Centralized controls: Smart software, networks trump smart hardware

Machine architectures require many field devices. For easier system design, engineers should select a core control platform that minimizes complexity, with a unified software platform and network. More streamlined hardware and networking architectures are easier to implement, support, and maintain.

06/03/2014


Machine design is easier with integrated PLC, safety, motion, robotics/kinematics, and communications functionality in software, with fewer auxiliary points of control (hardware), and software-based devices replacing hardware functions.

When determining the controls architecture of a machine, there are many important factors to consider and many different types of devices that will be included in the system. Some of those devices have built-in intelligence ("smart devices") while others are more basic, using a central controller to process and react to the data. When taking system reliability and maintainability into account, simplicity and flexibility are the most significant keys to success.

Figure 1: Removing network switches from the fieldbus system enables EtherCAT to deliver deterministic data from remote devices to the central controller. This brings with it the possibility to process the control algorithms in a central CPU, eliminating

Naturally, machine builders (original equipment manufacturers, OEMs) prefer that the automation and controls architecture help decrease the time and effort required to build and commission a system. Therefore, it is important for engineers to reuse as much preexisting engineering work as possible. This helps reduce configuration time for individual devices and systems. At the same time, the equipment end user needs the machine architecture to promote good overall equipment effectiveness (OEE) and minimize the impact on production when an unexpected failure or maintenance requirement occurs. The most efficient and reliable way to accomplish these goals is by increasing the use of smart software in the areas previously occupied by dedicated, special-purpose hardware.

Simplified controller architecture means that the core of the machine (or machines) is built around one controller and one network, which is all configured and programmed using one software platform that in itself has one primary development environment. Within one platform it is possible to integrate programmable logic controller (PLC), safety, motion, robotics/kinematics, and communications functions largely in software, while reducing the amount of auxiliary points of control (hardware), and replacing "smart" hardware devices in favor of "software" devices. Data from the field is gathered by standard I/O equipment for the centralized controller running smart software to process. This reduces the amount of "black box" hardware on machinery and the configuration that goes along with that hardware. This can lead to significant cost savings and fewer potential points of failure for simpler system commissioning and maintenance.

PC control is fast

Historically, cycle times of 10-20 ms were common in systems that relied upon multiple black box devices for special functionality. Usually, the communication to these devices is asynchronous and results in corresponding inaccuracies because of the lack of determinism associated with the system responses. However, technological advances in high-performance PC controllers have enabled reductions in cycle times easily in the 1-2 ms range, with the possibility to run as fast as 50 μs. This is a decrease in cycle times by a factor 10. As a result, control loops and time-sensitive algorithms can be moved from distributed hardware (smart devices) into the central machine controller, resulting in further cost savings and greater design flexibility within the application.

Centralized control technology can be particularly advantageous if the system can run faster and more powerful control algorithms than would be possible with many distributed small controllers that all require complex communication and handshaking. Modern industrial PCs (IPCs) typically offer significantly higher processing power and memory at lower cost than the sum of even a large number of small distributed controllers. 

Efficient engineering, flexible networks, software

This equation goes beyond hardware versus software; there are network factors to consider. For most controls engineers, complexity creeps into projects from the beginning, perhaps even starting with the first managed switch and subnet. This is followed by adding "smart" devices to TCP/IP based networks. Complexity quickly increases with each device added to the network as well as the required configuration time and effort for devices and networks.

Figure 2: Utilizing “core isolation” makes individual cores “invisible” to the operating system (OS), enabling them to be used exclusively by real-time automation and controls tasks. Resources on those isolated cores are then 100% available for use by Twi

By contrast, when implementing a true industrial Ethernet system, such as EtherCAT, users immediately simplify the system by eliminating the need for managed switches between the controller and the (remote) I/O. Removing network switches from the fieldbus system enables EtherCAT to deliver deterministic data from remote devices to the central controller. This brings with it the possibility to process the control algorithms in a central CPU, removing the need for a wide range of smart devices. 


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John , AZ, United States, 07/10/14 09:08 PM:

While I embrace the virtues of Ethercat and it variations... I question the use of general (COTS) PCs for realtime control of individual processes of installations expected to be operating for many years. Especially when controlling processes with over $100K of material being processed at a time. PC are still changing every 18 months. REAL TIME control performance that can be performed via ethercat (sub-microsecond)will be impacted by performance variations. Being faster, having more cores on replacement PC can create problems (too fast?).
And that nasty issue of which version of the OS (often - Windows) and compatible drivers will be supported and for how long? 5 years? 10 years? For my customers - not nearly long enough.
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