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High frequency strain gaging - Analyzing a signal

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Steelmesh

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I am doing research about if there would be a cost effective way to analyze a high frequency strain gage signal. I want to test the dynamics of valvetrain components in an engine. I would be using standard 120 ohm strain gages and a 3-5v excitation signal in a quarter-bridge circuit. A strain gage's resistance changes with strain, so it is typically one of the bridges in a wheatstone bridge circuit.

I found this inexpensive PC based analyzer:
PCS500

I would like to have a data point for every one degree of camshaft rotation. I may be going off into an alien world, please bear with me. So basically at 10,000 crankshaft RPMs the camshaft is turning 5,000 RPMs (a camshaft controls the engine's valvetrain). So there are 83 camshaft rotations per second. 1 camshaft rotation takes 0.012 seconds or 12 ms. I want a data point every 1 degree of camshaft rotation, so that is 0.033 ms per 1 degree camshaft rotations. If I got that right, I think I need to record at a frequency of 33 kHz.

Is the inexpensive analyzer like the PCS500 be able to obtain the measurements I need?

Do I need a boost converter to amplify the strain gage signal to get a readable measurement? (I believe using 120 ohm resistors it will output about 30-50 mV)

Thank you in advance,
Ken
 
That's less than 100hz... A PC sound card could pick that up if you adjust the voltage. And when I say adjust the voltage, I mean adjust it DOWN. Peak to peak on a sound card is usually about 1 volt.
 
I am doing research about if there would be a cost effective way to analyze a high frequency strain gage signal. I want to test the dynamics of valvetrain components in an engine. I would be using standard 120 ohm strain gages and a 3-5v excitation signal in a quarter-bridge circuit. A strain gage's resistance changes with strain, so it is typically one of the bridges in a wheatstone bridge circuit.

I found this inexpensive PC based analyzer:
PCS500

I would like to have a data point for every one degree of camshaft rotation. I may be going off into an alien world, please bear with me. So basically at 10,000 crankshaft RPMs the camshaft is turning 5,000 RPMs (a camshaft controls the engine's valvetrain). So there are 83 camshaft rotations per second. 1 camshaft rotation takes 0.012 seconds or 12 ms. I want a data point every 1 degree of camshaft rotation, so that is 0.033 ms per 1 degree camshaft rotations. If I got that right, I think I need to record at a frequency of 33 kHz.

Is the inexpensive analyzer like the PCS500 be able to obtain the measurements I need?

Do I need a boost converter to amplify the strain gage signal to get a readable measurement? (I believe using 120 ohm resistors it will output about 30-50 mV)

Thank you in advance,
Ken


Well your timing calcs seem to be correct at first reading. However I would first check the data sheet for your "standard" strain gauge and see what it's response time is as that my be the limit of how accurate and fast the readings can be. Also, how you going to mechanically couple the strain gauge to the cam shaft my be problematic as there are bound to be other engine vibrations that can mask the signal you are interested in.

I would suggest that you would first want to look at the strain gauge output signal with an oscilloscope to see if the wave form looks like something that will have the signature signal you are hoping to analyze. I suspect this is going to be much more difficult then might be imagined, but worth some investigation.

Lefty
 
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Well your timing calcs seem to be correct at first reading. However I would first check the data sheet for your "standard" strain gauge and see what it's response time is as that my be the limit of how accurate and fast the readings can be. Also, how you going to mechanically couple the strain gauge to the cam shaft my be problematic as there are bound to be other engine vibrations that can mask the signal you are interested in.

I would suggest that you would first want to look at the strain gauge output signal with an oscilloscope to see if the wave form looks like something that will have the signature signal you are hoping to analyze. I suspect this is going to be much more difficult then might be imagined, but worth some investigation.

Lefty

Hi Lefty,

I appreciate the advice!! I'm so lucky to have some of these issues you brought up already rectified. My employer already utilizes a series of Vishay strain gages for the dynamics testing that I'm proposing. Also, the components I would be looking at are; pushrods, rocker arms, and valve springs. I stepped into a system that has already been in place, so I'm just trying to decipher the setup so I can do this stuff at home and for my friends. They use a Nicolet Odyssey data acquisition system.

I did some searching last night on this forum, I see some stuff about amplifying the signal...is that pretty straight forward?
 
Hi Lefty,

I appreciate the advice!! I'm so lucky to have some of these issues you brought up already rectified. My employer already utilizes a series of Vishay strain gages for the dynamics testing that I'm proposing. Also, the components I would be looking at are; pushrods, rocker arms, and valve springs. I stepped into a system that has already been in place, so I'm just trying to decipher the setup so I can do this stuff at home and for my friends. They use a Nicolet Odyssey data acquisition system.

I did some searching last night on this forum, I see some stuff about amplifying the signal...is that pretty straight forward?

Pretty much, you will want to search on Instrumentation opamps as they are the best for low level signal processing. You can set the voltage gain with one resistor. You will need a stable noise free voltage source to excite the bridge and all cabling from the strain gage should be sheided and properly terminated. Filtering of the low level signal is sometime required to high a good signal to noise ratio.

Good luck, sounds like an interesting and challenging project.

Lefty
 
Hi
Steelmesh i am playing around with strain gauges at the moment did you read the posts (op amp resistors) .
How are you attaching s/g as mine are glued in posistion
Regards
bee
 
Hey Bee,

Yes, we use Vishay products to prepare the surface and glue to ferrous surfaces (i'm not advertising lol). The procedure for our applications may deviate from their instructions, but they seem to hold up well. We also use an oven at about 175F to speed up the cure times as well. If you need some help getting a durable and secure attachement let me know.

I will explore your post, thank you for the heads up!

Ken
 
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Hi
Thanks for the offer but that is one thing i seem to have mastered is the attachment it is glued to aluminium i machine a flat section 12*5 wide then polish &glue works well.I dont thing my electronics is accurate enough for what you need as i only need to take a reading then display 0-9 according to strain.
regards
bee
 
I'm back to work here. It looks like we are running 100kS/s sampling rate, so this PCS500 unit is capable of 50MS/s per channel, that is 50,000kS/s, I think. I'm going to suggest we pick up one of these units for a backup system, then I don't have to invest personally :)
 
What are you looking for within the valve train? Is there a resonant frequency .... some particular rpm where the rocker arm loses contact with the push rod, and the valve timing is affected?
 
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