HMI: A review of design fundamentals

By Control Engineering Staff March 9, 2006

One key to greater cost and efficiency savings in industrial processes is the human machine interface (HMI). Whether it is operating in tough industrial conditions or a sterile room, a well-designed HMI contributes to an improved working environment, better compatibility, and greater reliability. Here are some points to consider when designing and installing an HMI. Better design means better productivity. Good design means making the interface as user friendly as possible. Operator terminals should be compact, flexible, and easy to program, using intuitive software that allows application developers to write, test, and verify applications without having to have the terminals connected. On the hardware side, terminals should have a timeless design, take up a minimum amount of space, be mountable vertically and horizontally, and have some form of clip-on design for quick, easy, reliable installation without the need to drill holes.

Productivity is greatly improved when ergonomics and design are optimized. Be aware of the environment in which the operator terminals will be used. Ask what display sizes are available and how easily existing hardware can be replaced or improved.

Modern displays make reading and inputting information much easier. High-resolution TFT displays allow use of complex color tone differentiation. Other environments require dimmable cold cathode fluorescent lighting (CCFL) backlights, which are suitable for infrastructure and offshore applications. Consider whether your application requires burnout detection and if it needs to be controlled by a PLC.

Backward compatibility with existing products is important, in terms of hardware cutout size, existing drivers, cables, and accessories, as well as ability to use the same programming software and any current applications.

Microsoft CE .Net and Intel XScale offer power and reliability. Consider the amount of processing power your applications will require. Using a high-performance 416 MHz RISC CPU (Intel Xscale technology) with the integral 32MB of Intel’s StrataFlash and 64 MB RAM provide enough processing power and memory for just about any HMI application, from traditional industrial use to applications that require Internet connectivity.

Make sure that any integral compact flash slots can accommodate standard cards, should memory need to be expanded in the future. Compact flash memory cards can also be used for recipe storage, historical trending, and copying HMI project files directly into the terminal, thereby minimizing program transfer downtime.

CE .Net is becoming the de facto standard for HMI devices. It ensures more open system design, greater security, and system reliability. Using an open operating system helps ensure that future developments and connections can be easily incorporated into the HMI.

Don’t restrict communication. Minimize limitations on communication by ensuring there is a USB host port for connection to all PC peripherals. A built-in 10/100Mbit twisted pair Fast Ethernet connection is a must for remote operation. Think about integrating an expansion port that allows the terminal to use dedicated expansion modules, for example, to enable fieldbus communication.

Can the terminal be connected without special configuration to two different devices (such as PLCs or inverters from different manufacturers) at the same time? This capability not only saves money by allowing one HMI to service two pieces of control equipment, but also allows the two devices to exchange data.

When needed and appropriate, HMIs should be able to act as Web servers. By having their own Web addresses, they can store HTML pages that contain real-time data from terminals and associated control systems. This capability allows SMTP (simple mail transfer protocol) functionality so HMIs can send alarms and text reports as e-mails. Including FTP functionality means data, such as trend information and recipes, also can be accessed by the terminals.

Consider terminal reflection for remote management. Modern Internet-enabled HMIs let users connect over the Internet and view an exact copy (using an embedded applet sent to the user) of what the operator sees. This graphical copy of the HMI, or real-time terminal reflection, shows a picture of the HMI and real-time maneuverable objects on the HMI’s display. If the local operator changes anything on the local HMI (such as a value or screen), the remote reflection automatically updates to show the new data. These features are needed to run remote maintenance applications and allow true remote management.

Good HMI design helps employees and companies do better jobs. It should include flexibility for differing industrial environments, be user-friendly, and be easily integrated into existing product lines. Crucial characteristics are safe backup, even during power failures, an open operating system, remote use when needed, and unlimited communication to other devices.

Information for this article was provided by Beijer Electronics .

—Edited by Jeanine Katzel, senior editor, Control Engineering,