Human-machine interface (HMI) software is often driven by the hardware selected, such as operator interface terminal (OIT), embedded PC or PC-based. Often the hardware selection simplifies the HMI software development. Not until the decision to go with PC-based HMI do the HMI software possibilities open up.
The definition of an HMI is wide-ranging. Certainly anything with a graphical display that an operator can interact with can be called an HMI. Even a one-line alphanumeric display can be an HMI and can display operator-selected information, such as fault messages or production counts. In many cases, when the HMI is specified, the HMI software is chosen by default—it comes with the hardware—although many vendors require the development software be purchased separately. HMI software is often application-driven, and there is a lot of overlap in the choices available.
The basic OIT
For small applications, focus on the HMI hardware, functionality and cost of the HMI. Specify the HMI requirements and select the hardware; the software will come. Some important requirements for the small displays are screen size; color vs. black and white; and resolution. If just a few variables, counts and set points need to be monitored and adjusted and a couple buttons are needed for minor control functions, size your HMI screen size appropriately. It doesn't pay to get a larger display or to spend time on fancy graphics.
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Although among the lower-end HMIs, OITs may not connect to many controllers due to limited protocol availability and may have a limited feature set; the simple HMI software may still provide a built-in Web server enabling remote access to some of the data and may even have data logging, but limited memory to do so. While the features may be as limited as the screen size, the displays are easy to get connected and simple to program working well on may small machines.
Middle of the road
As the machine-application requirements move to more graphical screens, buttons, alarm history, recipe functions, trends and historian functionality, such as commonly offered by the PLC vendors, HMI software functionality increases. These middle-of-the-road HMIs, usually embedded PCs, still closely tie the HMI hardware and software together. These HMIs tend to use Windows Embedded, Window CE or similar operating systems, which open up more hardware options, such as solid-state drives, hard drives, USB ports and more communication options.
The HMI software for the embedded HMI is often purchased from the PLC vendor due to the close hardware ties. The software is usually just for development, such as configuring the communication protocol and tags and creating graphic screens. The HMI software also includes a download tool to convert the software into a runtime for the PLC vendor’s HMI hardware. Typically the software is configured and written on a development PC and then downloaded to the HMI with each HMI including the runtime.
At this level and as HMI features and functionality increase, not only is the PLC and HMI hardware closely related and from the same manufacturer, the software for the HMI uses the same development platform as the PLC programming software. It's the same software package—programming of PLC and HMI is merging. What for decades has been separate software is now the same. Sharing of tags or variables has never been easier.
As the HMI software moves to PC-based HMI, both the HMI hardware vendor's and HMI software-only vendor's capabilities overlap. The PLC/HMI software vendors software can run on embedded platforms or PC-based, as can that of the HMI software-only vendors. At the same time, the HMI software-only vendors have hundreds of communication protocols to tie all the hardware together. It's becoming easy to get connected.
Although it is happening at all levels from the basic OIT on up, it's at the PC-based HMI software level where edge devices and controllers are being tied to the Internet of Things. This is the HMI/SCADA level where the software can take advantage of the resources in a PC, such as more memory and storage capability, significant connectivity and strong graphics capabilities. If you need more, just upgrade, add to the hardware or add PCs and runtime license.
For the historian, database connections, reports and remote access, the PC-based HMI software provides IoT and cloud connectivity, which enables analytics with big data. The HMI can even reside in the cloud along with the data. The HMI software is not just about monitoring things such as machine status, graphical displays, alarms, variables and mode control on a machine or system. It's also about using the HMI and the data to make decisions.
While the HMI software can scale up to monitor and control large machines and processes, it is also shrinking back down to edge devices controlling small machines and process skids. In this case, the HMI software is mated to the machine-control software on the same mini-PC no bigger than a deck of cards. While the HMI software is installed, there is no HMI hardware, but the included Wi-Fi connects the controller to the HMI in your pocket—the smartphone.
In the end, the HMI software is tied closely to the HMI hardware and application. It also plays a strong role in the user experience when operating the equipment. If the HMI software makes the operator and occasional engineer who works on the machine happy, it's the correct HMI software and written properly.
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