MICROBAROMETER IN A VIRTUAL SYSTEM Žarko Gorup University of Ljubljana, Faculty of Electrical Engineering, Ljubljana, Slovenia Key words: Virtual instruments, microbarometer, data acquisition systems, signal processing Abstract: A new virtual system for the detection of small pressure variations in ttie infrasound band of the characteristic Bora wind from the North Adriatic coast has been developed. It is composed of a precision pressure transducer (PPT), standard data acquisition (DAQ) card and virtual instruments (Vis) in the LabVIEW environment. Data collection and analysis of small atmospheric pressure variations during the Bora need a unique pressure sensor and signal conditioning electronics. We have fulfilled several major requirements of the measuring system: sufficient frequency bandwidth (0 Hz to 20 Hz), the resolution of the PPT response and low-noise design, a changeable and adjustable graphic user interface, configurability of the system, and analytical tools for data elaboration. Sufficient linearity is obtained by appropriate selection of a PPT with a signal conditioning circuit. Several efficient methods for the reduction of noise in electronic circuits have been used. The best effect in signal conditioning is achieved with low-noise electronic components, an appropriate circuit layout, and correct cabling and ground connection. Signals generated in the PPT are conditioned, acquired and analyzed under the control of a particular virtual instrument (VI) and subinstruments (subVI). DAQ and analysis are supervised from the front panel of a particular VI. The principal aim of the data analysis is the power density function (PDF) obtained with certain complex virtual Vis. The custom-designed Power Spectrum Analyzer (PSA) VI offers many useful options: graphical presentation of time and frequency, data records selection, changeable windowing and measurement of noise level. Mikrobarometer v virtualnem sistemu Kjučne besede: Virtuaini instrumenti, mikrobarometer, sistemi za zajemanje podatkov, procesiranje signalov Izvleček: Delo opisuje virtuaini sistem za zaznavanje in merjenje majhnih sprememb zračnega tlaka. Merjene spremembe povzroča burja, ki je značilen veter severnega Jadrana. Merilno napravo sestavlja precizen senzor tlaka, standardna večfunkcijska kartica za zajemanje podatkov in virtuaini instrumenti v programskem okolju LabVIEW. Zbiranje in analiza podatkov o spremembah zračnega tlaka, ki jih povzroča buria, zahteva poseben senzor tlaka in elektronsko vezje za prilagajanje signalov. Realizirani sistem izpolnjuje vse glavne zahteve merilnega sistema: ustrezen frekvenčni pas (O Hz do 20 Hz), ločljivost preciznega senzorja tlaka in malošumna izvedba vhodnega vezja. Poleg tega načrtovani sistem omogoča spremenljiv in prilagodljiv uporabniški vmesnik, fleksibilno zgradbo sistema in analitična orodja za obdelavo podatkov. Linearnost odziva smo dosegli s primernim senzorjem in vezjem za prilagajanje signalov. Zmanjšanje šuma smo dosegli z uporabo metod načrtovanja malošumnih vezij. V vezju za prilagajanje signalov smo uporabili malo-šumne elektronske komponente, primerno načrtovano tiskano vezje ter pravilne medsebojne povezave in povezavo mas. Signali, kijih generira senzor se prilagajajo, zajemajo in analizirajo pod nadzorom načrtovanega virtualnega instrumenta (VI) in virtualnih podinstrumentov. Zajemanje podatkov in analizo lahko spremljamo prek čelne plošče določenega VI. Glavni cilj analize podatkov je izračun in prikaz funkcije gostote močnostnega spektra, do katere pridemo z gradnjo kompleksnih VI. Zgradili smo uporabniško orientiran VI "analizator močnostnega spektra", ki nudi več uporabnih funkcij: grafično predstavitev spektra v časovnem in frekvenčnem prostoru, izbiro podatkovnega niza, izbiro okenske funkcije in meritev šumnega nivoja. 1 Introduction Infrasound is inaudible sound with frequencies below the human hearing threshold of 20 Hz. The lower frequency cut-off of infrasound is limited by the thickness of the atmosphere. In general, infrasound is measured within a frequency range of 0.005 (T=200 s) to 20 Hz (1=50 ms). Within this frequency band, many sources of both known and unknown origin generate infrasound. Sources that can often be detected for seconds, minutes or hours are: winds, volcanoes, sea waves, explosions, meteors, mountain-associated waves and aurora. Infrasound can be measured with either a low-frequency microphone or a high-frequency barometer. In the project we preferred the use of a microbarometer because of its robustness with respect to field application and durability. Furthermore, microbarom-eterscan measure much longer periods than microphones. Measurements of small and rapid variations of