Contact and Coil | Nearly In Control

TAG | ladder-logic

Mar/18

18

Idiomatic Ladder Logic

I want to talk about the concept of “idioms” or the idea of “idomatic” when it applies to programming languages.

Python is said to have “strong idioms“:

One reason for the high readability of Python code is its relatively complete set of Code Style guidelines and “Pythonic” idioms.

When a veteran Python developer (a Pythonista) calls portions of code not “Pythonic”, they usually mean that these lines of code do not follow the common guidelines and fail to express its intent in what is considered the best (hear: most readable) way.

Many years ago I did some programming in Perl. A main philosophy of Perl is the acronym TMTOWTDI (There’s more than one way to do it). That’s an example of a language with “weak idioms”. This goes hand-in-hand with Perl’s apparent lack of focus on readability. In fact, some detractors of the language claim it’s a “write-once, read-never” language.

When I started writing the Patterns of Ladder Logic Programming page, I wasn’t thinking of it at the time, but in retrospect I was trying to document “Idiomatic Ladder Logic.” Yes, there are many ways to accomplish the same task in a PLC, but you should stick to idioms when they help communicate the meaning of your code.

When you deviate from these idioms, you’re communicating to the reader that something about this case is different. If I expect a fault coil to be sealed in, and you make it a set-reset, you’re communicating to me that this fault condition needs to survive a power outage. That’s useful information. If your fault coil isn’t sealed in, that must mean you intend it to be self-clearing.

Even your deviations should be idiomatic. Don’t make fault coils self-clearing by putting a Reset instruction for the same coil somewhere else in your code. That won’t be obvious to the reader, and is the reason for the idiom “don’t use a coil more than one place in your logic.”

Most or all new PLCs can be programmed in multiple languages, from the IEC-61131-3 specification. These languages are very different, and the idiomatic way to do something in one language isn’t necessarily the way to do it in another language. For instance, doing any kind of loop (for, while) is non-idiomatic in ladder logic, but is certainly an idiomatic construct in structured text. That means part of our job as programmers is to pick the correct language to express our intent.

Data collection, string parsing, and math is naturally expressed in structured text (ST), but control logic is naturally expressed in ladder diagram (LD). Part tracking logic can go either way. Sequential function chart (SFC) is perfect for expressing a sequence, ladder diagram is a good runner up using the Step Pattern, and structured text requires that you define a state machine, which is the least expressive option.

I once wrote a bubble-sort routine in ladder logic for a SLC500 PLC. I wish I’d had structured text back then. First pick the right language, and then pick the right idiom.

·

Mar/17

26

PLC Programming goes Imperative

Decades ago, computer science emerged from the dark ages of assembly language programming and created two new languages: Lisp and Fortran. These are two very important computer languages because they exist at opposite ends of an imagined spectrum in the eyes of computer scientists: functional languages vs. imperative languages.

Fortran “won” the first battle, not least because imperative languages are closer to how the CPU actually does things, so back in the day when every little CPU cycle mattered it was easier to understand the performance implications of a Fortran program than a Lisp program. Plus, if you were already programming in assembly, then you were already thinking about how the computer was executing your code. In fact, the next big imperative language, C, is often referred to as “portable assembly language.” Fast forward to now, and modern languages like C#, Java, Python and Ruby have all grafted a lot of functional programming features onto their imperative programming basic syntax. In C#, for instance, Linq is a direct rip-off of Lisp’s S-Expressions and it now has Closures and lambda functions. Functional languages provide ways to think at a higher level than imperative languages. In a functional program you describe what you want and in an imperative program you describe how to do it.

Here’s an example in C#, using imperative programming:

var data = new int[] { 1, 2, 3, 4, 5 };
var sumOfSquares = 0;
for(var i = 0; i < data.Length; i++)
{
    sumOfSquares += data[i] * data[i];
}

…and the same thing done functionally:

var data = new int[] { 1, 2, 3, 4, 5 };
var sumOfSquares = data.Select(x => x * x).Sum();

In the second case, I’m taking the list of numbers, using Select to translate that into a list of their squares (also known as a Map operation) and then using Sum on the resulting list to compute an aggregate sum (also known as a Reduce operation). It has some interesting advantages. For instance, the original code can’t be split across multiple cores, but the latter can. Also, if you know both syntaxes, the latter is easier to read and understand.

