6/6/2014 11:21 AM | |
Posts: 12 Rating: (0) |
Hello everybody, As it is mentioned in Siemens website about S7-200 fast analog input, its conversion time is 25 μs which is enough for my application. (http://w3.siemens.com/mcms/programmable-logic-controller/en/simatic-s7-controller/s7-200/cpu/Pages/Default.aspx) But as it is mentioned in S7-200 user manual (page 412), the conversion time for analog inputs is 250 μs which is not enough for my application. (https://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&lang=en&objID=10805150&subtype=133300) Is there anyone who knows which one is true? , I think only one analog to digital convertor is used in the AI cards so the conversion time would be shared between channels, is it true? I should mention that I have checked S7-300&400 AI cards, but none of them meet my requirements. |
6/9/2014 10:47 PM | |
Posts: 12 Rating: (0) |
Hello my friend dear William. Thanks so much for wetting your feet for me J and be noted that it’s a long time that my feet are also wet for this project but do not be in hurry because I am waiting for the budget of this project and it is a time consuming process here. Anyway I am trying my best to complete my knowledge and to become assured of everything I am going to use in the project in order be 100 percent ready when budget comes. I understand you when you say that the fast analog input or processor may not be needed because I am in touch with industrial control loops as I am working in Oil & Gas industry. The Processes we have in the factory are so slow; we have PCS7 as control system with normal sm331 analog input cards which are so fast for such processes. But Active Magnetic Bearing (AMB) is another story, before starting to talk about AMB I think this forum may not be a good place for this discussion(which I like it to be continued) because from now on, a minimum part of the paragraphs could be about Siemens products. - AMBs in our industry are installed on high Speed rotating equipments with minimum 17000 revolutions per minute (RPM) to more than 40000RPM e.g. “Turbo Expander” with heavy shafts up to 50 tons, in my project the shaft is not that heavy, it is about 500 kilograms; this shaft should be levitated in the center of rotating space by AMBs which are placed around the shaft. The electro magnets (as I think you know better than me) make electromagnetic force by the current flown in their windings, this force is opposed to the shaft in three dimensions (Radial and axial). The current is controlled by the control system (DSP or PLC) via a switching power amplifier. The position sensors light the position of shaft for the control system. As I told before the air gap between shaft and body is 0.6 millimeters so the set point for radial AMBs are 0.6 millimeter (assume that sensors are installed aligned with body) and if any outer force would be opposed to the shaft (a normal event in the process), the control system has very short time to readjust the current of AMBs to make a force equal and in opposite side of outer force to hold shaft in the center point. If the control system fails to response fast enough, the shaft may touch the body. Just Imagine if shaft touches the body in such high speed what could happen. If you want to know more about AMBs there are good references for this subject here and there on the web and also I have attached a file which explains how it works. - About the sensor I think I should clarify its mechanism; vibration sensors are in fact distance sensors which can measure the distance between sensor and the target object (shaft) with very high precision. Their maximum range is usually 2 centimeters and their accuracy is 1 micro meter in well known brands. They use eddy current method to find out the distance but the output (which is in mill volt) of the sensor does not change linearly with the distance so a signal processing is needed to make the output linear. This signal processor is called probe driver which is installed near the sensor because the length of sensor cable is limited by the manufacturer to nearly 40 centimeters to minimize the noise. The output of probe driver is 0 to 5 volts for 0 to 2 centimeters. Now let’s talk about frequency: if the frequency of the vibration of the shaft rise high to 15khz, the vibration sensor can measure the distance accurately but with vibration frequency higher than this level, sensor output will not remain accurate. I should mention that in order to control the position, the main thing which should be measured is distance not the vibration, but as I told the response time of the control system must be very low therefore position feedback should be updated very fast. I have checked multiple similar completed projects on the web. The lowest frequency for analog input was 15 kHz (40microsecond) and also for future developments I am thinking about optimizing the control system to have the ability of vibration canceling. For that goal I need fast analog inputs to monitor the vibration accurately. - About the control system for the AMBs, DSP is the most common and analog controllers are also used in earlier models, I think the PLC is not used until now to control the AMBs (we may do the first one). PLC does not make a fundamental change in control system but will make programming and monitoring easier which is important for me while I am testing mechanical and magnetic parts. You mentioned about a DSP with FBD language, which I think is a good option, I myself have chosen Texas Instrument C2000 family which has the ability of real-time control but should be programmed by C codes while I prefer FBD, I like to know about the DSP you have used. Wish not to become bored of my long explanation… I hope it would be complete…. I would be pleased to have your feedback…. And please tell me where else do you prefer to continue talking? Attachmentjsrao.pdf (331 Downloads) |
Last edited by: Ayoob at: 6/10/2014 10:22 AM |
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