Author(s):
Kent L. Gee -
Department of Physics and Astronomy, Brigham Young University
The Acoustics Research Group (ARG) at Brigham Young University (BYU) comprises faculty and students from the physics and mechanical and electrical engineering departments. Using LabVIEW for data acquisition system configuration and control was convenient because BYU already had an NI Academic Site License for LabVIEW and its toolboxes.
Data Acquisition System
The foundation of our current DAQ system is the NI PXI-1045 18-slot chassis with eight NI PXI-4462 DSA modules, each with four analog inputs. The 24-bit DSA modules feature six selectable analog input gains and a maximum simultaneous sampling frequency of 204.8 kS/s. Additionally, analog output capability is provided by an NI PXI-4461 data acquisition module, which has two analog inputs and two analog outputs. In all, the system has 34 input channels and two output channels. We also use an NI PXI-7344 mid-range stepper/servo controller for our automated motion control applications.
Based on its superior performance and anticipated portability, the controller we selected for the PXI chassis was the NI PXI-ExpressCard8360, which is used for control with laptops. The NI PXI-ExpressCard8360 offers significantly greater throughput (up to 60 percent) over the legacy CardBus, and most laptop manufacturers provide ExpressCard slots on at least some of their laptops. We purchased two laptops to control the PXI system: the Dell XPS M140, a mid-size laptop with a single ExpressCard slot, and the Dell Latitude D820, a somewhat larger laptop with both ExpressCard and CardBus slots, a SATA hard drive, 2 GB of RAM, and a high-performance dual-core processor. Consequently, we use the XPS M140 for most everyday applications and the Latitude D820 primarily for high-channel-count applications where we need more horsepower. Control of the PXI chassis and DSA modules with LabVIEW and the NI-DAQmx platform from these two laptops has been relatively straightforward. The chassis and the XPS M140 are shown in Figure 1.
Figure 1. The NI PXI-1045 Chassis and Dell XPS M140 Laptop
Because the PXI-446x modules feature integral electronic piezoelectric (IEPE) technology, which is also known as CCP power, use of measurement sensors for our high-precision acoustics research is both economical and convenient – we do not need a separate transducer power supply. The BYU ARG has approximately 60 half-inch Larson Davis, PCB, and GRAS IEPE microphones. In addition, BYU recently purchased 32 quarter-inch G.R.A.S. Type 46BE IEPE microphones for measurements where smaller sensor footprint, larger bandwidth, or low sensitivity is required. The only disadvantage to these sensors is a limitation in useable bandwidth for large cable runs because of the inherent capacitance in the coaxial cable. To overcome this problem, we use low-capacitance coaxial cable in situations where we need to make high-frequency, long-range measurements.
Fan Noise Setups
The ARG features two experimental setups for fan noise characterization. The first is a stepper motor-controlled semicircular measurement boom, shown in Figure 2. Thirteen half-inch IEPE microphones are attached to the boom, which is then rotated to provide a hemispherical directivity measurement. The PXI-7344 controller drives an Oriental Motor stepper drive and motor, which, when coupled with various gears, rotate the boom with a precision of 1/60 of a degree. Screenshots of the LabVIEW measurement software interface are shown in figures 3 and 4.
One of the features of the measurement (see the upper right corner of Figure 4) is an incremental directivity plot for a given frequency as the program steps through the measurement process. This boom and measurement program yields the far-field directivity for the case-mounted fan as a function of frequency and flow condition. These directivity measurements are also performed while the fan noise is being actively reduced to provide information about the global noise reduction.
Figure 2. In this image of a semicircular measurement boom in BYU’s fully anechoic chamber, a 60 mm axial fan is mounted on the top plate of the aluminum chassis, and the device that is mounted on the tripod is a phototachometer for monitoring the fan’s rotational speed.
Figure 3. A Screenshot of the Boom Measurement Program
Figure 4. This screenshot shows how the measurement and motion screen of the boom measurement program displays the power spectrum for any of the channels as well as a directivity plot for a given frequency.
The second setup is a near-field linear array that we are using to explore characteristics such as the tonality and stationarity of the fan noise near the source. The linear array, which consists of 23 quarter-inch GRAS microphones mounted with half-inch spacing, is displayed in Figure 5. The array is then manually stepped over the chassis surface at quarter- or half-inch increments to map out a 2D acoustic pressure field. Because airflow over the microphones directly over the fan creates additional noise, only microphones outside the flow of the fan are used in the 2D pressure mapping. Also visible in Figure 5 are the four small circular loudspeakers used to actively control the sound field. The linear array is being used to determine the changes that occur in the acoustic near field when active control is engaged.
Figure 5. The Linear Array Used for Near-Field Fan Noise Characterization
These experimental setups bring up another issue – microphone calibration. High-precision measurements require frequent calibrations, which can be quite time consuming for high-channel-count applications. To streamline this process, we developed a “smart” microphone calibration for the PXI system. During the calibration process, a single researcher can start the program on the laptop located in the control room and then go into the chamber with the calibrator. The software simultaneously acquires data from all active channels in short increments and then searches the data for a valid calibration signal (250 or 1000 Hz) using the Frequency/Amplitude Detection VI in LabVIEW. This process continues in a loop until a valid calibration signal is detected on a microphone channel. Once that channel is found, the researcher can use the SVL Calibrate Microphone VI included with the NI Sound and Vibration Toolkit to calibrate that channel.
An LED in the anechoic chamber keeps the researcher apprised of the calibration status (see the red and white wires in Figure 2). The LED is driven directly using the analog outputs on the PXI-4461 module with a 12 V square wave generated in LabVIEW. The frequency and duration of the square wave pulse let the researcher know if the calibration was performed successfully or if there was an error. This calibration process is very efficient. One person can now calibrate 23 microphones in approximately five minutes. Other data acquisition platforms at BYU usually require two people and significantly more time to calibrate the same number of microphones. The LabVIEW program makes frequent calibration much more feasible.
Developing Applications
The fan noise characterization systems were developed in the few months that the ARG has had the PXI system. However, these measurements do not require even a significant fraction of the PXI-446x capabilities. The system was purchased for rocket and jet noise measurements, which are much more demanding.
Characterization of high-amplitude rocket and jet noise is an ongoing research challenge. Because acoustic shocks, which have fast rise times and important high-frequency content, often exist in the near field of these sources, high-bandwidth measurements are needed. In addition, variable acoustic amplitudes with frequency and with changes in engine condition require a large dynamic range.
With the PXI-446x modules, the PXI and laptop-based system provides a high degree of flexibility in making these measurements. In addition, the ability to stream large amounts of data in real time makes array measurements possible, thus maximizing measurement potential for engine run time.
Benchmarks with the Latitude D820 have shown that it is capable of streaming single-precision data in real time to an external hard drive for all 34 channels while sampling at approximately 150 kHz. If the internal SATA hard drive is used, real-time data acquisition for all 34 channels can occur at the maximum sampling frequency of 204.8 kHz. With its laptop controller, large dynamic range and bandwidth, and IEPE transducer capability, this system is near state-of-the-art for a maximally portable, high-precision field measurement system, which we have already tested in jet noise measurement studies.
Figure 6. Jet Noise Measurements With an F-16
Author Information:
KentL. Gee
Department of Physics and Astronomy, Brigham Young University
N319 ESC
Provo, UT 84602
kentgee@byu.edu
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