1. Currently there are two main stream types of control systems – PLC-SCADA and Distributed Control Systems (DCS).
2. DCSs were traditionally used to control large processes, while PLC systems were used to control machines. Since the introduction of PC Windows-based SCADA systems and Ethernet networks, the dividing lines between DCS and PLC-SCADA systems have become blurred.
PROGRAMMABLE LOGIC CONTROLLER
1. The primary difference between a PLC (or Programmable Logic Controller) and SCADA (Supervisory Control and Data Acquisition) is the fact that a PLC is hardware and SCADA is (generally) software, though some would argue that SCADA is a plant’s overall control system using hardware and software elements.
2. Regardless of semantics, both are used in industrial settings, and they’re typically used together, but they are still two very distinct systems.
3. PLCs are designed to control complex industrial processes, such as running machines and motors. They are simple to program and fully scalable to an operation’s requirements. They’re also used to collect data from the systems they control.
4. The PLC hardware is designed taking into account harsh industrial environments, requirements of modularity and scalability etc. Programming languages are simple. Programs can be modified online without stopping the PLC or the operations it controls.
5. They’re an upgrade over the old relays and timers previously used to control industrial machinery since PLCs are capable of performing much more complex tasks.
6. SCADA is essential a piece of software that is installed on a PC/computer. One of its major functions is to act as Human Machine Interface (HMI).
7. SCADA systems are often used in conjunction with PLCs and other devices (in fact, some would say that a PLC would be part of a SCADA system). Data from PLCs and Remote Terminal Units (RTUs) are relayed to the system, and commands are entered into the HMI to make adjustments to the processes they control.
RELATIONSHIP BETWEEN PLC AND SCADA
1. Used together, SCADA software and PLCs form an automatic system for prescribing maintenance tasks, forming the core of a predictive maintenance program. It works something like this:
2. PLCs are a part of the system that SCADA oversees. The PLCs need SCADA to control their function, but SCADA relies on data from the PLCs to complete its overview.
-Data from sensors on individual assets is transmitted to the PLC
-The PLC translates that data into a format that can be used by the software
-Users access the data through the HMI on the software
-If the data crosses certain thresholds, a maintenance work order is created
EXAMPLES
1. For instance, if a turbine is showing too much vibration, sensors transmit that data through the system, and the readouts on the user end would trigger a work order. In this application, SCADA software controls the entire system, while PLCs act as relay points and controllers for specific assets.
DISTRIBUTED CONTROL SYSTEMS (DCS)
1. Distributed control systems are process-oriented platforms that rely on a network of interconnected sensors, controllers, terminals, and actuators to act as a centralized master controller for all of a facility’s production operations.
2. Thus, a DCS focuses on controlling and monitoring processes and providing facility operators with the ability to see all facility operations in one place.
3. At the same time, a DCS allows you to implement more advanced process automation strategies because it operates on a closed loop control platform.
4.That makes DCSs ideal for controlling operations at a single facility or factory.
5. Ultimately, a DCS is critical for maximizing the visibility of a facility’s day-to-day operational processes.
DCS VS SCADA SYSTEMS
1. While a DCS emphasizes process-level operations, a SCADA is event-driven and prioritizes data gathering.
2. A DCS delivers data to operators, and at the same time, a SCADA concentrates on the acquisition of that data.
3. SCADA systems are also more scalable and flexible, so they can be used to collect data from a single factory or a dozen without geographical restrictions.
4. Makes a SCADA solution ideal for organization’s that want to deploy remote monitoring platform.
5. In short, a SCADA is geared towards understanding and collecting data on processes, while a DCS emphasizes process control.
6. Both are necessary if you want to take advantage of the latest in Industry 4.0 technology. Today, many DCS solutions integrate SCADA systems into their operations to form a unified whole.
7. While SCADA and DCS platforms started as distinct operations, they’re now merging to create a singular supervisory system for all of your facility’s needs.
8. PLC-SCADA solutions cater to meet specific client needs but require extensive work to manage the associated complexity of the PLC and SCADA software. A PLC-SCADA solution is characterised by its open nature, flexibility, complexity and can be considered to be a blank canvas.
9. DCS offer validated software modules, enforce standardisation, modularity and consistency at the cost of flexibility and are accompanied with vendor locking and restrictions. A DCS is characterised by its consistency/rigidity, closed and proprietary nature, and requires specific system knowledge. It can be considered to be a paint-by-numbers solution.
10. Today, DCS and DCS-like configurations aim to provide solutions for most types of processes from the field components, across control functionality and visualisation, to MES functionality (vertical solutions).
11. These solutions are often comprehensive and are limited to the products available from that manufacturer because these vertical solutions require a great amount of integration between PLC/controller software, SCADA/visualisation software and MES software.
12. DCS manufacturers don’t allow end users to change the standard and validated software modules, as these changes would affect the integrated functionalities. These limitations reduce the flexibility of the control software and functionality. This gives a DCS a more proprietary and rigid nature.
13. It also shifts the role of the software engineer from creating software to configuring software by knowing the features (secrets) of the standard software provided by the manufacturer.
14. As a PLC is a blank canvas, subsequently the quality of the software is highly dependent on the skill set and experience of the engineer(s). PLC-SCADA solutions tend to be less comprehensive than DCS solutions and are therefore leaner.
15. Software is custom made and is open, therefore changes can be made to suit the client’s specific needs at any time. A PLC-SCADA solution is somewhat limited to the PLC and SCADA functionality and does not have standard integrated MES functionalities (these would need to be custom made).
16. PLC-SCADA systems offer freedom and require a high level of discipline in regards to structuring and managing of software. If discipline is not adhered to, software can become complex and unreliable.
17. What both systems have in common is that they follow the ISA88 guidelines and support the IEC 61131 programming languages.
18. The ISA88 guidelines contribute to the establishment of a paradigm in the industrial automation community in regards to how control software is structured and created.
19. This paradigm results in a consistent high level of complexity of control software in the majority of the most prevalent systems (PLCs and DCS). For an engineer, the difference between the two systems is that programming a PLC-SCADA system relies more on extensive skills, while configuring a DCS relies more on extensive and up-to-date knowledge.
Source:
https://www.onupkeep.com/answers/asset-management/plc-vs-scada/
https://www.hexaengineers.us/the-differences-between-dcs-and-scada/
https://pacetoday.com.au/plc-scada-vs-dcs/
