13-08-2013, 04:56 PM
Automation using PLCs SCADA and Telemetry Process Control and Data Acquisition
[attachment=57142]
Automation Using PLCs
A PLC or programmable controller is a computer based solid state device that
controls industrial equipment and processes. Initially designed to perform the
logic functions executed by relays, drum switches and mechanical timer/counters,
it has been extended to analog control as well.
A typical PLC system consists of a processor and an input/output system all
mounted in a rack like system. The PLC system is a cost effective solution for
applications with a high ratio of digital to analog points in a system. There are
numerous third party vendors supplying software packages that allow the PLC to
be interfaced to a PC based operator interface package. The typical method of
programming PLCs is using ladderlogic.
Basic Rules of Ladderlogic
The basic rules of ladderlogic can be stated as:
• The vertical lines indicate the 'Power supply' for the control system. The logical
'power flow' is visualized to move from left to right, and cannot flow from
right to left (unlike 'real' wires).
• Read the ladder diagram from top to bottom and left to right (as in the normal
Western convention of reading a book).
• Electrical devices are normally shown in their de-energized condition. This
can sometimes be confusing and special care needs to be taken to ensure
consistency.
Output Energize Coil
When the complete ladderlogic rung is set to a TRUE or ON condition, the output
energize instruction sets its memory location to an ON condition; otherwise if the
ladderlogic rung is set to a FALSE or OFF condition, the output energize coil sets
its memory location to an OFF condition.
Master Control Relays (MCR)
An example of this is given in Figure 1.8. Essentially when the MCR is energized,
the output coils for each rung following can be driven by their appropriate logic.
Whenever the MCR is de-energized, the output coils for each rung following cannot
be energized even if the appropriate logic for that coil attempts to drive it into the
energized or true state.
File or Block Manipulation
Words in a PLC are defined as 16-bit locations in the memory. They can be used
to store the contents of an A/D input module with 16-bit resolution or the states
of digital inputs and outputs (external or internal). A file or block on the other
hand is considered to be a collection of contiguous words. Files are also referred
to as data tables.
Jump to a Label or Skip
The JUMP instruction allows the processor to proceed to any part of the program
(either forwards [ahead] of the current JUMP instruction or backwards [behind]
the current JUMP instruction). The JUMP instruction proceeds to a defined label
when the rung on which it is situated becomes true. An example is given in the
following figure below.
SCADA and Telemetry Fundamentals
Supervisory Control and Data Acquisition (SCADA) systems have been in use in
various forms for over thirty years. Telemetry systems are a key element of a
SCADA system providing the necessary transfer of analog and digital data from
the Remote Terminal Units (RTUs) to the master stations. The term SCADA
implies that there are two activities that are necessary:
• The acquisition of data and subsequent transfer to some central location
(or group of central locations), and
• The control of some process or equipment from these central locations.
There are four components to a SCADA system:
• The central site which is the controlling station for the entire system, normally
providing the operator interface for display of information and control of
remote sites.
Basic Definitions
In a control system, the variable, we want to control, is called the Process
Variable or PV. In industrial process control, the PV is measured by an instrument
in the field and acts as an input to an automatic controller (which is computer
based) which takes action based on the value of the PV. Alternatively the PV can
be input to computer based hardware and displayed so that the operator can perform
manual control and supervision. The variable to be manipulated, in order to have
control over the PV, is called the Manipulated Variable. If we control a particular
flow for instance, we manipulate a valve to control the flow. Here, the valve
position is called the manipulated variable and the measured flow becomes the
process variable. In the case of a simple automatic controller, the Controller
Output Signal (OP) drives the manipulated variable. In more complex automatic
control systems, a controller output signal may not always drive a manipulated
variable in the field. In practice, the term Manipulated Variable is rarely used.
Most people involved in process control refer to the OP (output) of a controller
and it is assumed that one knows the purpose of it. The ideal value of the PV
(Process Variable) is often called Target Value. In the case of automatic control
the term Set Point Value (SP) is preferred.
