Design trends in PLC-based control systems
Equipment used in industrial control, such as a programmable logic controller (PLC), has increasing levels of connectivity and embedded processing. Additionally, there is a desire to make machinery functionally safe to operate. A few key functional product specifications for a small sample of automation vendors, summarized in Table 1, illustrates the emphasis on features in connectivity, embedded processing, and functional safety.
Delving deeper into the architecture and design of I/O modules in these systems, we observe a mutually reinforcing cycle between advances in automation and proliferation of electronic technology. Let’s examine each of these automation trends and the corresponding effect on the underlying electronics.
From a simple industrial control system to a large-scale processing plant, automation infrastructure is usually classified into three levels (Figure 1). At the top is the operations and enterprise level. Next is the control level, consisting of the main processing units used to control machinery and processes on the factory floor. Finally, the actual sensors and actuators are located in the field level. There is a high level of connectivity between these levels, and the underlying theme to inflection today is a need to embed intelligence in the field devices and to control and communicate with them for better efficiencies and planning.
Automation trend: Previously, the main mode of communication between field devices, controllers, and other equipment was 4-20 mA loops or industrial fieldbuses. While these will continue to be used for many years, Ethernet is being adopted in industrial control systems with several fieldbus protocols having corresponding industrial Ethernet versions. Variants include Profinet, EtherNet/IP, EtherCAT, Sercos, Powerlink, and many more. In fact, deterministic protocols are gaining adoption in performance-critical applications like motion control. The benefits include increased throughput, node density, and standard hardware. Data from the manufacturing floor is available to operations planning software with corresponding benefits. In addition to Ethernet, you can see a renewed interest in protocols like HART and IO-Link allowing digital communication with field devices. Finally, wireless technology is making headway, sometimes to augment control loops as a secondary option, in monitoring process variables like tank levels.
Design/architecture trend: The key trend in electronic components enabling connectivity is programmability and flexibility. Given the variety of industrial Ethernet protocols, IC (integrated circuit) manufacturers now offer programmable hardware and firmware to allow system designers the flexibility to support multiple protocols with the same processor. In fact, several processors and FPGAs include hardware engines to enable the cycle-to-cycle determinism required by industrial Ethernet protocols. Moreover, IC manufacturers offer industrial grade products to provide the analog interface for many of the fieldbuses mentioned above, with some interfaces integrated into the processor as peripheral components. Finally, wireless chips and transceivers include encryption engines and increased flash memory to store the protocol stacks required by the various communication interfaces.
Automation trend: Advances in connectivity have led to two system trends: an increasing number of channels per system, and an increase in remotely located systems. For example, digital inline I/O modules from Phoenix Contact go up to 32 channels. The increased channels require an increase in processing capability, and to support the remote I/O, most products today are much smaller in form factor. The control algorithms currently used have tremendous complexity and also need significant processing power. For example, the DeltaV series from Emerson includes the Charms I/O card, which enables remote I/O. DeltaV also touts embedded intelligent control through techniques like fuzzy logic, neural networks, and multivariable predictive control. These advances in processing complexity and size are achieved through embedded processing. Furthermore, there is a trend to move processing from a central controller to distributed local control. Finally, the field devices themselves have increasing levels of embedded intelligence to communicate with their control network and also to perform diagnostics and preventive maintenance.
Design/architecture trend: The key drivers for enabling these trends are integration, packaging, and lower power. Nowhere is this integration more evident than in the increasing level of integrated functions available in microcontrollers and microprocessors today. The simplicity, ubiquity, and the IP licensing model of ARM architecture, coupled with its lower power, have enabled significant advances in families of embedded processors offered by IC manufacturers. Semiconductor process technology, with ever-shrinking transistor sizes, has been another contributor. These advances have allowed significant integration of peripheral components and connectivity options onto these processors, allowing the level of embedded intelligence seen today.
