National Instruments Italy

Presentazione sul tema: "National Instruments Italy"— Transcript della presentazione:

1 National Instruments Italy
L’utilizzo della Strumentazione Virtuale per le Misure Industriali

2 Agenda Introduzione alla strumentazione Virtuale
Elementi di una catena di misura Dimostrazione di LabVIEW Esercitazioni e supporti

3 National Instruments Italy
Fondata nel 1989 50+ dipendenti Uffici a Milano e a Roma Divisione Commerciale Divisione Tecnica Divisione Marketing Divisione Didattica e Ricerca Sito - Didattica e Ricerca Dispense Esercitazioni Opportunita’ di lavoro ISO 9002 Certified

4 Divisione Didattica e Ricerca
Fondata nel 1997 Strutture dedicate esterne Uffici a Milano e a Roma, Torino,Firenze,Genova,Padova 100% operativi nel mondo accademico Politiche Prodotti Attivita’ di training …certificazione! ISO 9002 Certified

5 Computer-Based Instruments

6 Strumento standard vs strumento virtuale
PROCESSOR BUS Conditioning Timing A/D D/A DI/O TI/O DISPLAY AND CONTROL 488 PORT µP Math MEMORY ROM Control Panel Flow Pressure Alarm Conditions STOP Temperature PROCESSOR BUS Conditioning Timing A/D D/A DI/O TI/O DISPLAY AND CONTROL 488 PORT µP Math MEMORY ROM Register-mapped I/O Limitate capacità di espansione Funzionalità fisse Interfaccia esterna Memory mapped I/O Processamento Dati Veloce Connessione Internet/intranet Online data logging/trending Online report generation Memoria Espandibile Today, stand-alone instruments are quite common. Most are dedicated, special purpose instruments, but some measurements can be taken significantly faster and for a lot less money with computer-based instruments. Stand-alone instruments have a fixed set of functionality defined entirely by the instrument manufacturer. These instruments pass information back to a host computer over an interface such as GPIB or serial which is typically quite slow. On the other hand, computer-based instruments are user- defined. This means that their complete functionality is determined by how you program them and what kind of analysis and display capabilities you wish to use for your measurements. With a computer-based instrument, your instrument is essentially the PC so you can connect to a network, monitor and/or control over the Internet, and even process or log data online. Computer-based instruments are extremely fast and 100% customizable. To increase measurement throughput, lower the cost of taking measurements, simplify multiple-instrument synchronization, and maximize performance, computer-based instruments are the right choice. The key to higher measurement throughput is speeding up the transfer of data to your computer. The advantage of PCI or PXI/CompactPCI plug-in devices is that they are much faster than GPIB-based instruments for data transfers (132 MB/s versus 8 MB/s). This disparity becomes increasingly evident when performing several I/O operations within a single test or program. Definite dal costruttore Definite dall'utente ©National Instruments Corporation 67 LabVIEW—Proven Productivity

7 IL PC dentro lo strumento
Vantaggi Interfaccia Windows familiare, aggiornamento software automatico, connettività di rete Potenza di processamento a più basso costo Sistemi operativi standard Aggiornamento software (on line) più facile

8 IL PC dentro lo strumento
Un esempio : HP Infinium

9 Lo Strumento nel PC L’utilizzatore può scegliere il computer
L’utilizzatore acquista solo le funzionalità che utilizza L’utilizzatore ha il controllo TOTALE del sistema L’utilizzatore si avvantaggia delle nuove tecnologie Gli strumenti nel PC sono il REALE vantaggio per l’ utente, permettendo di fruire appieno della rivoluzione tecnologia dei personal computer Costi minori vs prestazioni migliori

