TAG | plc
I’m not a lawyer and what follows is not legal advice. What I’d like to do here is point out some minimum things that you need to be aware of if you’re doing control system integration when it comes to intellectual property. I hope you can appreciate that there are subtle details that can affect the reality of your situation and you should consult a lawyer depending on your situation, geographic location, and product.
If you do control system integration for a living then you deal every day with intellectual property. Ladder logic (or structured text, etc.) is code, and code is automatically copyrighted. Electrical designs are also intellectual property. The important thing to realize is that somebody owns that intellectual property, and people are sometimes surprised by who owns it.
Here’s a typical situation: you work for a control system integrator and they’re doing a fixed price (or “turn-key”) project for a customer. In this case the ownership of the copyright should be spelled out in the contract, and most control system integrators will specify that the copyright belongs to the integrator, but part of the purchase price of the project covers a license to use that software in their facility. There are very good reasons for this. The integrator works on many different projects for many different customers and over time you’re going to see the same problem over and over so you want the ability to maintain an internal library of tested code that you can re-use on future projects. Not only does this make you more efficient, but it’s almost impossible to avoid writing the same code again when you see a problem that you’ve solved before.
In this case the customer, to protect themselves, needs to insist on a license that will cover all of their needs, including the ability to see and modify the code, including having someone else come in and modify it, though only for use in that facility or on that machine, as the case may be.
Now consider a very different situation. You’re a control system integrator doing a time-and-material project for a customer. They have their own internal PLC ladder logic standard, including a library of function blocks, and they know how they want everything written. They give you a sample program and tell you to start with that and modify it to suit the machine you’re building. This is a completely different case. The copyright of the finished PLC program is almost certainly owned by the customer, not the integrator. The integrator and the customer should both be making sure that the contract specifies this in writing. This creates some interesting differences for the integrator.
The integrator cannot (and should not) be using their own internal library of code on this project, or the customer could end up claiming copyright on the integrator’s code. Even more importantly, the integrator should definitely NOT take code they see in the customer’s standard library and put it in their own internal library. Here’s what could happen: you take code from customer A and put it in your library. You use that code on a project for customer B, and it happens that customer B is a competitor to customer A. Customer A’s lawyers can now use the legal system to prevent customer B from using that machine without paying royalties to customer A.
You may be wondering, “how would customer A even find out that this happened?” There are several ways… for one, people in the control system industry move around. A lot. Trust me, it’s a small group of people and everybody knows each other. It’s not a big stretch for someone who worked at customer B to end up at customer A. For another, sometimes employees just talk. I’m writing this because just the other day I witnessed an employee from a local integrator say out loud that they’d done just this… took some function blocks from their customer and used it on other projects for other customers… and they said this in front of the customer they took them from! Now, the representative from the customer seemed to think this was OK, but I doubt their corporate lawyer would agree, and I doubt this agreement was in writing. If some future customer of the integrator ever got in trouble legally, what do you think would happen to that integrator?
I realize it’s sometimes hard to adjust from a university environment with the free flow of ideas, where everyone’s downloading copyrighted movies and songs from peer-to-peer networks all day long, and then work in a corporate environment where the stakes are much higher, where lawyers and executives on both sides would love to find some leverage over their competition. Seriously, it’s not worth your career. Take a moment to school yourself on intellectual property law and how it affects your profession. This can be a big deal. Be careful.
Over the past year I’ve been working on a TwinCAT 3 Tutorial, and with the posting of the 13th chapter about the TwinCAT 3 Scope View, I’m considering it “complete.” I’ve covered all the major topics that I wanted to cover.
Writing a tutorial like this was actually a learning experience for myself. Before I made any statement about the software I tried to validate my assumptions first, and many times I learned that I had preconceived notions about how it worked which may not have been completely accurate. If you’re a TwinCAT 3 programmer, then you need to know your tool set inside and out. If you’re not sure how something works, it’s best to setup a quick test and try it out. That’s one of the advantages of PC-based software: if you want to run a test, your office PC is a laboratory environment. Write some code and run it.
Thank you to everyone who offered feedback on the existing sections, and to those of you who kept pushing me to continue writing more. I sincerely hope this can be that missing link to take people from the world of traditional PLCs into the more interesting and ultimately more powerful world of PC-based control.
