Technical Notes
Sigma-Delta Digital Technology Enhances Acoustic/Vibration Measurements
Abstract - The traditional method for making acoustic and vibration measurements for aerospace and defense applications involves the use of a nalog tape recorders. After a test is completed the analog tapes must be digitized for analysis. This process could require weeks or months to accomplish, depending upon the number of channels involved. The new VXI-based acoustic/vibration system that is described uses sigma-delta analog conversion techniques to directly digitize a large number of channels at a high rate and store the digitized data. This results in an economical approach that reduces the initial analysis time to hours or days from the p revious weeks or months.
I. INTRODUCTION
Analog tape recorders are the primary method for making acoustic and vibration measurements for aerospace and defense applications. Many of these tests involve a relatively large number of channels, each with an analog bandwidth in the range of 25 kHz to 50 kHz. The duration of a test may range from 10 seconds to 5 minutes. After a test is completed the analog tapes must be digitized and then analyzed. This process could require weeks or months to accomplish, depending upon the number of channels involved. Tape recorders are often used for the large-channel-count systems because of the extremely high cost of directly digitizing the information and producing gigabytes of data for a typical test.
This paper describes a new acoustic/vibration test system that uses the sigma-delta analog conversion technique to directly digitize a large number of channels at a high rate and store the digitized data. This results in an economical ap proach that reduces the analysis time to hours or days from the previous weeks or months. This system uses the latest state-of-the-art sigma-delta converters with an analog bandwidth of 92 kHz and a maximum scan rate of 200,000 samples per second per channel. The sigma-delta converters contain digital anti-alias filter conditioning with near-perfect channel-to-channel phase matching. During a test the data is temporarily stored in a 32 megabyte memory on each multi-channel sigma-delta ADC module.
This highly modular acoustic/vibration system uses the VXI (VME eXtensions for Instrumentation) open standard for the system front-end [1]. In addition to the sigma-delta conversion and local memory, real-time processing, such as FFT, PSD, octave analysis, etc., can be performed by powerful digital signal processing (DSP) module(s) in the VXI chassis.
The system runs KineticSystems' Reality® software, which supports high-performance distributed data acquisition [2]. The software allows the streaming of data to removable disk or tape at a rate higher than 5 Mbyte/sec. in continuous mode to minimize the time between tests. More than one VXI chassis can be used along with multiple disk or tape drives to further increase the overall throughput to tape or disk.
(Editor's note: While this paper discribes a UNIX-based system, similar I/O performance can be obtained with our recently developed DAQ Director(tm) software package operating under Window s NT.)
II. ANALOG TAPE RECORDING
The current method for performing acoustic and vibration measurements for aerospace and defense applications such as jet aircraft and rocket engine tests often involves the use of tape recorders to acquire the data during the test. One of the more popular multichannel tape systems is the METRUM Model 101e [3]. The tape system discussed here is available with 7, 14, 28 and 32 track head configurations. Eight tape speeds are selectable fr om 0.937 ips (inches per second) to 120 ips.
If this test suite were performed using analog tape to record the data, ten 32-channel tape recorders of the type described would be required. A typical translator that can digitize the analog tape operates at 100,000 samples per second with 2 channels. This would allow playback at the same speed that the channels were recorded, but only 2 channels could be digitized at a time. Since there are 320 channels, digitizing would require 160 seconds for every 1 second of recording. Assuming a test run of 10 seconds every 70 seconds, approximately 275 hours of digitizing time would be required for every 12-hour data acquisition shift.
As the model position is changed between test runs, it is important to validate the data with a "quick look." Near-real-time analysis is difficult with this approach. Because of the analysis difficulties, teams of specialists have been used to find and analyze interesting events. Because of budget consideratio ns, these specialists are often not available.
III. ANALOG-TO-DIGITAL CONVERSION
Traditional data acquisition systems use analog-to-digital converters (ADCs) of the successive-approximation type. This style of ADC includes a digital-to-analog converter (DAC) and compares the output of the DAC to the input signal. The procedure works as follows:
- The A/D sets the highest-order bit on the DAC to 1 and all other bits to 0.
