HMI

Environmental conditions for the target application must be evaluated first because the HMI must be able to survive physically at a location to even be considered a viable choice . Most of these choices are intuitive, but some are not.

HMIs located in environments where they are subject to falling water or directed washdown will need compatible ratings in NEMA 250 enclosures for electrical equipment standard or UL 50 listings. This is critical when cleaning chemicals, process chemicals or other airborne contaminants could impact the HMI. A site pollution degree or material safety data sheet may be available to help define expected environmental conditions. Customer specifications, along with local installation codes or preferences, may drive the need for certain ratings or listings.

Another key concern is whether the HMI will be located indoors or outdoors. NEMA and UL ratings commonly are constrained as “indoor use only.” Even the brightest HMIs struggle with sunlight glare outdoors and direct sunlight can cause heat issues. Users should consider locating outdoor HMIs into more protected areas or outfitting them with sunshields.

HMIs typically contain sensitive display elements and backlights and are likely to have more restrictive allowable operating temperature ranges than other automation components. The display brightness (usually expressed in nits) and the average backlight lifetime should be evaluated carefully. Brightness is a key indicator of how easy it will be to view the display while the always-on backlight life will indicate how long the unit is likely to last.

Choosing the right display size is often a balancing act. Most operators would prefer the largest display size possible, especially if they need to select touch links, but also if the application is view-only and if they want to observe the display from a distance. Bigger displays provide more real estate for programmers to develop graphics. HMI hardware product lines with a range of sizes can help users learn one system that is applicable to many different models.

Of course larger HMIs cost more and can chew up precious installation space for control panels. The display needs to be installed at a working height suitable for most employees, often considered to be around 66 inches from finished floor to the middle of the display.

Most HMIs offer many colors, but in addition to the display physical size, users should make sure to compare the resolution in pixels, which determines how detailed the graphics can be.

For some applications—particularly involving data entry—users would want to use an on-board physical keypad or an external keyboard and mouse. However, most designers are following the lead of mobile devices and looking for touchscreens.

It is important to note many consumer touch devices tend to use capacitive multi-touch. This technology usually won't sense gloved fingers and can be affected by water, but it does have the capability to interpret multiple concurrent touches or gestures. On the other hand, industrial HMIs often employ resistive single-touch technology. Resistive touchscreens can be engaged with gloved fingers and only respond properly to one touch at a time, which often is preferred for industrial applications.

In addition to physical size, HMI offerings often are grouped with regards to memory and processor performance. They are rated for communicating a certain number of software tags and average displays. Careful designers will storyboard the anticipated graphics and do some rough estimates of tag counts to ensure they are within limits.

HMIs with expandable memory or multiple memory sizes may help to remove constraints. It's also important to remember advanced features such as on-board calculations and trending and recipes will impact HMI performance, so it's best to use conservative estimates.

It has become common for HMIs to support one or more SD Card slots so users can install their own removable storage for data logging. This feature is a must for any historizing and trending data applications.

Connectivity is also a key factor for choosing an HMI. Most commonly, users will want an Ethernet port so the HMI can communicate on the programmable logic controller (PLC) network. However, in some applications, it may still be desirable to use serial RS232C or RS422/485 connections. Serial connectors are most commonly 15-pin D-sub type, but they also can be RJ-12 or even terminal block style. It is imperative the PLC connection requirements be defined prior to finalizing an HMI selection.

USB ports come in two versions: type A and type B. Type A ports (like on PCs) let users add accessories like a keyboard, mouse, memory stick, barcode scanner or other compatible interface devices. Type B (often found on printers) allows users to connect an upload/download programming cable from the PC directly to the HMI without impacting the Ethernet network.

Another feature that is becoming a must-have is an HDMI output port. An HDMI connection can drive a second off-board monitor such as a large-format TV located in the plant or facility. An operator can work locally on the HMI display while other operators can see the larger display from a distance.