LVDT Servo-amplifier help 2

[mtariq85]

Dear All!

I am currently working on my very first project and i am having some problems. Any suggestion and help will be much appreciated.

In my project, i am trying to sense the position of a spool from LVDT sensor. In my circuit for LVDT, i am using a pic to generate a square wave. But as my LVDT operates on AC signal, therefore i am using a square to sinewave converter to convert my waveform into sinewave.That signal is passing through LVDT and goes to a precision rectifier to convert my sinewave back to dc signal. But i am having following problems:-

a) I am getting 60 to 80mV noise on the output. How can i get rid of it.
b) Is it possible to generate a sinewave directly from the pic? If so any help please.
c) Is there any suggestion to improve my precision rectifier (demodulator) and square to sinewave generator.

Please accept thanx in advance for all ur support.

Regards.

 

[Skogsgurra]

That's a very crude approach you are taking.

The LVDT output has phase information. Use that by having a phase-sensitive detector rectify your signal. That will take out most of the noise. Low pass filtering will take out the rest. Control the detector switches with control signals from the PIC - essentially the same square wave that you feed to the sine converter. You may even try to feed the square wave directly to the transducer. It has been done before.

Lots of information on the net. Google LVDT and "phase sensitive detector". The "quadrature" phenomenon is very real in these circuits. Google that too.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

The LVDT will produce its best performance when excited by a sinusoidal primary voltage. It is fairly easy to build a simple oscillator running at about 1kHz. There are numerous circuits available which don't require the sophistication of a PIC, and the frequency does not require the stability of a crystal clock. Amplitude stability should be fairly good.

What positional accuracy are you trying to achieve from the LVDT? 0.1% is certainly achievable using a fairly basic detector circuit built using precision components. A clean sinusoidal excitation signal will almost certainly reduce the need for a more sophisticated detector capable of dealing with the non-sinusoidal secondary waveform. I guess it depends on where you want to expend your effort.

Skogs,

Most of my LVDT experience is on long travel types ranging from 3" or so up to 3' or so. The majority are Schaevitz designs. Are the small ones much more difficult to deal with in terms of the electronics?


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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

No. Short travel ones were just as easy. Or easier. I used to design circuits for Schaevitz transducers as well. It was mostly for grinding and machine-tool applications (checking dimensions of machined pieces) so we seldom had more than 10 mm travel.Often less. The interchangability between transducers was the problem if you tried to run them off square waves. But that was mainly because they were factory calibrated with sine.

Using a square wave and a simple synchronous detector made from a 4066 or a couple of FETs was really the easiest way of doing it.

Gunnar Englund

http://www.gke.org

 

[mtariq85]

Thanx everyone for replying.

skogsgurra!!

I understand ur point. So do u mean to say that i will have to use a phase sensitive detector between my LVDT output and precision rectifier (demodulator)?. If so will i have to provide an AC signal to phase sensitive detector as well from the same source which is providing input signal to LVDT? If so, then why? i mean this is my first encounter with phase sensitive detectors. Could u please explain its operation a bit? How will it help in decreasing the sound level.

Thanks
a lot

 

[Skogsgurra]

I'm getting confused now. "Sound level"?

Have a look at

http://www.lab-systems.com/products/amplifier/a308.html

for the kind of application Iam thinking about.

Gunnar Englund

http://www.gke.org

 

[mtariq85]

i am sorry.not sound level..i mean reducing noise at the output

 

[mtariq85]

can anyone please explain how phase-sensitive detector works. how does it reduce noise.

 

[ScottyUK]

Useful link Gunnar. Those little boards might get me out of a fix one day! Thanks.


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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

A phase-sensitive detector is essentially two semiconductor switches connected to the secondary winding of the LVDT. The switches are synchronized to the zero crossings of the sine (or square) wave driving signal. Positive sine -> positive switch closed and opposite for the other polarity. The plus switch feed the input of a difference amplifier with a low pass filter. When both signals are equal (LVDT in zero position), the difference is zero. When LVDT travels positive the difference is positive and when it travels negative the difference is negative. Noise is usually (shall always be) random with regard to driving signal and the low pass filter averages noise contribution so it becomes zero.

Gunnar Englund

http://www.gke.org

 

[mtariq85]

i have one connection coming from the output of lvdt. how should i connect it to two semiconductor switches. should they be in parallel? then should i connect the output from them to my precision rectifier? is that it?

 

[ScottyUK]

How many wires does the LVDT have in total? Internally all have six: primary, ; secondary 1; secondary 2. Sometimes internal connections are made between windings, or even some signal processing is mounted on board, reducing the number of output cores.


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Sometimes I only open my mouth to swap feet...

 

[mtariq85]

I have total four wires connected to my lvdt sensor. Two are for primary coil. two secondary coils are connected internally and therefore only two more wires are coming out. But i am only using one of the wire of secondary coil because the other is connected to ground. So in short i have four wires. First one is my input (primary coil) with which my sinewave source is connected. The middle two are connected to ground and the last one is my outpuut which is currently connected to my precision rectifier, but now i want to learn how can i connect to phase-sensitive detector (two semi-conductor switches).

 

[ScottyUK]

Can you gain access to all six wires? You will get much less noise if you connect each of the secondaries to a differential amplifier via screened twisted pair cable with the screen connected to ground, and process the signal at the output of the diff amps.


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Sometimes I only open my mouth to swap feet...

 

[mtariq85]

No i can't. I can have only four wires. Is there any way with four wires? If yes can u plz explain in simple words how can i connect anything between my lvdt output and precision rectifier to reduce the noise. I am sorry i am very new to this field. And as i mentioned this is my very first project. And i will be awarded new projects based on my performance in this project. So i really have to fix this problem.Thanx.