It looks like TwinCAT 3 has a viable future ahead of it. Interest seems to be growing over time (according to Google Trends, anyway):
That makes TwinCAT 3 a valuable skill to learn if you’re an integrator, and an interesting technology to investigate for existing plants.
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.
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!
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!
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:
- Get a copy of the latest program from the PLC (a.k.a. an “upload”)
- Make your changes to the offline copy, and write down every change you had to make
- 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:
- Get a copy of the latest program
- Make your changes to the offline copy
- 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.
This information is for a Fanuc R30-iA, RJ3-iA or RJ3-iB controller but might work with other ones.
If you’re looking for a way to send the robot world TCP position (X, Y, Z, W, P, R) over to the PLC, it’s not actually that difficult. The robot can make the current joint and world position available in variables, and you can copy them to a group output in a background logic task. There is one caveat though: the values only update when you’re running a program. They don’t update while jogging. However, there is a work-around for this too.
First you should make sure that the feature to copy the position to variables is enabled. To get to the variables screen, use MENU, 0 (Next), 6 (System), F1 (Type), Variables.
Find this variable and set it to 1 (or True):
The name of that variable is Current position from machine pulse
Now create a new robot program, and in it write the following:
Note that I’ve multiplied the X, Y, and Z positions by 10, so you will have to divide by 10 in your PLC. Likewise I multiplied the W, P, and R angles by 100, so divide by 100 in the PLC.
To run this program in the background, use MENU, 6 (Setup), F1 (Type), 0 (Next), BG Logic. Configure it to run your new program as a background task.
Obviously you need to send these group outputs to the PLC. Ethernet/IP is great for this, but you can use hardwired interlocks too. You need to make sure that you have enough bits to handle the full range of motion. A 16-bit integer should work pretty well for all of these. Note that the robot will happily send negative numbers to a group output as two’s complement, so make sure you map the input to the PLC as a signed 16-bit integer (a.k.a. INT in most PLCs). For the X, Y, and Z positions, a 16-bit integer will give you from +3276.7 mm to -3276.8 mm of total range. For the W, P, and R angles you’ll get +327.67 deg to -327.68 deg. For most applications this is good (remember this is TCP, not joint angles). Please check that these are suitable though.
As I said, these numbers don’t update while you’re jogging, and won’t update until the robot starts a move in a program. One little trick is to do a move to the current position at the start of your program:
J PR[100:SCRATCH] 10% FINE
This starts sending the position without moving the robot. In my programs I typically enter a loop waiting for an input from the PLC, and inside this loop I turn a DO bit on and off. The PLC detects this as a “ready for command” heartbeat, and as long as the PLC sees this pulsing, then it knows the program is running and the position data is valid.
Another trick you can use is to detect when the robot has been jogged:
The name of this variable is Robot jogged. The description from the manual is: “When set to TRUE,the robot has been jogged since the last program motion. Execution of any user program will reset the flag.”
That’s how you get the world position of the TCP into the PLC. If you just want joint angles, you can use
$SCR_GRP.$MCH_ANG[n] as the variable, where “n” is the joint number.
Important note: The I/O will probably change asynchronously to the program scan, so what you want to do is make a copy of the X, Y, Z, W, P, R values coming into the PLC and compare the current values to the values from the last scan. If they haven’t changed, then update your actual values, otherwise throw them away because they might not be valid. If you have a fast scanning PLC and I/O then you should still be able to keep up with the robot even during a fast move. If you have a slow scan time on your PLC, then you might only get valid stable values when the robot is stopped.
Now what if you want to know what the TCP position is relative to one of your user frames? The robot controller doesn’t seem to give you access to this, but the PLC can at least calculate the X, Y, and Z positions of the TCP in your user frame itself, given the world position and the user frame parameters.
First you need to find the accurate user frame parameters. Under the normal frames screen you can only get one decimal point of accuracy, but you need the full 3 decimal points to have your numbers in the PLC match the user frame position given in the robot. You can find these accurate positions in a variable: use MENU – 0,6 (SYSTEM) – F1 (TYPE) – Variables – $MNUFRAME[1,9] – F2 (DETAIL). The second index in square bracket is the frame number, so
$MNUFRAME[1,1] is frame one and
$MNUFRAME[1,2] is frame 2. Copy these numbers down exactly.
