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Data acquisition is a process of measuring real world signals usually in voltage form and bringing that information in to the computer for processing, analysis, storage, and other data manipulation. The physical phenomenon represents real world signals such as speed, temperature, humidity, pressure, and radioactivity and so on. We use sensors, which are also called transducers to evaluate the physical phenomena and produce electrical signals proportionately. In laboratory research, scientists and engineers can use personal computers with PCI, PXI, PCMCIA, Parallel, and Serial ports for data acquisition for both test and measurement and industrial automation. To acquire the data and transfer it directly to a computer's memory, many of applications use devices which plug in directly to the PC, whereas, other devices use external data acquisition hardware which can be coupled via Ethernet, parallel or serial ports (RS-232 or USB usually).
Lab VIEW is a popular software tool for automated test and measurement. It can command data acquisition (DAQ) devices to read analog input signals (A/D conversion), generate output signals (D/A conversion), read and write digital signals and manipulate the on board counters for frequency measurement, pulse generation, quadrature encoder measurements and so on to interface with the sensor. In case of analog input, the voltage data from the sensor goes in to the plug in DAQ devices in the computer memory for storage, processing, or other manipulation.
GPIB (General Purpose Interface Bus):
GPIB facilitates the communication between computers and instruments. A bus is simply the means of which computers and the instruments transfer data and GPIB, provided a much needed specification and protocol to govern this communicatin.GPIB's basic purpose was to provide computer control of the test and measurement instruments. However, its use has expanded beyond these applications into other areas, such as computer-to-computer communication and control of millimetres, scanners, and oscilloscopes.
GPIB is a parallel bus that many instruments use for communication. It sends data in bytes and the massages transferred are frequently encoded as ASCII character strings. We can have many instruments and computers connected to the same GPIB bus. Every device including the computer interface board, must have a unique GPIB address between 0 and 30,so that the data source and destination can be specified by this data source and destination can be specified by this number. Where address 0 is normally assigned to the GPIB interface board, instruments connected to the bus can use address 1 through 30.The GPIB has one controller, usually the personal computer, which controls the bus management functions. To transfer the instrument commands and data on the bus the controller addresses one talker and one or more listener.thw data strings are then sent across the bus from the talker to the listeners. The lab view GPIB VI's automatically handles the addressing and most other bus management functions, saving the hassle of low level programming to the user.
To use GPIB as a part of virtual instrumentation system, we need a GPIB board or external box, a GPIB cable, Lab view software and a computer.
Data Acquisition Systems:
In the industrial automation parameters, one of the major characterising features of advanced automation is that, there is a lot of data flow in and out or up and down the systems, which does the real time computing which control the optimization techniques. All the data is seen collected using sensors so the data acquisition systems interface to the sensors on one side and the computers which transform the data, on the other side: so, through these systems the data (analog) will come through the sensors get converted from source physical form to an electrical forms and then through the data acquisitions will convert into digital data.
Data acquisition system is the collection of hardware and software components that enable a computer to receive physical signals.
ADC Digital Interfacing
Signal Conditioning Sample& holdData Acquisition System
Figure 1 Block diagram of a Data Acquisition system
Electrical signal conditioning
Multiplexing, sample and Hold
Interfacing with Computer
Storage, processing and display in computer with software
Amplification is very important in data acquisition because every A/D converter has a dynamic range, the analog signals presenting at the input port of the A/D converter to avoid the approximate errors. Therefore, it is important to amplify the signal to increase the resolution.
Isolation: Isolation is typically required because the analog channels generally come either has single ended or as differential, so when we have single ended it means that the value of the analog voltage with respect to the electrical ground of the A/D converter, so when we are applying in a single ended mode this voltage will be measured by the A/D converter with respect to its own ground and then convert it.
On the other hand, when it comes to the differential input, there are two inputs provided to the A/D converter, so we have both the positive and negative terminals and the difference in these two signals are provided. The potential of this can be different from the ground, so we place and isolation circuit such that the input side is actually galvanically isolated with the output.
It is required for noise removal, Anti Aliasing, and Linearization of signal conditioning.
In a standard data acquisition system, it is typically specify and analog channels simultaneously. From the above figure let us assume the four channels of analog signals are being presented at the four inputs which consists of four electronic switches which can be put ON or OFF, depending on the address signal, assuming each of the switches in Binary notation. When we close the first one, that particular signal gets connected to the sample and holds and then goes to the A/D converter. If we connect all the switches in quick succession then over a given time interval âˆ†T, so for every time interval 'âˆ†T' we receive four values of the signal.