- It then compares the DAC output vo ltage to the input signal (using an analog comparator).
- If the input is higher than the DAC signal, the bit is left at 1, if the input is lower, the bit it set to 0.
- This procedure is repeated with the next-lower-order bit, leaving the higher-order bit(s) with the previously determined value(s).
Thus, the digital value of the input signal is successively approximated until finally the least-significant bit (LSB) is determined, and the conversion is complete. This process is controlled by a clock.
Data acquisition at sample rates of 100,000 samples per second per channel and above have been rather expensive for the following reasons:
- Successive-approximation converters that operate at these rates are costly. A converter per channel is generally required.
- An expensive sample-hold amplifier is required for each channel to keep the signal constant during the conversion cycle.
- An expensive antialiasing filter is required for each cha nnel. The use of a sharp-rolloff lowpass filter prevents signal components above 50% of the sampling frequency from producing aliased data-false frequency elements that are produced by the difference between the sampling frequency and above-50% input information (usually called the Nyquist theorem).
IV. THE SIGMA-DELTA CONVERTER
The sigma-delta conversion technique has been in existence for many years. However, recent advances in integrated circuit design and fabrication have allowed the development of analog-to-digital converters of this type that have high resolution and accuracy along with high sampling throughput. The 16-bit ADC described here includes a sigma-delta modulator and a finite-input-response (FIR) digital. Additional information about the theory behind the operation of sigma-delta converters can be found in Harris Semiconductor Application Note AN9504 [4].
An Analog Devices AD7722 sigma-delta ADC is associated with each channel of the system descr ibed here. The analog input is continuously sampled by an analog modulator, eliminating the need for external sample-and-hold circuitry. The modulator output is processed by two FIR digital filters in series.
The oversampling and on-chip filtering reduce the external antialias requirements to first order in most cases. The sample rate, filter corner frequency, and output word rate are set by an external clock that is nominally 12.5 MHz. The use of a single bit DAC in the modulator guarantees excel lent linearity and dc accuracy. Endpoint accuracy is ensured by on-chip calibration. This calibration procedure minimizes the zero-scale and full-scale errors.
Information about the AD7722 can be found in the detailed data sheet for the converter [5]. With 16-bit resolution (1 part in 65,536) and a signal-to-noise ratio of 90 dB (compared to 35 to 41 dB for the analog tape system), this converter has ideal characteristics for acoustic and vibration measurements. At an output word rate of 200,000 s amples per second, the input antialiasing filter need only eliminate frequencies above 50% of the 12.5 MHz oversampling clock. This is sufficiently above the frequencies of interest that a simple filter can be used. The digital filter that is contained on the ADC provides decimation-the conversion of data from the oversampled rate to the output word rate. The extremely sharp cutoff of the filter allows the sampling of frequencies that approach the Nyquist limit.
V. MULTICHANNEL ADC MODULE
The V200 is a 16 or 32-channel ADC module in single-width VXI format [6]. Each channel includes the sigma-delta converter just described. Each channel includes an input multiplexer under software control. The differential input can be DC or AC, selectable on a channel-by-channel basis. In addition to the calibration built into each AD7722 ADC, the input multiplexer can select a short circuit (for zero calibration) as well as an internal or external calibrated voltage.
An R-C filter after the multiplexer attenuates high frequency signals. The instrumentation amplifier (IA) is programmable on a channel-by-channel basis. This is followed by a 3-pole fixed filter with a 1 MHz band-edge frequency to provide antialias protection. The output of this filter feeds the sigma-delta analog-to-digital converter (ADC). The digital output of each ADC is synchronously routed to a first-in/first-out (FIFO) memory, based upon the channel scanning order. Data words from the FIFO buffer are passed t hrough the digital signal processor (DSP). The DSP is used to provide full-bandwidth limit checking. The output of the DSP is passed through another FIFO buffer to the ping-pong buffer so that the data can be read over VXIbus.
In addition to the ping-pong buffer, two data output options are available. The Digi-busTM option allows data to flow over the backplane Local Bus to a DSP or RAM memory module. Multibuffer options are available with 4 to 32 megabytes of on-board memory. The measurement s ystem described here uses 32 megabyte multibuffers to capture the data for each test.
