Hi to all! In the company ,I am working for , I met very strange machine( you can see pics here http://forums.mrplc.com/index.php?act=modu...si&img=276)
This is a double mixer. This mixer has 2 chambers, 1 glue pump(dc motor) one water pump (ac motor) 1motor to rotate mixing srew for both chambers,glue and water valves. As you can see from pics. above each chamber they hooked the control panels. Now....... each control panel has PLC(telemechanik twido) 2 timers (!!!),2counter(!!!),and ech panel has DC Drive to control single glue pump +bunch of PB and water pump starters for single water pump. But that is not all......This is the really funny ---on the shaft of glue pump they installed inductive sensors to count the glue amount respecting glue pump RPM. This sensors connected to the STAND ALONE COUNTER(Look at the pics)!!!! water amount counts respecting to the time of opennig valve(simply flow meters would not do it more precisely?) and water amount sets by stand alone timer! (I know...You might started think that I am crazy...but look at the pics.) What is for they need 2!!! PLC??????? I traced PLC outputs wiring---almost all outputs used for flash lights tower
I relally confused ....and need any logical explanations . Any ideas?

Why do toasters have a setting that burns the toast so bad that no person would ever eat it?

Were these two panels in separate locations at one time? Were they mixed together (no pun!) to make the existing system? Does one have DC I/O and the other have AC I/O?

I worked for a parts supplier that seemed to have an affinity for other bankrupt suppliers' junk equipment. They seemed to think, despite the proven fact that the other companies couldn't make any money running those jobs on those machines, that somehow they could do it exactly the same way and somehow be profitable. Anyway.. they got several simple, manually-operated pedestal welders (and of course the jobs that were running on them) from this one joint that closed down. Like you, I struggled to comprehend the electrical complexity of these REALLY SIMPLE machines. One such welder had a SLC 500 rack, a Micrologix 1200, a Mitsubishi "brick" controller, and just to top it off, it also had a big can full of Flex I/O that served as a remote rack for some OTHER PLC that thankfully didn't come with it. I'm pretty sure I've got pictures of this contraption.. I should find them and add them to the "gallery" of shame :)

Anyway, this fine establishment I started working for knew JUST ENOUGH to rig the thing up to come down and weld when you hit the palm buttons. Well, it would do that most of the time. When it didn't, they'd all just scratch their heads and start monkeying with stuff. Eventually they'd jumper enough stuff out that they could get the weld head to fire again. Nobody really understood why it did what it was doing. Heck, they didn't have software or a cable for the Mitsubishi so even I didn't fully understand what was going on!

So why am I mentioning this? Because at one point, I'm sure that the way your dual-PLC was designed made sense to someone. That doesn't mean necessarily that it continues to make sense today. Would your process be more robust if it were redesigned to use only one PLC in one cabinet? How much downtime does it have? How much do you think it would have if it were rewired the way that makes better sense to you? Do you have any opportunity to take it out of production long enough to retrofit it? Can you get management to support this decision? These are the questions you need to get answers to. And if for one reason or another you CAN'T do anything to it, then I guess you are just going to have to understand it and make the best of it.

(btw.. YES I totally rewired the knee welders, and got them down to just one control enclosure and one PLC. The only one I never got around to doing was a robot cell that had a SLC 500 and Omron PLC, because I could never get it out of producton long enough to do anything with it. For the life of me I will not understand why anyone would install two PLCs of DIFFERENT BRAND! Guess it's no wonder why the place went out of business!)

Yes, this panels in separate locations and both of them have DC Drive and AC starter and 2PLCs and 3 timers and 2 counters just to control single existing DC glue pump and AC water pump.

So why am I mentioning this? Because at one point, I'm sure that the way your dual-PLC was designed made sense to someone. That doesn't mean necessarily that it continues to make sense today. Would your process be more robust if it were redesigned to use only one PLC in one cabinet? How much downtime does it have? How much do you think it would have if it were rewired the way that makes better sense to you? Do you have any opportunity to take it out of production long enough to retrofit it? Can you get management to support this decision? These are the questions you need to get answers to. And if for one reason or another you CAN'T do anything to it, then I guess you are just going to have to understand it and make the best of it.



I asked chef of engineering department what is for you need 2 PLC to control single Dc glue pump .and AC water pump as well as stay allone timers and counters.And I got the answer. Folks! If you are not sitting now you better set down.. He said-we need 2 plc because we have 2 chambers and plc do scan and one single plc can not make scanning for 2 chambers I was speechless about 2 hours...

