Schematic Design Methods

Something else they don’t teach you in college…

Despite what you may think there are many different ways to create control diagrams in industry today. I sometimes like to refer to it as Technical Graphic Arts! Nearly all of these methods use very expensive design software running on what just a few years ago might be considered super computers. The tools may have changed dramatically over the years but many designers still insist upon creating designs that have not changed in any meaningful way since the 1960’s. If you pulled out a control drawing from the 60’s and placed it next to a control drawing made today, you might only notice that the early drawing was done by hand. For most designers we have gone from paper & pencil and drafting boards to super computers that draw the lines and letters for us, but not much else has really changed.

There certainly is a long list of international, national, regional, industry and company “standards” but these are in most situations only recommendations. I could easily get side tracked into discussing IEC vs NFPA drawing standards but to be honest, in my opinion in actual use there is really not a whole lot of difference between them. The IEC standards organizations are at least active standards and updated fairly frequently where the NFPA standards are essentially a hot mess. Who is the governing body these days? NFPA or IEEE? And please, stop talking about the JIC! it has been defunct since the early 80’s and has not published a standard since 1967 (EMP-1-67/EGP-1-67) But all that has really nothing to do with this discussion, lets talk about the actual drawings.

Control panel drawings used in industry today have not changed significantly since the discovery of electricity, that’s not necessarily a bad thing but there is certainly room for a few new ideas. The basic circuit arrangement or layout style so far as I can tell possibly predates Nikola Tesla as the form is clearly recognizable to me in his patent filings which data back as early as 1891. In the document below which was filed in 1897 you can clearly see the B+ pile connections in the center of the page with the B- pile connections on the right, thus depicting an NFPA style current flow from left to right and clearly visible an recognizable ladder arrangement.



When I look back through the lineage of modern standards, this style is clearly defined in the ANSI Y14.5-1966. I suppose it predates even this, probably WW2, but I don’t have the paper trail at the moment. Suffice to say its old and has been around a long time. in the USA we tend to use the NFPA standard which in general has the higher potentials shown on the left side of the page and the lower potentials shown on the right and with the components that make up the circuits in between. In the rest of the world they use the IEC standard which in short, rotates the NFPA drawings -90 degrees so that now the higher potentials are shown on the top of the page and the lower potentials shown on the bottom.

ANSI Y14.5-1966

Swell, thanks for the history what?

In actual use there are essentially three types of control system drawing design methods in use today regardless of what national standard we are talking about. All of these styles have many similar design elements but the key difference lies in how they handle Logical IO. A system these days with more than a handful of relays in it often has a programmable controller (PLC) of some type to provide the smarts. These programmable devices were only objects of an engineers imaginations as recently as the early 1970’s and are commonly used today, they also have some special properties that require consideration when creating system drawings.

Here is a description of the three different styles of drawings:

Standard Ladder Diagrams
Think old school style drawings where the design is based on graphics that represent simplified rungs which are just wires connected to high and low potentials with components in the middle. The real differentiator here would be how the use of PLC is represented. In this style of drawing the PLC modules are drawn to closely represent the physical properties of the card, often including pin locations and orientations.


Card Based Ladder Diagram


Mechanical Model


These designs are usually pretty easy to create but they often leave out important information about field devices such as cables, connections, terminals and equipment locations. When I was was working on some of my first designs I recall asking senior engineers how to document the missing field devices and I was told not to worry about it, just let the electricians figure it out.  I am sure that I am not the only one to have been offered that nugget of wisdom. Times have certainly changed and many designers would agree that projects need to be fully designed and documented to help control costs and maintain schedules.

Another issue with these types of designs is that they require that you build your circuits from scratch on each and every project. From a logical standpoint machines are are simply a collection of functional systems, in the electrical world we often refer to these functional systems as circuits. Circuits are a collection of individual components interconnected and controlled in such a way as to provide the required function for the machine. These circuits are a often carefully designed and revised over time to provide the desired performance and the components are well know to the designers. However, when using this style of drawing the designer is forced to build these often tried and true designs each and every time from the individual components.

The reason for all this is that many designers (and their managers) feel that the IO needs to be wired directly to the PLC module in a manner that they are familiar with. PLC programmers also like it because it is very simple and makes it easy to also see what what’s been assigned and where.

Distributed IO style
This is a more modern style that I see more often in IEC designs but it works just fine with either format. In this style the PLC cards are still shown in overviews and on the panel layout but they are missing from the electrical design, instead they are replaced by a special symbol that represents a PLC connection point on a card. This design style now frees the designer to disassociate the IO from the PLC cards physical layout and thereby enabling the creation of reusable functional circuit sections. Now when you want to add a pump or a conveyor or a scale circuit to the design, you simply copy the circuit from a master repository and place it into your design, and all the required components need to make that circuit work are placed. Modern tools such a ePLAN can then automatically renumber the components to match the project requirements so that often only very limited manual editing will be required.

Distributed PLC Connection Point


Now don’t confuse the simple use of a distributed IO PLC symbol with a distributed IO design, very often designers will simply lay 8 of these symbols right next to each other and then you just have the same old Card based “Physical Design” that you had before, only now the designer is just using the different “distributed IO” PLC symbol. It still just represents a physical PLC card turned on its side.

IEC style PLC Card


The benefit only occurs when you move the connections away from each other. Below is a small circuit used to control a special water circuit in a machine, all the IO and control devices need for that circuit are right there. It can now be copied and pasted from one project to another quickly without the need of making unnecessary connections back to the PLC card.

NFPA Distributed IO Example


Functional Design style
Taking this idea to the next level we have what is referred to as the functional design style. This method essentially breaks the entire machine down into a collection of fundamental functional circuits. These circuits are often organized into a master reference project which contains all the standard circuits use in your organization. These circuits are often created and maintained by a senior engineer and made available for use in designs. In most cases these circuits can also be configured for several different optional circuit variants. When a new project presents itself, the designer simply makes a list of the various circuits required for the project and pulls the circuits from the premade macro project and places a copy in the schematic. This enables project design times to be greatly reduced as the process becomes much more modular and much quicker to complete. This method has the additional benefit that any improvements to standard circuits are easily communicated between team members and immediately made available for future designs.

In practice all the components needed for a functional circuit, from power feeds to IO, would be included on one or more macro pages. These pages would be placed into the current project and assigned an identifier that would serve as a higher level function ID (=), an example might be CONV1 or a circuit number or anything else you can think of. This has the huge benefit of enabling the project to be simply created by dragging and dropping the pre-approved basic circuits to build the final project. There are a certainly a few more steps required but very large projects can be created in a fraction of the time by using these more modern design styles.

Functional Design Example


If you would like to learn more about distributed or functional design methods, simply give me a call and lets discuss how I might help you on your next project.


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