S7 1217 with 5 PID Loops, 7 VFD's through Modbus, 100 DI's and 64 DO's

Sami2040

My Application is Foil Winding Machine and I have following Hardware:

S7 1217 DC/DC/DC 6ES7217-1AG40-0XB0

4 DI/DQ modules 16DI/16DQ

2 Modules 16DI

1 Module 4AI/2AQ

1 Modules CM 1241 RS422/RS485

I Need to Control 7 VFD via Modbus RTU and also control the tension of 5 motors with 5 dancers so I will use 5 PID Loops in cycling interrupt OB at the same time.

the question is that can my CPU handle all these kind of communication with 5 PID loops and handle all these numbers of 100DI/64DQ 

Your feedback will be highly appreciated.

 Hi,

• 7 MODBUS RTU connections: in the central rack you can only place 3 communication modules. If you are going to do a point-to-point with each slave, you would have to opt for an ET 200, but you can network several slaves and in principle, they could all be on the same network. The case here would be the network scanning time...
• PID: the S7-1200 has no exact limit on the amount of PID it can process, it depends on memory and processing time. 5 PID doesn't seem like much. But if the required processing time is absurdly short (for example, 1 ms), then it's a good idea to do practical tests before committing to using it.
• Program size: this is a recursive controversy... everyone creates their program in a different way. My suggestion in this case would be: put together some blocks with logics that don't need to be exactly those of your project, but that express the degree of complexity it should have. Right-click on the PLC, Assigment list, switch to the Resources tab. Evaluate the size of the program with one, two, five, ten, etc. of these blocks and make an extrapolation to the effective size of your project (be sure to include a safety factor, since “surprises” always appear during programming).
• Execution time: it is something similar to the size of the program: you can make estimates online (running "something" on the CPU) or offline (based on catalog data, execution time of binary instructions, with integers or with floating points). As in the other case, use a reasonable safety factor.

If your logic is written so it can smoothly transition from one MB VFD to another, and handles all exceptions your system can logically have in a way that prevents the COMMs from locking up, you can manage quite a few MB VFDs with a single CM1241.

Of course, all the VFDs need to be daisy-chained on the modbus network.

Of course, you need to be able to communicate with all the VFDs within the require time for whatever baud rate is being used so you do not get stuck waiting to send new requests before the previous responses have all been received - or timed out in the case of the last device on the MB network being offline.

Note that if an intermediate VFD is powered off, the daisy chain of MB devices will fail beyond that VFD.  If your process is naturally grouped with several VFDs can be off and other VFDs should still be operating, you may want to consider multiple CM1241's, one per VFD "grouping".

Sami2040

Denilson Pegaia

Sami2040

My Application is Foil Winding Machine and I have following Hardware:

S7 1217 DC/DC/DC 6ES7217-1AG40-0XB0

4 DI/DQ modules 16DI/16DQ

2 Modules 16DI

1 Module 4AI/2AQ

1 Modules CM 1241 RS422/RS485

I Need to Control 7 VFD via Modbus RTU and also control the tension of 5 motors with 5 dancers so I will use 5 PID Loops in cycling interrupt OB at the same time.

the question is that can my CPU handle all these kind of communication with 5 PID loops and handle all these numbers of 100DI/64DQ 

Your feedback will be highly appreciated.

 Hi,

In general terms, I would say there is a good chance it will fit, but you need to look at the details.

Examples:

• 7 MODBUS RTU connections: in the central rack you can only place 3 communication modules. If you are going to do a point-to-point with each slave, you would have to opt for an ET 200, but you can network several slaves and in principle, they could all be on the same network. The case here would be the network scanning time...
• ...
• ...

 A lot of thanks for your feedback, related to 7 Modbus Communication, do you think my CM 1241 will be enough to handle 7 VFD's at same time, here i need to control these drives (on/off based on working situation with required frequency commands as will as read dive status and data), is it enough with 1 CM 1241 or you suggest to have another communication resource (e.g. another CB 1241 on the CPU), does adding another resources will affect the execution time of CPU ??

Modbus is a daisy-chain network topology.  However, in my experience, _some_ VFDs when powered off will pull down the modbus network.  Therefore, I consider that and try to avoid situations where that can be a serious problem.

I have also seen many MB networks run flawlessly for years with one dead MB device in the midst of all the other MB devices.

This is not the same for Profinet devices using a daisy-chain topology.

