# Help with understanding PLC's in the field



## kg7879 (Feb 3, 2014)

I am working at a refinery and it looks like i will have to deal with some controls.

I understand basic ladder logic and I had to hook up a forward/reverse starter when I took my JW test a couple of years ago.

What I can not grasp my mind around is PLC's in the field. So for example what is the plc replacing or augmenting in a basic motor control circuit with a stop/start push button? 

My basic plc understanding is the push button would send an input signal to the plc and the plc would send an output signal to the coil to energize it. Is this correct? If so I do not understand the advantage of even having a plc.

Any help is appreciated.


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## xpertpc (Oct 11, 2012)

The PLC does not replace the inputs (switches and such) unless you go with an HMI like a panelview, the outputs are controlled either directly or with interposing relays.

The PLC is nothing but a black hole of voodoo that has all your relay timer logic tied together with one Coulomb of Higgs boson particles, which has a net weight of a photon at C or there about.


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## glen1971 (Oct 10, 2012)

To name a few....
PLCs also allow one input to do multiple operations alot easier than hard wiring to relays or contactors... Most will also save on wiring, as any alarms can be programmed instead of hard wiring to an indicator lamp.. They are alot more flexible and have the capability for better PID loop controls..


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## kg7879 (Feb 3, 2014)

I have no idea what a PID loop control is. 

So in a basic motor control example, could a plc time a motor to start after the start button is pushed without the use of a timer relay?

I am sorry if my questions are dumb. I tried to google this stuff and I either get over complicated programming answers or beginner ladder logic stuff. 

Just to clarify, I will not be programming anything. All I will be doing is install instruments and controls out in the field. I just want a better understanding of it.


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## dronai (Apr 11, 2011)

PLC has all internal relays, timers, counters, math, etc, so you eliminate the need for endless relays in the field.


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## xpertpc (Oct 11, 2012)

Absolutely - a plc can do all that with a minimum of wiring and the flexibility to make changes with the click of a mouse.

Don't fret about PID's they are part of the voodoo I spoke about, few people on earth really understand them and just hit auto-tune.


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## 51360 (Jun 9, 2014)

xpertpc said:


> Absolutely - a plc can do all that with a minimum of wiring and the flexibility to make changes with the click of a mouse.
> 
> *Don't fret about PID's they are part of the voodoo I spoke about, few people on earth really understand them *and just hit auto-tune.


PIDs are not that complicated, especially with the help of PLC software. Few loops actually use the " D " part of equation. :thumbsup:

Instrument guys like us all to think PIDs are complicated though. 

Borgi


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## 51360 (Jun 9, 2014)

kg7879 said:


> I have no idea what a PID loop control is.
> 
> So in a basic motor control example, could a plc time a motor to start after the start button is pushed without the use of a timer relay?
> 
> ...


Actually, that's an excellent attitude. Take some courses and you will do fine! :thumbsup:

Borgi


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## just the cowboy (Sep 4, 2013)

*Automation direct has free software*

AD has free HMI software that lets you simulate your project it's C-MORE micro.

And the Direct soft PLC software is free for up to 100 words of program, that is enough to learn on.

http://www.automationdirect.com/adc/Home/Home

Lots of free demos and videos


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## bill39 (Sep 4, 2009)

It's nice to find someone interested in learning. Seems like you have the right attitude.

Here are two PLC websites that are good places to learn and ask questions. I like the first one best. It also has a PLC simulator.
http://www.plctalk.net/qanda/forumdisplay.php?forumid=2
http://forums.mrplc.com/

Similar to what the others have said, in a PLC system to control a reversing MS you have described the following hardware would still be needed in the field:

Reversing motor starter with"
* a coil for forward (wired to a PLC output #1)
*a coil for reverse (wired to a PLC output #2)
Start PB (wired to PLC input #1)
Stop PB (wired to PLC input #2)
Aux contacts from Fwd MS ((wired to PLC input #3)
Aux contacts from Rev MS (wired to PLC input #4)
OL contact (wired to PLC input #5)
2-position Fwd/Rev Selector switch with:
*one contact block for forward (wired to PLC input #6)
*one contact block for reverse (wired to PLC input #7)

Note: realistically you wouldn't have to have the 2nd contact block on the Fwd/Rev switch. You could program it with only one input, but let's take one step at a time right now.