10 Gli Elementi di un sistema di Misura
Oscilloscopio Sorgente di Segnale Multimetri Matrici

11 Strumenti su scheda Alta risoluzione (8-24bit)
Trasferimento dati ad alta Velocita’ (AT CPCI/PXI) Fino a 100MS/sec Soluzioni: DMMs Oscilloscopi Analizzatori di spettro Frequenzimetri RF Analyzer (2.7GHz) Sofisticati sistemi di Triggering e Sincronizzazioni tra diversi dispositivi You can also choose from a wide variety of instruments. These instruments incorporate the digitizer and the appropriate signal conditioning to give you the capabilities of instruments with which you may already be familiar. National Instruments offers many different types of instruments, including digital multi-meters, oscilloscopes, function generators, arbitrary waveform generators, and dynamic signal acquisition cards with built-in DSP processing capabilities. These instruments plug right into your computer bus, so you can transfer data into PC memory at very high rates. In addition, they offer intricate timing and triggering capabilities. Therefore, you can create a super-instrument, where each component communicates with the other components, increasing system throughput and performance. ©National Instruments Corporation 35 Signal Conditioning Fundamentals

12 Una soluzione:Il PC Strumento!
Oscilloscopio Matrice Gen.di Funzioni Multimetro

13 Sistemi di Misura e Controllo
Hardware & Driver Software Software Applicativo GPIB Serial DAQ VXI Image Acquisition National Instruments provides you with each of these types of computer-based measurement and automation hardware and software tools. You can take all of your measurements from computers using different combinations of I/O that are appropriate for your application, such as: • rack-n-stack GPIB instruments (NI makes the GPIB host interface) • serial instruments (NI makes RS-232 & RS-485 host interfaces) • data acquisition boards with and without signal conditioning • VXI instruments (NI makes controllers, chassis, and DAQ boards) • image acquisition boards • motion control boards • PXI/CompactPCI instruments (NI makes controllers, chassis, GPIB controllers, DAQ boards, and IMAQ boards) The computers in a measurement and automation system are often networked together and today we will show you how LabVIEW allows you to quickly integrate networking and Internet technologies into your applications. LabVIEW excels at making you productive by allowing you to take measurements quickly and easily. Motion Control PXI Unita’ sotto test LabVIEW—Proven Productivity ©National Instruments Corporation

14 Componenti della Misura
Condiziona- mento Digitalizzazione Computer Segnali Sensori Termocoppie RTD Termistore Strain Gauge Pressioni Carichi Tensioni Correnti Digitali Amplificazione Attenuazione Isolamento Filtraggio Multiplexing Eccitazione SSH F-to-V Bridge Comp. Frequenza Risoluzione Analisi Presentazione Distribuzione Every measurement system has the same fundamental components, a signal source, signal conditioning, a digitizer, and a computer. While these components may be delivered in a single package, or divided up into multiple pieces, they still play the same role. Understanding this model opens new possibilities for creating your measurement system, one where you decide the features that are important for your application.

15 Le schede di acquisizione dati
Un classico esempio: scheda DAQ su PCI 8 canali ADC 12/16 bit Guadagno programmabile Range di ingresso selezionabile Da 20 a KS/s 2 canali DAC 12/16 bit Uscita fino a 42Volts Da 8 a 32 I/O digitali TTL 2 Contatori/Temporizzatori Quante applicazioni possono essere risolte?

16 Tecnologie presenti in una scheda DAQ

17 Caratteristiche delle schede DAQ
Convertitore ADC Numero di canali Risoluzione Velocita’ di Campionamento Ampiezza segnale Multiplexer o SS Filtri AntiAliasing Convertitore DAC Porte digitali Numero di linee Livello Segnale Direzionalita’ Contatori e/o Temporizzatori Frequenza

18 Schema a blocchi di una scheda DAQ
Multiplexer Amplificatore Convertitore Analogico/Digitale MUX ADC NI-PGIA Bus di sinconizzazione Analog Input Analog Output NI DAQ-STC Digital I/O There are two basic formats for the analog input design of a DAQ board: Multiplexed boards and simultaneous boards. Multiplexed boards are optimized for DC accuracy and channel density. Only one A/D converter is used to scan all of the channels, so the sampling rate has to be divided among the channels. Also, there is a slight delay between each channel sample. Simultaneous boards are optimized for faster sampling rates per channel and DC and dynamic measurements. Although they generally have lower channel density, there is no delay between channels when sampling. Counter I/O NI MITE Step-by-Step Data Acquisition ni.com

