Human-machine interface: Six factors help select the right HMI

The human machine interface (HMI) must be suitable for every class of user. How do you decide? The following considerations can help. See photos.


As the key component between a factory employee and an automated production line, the human machine interface (HMI) must be suitable for every class of user. A quick Google search reveals more than 250 HMI manufacturers from A (Arbonne) to Z (ZDAuto), and most offer multiple models. How do you decide? The following six factors can help with the decision-making.

1. What’s the operating environment?

A key factor to consider right away is the operating environment. Any unusual environmental conditions will narrow your HMI selection. For example, an abundance of electrical noise or vibration may require you to select a ruggedized or hardened industrial panel. An HMI operating in a washdown environment such as a food processing plant might require a high ingress protection (IP) rating. What about extreme temperatures or temperature ranges, such as an HMI placed near a blast furnace? If the HMI is to be used outside, will it require a daylight-readable screen? It’s better to plan ahead and consider a heavy-duty HMI than to go back later and replace a unit because it didn’t hold up.

Bosch Rexroth on choosing HMIs

The operating environment, such as

2. What’s the application and what else may the user want it to do?

This may sound obvious, but often it’s not discussed upfront. How many users know all their requirements at the beginning of a project? It may be wise to leave a little expansion room for additional memory or function keys, or for providing a color display even though the project may not explicitly call for it.

To help with HMI selection, know that while there are many different HMI applications, most fall into three broad categories:


The pushbutton replacer: Before the advent of the HMI and even today, many machines are/were controlled by pushbuttons and toggle switches. Process status and other data were displayed by indicator lights and BCD/LED displays. Many HMI applications today in essence replace hardwired inputs and outputs, with the HMI emulating buttons and switches via touchscreen keys or a keypad, and a screen that displays the status.

This type of application requires little computational power and can easily be handled by any manufacturer’s low-end “dummy” terminal. Keep in mind that the number of keys and statuses displayed should be considered when deciding the size of the screen. Some low-priced HMIs have a 3.5-in. or smaller screen, but the end-user may be frustrated using multiple screens to reach all the buttons and displays.

While many small HMIs with keys and two to four lines of displayed text can be used as a pushbutton replacer, they may lack beneficial features the customer might request down the road. For example, many smaller displays are black and white or monochromatic. The customer may decide later that it’s better to have some color– green for “GO” conditions or red to draw attention to alarm states, for example.


The data handler: Applications here might include functions such as recipes, data trending, data logging, and alarm handling/logging. In a flexible manufacturing line that runs multiple products, for example, a mid-level HMI will be required to handle this functionality, probably with a minimum of a 6 in. color screen to display all necessary data and graphs. Consideration should be given to the amount of data to be stored, and the size and number of recipes.

Data handling can be very memory intensive, so the amount of memory provided by an HMI will be an important factor in selection– at least 4 MB of RAM plus the ability to access a compact flash card. Ease of moving data in and out of the HMI may need to be considered. Will the recipes be stored onboard, or downloaded from a SCADA (supervisory control and data acquisition) PC? Will logged data be offloaded somewhere for analysis and storage? Is such data handling going to be automated or manual?

Answering yes to these questions will require an HMI with appropriate network connections and software tools—and lead to more questions. For example, is an Ethernet port capable of sending and receiving messages required, or is the use of a flash card mandated? In addition, some HMIs may only support data files in certain formats such as KSV, CSV or binary. The answers to these questions will depend on the other components in the system and company specifications.


The overseer: Does the application involve SCADA or maybe even MES (manufacturing execution system) functions such as data warehousing, database transactions, and interfacing with an enterprise-type system. With the current emphasis on real-time data analysis and cost reduction, the appropriate HMI could add significant value. An “overseer” application is usually going to require a full-fledged industrial PC that runs WinXP or an equivalent operating system. Consideration should be given to units with multiple Ethernet ports that run at a minimum of 100 MB, which is necessary for network segmentation to control network traffic speeds.

Another consideration here would be the desirability of a handheld or mobile HMI. A standard handheld HMI is connected to the machine via a cable and can be moved to a better vantage point to troubleshoot a problem. For large machines, a mobile HMI can be unplugged from one section and plugged in at another. Mobile HMIs can save on overall costs by reducing the number of HMIs needed to operate the equipment. Drawbacks to handheld HMIs usually involve cost, plus the potential for damage when moved around. Also note that reducing the number of HMIs will reduce the number of employees that can work on a machine at one time.


3. What will the HMI be connected to?

Choosing the appropriate HMI depends on other components in the system. Does the machine have a single PLC, several PLCs, or perhaps just a variable frequency drive that needs to be started/stopped and given a speed reference? Are all the products from one manufacturer or several? What are the connection methods—Ethernet, USB, or RS-485? Your HMI will need to support the connection methods required by the selected components.

Depending on the other components in the machine, having a PLC and HMI from one vendor can offer significant advantages. Technical support issues (in theory) are easier to resolve, because there is only one vendor to call, which removes the possibility of finger-pointing between two vendors. And for some systems, an HMI has increased capabilities when matched with a PLC from the same vendor. For example, a matched pair can allow for monitoring of special function modules and of ladder logic; a Rexroth HMI paired with a Rexroth controller simplifies addressing through the use of a common tag database.

