PLCs, PACs

Evaluate controller options for industrial applications

Choosing the best industrial controller requires evaluation of automation, communication and security needs.
By Benson Hougland July 30, 2019
Courtesy: Opto 22

Delivering reliable real-time control functionality has traditionally been a goal for those tasked with automating machinery, equipment and processes. If designers and engineers could pick a digital control platform, integrate it with field devices, program it to run as expected and deliver reliable system operation for years or decades, it was a job well done.

Today’s situation is better, but more involved. It’s better because many new control technology options are available; it’s more involved because users want optimized operational performance, improved visibility and superior connectivity to supervisory systems. Basic isn’t always enough anymore as end users demand more advanced features. In response, designers and original equipment manufacturers (OEMs) must evaluate the field of controller options to provide the right mix of characteristics for each automation application.

This effort begins with making fundamental decisions regarding whether controllers will physically and electrically work where they need to operate. Added to these requirements are more advanced criteria: programming options, networking connections, security, communications protocols, flexibility and scalability. This article compares several popular controller technologies based on these factors, helping designers focus on what is important to their applications.

Who are the players?

When it comes to automating machines and processes, industry has benefited from continual progress in digital control platforms. This technical field is often referred to as operations technology (OT). Any OT control platform encompasses controllers and associated products like input/output (I/O) modules, operator interface terminals (OITs), human-machine interfaces (HMIs), instrumentation and other devices (see Figure 1).

Figure 1: Industrial controllers such as this traditional PLC/PAC, are typically installed in control panels local to the equipment or processes they monitor and automate. Courtesy: Opto 22

Figure 1: Industrial controllers such as this traditional PLC/PAC, are typically installed in control panels local to the equipment or processes they monitor and automate. Courtesy: Opto 22

For all automation systems, the controller is the brain and therefore a central component that must be selected early, since it sets the detailed design path. While the product spectrum presents overlapping capabilities, here are the most popular controller categories for OT projects:

  • Programmable logic controllers (PLCs): A rugged and basic but capable control option, especially for discrete control applications
  • Programmable automation controllers (PACs): Similar to PLCs, but generally more advanced in the areas of communications, data handling and process control applications
  • Industrial personal computers (IPCs): Ruggedized full-fledged computers, requiring significant user effort to integrate hardware, software and remote input/output (I/O) for control
  • Edge programmable industrial controllers (edge controllers): Built in a PLC/PAC form factor, yet incorporating advanced programming and security options, native visualization and extensive communication abilities, much like a PC.

When choosing a controller for an application, there’s often more than one right answer. Choosing the correct controller requires evaluating several characteristics and perhaps considering multiple applications.

Figure 2: Industrial automation controllers must survive in field locations, directly control equipment, interact with other smart devices and communicate well with supervisory systems. Courtesy: Opto 22

Figure 2: Industrial automation controllers must survive in field locations, directly control equipment, interact with other smart devices and communicate well with supervisory systems. Courtesy: Opto 22

Strengths and weaknesses

Controllers play a central role within any automation project (see Figure 2). They may:

  • Monitor and command I/O points
  • Interact with other peer controllers
  • Connect with intelligent field devices
  • Interface with OIT and HMI visualization systems
  • Communicate with supervisory and enterprise level systems.

Every application is unique, so it is helpful to review a list of potentially important criteria to see which are relevant and what controller type best fits them. Table 1 presents a list of controller selection criteria, organized by topics such as form factor, programmability, security, etc. The suggested weightings are broad assessments and can vary for specific use cases.

Selecting the right controller for any application requires weighing many criteria. Courtesy: Opto 22

Selecting the right controller for any application requires weighing many criteria. Courtesy: Opto 22

PLCs and PACs, the classic choice

Both PLCs and PACs are built and optimized for the OT role and represent the classic choice. Certainly, PLCs and the more full-featured PACs have served as the principal control products for most machines and many processes. They are fast, reliable and affordable – and an exact fit for tough environments. Other complementary products like OITs and HMIs are developed in parallel to add visualization and other features to these platforms.

However, many PLCs and PACs were burdened with proprietary connectivity, specialized software and licensing costs. From a field device and I/O networking standpoint, this situation has improved greatly due to Ethernet and standardized industrial protocols. Also, software is becoming more open as PLCs/PACs are adopting IEC 61131-3 programming languages and cross-platform programming environments like CODESYS.

However, the legacy aspect of PLCs/PACs can act as a kind of inertia, resisting adoption of the latest technologies. This inertia is most apparent in generally weak support for advanced networking and security features. PLCs/PACs offer good connectivity at the OT level, but they tend to lack support for IT-centric programming languages and protocols such as HTTPS and messaging queuing telemetry transport (MQTT). For applications requiring built-in connectivity up to IT-centric enterprise applications and historians, users typically need to integrate extra hardware and software to bridge the gap.

