What are Virtual Instruments?

Virtual instruments (VI) are instruments based on computer software whose controls and displays are presented on a computer screen and whose operation and settings are controlled by the keyboard and mouse. Instead of purchasing a number of expensive, dedicated, standalone instruments, users find it less costly to purchase a computer with a multichannel, analog-to-digital converter (A/D) interface card of suitable speed and bit resolution or an outboard PXI system, and the virtual instrument software to control, process and display the measured quantities. The PXI in this case stands for PCI extensions for Instrumentation. The PXI standard defines a modular instrumentation platform (hardware modules and operating software) designed especially for robust measurement and automation applications. PXI modules typically have only input signal connectors and status LEDs on their narrow front panels. They are generally no control knobs or switches.

The interface A/D cards and outboard signal conditioning and A/D systems such as PXI modules connected to the computer through RS-232D, IVI-C or GPIB buses are supplied by National Instruments, CyberResearch, and so forth.

A true RMS voltmeter is an example of a typical VI instrument. A time varying analog signal, such as noise, is first anti-alias filtered, then converted from analog to digital form using A/D converter at appropriate rate, then M digital samples in each of N data sample epochs are each squared, averaged and the average of the squares, square rooted to generate numerical estimates of the true RMS value of the signal. Normally for noise and noisy signals, the N averages of the squares are averaged, and then square rooted to reduce the variance of the RMS estimate.

The RMS voltage is typically presented on the computer screen as the display of a VI RMS voltmeter shown in a window. The operator can adjust the VI meter’s range, the AAF’s cutoff frequency, its sampling rate, the epoch length, M, and the averaging algorithm by setting the knobs and switches on the VI display with the mouse. Pulse height window used in experimental neurophysiology and nuclear physics is another example of VI instrument.

Virtual instruments are used in science, engineering education, industry, and aerospace. They permit remote signal acquisition and conditioning modules to be located near the sensors and managed remotely by a host computer. The host presents data in a VI window display that allows the operator to adjust input signal conditioning parameters and observe the results in realtime. Furthermore, the host can store incoming data on a hard disc or other appropriate storage devices. The links to and from the peripheral modules can be copper, fiber optic or RF. Different types of bus protocols are available for communicating settings and data to and from modules.

In engineering education, Virtual Instruments offers an inexpensive way to implement oscilloscopes, bridges, meters, signal generators and control signals without the expense of purchasing complete, dedicated instruments. The host computer can do spectral analysis on signals by FFT and other DSP routines like cross correction of two signals.

In biomedical engineering/instrumentation laboratories, we have special Virtual Instruments systems employed. Students can observe ECG, EEG, and EMG waveforms and other physiological signals such as muscle form, blood pressure, pulse oximetry signals, and so forth utilizing medical isolation grade peripheral signal conditioners.

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