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Old October 21st, 2007, 12:12 PM   #1
Doug Hylton
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Hard wiring the PLC

Donít put logic in the I/O wiring
I know that its tempting to save some money and hardwire some contacts, selector switches and other stuff between the I/O device and the PLC but resist this temptation.

If you hardwire logic in your I/O wiring the chances are good that during start up you will be out there on the machine pulling wire instead of changing the program. Run everything directly to the devise or sensor. It makes the wiring simple and you will never regret it.

I first started programming PLCís on pulp bale finishing lines and all of the manual jog buttons were hard wired to the motor starters and only the automatic mode was done by the PLC. The idea was that if the PLC failed you could run the machine with the pushbuttons. I have seen I/O modules fail but I have never seen a PLC fail except for one and that one was connected to 220 Volts and it did smoke.

The problem with only part of the system wired to the PLC was that during start up we always ran into something we couldnít do because the device was not wired to the PLC.

Sinking or sourcing
I personally like sinking the inputs and sourcing the outputs. I think that sinking inputs are less susceptible to noise. A sourcing input is sitting there low all the time waiting for a high and it could get the high off of a wire laying next to it from a solenoid valve when the field is collapsing. A sinking input is sitting there high and its waiting for a low. I would source the outputs just to make it less confusing. The Japanese like every thing sinking and you see it more in the electronics field.

What happens when you hit the ESTOP
There are two schools of thought here. One is to just kill everything and the other is to leave the processor and inputs powered up and only kill the output power. Just killing the output power has an advantage if the machine parts or product can be moved manually with the ESTOP on. The PLC can then keep track of whatís going on and you can start back up again without a problem.

In the semiconductor manufacturing world the rules are you have to kill everything. Be careful about switching the negative. It not only makes trouble shooting a nightmare but in most places its illegal too.

If you have big motors running with frequency drives its safer to keep the power on the drive and allow the drive to decelerate the motors. Donít put contactors between the frequency drive and the motors. This is a no no and you can damage the frequency drive. Killing the power to a frequency drive with the motor running can also damage the drive.

Remember that the ESTOP is not just for emergencies often the operators will hit the ESTOP just to go to lunch or take a break.

The drawing
I like to use the I/O number for the wire number. You cant always do this because some customers have there own wire number convention but it makes trouble shooing easier when no translation from wire number to I/O point is needed. We say wire number but actually itís a circuit number. All conductors that are electrically the same are the same circuit number.

Retentive Memory
If you have the PLC keeping track of the machine positions and product on the conveyors all the bits and registers used for that purpose should be in retentive memory. Look at your PLC manual and find out what points are retentive. With retentive points when the power goes down the PLC will maintain the point status and when the power comes back on your machine can start up at the same place in the logic where it was shut down. Otherwise you might have to clear out the machine and start all over again. This can be all right in some cases but if you are moving ten ton rolls of paper it will be a problem.

Safety Circuits
If you have safety circuits for the protection of human life, bodily injury or major machine damage donít put them in the PLC. There are too many things that can go wrong with a PLC for safety systems. Just going into program mode will disable the circuit. Hard wire all safety circuits outside of the PLC.

Hot backup
You can use a PLC for safety systems with a hot backup. A hot backup is where you have two PLCs in the system with an identical program all powered by a UPS. Then you have a detection system that will swap PLCís if it detects the first one has failed. This is expensive but if you are using a PLC in a HAZMAT building for detecting gas leaks itís the way to go.

Startup
Donít even try starting up until you have checked out all the I/O. Do not load the program. Go through all the inputs one by one and verify the input is working. Have someone operate the device and watch the LED on the PLC. Then mark that circuit as OK on your drawing. Then force all the outputs on one by one and verify that its doing what its suppose to do. The reason for not loading the program is that for some PLCís an output cannot be forced on if there is a program in there telling the out put to turn off. Take care to mark any changes made to the hard wiring on the drawing.

Doug Hylton










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Last edited by Doug Hylton; October 21st, 2007 at 12:31 PM. Reason: spelling
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Old October 21st, 2007, 12:23 PM   #2
Tharon
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One quick question:

Quote:
The reason for not loading the program is that for some PLCís an output cannot be forced on if there is a program in there telling the out put to turn off.
Doesn't that defeat the purpose of a "FORCE"?
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Old October 21st, 2007, 12:53 PM   #3
PeterW
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Quote:
Originally Posted by Doug Hylton
Donít put logic in the I/O wiring
You quite often find hand/manual functions hard-wired, I don't see anything wrong with it as long as the hand/auto switch gives a signal to the PLC to alarm that its not in auto control.

