Electrical Engineering - Strain Gauge Oscilloscope Help

Author

Strain Gauge Oscilloscope Help

Whatever I Fear

2006-02-21, 3:21 pm

I am using Vishay 120 ohm Strain Gauges, (specifically EA-06-062AP-120)
to try and generalize certain readings taken from aircraft engines. We
have set up a small device to vibrate to simulate an engine on a small
scale. The strain gauge is mounted to this device and the leads are
then attached from the strain gauge terminals to a low noise signal
amplifier and then to a computer based oscilloscope.

Here is a link of a sample output image taken at 16.7 Hz. The small
wave is coming from a signal generator while the larger wave is from
the strain gauge and amplified by low noise amplifier.
http://www.brentless.com/Images/24_8_db.bmp

I understand that a strain gauge is in essence a resistor. However this
setup is just going through a low noise amplifier, and not a wheatstone
bridge. I understand what I would probably expect if I went through a
wheatstone bridge followed by the amplifier, but this is different. So
is what I am looking at in that graph the value of the resistance of
the the strain gauge with respect to the signal from the signal
generator? Can anyone help explain to me in english what exactly I am
looking at on this oscilloscope screen. What is the significance of the
Vrms value if it is a resistance? And I know the -24.8dBm is in
relation to the gain, but what does it mean?

The signal generator is at 16.7 Hz with a setting of 1.5V per division,
while the signal from the strain gauge is a setting of 50 mV per
division.

On a different note, the amplifier has a gain based on the resistance
value of a particular potentiometer mounted to a circuit board. Is
there a way to measure the resistance of that potentiometer while it is
mounted to the circuit board? Is it acceptable just to use an ohmmeter
across available terminals?

Any help is much appreciated.

daestrom

2006-02-21, 8:21 pm

"Whatever I Fear" <whatever.I.fear@gmail.com> wrote in message
news:1140548732.741436.273880@g44g2000cwa.googlegroups.com...
>I am using Vishay 120 ohm Strain Gauges, (specifically EA-06-062AP-120)
> to try and generalize certain readings taken from aircraft engines. We
> have set up a small device to vibrate to simulate an engine on a small
> scale. The strain gauge is mounted to this device and the leads are
> then attached from the strain gauge terminals to a low noise signal
> amplifier and then to a computer based oscilloscope.
>
> Here is a link of a sample output image taken at 16.7 Hz. The small
> wave is coming from a signal generator while the larger wave is from
> the strain gauge and amplified by low noise amplifier.
> http://www.brentless.com/Images/24_8_db.bmp
>
> I understand that a strain gauge is in essence a resistor. However this
> setup is just going through a low noise amplifier, and not a wheatstone
> bridge. I understand what I would probably expect if I went through a
> wheatstone bridge followed by the amplifier, but this is different. So
> is what I am looking at in that graph the value of the resistance of
> the the strain gauge with respect to the signal from the signal
> generator? Can anyone help explain to me in english what exactly I am
> looking at on this oscilloscope screen. What is the significance of the
> Vrms value if it is a resistance? And I know the -24.8dBm is in
> relation to the gain, but what does it mean?
>

An 'amplifier' by itself can't measure a resistance. There must be some
aspect of the circuit passing some current through the strain gauge. It may
be built in to the 'control amplifier' you mention. The scope is showing
you *voltage* output of the control amplifier. To work back and get the
resistance, you need to know something about that exact 'control amplifier'.
Sometimes referred to as 'excitation' current.

I would also guess that you have the scope on AC coupling. So any fixed DC
signal from the control amplifer is filtered out and you're only seeing the
changes in voltage (50 mV per div). So once you have the control
amplifier's particulars, this can tell you the variation in resistance (but
not the total resistance). Resistance variation may be enough if you have
data about the particular strain gauge.

> The signal generator is at 16.7 Hz with a setting of 1.5V per division,
> while the signal from the strain gauge is a setting of 50 mV per
> division.
>

So with a tiny amount of vibration, the strain gauge has a small amount of
resistance variation. Which in turn results in a tiny variation in 'control
amplifier' output.

