PLC is electronic systems that operate digitally and are designed for use in industrial environments, where the system uses programmable memory for internal storage instructions that implement specific functions such as logic, sequences, timing, enumeration and arithmetic operations to control machines
A Distributed Control System (DCS) usually refers to a control system of a manufacturing system, process or any type of dynamic system in which the controller elements are not centrally located (such as the brain) but are distributed throughout the system with one or more controllers controlling each component sub-system.
Difference between PLC & DCS:
Majority of process plant has both the PLC & DCS. PLC employs as the subsystem of DCS.
Core processes (food, pharmaceutical, refining, etc.) are usually controlled and managed by DCS. PLCs are used to regulate non-core process tasks including material handling, water treatment, engine control, plant equilibrium, air compressor control, packaging
Configuration vs. Programming:
DCS is designed to configure and PLC to program.
DCS configuration utilizes conventional control items which are automatically connected to the corresponding faceplate, simplifying setup and resulting in standardization.
PLC uses configured softwares to program process setpoints.
Skid & Packaged Systems:
Process plant PLCs on skid installed and packaged devices create control and automation problems.
Skid mounted and packaged systems are factory-built units that provide a particular plant-specific feature. Controls and automation on a skid are just as much component of the plant as controls and automation installed on site.
Control Network Communications
DCSs have extremely sophisticated and embedded Foundation Fieldbus and HART interfaces as well as appropriate interfaces to other networks of industrial automation.
PLC systems tend to have fewer sophisticated Foundation Fieldbus and HART interfaces. In many instances, they depend on hardware and software from third parties with more intensive setup.
Interestingly, discrete network interfaces can pose a problem with PLC systems, as there are many standards and bigger suppliers optimize the interface and software setup to their flagship protocols.
Modeling and optimizing real-time software is an evolving feature supplied by DCS providers to attain greater efficiencies. This optimization level is elevated, multivariable control based on real-time business management objectives, real feedstock data, demand for manufacturing and energy expenses
Advanced Process Control:
PLC lacks in process optimization compared to DCS. Usually offering a number of instruments to optimize control loops and more sophisticated options to enhance PID control efficiency. PLCs add these features to process control with their push.
DCS backbone network:
Typically, DCS backbone networks are conventional Ethernet hardware, but they use their own closed, high-performance protocols and support redundancy natively.
PLC systems use open published protocols to cover a broad variety of apps including easy discrete, synchronized movement control, engine control, and process control.
Value of the product being manufactured:
The PLC is the probable option if the price of each autonomous item being produced is comparatively small and/or downtime results in loss of manufacturing, but with little extra expense or harm to the process.
The choice should be DCS if the price of a batch is large, either in the price of raw material or market value, and downtime outcomes not only in lost production but possibly hazardous and harmful circumstances.
System speed, PLC vs. DCS
The PLC has been designed to satisfy the requirements of high-speed apps requiring scanning rates of 10 milliseconds or less, including movement control, high-speed interlocking, or motor and drive control activities.
Most of the time, the DCS doesn’t have to be that fast. The regulatory control loops usually scan within the range of 100 to 500 milliseconds. In some instances, executing control logic any quicker could be harmful–potentially causing excessive wear on final control components such as valves, leading to early maintenance and process problems.
PLC vs. DCS:
- DCS (Distributed Control System) is a CONTROL SYSTEM that operates with multiple controllers and coordinates all controller’s job. A distinct plant is handled by each controller. The PLC is linked to this controller.
- The PLC (Programmable Logic Controller) is a CONTROLLER that can be rescheduled. If the PLC is a stand-alone and not combined with other PLCs, it will be called DDC. It implies that PLC is a big system of sub-system called DCS.
- All system checks are performed in the central processor in DCS so that the entire power plant will also fail if it fails. Mostly DCS is used as a regulatory check.
- DCS is not a large PLC. Because system architecture of DCS and PLC are different.
- DCS is not an integrating PLC into a single big system. “Controller” is more intended for “Logic Controller” in the PLC, while “Controller” is more intended for “Process Controller” in the DCS.
- Both DCS and PLC is a configurable and reconfigurable.
Quick Look: PLC vs. DCS
PLC vs DCS
|PLC is controller||DCS is control system that operates multiple controllers.|
|The heart of the system is the controller||The heart of the system is the HMI|
|Analog Control: Simple PID only||Analog Control: Simple to advanced PID control
up to Advanced Process Control
|You can use the system offline to configure||Online configuration changes often required|
|Diagnostics to tell you when something is broken||Asset management warns you about what could break before it breaks|
|For motor or motion control, fast logic scanning (approx. 10ms) is needed||Control loops involve deterministic scanning at a velocity of between 100 and 500 ms|
|To create custom logic, high-level programming languages are accessible.||Custom logic from current blocks of functions|
|Use ladder logic to configure application||Use function block diagram to configuration
|System designed to be flexible||System intended to create processing apps "simple"|
|Program / configure and integrate individual components later (bottom-up).||Up-front design of complete system before implementation begins (top-down)|
|Customized routines usually required||Many algorithms (i.e. PID) are complex and
do not vary among applications
|The operator's primary role is to handle exceptions||The interaction of the operator is typically necessary to maintain the process within its target range of results|
|Status information (On / Off, Run / Stop) for the operator is critical||Faceplates and analog trends are essential in order to "see" what happens to the process.|