Here’s the math for calculating the TCP relative to your user frame. All variables are LREAL (which is a 64-bit floating point variable). I don’t know if you can use a regular 32-bit float or not.
Result is your TCP in user frame.
Point is your point in world frame (from the robot) and
Frame is the accurate user frame data you copied from the
Result.X_mm := Point.X_mm - Frame.X_mm;
Result.Y_mm := Point.Y_mm - Frame.Y_mm;
Result.Z_mm := Point.Z_mm - Frame.Z_mm;
RadiansW := DegreesToRadians(-Frame.W_deg);
CosOfAngleW := COS(RadiansW);
SinOfAngleW := SIN(RadiansW);
RadiansP := DegreesToRadians(-Frame.P_deg);
CosOfAngleP := COS(RadiansP);
SinOfAngleP := SIN(RadiansP);
RadiansR := DegreesToRadians(-Frame.R_deg);
CosOfAngleR := COS(RadiansR);
SinOfAngleR := SIN(RadiansR);
// Fanuc applies rotations WPR as W (around Z), P (around Y), R (around X)
// AROUND Z
temp := Result.X_mm;
Result.X_mm := Result.X_mm * CosOfAngleR - Result.Y_mm * SinOfAngleR;
Result.Y_mm := Result.Y_mm * CosOfAngleR + temp * SinOfAngleR;
// AROUND Y
temp := Result.Z_mm;
Result.Z_mm := Result.Z_mm * CosOfAngleP - Result.X_mm * SinOfAngleP;
Result.X_mm := Result.X_mm * CosOfAngleP + temp * SinOfAngleP;
// AROUND X
temp := Result.Y_mm;
Result.Y_mm := Result.Y_mm * CosOfAngleW - Result.Z_mm * SinOfAngleW;
Result.Z_mm := Result.Z_mm * CosOfAngleW + temp * SinOfAngleW;
DegreesToRadians() is just
Run that on your PLC and check that the values in your Result variable match the user frame TCP position reported on the teach pendant.
I haven’t gotten around to calculating the W, P, and R angles of the TCP in user frame yet. Currently I just look at W, P, and R in world frame if I need to know if I’m “pointed at” something. If you get the math to work for W, P, and R, I’d really appreciate if you could share it.
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.
I reviewed TwinCAT 3 in February of 2013 and it was a mixed bag. I lauded the amazing performance but warned about the reliability problems. I think it’s time to revisit the topic.
Things have improved greatly. When I wrote that review we had 2 production systems running TwinCAT 3 (the 32-bit version). We’re now up to 5 production systems with another on the way, all running version 3.1.4016.5 (which is a 64-bit version). The product has been more stable with each release. First we tried switching to a Beckhoff industrial PC, but we still experienced two blue screen crashes. We’ve then turned off anti-virus and disabled automatic windows updates. So far I haven’t seen another blue screen on that system, for about two months.
Manually installing windows updates isn’t a big deal, but it’s unfortunate to be running a PC-based control system with no anti-virus. Our industrial PCs are blocked from going online, and each one is behind a firewall that separates it from our corporate network, but it’s still a risk I don’t want to take. Industrial Control vendors continually tell us their products aren’t supported if you run anti-virus, and I don’t see how anyone can make statements like that in this day and age.
The performance of the runtime (ladder logic) and EtherCAT I/O is still absolutely amazing.
While the IDE is much better than the TwinCAT 2 system, the editor is still quite slow (even on a Core-i7 with a solid state drive).
The Scope is now integrated right into the IDE, and I can’t give that tool enough accolades. I recently had to use Rockwell’s integrated scope for ControlLogix 5000 and it’s pitiful in comparison to the TwinCAT 3 scope.