Now take ladder logic. I’ve made the claim before that basic ladder logic (with contacts and coils) is actually a functional language. A simple example might be ANDing two inputs to get an output, which in C# would look like this:

var output = inputA && inputB;

That’s actually functional. If I wanted to write it imperatively I’d have to do something like:

var output = false;
if(inputA && inputB)
{
  output = true;
}

In ladder logic, that would be the equivalent of using an unlatch (or reset) instruction to turn off an output and then using a latch (or set) instruction to turn on the output if the A and B contacts were true. Clearly that’s not considered “good” ladder logic.

Similarly, a start/stop circuit goes like this:

var run = (start || run) && !stop;

Now historically, mathematicians and physicists preferred functional languages because they just wanted to describe what they wanted, not how to do it. It’s worth noting that electricians, looking at ladder logic, prefer to see functional logic (with contacts and coils) rather than imperative logic (with sets, resets, and move instructions).

In recent years we’ve seen all major PLC brands start to include the full set of IEC-61131-3 languages, and the most popular alternative to ladder logic is structured text. Now that it’s available, there are a lot of newer automation programmers who only ever knew imperative programming and never took the time to learn ladder logic properly, and they just start writing all of their logic in structured text. That’s why we’re seeing automation programming slowly shift away from the functional language (ladder) towards the imperative language (structured text).

Now I’m not suggesting that structured text is bad. I prefer to have more tools at my disposal, and there are definitely times when structured text is the correct choice for automation programming. However, I’d like to point out that the history of computer science has been a progressive shift away from Fortran-like imperative languages towards Lisp-like functional languages. At the same time, we’re seeing automation programming move in the opposite direction, and I think alarm bells should be going off.

It’s up to each of us to make an intelligent decision about what language to choose. In that respect, I want everyone to think about how your brain is working when you program in an imperative style vs. a functional style.

When you’re doing imperative programming, you’re holding a model of the computer in your mind, with its memory locations and CPU and you’re “playing computer” in your head, simulating the effect of each instruction on the overall state of the CPU and memory. It’s only your intimate knowledge of how computers work that actually allows you to do this, and it’s the average electrician’s inability to do this which makes them dislike structured text, sets, resets, and move instruction. They know how relays work, and they don’t know how CPUs work.

If you know how CPUs work, then I understand why you want to use structured text for everything. However, if you want electricians to read your logic, then you can’t wish-away the fact that they aren’t going to “get” it.

As always, be honest with yourself about who will read your logic, and choose your implementation appropriately.

·

When traditional PC programmers see ladder logic, they think ladder logic programmers are terrible programmers. Being both a .NET developer and a ladder logic programmer, this has caused me a lot of frustration and confusion over the years. I have one foot in each world, and yet I choose to write C# programs one way and ladder logic programs another. Why?

Let’s ignore the fact that most traditional programmers just don’t grok ladder logic at all, because their minds think about programs sequentially rather than in parallel. The real reason they hate ladder logic is because ladder logic programmers avoid things like loops, indexed addressing and subroutines. To them, this means you’re programming at the level of an 8 year old.

The thing is, I know how to use loops, arrays, and subroutines, not to mention object oriented programming and functional programming constructs like s-expressions, closures and delegates. Still, I choose to write simple and straightforward ladder logic. Why would I, an experienced programmer, choose to write programs like an 8 year old? Do I know something they don’t?

I spend a lot of time trying to get people to think about why they do things a certain way. Everyone wants that simple rule of thumb, but it’s far more valuable to understand the first principles so you can apply that rule intelligently. Decades of computer science has given us some amazing tools. Unfortunately, a carpenter with twice as many tools in her tool box is simply twice as likely to pick the wrong tool for the job if she doesn’t understand the problem the person who invented that tool was trying to solve.

