Classification, Elements, Configuration of Data Acquisition System

(a) Classification of Data Acquisition System: 

Based on the data the DAS can be classified as:

• Analog data acquisition system and
• Digital data acquisition system

Based on the environment DAS can be classified as:

• System suitable for favorable environments i.e. with minimum RF interference and electromagnetic induction
• System suitable for hostile environment

(b) Elements of Data Acquisition System:

Analog data acquisition system will consist of some or all of the following elements:

1. Transducer
2. Signal conditioner
3. Visual display device
4. Graphic recording equipment
5. Magnetic recording equipment

A digital data acquisition system will consist of some or all of the following elements of

1. Transducer
2. Signal conditioner
3. Scanner or multiplexer
4. Signal converter
5. Analog to digital converter
6. Auxiliary equipment
7. Digital recorder.

System suitable for favorable environments:

These are laboratory instrument applications, test systems for collecting long term drift information and routine measuring equipment etc. These systems are designed to perform tasks oriented more towards making sensitive measurements. They do not concentrate on protecting the integrity of analog data.

System suitable for hostile environment:

These are designed to measure and protect the analog data under hostile conditions. Such conditions are encountered in aircraft control system, turbovisous in electrical power systems and in industrial process control systems. Hostile measurements require devices that are capable of withstanding wide range of temperature. They also require excellent shielding.

(c)  Aim of Data Acquisition System: 

The aims of data acquisition system are listed here under:
1. To acquire the necessary data at precise speed and at the correct time.
2. To inform system operator the status of plant using all the data collected.
3. To monitor the total plant operation and maintain on-line optimum safe operation.
4. To provide highly effective human communication and identify and notify problematic areas.
5. To collect and store data.
6. To compute unit performance indices using on-line real-time data.

(d) Factors that Determine the Configuration and Subsystems of DAS: 

The following are the important factors that determine the configuration and the subsystems of a data acquisition system

1. Resolution and accuracy expected
2. Number of channels to be monitored
3. Sampling rate per channel
4. Signal conditioning requirement of each channel
5. Cost

(e) Configuration of Data Acquisition System: 

The factors to be considered to determine the configuration for a DAS are listed below

1. Accuracy and resolution
2. Number of channels to be monitored
3. Whether the signal is analog or digital
4. Whether it is a single channel or multichannel system
5. Required sampling rate per channel
6. Signal conditioning requirements
7. The cost

The different configurations that are possible in a DAS are listed here under:

Single channel configurations: 

1. Direct conversion
2. Preamplification and direct conversion
3. Sample and hold and conversion
4. Preamplification, sample and hold and conversion
5. Preamplification, signal conditioning and any of the above

Multichannel configurations: 

1. Multiplexing the output of single channel converters
2. Multiplexing the output of sample hold
3. Multiplexing the inputs of sample hold
4, Multiplexing the low level data
5. More than one multiplexers

Signal conditioning methods: 

1. Ratiometric Conversion
2. Wide dynamic range options:
a. High resolution conversion
b. Range biasing
c. Automatic gain switching.
d. Logarithm conversion

3. Noise reduction options:
a. Filtering
b. Integrating converters.
c. Digital processing

System measurement time, error and cost are some of the useful estimates for the finalization of system configuration.

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