Sample & Hold Control signal Addressing Signal
Sample and Hold:
When the A/D converter is converting the signal, it is necessary to maintain the same values as at the input. Therefore, the sample and hold has an additional circuit such that we put ON the circuit, let this voltage sensed by the circuit which holds the voltage value even if the circuit opened, this occurs for every cycle until the new value arrives.
âˆ†T âˆ†T âˆ†T/4
0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 Multiplexed Signal After Sample & Hold
We use the sample and hold to avoid the errors,the above is the sample and hold circuit, which consists of two amplifiers which acts as a Buffer, where whenever we turns the switch on the tow points in the MOS are connected, the applied voltage V1 appears through the switch and charges the capacitor which is the sampling phase. So, the moment the switch if off the capacitance voltage has high impedance on either sides which does not allow the charge to escape so, the last voltage the capacitor had is held and the stored voltage is passed on to the next buffer A2.
A1 Switch (MOS) A2 V0
Gate control voltage
Analog -Digital Converter:
N-Bit ADC Analog Input N-Bit Digital Out put
Considering the 3-bit ADC, where with 3 bits we can represent 8 numbers (23-1) as every ADC has an analog reference voltage which decides where the number '0' corresponds to 0 volts. The binary value increases when the signal voltage increases by the principle of quantization, apparently the higher the number of bits the smaller is the quantization interval and the better is the resolution. To get the better resolution it is always important to amplify the signal with programmable gain amplifier when under the software control. Each division is represented by a binary code between 000 and 111. Clearly, the digital representation is not a good representation of the original analog signal because information has been lost in the conversion. By increasing the resolution to 16 bits, however, the number of codes from the ADC increases from 8 to 65,536, and you can therefore obtain an extremely accurate digital representation of the analog signal if the rest of the analog input circuitry is properly designed.
Digitized Sine Wave with a Resolution of Three Bits
In the recent past before the computers were not widely introduced in to the industrial automation systems,all the faults that occured in industrial processes were checked and dealt with by trained or experienced operators by monitering the operational conditions manually or in a semiautomatic manner ,which however had some major drawbacks.For instance considering the industrial experience the operator had to do the majority of the work by hand,the abnormal conditions could not be monitered and handled in the real time ,the remote measurement parameters could not be effectively monitered and there by operators were prone to make mistakes in recording and manupulating a large amount of data.Therefore it is highly necessary to automate the measurement operations as well as to improve the operating efficiency.
In recent decades,this picture has been dramatically changed due to the wide adoption of technology in a wide range of industrial applications.Information technologies have been rapidly developed in recent years,and they have provided sufficient technical support for building modern industrial automation systems with more open architecture with respect to the previous ones.It turns out that the computerized real time monitering analysis can realize the full automation of an industrial test & measurement system.It will significantly improve the working efficiency of the system operators and decision makers.As a result,developing such systems with the aforementioned characteristics for acheiving induatrial automation has a positive practical significance in both economy and technical perspectives.A typical industrial automation system,as illustrated in the figure below is usually made up of the physical system,sensors/transducers,device drivers and data I/O,host computer,network server and remote computers.
A modern industrial automation is made up of a variety of independently functioning but interacting modules.It opens a new window of oppurtunity to increase productivity and management effectiveness of industrial process.the wider use of distributed supervisory control and data acquisition across the industrial applications turns out to be able to enhance the productivity and the profitabliity significantly.Future trends in automation include improving the industrial automationsystem reliabilty/availability,responsiveness,scalability,expandability,flexibility,interoperability and so on. The development trends in the modern industrial automation systems were carried out in two directions,firstly,the size of the instruments are portable and universal which controls the running of the plant floor away from the plant by collecting the signals using the general purpose software burned in the system it self which is further connected to a computer for analyzing the data through the general instrument buses like IEEE-488 and RS-232.Secondly,the development focused on the continous,online,real time measurement and control systems.This functions include every option needed with regard to automation but the cost is much higher as they have more capability for data acquisition and signal conditioning using electrical circuits such as operational amplifiers and filters.signal analyzers and effective algorithms are also used in this process for real time signal conditioning of the processed data which is collected from the main unit of the plant.
Introdution to Virtual instrumentation:
The technology has improved rapidly for the last to decades which resulted in the development of the microprocessors and the VLSI technologies in the field of electronic industrial automation test and measurementation.The virtual instrumentation system consists of the set of measurement devices with strong data acquisition capability and analysis software with efficient computation and presentation capabilities.Virtual instrumentation has an advantage of making use of the online resources through network technology to publish and share data through internet and intranet facilities where engineers can have access and operate the real time production and can also share the information to the other engineers across the globe for condition monitering and fault diagnosis.Virtual instrumentation is a promising technology which can be used in various automation fields like military, aerospace ,induastrial measurement and control, pharmaceuticals etc.virtual instrumentation is acheived by using the modular instruments which consists of both the hardware and the sofware that can offer excellent feature such as open structure,modularity,reuseablity, interoperablity with out effecting the external factors such as cable length, impedence, coefficient difference and so on.The functions in virtual instrumentation can be created at the user level which defines the instrument functionality that helps in tuning the existing device for modifying according to the future advancements in the modular instruments.
