Reading a PSD, can I get a mean frequency from this plot? 3

[Mahti86]

Hi!

I´ve tried to analyze the plot attached for a while now, but it does´nt make any sense to me. I´ve never encountered a PSD plot before and I need a frequency (mean) to continue my calculation on vibrations, but the PSD just makes me confused. Could anyone extract some information from it to me?

Acceleration data:

http://files.engineering.com/getfil...-4199-8f97-4c6b72b2fc37&file=Eng_help-Acc.jpg

PSD for the acceleration data:

http://files.engineering.com/getfil...4cfe-4e34-a257-9859aaa33e34&file=Eng_help.jpg

Kindly //

 

[Mahti86]

Yes, and my task is to evaluate the implementation possibilities within an application with a small rechargeable battery, with the task to power some LEDs from time to time. Models for power output has been developed and there are a bunch to try out, which is my plan before I´ll give myself into putting together a prototype.

The piezoelectronic will be mounted as a cantilever, which bends back and forth due to the vibrations. This is where my need for average frequency bandwidth comes in '

ps. Im not trying to achieve high current, I just mentioned current as the limiting one which I´ll have to investigate closer than the other factors.

 

[IRstuff]

OK, then it seems to me that this "average" frequency approach is not correct. Assuming that there is a frequency response curve for current output for the cantilever/piezo structure, the response curve should be multiplied by the input spectrum, and the resultant should be integrated over frequency to get the total current.

TTFN
faq731-376

 

[Mahti86]

The problem is that most piezoelements only harvests energy within a specified frequency, and the power output decreases heavily outside that spectrum. So I think that integrating the curve would give me false hopes of power output..

I´m currently halfway through the book "Energy Harvesting Technologies" by Priya & Inman, and I´ve read countless of articles. And the most of the equations rely on the electronics within the circuit and the surrounding kinetic energy (amplitude and frequency). Unfortunately the piezoelements are a bit too expensive for try-and-error solving, thats why Im bound to the calculations.

Btw, atm Im looking into this harvesting kit. And I´ve managed to sqeeze in two of them within my device (with CAD). Unfortunately it also has a higher frequency demand than I think I´ll have if the PSD would give me some actual frequency data.. So currently Im looking for a solution that offers a broad frequency band for energy harvesting.
I think that the average frequency bandwidth would lie around 20Hz, or am I totally lost now?
(My assumption is based on other data on the net about normal car driving accelerations & frequencies)

ps. It´s really hard to find anyone to rely on with discussions within piezoelectronics, so you are actually the one I´ve had the longest discussion with.

 

[IRstuff]

"piezoelements only harvests energy within a specified frequency, and the power output decreases heavily outside that spectrum."

That's precisely what I'm saying when I talk about the response curve. You simply multiply the response curve against the input PSD to get the weighted output PSD. The integral of that is the average power expected from the transducer.

TTFN
faq731-376

 

[amanuensis]

If PSD is not appropriated for analysing a random vibration, then you can also use another indicator: the Extreme Response Spectrum. It is the same as Shock Response Spectrum. The only difference is that ERS is used for random vibration.
If your signal is supposed to be random but not stationnary, then ERS is a good indicator for analysing random vibation.

 

[IRstuff]

You are confused, by the way. The device's datasheet simply states that 52Hz is its "max rated frequency," although it's probably its most efficient at that frequency. Your PSD clearly shows response at, and around, the device's operating frequency.

Of course, your device only puts out 7mW at rated frequency and deflection, which is pretty miniscule, when you get right down to it.

TTFN
faq731-376

 

[Mahti86]

I´d like to try this approach, but Im a bit confused with the details. The response curve you are talking about, is it available to get from some piezo institutes or do I need to order a piezoelectric kit and measure the curve myself?

For the PSD: do I get the equation of integration interest from doing a "reversed bode plot" on the PSD? (it´s many years ago I had a course with bode plots, but its the only thing I remember where I´ve constructed equations from a curve)?

"Of course, your device only puts out 7mW at rated frequency and deflection, which is pretty miniscule, when you get right down to it."
Is it possible to achieve these two requirements with three of the devices coupled together:
(1) overcome the required power to start charging a small battery (around 120mAh)?
(2) to charge it in a reasonable time (Reasonable time = matter of hours, not days)?
Or is it completely out of question?

 

[IRstuff]

Depends on what the consumption is and what type of battery it is. Some batteries allow for high current (rapid) charging, and others do not. Are you attempting to run the electronics at the same time? What is its consumption?

Even in an ideal case, you would need something like 10 of these piezoelectric devices in parallel to get enough current to charge a fully drained battery within 2 hours, with no additional load.

Seems to me that you've not presented a sufficiently enough detailed operational scenario and requirements to even begin to figure out what you need to do.

TTFN
faq731-376

 

[Mahti86]

Seems to me that you've not presented a sufficiently enough detailed operational scenario and requirements to even begin to figure out what you need to do.

At my first try Im going with a Ni-Mh battery since it doesnt need extra circuitry to allow charging. The electronics are not going to be on at the same time as the battery is being charged (with and exception with the standby power of the circuitry ~1mA). The consumption of the electronics will hopefully not be over 50mA, but that is a bit uncertain for now with a choice of LEDs (the number of these has to be around 10 totally).

The 2 hours is a dream scenario, hopefully the idea is at least feasible and shows some charging activity which would allow more development and optimization.

The requirement of the lights today is that they can be activated 20times/day (8seconds/time) + one long term activation (60seconds), those are the main requirements for the energy source to handle.

Btw, I managed to get hold of the guy who produced the PSD today. He said that the PSD was constructed for one purpose only, and he has no idea why the data was sent to me since it cant be translated to something useful by anyone else. He is going to send me some PSDs and frequency data plots in the coming days that I actually can work with. But probably Im gonna need some help with those also

 

[IRstuff]

20*8+60 = 220 s
220s*50mA = 3 mAh plus 1mA standby

So there you have it. Your device puts out 1.4 mA at rated performance, and since you cannot have 100% charging efficiency, you'd need more than 8 hrs to recharge battery.

TTFN
faq731-376

 

[Mahti86]

I cant find it myself so I´ll have to ask, where did you get the 1.4mA from?