19 Multiplexers Scopo: incrementare il numero dei canali ADC
Another common signal conditioning technology is a multiplexer. A multiplexer expands the number of signals that can be routed to a single digitizer. Single-ended measurement systems only require one set of multiplexers, because all measurements are performed with respect to a single reference voltage. Differential measurement systems require multiple multiplexers, so you can route the positive and negative signal reference to the digitizer together. Multiplexers are generally less expensive than digitizers. Therefore, they provide an inexpensive method of increasing your systems channel count. Scopo: incrementare il numero dei canali

20 Acquisizione con Multiplexers
Interchannel Delay Phase Shift Each signal is routed through the multiplexer Time delay between sampling of each channel Phase shift is negligible for most applications When acquiring signals with a standard multiplex-based measurement system, there is a delay between reading one channel and reading the next channel. This is a common phenomenon resulting from the architecture of the system. This delay, called inter-channel delay, appears as a phase shift in your acquired signals. Good data acquisition systems reduce this effect by using additional clocks to change the multiplexer from one channel to the next at a fast rate. Therefore, while you may be acquiring data from any given channel at a slow rate, the time delay between each channel reading is very small. For most applications, this solution is completely sufficient. However, for applications where you are specifically examining the phase relationship between two or more channels, this phase shift may not be acceptable. Even a 1 MHz digitizer has an inter-channel delay of 1 μs. If this introduces too much error for your application, you have several options. You can use a separate digitizer for each input signal. This option is generally expensive and requires intricate timing. You can make adjustments in software, but this can be intensive. Your third option is to purchase a system with simultaneous sampling capabilities via track-and-hold amplifiers, described on the next page.

21 Campionamento Simultaneo
T/H T/H No Phase Shift Digitizer control signal locks the track-and-hold amplifiers Signals are routed through the multiplexer Track-and-hold amplifiers are released Simultaneous sampling systems replace standard amplifiers with special “track-and-hold” amplifiers. Track-and-hold amplifiers perform the same functions as standard amplifiers, except they can be commanded to hold their present signal level for a period of time. Once the amplifier is in hold mode, a multiplex-based system can then read the signal from each channel. Once all channels are scanned, the amplifiers are released to start tracking their incoming signals again. This method can reduce the inter-channel delay to values below 5 ηs. Since track-and-hold amplifiers must be allowed time to change between track-and-hold mode, they often offer slower acquisition rates than systems without track-and-hold amplifiers. However, they are a less expensive solution than the alternative, which is to use a separate A/D converter for each channel.

22 Tecniche di miglioramento del rapporto segnale rumore ( Dithering )

23 Tecniche per il miglioramento della risoluzione
Without Noise + Quantization Error (LSB) NI Dithering (12-bit only) Noise-rejecting op-amps Carefully designed (Gaussian) noise floor Number of Averaged Samples National Instruments data acquisition products have high-quality amplifiers with a high Common Mode Rejection Ratio, or CMRR. This means that they reject a significant portion of the noise experienced on both terminals of the amplifier, making your measurements less susceptible to noise that can decrease accuracy. NI designs E Series DAQ boards with separated ground planes that connect to a single ground reference. Separated grounded planes reduce noise by taking advantage of the fact that the A/D and D/A converter chips usually place analog and digital signals on either side of the chip respectively. By placing the converter chips so that they straddle the barrier between the analog and digital ground planes, noise generated on the digital side of the board does not affect the analog side of the chip or the traces residing around the analog ground plane. NI 12-bit E Series boards can actually improve their resolution beyond their specification with a hardware technique called dithering. Dithering is controlled in software, and, when enabled, adds approximately 0.5 LSBrms of Gaussian white noise to the input signal. Since the noise is random, you can then use averaging to essentially zoom past the specified resolution of the board, resulting in more accurate measurements. For instance, a 12-bit board can perform with 14-bit resolution with dithering enabled. You can disable dithering for high-speed applications that do not use averaging. © National Instruments Corporation Step-by-Step Data Acquisition

