WKla

Pseudo redundant system

6 posts in this topic

Hello,

Our customer asks for a PLC with redundant CPU. Because the process is not critical and from financial reasons we decided to came up with a Multi CPU system.

We are thinking there are 3 possibilities:

1. Multi CPU system: 2 CPU. Double each I/O module, one module for each CPU. The disadvantage is that when a error occurs in the CPU no. 1, the error occurs in all other CPU modules and the multiple CPU system stops. So this system is not suitable.

2. Multi CPU system: 3 CPU. The same system. The second and the third CPU will be responsible with controlling the process. First CPU will be dummy CPU, no written logic.

3. Two separate CPU systems each with his own base unit. Here there should be some communication between the two CPU's and one digital output from PLC1 to be connected in one digital input of PLC2 and vice versa.

The system has:

16 DI, 8 DO, 8 AI, one QJ71MB91 communication module and one QJ71E71-100 Ethernet module.

Q03UDE CPU.

One GOT 1455 - should communicate with both CPU's.

 

Has someone did this before? Or do you have some recommendation which system is simplest or more reliable?

Thank you.

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Hi WKla,

I did not implement such "redundant", but once pondered how to build it. The third is correct and most reliable, but expensive. The appropriate inputs and outputs on both PLCs are connected in parallel, the Modbus and Ethernet modules on reserve PLC should be connected to their networks, but should not be active there until the major PLC will get a failure.

At the same time, would like to say, there are a class of applications, where is not necessary to implement such expensive semi-redundant. While the system should just allow to provide maintenance with stopping a control from PLC and without stopping of a complete contol process. It can be done without hot substitutes.

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Hi Inntele,

Thanks a lot for your advices and expertise!

Yes, this is the way we were thinking to do: inputs and outputs to be connected in parallel, and modbus modules from the second PLC not active.

1. About the redundancy, these are their condition:

- the processor unit number: 2; processors operate redundantly.
- switching active processor does not influence the communication path and does not cause switching between communication paths (one channel would be modbus slave).

2. I don't know how GOT 1455 could or should communicate with both CPU's. The setting should be done in both CPU's because when the second PLC takes control it should run with the same settings.

Thank you!

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Hi WKla,

I see, the customer has a phobia, connected with configuration, where single CPU is used, and this phobia applies only to the CPU redundancy. 

Sorry for the demagogy. While a common rule says that a client is always right, I don't understand such customers, 'cause, on my opinion, this approach is quite stupid: any control system consists not only of PLCs, but of PSUs, sensors and actuators (relays, contactors, valves), which are more unreliable than PLC. A PLC also not only consist of CPU, but of I/Os and interface modules /ports. All abovementioned  components first takes a risk to be damaged, when an external factor like overvoltage, lightning etc take place. This risk is more real, than to get CPU's malfunction. And any problem with sensor, or actuator, or PLC input/output/interface port will require to stop and power off the control system for a time. What problem will be solved, if just two CPUs prop each other? To feed the customer's phobia??

If I'd build the pseudo redundant control system with required I/Os, then I decide to choose third configuration, i.e. a full complementary. And to save money I'd use FX3GE, instead of QnUDE. The advantage of such configuration is obvious: when one PLC get a failure, the second will take to itself all control and communication functions and the technical staff is free to make fault finding, module's replacement, reprogramming procedures on first PLC.

Because they ask only about CPU redundancy, then you could choose the second configuration. 

I don't see a problem with connection and communication one GOT HMI to multi CPUs. I'm not a great specialist in GOT HMIs, but seems the manual explain how to organize the desired, using Ehernet Hub/Switch and UDP.

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Thank you for your reply Inntele,

We had already bought the Q system and sent the technical data sheets to the customer, so we can't change the PLC. But we ordered a second CPU, power supply and main base unit for the second PLC system. Now we have 2 identical Q PLC's.

One output from de PLC1 is connected to the input of the PLC2 and one output of the PLC2 is connected to the input of the PLC1. If the PLC1 is master then all the outputs of the PLC2 are disabled, the modbus master is disabled (using the FB is easy to start/stop communication).

Other problems would be:

1. GOT HMI: how configure (write the same value in the same address in PLC1 and PLC2) both PLC's.

2. GOT HMI: how to control the actuators from the right PLC (base screen)

3. PLC: How to enable/disable modbus slave from one PLC or the other.

Anyway, the idea is that the system works... and easier than I expected.

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Nice to hear that you've managed to solve all issues.

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