PXI defines modular hardware platform based on the Intel 8086 microprocessor and a compact PCI bus. The typical configuration involves a PXI chassis, which holds its own PC controller running Microsoft windows and slots for all types of measurement modules like analog input, imaging, and motion control, sound delays, GPIB and VXI interfaces and more.
VXI-VME extensions for instrumentation is an instrumentation standard for instrument on a card system, introduced in 1987 which is based on the VME bus.VXI bus is higher-end usually more expensive system than PXI.VXI consists of a mainframe chassis with slots holding modular instruments on plug in boards.
A variety of instruments and mainframes size is available from numerous vendors and can use VME modules in VXI systems. VXI has wide array of uses in traditional automated test and measurement equipment applications. VXI is also popular in data acquisition and analysis for research and industrial control applications that require more number of channels. VXI instruments has the same features and the charateristics of the virtual instruments which is basically a modular instrument which has to be connected to the external devices like the computer to moniter the panel operations based on the VXI-bus architecture. The major characteristics of VXI instruments are their compatibility with many instrument manufactures globally based on their applications. It can also communicate with multiple processors of the back board with the shared data storage structure and has a high measurement system throughout. Modular instruments like PXI/VXI are designed to increase the system throughput by reducing the test time with increased capability. Unlike the traditional instruments modular devices are compact in size and reduces thr cost ove rthe lifetime of the system.
The main aim of industrial automation software is to monitor the system operating condition in real-time, and to perform certain tasks such as data processing, statistical analysis, report generation, printing, real-time alarming, etc.In this process, each and every part complements one another to achieve overall system functionality.
Basic framework of automated measurement system based on virtual instruments.
Features, which are considered while designing Industrial automation software:
Versatility: The designer of industrial automation software needs to consider various design requests from different industries such as petroleum, chemical, metallurgy, electric power, electric machinery, spinning etc. The designed software should be capable of satisfying various requirements and provide wide spread application
Comprehensive functions: Industrial monitoring software should provide several basic and high-level functions such as graphic monitoring display, trend analysis, report generation and printing, automatic data gathering, automatic memory/restore, real-time alarming etc.
System transparency: The system developed should be more flexible and should allow the user to deal with information according to their needs. The system should be more flexible for communication, capable of supporting different networks and different type of equipment interconnection, open control strategies, permission to add user defined control strategies, flexible representation and printing modes and effective interfaces for database communication with other systems.
User-oriented carefree configuration: The software should allow user for flexible configuration based on user demands through the configuration tool provided by the built in configuration system. The system should not enforce the user with fixed or rigid configuration patterns.
Man-machine Interface: The aim of this is to provide the user with high-quality graphic displays by the use of advanced graphic tools, use of multimedia technologies to improve the quality of user interface, and to improve user interface operation
Generally, textual programming languages take long time to grasp and learn. It requires significant programming skills. This can be achieved by a well developed code. Industrial automation engineers can use other programming languages such as visual and graphical programming languages to develop a code without much effort and skill.
A collection of visual development tools are developed based on windows API's or object windows of Borland C++. This type of programming is event-based software. For every event, the system automatically generates a message and passes them to the corresponding message handling functions. This function is automatically loaded in visual development tool.
Graphical Programming: Graphical programming language is a collection of many nodes which are connected together to carry a specific task. In this programming, developer can check the data flow after inserting a section of code. This includes the use of arrays, strings, and loops. It is much similar to the text based coding languages where we use nodes for developing the code instead of text and numbers
The introduction of Lab VIEW created a fundamental change in graphical programming field. This is mainly designed for data acquisition, device control, and data analysis and data representation. This can be easily understood and used while providing high flexibility. In the Lab VIEW development, the user can control the system and present the test results through graphical panels. This is compatible with almost all the operating systems like windows 9X/NT/2K/XP, MAC OS, Sun etc and can acquire the data from different devices like GPIB, VXI, PXI, Serial devices and other Plug and Play data acquisition devices which is then used to share the data online using ActiveX, DDE, and SQL. When the raw data is collected, the desired results can be used for data analysis, visualisation and debugging. Lab VIEW programming environment is user friendly and easy to understand and the flow of data makes the programming style as a natural thinking process.
PXI modules Used & in what manner