Hardware manufacturers have worked on the problem of addressing safe interaction with operator interfaces in hazardous environments for many years with varying degrees of success. One thread of thought was to reduce or eliminate the use of higher voltages from operator interfaces.
Early HMIs relied on cathode-ray tube (CRT) technology. CRT projects images by using an electron emitter to deflect electrons onto a phosphor-coated inner surface of a vacuum tube. The phosphor coating retains some light after the beam has deflected to a different location, and, by rapidly moving the emitter around, an image can be projected onto the screen.
The technology takes up a lot of space and early display monitors were large and bulky. This same technology was used in televisions of the day. Those early models were monochrome before multichromatic models and greater color displays were possible. The significant improvement came with liquid crystal display (LCD) and light emitting diode (LED) displays were developed. The size of the screens got bigger while the electronics needed to drive the images got smaller.
Another source of spark came from the need for a power supply in the device. If the screen could be operated on voltages lower than 50 V, then the potential for arc flash is pretty much eliminated. Motherboard technology gave way to single-chip designs, further reducing the size of the electronics driving the display device. Despite these advances in technology, there is still a risk of electronic arcing so the next step was to totally immerse the electronics in a substrate where no heat—spark—can get out.
Totally encased electronics are in common use today. The final step in the development has been to completely separate the head from the body of the display device. Most manufacturers today now offer a headless version of their display devices. The body, where all of the heat-producing components reside, is mechanically separated from visual display. The body is mounted in a remote cabinet or control panel, and just the graphic touchscreen is mounted in the production environment.
With this technology, just low-voltage power, 24 Vdc, for example, and a network connection such as Profinet or Ethernet/IP connects the safely isolated body from the field-mounted display. These devices can be used in Class I, Div. 1, and Class I, Div. 2, environments.
As it turns out, the last step is, in fact, not the last step. As often happens in the world of automation, advances in control devices parallel the development of devices for consumer markets. In this case, the development of smart phones and tablets has presented a means by which to put the head and body back into a single device.
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The device that we hold in the palm of our hands has all the power and more of a desktop computer and monitor that started the computer revolution in the early 1980s. Industrial versions of a tablet, in sizes matching the traditional HMI, can contain both the computer and display device, as well as the application to interact with the programmable logic controller (PLC) or soft-PLC computer running the machine or process. If running power out to a remote cabinet is still a risk, the display device can be battery-operated, just like a smartphone or tablet, and be swapped out for a freshly charged version that has been sitting on a charging station in a safe location nearby.
With the use of wireless networking technology, the application on the field device can be updated remotely as well with no need to plug in and download.
One of the biggest design-time decisions is where to locate the HMI or HMIs to best service the need to have important or risky actions in view of the operator. Some operations must be within line of sight and one of the best advantages of this new tablet-like interface is that it can be freely carried around the machine or process area because it isn’t mounted in a cabinet or tethered to a cable.
Some developers are using this to great advantage by leveraging the technology to use an application on a smartphone to access the HMI application or a subset of it. Through user management, the HMI application designer can give people access to particular screens and functions based on their assigned interaction with the machine or process. Maintenance or setup technicians get access to higher-level functions than operators. And engineers can override functions that the others can’t access.
The next time you pull your smartphone out for use, remember that the Voyager 1 space craft travelled all the way out to interstellar space with a 16-bit computer running 70 kilobytes of memory onboard, executing 8,000 instructions per second.
A smartphone, by comparison, can execute trillions of instructions per second. With the use of technology, hazardous environments don’t need to be hazardous to the people operating the equipment. Technology makes it easier for designers to make their creations safer and, in most cases, at a lower cost. It’s a win-win for everyone involved.
