Session 21 - DCS Introduction

DCS Introduction
Control System Training – Session 21

Classification: Restricted
Agenda:
• DCS Introduction
• PLC
• SCADA
• General architecture of DCS
• Process or application
• Scan time
• Input and Output requirement
• Redundancy
• RTU and LCU
• PLC vs DCS

DCS INTRODUCTION
PLC: Programmable Logic Controller
PLC is a controller device used for controlling a process or output depending
upon the input status and the program logic downloaded into the controller.
This device can also be called as Local Control Unit. The architecture of a LCU is
as follows.

DCS INTRODUCTION
The LCU receives inputs from measuring devices and commands from
operators and computes the control output needed to make the process follow
the commands. It then sends the outputs to the actuators, drives, pumps,
valves and other mechanical devices that regulate the process in field. The CPU
does this process and it communicates with all the devices in LCU through the
internal bus, which transmits addressing, data control and status information in
addition to the data.

DCS INTRODUCTION
SCADA: Supervisory Control And Data Acquisition
SCADA systems are used to collect data and control the processes at
supervisory level. In SCADA the control could be for a process or even specific
machinery, and the monitoring could be of any thing like an oil refinery, a
power generation plant to a simple switch. The Data Acquisition could be
collection of process data in terms of trends, alarms and events which could be
referred to provide efficient control aid.
The PLC and SCADA combination were being used in most of the industries for
automation, until the following factors became prominent ie the increase in
size and complexity of the industrial process and technology development to
maintain these processes. At this situation the DCS became popular because it
could support the above requirement.

DCS INTRODUCTION
DCS: Distributed Control System
Distributed Control System is a tightly integrated control system where in all
the components used to communicate with the field process could be
distributed around the plant.
The data from all these distributed systems are put together to control the
entire process industries like the oil and gas refineries, the power generation
plant etc..
The following architecture would explain it better.

DCS INTRODUCTION
General Architecture Of DCS System

DCS INTRODUCTION
PLC Vs DCS THE CHOICE
When choosing PLC or DCS for a process or application the following criteria
can be taken into consideration.
• THE PROCESS OR APPLICATION
• SCAN TIME
• INPUT AND OUTPUT REQUIREMENT
• REDUNDANCY
• RTU AND LCU

DCS INTRODUCTION
THE PROCESS OR APPLICATION
In Factory Automation, the process would be of assembling or manufacturing
specific items or things. In this process there would be lot of machines involved
and the items would be moved among these machines to have the final thing.
The products produced in these industries would be single product. The
procedure would remain the same which would be repeated over and over
again and such process is called as simple batch process. For batch process PLC
is the choice.
In Process Automation industries the process involves transformation of raw
materials through chemical reaction or introduction of physical changes to
produce a new and different product. In this kind of process the current
process always depends on the previous process or to say one or more process
units are piped together, and such a process can be called complex batch
process. For complex process DCS is the choice.

DCS INTRODUCTION
SCAN TIME
• PLC is designed for high speed applications which require scan rate of 10ms
or less. With this quick scan time the PLC can be used for control operations
involving motion control, or control of motors and drives where high scan
rates are essential for effective control of devices.
• When lot of regulatory analog loops are in the process like cascade loop, ratio
loop and feed forward loop the response time doesn’t have to be quick, it
requires a scan time around 100 to 500ms range. In such regulatory process if
the scan time is less then the final control element like valves could be
damaged quickly. It is in such conditions where the DCS is used. While
programming for the DCS device the scan time can be chosen.

DCS INTRODUCTION
INPUT and OUTPUT REQUIREMENT
• In Factory automation process the type of inputs and outputs required are
largely discrete, it also contains some analog control sequence to be
implemented, in such conditions PLC is the choice as it can handle.
• In Process automation the operation involves transformation of product and
this is done by controlling the analog parameters like level, pressure,
temperature, flow etc and for such condition the DCS is the choice because it
is designed to handle the large analog I/O requirement.