VI. A 320-CHANNEL ACQUISITION SYSTEM
An acoustic/vibration system for wind-tunnel testing of a rocket is described here. Because a scale model is used for the test, the frequencies of interest are increased by the ratio of full size to model size. Therefore, it is necessary to reproduce frequencies as high as 25 kilohertz. To do this each channel is scanned at 100,000 samples per second. Becau se of the requirement to attain a period of 60 seconds or less between tests, the system is divided into 4 segments.
Each segment serves 80 channels of PCB pressure transducers and accelerometers, interfaced to five 16-channel PCB signal conditioners [7]. The outputs of these signal conditioners are coupled to five 16-channel V200 ADC modules. Each V200 includes a 32 megabyte multibuffer. A test period of 10 seconds at a sample rate of 100,000 samples per second for 16 channels gives an aggregat e sample size of 16,000,000 16-bit words (32,000,000 bytes), which nearly fills the 32 megabyte (33,554,432 byte) multibuffer. One of the ADC modules in the system is considered to be the master and the remaining ADCs are synchronized with it via front-panel patch cords.
VII. REALITY-BASED DATA ACQUISITION
This system uses Reality data acquisition software for system configuration, channel setup, calibration, test execution, test sequencing and (as required) post-test analysis. Reality uses a UNIX workstation as the system host. This workstation is connected by Ethernet to each of the 80-channel subsystems. Each subsystem contains 5 PCB signal conditioning chassis, 5 ADC modules, a V150 local real-time processor module and a high-speed Sony cassette tape drive. The V150 uses a powerful 68060 processor and a real-time kernel, called VxWorks [8], [9].
Note that another module, the V387 discrete I/O, is only used with one of the 80-channel groups to accept the external sta rt trigger from the wind tunnel [10]. The data is acquired during the 10-second test and then written to the four tape drives in parallel. The writing process takes about 35 seconds, well within the 60-second maximum requirement. Three of the VXI subsystems use 6-slot minichassis, while the other subsystem uses a full-size chassis to accommodate the discrete I/O module used for triggering the system. The remainder of the racks are filled with the PCB signal conditioners and the Sony tape drives. Two add itional Sony tape drives are connected to the host workstation.
VIII. CONCLUSIONS
The availability of high-resolution, high-accuracy sigma-delta analog-to-digital converters with high sampling rates has led to the development of a multichannel ADC module that has ideal characteristics for acoustic and vibration measurements. A hardware/software system was shown to acquire acoustic/vibration data with high precision, and to be capable of rapidly transferring that data to tape or d isk. While a particular 320-channel system was described, the modular nature of the system lends itself to a wide variety of configurations. In this example, no real-time analysis was required, except for test purposes. However, powerful DSP modules are available to process the data in real time.
References
[1] IEEE-1155, "IEEE Standard for VMEbus Extensions for Instrumentation: VXIbus," Institute of Electrical and Electronic Engineers, 445 Hoes lane, Piscataway, NJ 0 8855.
[2] Robert T. Cleary, "New Standards-based Software Enhances Real-time I/O Performance," AUTOTESTCON, Dayton, OH, September 17, 1996.
[3] "METRUM Model 101e Datasheet," METRUM Instrumentation Products, P.O. Box 5227, Denver, CO 80217.
[4] "Harris Semiconductor Application Note AN9504,"http://www.semi.harris.com/appnotes/an9504.
[5] "Analog Devices AD7722 Data Sheet," http://www.analog.com/products/sheets/AD7722.html
[6] "KineticSystems V200 Data Sheet," http://www.kscorp.com/www/vxi/analogmd/v200.html
[7] PCB Piezotronics, Inc., http://www.pcb.com/
[8] "KineticSystems V150 Data Sheet," http://www.kscorp.com/www/vxi/syst_mod/v150.html
[9] "VxWorks," Wind River Systems, http://wrs.com/vxwks52.html
[10] "KineticSystems V387 Data Sheet," http://www.kscorp.com/www/vxi/digi_mod/v387.html
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