[attachment=57142]
Automation Using PLCs
A PLC or programmable controller is a computer based solid state device that
controls industrial equipment and processes. Initially designed to perform the
logic functions executed by relays, drum switches and mechanical timer/counters,
it has been extended to analog control as well.
A typical PLC system consists of a processor and an input/output system all
mounted in a rack like system. The PLC system is a cost effective solution for
applications with a high ratio of digital to analog points in a system. There are
numerous third party vendors supplying software packages that allow the PLC to
be interfaced to a PC based operator interface package. The typical method of
programming PLCs is using ladderlogic.
Basic Rules of Ladderlogic
The basic rules of ladderlogic can be stated as:
• The vertical lines indicate the 'Power supply' for the control system. The logical
'power flow' is visualized to move from left to right, and cannot flow from
right to left (unlike 'real' wires).
• Read the ladder diagram from top to bottom and left to right (as in the normal
Western convention of reading a book).
• Electrical devices are normally shown in their de-energized condition. This
can sometimes be confusing and special care needs to be taken to ensure
consistency.
Output Energize Coil
When the complete ladderlogic rung is set to a TRUE or ON condition, the output
energize instruction sets its memory location to an ON condition; otherwise if the
ladderlogic rung is set to a FALSE or OFF condition, the output energize coil sets
its memory location to an OFF condition.
Master Control Relays (MCR)
An example of this is given in Figure 1.8. Essentially when the MCR is energized,
the output coils for each rung following can be driven by their appropriate logic.
Whenever the MCR is de-energized, the output coils for each rung following cannot
be energized even if the appropriate logic for that coil attempts to drive it into the
energized or true state.
File or Block Manipulation
Words in a PLC are defined as 16-bit locations in the memory. They can be used
to store the contents of an A/D input module with 16-bit resolution or the states
of digital inputs and outputs (external or internal). A file or block on the other
hand is considered to be a collection of contiguous words. Files are also referred
to as data tables.
Jump to a Label or Skip
The JUMP instruction allows the processor to proceed to any part of the program
(either forwards [ahead] of the current JUMP instruction or backwards [behind]
the current JUMP instruction). The JUMP instruction proceeds to a defined label
when the rung on which it is situated becomes true. An example is given in the
following figure below.
SCADA and Telemetry Fundamentals
Supervisory Control and Data Acquisition (SCADA) systems have been in use in
various forms for over thirty years. Telemetry systems are a key element of a
SCADA system providing the necessary transfer of analog and digital data from
the Remote Terminal Units (RTUs) to the master stations. The term SCADA
implies that there are two activities that are necessary:
• The acquisition of data and subsequent transfer to some central location
(or group of central locations), and
• The control of some process or equipment from these central locations.
There are four components to a SCADA system:
• The central site which is the controlling station for the entire system, normally
providing the operator interface for display of information and control of
remote sites.
Basic Definitions
In a control system, the variable, we want to control, is called the Process
Variable or PV. In industrial process control, the PV is measured by an instrument
in the field and acts as an input to an automatic controller (which is computer
based) which takes action based on the value of the PV. Alternatively the PV can
be input to computer based hardware and displayed so that the operator can perform
manual control and supervision. The variable to be manipulated, in order to have
control over the PV, is called the Manipulated Variable. If we control a particular
flow for instance, we manipulate a valve to control the flow. Here, the valve
position is called the manipulated variable and the measured flow becomes the
process variable. In the case of a simple automatic controller, the Controller
Output Signal (OP) drives the manipulated variable. In more complex automatic
control systems, a controller output signal may not always drive a manipulated
variable in the field. In practice, the term Manipulated Variable is rarely used.
Most people involved in process control refer to the OP (output) of a controller
and it is assumed that one knows the purpose of it. The ideal value of the PV
(Process Variable) is often called Target Value. In the case of automatic control
the term Set Point Value (SP) is preferred.