DCS INTRODUCTION
REDUNDANCY
• This feature of the device comes into consideration based on the Downtime
effect in industries. Downtime is the time period between process stop to the
start of the same process.
• If the Downtime results in loss of production with additional cost and no
damage to the process, and if the value of the value of the independent
product is less then PLC is the choice. In such case the start up of the process
is easier compared to process industries.

DCS INTRODUCTION
REDUNDANCY
If the value of the products manufactured and raw materials used is high and if
the Downtime will not only cause loss in production but also create potentially
dangerous and damaging condition like for eg in some chemical applications
the process has to be maintained at steady state or else the product could
solidify in the pipe. In this condition DCS is the choice as it comes with optional
redundancy choice.
• CPU redundancy
• I/O Module redundancy
• Communication line redundancy
• Server redundancy

DCS INTRODUCTION
RTU AND LCU
RTU- Remote Terminal Unit: This device is used as the communication interface
between the process and the controller. The RTU device would be at the
remote location and would be containing the input output module, the process
data would be received by these modules and sent to the controller through
communication protocols.
LCU-Local Control Unit: This device is capable of performing closed loop control
and also communicating with the master DCS controller when the data is
required from the process controlled by the LCU unit.

DCS INTRODUCTION
PLC VS DCS
The following questions can be asked while choosing between a PLC and DCS.
1. What are you manufacturing, and how?
2. What is the value of the product being manufactured
and the cost of downtime?
3. What do you view as the "heart" of the system?
4. What does the operator need to be successful?
5. What system performance is required?
6. What degree of customization is required?
7. What are your engineering expectations?

DCS INTRODUCTION
1. What are you manufacturing, and how?
The way a product is manufactured, the performance needed, along with any
physical limitations of the process, all influence the system selection.

DCS INTRODUCTION
2. What is the value of the product being manufactured and the cost of
downtime?
Downtime: The time between the stopping of process to restarting it again.

DCS INTRODUCTION
3. What do you view as the "heart" of the system?
• In factory automation ,the control system is the controller (PLC), which
contains all of the logic to move the product in through the assembly line.
The HMI is often an on-machine panel or a PC-based station that provides the
operator with supplemental or exception data.
• In process automation, where the environment can be volatile and
dangerous, and where operators can't see the actual product, the HMI is
considered by most to be the heart of the system. In this scenario, the HMI is
a central control room console that provides the only complete "window"
into the process, enabling the operator to monitor and control the processes
which are occurring inside pipes and vessels located throughout the plant.

DCS INTRODUCTION
4. What does the operator need to be successful?
• In a PLC environment, the operator's primary role is to handle exceptions.
Status information and exception alarming help keep the operator aware of
what is happening in the process.
• The DCS plant requires an operator to make decisions and continuously
interact with the process to keep it running. The operator's process
knowledge is often critical to operational excellence and keeping the process
running optimally.

DCS INTRODUCTION
5. What system performance is required?
• The speed of logic execution is a key differentiator. The PLC has been designed to
meet the demands of high-speed applications that require scan rates of 10
milliseconds or less, including operations involving motion control, high-speed
interlocking, or control of motors and drives. Fast scan rates are necessary to be able
to effectively control these devices.
• The DCS doesn't have to be that quick – most of the time. The regulatory control loops
normally scan in the 100 to 500 millisecond range. In some cases, it could be
detrimental to have control logic execute any faster – possibly causing excessive wear
on final control elements such as valves, resulting in premature maintenance and
process issues.

DCS INTRODUCTION
6. What degree of customization is required?

DCS INTRODUCTION
7. What are your engineering expectations?

DCS INTRODUCTION
PLC VS DCS OVERVIEW

Topics to be covered in the next session:
• DCS continuation

Thank you!

Session 21 - DCS Introduction

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Session 21 - DCS Introduction