodwin]

ok i have read up on op amps... it seems that its justs a transistor with two resistors, with feedback.. is that correct.

No, it's an IC (LOT'S of transistors) with two resistors, the gain of the IC is so high that the eventual gain can be set by a couple of simple resistors.

 

[RadioRon]

While this is a relatively simple project, I wonder whether I would want my structural engineer to measure building vibration with home-brew electronics when he doesn't really know much about electronics. I know it might be fun to build such a thing, but isn't your paid time better spent doing structural engineering? When it comes to professional applications, especially those that impact public safety or my client's pocketbook, I would use professionaly designed electronics if I were you.

You might find something ready to use here:
www.omega.com such as this one:
**broken link removed**
although this one doesn't have a particularly wide output voltage range.
Then there is:
http://www.linearx.com/products/accessories/LP201/LP201_01.htm

I believe that Bruel & Kjaer (http://www.bksv.com) make accelerator preamps (liek the 2635), but I couldn't find them on their website.

To search for more, I found that "accelerometer preamp" or "...preamplifier" was the best search term.

If you still want to make your own, it is necessary to know something about the accelerometer like the type (piezoelectric, charge output or ?), its output impedance, is it amplified internally or not? If you can provide a maker and model number we can usually look it up. This is necessary because the input impedance and possible DC offset need to be estimated, and the circuitry for coupling the bias supply inline must also be added if needed.

Once completed, the amplifier must be calibrated in order for you to know exactly how much gain you have. This is a simple procedure, but requires a very good voltmeter.

And here is some theoretical info:
https://www.electro-tech-online.com/custompdfs/2007/06/TN_Accelerometer20Noise.pdf

 

[mark.arkinstall]

Thanks Radio Ron,

You can rest assured this project is really for my learning experience. My company does have several (2635) preamps which we use regularly.

THis project is about giving me a greater understanding. I would not be using this project for professional work. However to give a description for others reading this thread I had to explain the application it would be used for.

While I might be a novice at eletronics I am more than competent at Structural Engineering. For some of my work you could try a google search for "watercube" which will be the swimming centre for the Beijing olympics.

There is more info at **broken link removed**


cheers
Mark

 

[HarveyH42]

Semi-related question...

As a kid, I use to read a lot of Nikola Tesla stuff, and always wondered if there was much truth in the mechanical resonance stuff. He claimed to have caused an earthquake in NYC, said a small pocket-sized oscillator could topple a building. The right frequency could reduce a stainless steel rod to powder.
At the time, I just found it interesting reading, but didn't take everything as fact. Much later in life, I started to find out that many of the amazing things were true, but there was also a lot of stuff that is still debatable.

Just curious about how big a problem resonance is in designing a building.

 

[Bob Scott]

Semi-related question...
Just curious about how big a problem resonance is in designing a building.

Yes. One or two cycles of 0.2 Hz seems about right for the minor clock pendulum rattling shakes we get here, but...

Seriously now, do you have a degree in structural engineering? Take a look here for resonance in structures:

https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge#First_bridge

Did you maybe skip some classes? Even your 3 years of electrical classes should have covered resonance and damping.

Bob

 

[mark.arkinstall]

I can point you to several examples.

1. Try googling "Tacoma Narrows" Bridge . Its resonant cross wind vibration response caused its collapse. There was a university studying the bridge at the time and it actually collapsed on film !

2. Millennium Bridge lateral Vibration problem due to synchronised footfall vibration caused large lateral vibration resonance response which was eventually solved with 20% of critical damping in the form of viscous dampers

3. Flagpoles and street lights that vibrate in the wind even at low wind speeds. This is because the critical wind speed for different objects depends on their natural frequency f vibration and their shape and mass distribution.

4. The vibration of your car aerial when your driving down the freeway is a lateral cross wind resonant vibration response. High cycle low stress can lead to sudden fatigue failue.

5. Earthquake response of a building is significantly affected if the frequency content of the earthquake is similar to the natural frequencies of the building, possibly causing resonance.

6. Wind induced motion sickness in high rise buildings is due to resonance laterally from wind at a critical wind speed which causes vortex shedding that coincides with the buildings lateral frequency.

So in answer to your question... yes vibration can significantly affect the vibration of a building

 

[things]

mmmmm, mythbusters tested the "earthquake machine" and apparently it did work on a big bridge, but not so much that you could feel it yourself.

 

[things]

oh, and the thing they used was not pocket sized, they used a linear actuator and a huge controller

 

[RadioRon]

Thanks Radio Ron,

You can rest assured this project is really for my learning experience.
Mark

In this case, let's get to it. My point about knowing the source resistance, that is, the equivalent resistance of the accelerometer, is critical. I am no expert on accelerometers, but a quick search shows that they can be piezoelectric devices with extremely high source impedances. In fact, many are called "charge output" types and when you look up "charge" amplifiers they are calibrated in Coulombs/Volt or something like that. This implies that they treat the accelerometer as a capacitor that is piezoelectric and putting out a very small charge while modulated by vibration. So I think we need a FET input amplifier of some sort to provide the necessary impedance buffering. Do we need a differential or single ended input? This also depends on the accelerometer.