PLC-SCADA vs DCS

SCADA

PLC specialist Martÿn Hilbers talks about the differences between a PLC-SCADA control system and a DCS configuration.

Currently there are two main stream types of control systems – PLC-SCADA and Distributed Control Systems (DCS).

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.

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.

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.

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). 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.

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.

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.

PLC-SCADA systems are applied across many industries and provide clients with tailor-made solutions.

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.

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). 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.

What both systems have in common is that they follow the ISA88 guidelines and support the IEC 61131 programming languages. 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. 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.

Smart industry, or the fourth industrial revolution (Industry 4.0), includes the full utilisation of production and field data (other than control data). Currently, the information from field instruments is retrieved by the PLC and passed to the SCADA system where the data is made available to software that can analyse and use this field data. Currently this data exchange must be programmed in the PLC. However, the purpose of a PLC is to provide control functionality, not predictive maintenance. Programming additional data exchanges/handling that is not used for controlling the process, does not only require PLC programming expertise, but also poses risks to the continuity of the controlled process.

Schneider Electric does not have a traditional business model that is based on vendor locking or vendor restrictions, but applies an open philosophy to its automation products. This open philosophy allows unlimited possibilities for users to create solutions that meet their specific requirements.

The symbiosis of the PLC-Easy software and Schneider products results in modular solutions that require less time, efforts and skills to create, while providing reliability, flexibility to be customised to any requirement, and providing integration possibilities with many innovative products.

The largest expenses associated with the procurement of a control system is not the hardware nor the licenses, but the costs associated with the development of the control software. New hardware technology is developed at an increasingly faster rate, offering more and better solutions. Schneider-Electric guarantees all control software to be forward compatible, which allows the end client to take advantage to replace hardware with the latest technology without the expense of replacing the control software.

Schneider-Electric has developed a new PLC/PAC (M580) with an additional integrated network card for the purpose of collecting data from field devices. A Time Sensitive Network protocol and VLAN techniques are utilised by this additional network card to make non-essential data (to the control of the process) available to systems other than the PLC or SCADA system (without any PLC programing or any interference to the PLC’s hardware and data exchanges). Schneider-Electric is also the manufacturer of Wonderware, a MES and SCADA platform. The combination of a PLC, that is capable of passing field meta data without programming, with an open SCADA-MES platform that allows the integration of multiple vendor software solutions, could prove a to be a solution in the coming fourth industrial revolution.

PLC-Easy software breaks with the paradigms currently existing in the automation industry.

Through the use of a patented universal algorithm, an unconventional single layer software architecture can be utilised. A single layer architecture is simpler compared to the conventional architectures currently used in the industry.

By basing the PLC software library on a universal algorithm, the software modules have a nearly identical programmer’s interface for all levels of the ISA88 physical model. As the interface consists of eight parameters (connection pins), the learning curve is short and programming can be done with minimum skill and effort similar to that of building with Lego bricks. As the library is built with standard PLC programming techniques, the software modules and data structures of the library can be extended with user-defined data structures allowing for custom-made integration of control functionalities for batch control, motion control, MES and energy consumption.

Main Features of PLC-Easy are:

PLC-Easy offers the possibility to integrate any hardware diagnostic signals of any type of device from any vendor. This allows accurate and instantaneous fault diagnoses and sequence of events analysis by the operator.

Because more than 90 per cent of the software created with PLC-Easy consists of standard, tested software modules, the amount of unique software, and therefore complexity, is reduced by at least 50 per cent. The almost 100 per cent utilisation of standard, tested modules, makes the control software and functionality predictable and reliable as the complexity (and subsequent susceptibility to faults and downtime) is reduced.

PLC-Easy, theoretically, reduces the amount of software by at least 30 per cent, however empirically the reduction is a multitude of 30 per cent. For the recent pilot project, 47 sheets of code were replaced by five sheets of code (940 per cent).

It offers unlimited functional redundancy in any configuration and at any level of the ISA88 physical model (configured with basic programming skills).

Its speed (developing and diagnosing), ease of use and simplicity are key features. It not only empowers people with limited programming skills (like process engineers and electrical engineers) to create the more complex aspects of control software, it also empowers maintenance personnel and operators to diagnose and change control software.

PLC-Easy aims to separate the MES and DCS rigidity from the control software/functionality. The benefits of a DCS are combined with the benefits of a PLC, without the disadvantages of both systems.

PLC-Easy provides a lean-tested framework where functionality for every type of process and industry, as well as MES supporting functionality/data, can be easily created, added and modified by using basic programming skills. It provides the rigidity of a DCS and it provides the flexibility of a PLC-SCADA system without the proverbial “spaghetti programming”.