The TwinSAFE safety PLC editor is light years beyond the TwinCAT 2 editor, but it’s still clunky. It particularly sucks when you install a new revision of TwinCAT 3 and it has to upgrade the safety project to whatever new file format it has. We recently did this, then had to add a new 4-input safety card to the design, and it wouldn’t build the safety project because of a collision on the connection ID. It took us a couple hours of fiddling and we eventually had to manually set the connection ID to a valid value to get it to work. On another occasion, after a version upgrade, I had to go in and add missing lines in the safety program save file because it didn’t seem to upgrade the file format properly (I did this by comparing the save file to another one created in the new version).
The process of upgrading to a new TwinCAT 3 version often involves subtle problems. The rather infamous 3.1.4013 version actually broke the persistent variable feature, so if you restarted your controller, all the persistent variables would be lost. They quickly released a fix, but not before we experienced a bit of pain when I tried it on one of our systems. I’m really stunned that a bug this big and this obvious could actually be released. It’s almost as if Beckhoff doesn’t have a dedicated software testing department performing regression tests before new versions are released, but certainly nobody would develop commercial software like this without a software testing department, would they? That’s a frightening thought.
I ended my previous review by saying I couldn’t recommend TwinCAT 3 at this time. I’m prepared to change my tune a bit. I think TwinCAT 3 is now solid enough for a production environment, but I caution that it’s still a little rough around the edges.
Edit: Note that I’ve since added a TwinCAT 3 Tutorial section to this site.
Let’s assume you already know everything there is to know about motion control… you can jog a servo axis, home it, make it move to a position using trapezoidal or s-curve motion. Now what?
Sooner or later you’re going to find yourself with 2 or more axes and you’re going to want to do something fancy with them. Maybe you have an X/Y table and you want it to move on a perfect 45 degree angle, or you need it to follow a curved, but precise, path in the X/Y plane. Now you need coordinated motion.
Coordinated motion controllers are actually quite common. Every 3, 4, or 5 axis mill uses coordinated motion, every robot controller, and even those little RepRap 3D printers. What you may not know is that most integrated motion solutions you might encounter in the PLC world also offer coordinated motion (a.k.a. interpolated motion) control. If you’re from the Allen-Bradley world, the ControlLogix/CompactLogix line of PLCs allows you to use the Motion Coordinated Linear Move (MCLM) and Motion Coordinated Circular Move (MCCM) instructions along with a few others. If you’re from the Beckhoff world, you can purchase a license for their NC I product which offers a full G-code interpreter, which is the language milling machines and 3D printers speak.
Under the hood, a coordinated motion control solution offers several features necessary for a workable multi-axis solution. The first is a path planner, the second is synchronization.
The job of the path planner is fairly complex. If you say that you need to move your X/Y table from point 5,2 to point 8,3 then it needs to take the maximum motion parameters of both axes into account to make sure that neither axis exceeds it’s torque, velocity or acceleration/deceleration limits, and typically it will limit the “velocity vector” as well, meaning the actual speed of the point you’re moving in the X/Y plane. Furthermore, it must create a motion profile for each axis that, when combined, cause the tooling to move in a straight line between those points. After all, you may be trying to move a cutting tool along a precise path and you need to cut a straight line. To make matters far more complicated, after the motion is already in progress, if the controller receives another command (for instance to move to point 10,5 after the initial move to 8/3) then it will “blend” the first move into the second, depending on rules you give it.
For instance, let’s say you start at 0,0, then issue a move to 10,0 but then immediately issue a second move to 10,10. You have to option of specifying how that motion will move through the 10,0 point. If you issue a “fine” move then the X axis has to decelerate to a stop completely before the Y axis starts its motion. However, you can also tell it that you only care that you get within 1 unit of the point, in which case the Y axis will start moving as soon as you get to point 9,0 and will do a curved move through point 10,1 on its way to 10,10 without ever moving though point 10,0. This is actually useful if you’re more concerned with speed than accuracy. Another option you have is to issue a linear move to 9,0 followed by a circular move to 10,1 (with center at 9,1) followed by a linear move to 10,10. That will cause the tooling to follow a similar path, but in this case you’re in precise control of the curved path that it takes. In neither case will either axis stop until it gets to the final point.
