saving the inputs of my PLC

I've being doing some programming stuff for my company, for some years. last year I came across one of the machines I had installed a PLC into.
The inputs of the PLC were ZAPPED!!! I dont know if someone tried to do WELDING on the machine without disconnecting or probably dropped a LIVE wire on the machine.

There are reasons that all the machine's sensors cannot be replaced by elec. ones.
Most of these are mechanical type devices giving out signals (-0V or earth). The older systems used these so called detectors and were used in the interlocking.
As for some reason these cannot be replaced (only improved, somewhat), I want to evade such silly things. I wire the COMMON of the Input with a 24V and some NPN sensors and these "DETECTORS" are used as inputs. (and no the electrics here are not that good, and most safety devices, such as short circuit proctecting ones, ELCB, etc are disconnected)

can someone suggest a good way to protect (somewhat insulate) the inputs while using these "DETECTORS" in this environment!!


I was thinking of using DIODE at each of such INPUT, comments?

You can always try isolating the inputs but that means adding more hardware. I am not sure that would prevent feedback thru the ground but may if you used a floating DC supply.

You could use a terminal strip with small relays and have the input devices activate the relays which would power the inputs. Look at the Phoenix Contact line of terminal relays http://www.phoenixcon.com/

mjamil said:

There are reasons that all the machine's sensors cannot be replaced by elec. ones.

Click to expand...

Why is that? Insulated and/or electrically isolated sensors do exist. Non-contact sensors (ie. photoeyes and prox. switches) are useful as well.

mjamil said:

(and no the electrics here are not that good, and most safety devices, such as short circuit proctecting ones, ELCB, etc are disconnected)

can someone suggest a good way to protect (somewhat insulate) the inputs while using these "DETECTORS" in this environment!!

Click to expand...

Take this as an oppertunity to make this machine right. Using the machine chassis as a neutral and electrifying certain parts of the machine is a HUGE MISTAKE. The fact that you (or someone else) had to disconnect most of the safety devices to make your machine work should be your first indicator that this setup is not good.

You need to look into the right types of sensors for each input. Some of them will cost some money, but there is no question as to their value in the proper installation. (How much is your life worth if you get electrocuted on this machine with disabled safeties?)

Thanks RSDORAN for the info and JIMBO for your concern. ofcourse life is very very expensive, unfortunately, I work mostly in the asia region, and the I would say apart from the world renown companies working here, that is the way they work here. and beleive me or not dozens of ppl work in this environment, its not just a issue of this particular machine.

regarding your first comment that all type of sensors are available, that is a difficult part to understand since you have not seen the MECH. detectors on this machine, its a Cigarette Packing machine called HLP (stands for Hinge Lid packer). unfortunately the purpose these detectors fulfill cannot not be fullfilled by even after slight modification, I would have to set up a whole new company to make sensors for this machine. We just rebuild / modify these machines, so we cant go investing in sensor development program.

The problem is due to thier physical properties, the shape, the purpose, the size!!!
I have done some modifications, but am left with a couple which cannot be modified.
wish me luck!

I think that it is important to understand that the rebuild on these machines is not just achieved with degreaser and a paint brush - and having built these machines when they were new , I know what happens in the so called rebuild market . It is a testament to these machines that they still run after this treatment .

Regarding the detectors that you say cannot be changed , I assume that you are refering to the missing filter and loose end detectors , the missing cigarette and missing bundle detectors , missing innerframe and blank detectors , and let us not forget the good old delay drum and tridents .
All of this equipment can use modern detection devices , for example , a little application and machine work will allow you to change the ends detectors to fibre optic type (HLP4) and the same thing applies to all the rest of the machine equipment .
You are probably better not to PLC the machine unless you do the job properly , when the machine was designed in 1937 by Desmond Molins , it was designed to run at 60 ppm , and I would think that you are probably running at at least 120-150ppm which the old mechanical type detectors won't run .
You mention that because of the area that you work in , there is little money for development , this just isn't the case , there are no poor cigarette factories . Unfortunately you are a victim of your own circumstance because you guys offer the same "paintbrush" rebuild , your customers have become used to a low price , they don't really care about quality , if the odd missing filter gets through , no one minds too much .
You really have to decide you reason for PLC'ing the machine , if it is just to say " PLC Upgraded" then OK , but if you are interested in doing the job properly , then it is everything off down to the castings , rotary encoder and new detectors . I reckon that you can do the job with Omron for less than USD $4500 - and we used to get everything into one small cabinet that was connected using harting connectors and sat under the sealer bed .
If you want to see how others deal with the issues of upgrades , you can download many brochures from various other rebuild companies who do the job properly , you'll see the solutions they use and can devise you own.

