how do you know one PLC is performing better than the other

[FredPLC]

Hi all,

I always hear some say one PLC is better than the other, how do you actually rate and choose a PLC?

If we put money aside, do we look at the PLC scan time (fastest) next to decide how fast can a PLC run, which indirectly affect how fast a machine can run? Of course, such question is too general since machine speed does not only depend on how fast a PLC can run and also, scan time depends on how long the PLC program is.

Lets put it this way. If i use 2 different brands of PLC but with different speed to control a machine, do i experience different machine cycle speed when both PLC program has same number of rung?

Regards.

 

[Doug_Adam]

This is a question with no answer.

The best PLC is allways the one you feel most comfortable with.

How do we choose a PLC?
Actually, I have never chosen a PLC because I thought it was the best.
I choose a PLC because that is the brand that the company uses and all the electricians are familiar with it.
Depending on the application and setup, speed may or may not be important.
You also can never get around cost.
i.e, a good enough PLC that you can afford is better than the ideal PLC that you can't.

I think I confused the issue more than helped, but thats just the way things are.

Doug

 

[rsdoran]

I will reply to the thread because it was posted.

I hope that others will view this and let it act as the response.

This is a "it depends" situation, every application is different, for some scan time is applicable, for others many other factors apply.

Some of those factors could be or are:
1. Local and company support.
2. Familiarity ( a big big big big issue)
3. Pesonnel, this is an in house issue, who is most familiar with what
4. As you stated, size, I/O count, devices used, networking etc can be an issue. This is fast becoming a non issue.
5. Features or 3rd party addons
6. Communication capabilities..RS232, 422/485, Ethernet, Modbus, Profibus etc

There is no answer to what plc is the best because none are, overall they are the same to a point.

You, or your company, has to decide what fits your needs, applications and budgets.

As an afterthought you may want to look at this:
http://www.patchn.com/choosing_a_plc.htm

 

[DRThorne]

Most plc's nowadays are the same, what makes the difference is the programming software, take 2 software packages and see which one is easier to understand and you can program the fastest... like comparing RSLogix to Proworx...

 

[seppoalanen]

Equals program in Control Logix takes 1.4 ms and S7-315 35 ms.
My 1.st criterion is speed, 2'nd is openly for other systems.

And in last hand my Customer says what PLC I have to use.

 

[FredPLC]

Hi guys and gals,

Thank you very much for your insight in responding to the thread.

Personallly, i share the same view with what most of you said. I looked at reliability, affordable price, familiarity and most importantly, support from the vendor or represenstative. I have encountered very bad experience with some of local dealer from very reputable PLC brands. Sometime, just feel like we are begging them to give them money...

Anyway, thanks to all of you there again.

 

[Peter Nachtwey]

The advantage of faster scan times.

If you are doing machine control where checking inputs allows the transition from one state to the other then scan time can be very important. If a PLC has a scan time of 10 milliseconds, it will wait on the average of 5 milliseconds to transition to the next step. If a machine cycle has 6 steps then time wasted by the PLC is 30 milliseconds on the average and 60 milliseconds worst case. If a machine cycle is 3 seconds then 30 milliseconds is 1%. At this point one can start to ask about the ROI ( return on investment ) if one buys a PLC that is twice as fast. At this point it depends on the total cost of the machine. If the machine cost $1,000,000 then a .5% improvement in speed is worth about $5,000. From this I would say if the PLC speed can be doubled for less that $5000 that buying a faste PLC is worth it. Sometimes there are other benefits of going with the more advanced ( expensive ) PLC such as easier programming which will reduces costs.

Here in the Pacific North, West cutting wood is big business. There is much pressure to cut wood faster and some machines cost about $1,000,000 and have piece rate of 25 a minute. Buying a much faster PLC will definately improve production rates even if by only a small percentage. Yet people still buy PLC5s instead of ControlLogix just because they don't want to learn a new PLC even though the difference in processing speed will pay for itself many times over. The cost of ignorance high.

 

[seppoalanen]

Peter, you 'hit the nail on the head'.

 

[A.Char]

Best PLC..?.hmmmm
When I have trained in NAIT.(Nothern Alberta Institute of Technology)CANADA...I think AB..nice...Good Proessor there..
When I have visited Israel..I have seen some training kit with Modicon
and a very nice HMI. graphic software..very interesting
When I studied in Hamburg..Germany ..I think Siemens ..A lot of books
explain with a nice picture very clear (except language)..

When I am teaching here..(RIT.Nonthaburi Thailand)I am using Mitsubishi
and Siemens..but PLC Brand above still good in my Idia..

According to all of the answer from Profs(Rsdoran,Peter,DRThorne,Doug-ADAM ,Seppoalanen)....You will see some advantage and disadvantage
of each brand when you want to install in each machine or each process..also.
Some process plant...need a good communication network.
Some machine need a good scan time (very fast scan time)
Some factory location is close to some plc dealer..That sound like a quick support .
Some Plc easy to write the program in sequence..or multiple flow..but lag in some case..
and etc...
Anyway...If you found some nice controller that can make people
Good and very kind ..please do not hesitate to contact me..(Smiles)

A.Char
Mechatronics RIT.Non

 

[rsdoran]

Me a Professor?

Where is the money?

 

[A.Char]

Where is the money..?

Attention to Prof.Rsdoran..

I called you Prof. ..because..of I have seen you quite sevaral times
in plcs.net..and some answer explore my idia in this area..

So..may I call you an internet plc professor? (smile)
Also..another plcs expert guy too.

You are very kind..
Why need a big money? in this plc world...
I give you 1,000,000 US dollars that I have sent by this post..(1000 smiles too)..

A.Char
Mechatronics RIT.Non