If your experience with your particular MB devices is like the flawless MB network operation I've seen, then by all means, use and rely on it.  Just keep in mind, there are some MB devices with bad failure modes out there and until you know for sure, you need to ask around (using your favorite chatbot, service rep, or other online resource) to see if that applies to any MB VFDs you are using.  If the product family has been around for more than a year, they have likely addressed any bad failure modes with hardware revisions.

Sami2040

0_Einstein

If your logic is written so it can smoothly transition from one MB VFD to another, and handles all exceptions your system can logically have in a way that prevents the COMMs from locking up, you can manage quite a few MB VFDs with a single CM1241.

Of course, all the VFDs need to be daisy-chained on the modbus network.

Of course, you need to be able to communicate with all the VFDs within the require time for whatever baud rate is being used so you do not get stuck waiting to send new requests before the previous responses have all been received - or timed out in the case of the last device on the MB network being offline.

Note that if an intermediate VFD is powered off, the daisy chain of MB devices will fail beyond that VFD.  If your process is naturally grouped with several VFDs can be off and other VFDs should still be operating, you may want to consider multiple CM1241's, one per VFD "grouping".

Sami2040

Denilson Pegaia

Sami2040

My Application is Foil Winding Machine and I have following Hardware:

S7 1217 DC/DC/DC 6ES7217-1AG40-0XB0

4 DI/DQ modules 16DI/16DQ

2 Modules 16DI

1 Module 4AI/2AQ

1 Modules CM 1241 RS422/RS485

I Need to Control 7 VFD via Modbus RTU and also control the tension of 5 motors with 5 dancers so I will use 5 PID Loops in cycling interrupt OB at the same time.

the question is that can my CPU handle all these kind of communication with 5 PID loops and handle all these numbers of 100DI/64DQ 

Your feedback will be highly appreciated.

 Hi,

In general terms, I would say there is a good chance it will fit, but you need to look at the details.

Examples:

• 7 MODBUS RTU connections: in the central rack you can only place 3 communication modules. If you are going to do a point-to-point with each slave, you would have to opt for an ET 200, but you can network several slaves and in principle, they could all be on the same network. The case here would be the network scanning time...
• ...
• ...

 A lot of thanks for your feedback, related to 7 Modbus Communication, do you think my CM 1241 will be enough to handle 7 VFD's at same time, here i need to control these drives (on/off based on working situation with required frequency commands as will as read dive status and data), is it enough with 1 CM 1241 or you suggest to have another communication resource (e.g. another CB 1241 on the CPU), does adding another resources will affect the execution time of CPU 

 Thank you for your valuable feedback, just want a clarification on:

"( Note that if an intermediate VFD is powered off, the daisy chain of MB devices will fail beyond that VFD.  If your process is naturally grouped with several VFDs can be off and other VFDs should still be operating, you may want to consider multiple CM1241's, one per VFD "grouping")"

how the communication will be affected once intermediate VFD is switched off? all modbus slaves are connected in parallel to the bus I am right? could you pls. explain more about this point.

For S7-1200 in particular, you want time space out the PLC workload so it does not get stuck doing too much work in one PLC scan.  That can cause loss of control in many situations.

Let's say each PID controller is called once every 100ms.

For 5 PID controllers, set the cyclic interrupt OB time to 20ms with a 10ms offset.

In OB100, set the PID_counter = 1 so it always starts with PID #1.

In OB30 (or OB3n):

1. Insert a ---[==]--- {PID_counter =?= 1} in front of the FB called with all the PID details for PID #1.
2. Insert a ---[==]--- {PID_counter =?= 2} in front of the FB called with all the PID details for PID #2.
3. ...
4. ...
5. Insert a ---[==]--- {PID_counter =?= 5} in front of the FB called with all the PID details for PID #5.
6. INC PID_Counter by 1

Each time OB30 is called, it services the next PID.

This spaces the work out, with one PID loop serviced every 20ms.

If there is any extra logic in that OB3n where the PIDs are being serviced and it ends up taking closer to or more than 20ms, you have a problem and may need to upgrade to a s7-1500 for that project.

Sami2040

thank you very much, it is clear now, my drive set will be Delta MS300 Series through Modbus RTU with my CM 1241, I guess it should work without any problems.

Just last question pls., regarding PID loops, what is best practice to call all 5 PID Loops in 1 cyclic interrupt OB or I should call each one individually in separate OB