Data Acqusition Systems Generalit.

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Course 1 - DATA ACQUISITION SYSTEM

Table of contents:

Course 1 - DATA ACQUISITION SYSTEM ................................................................................................... 1

DATA ACQUISITION SYSTEM ..................................................................................................................... 1

Generalities of data acquisitions..................................................................................................................... 1

Definition ......................................................................................................................................................... 1

The structure of a data acquisition system ..................................................................................................... 2

Components of a data acquisition: ................................................................................................................. 3

Software for data acquisition .......................................................................................................................... 4

Computing systems ......................................................................................................................................... 5

Classification of data acquisition systems: .................................................................................................... 6

DATA ACQUISITION SYSTEM

Generalities of data acquisitions

Definition

In the most general sense "data acquisition" means the process of obtaining

data from another source outside of the system.

There are many situations in which data (information) from several pointssimultaneously must be stored, transmitted or processed further to its use as control

data. Information processing can consist of simple operations (comparisons) to

complicated mathematical processing (integration, differentiation, mediation,

computing the Fourier transformation, etc. Until recently the data was processed as

analog, but as systems become faster and more complex it is necessary to store

them in digital form, so that they can be processed by a computer which can read



more information and process it more quickly - to an operator - to reach an

essential attribute of procurement systems: real-time control data.

Data acquisition calls a branch of engineering that deals with collectinginformation from a number of numerical and / or analog sources, the conversion of

these data into digital form, processing, storage and transmission of data, such as a

computer, alphanumeric display or to a printer.

To control a physical process it is necessary to extract information about the

process by using transducers. The electrical signal obtained at the output of the

transducer is converted into an electrical signal with different parameters (current,

voltage, etc) through conditioning circuits. Numerical control of physical process

requires the conversion of analogue signals into supported digital signals for digital processing system. Digital signals are obtained by taking data at any given time, of

the analog values and converting these values in numerical form through digital

analog converters, CAD. Electrical signals obtained are used for command

execution and control elements (numeric and alphanumeric displays, relays,

solenoid valves, etc). For analog signals of command execution and control

elements (motors, recorders, etc) it is necessary to convert digital signals into

analog signals through data distribution systems, SDS.

Acquisition of data is found in many of today's fields of activity:

- In industry: the computers that monitor and regulate the process of

technological installations;

- Scientific research: measuring and processing a wide range of electrical

and non-electrical quantities;

- Communications: for monitoring and measurement of communication

lines, and even in everyday life;

- On-board computers fitted in many modern cars.

The structure of a data acquisition system

In the narrower sense, a data acquisition system must be able to perform

three basic functions:

- Converting of a physical phenomenon into a signal which can be

measured;

- Measuring signals generated by sensors in order to extract information;



- Analyzing data and presenting them in a usable form;

Most modern systems of data acquisition are using a personal computer as a

controller. So taking into account the ones mentioned above, the typical structureof a data acquisition system that is based on a PC are as follows:

- Sensors have transducers that convert physical phenomenon into an

electrical signal that can be measured;

-Adjustment circuits for signal isolation, conversion and / or amplification of

signal from the transducer;

-A data acquisition subsystem (which may include multiplexers and

analogue – digital converters);

- A computing system;-Data-acquisition software.

Figure 1.1 Typical structure of a data acquisition system

Components of a data acquisition:

The most common components of a data acquisition system are:

Sensors: are devices that convert the measured size (measuring) into a proportional electrical signal.

Signal conditioning circuits: shall include any device that converts the signal

from the sensor to an acceptable level of A / D (analog digital). These circuits are

usually composed of amplifiers, filters and converters AC / DC.

Anti-aliasing filters: are used to remove (cut) high frequency signals that

cannot be converted correctly by A / D converter.

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MĂRIME FIZICĂ

SENZOR

SEMNAL

ELECTRIC CU

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CIRCUITE DE

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Sample and hold circuits: are circuits that are used before multiplexing, the

sampling signals from multiple simultaneous channels or before A / D converter

Multiplexer (MUX): is a selector which connects one channel at a time to theA / D converter.

