PLC & Control

A PLC is not the alone best for authoritative a process. Sticking with alone basal relays may be of a account depending aloft your application. Yet, on the added hand, a computer ability be the way to go. The PLC vs. PC agitation has been activity on for a continued time. More generally admitting it doesn't appear bottomward to an "either or" bearings but involves a mix of technologies.

PLC vs Relay

When I first started programming PLCs it was still ambiguous if a PLC was all-important over aloof broadcast control. With PLC prices activity down, admeasurement shrinking, and achievement of PLCs convalescent over the years this has become beneath of a battle. Yet the designer has to ask themselves if a PLC is absolutely abstract for their application. Some questions should be asked.

• Is there a need for flexibility in control logic changes? Will there be frequent control logic changes? Will there be a need for rapid modification? A lot companies believe they will never change a design but more often then not ideas and goals do change and modifications will need to be made. Do you want to do that in hardware (relays) or software (PLC)?
• Must similar control logic be used on different machines? It's so much easier to download a program then build another panel.
• Is downtime a concern? Any change or troubleshooting on a relay system means the system might have to go offline. Changes in a PLC can often be made online with no downtime.
• Are space requirements important? Based on the number of relays a PLC can be a real space saver.
• Are increased capability and output required? PLCs can be faster then their mechanical counterparts.
• Are there data collection and communications required? Only possible with a PLC or computer.
• What are the overall costs? There's a certain price point comparison but in these days it's very low in favor of a PLC.

• Is there a need for flexibility in control logic changes? Will there be frequent control logic changes? Will there be a need for rapid modification? A lot companies believe they will never change a design but more often then not ideas and goals do change and modifications will need to be made. Do you want to do that in hardware (relays) or software (PLC)?
• Must similar control logic be used on different machines? It's so much easier to download a program then build another panel.
• Is there a need for future growth? A PLC can easily accept a new module in a slot or get an expansion base.
• Is there a need for high reliability? PLCs are seen as more robust over individual components.
• Is downtime a concern? Any change or troubleshooting on a relay system means the system might have to go offline. Changes in a PLC can often be made online with no downtime.
• Are space requirements important? Based on the number of relays a PLC can be a real space saver.
• Are increased capability and output required? PLCs can be faster then their mechanical counterparts.
• Are there data collection and communications required? Only possible with a PLC or computer.
• What are the overall costs? There's a certain price point comparison but in these days it's very low in favor of a PLC.

PLC vs. Dedicated Controller

A dedicated controller is a single instrument that is dedicated to controlling one parameter such as a title="Acronym for Proportional, Integral, Derivative. Control algorithm that is used to closely control processes such as temperature, mixture, position, and velocity. The proportional portion takes care of the magnitude of the error. The derivative compensates for the rate of error change. The integral takes care of small errors over time.">PID controller measuring a temperature for heating control. They have the advantages of an all in one package, typically with display and buttons. This can be a very good thing to use in simple applications. A PLC these days can compete price wise and functionally with these controllers especially if you more then one controller is needed. PLCs offer a greater degree of flexibility too because the can be programmed to handle all sorts of different scenarios.

PLC vs. PC (Personal Computers)

The PLC vs. PC debate has been going on for years and I'm not going to attempt to give the definitive answer. They both have their pros and cons. What often happens is that the two are used for their strengths in different parts of the factory.

 

Â	PLC	PC
Environment	The PLC was specifically designed for harsh conditions with electrical noise, magnetic fields, vibration, extreme temperatures or humidity.	Common PCs are not designed for harsh environments. Industrial PCs are available but cost more.
Ease of Use	By design PLCs are friendlier to technicians since they are in ladder logic and have easy connections.	Operating systems like Windows are common. Connecting I/O to the PC is not always as easy.
Flexibility	PLCs in rack form are easy to exchange and add parts. They are designed for modularity and expansion.	Typical PCs are limited by the number of cards they can accommodate and are not easily expandable.
Speed	PLCs execute a single program in sequential order. The have better ability to handle events in real time.	PCs, by design, are meant to handle simultaneous tasks. They have difficulty handling real time events.
Reliability	A PLC never crashes over long periods of time. ("Never" may not be the right word but its close enough to be true.)	A PC locking up and crashing is frequent.
Programming languages	Languages are typically fixed to ladder logic, function block or structured text.	A PC is very flexible and powerful in what to use for programming.
Data management	Memory is limited in its ability to store a lot of data.	This is where the PC excels because of it's hard drive. Any long term data storage, history and trending is best done on a PC.
Cost	Just too hard to compare pricing with so many variables like I/O counts, hardware needed, programming software, etc.

