Virtual instrumentation combines mainstream commercial technologies, such as the PC, with flexible software and a wide variety of measurement and control hardware. Engineers use virtual instrumentation to bring the power of flexible software and PC technology to test, control, and design applications making accurate analog and digital measurements. Engineers and scientists can create user-defined systems that meet their exact application needs. Industries with automated processes, such as chemical or manufacturing plants use virtual instrumentation with the goal of improving system productivity, reliability, safety, optimization, and stability. Virtual instrumentation is computer software that a user would employ to develop a computerized test and measurement system for controlling from a computer desktop, an external measurement hardware device, and for displaying, test or measurement data collected by the external device on instrument-like panels on a computer screen. It extends to computerized systems for controlling processes based on data collected and processed by a computerized instrumentation system. The front panel control function of the existing instrument is duplicated through the computer interface. The application ranges from simple laboratory experiments to large automation applications.
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Virtual instrumentation as shown in Figure 1.10 uses highly productive software, modular I/O, and commercial platforms. National Instruments LabVIEW, a premier virtual instrumentation graphical development environment, uses symbolic or graphical representations to speed up development. The software symbolically represents functions. Consolidating functions within rapidly deployed graphical blocks further speeds up development.
Figure: Virtual instrumentation combines productive software, modular I/O and scalable platforms.
Another virtual instrumentation component is modular I/O, designed to be rapidly combined in any order or quantity to ensure that virtual instrumentation can both monitor and control any development aspect. Using well-designed software drivers for modular I/O, engineers and scientists quickly can access functions during concurrent operation.
The third virtual instrumentation element using commercial platforms, often enhanced with accurate synchronization, ensures that virtual instrumentation takes advantage of the very latest computer capabilities and data transfer technologies. This element delivers virtual instrumentation on a long-term technology base that scales with the high investments made in processors, buses and more.
In summary, as innovation mandates use of software to accelerate a new concept and product development, it also requires instrumentation to rapidly adapt to new functionality. Because virtual instrumentation applies software, modular I/O and commercial platforms, it delivers instrumentation capabilities uniquely qualified to keep pace with today’s concept and product development.
A traditional instrument is designed to collect data from an environment, or from a unit under test, and to display information to a user based on the collected data. Such an instrument may employ a transducer to sense changes in a physical parameter such as temperature or pressure, and to convert the sensed information into electrical signals such as voltage or frequency variations. The term “instrument” may also cover a physical or software device that performs an analysis on data acquired from another instrument and then outputs the processed data to display or recording means. This second category of instruments includes oscilloscopes, spectrum analyzers and digital millimeters. The types of source data collected and analyzed by instruments may thus vary widely, including both physical parameters such as temperature, pressure, distance, light and sound frequencies and amplitudes, and also electrical parameters including voltage, current and frequency.
A virtual instrument (VI) is defined as an industry-standard computer equipped with user-friendly application software, cost-effective hardware and driver software that together perform the functions of traditional instruments. Simulated physical instruments are called virtual instruments (VIs). Virtual instrumentation software based on user requirements defines general-purpose measurement and control hardware functionality. With virtual instrumentation, engineers and scientists reduce development time, design higher quality products, and lower their design costs. In test, measurement and control, engineers have used virtual instrumentation to downsize automated test equipment (ATE) while experiencing up to a several times increase in productivity gains at a fraction of the cost of traditional instrument solutions.
Virtual instrumentation is necessary because it is flexible. It delivers instrumentation with the rapid adaptability required for today’s concept, product and process design, development and delivery. Only with virtual instrumentation, engineers and scientists can create the user-defined instruments required to keep up with the world’s demands. To meet the ever-increasing demand to innovate and deliver ideas and products faster, scientists and engineers are turning to advanced electronics, processors and software. Consider modern cell phones. Most of them contain the latest features of the last generation, including audio, a phone book and text messaging capabilities. New versions include a camera, MP3 player, and Bluetooth networking and Internet browsing.
Virtual instruments are defined by the user while traditional instruments have fixed vendor-defined functionality. In a conventional instrument, the set of components that comprise the instrument is fixed and permanently associated with each other. Nevertheless, there is some software that understands these associations. Thus the primary difference between a virtual instrument and a conventional instrument is merely that the associations within a virtual instrument are not fixed but rather managed by software.
Every virtual instrument consists of two parts—software and hardware. A virtual instrument typically has a sticker price comparable to and many times less than a similar traditional instrument for the current measurement task. However, the savings compound over time, because virtual instruments are much more flexible when changing measurement tasks. By not using vendor-defined, prepackaged software and hardware, engineers and scientists get maximum user-defined flexibility. A traditional instrument provides them with all software and measurement circuitry packaged into a product with a finite list of fixed-functionality using the instrument front panel. A virtual instrument provides all the software and hardware needed to accomplish the measurement or control task. In addition, with a virtual instrument, engineers and scientists can customize the acquisition, analysis, storage, sharing and presentation functionality using productive, powerful software.
Without the displays, knobs and switches of a conventional, external box-based instrumentation products, a virtual instrument uses a personal computer for all user interaction and control. In many common measurement applications, a data acquisition board or card, with a personal computer and software, can be used to create an instrument. In fact, a multiple-purpose virtual instrument can be made by using a single data acquisition board or card. The primary benefits of apply data acquisition technology to configure virtual instrumentation include costs, size, and flexibility and ease of programming. The cost to configure a virtual instrumentation-based system using a data acquisition board or cards can be as little as 25% of the cost of a conventional instrument.