The other important feature of coordinated motion is synchronization of the axes. Typically the controller delegates lower level control of the axes to traditional axis controllers, and the coordinated motion controller just feeds the motion profiles to each axis. However, it’s imperative that each axis starts its motion at precisely the same time, or the path won’t be correct in the multi-dimensional space. That requires some kind of clock synchronization, and that’s the reason why you see options for things like Coordinated System Time Masters on ControlLogix and CompactLogix processors.
That was very brief, but I hope it was informative. If you do have to tackle coordinated motion on your next project, definitely allocate some time for reading your manufacturer’s literature on the subject because it’s a fairly steep learning curve, but clearly necessary if your project demands it.
One of the most confusing things that new programmers face is how to break down their program into smaller pieces. Sometimes we call this architecture, but I’m not sure that gives the right feel to the process. Maybe it’s more like collecting insects…
Imagine you’re looking at your ladder logic program under a microscope, and you pick out two rungs of logic at random. Now ask yourself, in the collection of rungs that is your program, do these two rungs belong close together, or further apart? How do you make that decision? Obviously we don’t just put rungs that look the same next to each other, like we would if we were entomologists, but there’s clearly some measure of what belongs together, and what doesn’t.
Somehow this is related to the concept of Cohesion from computer science. Cohesion is this nebulous measure of how well the things inside of a single software “module” fit together. That, of course, makes you wonder how they defined a software module…
There are many different ways of structuring your ladder logic. One obvious restriction is order of execution. Sometimes you must execute one rung before another rung for your program to operate correctly, and that puts a one-way restriction on the location of these two rungs, but they could still be located a long way away.
Another obvious method of grouping rungs is by the physical concept of the machine. For instance, all the rungs for starting and stopping a motor, detecting a fault with that motor (failure to start), and summarizing the condition of that motor are typically together in one module (or file/program/function block/whatever).
Still we sometimes break that rule. We might, for instance, have the motor-failed-to-start-fault in the motor program, but likely want to map this fault to an alarm on the HMI, and that alarm will be driven in some rung under the HMI alarms program, potentially a long way away from the motor program. Why did we draw the line there? Why not put the alarm definition right beside the fault definition that drives it? In most cases it’s because the alarms, by necessity, are addressed by a number (or a bit position in a bit array) and we have to make sure we don’t double-allocate an alarm number. That’s why we put all the alarms in one file in numerical order, so we can see which numbers we’ve already used, and also so that when alarm # 153 comes up on the HMI, we can quickly find that alarm bit in the PLC by scrolling down to rung 153 (if we planned it right) in that alarms program.
I just want to point out that this is only a restriction of the HMI (and communication) technology. We group alarms into bit arrays for faster communication, but with PC-based control systems we’re nearing the day when we can just configure alarms without a numbering scheme. If you could configure your HMI software to alarm when any given tag in the PLC turned on, you wouldn’t even need an alarms file, let alone the necessity of separating the alarms from the fault logic.
Within a program, how should we order our rungs? Assuming you satisfied the execution order requirement I talked about above, I usually fall back on three rules of thumb: (1) tell a story, (2) keep things that change together grouped together, and (3) separate the “why” from the “how”.
What I mean by “tell a story” is that the rungs should be organized into a coherent thought process. The reader should be able to understand what you were thinking. First we calculate the whozit #1, then the whozit #2, then the whozit #3, and then we average the 3. That’s better than mixing the summing/averaging with the calculating of individual whozits.
Point #2 (keep things that change together grouped together) is a pragmatic rule. In general it would be nice if your code was structured in a way that you only had to make changes in one place, but as we know, that’s not always the case. If you do have a situation where changing one piece of logic means you likely have to change another piece of logic, consider putting those two rungs together, as close as you can manage.
Finally, “separate the why from the how”. Sometimes the “how” is as simple as turning on an output, and in that case this rule doesn’t apply, but sometimes you run across complicated logic just to, say, send a message to another controller, or search for something in an array based on a loop (yikes). In that case, try to remove the complexity of the “how” away from the upper level flow of the “why”. Don’t interrupt the story with gory details, just put that into an appendix. Either stuff that “how” code in the bottom of the same program, or, better yet, move it into its own program or function block.
None of these are hard and fast rules, but they are generally accepted ways of managing the complexity of your program. You have to draw those lines somewhere, so give a little thought to it at first, and save your reader a boatload of confusion.