> On a different note, the amplifier has a gain based on the resistance
> value of a particular potentiometer mounted to a circuit board. Is
> there a way to measure the resistance of that potentiometer while it is
> mounted to the circuit board? Is it acceptable just to use an ohmmeter
> across available terminals?
>

Generally, no. Your meter will be reading the combination of the
potentiometer and any/all circuitry that is in parallel with it. So you
will get a lower reading than what the potentiometer is actually set for.
To accurately read the potentiometer (although, I can't see what good that
would do), you'll have to somehow isolate it from the remaining circuitry.

daestrom

dave y.

2006-02-22, 12:21 am

You need to find out

1) The wiring diagram of how the strain gage is wired.
There must be some bridge completion resistors in the
circuit. It's probably a wheatstone bridge circuit, but
how many legs are active strain gages? You must
know this.

2) Strain gages have a gage factor which relates
strain to change in resistance. You should know what
this is.

3) You need to calibrate your instrumentation by

a) replacing the gage with a voltage source or

b) attaching a calibration resistor to the circuit or

c) applying a known physical load to the specimen

4) Lastly (actually firstly) you need to get a book on
how to do all this and read it.

dave y.

Mark Walter

2006-02-22, 1:21 am

bob mcree

2006-02-25, 12:21 am

Whatever I Fear wrote:
> I am using Vishay 120 ohm Strain Gauges, (specifically EA-06-062AP-120)
> to try and generalize certain readings taken from aircraft engines. We
> have set up a small device to vibrate to simulate an engine on a small
> scale. The strain gauge is mounted to this device and the leads are
> then attached from the strain gauge terminals to a low noise signal
> amplifier and then to a computer based oscilloscope.
>
> Here is a link of a sample output image taken at 16.7 Hz. The small
> wave is coming from a signal generator while the larger wave is from
> the strain gauge and amplified by low noise amplifier.
> http://www.brentless.com/Images/24_8_db.bmp
>
> I understand that a strain gauge is in essence a resistor. However this
> setup is just going through a low noise amplifier, and not a wheatstone
> bridge. I understand what I would probably expect if I went through a
> wheatstone bridge followed by the amplifier, but this is different. So
> is what I am looking at in that graph the value of the resistance of
> the the strain gauge with respect to the signal from the signal
> generator? Can anyone help explain to me in english what exactly I am
> looking at on this oscilloscope screen. What is the significance of the
> Vrms value if it is a resistance? And I know the -24.8dBm is in
> relation to the gain, but what does it mean?
>
the -24.8 dbm is a measure of the power usually assuming a 600 ohm load
and is really just a voltage measurement.

when you use a strain gauge out of a bridge circuit there are loading
considerations that will affect the reading. the balanced nature of the
bridge gives greater sensitivity. if you are using the strain gauge in
a different configuration it will be a voltage divider including the
input impedance of the meter.

> The signal generator is at 16.7 Hz with a setting of 1.5V per division,
> while the signal from the strain gauge is a setting of 50 mV per
> division.
>
> On a different note, the amplifier has a gain based on the resistance
> value of a particular potentiometer mounted to a circuit board. Is
> there a way to measure the resistance of that potentiometer while it is
> mounted to the circuit board? Is it acceptable just to use an ohmmeter
> across available terminals?
>
it depends on the external circuitry but the resistance across the pot
in circuit with the power off will usually be close. most pots have the
value printed on them.

> Any help is much appreciated.
>

NonDigital@gmail.com

2006-02-25, 10:21 am

hI-

1. Try getting a free copy of PMV2000 image viewer. Then convert
any images to be shared to jpg if you expect people to download them.
BMP are large files and take too long.

2. To measure a pot on the fly, first measure the total
resistance of the pot. Then when the pot is installed, connect the
ohmmeter to the unused portion of the pot and this should not interfere
so long as the ohmmeter leads are to the op amp output and to the
unused section of the pot.

3. Analog Devices has a little monthly rag called Analog Dialog.
They have discussed just about everything at some time.

4. Try looking for the info you need at Tinaja.com. This fellow
has an encyclopedic site of just about everything electronic.

5. I couldn't understand your problem from a quick read of your
message. Try writing more clearly so you can benefit from these
contacts. I couldn't understand my own first writings and I knew the
problem. Look at some patents and there find a good example of the most
clear, unequivocal technical writing on the planet.

Charles Gilbert
Consultant
NonDigital.Netfirms.com

LinkBot