Hi,

I'm new in the field of Industrial Automation and I have a big issue:

I work for an Italian Company building Power Plant, and in one of our PP (using steam) we've started to use the Profibus DP as a fieldbus, connected to a Siemens DCS.

So, I have to check if Profinet can help us to optimize performances of the regulators in case of either normal operation (Load Control, Pressure Control, Temperature Control) or Primary Frequency Regulation. Obviously, they are relatively slow controls, around fractions of second, so we don't need Real-Time responses.

As in Europe, also in Italy Profibus is very common in factories, but I need to compare it with Fieldbus Foundation HSE/H1, to see if this protocol could be more practical to use.

Moreover, I'd like to compare also the Troubleshooting and the Costs in both protocols.

I think to have clearly explained myself (despite my english...), I hope you can help me...

Best Regards, Fabrizio

 

Dear Fabricio,

I assume you are talking about PID control loops here? In my personal opinion Foundation fieldbus (FF) H1 is a better choice here because it is precisely periodic (isochronous). That is, it has a constant sampling interval "scan time" as required by the PID algorithm due to the 'dt' in the integral and derivative terms for best performance. Moreover, in FF the execution of function blocks is scheduled and time synchronized with the communication for the same reason. This is the biggest difference between Fieldbus and PROFIBUS which is 'free running'.

Meeting fraction of a second period is possible provided you use fast
devices.

Cheers,
Jonas

 

PROFIBUS-DP V2 provides also precisely isochronous operation. This is
also the case with ProfiNet IRT.

Best Regards,
Armin Steinhoff

 

Dear Mr. Jonas,

Is it possible to explain more about this difference (Isochronous) and the effect which it does have on a control loop? Does it affect only the control loops? As stated in one of the replies, Profibus DP-V2 or Net could support this feature, may I ask you to give me your idea about it?

Best regards,
Vahid

 

Dear Mr. Vahid: While Profibus DP and Profinet are excellent fieldbus technologies, they have not been designed for closed loop process control. They may be fast, and in actual usage they MAY be fast enough, but there is no clocking structure within the protocol to synchronize measurements with the demands of a sampled data controller. Foundation Fieldbus H1 and HSE have such synchronization built into the protocol.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720 USA
E-mail: RCaro [at] CMC.us
Subscribe to the CMC Wireless Report <http://www.CMC.us>
===========================================

 

> Dear Mr. Vahid: While Profibus DP and Profinet are excellent fieldbus technologies, they have not been designed for closed loop process control. They may be fast, and in actual usage they MAY be fast enough, but there is no clocking structure within the protocol to synchronize measurements with the demands of a sampled data controller. <

The Profibus DP-V2 specification includes:
* Data exchange Broadcast - broadcast data over the bus.
* Synchronous (Isochrone) Mode - trigger applications simultaneously.
* Prioritized Communication - create network priorities.
* Redundancy - for process automation resilience.
* Clock Synchronisation - keep things in sync.

This supports deterministic behaviour in motion applications and control loops/regulators etc.

The same features are also offered by PROFINET IRT ... IRT means:
Isochronous Real Time

Best Regards,
Armin Steinhoff

 

Thank you all for the answers, but I have a new issue: I know that FF premise to substitute easily one device with few changes of software (I mean, FF assign automatically a new address to a new device). Well, can Profisafe fulfill the same issue? Or this profile is necessary only for safe parameters (F-Parameters)?

Ing. Fabrizio Di Paolo

 

Dear Fabrizio,

For comparing FF and PROFIBUS, there is a good white paper on www.us.profibus.com:

us.profibus.com/resources.aspx?pagetype=white_papers

It is called "PROFIBUS and FF a cost comparison".

For doing control in either PROFIBUS or FF, network design is very important. In both buses, you can easily have a large variation in update times, or a small. Some application can handle a very large variation in update times, others can not. In either case, PROFIBUS offers the advantage that you get very fast update times.

For PROFIBUS in PCS7, you will need to watch your controller cycle time and overall bus cycle time. Using timed interrupts, you can easily bring down the update variation to a point for optimal control.

For more information on PROFIBUS cycle time, there is a very good introductory level book on PROFIBUS called 'Catching the process fieldbus - an introduction to Profibus for Process Automation'. You can order a copy from www.measuremax.ca or get a copy from your local Siemens representative. The Siemens part number is 7ML19915BB01.

Best Regards,
James

 

The comments made by Powell and Steinhoff are very true for modern versions of Profibus and Profinet. Isochronous protocol has been added for synchronization of motion control applications. However, there are no Profibus-based instruments capable of performing closed loop process control (PID) in field instruments. It would be possible in the future using isochronous protocol, but none do at present. The basis of Profibus process control is that control is a host function (in the controller.)

Only Foundation Fieldbus is designed for field control, performing closed loop control entirely in the field devices. Many companies are using Foundation Fieldbus in this way with great success, lower system cost (fewer controllers), and more responsive control loops. The cost of wiring is about the same with either technology.