 

Hybrids of PLC/PCs are common now (e.g. WinPLC). This type of hardware tries to mix the two platforms using the strengths of both. So the CPU might be able to run Windows CE or Linux in a rack that can accept common I/O modules.

Off the Shelf vs. Build Your Own

For some manufacturers the choice these days is coming down to buy a PLC or make your own. The benefits of PLCs have become so widely known that manufacturers looking to cut cost can engineer their own solutions and build them more cost effectively. An example of this is the Divelbiss "PLC on a Chip" with the accompanying EZ LADDER programming software.

Please visit us at  http://plc2u.com/index.php/plc-article/plc/plc-vs-other-type-of-control

It’s inevitable. Left on your board is an old PLC agenda with a chicken post-it agenda on it that artlessly says, “Bad Card”. So you assay it out and abiding abundant article is mysteriously amiss with the cyberbanking audacity of this big-ticket little device. Your aggregation may accept some action for abortion assay and backup parts, but, added generally than not, there’s a chiffonier or drawer about with a lot of bad parts, bashed with cryptic adhesive notes, that you don’t accept the affection to bandy out. As those genitalia get added numerous, adored and cher it makes faculty to analysis into companies that specialize in PLC repair.

Please visit us at  http://plc2u.com/index.php/plc-article/plc/repair-plc

PLC Repair Companies

There are absolutely a few companies whose capital business focus is electronics adjustment including PLC boards. Some of them are alike absolutely PLC accouterments accompanying but best additionally account drives, DCS accessories and added ascendancy devices. Not alone will they adjustment PLC genitalia but they additionally accept a ample accumulation of surplus (new in box) and refurbished genitalia for replacement. This abnormally becomes important in earlier bequest systems. Alike admitting genitalia can be upwards of thirty years old they still abutment aliment on the brand of Allen Bradley PLC-2/PLC-3, Modicon 484/584, Siemens S5/S7 and Texas Instruments 500/505. PLC adjustment companies action the experience, assets and the abstruse libraries of schematics and manuals that would contrarily be adamantine to appear by.

Am I a Candidate?

Basically any aggregation complex in PLCs can account from PLC repair. Mid-size to ample factories are archetypal candidates but arrangement integrators and resellers additionally apprehend the advantages. The charge for bound controls, minimum blow and acid costs accomplish PLC adjustment needful for all sorts of industries from packaging, aliment processing, pharmaceutical, automotive and so on.

If you assignment in an earlier bulb you apperceive how able-bodied a PLC can be alive year afterwards year. You additionally apperceive as a artisan that your accessories will eventually fail. It’s not a amount of “if” but “when” and that’s back you’ll charge adjustment services. One of the problems is that the amount of advance is aloof not account it back it comes to factoring in removing the old system, programming the fresh one, installing hardware, alive out the bugs and acquirements how to use the fresh system. Having a reliable adjustment account ensures that you can accumulate the arrangement active at basal cost.

How Much Can I Save?

First of all, it doesn’t amount annihilation to acquisition out how abundant the accident will be in dollars. The majority of the PLC adjustment companies do not allegation annihilation for ciphering a adjustment cost. You would alone be out the amount of shipping.

Second of all, there are no accuse for time and materials. A archetypal adjustment account has a database of genitalia and their costs. These companies accept been about for awhile and accept accumulated the ability of archetypal genitalia that go bad and accordingly accept a abject amount for repairs.

The basal band is that it should amount you no added the 50% of the amount of new. Greg Carter of Radwell International says, “We alarm an account ‘Beyond Economical Repair’ aback the amount of genitalia and activity beat 45% of affairs new. We try to accomplish adjustment a actual adorable advantage --- 73% of items are angry about in 5 canicule or less.” Aback I asked Willy Williamson of Electrical South about a aphorism of deride he said, “Repairs should about not amount added again 50% of the amount of a fresh archetype of the aforementioned allotment number. If you run into instances area you apperceive a fresh one can be bought for beneath than the quoted adjustment price, allocution to your sales representative.”

