15A.3 Scaling and Calibration

Back

This Appendix discusses using the SVL Scale Voltage to EU VI located on the Scaling palette to scale a

signal to engineering units (EU) and using the Calibration VIs located on the Calibration palette.

15A.3.1 Scaling to Engineering Units

This section discusses scaling data to the appropriate EU so one can perform measurement analysis.

Typically, scaling a signal to the appropriate EU occurs before any analysis is performed. Use the SVL

Scale Voltage to EU VI to scale the signal to the appropriate EU. All measurement VIs in the Sound and

TABLE 15A.3 NI PXI-4461 Gain Ranges

Gain (Referenced to ^ 10 Vpk)

(dB)

Voltage Range

(Vpk)

0 ^ 10

2 20 ^ 1

2 40 ^ 0.1

Virtual Instrumentation for Data Acquisition, Analysis, and Presentation 15-79

© 2005 by Taylor & Francis Group, LLC

Vibration Toolkit expect input signals and return results with the appropriate units, such as time-domain

signals in the correct EU, frequency spectra in decibels with the proper reference, phase information in

degrees or radians, and so on. To handle units properly, the high-level VIs need the signal to be scaled to

the appropriate EU.

If you use any method outside of the Sound and Vibration Toolkit to apply scaling to a waveform, do not

use the SVL Scale Voltage to EU VI. NI provides several tools and methods to apply scaling to a waveform.

These include, but are not limited to, NI-DAQmx tasks or global channels created with Measurement

and Automation Explorer (MAX), the DAQ Assistant, or the DAQmx Create Virtual Channel VI.

15A.3.2 Performing System Calibration

One typically performs system calibration with a dedicated calibrator, such as a pistonphone for

microphones or a handheld shaker for accelerometers. If you are calibrating a microphone, consider

using the SVL Calibrate Microphone VI. If you are calibrating an accelerometer, consider using the SVL

Calibrate Accelerometer VI. These VIs are very similar to the general-purpose SVL Calibrate Sensor VI,

but they offer the advantage of having default values commonly found for pistonphones or hand-held

shakers. All of the Calibration VIs use the characteristics of the calibrator, such as reference calibration

value and frequency, to perform the calibration.

15A.3.2.1 Propagation Delay Calibration

The Sound and Vibration Toolkit provides VIs for calibrating the propagation delay of the measurement

system. National Instruments DSA devices like the NI PXI-4461 and NI PCI-4451 can acquire and

generate signals on the same device. The input and output channels have analog and digital circuitry,

such as antialiasing and anti-imaging filters, that introduce a certain delay to the signal. The propagation

delay is the number of samples ranging from the time a sample is first written to the output channel, to

the time when that sample is digitized on the input channel, assuming there is no delay from the output

channel to the input channel. This delay varies by

DSA device.

There are two ways to determine the propagation

delay of the DSA device. You can refer to the

documentation for the DSA device to find the

propagation delay specifications, also referred to as

group delay. You also can measure the propagation

delay in samples with the SVL Measure Propagation

Delay VIs. The SVL Measure Propagation

Delay VIs allow you to measure the delay

introduced in the input and output circuitry for

a specific device at the desired sample rate.

Connect the DSA device output channel directly

to the input channel, as displayed in Figure 15A.2,

to measure the device propagation delay.

Note: Do not put a DUT in the signal path when

measuring the propagation delay for a DAQ device.

For an E or S Series DAQ device from NI, you

should expect to measure a one-sample propagation

delay due to the time required for the signal

to traverse the signal path between the DAC on the

analog output channel and the ADC on the analog

input channel. Figure 15A.3 shows the time

domain data for the propagation delay measurement

of an NI PCI-6052E.

FIGURE 15A.2 15A.2 Measuring the device propagation

delay.

15-80 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

For DSA devices, or any other device which

has onboard filtering on either the input, output,

or both channels, you should expect to measure

a propagation delay consistent with the sum of

the delays specified for the onboard filters on the

input and output channels. Figure 15A.4 shows

the delay of a smooth pulse generated and

acquired by an NI PXI-4461 with a 204.8 kHz

sample rate.

Not all DSA devices have a constant propagation

delay across the entire range of supported sample

rates. For example, the NI PXI-4461 propagation

delay is dependent on the output update rate.

Figure 15A.5 shows the total propagation delay vs. sample rate relationship for the NI PXI-4461 from

output to input as a function of the sample rate.

As illustrated by Figure 15A.3, Figure 15A.4, and Figure 15A.5, the propagation delay can vary

significantly with different sample rates and devices. To ensure measurement accuracy in your I/O

applications, determine and account for the propagation delay of the DAQ device at the same sample rate

used in your application.

It is important to remove the effects of the delay due to the data acquisition system for two reasons.

First, there is always a delay between the generated output signal and the acquired input on the device

even when the output and input channels are hardware synchronized. Second, the anti-imaging and

antialiasing filters of the device introduce additional delays. You must account for this delay to perform

accurate dynamic measurements. Use the device propagation delay [samples ] input on the examples

found in the LabVIEW program directory under “\examples\Sound” and “Vibration\Audio

Measurements\” to remove the delay due to the DAQ device.

The anti-imaging and antialiasing filters have a low-pass filter effect on the data. This effect results in a

transient response at sharp transitions in the data. These transitions are common at the start and stop of a

FIGURE 15A.3 Propagation delay measurement of an NI PCI-6052E.

FIGURE 15A.4 NI PXI-4461 propagation delay with a 204.8 kHz sample rate.

FIGURE 15A.5 NI PXI-4461 propagation delay vs.

sample rate.

Virtual Instrumentation for Data Acquisition, Analysis, and Presentation 15-81

© 2005 by Taylor & Francis Group, LLC

generation, at a change in frequency (swept sine),

and when the amplitude changes (amplitude

sweep). The swept-sine analysis and audio

measurements examples in the Sound and

Vibration Toolkit account for this transient

behavior in the device response to achieve the

highest degree of accuracy.

The propagation delay of the DUT is also an

important specification in some applications. For

example, the propagation delay for the DUT is a

required input when performing audio measurements

and when measuring the frequency

response using swept sine. If the DUT and the

propagation medium can successfully pass the

pulse signal used by the SVL Measure Propagation

Delay VIs without excessive attenuation, then this measurement also applies when measuring the

propagation delay of the DUT and the propagation medium. Figure 15A.6 shows the wiring diagram for

this configuration.

The DUT propagation delay is the delay of the entire system minus the device delay. Remember to

measure the device delay without the DUT connected.

The propagation delay for an analog DUT is a constant time delay rather than a delay of samples.

Use the following equation to convert the measured delay in samples to the equivalent delay in seconds:

delay½sec􀀉 ¼ delay½samples􀀉= sample rate½Hz􀀉

Навигация