atmospheric pressure have historically been made by a variety of methods. All of them are enabled by means of different types of pressure transducers /1 / connected to data acquisition, elaboration and storage systems. Standard ana- log barometers have resolutions in the range of hPa (or mb). Precise measurements of atmospheric pressure changes are made with microbarometers with resolution in the range of 0.1 hPa to 0.002 hPa /2/, /3/. Such instruments are suitable for observation of fluctuations of the pressure of the wind. The Bora in the Karst region (Kras, Carso) is a particular type of wind in which the energy peaks are pulsations. They are of particular interest because they disappear and reappear within an episode /4/. Spectral analysis of the Bora reveals information about energy content, time distribution and local characteristics. A better understanding of the principal physical characteristics of the Bora provides a key to solving local problems in the areas of construction, architecture, environmental planning, agriculture, traffic control, etc. This can lead to cost reductions and an increase in the quality of life. A preliminary spectrum analysis was carried out on patterns of atmospheric pressure oscillations generated by the Bora, as recorded in Trieste (Italy) /5/. Bora wind gusts produce immediate changes of pressure that the pressure transducer has to convert proportionally to a voltage. A microbarometer with a very accurate, sta- ble and sensitive built-in pressure transducer is needed for such purposes. Some commercially available pressure transducers match the specifications for accurate continuous measurements of instantaneous atmospheric pressure changes. The microbarometer employed uses a high-accuracy pressure transducer system made by Setra Systems Inc. /6/and /7/. The PPT is connected to a custom-designed circuit which performs conditioning of the analog signal. All measurements of low-level signals are subject to noise. Various methods describe the reduction of noise in signal conditioning circuits, but they are difficult to implement in practice /8/. With regard to noise, there are some significant factors, among which the most influential are grounding, shielding, proper ground connection and suitable circuit layout strategy. On the other hand, there are several methods for lowering noise in the measuring signal. The most well known is analog filtehng, which is always used in antialiasing circuits. Another method is averaging, which can be performed in hardware or in the software of the measuring system. The analog signal terminates in a DAQ system. Many standard PC-compatible plug-in DAQ cards offer many advantages in comparison with classic measuring instruments. Today's multifunction cards are reliable, accurate and cost effective. Moreover, they use plug-and-play technology and are compatible with the Windows operating system and with most software packages designed for building Vis. General purpose Vis, such as multimeters, function generators and oscilloscopes, are frequently offered, but special measuring Vis are rarely available on the market. Choosing the right DAQ card is very important for the design of a measuring system. The selection should be made on the basis of electronic specifications, with close correlation of the card's characteristics and measurement needs. Full hardware compatibility with LabVIEW plays an important role in the final choice of a DAQ card. A virtual instrument (VI) can be created in the LabVIEW environment. Using Vis, a virtual measuring system can be set up which enables the user to carry out measurements on various subassemblies under specific program control. This kind of instrument has some advantages compared with classical or traditional measuring instruments /9/. The challenge of the project was to design a Vl-based microbarometer for spectral analysis of the Bora. The system must be capable of acquiring samples from the PPT, performing measurements using an appropriate standard DAQ card /10/, and processing and storing the acquired signal. The solution was combining a high-performance plug-in data acquisition card and signal conditioning in order to obtain a precise measurement of wind parameters. The VI enables the user to acquire data from the PPT (located outdoor) and process the data, calculate performance results, log data and generate reports /11/. 