In addition to processors, there has been an increase in the level of integration offered in analog semiconductor chips. To understand the level of integration being achieved, let’s look at a system block diagram of a typical architecture used for the design of an analog input module as illustrated in Figure 2.
Today, most IC vendors provide products that integrate the analog multiplexer, the amplifier, the analog-to-digital converter (ADC), the voltage reference, and the buffer. Essentially, most of the key components on the left side of this block diagram are available in a package of size less than 40 sq. mm. In addition, advances in packaging allow a significant portion of the power management to be integrated into modules or even single packages in some cases. This level of integration, coupled with the advances in processing, allows small form factors enabling products like the MicroLogix PLC family from Rockwell Automation.
In addition to integration, there is power dissipation to consider. Figure 3 shows the system block diagram of a typical analog-output module. One such module with four active channels (only three channels are shown in the figure), each driving a full scale 20 mA signal onto a light load, can dissipate up to 1.5 W on the module itself. That is a large amount of power in a very small space. However, some clever design techniques and advances in integration and packaging will allow the digital-to-analog converter (DAC), the amplifier, and potentially a significant portion of power management to be available in one single package.
Other examples of enabling higher channel density are recent semiconductor chips that allow a large number of digital I/O to be aggregated together in a single IC. As with other electronic equipment, the trend of integration and increasing embedded processing is expected to continue into the future.
Automation trend: Increased connectivity and higher levels of embedded processing have led to tremendous complexity in the automation systems being developed. Therefore, there is a much higher cost to both machinery as well as humans in case of failure. At the same time, regulatory agencies have started to demand higher levels of safety. For example, Europe has issued machinery directive 2006/42/EG on health and safety requirements related to machinery. Such standards are important for certification and typically include specific ratings. For example, IEC standards include the safety integrated level (SIL) ratings. To address these needs, a lot of industrial Ethernet protocols have a complementary safety extension for safety-related data. For example, Profisafe integrates safety into the existing Profibus and Profinet fieldbuses. To meet all of these requirements, automation vendors have started to incorporate more safety-related and diagnostic features to enable functional safety in machinery. For example, Siemens offers certified products to meet various safety standards, and the Simatic line of safety integrated systems combines safety into the standard controller, avoiding the need for a separate safety controller. Several manufacturers offer redundant channels to ensure fail-safe operation. On top of safety, these options provide the added benefit of reduced risk of downtime in machinery.
Design/architecture trend: The key trends that will enable functional safety are integrated diagnostics and safety-enabling features in electronic components. Dual core processors running in lockstep have been used in automotive applications and should migrate well into industrial safety. Although most applications envision dual redundant channels, it is possible that the second channel could use a lower cost processor and process only the safety-related data. Along with the processor, significant diagnostic and fail-safe options exist in the peripheral electronics. Several Ethernet transceivers withstand industrial temperature ranges and together with embedded processors support diagnostic features like time domain reflectometry (TDR) to detect faults and perform cable diagnostics, which is very important in a plant environment. Finally, a number of interface ICs for fieldbus protocols have options like being immune to cross-wire connections using SymPol technology, higher voltage protection, and many other features in case of failure. The next wave of products will include options to perform self-test on a number of these products so that they can be isolated from the rest of the system to enable error checking and improve the reliability of the system.
Advances in automation and the corresponding proliferation of electronics technology create a mutually reinforcing cycle as shown in Figure 4. The focus on connectivity, embedded processing, and functional safety in new PLC I/O products has led to higher channel densities and remote I/Os. This has led IC manufacturers to offer programmable, integrated solutions with lower power consumption and diagnostic features in innovative packages.
A related automation trend to watch is the potential emergence of lower cost solutions driven by Asian markets. This mutual reinforcement between automation trends and the underlying electronics is expected to continue further with advances in automation, driving new electronic designs and vice-versa.
Navin Venkata Kommaraju is business development manager, end equipment solutions marketing at Texas Instruments.
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