AJAX cleans up HMI performance
Human machine interfaces (HMIs) have been in place for more than 20 years, but some basic rules have not changed. If you want a responsive system, you have to install specialty software on the operator stations, qualify the software, and validate and maintain the installations. HMI applications that require specialty software installed on the operation station are called "thick client" applica...
Human machine interfaces (HMIs) have been in place for more than 20 years, but some basic rules have not changed. If you want a responsive system, you have to install specialty software on the operator stations, qualify the software, and validate and maintain the installations.
HMI applications that require specialty software installed on the operation station are called "thick client" applications. The alternative, "thin client" solutions, do not require any specialty software and instead used Microsoft's Internet Explorer browser as the display software.
Thin clients have major advantages for manufacturing IT departments. They are easier to install, since the required software is installed automatically when the browser points to the HMI server. The maintenance cost is low, because changes are effectively rolled out immediately. Lastly, replacement of operator station hardware can be fast and easy to validate. Thin clients suffer from poor responsiveness, however, because of the large communication bandwidth required between the HMI browser and the HMI server. In many cases, large bandwidth was required for complete redisplays of screens, even when only a small part of the screen information changed.
AJAX also significantly reduces the network traffic and bandwidth required versus other thin client solutions, and it can be almost as bandwidth efficient as thick clients. AJAX is a technique for utilizing features already available on Web-based systems, so it is rapidly entering mainstream, rich interface, thin client applications.
AJAX does offer some unique advantages for HMI applications and, because tool vendors are currently adding AJAX support into their programming environments, it is ready for use in custom manufacturing applications. The typical custom HMI application is a Visual Basic program running as a thick client. While this works for single-station systems, it does not scale to multiple display stations and operators. In an AJAX-enabled application, the custom HMI would be split into display and input elements, and the data access elements. This is a good practice in any custom HMI design, and AJAX encourages this good practice.
For example, the server-side application could require a write-key to be obtained (using an asynchronous XML request) before any write commands would be accepted and could prevent any write-keys from being sent to clients outside the local protected network.
Dennis Brandl, email@example.com , is the president of BR&L Consulting, Cary, N.C., which is focused on manufacturing IT solutions.