Any timers or counters would only have to exist in the PLC program.

Hope this helps.


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## Michigan Master (Feb 25, 2013)

Very basic explanation here, with pictures, but a real good starting point.
http://www.allaboutcircuits.com/vol_4/chpt_6/6.html

No black magic or voodoo, although there _are_ those imaginary bits. 

For a PLC application there will likely be a lot more than just a motor control circuit; the advantage is, the PLC program is able to monitor and control _many_ different things without the need for a ton of hard-wired relays or timers, and logic changes can be made much more quickly.


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## gnuuser (Jan 13, 2013)

plc systems only need to hard wired once 
but with the ability to change a machines behavior via program 
the flexibility of the program is what allows for the changes required without tearing everything apart and rewiring over and over to change an operation
(been there and done that too much)

hardwired timers are dependable but has its limits.
you can write some extremely complex programs to make your machine operate in the manner you choose 

take any opportunity to get training in plc/slc 
design your program on paper first 
then write the program (verify each rung)
test your program first with no one near the machine.
and always keep in mind all the safety equipment that you will need to configure.


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## ScooterMcGavin (Jan 24, 2011)

So that you can work on this:









Instead of this:








Another huge advantage of PLC's is that you can see the logic in real time on a computer as its happening and trace problems a lot easier. If setup properly you can even look at and change logic from home and save yourself from having to go into work when there is a problem. With hard wire relays you have to use a meter to check logic or use tell tale lights. 

If you're going to being working with field instrumentation take a look at this free ebook from Tony Kuphaldt. This will set you straight on any questions you may have.

http://www.ibiblio.org/kuphaldt/socratic/sinst/


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## JRaef (Mar 23, 2009)

> _I have no idea what a PID loop control is. _


You probably do, but you might not realized it.

PID-101:
PID stands for Proportional, Derivative, Integral control, a way of exerting steady state control over some variable process to avoid having it over correct. That's the geeky answer, here's the common example almost everyone has used at one point or another:

_*Cruise control in your car is a PID* Controller.*_ When you hit the "Set" button, you are programming the "Setpoint Variable" (SV) in a PID loop controller. That means, this is the steady state that you want to maintain, let's say 80MPH. Once you do that, you are letting the controller take over operation of your throttle, by virtue of a throttle servo controller, that regulates your engine output (if you've ever rested your foot on the gas pedal while in cruise control and feel the pedal get pulled down under it, that's the throttle servo motor). This is called the "Control Variable" (CV), the action that you take to maintain the SV. Then in order to see what the heck is going on, the cruise controller looks at your speed using a speedometer sensor, called the "Process Variable" (PV) that is telling your cruise controller how fast the car is actually going. So on flat ground, everything goes along smooth. If you start to go up a small hill however, the speedometer sensor tells the controller that the car slowed down to 75MPH, the controller tells the throttle servo to goose the gas a _little_, the engine puts out more HP and gets the car back up to 80MPH. The response, the little goosing of the gas, is PROPORTIONAL to the fact that the speed only dropped by 5MPH. If it was a steep hill and the car started slowing down to 50MPH, the controller would tell the throttle to go down hard to goose it more. 

And if the change in speed happens really quickly, the controller INTEGRATES the change in speed and the length of time in which it changed to make sure the throttle responds _quicker_. 

The DERIVATIVE (D*) function on a PID controller doesn't relate to most cruise controllers, until you get into maybe Caddy or Benz price ranges. What that would do is start to track the number of speed changes over time so as to start predicting how often it is changing and _anticipate _it rather than wait for the change to happen.

I know this is way more than you were looking for, I just like dispelling this fear factor people have when it comes to PID loop controls. Engineers tend to try to describe them to people with math formulas, which makes them scream and run away. But a LOT of people have used simple things like cruise control without ever having to make one from scratch. Let the geeks do the math, we just set it and use it.


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## Cincycaddy (Sep 18, 2014)

Max Rabiee was my professor for two PLC Classes when I was in college for Electrical Engineering Technology. Below is one of the books he has written. I believe it may also come with a SLC 500 simulator or license to download one. It's been 7 yrs since I took his classes. Anyway he does a pretty good job of explaining how PLCs work and the kind of things that can be accomplished using them. 

http://www.amazon.com/Programmable-Logic-Controllers-Hardware-Programming/dp/1605259454#


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