2 Transducer unit Precision pressure transducer The Precision Pressure Transducer (PPT) is a compact sensor device that generates a very sensitive analog output signal proportional to the pressure being measured. The PPT unit has an analog output and is individually calibrated at the factory for temperature variations over the full scale pressure span across a -40 to 85°C range. The PPT used is a Setra Systems Inc. Model 270 analog sensor which measures absolute (atmospheric) pressure. It contains a capacitor as a sensing element, which results in a simple, durable and fundamentally stable device. The sensing scheme is presented in Figure 1. atmosphßjlü pressure dtaphragm rrißvfr'jg &l(öclmd# ■nics vakiajTO fixed etectroda xsbl (varylRg cafAöcälan«) Fig. 1: Cross-section of a capacitive absolute pressure sensor In atypical configuration, a compact housing encloses two closely-spaced, parallel, electrically-isolated metallic surfaces. One surface is essentially a diaphragm capable of slight flexing under applied air pressure. The diaphragm is constructed of a low-hysteresis material, such as 17-4 PH SS or a proprietary compound of fused glass and ceramic: Setraceram. These firmly secured surfaces (or plates) are mounted so that a slight mechanical flexing of the assembly Dx, caused by a change in applied pressure, alters the gap x between them to x ± Dx (creating, in effect, a variable capacitor), as depicted in Figure 1. Capacitive pressure transducers are not common but do have higher sensitivity to pressure changes than do other pres-sure-sensing devices (typically 10 to 100 times). They are much less sensitive to thermal stresses and local diaphragm stresses since capacitive transducers integrate the movement of the entire surface of the diaphragm, while piezoresistive (PR) and piezoelectric (PE) devices use localized strain measurements. Capacitive transducers commonly have small capacities and generally are more expensive and larger than other devices because they usually carry their signal conditioning circuitry on the same board as the sensor. The sensor alone is very sensitive to environmental coupling, so it must be mounted in a mechanically and electrically protected space. In the case of Setra's PPT protection is afforded by a stainless steel housing to cover the sensor and functional electronics (excitation circuit, amplifier, power supply, analog buffer, etc.). To reduce noise and increase the signal-to-noise ratio, the amplifier unit must be located as close as possible to the sensor element. The capacitance of the sensor is a pure measuring value, so it must not be changed in the remaining circuitry. Signals containing the measurement are conducted to the signal conditioning circuit. The sensitivity of the signal-conducting wires is such that all other conductors need a shield. 3 Data acquisition The data acquisition (DAQ) unit is an important subassembly with the main functions of conditioning and multiplexing input channels, digitalization of input analog signals, timing control, synchronization and providing a digital I/O port. Usually we consider the DAQ card as a DAQ system, but in this case we must distinguish between the DAQ card and DAQ process. The DAQ process includes the conditioning part of the PPT and relevant software control. The data acquisition system (DAS) contains two main sub-units: signal conditioning and signal conversion. Signal conditioning is partially accomplished by the PPT unit, while the conversion is on the input stage of the DAQ multifunction card. Analog-to-digital (ADC) signal conversion with all the necessary logic and data path circuitry is part of the multifunction DAQ card. 3.1 Signal and data flow Once an analog signal from the sensor output is achieved, signal conditioning is applied. The main premise of the signal conditioning is linearity and response of the analog signal. After that, signal conversion follows and the data become digital information. The basic scheme of signal and data flow is presented in Figure 2. analog signal digital data sensor conditioning si^al conversicMi DAQ analysis {lata storage data pmseniatlon PC Analog signal flow begins with the PPT as the generator and ends with the ADC. Because of the very low-level signals of the first part of the circuit, they are very sensitive to noise and other disturbances from the surrounding environment. The signal path is followed by temporary data storage and data elaboration. Under the control of software, suitable data records are created and signal analysis is performed. The virtual system permits the user to observe the results graphically in quasi real time. At the same time, data are stored on two independent memory media for later processing and analysis. 