My minimum LVDTs voltage is 0.9 VDC, is it necessary to have 0.7 VDC for LVDT, since linearity is the concern

 

Linearity is the concern, but you haven't provided enough information to respond to your main question.

First, I'm not familiar with measuring the output of an LVDT in VDC; so I can't comment on that.

Second, what does the LVDT manufacturer's specifications say for the linear output range of the LVDT? Over what range of travel is the output of the LVDT linear?

Third, what is the range of travel of the device the LVDT is attached to?

In other words, with the "minimum voltage" (DC or AC) set at 0.9 VDC, does the output of the LVDT remain linear over the range of travel of the device the LVDT is attached to? Have you stroked the device to which the LVDT is attached while measuring the LVDT output voltage? Is the LVDT output voltage linear over the entire range of travel of the device of the device to which it is attached?

Fourth, what is the LVDT being used to measure?

Fifth, what is the control system the LVDT is providing the indication to? Does the control system require some specific voltage feedback range to operate properly?

Sixth, where did the 0.7 VDC "minimum voltage" value come from?

You have provided very little useful information in your post. The question you asked is like someone asking you, "If I close the switch of the electric light in my bedroom will it provide more or less light than what is coming in through a partially open window?" There's just not enough information to provide a useful response.

If you have measured the LVDT output over the entire range of travel of the device it is attached to, and if the LVDT output voltage remains linear with the "minimum voltage" set at 0.9 VDC, and if the control system to which the LVDT is providing the indication can work with the range of voltage being provided by the LVDT, then the answer is probably that everything is okay. Because you will have proven that the LVDT output is linear over the range of travel of the device it is attached to with the minimum voltage set at 0.9 VDC, and if the control system can work with the voltage range then everything is most likely fine.

But, we don't have enough information about the LVDT, the device it as attached to, the specifications of the LVDT, and the control system it is providing the indication to in order to provide a meaningful response.

 

> Linearity is the concern, but you haven't provided enough information to
>respond to your main question.
>
> First, I'm not familiar with measuring the output of an LVDT in VDC; so I can't
>comment on that.
>
> Second, what does the LVDT manufacturer's specifications say for
> the linear output range of the LVDT? Over what range of travel is the output
>of the LVDT linear?

0,7 V is the traditional closed value described in the control specification that GE uses for their Gas Turbines. GE steam turbines don't worry so much about this value and even use 1 V or 5V depending on the phylosophy used for fail safe.

I would check linearity and also check the value of the adjacent LVDT. I guess you have unscrewed the locknut and tried to adjust the 'faulty' LVDT.In case you have any doubt it is recommended replacement being such an important device. Also try to see the voltage seen by the controller,this can be done in both MkV and MkVI.

Regards.

 

> Linearity is the concern, but you haven't provided enough information to
>respond to your main question.

LVDT linearity and settings

Linearity will not be a problem as long as you are using the LVDT as per the manufacturer specifications. the important thing is to know operating range of the value being measured and the corresponding LVDT voltage range. Let us take an example of a valve opening.<pre>
Valve position LVDT voltage LVDT displacement

o% (Pmin) 0.7 (Vmin) 50mm
100 %(Pmax) 2.5 (Vmax) 200mm

the valve position at a voltage V' of the LVDT is calculated by the formula

Vlave position = ((V' - Vmin)/(Vmax-Vmin))*(Pmax - Pmin) </pre>
This is general equation used for single sided measurement using LVDT. Care should be taken that the LVDT displacement is within the manufacturer specification for obtaining best possible accuracy.

From the above you see that you need to know the actuator min position , max position and the corresponding LVDT voltage for the same. If you get the above four parameters right you will have a accurate position indication at hand. Problem mostly happens when you wrongly configure the Pmin and Pmax setting in the controller. you can use the above info to intuitively configure what ever that you are trying to measure with the LVDT.

 

> 0,7 V is the traditional closed value described in the control specification
> that GE uses for their Gas Turbines.

The GE Control Specification usually lists <b>0.700 VAC RMS</b> as the "zero stroke" voltage setting for the LVDT feedback. Not 0.7 VDC. But we don't know what the application is, or if this is for an LVDT being used on a GE-design heavy duty gas turbine with a Speedtronic turbine control panel.

The original post did not have sufficient information. It was hoped the original poster would provide more information and clarification.

 

sorry typo, i was revised the thread to 0.9VAC and now i know what the PREVIEW function is
you're correct the voltage is VAC rms.
thanks for the replies,
since it's VAC does it matter with polarity?

 

I'm trying very hard to understand how to tell you that you will get a much better, more concise, and faster response if you provide as much information as you can in your original post.

All LVDTs are not the same. All applications of LVDTs are not the same. All control systems do not handle LVDT inputs in the same way.

If you would tell us what you are doing and what it is you're trying to understand we could be more helpful.

If you don't want to take the time to tell us what you're doing and what you're trying to understand, then use Google and do some searches for LVDT and you will find lots of videos about LVDTs and how they operate. You may still have some questions, but you might be better able to state what your concerns and questions are.

We aren't there alongside you and we don't know what you know, and what you don't. The more information you can provide us, the better the reply you will receive in return.

As for polarity, no; it's not important. HOWEVER, the connections of the LVDT to the control system are very important. If you're replacing an LVDT with a like LVDT, they both probably have colored leads and you should connect the colored leads of the new LVDT to the same terminals as colored leads of the LVDT being replaced. That should ensure (presuming the new LVDT is a direct replacement for the old LVDT) the connections are correct.

Then, the next thing to do is to set the minimum stroke voltage to ensure the LVDT output is on the linear range of output.

If the new LVDT is a direct replacement for the old LVDT, that should be all that's required, except for the calibration of LVDT feedback.

If the new LVDT is NOT a direct replacement for the old LVDT, then you have other work to do.

BUT, we don't know what you're trying to do. Because you haven't told us. And because we're not not there beside you and you haven't told us we can't provide much in the way of useful information.

When writing to a forum such as this, the more information you provide the more satisfied you will be with the response.

We can make (and some of us have made) assumptions, but they may or may not be correct.

Best of luck in your endeavor!

 

By the way, you must use a True AC RMS voltmeter to measure the LVDT output. Just any AC voltmeter won't do the trick, and even some models of very expensive Fluke calibrators are NOT accurate at measuring VRMS above 1 KHz, so check the specifications of the meter you are using carefully.

You will be VERY surprised at the differences in readings between a True AC RMS voltmeter and one that is not a True AC RMS voltmeter.

If you don't have access to the specifications for the voltmeter you are using and cannot find them on the Internet, if it doesn't say on the face of the meter that it's a True AC RMS voltmeter--it's probably not. One pays extra for this accuracy, and the meter manufacturers usually "advertise" the feature prominently somewhere on the face of the meter. So, in other words, if in doubt--it's probably NOT a True AC RMS voltmeter.

 

Dear CSA,

Thank you, i was installing new LVDTs for IGV, and using TSM to see the voltage. it's good to have all of your thoughts especially to deal with GE guys there.

...is it necessary to employed by GE to be good one?

 

koskos,

Well, you seem to be on a parallel path, rarely speaking in full sentences, assuming others know exactly what you're talking about all the time, providing no real data preferring anecdotal information to actionable information, and misreading and misquoting most documentation.

 

More info on LVDT's here

http://www.daenotes.com/electronics...LVDT-linear-variable-differential-transformer

 

Yes 0.7VDC even 0.5VDC is the traditional closed value for many LVDT. For more information of LVDT, please refer to www.itargetsensors.com