If a allotment is accounted as un-repairable, as a aback up, these companies action refurbished or surplus genitalia at cogent savings. Bill Zalot, President of TEK Supply says, “In best cases if a assemblage is un-repairable for any reason, we are able to accommodate a backup for the chump at a amount alone hardly added than adjustment cost.” These surplus or repaired genitalia are additionally acceptable avenues to accomplish abiding you accept a aback up on your shelf for emergency situations.

What about Quality?

Of advance acclimation a allotment may not be as reliable as affairs a cast fresh one. The added acceptable adjustment companies admitting booty a abundant accord of pride in their assignment and ensure a affection repair. A acceptable adjustment aggregation will action a assurance on the allotment or alike the accomplished PLC. It seems the about assurance runs for one year but they can differ. It’s important to pay absorption to the assurance and analyze it to what others are offering. It’s additionally account allurement about their testing and analysis procedures. Make abiding they accept the adeptness to put the allotment in a arbor to absolutely exercise it afore sending it aback to you. How important is this? “We go to abundant lengths to absolutely exercise the PLCs we repair. We use analysis programs to absolutely exercise the memories and functions of the PLC. We absolutely don’t appetite to see it appear aback as a warranty. We additionally adopt to accumulate our barter advancing aback to us with added repairs, and acceptable affection is a above contributor to abiding relationships”, said Willy Williamson of Electrical South.

This handbook is a accumulating of programming overviews, notes, helps, bluff bedding and whatever that can advice you (and me) affairs a Siemens PLC.

• Siemens Website Quick Links
• Review of Siemens SIMATIC Step 7 Lite Programming Software
• Step 7 Connecting, Downloading and Uploading
• Step 7 Lite, Step 7 and Step 7 Professional Differences
• The Case of the Missing SIMATIC Step 7 Documentation
• S7 Library Functions
• Siemens Technical Terms
• Step 7 Elementary Data Types
• Symbol Table Allowed Addresses and Data Types
• Statement List (STL) Cheat Sheets
• Connecting IFIX SCADA to Siemens S7 using TCP/IP

For years now, Object Oriented Programming archetype (or OOP) has been a frequently acclimated programming practice, and has of advance begin its way into automated automation as well.

In the beggarly time, best PLC manufacturers accept begin agency to accomplish the programmers activity easier by introducing User Defined Types or UDTs. The name says it all; it is a ‘type’ that you, the programmer, can ascertain all by yourself. This agency that your programming ambiance will not alone accept the approved integers (INT) and Booleans (BOOL), but could additionally accept a ‘VALVE’ blazon or a ‘MOTOR’ type.

I can’t allege for added brands of PLC’s, but the Allen Bradley ControlLogix alternation of PLC’s, calm with RSLogix 5000 programming software, makes it actual accessible to assignment with these UDTs, and back the addition of RSLogix adaptation 17 beforehand this year, it is now alike accessible to adapt your UDTs while online with a active system.

The Controller Organizer has a binder alleged Data Types > User-Defined with all the UDTs in the project.

I am of the assessment that every PLC affairs should await heavily on UDTs to advance readability, and if you are an OOP adept, it can be a abundant advice to adapt your classes.

Let’s go over the fundamentals of OOP for a little bit:

• Classes: Classes define the abstract characteristics and behavior of an object. For example, a simple ‘VALVE’ class would have the characteristics (or attributes) that it can be open or closed (the things it can be), and as far as behavior goes, it could have the methods ‘to open’ and ‘to close’ (the things it can do)
• Objects: An object is an instance (occurrence) of a class. In our example, there could be a Valve_001 and a Valve_002, which are both instances of the class ‘VALVE’, with the same attributes and methods.

Of course the definition of OOP goes a lot further than this. There is a very understandable explanation here: http://en.wikipedia.org/wiki/Object-oriented_programming#Fundamental_concepts for those who would like to read more. For now, let’s leave it at this, and see how we can apply this to an industrial environment.

