My control system isn't on the Internet! Why should I care about web based HMI or SCADA systems? I've been discussing these points for 5 years now, which is at least 10 years behind the corporate world. My ultimate goal is to educate end users on the pros and cons of a technology that may be very relevant to them.
"You'll sail upwind by a fire under your deck - I don't have time for such nonsense", my recollection of a quote from a Civilization video game. The point is that the application of new technologies isn't always immediately obvious. It helps to take a step back to consider what you're trying to accomplish.
Let's break down your SCADA system. Visibility into your process is probably the single most important aspect, which may come in the form of numbers, pretty graphics, etc. You need to be able to see the realtime and historical status of your operation. Next comes control. The ability to adjust setpoints, start and stop things, etc. After that it probably branches into: data input, reporting, alarming, interfacing with other systems, specialized applications, etc.
Sound like things a stand alone computer software package can do? Yes, that's an ideal version of the HMI that you're used to. Sound like something an HTML web page can do? Absolutely not! The first thing to realize is that "web based" is referring to a technology set that's prevalent on the Internet.
Now think of all the cool animated web sites you've been to. Java, Flash, ActiveX, DHTML and AJAX, and other technologies enable rich multimedia content. The big advantage is that the framework exists to run such applications with only a web browser. Instead of dragging CDs along to each HMI seat to install the software, then necessarily configuring: projects, data connections, PLC connections, server connections, etc, a new "web based" client need only the URL and permissions to access the system.
The next advantage has to do with routing and existing infastructure. How easy is it to connect a new computer on the other side of the plant to your legacy (non-ethernet) PLC network? It can get pretty tough! You probably already have an IP network throughout your plant to ride. What about realtime connections to your other locations? Again, our existing transport infastructure simplifies things with a "web based" approach. Does this mean that it has to be on the Internet? No, but it can. Or you can establish secure tunnels over a public network (Internet).
OK - so a "web based" approach brings nearly the same capabilities to the table, and can be easily launched from anywhere without the "traditional" lengthy installation and configuration process. What about safety, responsiveness, stability, and security?
It's true that the Industrial arena has considerations that don't exist elsewhere. This is why it's important to deal with people and companies that specialize in your trade. However, dealing with proven standards and technologies on the scale of decades of development and millions of users makes more sense than blindly trusting the "big" industrial software companies homebrewed nightmares. They are small as software companies go, and began with single terminal requirements that were much simpler. It's in their best interst to stretch their legacy junk in favor of backward compatibility, but even they are slowly turning to standards based approaches.
Remember that the Internet didn't grow out of the necessity to email grandma - it was the evolution of a hardened system designed for C2 (command and control), specifically launching missiles. Keep in mind that standard encryption based security schemes keep your investments safe from hackers. Web based SCADA provides a very capable, centrally managable, scalable and secure, IT supportable system. Wow!