Traditional instruments and software-based virtual instruments largely share the same architectural components, but radically different philosophies as shown in Figure 1.11. Conventional instruments as compared to a virtual instrumentation can be very large and cumbersome. They also require a lot of power, and often have excessive amounts of features that are rarely, if ever used. Most conventional instruments do not have any computational power as compared to a virtual instrument. Since the virtual instrument is part of a person computer configuration, the personal computer’s computational as well as controlling capability can be applied into a test configuration. Virtual instruments are compatible with traditional instruments almost without exception. Virtual instrumentation software typically provides libraries for interfacing with common ordinary instrument buses such as GPIB, serial or Ethernet.
Figure: Traditional instruments (left) and software based virtual instruments (right).
Except for the specialized components and circuitry found in traditional instruments, the general architecture of stand-alone instruments is very similar to that of a PC-based virtual instrument. Both require one or more microprocessors, communication ports (for example, serial and GPIB), and display capabilities, as well as data acquisition modules. What makes one different from the other is their flexibility and the fact that we can modify and adapt the instrument to our particular needs. A traditional instrument might contain an integrated circuit to perform a particular set of data processing functions; in a virtual instrument, these functions would be performed by software running on the PC processor. We can extend the set of functions easily, limited only by the power of the software used. By employing virtual instrumentation solutions, we can lower capital costs, system development costs, and system maintenance costs, while improving time to market and the quality of our own products.
There is a wide variety of hardware devices available which we can either plug into the computer or access through a network. These devices offer a wide range of data acquisition capabilities at a significantly lower cost than that of dedicated devices. As integrated circuit technology advances, and off-the-shelf components become cheaper and more powerful, so do the boards that use them. With these advances in technology, comes an increase in data acquisition rates, measurement accuracy, precision and better signal isolation. Depending on the particular application, the hardware we choose might include analog input or output, digital input or output, counters, timers, filters, simultaneous sampling, and waveform generation capabilities.
Virtual instrumentation has achieved mainstream adoption by providing a new model for building measurement and automation systems. Keys to its success include rapid PC advancement; explosive low-cost, high-performance data converter (semiconductor) development; and system design software emergence. These factors make virtual instrumentation systems accessible to a very broad base of users.
Virtual instruments take advantage of PC performance increase by analyzing measurements and solving new application challenges with each new-generation PC processor, hard drive, display and I/O bus. These rapid advancements combined with the general trend that technical and computer literacy starts early in school, contribute to successful computer-based virtual instrumentation adoption. The virtual instrumentation driver is the proliferation of high-performance, low-cost analog-to-digital (ADC) and digital-to-analog (DAC) converters. Applications such as wireless communication and high-definition video impact these technologies relentlessly. Virtual instrumentation hardware uses widely available semiconductors to deliver high-performance measurement front ends. Finally, system design software that provides an intuitive interface for designing custom instrumentation systems furthers virtual instrumentation. Various interface standards are used to connect external devices to the computer. PC is the dominant computer system in the world today. VI is supported on the PC under Windows, Linux, Macintosh, Sun, and HP operating systems. All VI platforms provide powerful Graphical User Interfaces (GUIs) for the development and implementation of the solutions.
Input/Output plays a critical role in virtual instrumentation. To accelerate test, control and design, I/O hardware must be rapidly adaptable to new concepts and products. Virtual instrumentation delivers this capability in the form of modularity within scalable hardware platforms. Virtual instrumentation is software-based; if we can digitize it, we can measure it. Standard hardware platforms that house the I/O are important to I/O modularity. Laptops and desktop computers provide an excellent platform where virtual instrumentation can make the most of existing standards such as the USB, PCI, Ethernet, and PCMCIA buses.
Software is the most important component of a virtual instrument. With the right software tool, engineers and scientists can efficiently create their own applications by designing and integrating the routines that a particular process requires. You can also create an appropriate user interface that best suits the purpose of the application and those who will interact with it. You can define how and when the application acquires data from the device, how it processes, manipulates and stores the data, and how the results are presented to the user. With powerful software, we can build intelligence and decision-making capabilities into the instrument so that it adapts when measured signals change inadvertently or when more or less processing power is required. An important advantage that software provides is modularity. When dealing with a large project, engineers and scientists generally approach the task by breaking it down into functional solvable units. These subtasks are more manageable and easier to test, given the reduced dependencies that might cause unexpected behaviour. We can design a virtual instrument to solve each of these subtasks, and then join them into a complete system to solve the larger task. The ease with which we can accomplish this division of tasks depends greatly on the underlying architecture of the software.
A virtual instrument is not limited or confined to a stand-alone PC. In fact, with recent developments in networking technologies and the Internet, it is more common for instruments to use the power of connectivity for the purpose of task sharing. Typical examples include supercomputers, distributed monitoring and control devices, as well as data or result visualization from multiple locations. Every virtual instrument is built upon flexible, powerful software by an innovative engineer or scientist applying domain expertise to customize the measurement and control application. The result is a user-defined instrument specific to the application needs. Virtual instrumentation software can be divided into several different layers like the application software, test and data management software, measurement and control services software as shown in Figure 1.12.
Figure: Layers of virtual instrumentation software.
Most people think immediately of the application software layer. This is the primary development environment for building an application. It includes software such as LabVIEW, LabWindows/CVI (ANSI C), Measurement Studio (Visual Studio programming languages), Signal Express and VI Logger. Above the application software layer is the test executive and data management software layer. This layer of software incorporates all of the functionality developed by the application layer and provides system-wide data management. Measurement and control services software is equivalent to the I/O driver software layer. It is one of the most crucial elements of rapid application development. This software connects the virtual instrumentation software and the hardware for measurement and control. It includes intuitive application programming interfaces, instrument drivers, configuration tools, I/O assistants and other software included with the purchase of hardware. This software offers optimized integration with both hardware and application development environments.