Quote:
Originally Posted by Doug Hylton
What happens when you hit the ESTOP
I'd never kill the PLC, what does that achieve? The fuction of the E-Stop is to lose 'motive' power.


Quote:
Originally Posted by Doug Hylton
If you have big motors running with frequency drives its safer to keep the power on the drive and allow the drive to decelerate the motors. Donít put contactors between the frequency drive and the motors. This is a no no and you can damage the frequency drive. Killing the power to a frequency drive with the motor running can also damage the drive.
That is correct.

Quote:
Originally Posted by Doug Hylton
Remember that the ESTOP is not just for emergencies often the operators will hit the ESTOP just to go to lunch or take a break.
ARE YOU CRAZY?

The E-Stop should only be used in an Emergency, operators who press an e-stop to go for a break should be educated on the purpose of the E-Stop and if they persist send them out of the door.

I realise that safety has not hit the North American continent yet, I'm working on systems where they zone the E-Stops, so you need to know what the E-Stops stop, this is crazy, you see someone in trouble you want the press the NEAREST E-Stop.

The level of the safety system should be determined by a risk assessment.

If operators abuse E-Stop's where does it stop, press an E-Stop then stick you hand in out of site of the E-Stop?

Quote:
Originally Posted by Doug Hylton
The drawing
Or related directly to a drawing and line.

Quote:
Originally Posted by Doug Hylton
Retentive Memory
Pure choice on what you are doing.


Quote:
Originally Posted by Doug Hylton
Safety Circuits
On the whole I agree, although there are machine builders (Krones is one) that certify safety logic and partially put the safety into the PLC code. You cannot change the code.

Also nowadays there are safety PLC's (like PILZ for example) where all the safety is done safely in the PLC.

I don't like what Krones have done and would never sanction safetyu code in a normal PLC but safety PLC's are a different animal made for purpose.

Quote:
Originally Posted by Doug Hylton
Hot backup
horses for courses, very few applications I have worked on would warrant one.


Quote:
Originally Posted by Doug Hylton
Startup
standard commissioning really, athough saying that when using a Siemens I would normally load the full program and place a BEU at the start if there was no interupt blocks switching I/O.

(EDIT: Actually the reason I wouldn't want the program to be running is because I wouldn't want an uncontrolled accident to happen if something wasn't wired correct)


So

Doug Hylton

What prompted you to suddenly enlighten us with your phylosophy










.[/quote]
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Old October 21st, 2007, 12:55 PM   #4
OkiePC
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My first challenge as a PLC programmer was dealing ISSC 8 bit controllers with a hardwired manual mode


these 16k ram machines were flaky as hell, and controlled a tire assembly machine sequence that was very complex and detailed, but when the CPU puked, it was intended that a person could "jog out tires" with buttons and selector switches controlling stuff in parallel with the PLC.

When we migrated to PLC5 they had 10 machines done with six evolved versions of other modification mixed in. They were each different, with various other modifications, but they were also wired in parallel with the manual controls, just like the junk they replaced! It sucked big time. I am talking about 20 year old safety relays and mode control realys and pushbuttons with 12 sets of contacts on them. It really sucked.

I eliminated most of that with my 1st iteration after I helped startup AB machine numbers 11 and 12 of 36 total.

There are rare cases where you want series or parallel inputs, and hardwired overtravel limits between the brain and some loads, but aside from that, I agree, put the logic in the controller, except for personnel safety.
Nice write-up!
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Last edited by OkiePC; October 21st, 2007 at 12:58 PM.
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Old October 21st, 2007, 01:07 PM   #5
Stationmaster
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PeterW,

I'm enjoying the philosophical debate immensely, but you lost me a little bit:

Quote:

If operators abuse E-Stop's where does it stop, press an E-Stop then stick you hand in out of site of the E-Stop?

end quote.

BTW, I work at a plant on occasion where they have to have raised LIGHTS on the E-stop locations so one can determine which estop has been "abused".

