LVDT calibration

I want to know the correct procedure for calibration of LVDT of GCV/SRV in Mark_VI Machine. And how to download the .m6b file to controller after changing the null bias value. Any body please suggest Reply to this post... Posted by Process Value on 14 November, 2010 - 2:02 am It is a pretty straight forward procedure. I am uploading a small document here , in that you will find the detailed procedure including the screen shots of how to do the calibration. http://www.2shared.com/document/TSYug7uX/sevo_calibration.html Hope this will help you in clearing your doubts in this regard. Reply to this post... Posted by chris on 15 November, 2010 - 8:20 am >I am uploading a small document here , in that you will find the >detailed procedure including the screen shots of how to do the >calibration. > >http://www.2shared.com/document/TSYug7uX/sevo_calibration.html Link is not working ... Getting message invalid link. Is it possible for anyone/public to download ? Please advise Reply to this post... Posted by Process Value on 18 November, 2010 - 12:07 am Chris the link is working well , only problem is that they have disguised the download button very well. in the bottom of the page you will have a small button which will say "save file to computer" you need to click it to down load the doc. till now 20 downloads have been done so i think it has worked for that many people. Reply to this post... Posted by CSA on 15 November, 2010 - 11:16 am ProcessValue, Can you explain specifically your section on null bias current? I'm not sure what the intent is nor how to accomplish what is to be done. Reply to this post...

Posted by Process Value on 18 November, 2010 - 12:03 am Servo null bias configuration. Here comes the long answer once again :) Servo valve operation : to explain in detail about the servo null bias one should know how a 2 way servo valve actually works. i am uploading a small pic here in which i have explained in detail the working of a flapper/nozzle servo which is used by moog. the pics that you will see represent a very similar design used by moog and by all servo manufacturers. a. first part - the main parts of a servo is explained. b. operation of a servo valve to move the actuator to the right is explained ( the reverse operation is vice versa) c. the resetting of the servo valve after the required movement of the actuator is explained http://www.2shared.com/photo/_LkyHPfy/servo_expain.html please read through the above so that what i am going to say below can be understood in greater detail. In the pic above the centering spring is present on both sides of the servo spool piece. This centering spring is responsible to bring back the servo spool to the initial position so that there is no control oil flow into the actuator chamber. in our case there is only one spring installed in the system. this spring is called the failsafe spring/ compression spring . in the absence of any electrical commands this pushes the spool piece to open a very small port which causes the actuator rod to retract (open/close the vavle depending on the design). During normal operation , such a thing should not happen , and thus the current required to over come the spring force and return the servo to null (zero control flow) position is called the servo null bias.The servo null bias is often expressed as a percentage of the full scale servo current. in moog design its 10 ma full scale and for a null bias current of 0.2 ma means a null bais of 2%. i am uploading another pic here , in that you will see the servo chara for a typical moog valve ( control flow vs. input currnet ) in the same i have marked the servo null current for better understanding. http://www.2shared.com/photo/4CNdVGGl/moog.html now coming to the null bias setting in mark Vi.

in mark vi TMR systems the servo is a three coil one , with each of the three processors supplying the current to the servo. in GE designs the servo feedback is accomplised by two methods. a. LVDT - IGV / Gas based designs b. pulse rate inputs - fuel valve in distillates the servo null bias has a range of about -0.133 ma to -0.4 ma. the required null current will be somewhere between this value. this is the usual norm but it may not be true for all cases. i have seen a site with a null current of -0.75 ma in IGV conrol servo working perfectly. please see the servo data sheet to get the actual value. for an example let us say that we have fixed the null value at -0.15 in the servo configuration (-1.5%) . now given that the processors are online and you are in the servo calibration page use the manual command to give a stroke of 50%. if you have a LVDT then you can see the value easily other wise you need a dial guage to measure the valve stroke. let us say for the stroke of 50% we get the following values. R - 47.5 s - 47.8 t - 47.7 dial guage - 48. now this means that the control oil ported is not enough to attain the correct stroke and you need to increase the null bias value , theoretically a 2% increase in the null bias ie from 1.5% to 3.5% must get you to 50% value (assuming that the dial gauge reading is most accurate), but it rarely happens; with three processors and three coils and three feedback inputs attaining a perfect 50% is well quite difficult , anything in the 49-51% range is acceptable (at least for me). further calibration is quite tedious and frankly a waste of time. now enter the new null value (3.5%) in the servo calibration page and do the stroking once again , you will get a value close to 50%. if you still want to have some fun you can change the resolution of the servo null bias and try to get the perfect fifty ( upto 5 decimal places are accepted if i remember correctly). most probably you will see that in GE machine the null bias value to be at -2.667 %. this is the median value of the range specified by moog , and yes this will work most of the cases. it is quite rare that you need to change this value. this is to the best of my knowledge how a servo works and this is the method to calibrate the servo null current. hope CSA will be satisfied by the explanation and can give more pointers in the area :) . Reply to this post...