The first time you show a new programmer a “for” loop, they think, “Amazing! Instead of typing the same line out a hundred times, I can just type 3 lines and the computer does the same thing! I can save so much typing!” They think this because they’re still an idiot. Don’t get me wrong, I was an idiot about this too, and I’m still an idiot about most things. What I do know, however, is that for loops solve a much more important problem than saving you keystrokes. For loops are one of many tools for following the Once and Only Once (OAOO) Principle of software development.

The OAOO principle focuses on removing duplication from software. This is one of the most fundamental principles of software development, to the point where it’s followed religiously. This principle is why PC programmers look at ladder logic and instantly feel disgust. Ladder logic is full of duplication. I mean, insanely full of duplication. So how can you blame them? God said, “let there not be duplication in software,” and ladder logic is full of duplication, thus ladder logic is the spawn of Satan.

That’s because programmers who believe the OAOO principle is about removing duplication are idiots too. Don’t they ever wonder, “why is it so important to remove duplication from our code?” Should we really worry about saving a few bytes or keystrokes? NO! We focus on:

  1. Making it do what it’s supposed to do
  2. Making it obvious to the reader what the program does
  3. Making it easy to make changes when the requirements change

… in that order.

In fact, #3 is the real kicker. First of all, satisfying #3 implies you must have satisfied #2, so ease of understanding is doubly important, and secondly, satisfying #3 implies you can predict what will change.

Imagine if you have to print the numbers from 1 to 5. If I asked a C# programmer to write this, they’d likely write something like this:

for(var i = 1; i <= 5; i++)
{
    Console.WriteLine("{0}", i);
}

… of course I could write this:

Console.WriteLine("1");
Console.WriteLine("2");
Console.WriteLine("3");
Console.WriteLine("4");
Console.WriteLine("5");

Why is the first way better? Is it because it uses fewer keystrokes? No. To answer this question, you need to know how the requirements of this piece of code might change in the future. The for loop is better because many things that might change are only expressed once. For instance:

  • The starting number (1)
  • The ending number (5)
  • What to repeat (write something to the screen)
  • What number to print
  • How to format the number it prints

If the requirements of any of these things change, it’s easy to change the software to meet the new requirements in the first case. If you wanted to change the code so it prints every number with one decimal place, the second way clearly requires 5 changes, where the first way only requires one change.

However, what if the requirements changed like this: print the numbers from 1 to 5, but for the number 2, spell out the number instead of printing the digit.

Okay, so here’s the first way:

for(var i = 1; i <= 5; i++)
{
    if(i == 2)
    {
        Console.WriteLine("two");
    }
    else
    {
        Console.WriteLine("{0}", i);
    }
}

… or if you wanted to be more concise (but not much more readable):

for(var i = 1; i <= 5; i++)
{
    Console.WriteLine(i == 2 ? "two" : i.ToString());
}

Here’s the change using the second way:

Console.WriteLine("1");
Console.WriteLine("two");
Console.WriteLine("3");
Console.WriteLine("4");
Console.WriteLine("5");

Here’s the thing… given the new requirements, the second way is actually more readable and more clearly highlights the “weirdness”. Does the code do what it’s supposed to do? Yes. Can you understand what it does? Yes. Would you be able to easily make changes to it in the future? Well, that depends what the changes are…

Now think about some real-life ladder logic examples. Let’s say you have a machine with some pumps… maybe a coolant pump and an oil pump. Your programmer mind immediately starts listing off the things that these pumps have in common… both have motor starters with an overload, and both likely have a pressure switch, and we might have filters with sensors to detect if the filters need changing, etc. Clearly we should just make a generic “pump” function block that can control both and use it twice, right?

NO!

Look, I admit that there might be some advantage to this approach during the design phase if you had a system with 25 identical coolant pumps and your purchasing guy says, “Hey, they don’t have the MCP-1250 model in stock so it’s going to be 8 weeks lead time, but they have the newer model 2100 in stock and he can give them to us for the same price.” Maybe it turns out the 2100 model has two extra sensors you have to monitor so having a common function block means it takes you… 20 minutes to make this change instead of an hour. We all know how much you hate repetitive typing and clicking.