24 Dithering 12-bit Without Dithering 9 Actual Signal
1 bit (4.8 mV for 12-bit board with +/- 10 V input range) Actual Signal Applying .5 LSB of Gaussian white noise to the original signal (dithering) causes the board to “flip bits”, whereas the original signal may have only been interpreted as the upper level. However, since this signal resides in the top half of the code width, the “bit flipping” will be weighted, so that, when you average the resulting signal over a large number of points, it will essentially represent the signal as accurately as a 14-bit board. Weighted Average = 4.8mV Actual Signal = 3.3mV

25 Dithering 6 Dithering Applied 3 Weighted Average = 4.8mV
1 bit (4.8 mV for 12-bit board with +/- 10 V input range) Dithering Applied 3 Applying .5 LSB of Gaussian white noise to the original signal (dithering) causes the board to “flip bits”, whereas the original signal may have only been interpreted as the upper level. However, since this signal resides in the top half of the code width, the “bit flipping” will be weighted, so that, when you average the resulting signal over a large number of points, it will essentially represent the signal as accurately as a 14-bit board. Weighted Average = 4.8mV Actual Signal = 3.3mV Dithered Weighted Average = 3.2mV

26 Tecniche di miglioramento del rapporto segnale rumore Range & Guadagno

27 Range La risoluzione dell’ A/D è distribuita all’ interno del range di acquisizione Massima Risoluzione = Range Corretto 100 200 150 50 Time (ms) -7.50 -10.00 -5.00 -2.50 2.50 5.00 7.50 10.00 Amplitude (volts) Range = -10 to +10 volts (5kHz Sine Wave) 3-bit resolution 000 001 010 011 101 110 111 | Choosing the proper range for a signal is very important to help maximize the resolution of our ADC. To illustrate this, let us revisit our sine wave and our 3-bit ADC. Due to poor resolution we are still not going to be able to represent our sine wave very well. However, an improper choice of range can make our representation of the sine wave even worse. Our sine wave has a minimum value of 0 Volts and a maximum value of +10 Volts. If we choose our range as Volts we will have 8 different voltage levels we can represent. If we were to improperly choose a range of -10 to +10 Volts we would now only have 4 voltage levels to represent our signal, because the other 4 levels would be used by the 0 to -10 Volt range. Our smallest detectable voltage would change from 1.25 to 2.50 and we would get a worse representation of our sine wave. As you can see improperly choosing the range will negatively impact the representation of your signal. However, we do not always have a choice as to what range to pick. For instance, if our sine wave actually went from -2 to +8 Volts, we could not choose 0 to +10 Volts as our range, because the signal does not fit within that range. We would be forced to choose a range of -10 to + 10, even though it spreads out our resolution.

28 Condizionamento: amplificazione
Amplifier Ottimizza la risoluzione nel range di misura scelto 16-bit Digitizer 10 V signal 65,536 livelli di risoluzione 16-bit Digitizer 10 mV signal Solo 32 livelli di risoluzione! One benefit of amplification is that it takes full advantage of all of the possible measurement values associated with the resolution of the analog-to-digital converter. Consequently, it can increase the accuracy of your measurement by a factor of 100 times or more. Amplification: Increases the amplitude of your signal Provides better match to the input range of your ADC Increases sensitivity of your measurement One of the most common types of signal conditioning Range +/-10 Volts Step-by-Step Data Acquisition ni.com