Posted by cikenkari on 27 December, 2010 - 8:10 am Process Value, can you reupload the files, seems the link is dead. Thks Reply to this post... Posted by Process Value on 27 December, 2010 - 12:39 pm Sigh ... well the links are indeed working, as i have said in the earlier post at the bottom right side of the page there is a posting like this Save file to your PC: click here click the link and you will able to download the file. from the last time i saw the screen it is now downloaded 44 times :) hurray :). Reply to this post... Posted by cikenkari on 28 December, 2010 - 4:13 am i dunno, but the message says "The file link that you requested is not valid. Please contact link publisher or try to make a search. " Please upload to another file sharing, or send email to cikenkari(at)gmx.com Thanks ya Reply to this post... Posted by cikenkari on 28 December, 2010 - 4:18 am Seems the link can't find the file, so using search to find "sevo_calibration", thanks Process Value, very nice procedure Reply to this post... Posted by Process Value on 28 December, 2010 - 8:12 am Good to hear that you did download the file , and as far as the invalid link i do not know what the problem is i will upload to a different site and post here. and yes glad to be of help :) Reply to this post... Posted by CSA on 2 January, 2011 - 10:17 pm Well, I'm going to have to take exception to some of the material presented here. While the basic premise is correct, the drawings used to explain the concept along with the disclaimers about the drawings, and the math doesn't seem to work out.

If there is a centering spring that returns the servo spool piece to a condition that blocks oil and prevents opening or closing of the servo-operated actuator, isn't that contrary to the "fail-safe" concept? In other words, in the absence of any servo current, the servo valve is supposed to port oil to the actuator to shut off the flow of fuel or air or steam. If there were no current applied to the coil in the drawing, the centering spring would just "block" the actuator in its current position, which could keep fuel or air or steam flowing in a "loss of control" situation (no servo current). So, this doesn't seem to make sense. ProcessValue then says, oh, forget about the centering spring, because the servos at his site only have a spring at one end. A spring at only one end would cause a properly applied servo to shut off the flow of fuel or air or steam in the event of loss of servo current to the coils--which meets the definition of "fail-safe". So, we're given some drawings with explanations that we're told to read and study, and then we're told, "Oh, that's okay, but that's not what really happens." The Speedtronic servo regulators used by GE put out 0.00 mA when the actual position is equal to the reference (the error is zero), and then the null bias current value is added to that to overcome the null bias spring tension in order to keep the actuator/device at the desired position. GE specifications for servos used for heavy duty gas turbines state that the null bias current (total) should be -0.8 mA, +/- 0.4 mA. Take that value and divide it by three for a three-coil servo and you get -0.267 mA per coil, for a total of -0.8 mA. Presuming that no one has adjusted the spring tension of the single fail-safe spring of the servo (which should never be necessary except under extremely unusual circumstances), an individual null bias current of between -0.133 mA to -0.400 mA should be sufficient to make the measured position match the indicated position (from the LVDT feedback). ProcessValue makes a very important and excellent point in the explanation: The null bias current adjustment should only be used to make the actual position (and, hence, indicated position) equal to the reference. It's not to be used to make the indicated position (feedback value displayed on the screen) equal to the reference if the actual position isn't being measured! In other words, the calibrated feedback value needs to be very nearly equal to the actual physical position as measured (using a dial indicator or vernier caliper, or machinists' protractor, as appropriate) and then and only then should the null bias current be adjusted to make the physical position (feedback) equal to the reference value. So, if the reference is 50% and the actual position is 48.9 and the indicated position is 49.0, then one can adjust the null bias current to make the actual position equal to the reference value. And we're speaking about averages here for a three coil servo. If one processor's indicated feedback is radically different from the actual position (and hence the indicated

