By Linda Htay, IDEC Corporation
When it comes to industrial automation applications, there has long been a degree of separation between “control” and “visualization” functions. Programmable logic controllers (PLCs) and human-machine interfaces (HMIs) have evolved technologically over decades to perform distinct but closely related operational roles. A closer look reveals why this has been the case historically, and it can also show how uniting both PLC control and HMI visualization roles into an all-in-one PLC and HMI (PLC+HMI) device can be the best solution for certain industrial automation applications (Figure 1).
A combined PLC+HMI provides essential visualization, I/O, controller, and communications connectivity in an ultra-compact industrial form factor, making it a leading choice for many applications across diverse industries.
Control and visualization differences
As digital controllers first superseded hardwired automation decades ago, they were architected with programming and execution concepts designed to ensure reliable monitoring and operation of critical devices on machinery and equipment. For industrial applications, the PLC was particularly adapted to work in harsh environments, interact with all the common signaling voltages and types, and provide the required rigorous programming instructions. Sometimes, these instructions mimicked physical elements, such as electrical contacts and drum sequencers, but at other times, they advanced into more esoteric areas of math and logic.
Original operator interface methods for PLCs were primarily oriented around panel-mount devices like buttons, switches, lights, and indicators. But as digital displays and PCs developed, they were quickly adapted into more capable graphical HMIs. The focus of HMI configuration was on accessing PLC data and displaying it in a useful manner for operators, while also empowering them to interact with the data via virtual pushbuttons, data entry fields, and other means.
Both PLCs and HMIs perform important industrial automation roles, even though the core functionality of each is very distinct from the other. Close interaction between PLCs and HMIs is essential, yet developers often must pursue two parallel development paths before eventually integrating the two different work products. In some applications, the additional physical space demanded by separate PLCs and HMIs becomes a concern, as well as integration issues, cost, and complexity. For these and other reasons, and depending on the application, designers should consider an all-in-one PLC+HMI device to best meet their industrial automation needs.
Automation can be better together
Combined PLC+HMI devices have been available for many years and usually fit a targeted use case of smaller input/output (I/O) count applications with significant physical space constraints. Certain devices have been based on PLCs with some visualization capabilities added, while other examples are created from HMIs with selected control functionality incorporated.
The idea of selecting a combined PLC+HMI device is attractive to designers for many applications, but few implementers will be willing to sacrifice the control, visualization, and overall performance they have come to expect with individual devices. New solutions address this issue, and here are some of the key considerations for evaluating a combined PLC+HMI.
Form factor
A lack of installation space is one of the main driving factors for moving to a combined PLC+HMI automation solution (Figure 2). This concern becomes increasingly important as equipment footprint shrinks, machines are more highly instrumented, and automated functionality increases.
An all-in-one PLC+HMI device provides significant space savings and simplifies installation while providing suitable control and visualization capabilities for many applications.
PLC control performance
PLCs demand real-time I/O scan performance, while HMIs require computational processing adapted for graphics presentation. Reconciling these two different purposes in a single central processing unit (CPU) is possible, but a better approach for PLC+HMI units is incorporating two CPUs, with one dedicated to each role. The two CPUs can be designed to communicate natively with each other so that users will experience the best computing performance and streamlined connectivity.
Users will also expect a PLC+HMI to feature the full PLC instruction set, including PID analog loop control, without any limitations that would negatively impact implementation. Because PLC+HMI is commonly used for small to medium applications, best practice is for the device to incorporate a reasonable number of built-in I/O. The I/O types should be a selection of discrete and analog inputs and outputs suitable for the most common 24VDC, 0-10VDC, and 4-20mA signaling voltages or currents. High-speed inputs and RTD inputs provide even greater design flexibility. Relay output versions can provide a higher current-carrying option, while transistor output versions can operate at higher speeds and even work as pulse train outputs for motor drives. For slightly larger applications, the ability of a PLC+HMI to use expansion modules to increase the I/O count is a plus.
Leading-edge HMI design
An HMI display touchscreen can represent a vulnerable weak point because it is exposed to the environment and user interaction. For this reason, only industrial-grade hardware will serve; designers should look for devices rated for operation in extreme temperatures and washdown environments. However, older technologies have used membrane pads or infrared light, making them susceptible to damage and false operation due to dirt or water drops.
More modern HMIs are now incorporating projected capacitive touch panel (PCAP) advanced technology, which is similar to what is used for smartphones and tablets (Figure 3). The tech has been toughened to resist water and scratches while maintaining multi-touch high performance, and resisting false signals due to dirt and water droplets on the screen face. Users can even interact with a PCAP display while wearing thin gloves.
Since PCAP has a glass surface and uses fewer layers than older tech, it has much-improved brightness and clarity. PCAP also works well with slim bezels, resulting in the maximum possible display area for a given installation footprint and a very shallow mounting depth clearance requirement.
The IDEC FT2J PLC+HMI incorporates PCAP touchscreen display technology similar to smartphones and tablets, but toughened for industrial use.
Advanced connectivity, simplified software
As the intelligence level increases for automated machines, users need ever more connectivity options for the control and visualization platforms, with Ethernet the de facto standard for today’s equipment. Industrial protocols such as Modbus TCP can be used to access many common types of field devices, while IT-type functionality such as the FTP protocol, SMTPS secure emailing, custom web page hosting, and even X, formerly known as Twitter, integration can provide many ways for the automation system to interact with host systems and remote users.
To ensure easy implementation, users should investigate the programming/configuration development environment. A single integrated development environment harmonizes the PLC programming and HMI configuration effort, streamlining developer work. Users should also look for ease-of-use features such as an intuitive drag-and-drop interface, built-in symbol libraries, and unlimited no-cost updates for the life of the product.
PLC+HMI as the adaptable answer
Individual PLCs and HMIs have a long history of excelling throughout all types of industrial applications. While prudent designers may have fair concerns about whether a combined PLC+HMI might introduce some types of compromises, the fact is that modern hardware architectures, the latest PCAP touch display technology, and well-designed software have overcome many of these challenges.
IDEC
us.idec.com
Filed Under: Displays • HMIs • operator interfaces • monitors
