I was approached by a client who had a very demanding application. Client had a piece of equipment that they wished to control with a PLC and HMI combination. No problem, right? Wrong. The application was in a Class 1, Division 1, A, B, C, D environment. My budget for this application was tight so I had to be very selective on the components that I chose. For the Class 1, Div. 1 area classification, I had two (2) choices available to me: Put the controls into an NEMA 7, Aluminum Enclosure or utilize an X-Purge system from either Bebco (Pepperl-Fuchs) or Expo International. Being that there will be an HMI involved, I decided to go with putting the controls within a NEMA 7 enclosure. The NEMA 7 enclosure option solved all of my problems without creating any. Going with a purging system would require that the HMI be exposed to the outside environment; therefore, you would have to utilize a HMI which would be suitable for the area classification (ching!). Also, not all HMI manufacturers can give you a straight answer on whether or not their screens can go into a purged environment. I once had a very bad experience when I put a Magelis 12" terminal in a purged environment: The screen protector on the front of the LCD bulged out to such an extent that the HMI was rendered unusable. That was a very expensive lesson with regards to HMI's in purged environments.

Using a NEMA 7 enclosure with an HMI meant that I would need a window kit installed with interfacing pushbuttons. I would also need to utilize an HMI which could use a PLC register for the screen selector. Believe it or not, not all HMI's have the ability to reference a pointer register within the PLC for screen navigation. I was surprised; however, to find that the AutomationDirect C-More Micro-Graphic panels actually could be setup to have screens navigated by referencing a PLC register. That was a great discovery, being that I was on a tight budget. All I needed to do now was select an appropriately sized PLC to run my application. I chose the 36 I/O AutomationDirect (Koyo) DL-06 PLC. The 20 inputs and 16 relay outputs gave me plenty of I/O for my small application and the two (2) serial ports were perfect for a PLC/HMI combination.

To have the HMI work with the NEMA 7 enclosure, I would have to get a window kit installed. A 6" window kit was sufficient to view the entire C-More Micro-Graphic screen. I mounted the HMI on a subpanel which was screw fastened to the back side of the enclosure door via ½" deep ¼-20 holes. I used 2" standoffs to hold the mounting panel with the HMI cutout. When completed, I had a perfectly viewable HMI thru the window kit of my NEMA7 enclosure. I used six (6) thru-panel push buttons which were rated Class 1, Div. 1, A-D to interface with the PLC to give me plenty of options on each screen. I had to design each screen with as many scroll or numerical selection functions as possible. For numerical entry functions, I had to use one (1) pushbutton to page up and one (1) pushbutton to page down for parameter selection. When I had a numerical parameter selected to edit, I used increment and decrement functions to adjust the parameter. I programmed it so that if you held the button down , the function would increment or decrement one (1) unit per second. Holding the button down longer than five (5) seconds would increment or decrement the variable ten (10) units per second. This was a sort of "coarse" and "fine" adjustment. Although the screen navigation and manipulation took a bit longer than without the pushbuttons, there was really no other way.

The program development for this application took a bit longer than I had liked; however, the solution which I put together worked very well for this application. At the end of the day, my client was happy - so I was happy. This was another great project which not only challenged my controls and programming skills, but my general electrical engineering skills as well.

For more details regarding this project, please use the form on the contact page and reference "Compressed Natural Gas PLC/HMI" in the message body.