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Sunday, 3 February 2019

Analog and Digital Data Acquisition Systems

Trends in Data Acquisition System:

Size and design time of system are reduced. Modular systems with high versatility are being made available for use in DAS. Microprocessor based or PC based DAS are available that contain all the components necessary for a DAS. The following facilities are offered by the currently available DAS.

1. Display of information instantly for easy assimilation
2. Display spatial as well as time variation
3. Display of important parameters logically and concisely grouped
4. Video display of plants and subsystems
5. User friendly messages on problems
6. Ability to analyze data and perform computations to display performance of equipment and plants.
7. Operate alarms using audio visual techniques under abnormal plant conditions
8. Ability to provide dynamic updating of parameters
9. Ability to provide hard copy record of different events of the plant and plant logs.

Data acquisition and control systems with latest features are available. As an example some of the advanced features of such an equipment available currently are :

Availability of as many as 1440 channels for input and output

For computer link up RS232 or RS485 ports

Flexible configuration for measurement and control

Front panel provided with digital display and membrane key pad.

In built signal conditioning for different transducers like:

Thermocouples, RTDs. Strain Gages, 4 to 20 Ma transmitters, AC LVDTs etc

Analog and Digital Data Acquisition Systems: 

The most important type of an instrument system is the data-acquisition system. The need for this comes from the necessity to measure and record large quantities of data in research, development and production. If the data has to be acquired quickly and repetitively, automatic data-acquisition systems are needed. The most common systems commercially available are useful for the measurement of voltage, frequency, resistance, etc. The data collected can be printed out on a paper-tape strip or stored on a punched paper tape or magnetic tape. If non electrical measured, transducers are used to convert these physical variables into equivalent electrical signals which are then handled by the data-acquisition system. Signal conditioning and processing m may be necessary before the data are made suitable for being fed as the input to the data-acquisition system. The data-acquisition system can be computer-controlled. The analog data-acquisition system shown in Figure consists of:

(1) transducers to convert physical parameters to electrical signals,
(2) signal conditioners for amplifying, refining or selecting certain portions of these signals,
(3) a multiplexer for scanning different signal sources,
(4) visual-display devices for continuous monitoring of the signal,
(5) graphic recorders for obtaining permanent records of input data and
(6) magnetic recorders for preserving and reproducing data.

Block diagram of analog data acquisition system
The digital data-acquisition system shown in Figure includes transducers, a signal conditioner, multiplexer, signal converter to translate the analog signal to a form acceptable by the A/D converter, digital recorder to record information on punched cards, perforated tape, magnetic tape, and programmer to control the data processing. Analog systems are used when wide bandwidth is required or when lower accuracy can be allowed. Digital systems are used when the physical process being monitored slowly varies and when high accuracy and low per-channel cost is required.
Block diagram of digital data acquisition system
Computer-controlled data-acquisition systems are used when very high-speed operation is required or where on-line acquisition and manipulation of the data are necessary.

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