On the other hand, when this system goes live, making an identical change to every single pump at exactly the same time is very rare. In fact, it’s so rare that it’s effectively never. And even if that were to ever actually happen, the amount of programming time it actually saves you is so tiny compared to the labor cost of actually physically modifying all those pumps that it’s effectively zero.

However, since these are physically different pumps, you’re very likely to have a problem with one pump. When your machine is down and you’re trying to troubleshoot that pump, do you want to be reading through some generic function block that’s got complicated conditional code in it for controlling all 50 different types of pumps you’ve ever used in your facility, or do you want to look at code that’s specific to that pump? And maybe the motor overload on that pump is acting up and you need to put a temporary bypass in to override that fault. Do you really want to modify a common function block that affects all the other pumps, or do you want to modify the logic that only deals with this one pump? What’s more likely to cause unintended consequences?

So this is why ladder logic written by experienced automation programmers looks like it was written by an 8 year old who just started learning Visual Basic .NET last week. Because it’s better and we actually know why.

·

Feb/16

20

How to Write a Big PLC Program

Staring down the barrel of a big automation programming project is intimidating. It’s hard to even know where to start. Even when you’ve done a few before, you’re only marginally more confident the next time.

I have quite a few big automation programming projects under my belt, so I think I can generalize the process a bit. Here goes:

1. Get the Prints

There’s almost no point in starting to program unless you have an almost final set of electrical drawings. If you don’t have them yet, push for them, and go do something else productive until you get them.

2. Create a Functional Specification

You don’t always have to write out a functional specification, but it at least needs to exist very clearly in your head. If at any point, you don’t know exactly how the machine is supposed to work in excruciating detail, stop writing code and go figure it out. Ask stakeholders, talk to operators, whatever it takes. Functional specifications are best written as a list of “user stories”. If you’re not sure what a functional spec should look like, check out Painless Functional Specifications by Joel Spolsky.

3. Shamelessly Copy

Identify what other projects you can find with logic that you can steal. Any code that works in another machine has the advantage of already being debugged. Don’t re-invent the wheel. (At the same time, never blindly copy logic without understanding it. Copying the code by re-typing it one rung at a time is still faster than writing it from scratch, and it’s a form of software review.)

4. Structure Your Project

Now you break open the ladder logic programming software and start creating your project. Pick your CPU type, setup the I/O cards based on the electrical drawings. Map your inputs. Plan out your program by creating programs or routines for each functional unit of the machine. Setup your fault summary rungs and your alarm logic.

5. Write the Manual Mode Logic

PLC logic is typically written “bottom up.” Manual mode logic is the lowest level of logic because it deals directly with individual functions in the machine. Advance cylinder. Retract cylinder. Home axis. Jog axis. While you’re writing the manual mode, this is when you take extreme care making sure that actions are interlocked so the machine can’t crash. If you’re using the Five Rung Pattern, this means paying attention to what goes in the Safe rung. Does cylinder A always have to advance before cylinder B can advance? The Safe rungs should reflect that. Make sure that even in manual mode, the operator (or you) can’t break the machine. Make sure to hook your faults and alarms into the applicable fault summary rungs and alarm logic.

6. Write Part Tracking Logic

Now that manual mode is complete, write the logic that tracks parts (and their state) through the machine. Remember, you should be able to run the machine in manual mode, and the part tracking should (ideally) work correctly. I know this isn’t always the case but surprisingly part tracking in manual mode can work 95% of the time. That means part tracking works based on the state of the machine. Closing the gripper with the robot in the pick position and the part present in fixture sensor on should latch a bit “remembering” that the gripper has a part in it.

Once you’ve written your part tracking logic, go back and use the part tracking and state bits to condition your Safe rungs. Don’t let the operator (or you) mistakenly open the gripper if the gripper has a part and isn’t in a safe position to let go of the part. Of course, you may need to add a way to manually override this (that’s what output forcing was created for), but in most cases you want to prevent improper operation.

