measurement of the dynamic pressure by IEPE sensor

[przemo]

Hello, I work on cylinder pressure measurement in combustion engine. I have a piezoelectric pressure sensor with charge output 1 pC/psi and an inline charge amplifier 20 mV/pC

what gives overall sensitivity 20 mV/psi.

As my instrumenst are of an IEPE standard I decided to use NI9233 acquisition board. Unfortunately the output signal is far from the truth. I have checked sensor/amplifier with another acquisition device and they work correctly. I have 2 modules and both give same results. I have checked some parameters  and seems that the output signal is proportional to the logarithm from the pressure differential.  When I try to set the module configuration in MAX there is no pressure measurement available however there is force and acceleration.

Is it possible to use this module for pressure measurement? Somebody experienced similar problems?

Regards,

Przemek

[preston_johnson]

Thank you for your post. 

It sounds like your charge based peizoelectric pressor sensor has the signal conditioner giving you a voltage output of 20mV/psi.  Unfortunately, the 9233 always has the IEPE power turned on.  The 9233 cannot have the IEPE turned off.  The 9234 came out a few years ago and allows the ability to turn off the IEPE so it is possible to work with voltage sensors.  It is likely that the IEPE on (20V bias and 2ma current) is affecting your measurement. 

Yes in MAX, DAQmx does not have a pressure measurement.  You can use a custom scale, name it PSI, and set the scaling to 20mV / psi.  You can also make this multiplication in your source code, such as LabVIEW. 

Do you have another C series voltage input module to try?  The 9239, 9229, and 9234 are excellent 24bit choices.  The 9215 and 9205 are good 16bit choices. 

[przemo]

Many thanks for fast reply.

But my signal conditioner is of a IEPE standard and it needs a 2mA excitation fromthe 9233 module.

When I was collecting all istrumentation it was all quite easy: charge sensor + charge amplifier of IEPE standard + module 9233 to supply IEPE sensor = succesful measurement.

Now it is not working and I can not trace what is wrong.

Hope I get some clue at this board.

[fanman]

If you are using an IEPE pressure transducer you don't need the charge amplifier as the 9233 servers that function. IEPE pressure sensor + 9233 module = successful measurement.

[preston_johnson]

Can you share the model of the sensor and the charge converter you mentioned in your first post?

Do you have a graph of the time waveform you digitized with the 9233?

[przemo]

Here are the parameters: Sensor Maker: Dytran, type: 2200C5

Charge amplifier Maker: Dytran, Type: 4705M5, charge amplifier is of an IEPE standard.

 I have attached their specification cards too.

In the file signals.jpg you can find my calibration experiment record. I was applying step-like pressure (blue line) to simulate rapid pressure increase. the value was 120 bar. I was expecting that the answer signal will be a pulse proportional to the step pressure. I have noticed that the answer depends from how fast the pressure increase, that means from the derivative of the pressure signal. I will attach one more file where you can see that it is proportional to the logarithm from that derivative.

Honestly I do not understand it at all now.

[przemo]

Here one more file. One digram is pressure versus dytran signal (peak value)

next diagram is derivative versus dytran peak signal.

[preston_johnson]

Interesting,

can you try a slow increase and decrease in pressure?  With the step function in pressure, you may be exciting the resonant frequency of the pressure transducer. 

[przemo]

As you can see from the file digrams.pdf, I have done the test for 5 different speeds of pressure step (lower diagram). The speed of the pressure increase was from approx 190 bar/s up to 1050 bar/s. the final value of a pressure stem was from 114 to 121 bar (upper diagram). When I increase the pressure very slow there is almost no answer from the measuring chain - output voltage barely higher from noise.

When I apply the sensors in the engine combustion chamber I have pressure increase with speed up to 12000 bar/s and the resulting values are also incorrect.

I was trying to answer the question: there is problem with sensor/amplifier or with the 9233 module or perhaps module doeas not fit the sensor/amplifier?

this question is still open for me.

If you

[przemo]

Hello,

I have a diagram with a very slow pressure increase speed, there are two faster steps too. Upper blue course is a pressure signal when lower green course is an answer from dytran read by 9233. The final step pressure was still approx 120-125 bar. For testing I use compressed air from diving aparatus cylinder so after every discharge the next final pressure is slightly lower. the difference from step to step is no more than 1 bar.