Accelerometer & Hammer range settings ineffective

[Dowrin]

Hello,

I am making a software (in Labview), that measures natural frequencies and modal parameters. For this purpose I am using NI 9234 module that collects data from both an accelerometer PCB 303A (measures response acceleration in g) and a hammer PCB 086D05 (measures stimulus force in Newtons). My data are coming through NI MAX where I create 2 global channels for hammer and accelerometer. There are settings of 1) range in g or N, 2) sensitivity in mV/g or mV/N, 3) excitation current in mA.

So far (from other discussions and several papers) I have understood that while both hammer and accelerometer can be excited by 2-20 mA, I can use the primary setting of NI module internal 2,1 mA. Both instruments, according to their spec. sheets, should have excitation voltage less or equal to 19 V specified in NI 9234 manual.

Now I don't understand one thing: in NI MAX there is setting of range firstly of acceleration. My accelerometer has range of acceleration +-500 g (written in specifications). My usual signal has its maximum around -+50 g, so it's approx. 10% of the specified range. As my experience is, while measuring with an instrument, it's better to have range set to value that is no more than 2 times the expected value (for better accuracy). So I tried my measuring with different settings of range: 500 g, 100 g and 1 g. In ordinary (non-dynamic range) instrument, I would expect more accuracy in the 100 g setting and clipping in the 1 g setting. Nothing of that happened, the coherence of the Frequency Response Function was roughly the same in all cases as well as other outputs from my program and values had their peaks at still the same amplitude (~50 g).

The same story was with the hammer: specified range is 21 kN, my usual value is around 300 N and even when I had range set to +-1 N, there wasn't any clipping or change in coherence.

So here are my questions: what is the range setting for? Has it some other meaning? And finaly: what value should be set there?

Just to note that I have set the sensitivity for accelerometer to 10 mV/g and for hammer to 0,23 mV/N (as written in specs.). My measurement range of frequencies is 0 to 10 kHz. Coupling used: AC. Manual to all three instruments are in the attachements.

Thanks in advance for any answer form anyone.

Dalibor Preisler

The specifications are here: https://drive.google.com/folderview?id=0B55KozzmLYiHflFzaGhlb2dEeHg3cWdTZFVTb2FxV2wwZTlsQWMwWUVaOV95...

[jozef.liptak]

Dear Dalibor,

If you set any range, LV will set the nearest higher possible range what the used device supports. In case of NI 9234: this cRIO module has only one input rage +-5V. So if you set any range in the settings of MAX or LV the used input range will still be +-5V. Therefore you are experiencing the same behavior. If the cRIO card would support other rages you can select them. That is also what should be set there: the best practice is to set a value in the settings that is supported – that cases no confusion. If you want to check the supported input/output ranges refer to the specifications document of the device (you included it – page 21).

Have a nice day!