The IIoT reinvents the PLC

The original architecture of the PLC was well suited to its applications but that design is hitting its limitations and now requires a rethink. The original PLC architecture was designed to support three key factors - programmability, reliability and real time response. The programmability inherent to the architecture was comparatively basic. The key standard for defining how a PLC was to be programmed, IEC 61131, was designed to reinforce the reliability and real time behaviour of the PLC rather than software flexibility.

A key element of the design was the standalone nature of the PLC. The PLC was designed to react to local I/O, using internal control algorithms to react to changes and drive logic level and analogue outputs to control external actuators. IEC 61131 revolved around the implementation of a software configuration. This was the entire body of software – program and data – that controlled the PLC’s real time operation. The configuration equated to the program and data for just one PLC.

Although PLCs were increasingly networked using fieldbuses, the core architecture of IEC 61131 treated each PLC within a network as being logically independent with its own configuration. Within the configuration, programs consisted of interconnected function blocks, each of which could be written in IEC approved languages. These functions were triggered by tasks. Each task would be configured to execute continuously, in a loop, or triggered by inputs or a clock, to support periodic behaviour.
Such an architecture provides predictable results with a low probability of failure but becomes increasingly unwieldy in the face of developments in industrial automation that require much greater flexibility. Industry 4.0, or the industrial internet of things (IIoT), calls for control systems to cooperate much more deeply. Individual PLCs need not only to cooperate with each other but to work more closely with systems that lie beyond the factory itself, right up to servers in the cloud that process customer orders and requests in real time.

There is a further trend towards greater use of distributed control at the machine level. Rather than use one PLC to control the operation of a machine that may integrate multiple actuators and robotic manipulators, the architecture distributes real time control out to the individual subsystems. The ability of each of the PLCs in the network to react to each other and externally generated events, such as last minute changes in customer orders, improves response times and overall operational efficiency.

The increased importance of networking and distributed control calls for more powerful processors to be used in IIoT-capable PLCs. Key requirements are execution performance, the headroom to deal with security enhanced protocols such as Transaction-Layer Security (TLS) and sufficient memory capacity to handle internet protocol (IP) stacks. A 32-bit processor based on the ARM or a similar architecture can provide the core computing horsepower for the PLC.

The addition of digital signal processing (DSP) instructions supports the use of more advanced control algorithms, such as Kalman filtering, now becoming commonplace in motor driven systems. The transition to a full DSP architecture is not necessarily needed - the ARM Cortex-M4, for example, adds DSP-focused instructions to the general purpose ARM architecture. The Blackfin architecture from Analog Devices, which does provide high performance DSP, augments those functions with the general purpose instructions associated with MCU architectures.

For higher performance, an increasingly common approach is to combine a general purpose processor core for networking with one that adds elements of digital signal processing (DSP) for the real time control tasks. This architecture lets one processor handle supervisory, management, networking and high level processing tasks while one or more of the others focus on real time industrial I/O and interrupt handling.

Devices such as the Renesas RX600 add dedicated network processing hardware to offload packet handling tasks from the main 32-bit processor. Network technologies such as EtherCAT provide support for deterministic networking that provides the ability to support hard real time distributed control algorithms. Support for EtherCAT is built into Infineon Technologies’ XMC400 series of MCUs.

With redundant cores, it is possible to implement greater degrees of fault tolerance than is possible with traditional PLC architectures. Infineon’s AURIX family of multicore MCUs, for example, deploys three processor cores, two of which can operate in lockstep. Random errors in execution can be detected by discrepancies between the results. Once an error is detected, the operation can be repeated and checked or the system brought to a safe halt for checks by a technician before any product is damaged or operator safety is compromised.

Resilient operation also depends increasingly on the security of the PLC functions because of the presence of a network connection that extends all the way into the cloud. PLCs need to be authenticated before they are allowed to join a distributed control system and they, in return, need to authenticate the network itself. Any transactions that can affect operation need to be authenticated and encrypted to prevent interception and modification by hackers. A key requirement is for a hardware root of trust to be embedded in the core PLC hardware. This may be in the core MCU or provided using specialised cryptoprocessors and secure memory devices that do not allow the system to complete the startup process unless they are sure the boot image has not been compromised and that the devices attached to the PLC are valid.

The Cypress Semiconductor PSoC 6 brings the concepts of dual-core embedded processing and integrated security together. The MCU combines an ARM Cortex-M4 for high-level processing with an M0+ for rapid response to I/O events. Its trusted execution environment secures access to local data storage to prevent hackers gaining access to sensitive firmware.

Hand-in-hand with the trend towards distributed control is one of continuing miniaturisation, although this has to be offset against ease of maintenance and installation. I/O connections are expected to continue to be based on conventional screw-terminal blocks in many cases. Configurability of I/O connections is also vital. Although it may be possible to build a single PLC board that supports the most common configuration of analogue and digital I/O ports, it will make more sense to continue with a modular I/O architecture. This can be achieved through the use of I/O daughterboards that may either plug into a backplane or are mounted directly on the PLC motherboard.

To reduce down-time, support for warm- or hot-plug changeovers is likely to become an important requirement. The PLC may suspend normal operation but remain in control of other subsystems while a changeover of one of the I/O cards is performed. This requirement calls for a connector design that offers easy mating of daughterboards together with high pin density and a retention scheme that ensures that the cards cannot be separated except by a maintenance technician.

To prevent damage to the electronics during changeovers, hot-plug interface devices need to be used. To provide additional protection against overvoltage and overcurrent situations while the system is running – an ever-present threat in the industrial environment – isolation on the daughterboards needs to be in place. Although optocouplers have often been used to provide isolation, technologies based on transformers and similar electrical high voltage barriers enable compact isolation between external I/O and signal conditioning electronics and the more delicate ADCs and logic functions inside the core PLC. Silicon Labs’ Si8xxx series of digital isolators and Analog Devices’ iCoupler technology are examples of reliable, compact alternatives to optocouplers.

Configuration of the PLC and peripheral expansion may be provided through high-speed interfaces such as USB. Commercial connector designs will not withstand the rigours of the industrial environment. Specialised connectors, such as Amphenol’s MUSBR industrial-grade USB-C design, provide additional ruggedness with features such as IP67 seals. Similarly, for attachment to the Ethernet infrastructure, HARTING’s ix brings ruggedness to a miniature industrial connector design.

The power supply itself is an important consideration, as the drive to reduce size means many advanced PLCs will need to rely on convection and not use bulky fans for cooling. High-integration DC/DC now converters offer efficiencies in excess of 90%. Through multiphase operation, they can support such high efficiencies across a wide range of load conditions, allowing the PLC to drive high output currents when necessary to operate machinery, but also move into energy saving quiescent modes easily.

Through these various changes based on readily available components and subsystems, PLC architecture can accommodate the needs of the IIoT and bring increased resilience, robustness and cost-effectiveness to industrial automation.