29 Condizionamento: amplificazione
Amplifier Migliora il rapporto segnale/rumore (SNR) Rumore Amplificatore differenziale di classe strumentale + _ In addition to taking full advantage of all of the possible measurement values associated with the resolution of the analog-to-digital converter, amplification also increases the size of your signal compared to the noise around the wires and cabling. This allows you to detect small changes in your signal faster, and it can reduce calculations and measurement time. Add an amplifier with a gain of 1,000 To amplify to a 10 V signal To reduce the amplitude of noise with respect to the amplitude of the signal You must amplify your signal so that the noise has less effect on your signal You can do this by moving the amplification close to your signal source ADC Cavi Segnale di basso livello Amplificatore esterno Scheda DAQ Step-by-Step Data Acquisition ni.com

30 Esempio di amplificazione
Segnale d’ ingresso = Volts ADC Range = Volts Settaggio del guadagno dell’ amplificatore = 2 100 200 150 50 Time (ms) 1.25 5.00 2.50 3.75 6.25 7.50 8.75 10.00 Amplitude (volts) Different Gains for 16-bit Resolution (5kHz Sine Wave) Gain = 2 | Your Signal Gain = 1 Amplified Signal Applying a gain to an analog input signal is very similar to amplifying your voice with a microphone. If you tried speaking in a stadium for 100, 000 people without a microphone, very few of the 100,000 people will be able to hear your voice. However, if you amplify your voice with a microphone you can maximize the number of people that can hear you. In the same way a small signal will not be able to use the entire resolution of the ADC, unless a gain is applied to amplify the signal. Let us take a look at how the gain setting affects an analog input signal. Assume we have a sine wave with a range of 0 to +5 Volts and an ADC range of 0 to 10 Volts. As you can see above if we applied a gain of 1 (no change) to our signal we would only be taking up half of the range, and thus using only half of our resolution. However, if we apply a gain of 2 to our signal we now have a sine wave with a range of 0 to +10 Volts. Now our signal fits exactly in our range and we will be maximizing the use of our resolution. Now let us consider a sine wave with a range of 0 to +6 Volts with the same ADC range of 0 to +10 Volts. We can no longer apply a gain of 2, because our sine wave would have a range of 0 to +12 Volts which exceeds our ADC range. The only gain we can apply is a gain of 1. It is also important to note that if we put a 0 to +5 Volt signal into our device, our graph in LabVIEW will show a 0 to +5 Volt signal regardless of the gain that is applied. The gain setting is only used to maximize the use of the ADC resolution. It will not affect your measurement.

31 Signal to Noise Ratio (SNR)
Maggiore è l’ SNR, meglio è Obbiettivo: amplificare il segnale, NON il rumore Signal Voltage S.C.* Amplification Noise in Lead Wires DAQ Board Digitized SNR Amplify only at .01 V None .001 V x100 1.1 V 10 Amplify at S.C.* and DAQ Board x10 1.01 V 100 1.001 V 1000 The Signal to Noise Ratio (SNR) is a measure of how much noise exists in your signal compared to the signal itself. It is defined as the voltage level of your signal divided by the voltage level of the noise. The larger the Signal to Noise Ratio the better. As you can see above, the Signal to Noise Ratio is the best when only external amplification is used on your signal, and the worst when the signal is only amplified on the DAQ device. * S.C. = Signal Conditioning

32 Esempio : acquisizione di una termocoppia
DAQ Signal Accessory Scheda DAQ The SCXI-1112 provides 8 thermocouple inputs, each with a 2Hz low-pass filter. Also, the SCXI-1112 has open thermocouple detection circuitry, which indicates the presence of an open thermocouple.