If you look at a valve as an object in a typical industrial automation environment, you should note the following:

• It has inputs and outputs that are specific for the object (proximity switches and solenoids).
• It can be either ‘open’ or ‘closed’
• You can tell it to go ‘open’ or ‘close’.
• It could have an alarm timer, that would tell us if the valve did not open or close in a given time period after a command.
• It might have interlocks, which allow the valve to open or close under certain conditions.

A UDT for this class, would fit all these backdrop and methods in one simple type. But, as always, we can apprehend added complications of the chic ‘VALVE’ during the ability of a project. To be as adjustable as possible, I awful acclaim the convenance of nesting UDT’s, which will become bright forth the way.

Let’s alpha with defining our class, and accumulate in apperception that it will accept to be calmly attainable for aliment bodies or added programmers.

If we alpha at the I/O end, the best adjustment is to actualize sub-classes alleged VALVE_IN and VALVE_OUT, which will accommodate our I/O.

The afterward archetype uses RSLogix5000 V16. First, actualize the sub-classes. From the File card baddest New Component > Tag. The afterward chat box appears to actualize and adapt the associates of the UDT.

As you see, I am accustomed to booty the types I aloof created as the abstracts blazon in this UDT. The absolute advantage of this affection will become bright if you actualize a article alleged Valve001 of the blazon VALVE, and attending at the article in the ‘monitor tags’ window:

Wow! Just by creating a new tag of the type VALVE, it gets all these I/O points right away, and referenced in the program:

Of course, going further with this concept, everything for a valve can be included in one object. Allow me to skip some steps, and show you a possible final result:

The ‘VALVE’ class is now contained in a UDT called VALVE, which looks like this:

As you see, the class VALVE now consists of the sub-classes VALVE_IN, VALVE_OUT, VALVE_TIMER, VALVE_STATUS, etc.

And an instance of this class, the object Valve001, would look like this:

While adding stuff to my class, I did not have to re-create the object Valve001. RSLogix updated it for me, so all the properties and methods are available in my program.

Now, let’s say you’re working on this project with a couple hundred valves, and the customer decides to go with a different type of valve, that also has an analog input, that tells us the exact position of the valve. All we have to do is modify our VALVE_IN sub-class to add this to every instance of the type VALVE:

Of course, you would still have to write code to tell your program what to do with that information, but that is also the reason why PLC programmers still have a job.

For somebody that is not familiar with your program, it might be confusing to look at all your UDT’s. We just made eight UDT’s for one simple valve class! But remember, you only have to do this during the design phase. Once you have a solid design for all your classes (and made sure their names are self-explanatory), you will never have to look at your UDT folder again, and creating a new instance will be a breeze.

Please visit us at  http://plc2u.com/index.php/plc-article/plc/user-defined-data-types-udts-and-oop

NFPA 79 2002 Edition reference

Table E.1 Device and Component Designations ABE Alarm or Annunciator Bell ABU Alarm or Annunciator Buzzer AH Alarm or Annunciator Horn AM Ammeter AT Autotransformer CAP Capacitor CB Circuit Breaker CI Circuit Interrupter CNC Computerized Numerical Controller CON Contractor COs Cable-Operated (Emergency) Switch CPU Central Processing Unit CR Control Relay CRA Control Relay, Automatic CRH Control Relay, Manual CRL Control Relay, Latch CRM Control Relay, Master CRT Cathode Ray Tube, Monitor or Video Display Unit CRU Control Relay, Unlatch CS Cam Switch CT Current Transformer CTR Counter D Diode DISC Disconnect Switch DISP Display DR Drive EMO Emergency (Machine) Off Device END Encoder ESTOP Emergency Stop FLD Field FLS Flow Switch FS Float Switch FTS Foot Switch FU Fuse GEN Generator GRD, GND Ground GUI Graphical User Interface HM Hour Meter HTR Heating Element IC Integrated Circuit INST Instrument IOL Instantaneous Overload I/O Input/Output Device L Inductor LED Light Emitting Diode LS Limit Switch LT Pilot Light LVDT Linear Variable Differential Transformer M Motor Starter MD Motion Detector MF Motor Starter - Forward MG Motor Generator MR Motor Starter - Reverse MTR Motor OIT Operator Interface Terminal OL Overload Relay PB Pushbutton PBL Pushbutton, Illuminated PC Personal Computer PCB Printed Circuit Board PEC Photoelectric Device PL Plug PLC Programmable Logic Controller POT Potentiometer PRS Proximity Switch PS Pressure Switch PWS Power Supply Q Transistor QTM Thermistor REC Rectifier RECP Receptacle RES Resistor RH Rheostat S Switch SCR Silicon Controlled Rectifier SOL Solenoid SNSR Sensor SS Selector Switch SSL Selector Switch, Illuminated SSR Solid State Relay ST Saturable Transformer SUP Suppressor SYN Synchro or Resolver T Transformer TACH Tachometer Generator TAS Temperature-Actuated Switch TB Terminal Block T/C Thermocouple TR Timer Relay TSDR Transducer TWS Thumbwheel Switch V Electronic Tube VAR Varistor VM Voltmeter VR Voltage Regulator VS Vacuum Switch WLT Worklight WM Wattmeter X Reactor ZSS Zero Speed Switch