Bosch Rexroth control panel

Users might not know all the requirements at the start of an HMI project, so it's wise to leave a little expansion room for additional function keys or a color monitor, even if the project doesn't explicitly call for it. Source: Bosch Rexroth

Many systems do mix products from several different vendors, however, and for these applications an HMI from a company that offers a large number of third-party drivers may be the preferred option. In this case, you need to consider a few key points:

  • Does the company Website provide self-service for manuals and drivers?

  • How about for HMI driver updates or new HMI drivers?

  • Check the manual for the desired driver. How easy it is to configure?

  • Is there information on how fast the driver executes?

  • What is the connection method—Ethernet, serial or a PLC add-on module?

Not all drivers are created equal, so compare such data with all vendors under consideration.

4. What data entry tool best meets your needs?

For most HMIs, you will need to choose one of two methods for using the HMI: touchscreen or keypad. With today’s technology, touchscreens are just as robust as keypad HMIs, for the most part. So the decision here is mostly a matter of preference of the user. However, there are a couple of situations where a keypad HMI may be the clear favorite. If a large amount of numeric data is to be entered, a keypad HMI could allow quicker entry than the virtual keyboard offered by most touchscreens. A keypad might also be easier to use in a work environment requiring the operator to wear gloves.


5. What are your programming software preferences?

The complexity of the application and the preferences of the programmer will dictate the software required. The three main categories of that HMI programming software can fall into are proprietary, hardware independent with prebuilt functions, and open software. Here are some pros and cons of each:


Proprietary software : Most pushbutton replacers and data handlers can be programmed by standard software provided by the HMI vendor. Bosch Rexroth, for example, has its VI-Composer. Proprietary software tends to be simple to use and allows for quicker development times. One drawback is that the program developed with proprietary software will only run on that specific hardware platform. If another HMI vendor is specified for a future project, the application will need to be completely redeveloped.


Hardware independent software with prebuilt functions: This is third-party HMI software that can run on different types of HMI hardware. A popular example is InduSoft Web Studio. Unlike proprietary software, an application created with this type of software can be run on any open platform. This gives the programmer greater freedom to select HMI hardware based on features, quality of service, etc. Independent software often has many common prebuilt functions, such as pushbuttons and recipes. However, because it has to accommodate a wide variety of platforms, it may be less user-friendly than proprietary software, and may not have as many specialized functions.


Open software: For power programmers who want complete freedom to develop their own functions, there’s always the option of programming with open software. Examples of this software include Microsoft .Net, and Visual C. Applications created with this type of software are hardware-independent, allowing the machine designer to select a best-in-class HMI with flexible programming. While the programmer may have to start from scratch to build functions, there are a number of online communities where advice can be offered or where code can be downloaded.

Disadvantages include high initial development time, including debugging and testing, and possible lack of support after the project is completed. Despite the power inherent in this method of development, many end users may lack the capability to support a project developed in this method. Projects created via open tools are going to require staff with more software engineering expertise. More time may be required to alter or troubleshoot such a program. These costs could offset the initial software savings realized from purchasing open software tools.


For the most part, touch screens are just as robust as keypad HMIs. Source: Bosch Rexroth

6. Other considerations

There are a few other factors to address, such as control options, scalability of programs, geographic location, and technical support.

Control options refer to the ability of some HMIs to also function as a PLC or controller. Used properly, this capability can reduce the complexity of a system. Similarly, if a vendor provides a “scalable solution,” then the program created for one HMI easily can be altered for another higher-end or lower-end HMI from that company’s product line. This allows a programmer to save time by reusing previous work and adding or dropping features as desired.

Another factor to consider is the end destination for the machine. Some HMI vendors have a stronger presence in certain regions of the world. This may influence the HMI selection, since strong local support can be a sales strength and reduce the support requirements of the machine designer.

A final consideration is tech support. Can you call and talk to an individual, or is e-mail required? Is the support free or is there a charge? Is support available 24/7 or only during office hours? Does the Website provide easy-to-use self-service options for support (downloadable manuals, knowledge base) or is it strictly a sales tool? Determining your requirements for tech support will help narrow your choices for an HMI.


Working through these factors will result in a better decision. Notice that price comes last. There are so many other considerations that need to come first in order to make a proper decision. Choose the functionality you need and then compare pricing. Selecting an underpowered HMI based on price will eventually result in much greater costs in the long run, and a dissatisfied customer.

Ted Thayer is automation systems product manager for Bosch Rexroth Electric Drives and Controls , Hoffman Estates, IL. Rexroth offers six different lines of HMIs, with more than 30 models ranging from “dumb terminals” to full WinXP Industrial PCs.

Find other HMI suppliers at . Find system integrators with expertise in various engineering specialties including human machine interfaces at .

– Edited by Renee Robbins , senior editor
Control Engineering News Desk

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