By the way this system never been working and finally I created another one with single controller, flow meters insted of pumps and HMI instead of timers and conters.

Gotta love it Why is it that the decision-makers just don't "get it"? Sounds like someone that's well suited to being the "chef" of engineering :)

Good to hear that you've got a better system to replace it with!

I guess I should give a more serious reply... my first comment was intended to convey the idea that sometimes we find things on machines that just don't make any sense.

Your machine reminds me of a pair of vacuum furnaces a company I worked for aquired a few years ago. There were two vacuum furnace chamers and two sets of controls, but only one vacuum pump and only one heater power supply. Each chamber had its own PLC, temperature controller, recorder, etc. I think the idea was that one furnace would be heating while the other was cooling, so to save a few bucks the vacuum pump and heating power SCR/transformer was shared. But this kind of approach is stepping over dollars to pick up nickels. It required an expensive 1,000 amp buss switch and an extra pair of expensive vacuum gate valves. We decided we wanted to cut the frame in half and locate each furnace chamber in different parts of the area to facilitate product flow. A new vacuum pump and heating power supply were not that expensive.

From the looks of your mixer someone decided they could save a buck by sharing a couple of pumps. Its possible that the OEM makes both single and dual chamber models - and its also possible that someone at the OEM designed the control panel years ago and is long gone and they don't have the expertise to redesign it.

I know of one powder press company that in 1998 was still building presses with a panel full of about 60 AB700 relays. Someone who was long since dead designed the panel that way back in the late 60's, they were still shipping machines with copies of the original blue prints. They were good presses and did what we needed - though we had refitted a couple of them with PLCs. Anyways, we had purchased one of their machines in about '98 that was only a couple of years old and we were having a factory tech go through it. I asked him why they weren't using a PLC instead of spending thousands on all of the relays. He simply said that they didn't know PLCs and that this is what they knew. The company was not in business in 2005 when we wanted to buy another one. I think that there are many OEMs out there like this.

Good point.

We have 2 machines that are almost identical. The older machine is controlled by relays and the new one by PLC. It's funny to look at the electrical schematics of the old machine and the PLC program in the new one. The PLC mimics the old electrical drawings almost contact for contact.

There is nothing wrong with the way the new machine performs. I just thought it was a case as Alaric described.

Ok, I understood your point,but nothing to do with OEM. This "machine" were disigned internally 3 years ago and company just send out contract for control panels assembling. After controls panels arrived they hooked them up and call me to start up this creation.

In case someone you don't like comes over for breakfast?

I still can;t see the problem... with the design that is..... you got the short answer as to why are you even bothering asking me this, shouldn;t you be fixing something or get the thing going ... Why are you trying to redesign it now...type of answer fomr the engineer.

The intention might of been to have the ability to have totaly seperate glue pumps in the future... and it was probably easier to design a single system, and simply duplicate it. It gives a sense of redundancy and with their own timers and counters etc

All i can say is that i have seen a lot worse and more complicated systems... from the pic;s they seem pretty easy to follow and work out.... once youv'e been around the industry for a while you will soon found out what I mean.

Having had to sort out many similar machine control systems before, I can offer a likely scenario as to how it happened.

There was an original design concept that started when PLCs were big and expensive, so for a little task like this they designed it using relay logic and timers etc. Then by the time the budget got approval or the business model warranted executing the project, PLC prices had fallen to where the relay logic began to not make economic sense. But those small cheap PLCs didn't come with an inexpensive way to enter recipes or changes, so they kept the big dial timers as a form of cheap operator interface. So the PLCs just became a less expensive option for executing annunciation or fault sequencing, the run sequencing, i.e. the timers, was left in the old design mode. Most likely the designer had no time to learn and implement an HMI screen for entering timer values or settings. In addition, timer / counter processing feedback is a very old school way of doing things, so I would hazard a guess that the person who designed it was not willing to take the time to learn how to take advantage of all the PLC had to offer.

Here's my hypothesis. Your company had two of this particular model of PLC on the shelf at the time. Maybe they had the two enclosures, too. Somebody said, "we've got them, and they'll do the job. Go build it"

Have done that myself. On quite a few jobs I have used a PLC I already had. Have a PLC that is now in its 3rd machine. Salvaged it from the first machine right beofre they scrapped it. Second machine was for a very specific job that we stopped doing so I recovered the PLC yet again and put it in another machine I was upgrading. I do the same thing with sensors. I tend to stick with one brand and when I run across another I get rid of it once it fails.