A human machine interface — or HMI for short — is a control terminal on machinery to let operators monitor machine functions and adjust them as needed. This interaction is through a graphical user interface — GUI for short —that allows communications to and from supervisory controls and machine HMIs.

HMI hardware takes the form of compact controllers with embedded functions, usually in the form of ruggedized touchscreens — for example, LCDs with tempered glass and cast-aluminum frames or even sealed enclosures for outdoor applications or indoor plant applications with a lot of oil, dirt, and machining byproducts. Hardware standardization is increasingly common for HMIs on open-source and proprietary setups alike.


In addition, HMI hardware options can make the displays satisfy rather specific application requirements. Depending on the application’s complexity, myriad I/O options exist—including digital and analog.

Today’s HMI technologies include increasingly flexible setups and (much like in consumer electronics) ever-increasing resolution. There’s also been an uptick in affordable touchscreens and steady replacement of resistive displays with capacitive. In fact, these capacitive displays are excel medical and food-and-beverage applications needing bezel-free designs, as they’re sleeker and allow sterilization and easier cleaning. Solid-glass capacitive touchscreens also last longer than HMI hardware based on resistive technologies, because the screens don’t use pressure points to form circuits, so don’t wear or lose sensitivity over time. What’s more, many HMIs with capacitive displays have the multitouch capabilities of smartphones, which lets OEMs leverage user familiarity to offer intuitive interfaces.

In fact, innovation on the front of smartphones and tablets means ever-improving wireless communications, displays and portable processing power to let users connect to HMIs for industrial applications with tablets and smartphones for real controls.

Communication protocols range from simple RS-232 links to more advanced protocols such as CANOpen, SERCOS, and Ethernet-based communications. Depending on the HMI setup, multi-network communications usually go through ControlNet and DeviceNet. ControlNet is a control-level network for high-speed transmission of time-critical messaging data and I/O data. In contrast, DeviceNet handles industrial devices like drives, limit switches, motor starters, operator displays, photoelectric cells and valve manifolds—as well as PCs and PLCs that may connect to the machine. Designs that use both communications facilitate HMI data management between machines and operators.

More sophisticated HMIs also accept input from mobile devices and platforms such as Microsoft Windows CE—a scalable version of Windows for handheld devices. Such HMIs can often save designers money, as the operating systems are distributed on machine-level embedded HMIs, solid-state open HMI machines and distributed HMI servers, as well as relatively low-cost portable devices acting as HMI interfaces.

Just a quick note about HMI software: Programmers write HMI software for either machine-level HMI or supervisory-level HMI, with applications suitable for both types. Such software has high upfront cost, but is inexpensive long-term thanks to the way it reduces redundancies. Even for lower-tech applications (in which most machine interaction is via switches and pushbuttons) entry-level HMI offerings are making inroads — as they often reduce interface-part count and simplify controls. More sophisticated applications benefit in a different way: Pharmaceutical and medical machinery use the latest HMI features to differentiate from competitive offerings.

No matter the design, selecting HMI software starts with an analysis of product specifications and features. What kind of GUI will operators need? Will plant personnel need to see diagrams and digital photos and detailed system schematics — or are the requirements simpler? Other considerations include performance, system architecture, upfront and operating costs, integration, and the nature of the environment in which the HMI will operate.