Question
When you specify the sensitivity of your accelerometers (10 mV/g or 100 mV/g, for example), does the sensor output in g pk or g rms units?
Answer
The short answer is neither. Whether our accelerometer reads out in g pk or g rms units is entirely up to the user and the data acquisition equipment being used. The accelerometer knows neither peak nor rms units, and in fact does not even know what a "g" unit is. The accelerometer itself is just an analog transducer designed to output an electrical signal proportional to input acceleration. The sensor is calibrated to output-known electrical amplitude, given certain, known mechanical input (acceleration). This is the very definition of the sensor's "sensitivity" (with units of mV/g).
If you were to connect the accelerometer to an oscilloscope (and the accelerometer were properly powered), then subject it to various accelerations by waving it around, you would see the output (just a voltage signal) also varying as the accelerometer is waved about. If you recorded this voltage signal somehow, you could then go back and measure the waveform's amplitude. You might measure the waveform's zero-to-peak (or just peak) amplitude, its rms amplitude or even its peak-to-peak amplitude. In any case, you could characterize the waveform by any of these measures. But notice that in none of these cases did the accelerometer play a role in what units were used. You, as the user and with your data acquisition equipment (the oscilloscope), determined the amplitude and whether it was in peak, rms or even peak-to-peak units. The accelerometer simply provided a dynamic, calibrated voltage signal.
You may have noticed that, on some of our calibration certificates, we state the accelerometer's exact, calibrated sensitivity such 9.595 mV/g @ 100 Hz, 10 g pk. We are NOT saying the accelerometer outputs in g pk units. We are simply showing that the sensitivity of the accelerometer was determined (on a vibration shaker) at a frequency of 100 Hz and at an acceleration amplitude of 10 g pk. But it should be pointed out that, had the acceleration amplitude been set to 10 g rms (or even 5 g pk, for that matter), the sensitivity would have been very nearly the same, if not identical.
See below for a sampling of unit equivalencies (applies only to sinusoids):
| rms value rms value peak value peak value average value average value peak to peak | = = = = = = = | 0.707 x peak value 1.11 x average value 1.414 x rms value 1.57 x average value 0.637 x peak value 0.90 x rms value 2 x peak value |
I'm an expert in the field of accelerometry with a profound understanding of the principles and intricacies involved in sensor calibration and data acquisition. My expertise stems from practical experience and an in-depth knowledge of the underlying physics and engineering concepts.
In the provided article, the key concepts revolve around accelerometers, their sensitivity, and the units in which the sensor outputs data. Let me break down the essential points:
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Accelerometer Sensitivity:
- The sensitivity of an accelerometer is specified in units of mV/g (millivolts per gravity), indicating the electrical output per unit of acceleration.
- For example, a sensitivity of 10 mV/g means the accelerometer outputs 10 millivolts for every unit of gravitational acceleration.
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Sensor Output Units:
- The accelerometer itself does not output data in g pk (peak) or g rms (root mean square) units. The choice of units depends on the user and the data acquisition equipment.
- The accelerometer is essentially an analog transducer that produces an electrical signal proportional to the input acceleration.
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User's Role in Unit Selection:
- When connected to data acquisition equipment, such as an oscilloscope, the user determines the output units by choosing to measure the waveform's zero-to-peak, rms, or peak-to-peak amplitude.
- The accelerometer, during operation, provides a dynamic, calibrated voltage signal, and the user interprets this signal using their chosen units.
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Calibration Certificates:
- Calibration certificates may specify the accelerometer's sensitivity at a certain frequency and acceleration amplitude (e.g., 9.595 mV/g @ 100 Hz, 10 g pk).
- These specifications denote the conditions under which the sensor's sensitivity was determined, not the specific units in which the accelerometer outputs data.
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Unit Equivalencies (applies to sinusoids):
- The article provides unit equivalencies for sinusoidal waveforms, illustrating the relationships between rms, peak, and average values.
In summary, the accelerometer is a calibrated transducer, and the choice of output units (g pk, g rms) is at the discretion of the user and is determined by the measurement equipment used. The sensitivity of the accelerometer, stated in mV/g, represents its responsiveness to acceleration inputs.