Stationmaster
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Old October 21st, 2007, 01:17 PM   #6
PeterW
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What I meant was that E-Stops are EMERGENCY STOPS and as such should only be used in an Emergency, not to stop for another reason.

If people get used to pressing an E-Stop whenever they want to stop the system, then imagine someone seeing something caught in a conveyor, thinking he will do a favour and quickly sort it, presses the nearest E-Stop and jumps over the conveyor and starts clearing it, then ducks under to get at something underneath.

In the meantime someones walking the line to see why its stopped, see's the E-Stop pressed, curses whoever pressed it for messing about and resets the E-Stop. Ouch.

The point is the operation should not get used to the use of E-Stops for anything other than an Emergency.

In the time I have been in Canada I have seen lines that autoimatically restart on the E-Stop being pulled out, criminal in my mind.



Regarding the phylosophical debate, I was wondering if Doug was bored or frustrated at something to suddenly spill it all out.
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Old October 21st, 2007, 01:27 PM   #7
rsdoran
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Quote:
Doug Hylton

What prompted you to suddenly enlighten us with your phylosophy
I am curious about that too. For the most part if a new member takes the time they would see just about all these points have been discussed many times over the years.

ESTOPS, you mentioned risk analysis, most companies (at least Euro, UK, Canada, and US) know now there are categories for EStops that apply depending on the application. As mentioned Estops should not be used to go to lunch etc.
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Old October 21st, 2007, 01:35 PM   #8
Stationmaster
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PeterW,

OK, now I get what you were trying to get across. And I agree.

At least the plant I was referring to requires a bell to be rung 3 times before starting. I guess if no one yells during the startup bell they figure it's ok to start the system. I know they have power lockouts near the motors and conveyors....if they use them. I try to keep my head buried in a panel and stay out of the machine area if I can. It's out of my control......I just get called when needed. The owners/managers make those decisions, I just try to make the control system do what they want it to.

Stationmaster
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Old October 21st, 2007, 03:08 PM   #9
CJones
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Well, I'm no expert

But where else would you put the contactor, you mentioned never after the inverter, that leaves in front of the inverter which seems worse by far.

Even if you didn't use a contactor but had overloads shouldn't they go after the inverter?

Many times we will use a 20hp inverter and have 4 motors on a line with the one inverter powering all 4 motors. contactors after the inverter seems like the only way to go.

Clint
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Old October 21st, 2007, 03:28 PM   #10
PeterW
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Quote:
Originally Posted by CJones
Well, I'm no expert

But where else would you put the contactor, you mentioned never after the inverter, that leaves in front of the inverter which seems worse by far.

Even if you didn't use a contactor but had overloads shouldn't they go after the inverter?

Many times we will use a 20hp inverter and have 4 motors on a line with the one inverter powering all 4 motors. contactors after the inverter seems like the only way to go.

Clint
The only contactor required for an inverter, would be the power on contactor, which could be the safety contactor, which would be BEFORE the inverter.

Doug is quite correct, you should not put a contactor between the inverter and the motor. You use the control signals to RUN/STOP the motor, not the contactor.

Overloads also go before the inverter as you are protecting the inverter, the inverter protects the motor.

Where the inverter supplies multiple motors, each motor should have its own protection after the inverter (in addition to the inverter protection), but if one trips the inverter should be stopped.

We had a HMI selection where we selected if a drive was enabled or not, therefore if one had a fault and tripped the feedback would be different to selection and the inverter commanded to stop. If the motor had a fault, then the HMI would be used to de-select that drive to enable restart without it. The breaker would be locked out. If someone turned on a breaker that was de-selected then again the inverter stopped.
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Old October 21st, 2007, 03:51 PM   #11
Stationmaster
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Some of the panels I work on (not the ones I build) have one VFD that "rotates", as the lead, among the available pumps. Thus, a different lead pump each start. The remaining pumps (lags) are started XL. This is all accomplished with mechanically interlocked contactors between the VFD and the motors. That is: for each motor, the VFD contactor cannot pull if the XL contactor is pulled and vice-versa. These are built by BIG pump station manufacturers, Industry leaders, they just know EVERYTHING (ask them). They do, however, make sure the contactor pulls before the VFD start signal is given (by monitoring an auxiliary contact), and that the VFD is stopped before the contactor is dropped.