Thanks Fred for your feedback. If I am not mistaken most of your experience was from PMD right?

anyway. You are right on most things, unfortunately that is the way things are, and I alone cannot change the way ppl think (and work) all I can do is try my best, negotiate to increase the quality.

To show you what I have to improve the old HLP-1s and 2s, (though I had a hard time fighting my superiors, and I have a bad impression in the company for over expenditures and too much paperwork)

1) The Cig. loose end and Filter end remain the same, and specially since I have to import most of the goods, any change in this sensor (for instance replacing with a fibre or other sensor - had done some working but not even close to our budget to propose) would be pretty high. Rarely have I seen CIG. LOOSE END detectors working, however the Filter end detectors are working no doubt.
Now if you refer to HLP4 or 5... we also have to consider that HLP2 will not sell at the price of HLP4 no matter what we install or whatever modification we do.

2) The Cigarette MISS detector has been modified, the cam has been removed, so now there are no issues of timing getting out.

3) Missing Bundle detector has been replaced by a Sensor, so has the Foil Miss. I am considering to replace the CARD REEL with a sensor, the disadvantage would be it would just measure the PRESENCE, and cannot measure the Thickness as the old mechanical ones do. Inner frame has been modified but still a bit mechanical.

4) Blank Miss and Blank feed have been replaced by a Thru beam sensor. (However Blank feed now cannot detect double BLANK presence - very rare does this occur though..)

5) We've added a Vaccum controlling unit, which diverts the vacuum from the SUCTION, so that NO BLANKS are picked up if there is a faulty Filter / Cigarette loose end or Cig. Missing in a packet.

I've spent about 3 years, to develop till here. and look forward to more improvements, however, you definately cannot compare the these improvements with those of MOLINS itself!
wish me luck!

My experience was fairly varied - I developed the chinese language interface for the Passim early generation , I rebuilt the first of the HLP5's , Wrapper5's and PACAMS with PLC and did various work on the makers , changing the old MK9 BCM from card to PLC control , and a few other bits on CMD equipment and tobacco transport systems . I still work on makers given half a chance , there is something quite special about seeing 14000 cigs/min come off the end of a maker .
There are a few updates you can use that work very well , double blank detector on the blank bed just after the blank magazine is easy to fit , blank transfer on the horizontal transfer is easy ( i very rairly use through beam as they are too affected by dust from the blanks - Molins use Omron E3SAD81 for most product detectors)
Don't forget broken card real/splice detector for the innerframe , the original one works just fine if set right , innerframe transfer on horizontal and vertical (Isle of Mann wheel) . Another nice mod is pressure wheel lift on the gravure gummer , this prevents glue being transfered to the next blank after ejection of bad bundle , or missing blank , and subsequent gumming up of the drying drum .
We have got HLP 2's up to 285 ppm with standard blank bed and 300 ppm when we throw away the blank bed and drop the blanks straight into the sealer pockets - the best I have got out of a stamper 2 was 385 ppm .
In conclusion , Desmond Molins was a clever old bugger , he invented the flexible manufacturing concept which Caterpillar took over under licence , various submachine guns and heavy artillary , synchronization of guns and propellors , many of the hydraulic principle that vickers took over (as you may know , Molins owned Vickers for a while) and many other things , with a few exceptions , most of this stuff is good , however it can benefit from updates with modern materials .

Thanks fred!

Your reply was pretty informative.

I'm also modifying the Cig. loose end and Filter end detectors, to show the condition, fault / no fault thru a LED. this would assist the maintenance guys (I think) I hate carrying a Multimeter for checking these sensors.

I havent heard of the Gummer unit you mentioned. do you possibly have a photograph that would assist.

I've also attached my semi-finished project in Microwin, in a separate post. If you have some time, let me know what areas dont need to altered (I doubt though)

regards,