Analog digital converter : is the heart of the acquisition system – it converts

analog signal into a digital signal. Mainly analog to digital conversion is an

operation of comparison; the signal is compared with a reference value and is

converted into a fraction, which is then represented as a digital coded number.

Clock generator : is a source of clock pulses required for all the devices in

order to function correctly.

RAM / ROM : are used for temporary or permanent storage of data. RAMused by the acquisition system may be that of a computer or can be placed directly

on-board the acquisition board in order to take and store data from the converter

output (until the computer will take tem over).

Analog digital converter : can be used when you want to obtain control of

analog signals

The interface: is a circuit that connects an element to another, for example to

connect an A / D to a processor bus.

Transmission of data: is done according to standards and methods of

transmission are adapted to the area in which the topology and the location of the

center of decision. Thus it can be used for data transmission standard RS232, 485

etc. (For serial transmission, if the transmission distance is less than 200 m), the

transmission on the parallel port (network) transmission through MODEM (long

distance) and not least to radio or GSM.

Software for data acquisition

Hard for data acquisition is completely useless without software - and

hardware acquisition with poorly supported software is almost useless. Therefore,

recently there has been a veritable explosion of software products for this area.

Alongside the continuous improvement of speed and resolution that users want for

measuring and testing equipment, scheduling software for data acquisition was

greatly facilitated by the emergence of software packages running under Windows

in addition to providing user-friendly interface - which is familiar to many

engineers, an opportunity that facilitates data exchange standards.



Consequently software is often an essential (sometimes decisive) in the

design of data acquisition systems, although hardware manufacturers do not like

this mentality. The truth is that we must choose the software first. It’s the samewhether we will run on a PC or an HP.

Although Windows drops speed in data acquisition and analysis, the

advantages that they offer surpasses this aspect. Demand for Windows application

for data acquisition applications is growing.

Another advantage of Windows is that it allows applications to be driven by

events, eliminating the need to carry out cyclic query (polling). This increases the

effectiveness of programming and provides the programmer more flexibility in

terms of exploiting the possibilities of multitasking in Windows. Suppose that in a polling system we should take over 1000 samples. The software should start

questioning the system operation and then after each sampling, asking whether the

sample with number 1000 has been acquired. A turn of events operation system

counts the samples and sends a message after the last sample acquisition. In polling

systems, the operation is controlled by the application, in a system controlled by

events is the control is maintained by the operating system.

Data acquisition hardware devices tend to be more and more a kind of

consumer goods. This trend causes differentiation of major software conversion

data acquisition systems. In very many cases the software may be the most

expensive component of such a system.

Computing systems

As a general rule one can say that it is not necessary that the computing

system for data acquisition to be extremely strong. However if the application

involves a combination of data acquisition and graphical data analysis

representation, your investment in a powerful platform must be justified. Recently,

a growing number of users of data acquisition systems raise the issue of portability.

Until recently, such a system by means of a portable computer laptop with sockets

to expand the interface of data acquisition.

But their size and power consumption is problematic in many applications.

Today more and more experts believe that the solution is the notebook computer.

These computers do not have data acquisition; they must be external boxes that use

serial or parallel ports of your computer. Today's notebooks (whose prices fall)



approaches more and more desktop PCs in terms of communication power,

covering about 90% of the needs of data acquisition that are "portable". The rest

can be external boxes.

Classification of data acquisition systems:

Data acquisition systems can be classified by the number of data acquisition

channels:

- Single-channel systems, those that takes data from a single measuring

system;

- Multi-channel systems- With analog multiplexing (switching input is analog);

- With digital multiplexing (switching input is after they have been converted).

The choice of multi-channel acquisition system with analog or digital

multiplexing is based on the type and number of measured quantities, the method

of variation of these sizes, speeds, etc. necessary purchase.

After analyzing the characteristics, functions and limitations that must

satisfy a system of measurement can advance purchase and a measuring system

architecture, modern acquisition and monitoring.

Data Acqusition Systems Generalit.

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