For a "no-no" I see (and have to work on) this arrangement quite often.

Stationmaster
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Old October 21st, 2007, 04:41 PM   #12
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I have to disagree with the original post overall. Not because his comments are wrong...they just aren't right either

There is no specific way to hard wire a PLC. It is going to be very application and plant dependent. Our opinions on this are going to vary tremendously based off of our experiences. I don't think Doug realizes this. In his world these probably are laws. But we work all over the world in all different industries.

Take the original post, turn it into a questionnaire, and give Jesper in Denmark, David working with his sheep droppings, Ron in his carnivals, Elevator Mike, and Tom Jenkins in the water industry. Various industries, various locations, and I guarantee you various answers

Here is the 4 I personally disagree with
Quote:
Donít put logic in the I/O wiring
Many of times this is just down right impossible. Take a burner control for example. Safety interlocks such as the flame check relays, none repeat relays, etc all must be wired into the PLC output.

Forward and reverse coming off of outputs to motor starters should be electricaly interlocked...I could go on and on with common situations.

Quote:
In the semiconductor manufacturing world the rules are you have to kill everything
That's strange, I've worked in the semiconductor industry and have never ran into this...

Quote:
Safety Circuits
If you have safety circuits for the protection of human life, bodily injury or major machine damage donít put them in the PLC. There are too many things that can go wrong with a PLC for safety systems. Just going into program mode will disable the circuit. Hard wire all safety circuits outside of the PLC.
Safety PLCs?
Quote:
Originally Posted by Stationmaster
Some of the panels I work on (not the ones I build) have one VFD that "rotates", as the lead, among the available pumps. Thus, a different lead pump each start. The remaining pumps (lags) are started XL. This is all accomplished with mechanically interlocked contactors between the VFD and the motors. That is: for each motor, the VFD contactor cannot pull if the XL contactor is pulled and vice-versa. These are built by BIG pump station manufacturers, Industry leaders, they just know EVERYTHING (ask them). They do, however, make sure the contactor pulls before the VFD start signal is given (by monitoring an auxiliary contact), and that the VFD is stopped before the contactor is dropped.

For a "no-no" I see (and have to work on) this arrangement quite often.

Stationmaster
Ditto on this. I've seen this many times.
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Old October 21st, 2007, 05:11 PM   #13
PeterW
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Quote:
Originally Posted by TWControls
Safety PLCs?
Not sure if your asking do such things exist, but that answer is yes.

To be fair to Doug, I think he was referring to putting the safety isolation as code in a normal PLC. As far as turning off the PLC then I would expect the 'off' condition to be the safe condition.

Having said that I would not put any safety isolation logic in a PLC over hard wired safety.

Regarding safety PLC's, I have worked recently on a multi-million pound project (might even have been billion), where the entire automated part of the site was protected by PILZ safety PLC's. All e-stops and guards were wired into the PILZ and logic in the PILZ enabled or disabled contactors for power to the systems.

Sometimes these were timed to allow VFD's to stop equipment faster than dropping the power would.

Quote:
Originally Posted by Stationmaster
For a "no-no" I see (and have to work on) this arrangement quite often.
I think what you described does not counter what I said. The contactors are for selection not for 'live' switching.
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Old October 21st, 2007, 05:20 PM   #14
CharlesM
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Quote:
There is no specific way to hard wire a PLC. It is going to be very application and plant dependent. Our opinions on this are going to vary tremendously based off of our experiences.
I agree.

I have found cases where people who don't understand PLC's have gone around them when they shouldn't. Just this last week my panel builder and I were working through a problem. He wanted to add a contact to a push button and use it break an output momentarily. This would have worked but the button was already an input to the plc. So he had the right idea just was unable to understand the plc could do it for him.
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Old October 21st, 2007, 05:35 PM   #15
Stationmaster
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PeterW,

I wasn't trying to counter what you said......I was just pointing out that there are valid instances where there are contactors between the VFD and the motor(s), and the precautions that must be taken when there are.

It was the OP, not you, who instructed everyone that this is a no-no, without explaining the difference between "switching" and "selecting".

I have to agree with TWControls when he says that different people in different industries will have different answers. Again, that's the power of this forum.

Stationmaster
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