positions of the other two processors) then adjusting the null bias current value is not the proper method for correcting this problem. And, lastly there's this statement about changing the null bias current by 2% to counter a 2% error between the actual physical position and the reference. I've never witnessed a one-to-one relationship between current and position reduce the error between actual and reference positions. It's not that simple, and a trial and error method is usually required. (And why was the original null bias value fixed at -1.5% to begin with? Why wasn't it -2.67%? The error would have been less had the original value been -2.67%, which is what the starting value should always be.) Again, I want to stress that the most valid and important points in ProcessValue's explanation are these: (1) The null bias current is to be adjusted to make the actual, measured physical position match the reference after the calibrated feedback has been proven to match the actual physical position by comparing the measured physical position to the calibrated feedback. If the actual, measured physical position is not very nearly equal to the calibrated feedback value on the display, then the feedback IS NOT calibrated properly, and adjusting the null bias current value isn't going to fix this error. (2) The normal range of null bias current values should be between -0.133 and -0.400 mA per processor, or, -1.33% to -4.00% (remember, in most Speedtronic systems the negative sign is not used; the positive null bias value on the display is "inverted" in the Speedtronic). Most sites never measure the valve or IGV position and just adjust the null bias current to try to make the feedback value on the display equal to the reference--and that's just WRONG! Or, they try to adjust the null bias value when one processor's indicated value is significantly different from the other processors' indicated value. By the way, the feedback spring in the drawings provided by ProcessValue would usually be the "fail-safe" element in a servo with a centering spring. Null bias current would be required to overcome the force that the feedback spring would apply to the coil armature in the loss of servo current to cause oil pressure to be ported to the actuator to shut off the flow of fuel or steam or air. Presuming the application required a fail-safe servo. Reply to this post... Posted by Process Value on 6 January, 2011 - 4:30 am > Well, I'm going to have to take exception to some of the material presented here. While the basic premise is correct, the drawings used to explain the concept along with the disclaimers about the drawings, and the math doesn't seem to work out. < well here are the answers to the "exceptions" found by CSA :) . we all love a healthy argument don't we :)

exception - 1 "If there is a centering spring that returns the servo spool piece to a condition that blocks oil and prevents opening or closing of the servo-operated actuator, isn't that contrary to the "fail-safe" concept? In other words, in the absence of any servo current, the servo valve is supposed to port oil to the actuator to shut off the flow of fuel or air or steam. If there were no current applied to the coil in the drawing, the centering spring would just "block" the actuator in its current position, which could keep fuel or air or steam flowing in a "loss of control" situation (no servo current). So, this doesn't seem to make sense." All servo valves manufactured need not have a "fail safe" concept , and fail safe concept need not necessarily mean a full open or a full close condition. an example of a servo system which does not have a "fail safe concept" of full open or full close is the damper control in an FD fan. these i have typically seen in boilers , and in some furnaces. most boilers follow a air rich or fuel rich control mode ,in both the concept the air flow is maintained by these dampers. in case we need to increase in the boiler load the damper opens first to let in more air and only if there is a air increase the fuel is increased to the boiler. Here the inlet air needs to be maintained at 3-4% excess of the stotiometric air. incase of absence of electrical signals the servo in the damper control just holds the damper in the last position. you cannot close the damper as the air inlet will be reduced , leading to black smoke then eventual tripping of the boiler , you cannot fully open the damper as you will let in excess air which leads to white smoke in the chimney , very inefficient operation , slight load reduction and lowering of stack temperature , lowering of temperature gradient in the boiler zone and a whole lot assorted problems. thus the servo just holds the damper in the last position. this servo "does have" a centering spring. Servos are hard to explain without any diagrams , hope you will agree with me on that one. i got the pic from the net , i modified it to a large extent to suit my explanation. I was trying to tell people how a servo actually works so that one can get a grasp of what the null current is. I do not think that anyone who does not know how a servo works will understand the concept of null bias current. i provided the explanation specifically for that. exception - 2 "ProcessValue then says, oh, forget about the centering spring, because the servos at his site only have a spring at one end. A spring at only one end would cause a properly applied servo to shut off the flow of fuel or air or steam in the event of loss of servo current to the coils--which meets the definition of "fail-safe". So, we're given some drawings with explanations that we're told to read and study, and then we're told, "Oh, that's okay, but that's not what really happens." " ha ha ha hurray for the tongue and cheek once again but i would like to add that my "Parent site" has all kinds of servos one with centering spring ( boiler FD fan in boilers ) ,