Security is a similar story. Most modern aspects of cybersecurity did not exist when the first PLCs/PACs where conceived. Features commonly included with PCs and edge controllers, such as secure user accounts, encrypted communications and virtual private networks (VPNs), must be bolted onto PLCs/PACs, requiring significant end user expense, effort and risk. A better alternative is to make sure any required security features are native to the control platform.

IPCs for power users

As PC technology became readily available and affordable, many users looked to PCs for industrial control. The commercial roots and economies of scale for PCs ensured the hardware and operating system platforms were well understood, and a PC solution had plenty of computing power and display options.

However, users soon found out commercial PCs were not robust enough for many industrial controller applications. That was remedied when vendors offered ruggedized PCs, or IPCs, so they could operate reliably in machine environments. Therefore, some users could turn to IPCs as an all-in-one control platform for automation, visualization and communication.

IPCs also offer users a way to perform more advanced programming or data manipulation options, and they are better positioned than PLCs/PACs for IT-centric communications and many IoT interfaces.

However, IPC-based control is very much a do-it-yourself endeavor. The end user must select a bare-bones IPC and assemble a suite of other hardware and software products for control, visualization, communications protocols and remote input/output (I/O) solutions, hopefully resulting in a cohesive package. The result could be an ideal fit, but it also could become a custom system presenting support challenges, which can be difficult to manage over time.

Although the IPC’s flexibility makes it attractive for the role of an industrial controller, there are many hurdles to creating and maintaining a complete system.

Edge controllers hit the sweet spot

Edge controller devices are a recent development on the industrial automation front. This newer generation of controllers was designed to take advantage of the latest IT communication and IoT developments, while maintaining the OT heritage that has made PLCs/PACs so popular. For many applications, this hybrid combination of technologies for meeting multiple requirements make an edge controller an excellent fit.

Edge controllers provide all the benefits of OT solutions like PLCs and PACs because they are robust, rated for extreme temperatures and offer a wide range of integrated I/O. Some edge controllers can be programmed with a choice of flowcharts or IEC 61131 languages (see Figure 3). An edge controller can act as a direct replacement in an existing PLC/PAC application or become the foundation for a new project.

Figure 3: Edge programmable industrial controllers include traditional OT-centric controller functionality, while natively incorporating IT-centric networking and mobile technologies. Courtesy: Opto 22

Figure 3: Edge programmable industrial controllers include traditional OT-centric controller functionality, while natively incorporating IT-centric networking and mobile technologies. Courtesy: Opto 22

An edge controller’s focus on OT-friendliness extends to other possible benefits. For example, one available edge controller houses an on-board touchscreen display that can act as a local HMI. It can also be used for system configuration and diagnostics, freeing support personnel from the need to rely on other devices. Built-in networking, USB and HDMI ports make it easy for users to interface with an edge controller. Under the hood, it runs embedded Linux, and other edge controllers use this or a similar embedded operating system. Although users can interact with it like they would a PLC/PAC, more advanced capabilities are possible. The PC pedigree of an edge controller enables it to use extended programming languages like C++, Java and Python – all concurrently – giving users programming flexibility.

Edge controllers can run locally in a standalone mode for any control application. Added value is realized when they connect to edge-located sensors and devices, filter and process Internet of Things (IoT) data and communicate with enterprise systems located on-premises or in the cloud. This is especially the case if they include protocols for device-originated communications, and for configuring drag-and-drop data connectivity. An edge controller with these capabilities removes the need for interposing layers of hardware or software (see Figure 4).

Figure 4: Native IT and OT communication capabilities enable edge controllers to flatten automation architectures, avoiding complex interposing layers of hardware and software. Courtesy: Opto 22

Figure 4: Native IT and OT communication capabilities enable edge controllers to flatten automation architectures, avoiding complex interposing layers of hardware and software. Courtesy: Opto 22

Unlike PLCs/PACs, security is built into edge controllers, especially those offering a pair of segmented, non-routable Ethernet ports – one for a trusted network, like the OT field side, and one for an untrusted network, like those with direct internet connections. User security accounts are handled right at the controller level, data communications are encrypted and a built-in VPN is included. All these provisions help make control systems mobile-ready and cyber secure.

An edge controller can provide a flexible OT control platform like a PLC/PAC for industrial automation applications, but with the added IT advantages of an IPC. As a comprehensive all-in-one solution with on-board visualization and secure connectivity, an edge controller is often the most cost-effective controller choice.

Control for today and the future

More choices are better for end users. For automation and control platforms, end users and OEMs have enjoyed reliable options over the years in the form of PLCs, PACs and IPCs. However, modern edge controllers present an attractive option because they combine the best aspects of the other three leading platforms.

Benson Hougland has 30 years of experience in IT and industrial automation. He drives strategy for Opto 22 products connecting the real world to computer networks. He speaks at trade shows and conferences, including IBM Think, ARC Forum and ISA. His 2014 TEDx Talk introduces non-technical people to the IoT.

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering.

– See other articles from the supplement below.


Benson Hougland
Author Bio: Benson Hougland is vice president of marketing, Opto 22.