33 Un amplificatore in classe strumentale: NI-PGIA
Other Settling Time (LSB) NI Garantisce un tempo di assestamento bassissimo, anche a frequenze di campionamento elevate Sampling Rate (kS/s) You can use the National Instruments programmable instrumentation amplifier, the NI-PGIA, on most E Series devices, to deliver full 12- and 16-bit accuracy, even when scanning multiple channels at high gains and fast rates. E Series devices can sample channels in any order at the maximum conversion rate. Furthermore, you can individually program each channel in the scan with a different gain, as bipolar or unipolar, and as differential or single-ended. Step-by-Step Data Acquisition ni.com

34 Altre tecniche: auto calibrazione
Other Drift Error (%) NI Misure migliori e più stabili nel tempo Riduzione dell’ effetto del drift in temperatura dei componenti Time The E Series analog inputs and outputs have calibration circuitry to correct gain and offset errors. You can calibrate the device in software to avoid errors caused by time and temperature drift at run time. No external circuitry is necessary; a highly-stable internal voltage reference ensures high accuracy and stability over time and temperature. Factory-calibration constants are permanently stored in an onboard EEPROM and cannot be modified. A modifiable section of the EEPROM stores user-modifiable constants. You can return the devices to their initial factory calibration by accessing the unmodified factory constants.

35 Circuito di protezione dal drift in temperatura
Other Temperature Error (%) Uso di reti di compensazione e componentistica di grado superiore Auto calibrazione basata su una sorgente a bordo precisa Sensore di temperatura a bordo NI Temperature (°C) The temperature in your computer or bench top instrument can and will fluctuate. Measurement Ready DAQ boards guarantee accurate measurements from 0 to 55 °C. Custom resistor networks and high-grade components help keep temperature drift to within 6 ppm/°C. In addition, the board can also perform a self-calibration with a single function call that will bring the temperature drift even lower to approximately 0.6 ppm/°C. A temperature sensor comes on all NI E series and NI S series boards to measure ambient temperature. You can access this temperature sensor programmatically with a simple function call to ensure that your device is operating within the specified range. Tutto ciò assicura un comportamento uniforme a standard elevati a prescindere dalla temperatura ambiente

36 Caratterizazione del convertitore analogico/digitale

37 Risoluzione di un convertitore AD
100 200 150 50 Time (ms) 1.25 5.00 2.50 3.75 6.25 7.50 8.75 10.00 Amplitude (volts) 16-Bit Versus 3-Bit Resolution (5kHz Sine Wave) 16-bit 3-bit 000 001 010 011 101 110 111 | One of the most important features of the measurement device is the resolution of its A/D converter. The resolution of an A/D converter describes the number of discrete voltage levels it can digitize over a specified range. National Instruments offers plug-in DAQ devices with either 12-bit or 16-bit resolution. To understand how resolution affects your measurement, consider the signal in the graph above. With a 3-bit A/D converter, the analog input circuit only has 2^3 (or 8) discrete levels to represent the incoming signal. Therefore, the signal must change 1.25V for the A/D converter to represent the incoming signal with a new number. The resulting waveform representation in computer memory looks like the stair-step waveform shown above. A 12-bit A/D converter offers 2^12 (4096) discrete levels over a specified input range. For a 0-10 V input range, this relates to 2.44 mV of resolution. In this configuration, a signal must change more than 2.44 mV for the DAQ device to detect a change. A 16-bit A/D converter offers 2^16 (65,536) discrete levels over a specified input range. For the same 0-10 V input range, this relates to ~150 uV of resolution. As a side note, you should realize that resolution does not mean accuracy. Resolution does affect accuracy, but it is not the only factor. Other factors include the linearity and offset characteristics of your amplifier, and system noise. La dinamica di conversione può essere migliorata giocando con il range ed il guadagno

