Reinventing the PLC for Industry 4.0
The Industrial Internet of Things (IIoT), Industry 4.0 – call it what you will – is moving swiftly into the fourth industrial revolution. Today’s mainstay of industrial automation, the programmable logic controller (PLC), is set for some major changes.
The technology was originally intended to replace inflexible relays – imagine a huge, 50 ft. long cabinet full of relays and wires that worked together to control a machine. Any problem or design change required the whole system to be worked out on paper before shutting down the machine, adding relays, moving wires, debugging and starting over. In 1968 GM engineers set out the design criteria for a ‘standard machine controller’. Bedford Associates won the contract and developed their new MODICON technology. In 1973 the 184 model was released and MODICON (MOdular DIgital CONtrol) became the early leader in programmable controllers.
Early PLCs were designed to support three main factors: programmability, real-time response and reliability. Well-suited to these original applications, the PLC is now hitting its limitations and requires a rethink. The key standard defining PLC programming – IEC 61131 – was designed to reinforce real-time behaviour and reliability, rather than the software flexibility that we’ve become accustomed to.
PLCs were designed as standalone devices reacting to local inputs and outputs. Internal control algorithms reacted to changes driving logic-level and analogue outputs which, in turn, controlled external actuators. The IEC 61131 standard revolved around implementing the software configuration – program and data – to control the real-time operation of the PLC. Program and data configuration was for each individual, standalone PLC.
As technology developed, PLCs were increasingly networked with the use of Fieldbus (IEC 61158), but the core architecture of IEC 61131 treated every PLC within the network as logically independent with its own, individual configuration. Programs consisted of interconnected function blocks which could each be written in IEC-approved languages. Tasks triggered these functions, and each task was configured to execute in a set way – in a loop, continuously, triggered by a clock or triggered by inputs – supporting periodic behaviour.
An architecture like this does provide predictable results with a low probability of failure, but on the flipside, it becomes increasingly cumbersome when faced with developments in industrial automation that require much greater flexibility. The IIoT requires control systems that can co-operate on a much deeper level. Not only do individual PLCs need to co-operate with each other, they also need to work much more closely with other systems within the factory and externally, to the cloud for example.
Today we tend to make greater use of distributed control at the machine level. So, instead of using one PLC to control operation of a machine integrating robotic manipulators and multiple actuators, real-time control is distributed by the architecture to the individual subsystems. Allowing each of the PLCs in the network to react to each other – or externally generated events such as last-minute changes to customer orders – improves response times as well as overall operational efficiency.
Networking and distributed control are of utmost importance to IIoT-capable PLCs, and more powerful processors are required. Key requirements are:
- Execution performance
- Ability to deal with security-enhanced protocols such as Transaction-Layer Security (TLS)
- Sufficient memory capacity to handle internet-protocol (IP) stacks
Digital signal processing
A 32-bit processor based on ARM or a similar architecture can provide the core computing power for the PLC. The use of more advanced control algorithms – such as Kalman filtering – is supported with the addition of digital signal processing (DSP) instructions, commonplace in today’s motor driven systems. Transitioning to a full DSP architecture, however, is not necessarily required. Some processors add DSP instructions. For example, the ARM Cortex-M4 which adds DSP instructions to general purpose ARM architecture, or the Blackfin architecture from Analog Devices which provides high-performance DSP, augmenting functions with the general-purpose instructions associated with MCU architectures.
It’s becoming an increasingly common approach to combine a general-purpose processor core for networking with another processor that brings some elements of DSP to handle the real-time control tasks. For higher performance requirements, this architecture allows one processor to handle networking, management, supervisory and high-level processing tasks, while the other (or others) look after real-time I/O, in addition to interrupt handling.
Some devices add dedicated network processing hardware which offloads packet-handling tasks from the main 32-bit processor – the Renesas RX600 for example. EtherCAT and other network technologies support advanced deterministic networking, which, in turn, supports real-time distributed control algorithms. Look for devices that support the networking technology you need, for example, Infineon Technologies’ XMC400 series of MCUs come with built-in EtherCAT support.