one with the fail safe spring at one end ( GT fuel recirculation moog valve for one ) , servos with no compression springs ( desalter rotary stirrer in CDU unit). In my opinion the explanation given in the diagram with the "centering spring" servos is correct , and i don't remember saying that is not happens (it happens at a lot of places as i explained) , i just said that is not what happens in GT fuel control. exception - 3 " And, lastly there's this statement about changing the null bias current by 2% to counter a 2% error between the actual physical position and the reference. I've never witnessed a one-to-one relationship between current and position reduce the error between actual and reference positions. It's not that simple, and a trial and error method is usually required. (And why was the original null bias value fixed at -1.5% to begin with? Why wasn't it -2.67%? The error would have been less had the original value been -2.67%, which is what the starting value should always be.) " i am copy pasting my exact wordings " theoretically a 2% increase in the null bias ie from 1.5% to 3.5% must get you to 50% value (assuming that the dial gauge reading is most accurate), but it rarely happens; with three processors and three coils and three feedback inputs attaining a perfect 50% is well quite difficult , anything in the 49-51% range is acceptable (at least for me) " . You just repeated what i have told ; it rarely happens , meaning there in never a perfect lenier relationship. I was giving an example by quoting that you begin with -1.5%., i have also mentioned in the post that a null bias value of -2.67% will work out in most cases and it is rare that you need to change them. and yes i always start with -2.67 % . so i hope that there are no points of singularity in my explanation , and the exceptions regarding the material , disclaimers ??!! and math are all resolved. further discussions , criticisms ;) from CSA are always welcome. Reply to this post... Posted by CSA on 6 January, 2011 - 7:09 pm We were, and aren't, talking about servos used for FD fans or rotary stirrers. The thread was about: >What is the correct procedure for calibration of >LVDT of GCV/SRV in Mark-VI machine. In the case of servos used for GE-design heavy duty gas turbines (as this thread regards), fail-safe operation is required. I don't see posts from ProcessValue in any threads other than GE-design heavy duty gas turbine-related threads (with the exception of speed/frequency control threads), so it's odd that FD fans and rotary stirrers are introduced into this thread. You say you've read and learned a lot from previous GE-design heavy duty gas turbinerelated threads on control.com, which were kept as "real world" as possible. The driving

concern has always been to present information that is relevant to the discussion, and most closely matches real world circumstances that the originator, or a subsequent poster, would encounter. Nothing more; nothing less. (Okay; an occasional editorial comment, meant to reinforce the information.) Diagrams of servos used on GE-design heavy duty gas turbines can usually be found in the Service/Instruction Manuals provided with the units; not always, but quite often (at least, they used to be included in years past in the Control System section). I agree, that it's difficult to explain servos without diagrams, but not impossible. And, some people have difficulty understanding any drawings, even with notes and circles and arrows and paragraphs; even engineers! (Sad, but true.) Your treatment of the null bias current adjustment issue in your write-up was lacking and superficial, and presumes that changes to the null bias current value are always necessary. And the explanation wasn't that much better. There are thousands of GE-design heavy duty gas turbines with TMR Speedtronic turbine control panels running around the world at this very instant (or any instant, for that matter) using null bias current values of -0.267 mA per coil, and they run just fine with no perceptible problems whatsoever. At the same instant in time, there are likely tens, if not more, GE-design heavy duty gas turbines running around the world with wildly incorrect null bias current values, and they, too, run just fine with no perceptible problems whatsoever (at least that people want to pay attention to; remember, Diagnostic Alarms are just nuisances, and as long as the unit doesn't trip, most people ignore them, and even if the unit trips, they ignore them still). I can probably count on one hand the number of sites that I've visited in nearly three decades that actually use a dial indicator (or machinist's protractor for the IGVs) when calibrating LVDT feedback, or that even use the scales provided at the valves/IGVs for checking the accuracy of the calibration. They just "click" and "click" and "click" and ass-u-me that the calibration is correct, without anyone ever observing the device, or without ever measuring the physical position. And, again, there are literally thousands of GE-design heavy duty gas turbines running without any perceptible problems around the world, but with inaccurate LVDT feedback calibrations (for a variety of reasons, not just improper null bias current values). In some cases, very inaccurate LVDT feedback calibrations, by the way. And, until some time in the last 10 years there was virtually NO null bias current adjustment ever made or even attempted during most LVDT feedback calibrations. It's really only since the advent of DLN combustion systems and the need for very accurate position feedback calibration in order to use fuel splits to control emissions very precisely