38 Frequenza di campionamento
E’ la frequenza di conversione dell’ A/D (Hertz) Va seguito il Teorema di Nyquist Fcampionamento>=2*Fsegnale Ben campionato Aliasato per sottocampionamento As we revisit filtering, we will discuss another common problem solved by signal conditioning called aliasing. Assume you are acquiring a signal of unknown frequency. If you sample this signal too slowly, it will appear as a signal whose frequency is much lower than it actually is. This process of misinterpreting the frequency of a signal is commonly called aliasing. While it may seem harmless, these aliased signals can amplify or cancel out other frequency components of your signal, causing measurement errors. The problem is that there is no easy method of detecting an aliased signal once it has been acquired, or to reconstruct the original waveform with the appropriate frequency information. Common sources of aliased signals are noise and harmonic frequencies of your signal. Harmonic signals are signals that have frequency components at a scalar multiple of your signal’s fundamental frequency. For example, a 30 Hz signal may have additional frequency components at 60 Hz, 90 Hz, 120 Hz, etc. The amplitude of the harmonic signals is typically smaller than the fundamental frequency, and typically attenuate at higher harmonics. Realize that noise sources may also introduce harmonic signals into your system. A 50/60 Hz noise signal from a nearby power source will also inject noise signals at 100/120 Hz, 150/180 Hz, etc.

39 Aliasing Sottocampionare un segnale analogico può dar vita all’ apparire di “frequenze fittizie” nella banda di interesse Un segnale aliasato non può più essere correttamente ricostruito As we revisit filtering, we will discuss another common problem solved by signal conditioning called aliasing. Assume you are acquiring a signal of unknown frequency. If you sample this signal too slowly, it will appear as a signal whose frequency is much lower than it actually is. This process of misinterpreting the frequency of a a signal is commonly called aliasing. While it may seem harmless, these aliased signals can amplify or cancel out other frequency components of your signal, causing measurement errors. The problem is that there is no easy method of detecting an aliased signal once it has been acquired, or to reconstruct the original waveform with the appropriate frequency information. Common sources of aliased signals are noise and harmonic frequencies of your signal. Harmonic signals are signals that have frequency components at a scalar multiple of your signal’s fundamental frequency. For example, a 30 Hz signal may have additional frequency components at 60 Hz, 90 Hz, 120 Hz, etc. The amplitude of the harmonic signals are typically smaller than the fundamental frequency, and typically attenuate at higher harmonics. Realize that noise sources may also introduce harmonic signals into your system. A 50/60 Hz noise signal from a nearby power source will also inject noise signals at 100/120 Hz, 150/180 Hz, etc.

40 Prevenire l’ aliasing Incrementare la frequenza di campionamento
Inserire un filtro passa-basso anti alias You have two weapons to attack and prevent aliasing. First, you can increase the sampling rate of your measurement system. The Nyquist Theorem states that you must sample your data at twice the rate of the highest frequency component of your signal to prevent aliasing. Many times this is impractical, as you may have no idea of all of the frequency components of your signal, especially those introduced by noise. The other method is to add lowpass filtering to your system. As mentioned before, lowpass filtering attenuates all unwanted signals, preventing them from affecting the signals you are trying to measure. A common practice is to implement both of these weapons. By setting your lowpass filter at a frequency just above the highest frequency you want to measure, you can eliminate noise signals and harmonics above that frequency. Then, you set your sampling rate just above two times this filter setting to obey the Nyquist Theorem and prevent your desired signals from being aliased.

41 Filtri Anti-Aliasing E’ un filtro analogico passa basso
Taglia fuori le componenti a frequenze superiore che potenzialmente possono dare alias You have two weapons to attack and prevent aliasing. First, you can increase the sampling rate of your measurement system. The Nyquist Theorem states that you must sample your data at twice the rate of the highest frequency component of your signal to prevent aliasing. Many times this is impractical, as you may have no idea of all of the frequency components of your signal, especially those introduced by noise. The other method is to add low-pass filtering to your system. As mentioned before, low-pass filtering attenuates all unwanted signals, preventing them from affecting the signals you are trying to measure. A common practice is to implement both of these weapons. By setting your low-pass filter at a frequency just above the highest frequency you want to measure, you can eliminate noise signals and harmonics above that frequency. Then, you set your sampling rate just above two times this filter setting to obey the Nyquist Theorem and prevent your desired signals from being aliased.