All of this is covered in my books on Automation Networks and Fieldbus Network Equipment.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
E-mail: RCaro [at] CMC.us
Subscribe to the new CMC Wireless Report <http://www.CMC.us>
Buy my books:
http://www.isa.org/books
Automation Network Selection
Wireless Networks for Industrial Automation
http://www.spitzerandboyes.com/Product/fbus.htm
The Consumer's Guide to Fieldbus Network Equipment for Process Control
Buy this book and save 50% or more on your next control system!!!
===========================================

 

Both Foundation Fieldbus and PROFIBUS have been used successfully to do close loop control. One of the differences is where the control is done. In PROFIBUS it is done in the controller. In FF, it is either done in the controller or in one of the field devices.

Isochoronous mode is available in many PROFIBUS DP masters now and can be applied to PROFIBUS PA, if the application really requires a fixed scan time that is 'more' fixed than regular PROFIBUS. Regular PROFIBUS already has a constant scan time except for a small variation. For most control applications, this would have no effect. The topic of fixed vs having a small variation is a bit of a hot topic in the literature. When you really dive into the protocol, you will see that both PROFIBUS and FF have a variation to their scan time. FF is less, but still there. Also, there is more to consider since most instruments have their transducer block runs asynchronously to the AI block. This whole area gets very complex very fast.

The best approach I know is to the take the practical approach: Find out what your process requires and design around it. The best situation is when you can test it out ahead of time. That way, you do not have to try to figure out who is telling the truth, since you can see it your self.

Good luck with your investigations.

Best Regards,
James

 

Dear Vahid,

PID controllers need constant sampling time because the "dt" in the integral and derivative terms of the PID algorithm are assumed to be constant. To understand more about sampling time for PID read chapter 4 of the classic book "Feedback Controllers for the Process Industries", F. G. Shinskey, 1994, ISBN 0-07-056905-3

Isochronous (precisely periodic) communication ensures constant sampling time in a distributed system. FF-H1 fieldbus provides this. And yet, PROFIBUS-DPv2 and one of the types of PROFINET also does it (PROFINET-IO-IRT). However, you don't find PROFIBS-DPv2 or PROFINET in transmitters and control valves. PROFIBUS-PA is based on PROFIBUS-DPv1 so it is not isochronous.

Cheers,
Jonas

 

I personally don't agree with the white paper:
* I don't quite agree to "either will do the job". 4-20 mA and on/off will also do the job. But there are significant differences that make busses suitable for different applications

* I don't think there is a price difference between FF and PA devices

* The paper equates the FF macrocycle with the PA bus cycle which is not correct because the FF macrocycle includes function block execution, not just communication. Control has to be taken into account. All things considered I think FF and PA can be loaded with the same number of devices and I believe FF with CIF will perform better because it closes the loop in just one cycle compared to many cycles for an asynch bus.

* Most systems do not require HSE linking devices, so no additional cost

* With the same practical number of devices per bus, there is no wiring and junction box cost difference

* I disagree FF devices are more complex. On the contrary, FF has protocol features that make FF devices easier to manage and are required for good PID control:

o automatic address assignment
o scheduling and time synchronization
o online firmware download
o ability to connect a handheld field communicator or laptop to a running bus
o etc.

For FF there is no variation in updates times to speak of. It is constant: precisely periodic, isochronous. This macrocycle was built in from the beginning so FF can meet the needs of process control; fixed PID sampling period.

I personally don't agree with several things in the book either, similar to the paper.

PROFIBUS PA devices use DPv1 not DPv2 so how can you make a PA bus run isochronously? It takes more than a the master, right?

It sounds like FF and PA are the same, but there are many big differences.

I don't quite agree FF has variation in scan time. As compared to other busses it is very constant, it was designed that way. FF jitter (bus cycle variation) is less than 0.2 ms (less than 0.1% of a typical control response period). As per your book, PROFIBUS variation is 100 ms. Considering the control loop is 300 ms for flow and pressure and 1000 ms for level and temperature, 100 ms variation is not 'small'. I personally disagree this will have "no affect" on control because 'dt' in the PID algorithm assume sampling time is constant. If it isn't, control will not be optimal.

You are correct that transducer blocks in FF devices need not be synchronized with the function blocks. However, there is a big difference here. Since the transducer block and its AI block are within the same device, they can rely on oversampling internally, many times faster than a bus. For instance, 22 times per second for a pressure transmitter, or 50 times per second for a positioner. This compares favorably against asynchronous behavior between different devices across a asynch bus which updates maybe 1, 2, or maybe 4 times per second - over and above any asynch behavior of the bus itself. Yes, I agree it is complex...

This is one reason I think FF is the better choice for instrumentation involved in PID loops and such. I would use PROFIBUS for motor controls like starters and variable speed drives. FF and PROFIBUS along with HART (for the 4-20 mA devices on the SIS) as well as WirelessHART integrate in the same system thanks to EDDL (www.eddl.org). See this article:

http://www.ceasiamag.com/article-6631-thedigitaldrive-LogisticsAsia.html

Cheers,
Jonas