Some questions about the impact example (S.E. 2009 SV)

[jeff_scharpf]

Hi,
I am trying to use the impact sample project for the Sound and Vibration Assistant (2009).

A couple of things that confuse me.

1) The units that are used in the example are acceleration, but my impact hammer has a force transducer. When I try to make one channel force, it "splits" the two analog inputs and so it messes up the trigger. Triggered data is supposed to come out with both channels. I can only get one since the trigger input does not accept two different types of channels. Not sure if I'm being clear with this. I am writing this from home so I will get the code tommorrow.

2) The response graph is confusing me. I have an object that I am striking. When I just look at the FFT from each hit, I always see a distinct value at 800 HZ, and I believe this is the correct resonant frequency of this object. But when I look at the frequency response, I not only see a large value at 800 HZ, but also much larger values beyone 3000HZ ..

Sorry if this is ambiguous, I am having trouble explaining what I'm seeing and it's two different issues.

Bottom line, I guess to start with, how do I convert the example which looks to me like it's expecting acceleration for both channels, into a force input and an acceleration?

Also I've been wondering how to select an appropriate sample rate, and more importantly, how many samples for each "hit"?

I'm using a 9233 in a USB carrier, a PCB 50mV/lb impacting hammer, and a PCB 35201 triaxial accelerometer (only using one channel atm).

My hammer has lost its plastic insert, so I've used electrical tape to try to eliminate ringing.

Again I apologize for this long post and I'm grateful if you've made it through this lol.

Jeff

[Ali M]

Hello Jeff,

The project does not ship with a custom scale, therefore we recommend that you create a custom scale for the hammer signal to convert g to N (or lbf). You can create a custom scale from the DAQmx Acquire Step Setup tab. Select the hammer channel and select 'Create New...' in the Custom Scaling control. To answer your second question, in order to select appropriate sample rates, double click on the DAQmx Acquire (Analog Input) step. The Acceleration configuration Window will launch and you can select a channel (under Channel Settings) then configure its sample rate (Timing Settings). The example's default number of samples is set to 2000 samples at 10KHz; however, these values can be changed to fit the needs of your application.

Best Regards,

Ali M

Applications Engineer

National Instruments

Message Edited by Ali M on 04-23-2010 03:04 PM

[jeff_scharpf]

Thanks for the reply. I would like to clarify.

I understand how to set the scalars, and setup the data acquisition.

My question is more about preferred settings and why I see alot of g's on the higher frequency when I don't think it's real.

I will try to attach some pictures of my settings, results, etc.