Part of writing the part tracking logic is adding “ghost buster” screens. Operators often need to remove parts from a cell, and if the machine can’t detect their removal, then you have to provide the operator with a way to clear these “ghosts.”

At this point you’re actually ready to dump the program in and start testing out the machine electrically and mechanically. While it’s ideal to have a fairly complete program when you go onsite, we all know that’s not always entirely possible. At the very least you want to get to this point before startup begins.

7. Write the Auto Mode Logic

The complexity of your auto mode logic depends on what type of machine you’re programming. You’ll always need a cycle start and a cycle stop feature. Even if you’re in auto mode, you usually don’t want the machine to start until the operator specifically tells it to start. Once it’s running, we call this “in auto cycle.”

In simple machines, you can write the auto logic by filling in the Trigger rungs in your Five Rung logic. Start by putting the In Cycle contact at the beginning of the rung, and then writing logic after that which expresses when the action should take place. For instance, an advance reject part cylinder’s Trigger rung could be as simple as In Cycle, Part Present, and Part Rejected. As long as the Part Present tracking bit gets cleared once the cylinder is in the advanced position, then this is all the auto mode logic you need for this motion. Have the retract Trigger rung be In Cycle, No Part Present and Not Retracted.

More complicated machines need more complicated auto mode logic. If your machine has to perform a series of steps (even if some of them are in parallel) then consider using the Step Pattern. If your machine needs to choose between several possible courses of action (commonly seen in a storage and retrieval system) then consider using the Mission Pattern.

8. Review

It’s hard to write correct logic. Review your functional specification, point by point, and make sure your logic meets all of the requirements. Check your logic for errors. A fresh look often uncovers incorrect assumptions, typos, and outright mistakes. The earlier you find and fix problems, the easier they are to fix.

Make a list of everything you have to do during startup. Starting up a machine is time consuming and therefore expensive. Anything you can do to prepare saves you time and money.

Good luck, and keep your fingers out of the pinch points!


· ·

Jan/16

17

Start your own Automation Blog!

One thing I’ve discovered about automation blogging is that it’s a pretty lonely place. Don’t get me wrong, there are a couple gems out there, but I don’t find many people writing about what it’s like in the trenches wading through rungs of ladder logic. In the .NET world there are tons of programming blogs with posts about every issue you could ever come across. Why such a dearth of information in the automation space?

It occurs to me that blogging seems difficult to a lot of people, as if you need to be a web programmer. That’s totally untrue (I’m absolutely not a web programmer). In fact there are a ton of inexpensive and simple options out there.

Maybe people wonder what to write about. That’s easy. At first I worried about writing something people didn’t already know, but it turns out there’s no shortage of shiny new graduates looking for any toe-hold they can get in this industry. Try to think back to when you didn’t know anything. How did you figure out how to get online with a PLC the first time? Did you have to call the office to ask someone, and feel foolish because you didn’t know what a Null Modem cable was? We were used to asking our peers for help, but the new generation grew up looking up how to do things on the web.

Maybe you’re worried about the cost. Shared hosting plans are very inexpensive. I use DreamHost, specifically because they’re inexpensive (starts at $9 per month including domain name, unlimited storage and bandwidth), the hosting is rock solid, they offer one-click installs of blogging software (such as WordPress), and their technical support is excellent.

It would really be great if I didn’t feel like the only voice in this cloud. Grab a blogging account and chime in. Write about a hard problem and how you solved it. Disagree with me! Help me learn something new!

·

Nov/15

1

Offline Changes to a PLC Program

As a PLC programmer, you’ll often be asked to do a change to an existing system. If there’s a significant amount of functionality to be added, you generally get your changes ready “offline” and then do all the changes during a short window of time to minimize disruption to the production schedule.

