21-12-2012, 05:32 PM
Virtual instrumentation
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Introduction
Virtual instrumentation is an interdisciplinary field that merges sensing, hardware and software technologies
in order to create flexible and sophisticated instruments for control and monitoring applications. There are
several definitions of a virtual instrument available in the open literature. Santori defines a virtual instrument
as "an instrument whose general function and capabilities are determined in software" [Santori91]. Goldberg
describes that “a virtual instrument is composed of some specialized subunits, some general-purpose
computers, some software, and a little know-how" [Goldberg00]. Although informal, these definition
capture the basic idea of virtual instrumentation and virtual concepts in general - provided with sufficient
resources, “any computer can simulate any other if we simply load it with software simulating the other
computer“ [Denning01]. This universality introduces one of the basic properties of a virtual instrument – its
ability to change form through software, enabling a user to modify its function at will to suit a wide range of
applications. The concept of virtual instrumentation was born in late 1970s, when microprocessor
technology enabled a machine's function to be more easily changed by changing its software [Santori91].
The flexibility is possible as the capabilities of a virtual instrument depend very little on dedicated hardware
- commonly, only application-specific signal conditioning module and the analog-to-digital converter used
as interface to the external world. Therefore, simple use of computers or specialized onboard processors in
instrument control and data acquisition cannot be defined as virtual instrumentation.
A Brief History of Virtual Instrumentation
A history of virtual instrumentation is characterized by continuous increase of flexibility and scalability of
measurement equipment. Starting from first manual-controlled vendor-defined electrical instruments, the
instrumentation field has made a great progress toward contemporary computer-controlled, user-defined,
sophisticated measuring equipment. Instrumentation had the following phases:
o Analog measurement devices,
o Data Acquisition and Processing devices,
o Digital Processing based on general purpose computing platform, and
o Distributed Virtual Instrumentation.
The first phase is represented by early "pure" analog measurement devices, such as oscilloscopes or EEG
recording systems. They were completely closed dedicated systems, which included power suppliers,
sensors, translators and displays [Geddes89]. They required manual settings, presenting results on various
counters, gauges, CRT displays, or on the paper. Further use of data was not part of the instrument package,
and an operator had to physically copy data to a paper notebook or a data sheet. Performing complex or
automated test procedures was rather complicated or impossible, as everything had to be set manually.
Second phase started in 1950s, as a result of demands from the industrial control field. Instruments
incorporated rudiment control systems, with relays, rate detectors, and integrators. That led to creation of
proportional-integral-derivative (PID) control systems, which allowed greater flexibility of test procedures
and automation of some phases of measuring process [Goldberg00]. Instruments started to digitalize
measured signals, allowing digital processing of data, and introducing more complex control or analytical
decisions. However, real-time digital processing requirements were too high for any but an onboard specialpurpose
computer or digital signal processor (DSP). The instruments still were standalone vendor defined
boxes.
Sensor module
The sensor module performs signal conditioning and transforms it into a digital form for further
manipulation. Once the data are in a digital form on a computer, they can be displayed, processed, mixed,
compared, stored in a database, or converted back to analog form for further process control. The database
can also store configuration settings and signal records.
The sensor module interfaces a virtual instrument to the external, mostly analog world transforming
measured signals into computer readable form. Table # summarizes some of the often used human
physiological signals [Charles99].
Sensor interface
There are many interfaces used for communication between sensors modules and the computer [Arpia01].
According to the type of connection, sensor interfaces can be classified as wired and wireless.
o Wired Interfaces are usually standard parallel interfaces, such as General Purpose Interface Bus
(GPIB), Small Computer Systems Interface (SCSI), system buses (PCI eXtension for
Instrumentation PXI or VME Extensions for Instrumentation (VXI), or serial buses (RS232 or USB
interfaces) [Tracht93].