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# Zero offset, zero drift or something else?

## Zero offset, zero drift or something else?

(OP)
I have some strange readings from an impact hammer and acceleromeeters. The hammer is from Piezotronics PCB Model 086D50 and the accelerometers are from Dytran model 3333A and 325M24.

On the first picture called Force you can see the plotted force magnitude in Newtons with number of samples. I have applied in total 4 impact with the hammer and the approx distance between each impact was approx 10 seconds. The force plot like this looks normal.

Figure 1 Force

However, if you zoom in the force plot close to the hammer response at rest, you can see that there are fluctuations in the data. This you can see on the second attached picture named ForceZoomedIn. Red arrows point on the events between impacts. Between each impact there is a distance of approx 10 sec and for this interval the force from the hammer should be around 0. But why do I have these jumps in the reading from x-axis when the hammer is at rest ? Can you clarify why this happens with the data reading from the hammer?

Figure 2 ForceZoomedIn

The same problem occurs with readings from the accelerometers. The attached figures are showing acceleration reading as result of 4 hammer impacts.
On the third picture called Acceleration you can see the plotted acceleration magnitude in m/s^2 with number of samples. The four acceleration peaks are result from four hammer impacts where distance between each impact is approx 10 sec. The acceleration plot like this looks normal.

Figure 3 Acceleration

Again, if you zoom in the acceleration plot close to the acceleration response at rest, you can see that there are drifts in the data over time. Red arrows point on the events between impacts. The fluctuations are present for both accelerometer models. This you can see on the fourth attached picture named AccelerometerZoomedIn.

Figure 4 AccelerometerZoomedIn

Between each event in the acceleration there is a distance of approx 10 sec and for this interval the acceleration reading should be around 0 because there is no excitation of the structure. But why do I have these jumps in the noise reading from x-axis when the vibration dissipates out?

Thank you!

Emina B.

### RE: Zero offset, zero drift or something else?

Sample Number means nothing without knowing the Sample Rate and F-max. Why not present your X-axis in Time units? Typically there is only one impact per time record, so why the closely (apparently) spaced impacts? It is hard to tell whether your data and concern about it is significant.

Walt

### RE: Zero offset, zero drift or something else?

(OP)
Here are the plots of force and acceleration with x-axis representing time in seconds.
Figure 1 Force

Figure 2 Force Zoomed IN

Figure 3 Acceleration

Figure 4 Acceleration Zoomed IN

Are the jumps in between impacts, or settling in the data for hammer and accelerometers in between impacts normal?
Can I consider these measurements as OK?
Is there a way how I can center the data around 0 on the x axis but for events in between impacts?

Emina B.

### RE: Zero offset, zero drift or something else?

I suggest that you read the application notes from the manufacturer: http://www.pcb.com/Resources/Technical-Information... which explain, in some part, why what you are doing is not necessarily valid.

TTFN (ta ta for now)
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### RE: Zero offset, zero drift or something else?

My first thought was discharge time constant but your low amplitude signal looks quite wavy. Can you share the plots of transfer function as well as the coherence from the 4 averages?

Kind regards,
Jason

### RE: Zero offset, zero drift or something else?

I have the same model PCB hammer. It has a Microdot cable connector on the end of the wood handle. Swinging a heavy sledge hammer with small cable and connector caused a lot of signal problems until I refitted a larger cable and connector. I can't comment further on the signal "noise", without knowing the entire instrument system and settings. I suspect the data shown is the result of poor test procedure and settings.

Walt

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