If you’re using an Allen-Bradley PLC, the procedure is typically this:

  1. Get a copy of the latest program from the PLC (a.k.a. an “upload”)
  2. Make your changes to the offline copy, and write down every change you had to make
  3. Go online with the PLC and apply your changes as online changes

Step 3 is much safer than just taking your modified program and doing a “download”. That’s mainly because when you download, you’re not just downloading the program, but the memory state of the PLC as well. The PLC typically has to track things in memory (like recipe data, part tracking, data collection, sequence numbers, machine counters, etc.). If you do a download, you’re going to overwrite all those values with previous values, and that can cause a lot of problems. The other thing step 3 saves you from is simultaneous changes that were done online while you were busy making offline changes.

The only other option you have is upload-change-download, but you really have to shut the machine down for the duration to make sure that the internal state doesn’t change.

When I did a lot of Allen-Bradley programming, I didn’t question that. It’s just how it was. I remember visiting a plant one time for a service call, and the local maintenance person was a bit suspicious of what I was going to do (after all, I was a young kid who had never seen this machine before). He decided to quiz me a bit, and one of the things he asked was, “when you go online, do you download or upload?” I said “it depends,” but his answer was, “you never download.” I agreed that someone in a maintenance role should never need to do a download unless they’re replacing a CPU, or recovering from a corrupted PLC program.

Now that I mostly do Beckhoff TwinCAT 3 programming, I realized one of the benefits are that offline changes are a breeze. It’s due to the fact that TwinCAT 3 completely separates the program from the memory data. The program is stored in local files on your hard drive and compiled into a TMC file. The persistent data is stored in a different place on your hard drive.

When I want to do offline changes to a TwinCAT 3 project, here’s the procedure:

  1. Get a copy of the latest program
  2. Make your changes to the offline copy
  3. Copy changes back to the machine (keeping a backup, of course), rebuild, and activate configuration

This makes offline changes go a lot more smoothly, of course. I don’t have to copy and paste my changes in while online, so it takes less time and eliminates the possibility of a copy/paste error.

Since we also use Mercurial for version control, getting a copy of the latest program is a matter of pulling the latest from the source control, and copying it to the machine is a matter of pulling the offline changes to the machine. Any changes that were done in parallel can be merged with Mercurial’s built-in diff and merge utilities. (Note: I/O changes can’t be merged nicely, so if someone changed the I/O while you were doing your offline changes, you have to copy those changes in manually, but that’s rare and at least it tells you that it can’t merge them.)

This got me thinking that Allen-Bradley probably has a better way of doing offline changes that most of us just don’t know about. I know that you can do an upload without uploading the memory. However, it seems like it requires you to download both the program and data at the same time. I wonder if anyone out there knows how to do better offline changes to a ControlLogix. If so, I would be interested to know that.

· · · ·

You may have noticed I recently added a new section to this site: Patterns of Ladder Logic Programming. My goal, as usual, is to try to help new ladder logic programmers come up to speed faster and without all the trial and error I had to go through.

The new Patterns section is an attempt to distill ladder logic programs into their component parts. I assume the reader already knows the basic elements of ladder logic programming, such as contacts, coils, timers, counters, and one-shots. The patterns describe ways of combining these elements into larger patterns that you’re likely to see when you look through real programs. In my experience, you can program 80% of the machines out there by combining these patterns in applicable ways.

The Patterns section isn’t complete yet, but I will be adding to it slowly over time. If you think of a pattern that’s blatantly missing, please send me a note so I can include it.

·

Motion control is pretty complicated.

There’s been something really bothering me about the “integrated” motion control you find in PLCs these days (notably Allen-Bradley ControlLogix and Beckhoff TwinCAT). Don’t get me wrong, they’re certainly integrated far better than stand-alone motion controllers. Still, it just doesn’t “feel” right when you’re programming motion control from ladder logic.

When I’m programming a cylinder motion in ladder logic, I would typically use a five-rung logic block for each motion (extend/retract). One of the 5 bits is a “command” bit. This is a bit that means “do such-and-such motion now”. Importantly, if I turn that bit off, it means “stop now!” This works well for a cylinder with a valve controlling it because when I turn off power to that valve, the cylinder will stop trying to move. It would be nice if integrated motion was this simple.

