Industrial Instrumentation and Control: Key differences between PLC and DCS systems

We have so many people with questions as to what are the similarities and differences if any between PLC and DCS systems. We will try to describe the working of these two systems and in the process help you understand the functions of each system.

Initially DCS was focused only on process control with analog signals that were used as main control system in process industries like Refining, Chemicals, and Petrochemicals etc. while PLC was focused on discrete automation with discrete on-off signals, that were used for example in Factory assembly lines and bottle lines but today DCS supports discrete I/O and some logic functions and PLCs support analog I/O with some control functions. In some instances Both PLC and DCS are used in the same plant i.e. PLCs are used on separate units on a plant floor which are then integrated with main plant-wide DCS for Control and Monitoring.

Let us now look at each system separately to help us understand more how they work.

PLC

Programmable Logic Controllers (PLCs) comes in different sizes which means various I/O and program capacities. Smallest sized PLCs are typically referred to as nano PLCs, micro PLCs and mini PLCs. They have fixed I/O and mainly used in stand-alone applications.

Large PLC support redundancy for CPU, power supply and possibly the control network, but typically not for I/O cards though there are large PLCs that support I/O redundancy by using duplicate I/O-subsystems with separate backplanes where the field instruments are wired in parallel to both I/O subsystems. The control network is typically a standard industrial Ethernet application protocol over Ethernet media and IP. The Field cabling comes directly onto the I/O card.

PLC usually support very fast scan times as required in discrete manufacturing but PID loops add to the CPU load, much more than discrete load thus making the scan time slower.

Loops are not handled individually in a PLC. Addition or change to loop requires a download of the entire program which affects other loops in the CPU as well.

PLCs are built around a given native protocol, this maybe: PROFIBUS, Modbus, DeviceNet etc. The PLC comes with its own native interface cards for native protocol supported by the PLC maker but relies on third-party interface cards for other Fieldbus protocols. The engineering software therefore automatically configures the communication interface card for the native protocol.

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Key Points to note on PLC

PLCs were designed to eliminate assembly-line relays during model changeovers. PLC is easier to change than relay panels; this has reduced the installation and operational cost of the control system compared with electromechanical relay systems.

BLOCK DIAGRAM OF A PLC SYSTEM

PLC offers the following advantages:

Ease of programming and reprogramming in the plant
Programming language is based on relay wiring symbols familiar to most plant electrical and instrumentation personnel
High Reliability and minimal maintenance
Small physical size
Ability to communicate with computer systems in the plant
Moderate to low initial investment cost
Available in modular designs

DCS

Distributed Control System (DCS) supports redundancy for controllers, power supply and control network as well as redundant I/O cards including fieldbus interface cards in the same backplane. The control network supports peer-to-peer communication between controllers. The control network is typically a proprietary application protocol over Ethernet media and IP. The field cabling in DCS lands on a Field Terminal Assembly (FTA) where a special system cable with a connector takes the signals to the I/O card.

Loops in a DCS are executed individually. The scan time in a DCS is set individually for each loop. Most loops run at 1000ms although 250ms is common for pressure and flow loops in refining and petrochemicals and even 100ms is also possible. The scan time is constant, and does not change with task loading. This is important for PID control and time-based functions such as integration/totalizing and lead-lag dynamic compensation.

Loops in a DCS are managed individually. A change and download to one loop doesn’t affect the other loops.

A DCS has an integrated development environment where I/O control strategy and operator graphics are created together and stored in a single database. This means once a tag is created in the DCS it automatically becomes available everywhere in the system with the same human readable tag name for use in basic control, advanced control, graphics, faceplates, trending, alarming, and turning etc. Without mapping data through registers or other tag names makes it easy to do changes or additions.

The Sensor & Actuator level “H1” Fieldbus network supported by DCS is basically FOUNDATION fieldbus for instrumentation and PROFIBUS-DP for motor controls.

The DCS comes with its own native Fieldbus interface cards. The engineering software therefore automatically configures the communication interface cards for the variables used in the control strategy and graphics.

Key Points to Note on DCS

DCS is miniaturized version of the multitasking, multivariable, multi-loop controller used for process control. It is functionally and geographically processing distributed system. Equipment making up a DCS is separated by function and is installed in two different work areas of a processing installation. Equipment for operator to monitor process condition and to manipulate the set point of the process operation is located in a central control room; from where the operator can view information transmitted from the process area and displayed on a video display unit and can change control condition from a keyboard. DCS systems are suitable for the following processes:

Where a single centralized system is not adequate i.e. Power, Steel, Pulp & Paper plants, Fertilizer etc.
Processes of different level of hierarchy
Processes which can be divided into different and functionally independent sections, based on functional scope and geographical distribution

DCS offers the following Advantages:

Compact to contain ON/OFF controllers
Reduced complexity and easy expandability
High Speed of the control processing
Control Algorithms changes do not call for hardware changes
Continuous trend data is available
User friendly but higher data security
Plant data are transparent on the network
Sequential, batching and feedback control are possible

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Differences between PLC and DCS based Systems

PLC	DCS
Redundancy not possible or limited	Redundancy possible at every level
It is used for low loop count	It is used for any loop count
Performance drops with increasing loop count	No change in performance with increasing loop count
Purely free running mode	Highly efficient multitasking mode
Analog processing simulated through digital computer	Analog processing done in real frequency domain function
No interplant connectivity	Fully Functional Inter-plant connectivity
Individual database for every node	System-wide global database
Typical performance: 100 PID loops/sec	Typical Performance: More than 1000 PID loops/sec.