04-01-2013, 03:59 PM
INSTRUMENTATION SYSTEMS
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Introduction
For any industry to be successful , quality control and scale of production at the viable economic level is a must . The quality control depends on the monitoring of vital processes and associated variables . The economy of operation is controlled by material and energy inputs into the production process / system . The monitoring of variables, the process thereby , energy and materials need instrumentation systems finally to achieve the automatic control of the given industrial process .
The integration of monitoring elements / devices and systems for each individual variable in the process is referred as data acquisition channel and a combination of all such channels is referred as Data Acquisition System ( DAS ). With the integration of DAS and the process being monitored and controller the DAS acquires the level of Instrumentation system , as shown in fig. 1.1.
Role of instrumentation systems
In any industrial system, measurement of variables, provides the information about their magnitude and time of occurrence viz. The status which may be utilized for one or more of the following activities:
i) Monitoring
ii) Control
iii) Behavioral analysis
Instrumentation professional is required to have appropriate choice of the system (or process, used synonymously) variables, measuring devices, their arrangement and method of utilization of the measurands so as to meet the desired objective(s). An arrangement satisfying the desired objective may also be called instrumentation system.
The objectives need to be defined more specifically, depending upon the type of system and the priorities associated with them. For example, the detailed objectives in the cases of oil refineries system, municipal water supply system, electric power generation system, meteorological system, aerospace vehicle system or a human physiological system have widely different objectives and therefore may lead to widely different instrumentation systems.
Elements of instrumentation systems
For understanding the instrumentation, their systematic organization and analysis, generalized approach is useful, as it makes possible to describe the operation and performance of the system, without referring to specific hardware.
A general instrumentation system can be categorized into two major classes - analog systems, dealing with the information available as continuous function throughout, and digital systems handling the data information in digital or discrete from. Both of these types will have three major elements as shown in figure 1.1.
Data transmission / telemetry is the stage of transferring the
data , corresponding to physical variable being measured , already brought into the format and level suitable for sending to the control room for monitoring by display , recording and control of the process. The format and the media for telemetry are large decided on the basis of distance over which the signal is required to be sent . For example the media could be coaxial cable for small distances with high fidelity, open telephone lines for medium distances with economy but open to noise contamination , through wireless media - the type of RF transmission analog or digital and modulation technique depending upon the distance and fidelity required but at a greater cost .
Use of Monitored Information:
The purpose of instrumentation in any industrial system/ process is to help in achieving desired output
quality, by utilizing the values of physical variables being monitored to implement the control
strategy appropriate for controlling the output variables . Here it will not be attempted to go into the
details of control strategies, yet a representative control system is presented to emphasize the role of
instrumentation. As shown in fig 1.2 the main elements of a control scheme for one of the variables of
the system are indicated.
electronic type :
Major developments have taken place in this type during the last 20 years and a stage has been reached so
that the new plants 10 years hence shall have 95% instrumentation of this type. This has been made
possible due to the advances made in electronics ,contribution to improvement in response time and
quality of data acquisition, data display facilities, recording and reproduction , faster processing and
analytical decision making to name a few. In view of the growing importance of this class of
instrumentation, the present text emphasizes this with some of the widely used configurations. The first
three types are not being presented, for the sake of brevity as these are becoming obsolete and within a
decade shall be replaced by electronic instrumentation. However, in the present systems wherever they are
in use , are being replaced slowly. Interested reader may look into bibliography.
Other Electrical Standards:
4-20mA standard described earlier is by far the most common yet there are some others also in
existence. Among these are 10-50 mA standard and 1-5V with zero off-set.
The voltage telemetry schemes shown in fig 1.5, suffer from a serious disadvantage of voltage drop
along the line , and this may necessitate the use of amplifiers / repeaters at the intermediate stations
between sending and receiving ends in case of long distances. The main advantage of voltage telemetry
lies in reduced power drain, almost to zero by using null deflection technique, but in dynamic situation it
is difficult to achieve this as it demands faster response from the equipment.
Often depending on type of output device, there may be a need of current to voltage (I-V) or voltage to
current converter (V-I), as part of signal conditioning operations.
