Throughout the history of automation, the human-machine interface (HMI) has evolved from simple lights on a board to sophisticated hardware with computers onboard and the possibility of combining the HMI and programmable logic controller (PLC) into the same physical package.
With sophistication comes the risk of exposure to the environment in which the HMI is installed. Just how do we keep the outside from getting inside? Perhaps, though, we might be jumping ahead of ourselves just a bit.
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The mention of a human-machine interface might conjure up the idea of a panel-mounted device that utilizes a high-resolution graphical interface. The internal components are very similar to a modern laptop or desktop computer. It has an operating system upon which an application is installed that provides a means to interact with a computer or programmable controller. Objects on the screen link to digital or analog memory locations in the PLC or computer.
If we look back far enough, however, we will discover that the simplest of a human-to-machine interface can be found in a pushbutton or pilot light. The controlling device receives an input from a pushbutton and makes a decision that will result in an output to a light, solenoid or motor starter, for example. Early HMIs didn’t involve a screen at all.
One of the earliest examples I encountered of a human-machine interface was a large board, upon which a graphic representing the various components of a water-treatment process was overlaid. At the various component locations, a hole was made through the board and a light (or lights) were installed to mimic the state of that particular component during operation. This was an example of a mimic board.
Many industries traditionally used mimic boards to represent, graphically, the status of the operation. Mimic boards were very popular in automotive and petrochemical where processes could stretch from hundreds of feet to miles in the concerned coverage area.
These boards would be isolated from the physical location of the process and, in such a manner, would be protected from the environment of the actual activity. The operator could follow the sequence of operation and make decisions about what processes to stop and start, via manual control buttons, to get the desired results.
The development of the cathode ray tube (CRT) and graphics cards provided a means to represent, electronically, the same machine or process that the mimic board accomplished, but on a more refined medium.
Early CRTs were of low resolution and monochromatic—one or just a few colors—so the results tended to be more box-like, but the advantage of being able to change the layout or add pop-up details was a vast improvement over a static board and some appropriately placed lights.
The evolution of the HMI parallels that of the personal computer, programmable logic controller and supervisory control and data acquisition (SCADA) systems—smaller, faster processors with huge amounts of base memory and removable storage devices.
Extreme high-resolution graphics cards provide images on ever faster and more resolute monitors. Higher resolution means more information can be represented on a single screen. Screen objects can be created that very closely represent real-world devices. Large amounts of onboard memory means that graphic images can be stored and displayed as part of interfacing with the human. Schematics and whole manuals can now be displayed on an HMI.
An HMI does not necessarily imply a high-resolution screen with complex graphics. Industrial robots, for example, utilize a pendant with an array of buttons on it and a small display to interact with the user.
A commercial blender might use a pendant that is buttons and lights only. This, too, is an HMI. An operator station for a press might utilize palm buttons that light up to prompt the user to interact with the buttons. Again, this is an HMI.
Operator hardware is robust these days and current designs protect the components when used in a harsh environment. For example, 30-mm buttons can be purchased with a rubber boot that fits over the operating components that are outside of the enclosure. This reduces the risk of water or other contaminants from getting into the physical device and the components that are on the backside—inside the enclosure. The backside components can be totally encased to prevent contamination.
Aside from an all-mechanical device, HMIs do tend to be more fragile in nature. Essentially, they are computers with exposed screens that use a touch interface to interact with the user. Under normal operating conditions, the hardware might survive operation, but, as most of us know, conditions are rarely normal.
Hardware manufacturers go to great lengths to protect HMIs from the environment in which they operate. Again, much of this has improved with technology. For example, an HMI originating in the 1980s might have an IP54 rating. This means that it would be protected from limited dust ingress and water spray in any direction.
While this might work for an environment where dry goods are produced, it would not stand up to intentional water spray or pressurized materials. By comparison, an HMI with a rating of IP69K means that the device is completely dustproof and can withstand washdown at pressures up to 1,450 psi for periods of up to 15 minutes at a time and temperatures up to 176 °F.
One approach that has gained traction lately is to use a tablet in place of a permanently mounted HMI. Tablets can be charged outside of the harsh environment and then brought into the area for use. Tablets, much like our smartphones, are designed to withstand environments that other devices could not. There are some rules to follow. For example, the application on the tablet can only control devices that are within eyesight of the operator. This means a means of locating the device must be made or the application on the device must be restricted to the immediate vicinity of the wireless access point (WAP) that the device connects with.
Another means of protecting the HMI is to mount it, as usual, in an enclosure and then add additional methods of protection. For example, if the HMI is used mostly as a visual tool, then a see-through cover could be added to the enclosure face to go over the HMI. If the operator needs to access the HMI directly, the cover can be unlatched and moved out of the way while in use and then latched back into place when done.
The enclosure itself can be positively charged with air or an inert gas to, effectively, push any foreign particles back out any small crevice that might be a point of entry otherwise.
Another approach that has merit is the use of a thin client. In these situations, the smarts of the HMI are located remotely, and only the display is mounted in the field. The screen itself can have ratings of IP66 or IP69K, making it an excellent choice for this application. Often, the membrane of the screen can be replaced in the field, and, if there is a failure, you are only replacing the touchscreen monitor and not the whole system.
With proper implementation, the HMI can last a very long time. Protecting these devices is very important to the control system design. Manufacturers go to great efforts to provide us with a sufficient set of options to make sure that the device suits the need, and we don’t have to put an expensive piece of hardware in our design if we don’t really require it for our operating environment.
Happily, in most cases, these various degrees of construction are within a single product line, and an application design can easily be scaled up and down to meet the different physical device characteristics without reinventing our usual operator station design.
The HMI is an essential part of a control system design. The device provides direct interaction with the controller, as well as endless capabilities to provide troubleshooting for an operator or line technician.
With the high-resolution graphics, actual photos of the area of concern can be shown right on the screen with interactive popups to provide further details of the various components that make up the control system. The approach at my place of work is to employ the HMI as a means of keeping people out of our control cabinets whenever possible.
To this end, we use graphical representations of most of our in-panel components and super-impose status lights right over the appropriate spot on the image. We then use soft buttons to pop up information that describes the device and what the various lights and patterns mean.
This approach keeps our control enclosures clean and reduces/eliminates the need to open the door to perform basic troubleshooting.
