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Analog input value sending from one PLC to another PLC speed problem

Created by: Borut23 at: 1/16/2011 7:57 PM (7 Replies)

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1/16/2011 7:57 PM	Rate (0)
Borut23 Posts: 15 Rating: (1)	Hi, if someone can help me on this problem: I have laser distance meter which gives output 0-10V or 4-20mA. I measure this value with PLC1 and then send this data to PLC2 for further use thru Profinet. PLC2 uses this data to control a frequency inverter which drives a motor of a small train so that this train stops on predefined locations. My problem is that data comes to PLC2 to late so my train never stops on the proper location - it always stops to late. I don't have much experience with this kind of controling so can someone help me? I think I have a couple of issues here - first is the noise reduction integration time of the analog inputs (possible to cancel this noise reduction) which delays my data, then how to send the data as fast as possible to PLC2 thru profinet. Is there a way to send data using TSEND_C on every pass of OB1 (my TSEND_C is in OB1 and currently I'm using M0.1 as trigger for send request) and not using system clock markers? Any other way in generating a positive edge pulse to use with TSEND_C block for trigering the send commadnd so I would send data on every pass of OB1? Please give me suggestions as this is the only problem I have in my machine. Thank you very much, Borut
Borut23 Posts: 15 Rating: (1)
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1/18/2011 8:26 AM	Rate (0)
Borut23 Posts: 15 Rating: (1)	Nobody? Realy?
Borut23 Posts: 15 Rating: (1)
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1/18/2011 1:43 PM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	Your problem is not really the delay, even if it could be near zero the same problem will occur. You have a regulation loop stability problem. If you explain how you compute output to inverter with input signal, maybe I can tell what you are doing wrong. Don't waste your time to search how to speed up the AD conversion, make sure that conversion and comunication is determinstic, let's say every 1 sec or less, but the true data and filtered of course. Deterministic = use time interrupt to make AD conversion and to send data, not to send every OB1 scan! The frequency of sending must be at least 2 times lower than PLC2's scan frequency.
	Last edited by: Marko Bursic at: 1/18/2011 2:02 PM
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1/18/2011 2:05 PM	Rate (0)
Borut23 Posts: 15 Rating: (1)	Hi, the motors are not driven with analog values.Only predefined speeds and digital outputs. And the speed of communication IS the problem. Because the real distance measuret at laser meter and the distance communicated from PLC1 to PLC2 were not the same. One thing that will help will be to lower the speed of the motor very much when train will be close to the destination and form then on slowly comming to final point. Also I got some hints on how to speed up the communication - using DONE bit and ERROR bit for triggering the TSEND_C block butI don't know how to create a trigger from this two signals. I know this is basic STEP 7 but I'm lost here :) Using P_TRIG or which block would be the right one here? Thanks, Borut
Borut23 Posts: 15 Rating: (1)
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1/18/2011 2:26 PM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	Well, if I understood, you are searching edge triger function, thats pretty easy: make a contact with your bit Done, put in series another contact , scroll down list and choose a contact in middle P or N (positive or negative edge) at last put a coilfor example a bit m0.2 and name it, lets sayDonePulse. Thats it, the DonePulse will last one scan cycle when Done will make a prasition from0 to 1 (if Pisused).

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2/2/2011 9:46 PM	Rate (1)
Galen Advanced Member Joined: 10/1/2008 Last visit: 1/27/2025 Posts: 21 Rating: (7)	Hi, Using the TSEND_C and TSEND_C blocks to send data that is required for fast motion control is not a good idea. I would recommend using an ET200S CPU with a remote ET200 I/O drop. If your application is slow enough then here are a few points that will help when using the TCP blocks in the S7-1200. 1. Increasing the Communication load from 20% to 50% in the hardware properties of both CPUs. This requires a CPU with firmware 1.0.2 or later. This will reduce the time to send a short message from 10-20ms to 3-5ms. 2. Make sure that the TRCV_C block in CPU2 is executedmore oftenthan the TSEND_C in CPU1. The Done output on the TSEND_C shows that CPU2 has placed the message on an internal buffer, not that the TRCV_C has read it. Multiple messages can be placed in the buffer at the same time which will cause a delay in sending your data. For example: if the rate you are triggering the TSEND_C to send data is every 10ms but you are only calling the TRCV_C block in CPU2 every 15ms then buffer in the CPU2 will start to fill up with old data causing a delay in getting the most current data. Otherwise, you need to send a handshake message back from CPU2 to CPU1 when new data is received so it will know when to send the next message.

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2/2/2011 10:37 PM	Rate (0)
Borut23 Posts: 15 Rating: (1)	Hi, thank you very much for this. Well using different hardware is not an option as the system is already built. So I increased the rate at which data is sent, also I increased the rate at which laser detector is sampling distance and also I decreased the speed of the motor when am come to proximity of the desired location. And after the last tests this is now working without any problems. So it seems it is solved. I will do some more tests but for now all is good. Thank you for your advice. I will remember this as everything can help. Borut
Borut23 Posts: 15 Rating: (1)
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2/3/2011 2:31 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 3/15/2025 Posts: 3042 Rating: (1061)	Hello Borut23 perhaps a bit late, but it may still help you now and/or in similar applications in the future: The lastest S7 PN CPU's (finally) support so called "I-device" functionality. What this means is that they can act as a Profinet I/O Controller as well as an intelligent Profinet I/O device at the same time. You could then have one CPU as an I-device of the "main" CPU and exchangevalues between them via Profinet I/O's. This will be much faster than any Send/Receive or S7 communciation based linkthat you can configure/program betweenthem and you can find out more about it at the link below: Which CPU can you operate as a PROFINET I device? I hope this helps
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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