Smart machines/PLC overlap?

During my tour at a oilfield pipe making facility I noticed that the PLCs ran the show, but werent just telling dumb motors to energize or reading dumb encoders and switches etc.

There was some fairly smart equipment there as well. For example the saw (picture a chop saw but with a 3 ft blade) had a big console with a touch screen display and must of had some circuit boards and brains all its own inside. The maint guy was pointing out a schematic he had pulled out that the italian nomenclature was different for circuit symbols on this italian made behemoth.

The console was remotely mounted about 10 feet from the buisness end of the saw which was completley enclosed (I could imagine the noise)in a booth with a circulating cutting fluid system.

So Im wondering, the saw is smart and can be commanded by the operator but a whole cabinet of brand new PLCs was installed right behind the saw.

There was also a high tech smart welder with its touch screen on the line too along with some inspection equipment that I didnt get to see close up.

My question is if PLCs call the shots but are simple I/O boards that cant talk to computers thru their simple inputs how do they command a smart machine, does the saw just take a order from the PLC, "Cut pipe until further notice" and the saw takes it from there cutting 40 foot joints with it own optical encoder and controlling its own functions and then just set an output that indicates a cut is complete and the PLC then tells a motor to pull the new pipe away?

Why would you need a whole cabinet full of PLCs? All that happens after the saw is the pipes are moved down racks to be blown out with air and inspected, simple hydraulic arms halt and then let pipes roll with gravity once past the saw.

Do the PLCs stick their nose into every aspect of the saws operation? Where do smart machines and PLCs functions meet? can they talk to each other on the rs-232 bus? Are they made to work with plcs? Is there ever any conflict or safety concern if the smart machine has issues and "lies" to the PLC?

Couldnt you just have a production line with smart machines and no PLCs, everything talking on a ethernet or whatever? You could have smart conveyor belts and material handling machines for that matter. Every machine could be on a bus and a terminal in a control room could display everything.

I know industry didnt go that route, just curious. TIA I know you guys are busy and I do appreciate the feedback to beginner questions like this, Im just trying to wrap my mind around the big picture.

HMI's allow the entry of parameters, display alarms etc, its the operator's means of seeing what is happening and giving commands.

I guess you are not seeing panels full of PLC's but panels with perhaps 1 PLC and associated IO or remote racks just containing IO, plus of course drives, instrumentation etc.

You have insight

carwashblues said:

Couldnt you just have a production line with smart machines and no PLCs, everything talking on a ethernet or whatever? You could have smart conveyor belts and material handling machines for that matter. Every machine could be on a bus and a terminal in a control room could display everything.

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Yes

I know industry didnt go that route, just curious.

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What make you think that? You have seen one or two oil field operations. Don't judge the whole world from the 0.01% you have seen.
Many oil field operations require class 1 div 2 certification for their electronic controls. Not all PLCs have this certification.
Here is an example of 'smart' machines tied together with Ethernet.
http://www.hydraulicspneumatics.com/200/Issue/Article/False/86388/Issue
I like the way Frank Braswell put in the extra effort to simulate and test as much as possible before going to the field. Note, this kind of controls is not that common either. Oil field operations are move to new technology slowly.

Here is another one.
http://www.youtube.com/watch?v=P69LDLHAN9g
This one is probably controlled by an SLC500 and a motion controller using Ethernet but the new ones will use just the motion controller. The math required to do the robotics is extensive. In the new version the motion controller keeps track of where all the pipe are and how to get to them. The smart machine does the racking when the pipes are pulled out of the ground and and unracking when the pipe are put back into the ground. This may not look as impressive or fast as most industrial robot but most industrial robots are massive compared to the loads they move. You can see that the pipes weigh as much as or more the robot arm that is moving them.

There may also be a SCADA system over the top of the PLCs and data being entered fron a remote point that way.
Also be aware that PLCs are not just dumb I/O driven devices. It is quite common for a PLC to be attached to VSDs and instructions being passed to the VSDs via HLI, and other devices.
PLC screens often have inspection devices etc attached to them such as cameras. Also printers and almost everything else.
PLCs are often used to interface between all types of devices and quite often are used to track things through plants and adjust speeds, positions, lengths and all sorts of things.
PLCs are also often used for position control as well and can then also control the feed rates of product, monitor daily output - almost anything you could possibly imagine.
They are often the 'binding' in the plant and carry out the overall control.

Im guessing VSD is "variable speed drive" and the HLI is an interface between the PLCs output and the VSD and is an electronic device that sets an industrial motors speed? Nope, were not talking a handheld drill here,on second thought I would imagine big industrial motors like to run at a constant rpm, so I guessing the VSD would adjust the drive ratio of some drive system to achieve faster or slower speeds on the line.

If this is the case, and I dont know all the acronyms yet, then how does the PLC talk to the HLI? A digital instruction ? does it send an analog signal, more voltage means faster or would it just toggle dedicated inputs of the HLI, each one correlating to a speed programmed into the HLI?

How would it handle infinite variable speed control like the drive systems with the conical pulleys on belt driven systems.