[attachment=46508]
Introduction
For any industry to be successful , quality control and scale of production at the viable economic level is a must . The quality control depends on the monitoring of vital processes and associated variables . The economy of operation is controlled by material and energy inputs into the production process / system . The monitoring of variables, the process thereby , energy and materials need instrumentation systems finally to achieve the automatic control of the given industrial process .
The integration of monitoring elements / devices and systems for each individual variable in the process is referred as data acquisition channel and a combination of all such channels is referred as Data Acquisition System ( DAS ). With the integration of DAS and the process being monitored and controller the DAS acquires the level of Instrumentation system , as shown in fig. 1.1.
Role of instrumentation systems
In any industrial system, measurement of variables, provides the information about their magnitude and time of occurrence viz. The status which may be utilized for one or more of the following activities:
i) Monitoring
ii) Control
iii) Behavioral analysis
Instrumentation professional is required to have appropriate choice of the system (or process, used synonymously) variables, measuring devices, their arrangement and method of utilization of the measurands so as to meet the desired objective(s). An arrangement satisfying the desired objective may also be called instrumentation system.
The objectives need to be defined more specifically, depending upon the type of system and the priorities associated with them. For example, the detailed objectives in the cases of oil refineries system, municipal water supply system, electric power generation system, meteorological system, aerospace vehicle system or a human physiological system have widely different objectives and therefore may lead to widely different instrumentation systems.
Elements of instrumentation systems
For understanding the instrumentation, their systematic organization and analysis, generalized approach is useful, as it makes possible to describe the operation and performance of the system, without referring to specific hardware.
A general instrumentation system can be categorized into two major classes - analog systems, dealing with the information available as continuous function throughout, and digital systems handling the data information in digital or discrete from. Both of these types will have three major elements as shown in figure 1.1.
Data transmission / telemetry is the stage of transferring the
data , corresponding to physical variable being measured , already brought into the format and level suitable for sending to the control room for monitoring by display , recording and control of the process. The format and the media for telemetry are large decided on the basis of distance over which the signal is required to be sent . For example the media could be coaxial cable for small distances with high fidelity, open telephone lines for medium distances with economy but open to noise contamination , through wireless media - the type of RF transmission analog or digital and modulation technique depending upon the distance and fidelity required but at a greater cost .
Use of Monitored Information:
The purpose of instrumentation in any industrial system/ process is to help in achieving desired output
quality, by utilizing the values of physical variables being monitored to implement the control
strategy appropriate for controlling the output variables . Here it will not be attempted to go into the
details of control strategies, yet a representative control system is presented to emphasize the role of
instrumentation. As shown in fig 1.2 the main elements of a control scheme for one of the variables of
the system are indicated.
electronic type :
Major developments have taken place in this type during the last 20 years and a stage has been reached so
that the new plants 10 years hence shall have 95% instrumentation of this type. This has been made
possible due to the advances made in electronics ,contribution to improvement in response time and
quality of data acquisition, data display facilities, recording and reproduction , faster processing and
analytical decision making to name a few. In view of the growing importance of this class of
instrumentation, the present text emphasizes this with some of the widely used configurations. The first
three types are not being presented, for the sake of brevity as these are becoming obsolete and within a
decade shall be replaced by electronic instrumentation. However, in the present systems wherever they are
in use , are being replaced slowly. Interested reader may look into bibliography.
Other Electrical Standards:
4-20mA standard described earlier is by far the most common yet there are some others also in
existence. Among these are 10-50 mA standard and 1-5V with zero off-set.
The voltage telemetry schemes shown in fig 1.5, suffer from a serious disadvantage of voltage drop
along the line , and this may necessitate the use of amplifiers / repeaters at the intermediate stations
between sending and receiving ends in case of long distances. The main advantage of voltage telemetry
lies in reduced power drain, almost to zero by using null deflection technique, but in dynamic situation it
is difficult to achieve this as it demands faster response from the equipment.
Often depending on type of output device, there may be a need of current to voltage (I-V) or voltage to
current converter (V-I), as part of signal conditioning operations.