After begging and pleading, kicking and screaming at my boss for some "advanced" PLC training, I got some "advanced" S7200 training from a distributor. This "advanced" training went like..."PLC stands for...", "this switch puts the PLC in run mode", "this is how you make a NC contact", etc... You get the picture.

I have decided to try and teach myself some "advanced" PLC instructions. I read Hugh Jacks tutorial and it was very informative, but you cant ask a book questions or pick its brain.

I'm at the point where I feel comfortable with contacts, coils, timers and counters but get confused when dealing with memory locations (shift registers, math instructions, etc.). Ive been reading manuals, forums and it just doesnt "click". Its like i need to hear the right thing to make it all come together. To say "AHHHHHHHH, THATS IT! now it makes sense!"

Anyway, I downloaded the free DirectSoft5 software and then downloaded Peter Nachtweys sliding mode control program. Could someone take the time and please explain to me, in as simple terms as possible whats going on from rung 6 down? I would be greatful

I have never looked at his AD program but i might suggest that you look at this thread from many moons ago introducing sliding mode control.

Thread

Thanks for the reply Sleepy. Maybe I'm in over my head trying to understand that program.....let me make an example. Lets say a number is entered into an HMI to keep a cooler a certain temp. Does the PLC see that an HMI is there and request data? or does the HMI just put data in a PLC's memory location? And am I right in assuming that that number (say 40f) is converted to binary? And what if the actual temp. is to be displayed on the HMI, how is that calculation made and how is it sent to the HMI?

I believe the HMI's poll for the data using the PLC's protocol. So, whatever is programmed to display on the HMI, the HMI polls for the data then display's it after it receives a response from the PLC.

Scaling analog values is usually done in the HMI, but can be done in the PLC. I usually do it in the HMI. A full scale value from the PLC will usually be 0-4095. Then in the HMI, for example, just tell it 0-4095 equals 0-100%.

I think what you really need is a small plc to play with. Hook up some switches and play. For an HMI, you'll have to buy something and play. Practice some of the examples that come with it.

Well lots of "data" that an HMI has on display is either static, or dynamic. Static displays such as text or graphics are stored in the HMI and displayed only on the operator's screen. Dynamic information is stored in the PLC. Anything that changes with the machine that is displayed would be dynamic. There are two forms of dynamic info on a HMI screen. One form of info would be your temperature set point. You enter that and it is written to an address you assign for the plc to look, the other is information written to the HMI from an assigned address, such as the actual temperature of your cooling process.

Inside the PLC "junk" is always stored in binary, but the development software usually allows you to display it in what ever format you desire, ASCII, binary, hex, decimal, octal and what ever else someone has thought handy. Where the fun starts is how you want the HMI to interpret or write stuff. Some of the more programmer friendly HMI's allow you to enter ASCII strings as text, and some of the more stone age units require you to enter the ASCII strings as segments of ASCII Code.

Just remember that no matter what, PLC's and HMI's only do what you tell em to do. Not what you wanted them to do, so take your time and just have fun playing with stuff. I think most of us learned by examples, thought, experimentation, and picking apart other people's programs, and I know I still have favorite subroutines I have "found" or written myself over the last ten years or so.

Hope that helps a lil bit