2 PLC's controlling Devices

Here’s the deal I have a client that that is revamping an old system and one part of the system has a standalone machine that has its own PLC. They now want to control it remotely through another PLC . We are not allowed to access the original PLC or even to communicate to it in any way. My client came up with the bright idea of controlling the motor starters, solenoid valves and other devices directly with the new PLC and still leave the existing PLC running and connected to the devices. In short both PLC’s will be controlling the devices at the same time without communicating with each other.
They got quite testy when I told them that it’s not a good idea and in fact could be dangers to both man and machine. They are not leaving me with many options either do it their way or leave the job.
I just wanted to get some thoughts on this type of operation.

doesnt sound like a good situation.

When I was an employee and any boss came with such brilliant idea, I asked for a signed paper releasing me from any liability in case of accident or death. That was enough for discourage the boss.
Now it's easier: I just walk to the next client.

either do it their way or leave the job

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Does the client have a track record of successfully implementing out-of -the-box designs? Do they make a compelling case for their demands? If the answer to either of those questions is no, take the second option.

Put in a six figure quote and wipe your hands of it.

Garys,

Two plc's acting independent of each other just won't work !!
one plc is in manual homing the machine and the other is running in automatic. Sorry, it just won't work.

Unfortunately, I would more than likely walk away if they won't listen to reason.

If you are still not wanting to walk away, I would have your lawyer draw up a document stating the following:
1. your concerns and the reason for that concern.
2. possible machine damage.
3. possible harm to people including loss of life.
4. you, your company, your employees, and your family members are not liable for any damages to the equipment, loss of production and sales, harm to others, or loss of life.
5. you state exactly what you are providing. dot every I and cross every t.
6. any add on's, modifications, or deviation from your scope of work must be agreed on and a separate po issued for the work WITH and I repeat WITH item 4 and 5 spelled out.
7. Your client agrees that his company is aware of your concerns and assumes full responsibility for all damages, loss of production, harm to others, or death and they will pay any and all medical bills, funeral costs, and legal fees for you in case of a lawsuit.

Again, I would walk away.
IF you do this and the project goes bad (you know it will), your reputation will be in jeapordy as well as your career.

regards,
james

A bit far fetched.......but...You could run the old PLC outputs to inputs on the new PLC for secondary decision.
You definitely would not want both PLCs controlling the same device.....but 1 PLC could ask for a device and the other PLC can turn it on.

I have also ran to a few output cards that do not like being "back-fed". Especially from different sources.
This may be your way out.
See if the existing PLC outputs can be "back-fed", especially by a different power source.

Sometimes you have to bend to the will of your customer. When they have a strong desire to be dangerously stupid, it is better not to bend, but break and run.

Put forward the technical and safety reasons why it cannot be done and walk way if they dont see the light

It is the most ridiculous idea i have heard in a long time..takes the biscuit

Quote,

There can be only one Captain to a ship.

Thomas John Barnardo

They would need some form of selector switch to decide which PLC was controlling all of the devices, such that only one PLC can take control
at any one time.
Probably using auxiliary contacts at each device to wire them independently.


Still a dangerous idea though.....I would refuse....

The Ugly Baby Scenario

People get defensive and rigid when someone calls their baby ugly, especially in public. The scenario: client has a 'brilliant' design concept (their baby) that will leave a 'positive' mark on the company's operation. The baby is ugly.

In these situations, it's best to tease out the goals and constraints of the project and work with the client to 'refine' the design in a way that makes the design palatable, yet still preserves it as their baby. Easier said than done.

Try to avoid public condemnation of his baby. Find out what the goals/constraints are. Point out concerns and present choices for improving the design that the client can choose from and adopt as their own (they preserve some design ownership/credit). Hopefully, you gain an ally in the process.

It may be too late, and some people can't help themselves or be helped with their garbage procreation.

Good luck.

NetNathan said:

You could run the old PLC outputs to inputs on the new PLC for secondary decision.
You definitely would not want both PLCs controlling the same device.....but 1 PLC could ask for a device and the other PLC can turn it on.

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This was going to be my suggestion

Mark

GaryS said:

the bright idea of controlling

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= Gobber_Gobller_Bubba

James Mcquade said:

...4. you, your company, your employees, and your family members are not liable for any damages to the equipment, loss of production and sales, harm to others, or loss of life....

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Unfortunately, in this sue-happy country, even this clause would not stop a programmer or company from being sued and held liable if someone got injured or killed by a machine they made any modifications on.