2130 measurement cautions

Title: Analog vs Digital for Overall Measurement

Product: Data Collectors

Program: 2115/2117

Version: Not Applicable

Technology: Vibration

Technote Number: 95-00008

On the analyzer under UTILITY - CHANGE SETUP - MEASUREMENT MODE - OVERALL LEVEL MODE, the user can select either DIGITAL or ANALOG.

The Overall value, calculated in DIGITAL mode, will calculate the value from the low frequency cutoff to the maximum frequency specified. In other words, the overall is calculated over the defined frequency range of the spectrum.

The Overall calculated in ANALOG mode will calculate the value from 2 Hz to the bandwidth of the instrument (2110 -20khz, 2115 - 30khz, 2120 - 80khz). The Analog overall value is sometimes referred to as an Unfiltered reading and is calculated from the waveform.

Title: Analog vs Digital Integration Mode for Measurement

Product: Application Notes

Program: App Notes




On the analyzer under UTILITY - CHANGE SETUP - MEASUREMENT MODE - SIGNAL INTEGR MODE there is the choice of ANALOG or DIGITAL.

ANALOG integration integrates data in the waveform then performs the FFT - Fast Fourier Transform. The end result is that the waveform stored is in the integrated units and the spectrum is stored in the integrated units.

DIGITAL integration performs the FFT first then the integration is performed on the spectrum. The end result is that the waveform is stored in the raw units of the sensor and the spectrum is stored in the integrated units. A common setup is to use an Accelerometer and integrate the signal to Velocity units. If DIGITAL integration is used then the waveform will be stored in G's and the spectrum will be stored in IN/SEC. However, one disadvantage to using DIGITAL integration is low frequencies will be enhanced. This causes a "ski slope" effect or "integration leg" on the low end of the spectrum. Typically this does not cause a problem unless the user is analyzing slow speed machinery.

The ANALOG integration steps are:

1. The Waveform is collected in Acceleration units of G's

2. The Integration is performed on the waveform.

3. The last Waveform collected is stored in Velocity units of IN/SEC

4. The FFT is performed.

5. The averaged spectrum is stored in Velocity units of IN/SEC

The DIGITAL integration steps are:

1. The Waveform is collected in Acceleration units of G's

2. The last Waveform collected is stored in Acceleration units of G's

3. The FFT is performed.

4. The spectrum is averaged in Acceleration units of G's

5. The Integration is performed on the spectrum

6. The averaged spectrum is stored in Velocity units of IN/SEC

Title: Analog/Digital Overall/Integration - 4 Cases, 1 Exception


Program: App Notes




Four possible cases for setup include:

Overall Level Mode: Digital Digital Analog Analog

Signal Integr Mode: Analog Digital Analog Digital

If a customer indicates overall measurement in the data collector shows as digital, but upon checking setup in collector for overall measurement mode he has actually chosen analog.

For overall mode in analog, the frequency range for data acquisition is 2 Hz to 20 Khz. For overall mode in digital, the frequency range is 2 x delta f to the F max specified in the analysis parameter set.

For integration mode in analog, the initial waveform signal is integrated when collected. The waveform is then stored in the integrated units. The FFT is applied to this waveform to get the spectrum. Since integration is performed on waveform, the mathematical amplification of spectrum at low frequencies is eliminated. For integration mode in digital, the waveform is collected and an FFT is applied to it to get the spectrum. This spectrum is then digitally integrated to get required result. The waveform is saved in native units of sensor, but "ski slope" has been introduced at low frequencies of spectrum.

The Exception:

Assuming integration is being performed on the point, with the combination of analog overall and digital integration modes, the expected result does not occur. The overall measurement is saved in digital not analog. This is because to get an analog overall, an analog integrated time waveform is required. The waveform is already in digital mode to get the requested digital spectrum.

In this situation the customer wanted overall in analog to be able to have any high frequency abnormalities indicated be trending overall values. The integration was in digital to keep waveform units in sensor units, Gs in this case.

CSI's recommendation is to take overall in digital and specify Fmax large enough to encompass the frequencies in question.Usually, problems occurring at high frequencies are so small that 1xRPM or other multiples will mask any changes. CSI now has the ability to save 12 trend parameters and this should be adequate to include any high frequency bands.

The integration mode should be in analog to eliminate amplification of values at low frequencies. A special time waveform may be specified to be collected in the analysis parameter set. This special time waveform will replace the original after FFT is performed, and can be specified to be collected in native units of sensor.

Title: Why Analog Overall is not recommended when collecting PeakVue Data


Program: App Notes

Version:



A customer asks what the difference is in the Overall collection mode and how it affects PeakVue data.

The signal processing method is the same whether Digital or Analog Overall is used. The spectrums shown below were collected on a rotor kit without moving the accelerometer, just the overall parameter was changed. The top spectrum was in Analog Overall and shows a value of .1246 G's as compared to the lower spectrum collected in Digital which is shown at .0540 G's. The Analog Overall will calculate a value from Fmin to the high end of the frequency response of the accelerometer. The Digital Overall will calculate a value from the Fmin to Fmax but because the HP Filter is set to 1KHz those values are filtered out and not included in the overall calculation.

Accel-Accel

500Hz Fmax

1000 HZ HP filter used for PeakVue

1 Average

Also note that the spectrums below taken from the rotor kit show all the energy below the High Pass FIlter selection. If no energy is present both Overall values could be very close.

The above spectrum was collected with Analog Overall

The above spectrum was collected with Digital RMS Overall

2130 measurement cautions

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