3.2 Signal conditioning A linear transformation between signal excitation and response must be achieved by the signal conditioning circuit. Components that can be driven to the limits of the supply voltage (rail-to-rail) make the maximum dynamic range available. Signal conditioning is performed in two successive phases: the first phase is accomplished on the sensor unit and the second is performed on the standard DAQ multifunction board. Fig. 2: Signal and data flow from PPT to data storage. Fig. 3: Signal conditioning circuit. The main task of the signal conditioning circuit is adequate signal propagation, during which the signal is set to fit the ADC input voltage swing. Signal amplification reduces noise and protects signals from other disturbances. To increase dynamic range in this low-voltage environment, the first operational amplifier A in Figure 3 processes rail-to-rail input signals and drives rail-to-rail output signals. A circuit that maintains constant transconductance gm over the input common mode ranges reduces distortion. The bandwidth of the signal conditioning circuit must provide a frequency response within the range of interest for the fastest rate of change of the signal. The antialiasing filter is one of the most important elements of the DAQ system /12/. It is impossible to differentiate between noise with frequencies in the band and out of the band of interest. Only an analog filter can preserve signal integrity and extract real frequencies from folded frequency components. The cut-off frequency of the antialiasing filter must lie below the Nyquist frequency. The method for determining and implementing the appropriate analog filter parameters is controlled using Microchip's "FilterLab" software. The proposed circuit has been exported and simulated by PSPICE (Cadence Inc.) and the result is presented in Figure 4. '"■-T» T............. ! V;' ;■ ¥ ' \ ; \ ■ ^-----, .................;...........\ T i,<|ßJ«{UflJS:0:tJT)) f LOG18) Frs^ttencp Fig. 4: Calculated (A) and realized (B) transfer function of the antialiasing filter. Implementation with a Butterworth filter design has been carried out. A Butterworth filter is used in the filter implementation of the antialiasing filter For this circuit, an S'^-order filter is used with a cut-off frequency of 10 Hz. Four active Sallen-Key filters are used. This filter attenuates the pass band signal to 80dB at a frequency of 20 Hz. The effect of the filter is clearly visible in Figure 5. The output signal with noise is measured directly in the illustration above and passing the filter stage in the illustration below. 3.3 Signal Conversion The ADC is an Analog Devices successive approximation register type with a maximum 200 kS/s conversion rate. It operates from a single 5 V power supply. The resolution of the ADC is 16-bit, or 1-in-65536. The converter's integrated circuit contains a high-speed 16-bit sampling ADC, an internal conversion clock, internal reference, error correction circuits, and both serial and parallel system interface ports. The ADC is fabricated using a high-performance, 0.6 mm CMOS processor and is operable from -40°C to +85°C. The single 5 V supply of the ADC typically dissipates only 35 mW. Its power dissipation decreases with throughput to, for instance, only 15 |jW at a 100 SPS throughput. It consumes 7 pW maximum when in power-down mode. The circuit has superior integral nonlinearity (INL). A maximum Fig. 5: Output noise level of the output signal without (above) and with filtering (below/). INL of 3 LSB with no missing 16-bit code is achievable. Serial or versatile parallel interfaces (8 bits or 16 bits) or a 2-wire serial interface arrangement compatible with both 3 V and 5 V logic are available. Conversion is also possible in burst mode as a software-selectable option at a burst rate of T = 5 ps. The ADC uses a RAM buffer which holds 8 K samples. Data transfer can be programmed input-output (I/O) or direct memory access (DMA). DMA modes are "demand" or "non-demand" using scatter/gather operations. Configuration memory contains up to 8 K elements, which can be stored data for channel programming, gain, and offset of the on-board signal conditioning circuit. Maximum sampling rate reaches 200 kS/s with single channel operation and a 15-minute warm-up. Accuracy is assured by internal calibration. Measurements are valid for operational temperatures within ±1 °C of internal calibration temperature and +10 °C of factory calibration temperature. All hardware configuration options on the multifunction card are software controlled. Some configuration options, such as digital channel configuration (input or output), have been configured with installation software (InstaCal). Once selected, any program that uses the Universal Library can initialize the hardware according to these settings. Smrorg Cirrifj-aiim Chi'XK!) '.'J I ./J Cäi LxJi Tsimrrfi itJasD^iJ .IliiiSliiiilliii wrds«,' ^jHiSTirKj r- '»fs;»';! ' ^ IS ^^(I^ITSjÖÜ wiid'^ilfl Fii-ide-■r -'V.-: owfißtoic Co"»j ?t