Basically Im asking by what means does a PLC talk to a smart machine; it would have to send something more than an open or close 1 or 0 from its output or else you only get 2 speeds unless it was a serial data thing. Tying up 16 outputs to send a digital command dosent seem very elegant. Im sure its a simple answer. Are there special digital output cards for PLCs that can send any hexadecimal command anywhere the programmer wants to?

Sorry for the annoying questions, I did google VSD, but it turns out to be a heart condition! It seems all 3 letter acronyms have been appropriated by one profession or another.

Thanks for your patience!

carwashblues said:

Basically Im asking by what means does a PLC talk to a smart machine; it would have to send something more than an open or close 1 or 0 from its output or else you only get 2 speeds unless it was a serial data thing. Tying up 16 outputs to send a digital command dosent seem very elegant. Im sure its a simple answer. Are there special digital output cards for PLCs that can send any hexadecimal command anywhere the programmer wants to?

Sorry for the annoying questions, I did google VSD, but it turns out to be a heart condition! It seems all 3 letter acronyms have been appropriated by one profession or another.

Thanks for your patience!

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Almost every PLC has a means of communications. The simpler ones are serial based (RS232/RS485) while the more advanced ones tend to be Ethernet Based.

Over this communication pipeline, there can be proprietary communication protocols for programming or monitoring the PLC and also for sending information to the Operator interface panel. Over this same communication pipeline there can be industry standard protocols used for the same type of data transfer.

This data goes beyond on/off signals and can include anything that can be stuffed into a packet. Think integer and real numbers, strings, arrays etc.

PLC's can also interface with "smart" devices like printers, scanners, scales, measuring instrumentation etc.

PLC's can even be programmed to send e-mails and text messages over their Ethernet interfaces.

For all practical purposes, the modern PLC with an Ethernet interface can be thought of as a computer with a real time operating system that runs a proprietary program (or programs as some are multi-tasking).

If you can dream a control scenario up a PLC can likely do it...

...even if the job is not what a PLC was specifically designed for.

We have set up PLCs to run numerous conveyor systems. We have also have set PLCs up to run machines like bottle fillers, palletizers, packers, box erectors, etc. They are used on US Navy ships and amusement park rides. They get data from HMIs and barcode scanners, temp sensors and speed sensors. They send data to HMIs and VFDs (VSDs) heater controls and weld controllers. They can send data to computers and Microsoft Excel...

In a water bottling facility I worked at, for example, bottles came off of a depalletizer, went across an elevated air conveyor, through a rinser into the filler and onto a bottle conveyor. The conveyor led to a shrink wrapper to get "6-packed" and the 6-packs hit another conveyor. Through a drop packer onto more conveyor to the palletizer and then to a pallet shrink wrapper.

Guess what controlled every component I described above? Each PLC communicated with every other PLC. When we filled half-liter bottles the entire scheme was set up differently than if we ran 1 liter bottles (conveyor speeds, timing setpoints, fan speeds, etc).

Did I mention that the water control system that controlled three different types of water ran off a PLC? Or that there were a total of four different lines? This plant had so many PLCs that AB sent us Christmas cards every year.

Just like in your car or truck there is a central "brain" but that component won't be the device that actually sends gas to your engine or stops the vehicle. It will likely control the devices that do, though.

In the water plant I described above, the conveyors' PLC had ultimate command of the PLCs in each machine on their line. I could then offer that the PLC bank you saw behind the other equipment had something to do with the process--just not machine control per say.

I have worked with a wax injection machine that injects wax into molds that have an RFID label on them. We set the PLC/HMI up so the RFID reader sends the data to the PLC, which sends the mold number to the company's SQL database where all the info for that mold is kept. The operator scan a traveller card with the Work Order on it and returns certain info about that WO to the PLC--Clamp pressure, injection pressure, dwell time, quantity, etc). The PLC uses that data to control the actual press which is located in another room nearby. ALL of this is done via ethernet.

I'm pretty sure that one day with me or Alaric, or Peter, or Ron or Tom or countless other members of this forum would prove just how friggin cool PLCs can be.

Thanks Norm. That makes sense. Of course this raises my next question!

If the PLC is receiving data over an rs-232 bus, from a giant smart saw for example, couldnt the smart saw be not so smart occasionally as a result of an intermittent electronic fault in the saws circuitry, and send bogus data to the PLC? Like not telling the plc that its done cutting or telling it that there is some kind of overload when there is none?
If a smart machine acts up and hangs up the bus or causes a PLC to shutdown, but only once in a while, that is still considered bad, right?, with the 5000/min down time and all.

If 10 minutes of downtime cost 50k, then having an electronics technician like myself go deeper in the saw with smart standalone test equipment and find the issue with the saw would be cost effective. And yes I can run probes into the saws control cabinet and conduct my testing with the line running and all panels closed

I still think an electronic tech on a maint team would be an asset. If I only resolved that one issue the whole year, I would save more than my salary and still be there to assist my co-workers with regular tasks.

Thoughts? anyone.

Thanks Rube, excellent response, I appreciate everyones feedback.