It’s interesting to note that manual moves (a.k.a. “jogging”) are usually this simple. You drop a function block on a rung, give it a speed and direction, and when you execute it based on a push-button, the axis jogs in that direction, and when the push-button turns off, it stops jogging. Unfortunately none of the other features are that simple.

All other moves start motion with one function block and require you to stop it with another. The reason it works like this is because motion controllers also support blended moves. That is, I can first start a move to position (5,3) and after it’s moving there I can queue a second move to position (10,1) and it will guide the axes through a curved geometry that takes it arbitrarily close to my first point (based on parameters I give it) and then continue on to the second point without stopping. In fact you can program arbitrarily complex paths and the motion controller will perform them flawlessly. Unfortunately this means that 90% of the motion control logic out there is much more complex than it needs to be.

Aside: in object-oriented programming, such as in Java or .NET, it’s pretty normal to have to interface with a relational database such as MySQL or Microsoft SQL Server. However, when you try to mesh the two worlds of object-oriented programming and relational databases, you typically run into insidious little problems. Programmers call this the Object-relational impedance mismatch. I’m sure that if you added it up, literally billions of dollars have been spent trying to overcome these issues.

My point is that there is a similar Ladder logic-motion control impedance mismatch. The vast majority of PLC-based motion control is simple point-to-point motion. In that case, the ideal interface from ladder would be a single instance of a “go-to” function block with the following parameters:

  • Target Position (X, Y…)
  • Max Velocity
  • Acceleration
  • Deceleration
  • Acceleration Jerk
  • Deceleration Jerk

When the rung-in-condition goes true on this block, the motion control system moves to the target position with the given parameters, and when the rung-in-condition goes false, it stops. Furthermore, we should be able to change any of those parameters in real-time, and the motion controller should do its best to adjust the trajectory and dynamics to keep up. That would be all you need for most applications.

The remaining applications are cases where you need more complex geometries. Typically this is with multi-axis systems where you want to move through a series of intermediate points without stopping, or you want to follow a curved path through 2D or 3D-space. In my opinion, the ideal solution would be a combination of a path editor (where you use an editing tool to define a path, and it’s stored in an array of structures in the PLC) and a “follow path” function block with the following parameters:

  • Path
  • Path Tolerance

When the rung-in-condition is true, it moves forward along that path, and when it turns off, it stops. You could even add a BOOL parameter called Reverse which makes it go backwards along the path. The second parameter, “Path Tolerance” would limit how far off the path it can be before you get a motion error. I think this parameter is a good idea because it (a) allows you to initiate the instruction as long as your position is anywhere along that path, and (b) makes sure you’re not going to initiate some wild move as it tries to get to the first point.

A neat additional function block would be a way to calculate the nearest point on a path from a given position, so you could recover by jogging onto a path and continue on the path after a fault.

Obviously there needs to be a way for the PLC to generate or edit paths dynamically, but that’s hardly a big deal.

Anyway, these are my ideas. For now we’re stuck with this clunky way of writing motion control logic. Hopefully someone’s listening to us poor saps in the trenches! 🙂

· ·

Jul/14

1

Ladder Logic running on an Arduino UNO

Happy Canada Day!

Some of you may wonder if I’d fallen off the face of the Earth, but the truth is life just gets busy from time to time. Just for interest’s sake, here’s my latest fun project: an Arduino UNO running ladder logic!

Ladder Logic on a UNO

You may remember I wrote a ladder logic editor about 5 or so years ago called SoapBox Snap. It only had the ability to run the ladder logic in a “soft” runtime (on the PC itself). This is an upgrade for SoapBox Snap so that it can download the ladder logic to an Arduino and even do online debugging and force I/O:

Arduino UNO Ladder

I haven’t released the new version yet, but it’s very close (like a few days away probably).

Edit: I’ve now released it and here is a complete tutorial on programming an Arduino in Ladder Logic using SoapBox Snap.

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There’s a lot of stuff that falls under the term “functional programming,” but I’m just going to focus on the “functional” part right now, meaning when you define the value of something as a function of something else.

