How to convert an accelerometer waveform value to vibration level
Vibration waveform of NP series accelerometer (voltage output PS-601) is measured by an oscilloscope. How can the temporal axis waveform read-out value be expressed in the vibration level?
"Vibration level" must be measured by "vibration level meter". Vibration level is the value that is expressed as a result of applying weighting character of excitation sensation (leveling after performing frequency correction

) to the vibration signal, obtaining the scaling factor of the value against 10-5 m/s2, and applying a logarithm. Since the electrical configuration is the same as that of a noise meter, refer to "VR-6100 vibration level meter" for the details. The value obtained by leveling without applying frequency correction is referred to as a "vibration acceleration level". Since frequency correction cannot be applied this time, the method for "converting the value to that equivalent to the vibration acceleration level" is explained here. In this method, JIS-specified leveling processing is not performed. Therefore, use this value as the reference only.

 

Calculation example

For instance, if the voltage value is 1.39 mV (P-P), the following result is obtained as a result of 0-P conversion.

 

P-P = 1.39mV / 2 = 0.695mV (V0-P)

 

Convert the voltage to the vibration acceleration B m/s2.

 

B = 0.695 / A (m/s2)

 

Assume that sensitivity mV (m/s2) of A = NP-3120 is 1.00mV.

 

B = 0.695/1 m/s2 = 0.695 (m/s20-P)

 

Convert the value to effective value C.

 

C = B/√2 = 0.695/√2 = 0.491 (m/s2 rms)

 

Apply a logarithm for the ratio to 10-5 m/s2 to convert the value to oscillation acceleration level D

 

D = 20 Log (0.491/10-5) = 20 Log (0.491) + 100 = 93.8 dB

 

(Since a temporal waveform comprises various frequencies, frequency correction cannot be applied so that the value cannot be expressed as vibration level.)

 

If the gain of amplifier PS-601 is increased by ten times, is 1/10 of the read-out value the actual value?

That’s right. At calculation, assume 10 times (= 10.0 mV) of the sensitivity in <calculation example> indicated above as the sensitivity of A.

Revised:2002/08/19


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