I stopped building systems without HMI's (Human-Machine Interface, btw) a long time ago because that's a great place to show why a system is not doing what it's supposed to be doing. I've designed systems where fully 1/4 to 1/3 of the program in dedicated to monitoring or alarms. A properly used HMI will let operators know if they should correct a situation or call for help. It can also let Maintenance know what to go look for, thus, in both cases, decreasing downtime.

Production data can also be displayed on the HMI and let's not forget about batch processing and recipes.

So the saw can get it's instuctions from the HMI--how many pieces, how long etc. And then it goes and does the actual cutting. If something goes wrong; piece jams, not enough material for the cut, blade breaks, etc., the PLC, or PLCs stop everything. Bells, lights and messages on an HMI let the operator and/or Maintenance know what to do to get running again.

PLC's would never talk to PLC's via Serial RS232. You might find scanners or load cells, i.e. a component of a machine that may be serial.

A lot of this component level equipment nowadays would be able to communicate with numerous communication methods, most VFD's for example can have options of cards of different communication type.

PLC's would use other means to talk to each other, ControlNet for AB, Profibus or Profinet for Siemens, Modbus Plus for Modicon and Ethernet on most.

If a machine gave incorrect messages, it would not be due to an intermittent fault with the communication equipment, you are more likely to lose the comms altogether.

Software faults should be eradicated during the commissioning stage, all alarms would be tested during this stage so incorrect messages for a fault should not happen, software does not suddenly become faulty although there is the extremely rare occasion where there is the exception to the rule or not all things are tested.

Thanks everyone Im getting the picture

Thanks Rube and PeterW, So the hierarchy is SCADA, The networked PLCs all over the plant and then the smart equipment like the welder and saw with their own HMI (touchscreens) and less smart equipment like a VSD or VFD.

Peter I agree that software once debugged and all the digital circuitry are quite reliable. By faulty data I really meant the saw could send perfectly valid data to the PLCs indicating a problem just as designed, only there is really no problem. The saw might be relying on monitoring circuits far from the digital I/O to provide the program with assurance that every thing is working. A fault in those circuits could fool the saw and in turn fool the PLC.

Rube, what if the HMI onscreen instructions say something like "error 100: motor current out of range" and give a list of things to check like a jammed up pipe, but none of the things on the list are present? I bet the operator calls maintenance and they come out with their flukes in hand, but what if the last little trick they used like pulling a board out and sticking it back in dosent work this time? Eventually it wont work because the connector will get worn out from all the board swapping.

Nope this time the board reseating trick dosent work along with all the reset this and that tricks and they put all 3 spare boards in the machine with no effect.

At a time like that 5000 bucks a minute or not, unless someone can hack the machine to ignore error 100, no pipes will be cut for a good long time.

They could then sure use someone who could break out the schematics, find the circuit that monitors current and will set code 100 and do some real diagnostic work. Someone who done stuff like that for many years and always found the problem. I would bet a few paychecks they would be more than happy they hired me then , instead of another electrician who can poke around PLCs.

Thats my new improved pitch anyway. Ive toned it down from "Real electronics techs can do a lot more for your maint efforts" to more like "Hey you might not need my skills very often, but when you do I could save the day"

If all that fails Im going to get some service job somewhere and take online classes and get a degree in industrial technology!

carwashblues said:

They could then sure use someone who could break out the schematics, find the circuit that monitors current and will set code 100 and do some real diagnostic work. Someone who done stuff like that for many years and always found the problem. I would bet a few paychecks they would be more than happy they hired me then , instead of another electrician who can poke around PLCs.

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Don't misunderstand, please! A good tech (or a great one) is VERY valuable to any company that runs machinery. I, as I'm sure many others have, started out as a "wrench turner" then got into electrical, then PLCs, then so on. Now decades later, I'm pretty versatile and actually design control systems and design panels. See some of them in the panels threads from a few years back for photos of some of the panels forum members have designed and built.

But having said that, I still maintain that for you a next logical step would be PLCs. Yeah you can hang a Fluke around your neck and get hired and work all day wondering "what if". You could hang a 7/16" open end (or a hacksaw to cut pipes) on your belt and never get the thrill of tagging a 277vac line.

But if you hang that wrench on your belt you won't need to visit here for all this stimulating conversation.

I wish I could hire you--I would just to prove it to you. I would prove to you that with a PLC and a $200 sensor mounted on that jammed pipe, that the HMI REALLY had the message: "Flow below limits--check for broken/plugged pipe".

Depending upon the necessary speed of communication you can send analog information over a single on off line using pulse width modulation. This is a cheap way to create an analog input and output. It works really well if you are using a solid state switch to control a heating element since your cycle time can be quite long. I've had excellent results making PID loops which also anticipate startup and state changes including changes in operating speeds.

My skills are very limited in comparison to what most of you do. Not even close. We set up a system last year to do the most basic start stop sequnces. Timed. On alarm, the HMI would give the operator, the MCC # and bucket location, and suggest corrective action if a process failed to start or stopped. We had studied this for months so we had a pretty good idea where the problem areas were. A few days after startup the operator complained to one of my electricians that a motor would not start. The HMI instructed them to "...Check Chain". They had not looked. It was laying on the ground. The biggest problem area we missed were the operators themselves.