In ladder logic, we define the values of internal state (internal coils or registers) and outputs. We define these as functions of the inputs and internal state. We call each function a “rung”, and one rung might look like this:

Ladder diagram of Inputs A and B, and Internal State C

There’s something slightly odd going on in that rung though. You might say that we’ve defined C recursively, because C is a function of A, B, and itself. We all know, of course, that the PLC has no problem executing this code, and it executes as you would expect. That’s because the C on the right is not the same as the C on the left. The C on the right is the next state of C and the C on the left is the previous state of C.

Each time we scan, we redefine the value of C. That means C is an infinite time-series of true/false values. Huh?

Ok, imagine an array of true/false (boolean) values called “C”. The lower bound on the array index is zero, but the upper bound is infinite. C[0] is false (the value when we start the program). Then we start scan number 1, and we get to the rung above, and the PLC is really solving for is this:

Ladder logic defining C[1] as a function of A, B, and C[0]

If that were actually true (if it had an infinite array to store each coil’s value), then the ladder logic would be a truly functional programming language. But it’s not. Consider this:

Two ladder logic rungs with inputs A and B, internal coil C, and output D

In all modern PLCs, the first rung overwrites the value of C, so the second rung effectively uses the newly computed value for C when evaluating D. That means D[1] is defined as being equal to C[1] (the current state value of C). Why is this weird? Consider this:

Two previous rungs with the rung order reversed

By reversing the order of the rungs, I’ve changed the definition of D. After the re-ordering, D is now defined as C[0] (the previous state value of C) rather than C[1]. This isn’t a trivial difference. In an older PLC your scan time can be in the hundreds of milliseconds, so the D output can react noticeably slower in this case.

In a truly functional language, the re-ordering either wouldn’t be allowed (you can’t define D, which depends on C, before you define C) or the compiler would be able to determine the dependencies and re-order the evaluation so that C is evaluated before D. It would likely complain if it found a circular dependency between C and D, even though a PLC wouldn’t care about circular dependencies.

There are a few of reasons why PLCs are implemented like this. First, it saves memory. We would have to double our memory requirements if we always wanted to keep the last state and the next state around at the same time. Secondly, it’s easier to understand and troubleshoot. Not only does the PLC avoid keeping around two copies of each coil, but the programmer only has to worry about one value of each coil at any given point in the program. Third, the PLC runtime implementation is much simpler. It can be (and is) compiled to a kind of assembly language that can run efficiently on single threaded CPUs, which were the only CPUs available until recently.

Of course this comes with a trade-off. Imagine, for a moment, if rung-ordering didn’t matter. If you could solve the rungs in any order, that means you could also solve the rungs in parallel. That means if you upgraded to a dual-core CPU, you could instantly cut your scan time in half. Alas, the nature of ladder logic makes it very difficult to execute rungs in parallel.

On the other hand, we can still enforce a functional programming paradigm in our ladder logic programs if we follow these rules:

  • Never define a coil more than once in your program.
  • Don’t use a contact until after the rung where the associated coil has been defined.

That means there should only be one destructive write to any single memory location in your program. (It’s acceptable to use Set/Reset or a group of Move instructions that write to the same memory location as long as they’re on the same or adjacent rungs).

It also means that if coil C is defined on rung 5, then rungs 1 through 4 shouldn’t contain any contacts of coil C. This is the harder rule to follow. If you find you want to reference a coil before it’s defined, ask yourself if your logic couldn’t be re-organized to make it flow better.

Remember, someone trying to solve a problem in a PLC program starts at an output and uses cross references to move back through the program trying to understand it. Cross referencing from a contact to a coil that moves you forward in the program doesn’t require any logical leaps, but cross referencing to a coil later in the program means you need to logically think one scan backwards in time.

Benefits

While ladder logic isn’t a truly functional language, you can write ladder logic programs in the functional programming paradigm. If you do, you’ll find that your outputs react faster, and your programs are easier to understand and troubleshoot.

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