did this issue ever arise. And, it has arisen with virtually no proper documentation or instructions from GE or its packagers whatsoever. Bad documentation is worse than no documentation. It causes people not to trust, and even not to read, documentation going forward. It causes people to put stock in myths, legends, and wives' tales, and worse, to perpetuate them over time. That's something which should be ended. To quote a GE T.A. I worked with many years ago, "This stuff ain't rocket science!" No matter how people try to make it out to be rocket science. Providing concise and correct information with real world application and examples relevant to the questions being asked is the goal. The only thing I'm going to argue is whether or not correct and relevant information is being provided. This is trying to keep the information presented on track, relevant, and correct. It seems that, as with the incorrect usage of the word "doubt" to express a question or questions, some want to use the word "argue" to mean discussion. When I see incorrect and irrelevant information being provided, I'm going to call attention to it. Just as others have chosen to do. Reply to this post... Posted by Process Value on 7 January, 2011 - 8:43 am I still do not understand what is irrelevant or wrong about the information i have given. I did not write about Boiler internals in a servo related thread. The simple thing is that the picture presented there provided a clear idea about a servo working. i have MOOG drawings , data sheets etc , but they have cross sectional diagrams , and it did not have the actuator part in it as in the one i have presented. I do not know how a 10 point explanation about the working can be called superficial. If you have a better explanation please post it in here i would be glad to read it. i have gone through many control.com posts but i have not seen anyone post a picture or an accompanying explanation. It is not that i reply to gas turbine related posts alone, i have also replied to steam turbine and boiler related posts also.It just so happens that not many people here ask about boilers. My career started with the commissioning of a co-generation steam turbine power plant. and i have been maintenance coordinator for boiler and HRSG shutdowns. " Your treatment of the null bias current adjustment issue in your write-up was lacking and superficial, and presumes that changes to the null bias current value are always necessary. And the explanation wasn't that much better." i have never said in my post that null bias changes are always necessary , i have said quite in the contrary , the value of -2.67 % almost never needs to be changed ; please go

through my earlier posts. Explanations are liked by some , disliked by some , if you did not like it , tough luck ... but the explanations are the best i can do and for the given diagram they are correct , and i am pretty sure a lot of people out there would have gained "correct" and knowledgeable information from it.you probably do not need them , as you have seen them for a long time. but let me assure you control.com is visited by a lot of new and young engineers , i was one of them a couple of years back. Information and knowledge that you take for granted is not the same for all people , i have been there first hand. And providing a additional bit of information is not going to hurt them or anybody. This is my way of giving back to the community from which i have learned. " There are thousands of GE-design heavy duty gas turbines with TMR Speedtronic turbine control panels running around the world at this very instant (or any instant, for that matter) using null bias current values of -0.267 mA per coil, and they run just fine with no perceptible problems whatsoever." The question was about LVDT calibration , yes it is true that thousands of machine are running with the standard value but what was i supposed to say. Just fix up -2.675 it will work fine. i just uploaded the document which had the procedure. then you asked to explain more in detail about null bias current , which is why i posted the second thread with the explanation about the servo and all. i agree that most of the units do not use the dial gauge , I do , the people who taught me to do it did the same ; so you can add one more to your count on that. " To quote a GE T.A. I worked with many years ago, "This stuff ain't rocket science!" No matter how people try to make it out to be rocket science. Providing concise and correct information with real world application and examples relevant to the questions being asked is the goal. " Actually Moog is more famous for using its servo valves in flight controls , missile controls and yes Space craft controls. Servos form a integral part of flight control and missile control systems , so they do form a small part of rocket science. And lastly , you would probably know this but i am not getting anything out of posting servo workings in control.com , The explanation that you deem superficial took me a whole 2 hours ; to hunt a good picture , modify and present. The only thing that i am getting out of posting the irrelevant servo information is the satisfaction that i am giving back to the community which had helped me out. you have every right to raise an attention when you see a irrelevant material , but try to appreciate the fact that i am taking the time to help out young engineers and readers with information which is to the best of my knowledge correct. This will be my last posting in this thread.