Greater degrees of fault tolerance can be implemented with redundant cores than was possible with traditional PLC architectures. For example, the AURIX family of multicore MCUs from Infineon deploys 3 processor cores, allowing 2 to operate in lockstep. Discrepancies between the results detect random errors in execution, and, once detected can be checked and repeated, or the system halted for technical checks before products are damaged or safety compromised.
Because they no longer operate in isolation and are generally connected to the cloud via a network connection, security of PLC functions is essential to resilient operation. PLCs must be authenticated before being allowed to join the distributed control system and, in turn, the PLC needs to authenticate the network itself. Transactions affecting operation need to be encrypted as well as authenticated to prevent interception, and even modification, by hackers. A hardware root of trust embedded into the core PLC hardware is a key requirement, either in the core MCU or provided using specialist cryptoprocessors and secure memory devices. Essentially, these will not allow the start-up process to be completed without ensuring that the boot image has not been compromised and all devices attached to the PLC are valid.
Integrated security and dual-core embedded processing are brought together with the PSoC 6 from Cypress Semiconductor. High-level processing in the form of an ARM Cortex-M4 is combined with an M0+ providing rapid response to I/O events. Hackers are prevented from gaining access to sensitive firmware with its trusted execution environment securing access to local data storage.
In addition to the ongoing trend towards distributed control is that of continuing miniaturisation. This must be balanced to allow for ease of installation and maintenance once installed. For the medium-term, at least, I/O connections are expected to continue as conventional screw terminal blocks for the most part. The ability to configure I/O connections is vital too. It is possible to build a single PLC board supporting common analogue and digital I/O port configurations but makes more sense to continue with a modular I/O architecture using daughter boards plugged into a backplane or mounted directly onto the PLC motherboard.
Downtime can be reduced by providing support for warm or hot-plug changeovers, allowing the PLC to suspend normal operation while remaining in control of other subsystems during the changeover of one of the I/O cards. If this functionality is required, the connector design needs to provide easy mating of the daughter boards with high pin density and retention scheme so that the cards cannot become accidentally separated.
Hot-plug interface devices should be used to prevent damage to the electronics during changeovers. In addition, isolation on the daughter-boards needs to be in place to provide additional protection against overcurrent and overvoltage situations – an ever-present threat in the industrial environment – while the system is running. Traditionally optocouplers have been used to provide isolation, however, newer technologies based on transformers or similar high-voltage electrical barriers are available that enable compact isolation between signal-conditioning electronics and external I/O and the more delicate logic functions and ADCs inside the PLC’s core. Examples of compact and reliable alternatives to optocouplers include Analog Devices’ iCoupler technology and the Si8xxx series of digital isolators from Silicon Labs.
High-speed interfaces such as USB enable PLC configuration and peripheral expansion, however, commercial connector designs may not be able to withstand the rigours of the industrial environment. Specialised connectors can be used to provide ruggedness and additional features for harsh, industrial environments, such as IP67 seals. Examples include the ix range of rugged industrial connectors – for attachment to Ethernet infrastructure – from HARTING or Amphenol’s MUSBR industrial-grade USB-C connectors.
Last but certainly not least, we must consider the power supply itself. As we continually drive to reduce size many of today’s advanced PLCs must rely on convection rather than using bulky fans for cooling. Efficiencies in excess of 90% can now be achieved with high-integration DC/DC convertors. Multiphase operation means that high efficiencies across a wide range of load conditions can be supported. This, in turn, allows the PLC to drive high output currents when it needs to in order to operate machinery but means it can also move into energy-saving quiescent modes easily.
All in all, the technologies, subsystems and components are readily available to make the changes required to PLC architecture that will bring the increased robustness, resilience and cost-effectiveness to industrial automation required to accommodate today’s Industrial Internet of Things.
Are you designing smarter for IIoT? Find out more about what’s inside a PLC at RS online today.