42 L’ importanza del driver di misura: Measurement and Automation Explorer
In the past, data acquisition system developers spent a large amount of time defining the signal types, connections, transducer equations, and unit conversions of the system before ever beginning actual development. However, Measurement and Automation Explorer greatly simplifies these tasks by providing access to all your National Instruments DAQ, GPIB, IMAQ, IVI, Motion, VISA, and VXI devices from a single location. With Measurement & Automation Explorer, you can configure your National Instruments hardware and software; add new channels, interfaces, and virtual instruments; execute system diagnostics; and view devices and instruments connected to your system. The Data Neighborhood category of Measurement and Automation Explorer provides access to all your virtual channels which are shortcuts to configured channels in your system. These virtual channels allow you to descriptively name physical channels and transparently apply modifications, such as scaling, to your measurements. The Scales category provides access to the custom scales you have configured for operating on acquired data. You can even view, launch, and update the National Instruments software installed on your system with the Software category. Because of the tight integration between LabVIEW and Measurement and Automation Explorer, you can access, edit, and insert channels directly in Measurement and Automation Explorer from LabVIEW to decrease your application development time. Measurement and Automation Explorer makes you more productive by minimizing the time you spend configuring and accessing your system hardware so that you can spend time on more meaningful tasks like taking measurements. ©National Instruments Corporation 49 LabVIEW—Proven Productivity

43 LabVIEW™ Pannello Frontale Diagramma a Blocchi
Interfaccia Utente Grafica Indicatori e Controlli Diagramma a Blocchi Codice Sorgente Libreria delle “funzioni” Rapido sviluppo di codice Auto-documentante LabVIEW programs are called “Virtual Instruments” or “VIs” for short. They are modeled after traditional benchtop instruments which generally possess a panel for inputs and outputs and user interaction and behind the panel is the instrument’s hardware. LabVIEW Virtual Instruments are similar—they have a Front Panel which serves as the graphical user interface containing program inputs and outputs, and a graphical source code area called the Block Diagram. Program inputs on the Front Panel are called controls and indicators are program outputs. Every object on the Front Panel has an associated terminal on the Block Diagram that allows you to perform various operations and analysis on the data it contains or displays. LabVIEW—Proven Productivity ©National Instruments Corporation

44 LabVIEW Programming Compiled graphical programming Wires and icons
Development time reduction by 4 to 10X Full-fledged programming environment LabVIEW is a compiled programming language just like other programming languages, such as Visual Basic or Visual C++. However, LabVIEW is a high-level language. In text-based programming languages, you are as concerned about the code as you are about what you are trying to do. You must pay close attention to the syntax (commas, periods, semicolons, square brackets, curly brackets, round brackets, etc) which may be quite tedious. LabVIEW is higher-level – it uses icons to represent subroutines, and you wire icons together to determine the flow of data through your program. It is similar to flow-charting your code as you are writing it. LabVIEW makes you productive because you can write your program in significantly less time than if you wrote it in a text-based programming language. LabVIEW—Proven Productivity ©National Instruments Corporation

45 Dataflow Programming Plot Execute In Parallel RMS Save
Wires pass data (nonlinear) Data flows from sources to sinks Code can execute multiple operations in parallel LabVIEW is technically a dataflow programming language. This means that program data flows from a data source to one or more sinks and it propagates through the program in this fashion. Because of dataflow, LabVIEW is not linear like text-based languages and can execute multiple operations in parallel. For example, as you can see in this picture, we are plotting a waveform, calculating the RMS value, and saving the waveform to disk all in parallel. ©National Instruments Corporation 9 LabVIEW—Proven Productivity

46 Hierarchy of VIs Modular design Reusable building blocks
Hierarchal system LabVIEW lends itself well to modular programming techniques—you can write VIs and use them as subroutines in a higher-level program. VIs used inside of other VIs are called subVIs. LabVIEW—Proven Productivity ©National Instruments Corporation

47 Alcuni esempi di Strumenti Virtuali
Esempio n°1 Esempio n°2 Esempio n°3