Please visit us at  http://plc2u.com/index.php/plc-article/plc/how-to-symbol-program-with-allen-bradley-rs-logix-500

Your symbols will start to pop up in a quick pick list. Arrow down to find your proximity switch if you can't remember it, or better yet keep typing it all out and press enter when done. Viola! You've just typed your first symbol program.

Other symbols programming tips I can offer is a grouping of symbol names such as STATION 1 could mean S1. If you are state logic programming, you might remember a station more so by its function. Let's say you have a station on some automation that presses a part assembly together. I would call all the internal bits by "PRESS" to indicate all these bits that begin with PRESS have the same functionally with my press station.

The fun doesn't stop there. Can't remember which control relay enables the fast speed index? Just type CR to bring up the list of all your control relays.

How about when you are creating bits that transfer information from your Panel View to your PLC? You could also group all those symbol names together with a PV.

I hope you can see based on the samples shown that trying to memorize your program by the address names is pretty tough. Giving each of your symbols a name and you should be able to program faster than before. Group your symbols names to speed up symbol searches. Come up with your own "short hand" that you can remember easily or use the NPFA chart for some ideas.

Symbols are used in RS Logix to identify a bit or memory area of a PLC by a name that you can more easily remember. See if you can see the symbols in this screen shot:

If you haven't found them by now, they are the descriptions in GREEN.

AUTO_MODE, RUNNING, POWER_ON, ALL_HOME, INDEX_ON_STAT, etc...

RS Logix makes it extremely easy to use symbols in your program. When you enter a new instruction with a question prompt, you can start typing your symbol name and a pop out window will start narrowing your search down to the symbol you have already created in the database. Notice I said "already created". That means it would be a good idea if you made up all your symbols ahead of time using the spread sheet template downloadable here

That's ok that you can't do them all before you start. Just keep creating them on the fly with terms or "short hand" that you can remember. Here are some tips for common short hand naming that I use myself.

When I am programming a hardware device such as a proximity switch or a photoeye input, I use the NFPA's Table E.1 Device and Component Designations as guidance to name all my hardware I/O. Below is a list of codes at the end of this article. This bascially means if I have a proximity switch connected to I:1.0/14 I could give it the symbol name

PRS_TRANSFER_EXT

PRS comes from the NFPA table which means "Proximity Switch TRANSFER means that this prox is connected to a transfer device of some sort EXT means that the direction the prox is detecting is You aren't required to use the NFPA's component designation, but this will give you some bases to start if you need ideas. More recently I have tried to trim my own symbols to the first two letters so I can drill down my symbol list very quickly in RS Logix. I've changed my own personal preference to PX for Proximity Switch. So if I enter an instruction in a rung, I can just type two letters "PX" at the question mark and low an behold EVERY SINGLE proximity switch on my automation system will come up in my list to choose from.

File Information
Name: InView 2706-P42C w/Ethernet IP ASCII Example Code Author: chakorules Submitted By: chakorules Submitted: Wed, Mar 18 2009 10:40 am Views: 218 Downloads: 60

Download File