Pressure Sensor Design Questions

[jlwilson]

I am wanting to design a tire inflator that stops the compressor after
a set pressure has been reached in the tire. I would like to use a
pressure sensor but I am not sure exactly how these animals work. I
have read a little and from what I have gotten it seems that their
resistance changes with pressure which in turn changes the output
voltage. I am thinking take this output voltage and put it through an
A/D converter and go from there.

Is there any way to determine what voltage represents what pressure and
vice versa?

The sensor that I ordered as a sample was MSI Sensors part number
1451-100G-T. I got it from mouser and their part number is
824-1451-100G-T. It ranges from 0-100psig. The only thing that I have
connected it to so far was a small aquarium pump which only pumped
about 1psi. It didnt register anything whatsoever on the gauge by
looking at the voltage with my Oscilliscope.

Has anyone done this before? Hopefully you guys can shed some light on
this a little better for me.

Thanks

 

[Don Foreman]

I am wanting to design a tire inflator that stops the compressor after
a set pressure has been reached in the tire. I would like to use a
pressure sensor but I am not sure exactly how these animals work. I
have read a little and from what I have gotten it seems that their
resistance changes with pressure which in turn changes the output
voltage. I am thinking take this output voltage and put it through an
A/D converter and go from there.

Is there any way to determine what voltage represents what pressure and
vice versa?

Yes. Read the specsheet. You'll need an instrumentation opamp which
will cost under $5. Appnotes showing ciruits are readily available.
Look to Sensym, TI, Burr-Brown, Linear Technologies for details.

 

[jlwilson]

Yes. Read the specsheet. You'll need an instrumentation opamp which
will cost under $5. Appnotes showing ciruits are readily available.
Look to Sensym, TI, Burr-Brown, Linear Technologies for details.

Thank you. I read the specsheet on the pressure sensor and it wasnt
very helpful. I will look for the application notes as you suggested.

 

[Chris]

I am wanting to design a tire inflator that stops the compressor after
a set pressure has been reached in the tire. I would like to use a
pressure sensor but I am not sure exactly how these animals work. I
have read a little and from what I have gotten it seems that their
resistance changes with pressure which in turn changes the output
voltage. I am thinking take this output voltage and put it through an
A/D converter and go from there.

Is there any way to determine what voltage represents what pressure and
vice versa?

The sensor that I ordered as a sample was MSI Sensors part number
1451-100G-T. I got it from mouser and their part number is
824-1451-100G-T. It ranges from 0-100psig. The only thing that I have
connected it to so far was a small aquarium pump which only pumped
about 1psi. It didnt register anything whatsoever on the gauge by
looking at the voltage with my Oscilliscope.

Has anyone done this before? Hopefully you guys can shed some light on
this a little better for me.

Thanks

Hi, Wilson. Oh, my. [/Rumsfeld]

First off, spending a little time at the manufacturer's website will
dredge up this link, which might be helpful on a couple of fronts:

http://www.meas-spec.com/myMeas/download/pdf/english/application/temperature_compensation.pdf

http://tinyurl.com/glzxg

The tinyurl duplicates the above.

Since a couple percent total accuracy is more than sufficient, you'll
probably want to save a few bucks and roll your own instrumentation amp
from 3/4 of a quad and a couple of pairs of 1% or 0.5% resistors. Look
at the top circuit on page 14 of the venerable National Semiconductor
AN-31, "Op Amp Circuit Collection"

http://www.national.com/an/AN/AN-31.pdf#page=1

Setting things for a gain of 20 or so looks about right -- it kind of
depends on what you want to do here.

Also, if this is a senior project, I'm afraid it's a little too simple.

But if you want more help, feel free to post at sci.electronics.basics.

Go Trojans!
Chris

 

[Don Foreman]

Thank you. I read the specsheet on the pressure sensor and it wasnt
very helpful. I will look for the application notes as you suggested.

Go to
http://focus.ti.com/lit/ds/symlink/ina128.pdf

This is the datasheet for the INA128 instrumentation opamp. Look at
figure 4 on page 11.

Also search on straingage amplifiers. Pressure sensors are
electrically very similar to straingages -- because they are
straingages!

 

[Stanislaw Flatto]

I am wanting to design a tire inflator that stops the compressor after
a set pressure has been reached in the tire. I would like to use a
pressure sensor but I am not sure exactly how these animals work. I
have read a little and from what I have gotten it seems that their
resistance changes with pressure which in turn changes the output
voltage. I am thinking take this output voltage and put it through an
A/D converter and go from there.

Is there any way to determine what voltage represents what pressure and
vice versa?

The sensor that I ordered as a sample was MSI Sensors part number
1451-100G-T. I got it from mouser and their part number is
824-1451-100G-T. It ranges from 0-100psig. The only thing that I have
connected it to so far was a small aquarium pump which only pumped
about 1psi. It didnt register anything whatsoever on the gauge by
looking at the voltage with my Oscilliscope.

Has anyone done this before? Hopefully you guys can shed some light on
this a little better for me.

Thanks

Hi!
As I used pressure transducers (NOT sensors!) before so I d/l the specs
pdf of this item and tried to make sense of the specs.
a) the range of output voltages at 3V DC excitation is from 30-120mV at
full scale, so where are you? Do you have a standard presure to find out
what sensitivity your sample produces?
BTW did you excite the sensor when trying to measure anything on output?
And don't try to read on grounded probe, you short part of the measuring
bridge, a floating AVO is recommended till you have your diff. amplifier
functioning.
b) Some measuring bridges can accept AC excitation producing AC outputs,
much easier on resulting amplification stages. Nothing in specs of this
item.
c) Important, on any transducer, simple disregard the lower 30% of the
scale if you don't have access to calibrating equipment where you can
define your own part range sensitivity.

HTH
Can go on and on but if you have a querry give a sign.

Stanislaw
Slack user from Ulladulla.

 

[jlwilson]

Also, if this is a senior project, I'm afraid it's a little too simple.

Thanks,

No this isnt for a senior design.

A little more info though:

I am wanting to be able to adjust the pressure by pressing an up or
down button. I plan on using a microcontroller.

thanks for all of the links guys... now I must get to reading up on
them.

 

[jlwilson]

Hi!
As I used pressure transducers (NOT sensors!) before so I d/l the specs
pdf of this item and tried to make sense of the specs.
a) the range of output voltages at 3V DC excitation is from 30-120mV at
full scale, so where are you? Do you have a standard presure to find out
what sensitivity your sample produces?
BTW did you excite the sensor when trying to measure anything on output?
And don't try to read on grounded probe, you short part of the measuring
bridge, a floating AVO is recommended till you have your diff. amplifier
functioning.
b) Some measuring bridges can accept AC excitation producing AC outputs,
much easier on resulting amplification stages. Nothing in specs of this
item.
c) Important, on any transducer, simple disregard the lower 30% of the
scale if you don't have access to calibrating equipment where you can
define your own part range sensitivity.

HTH
Can go on and on but if you have a querry give a sign.

Stanislaw
Slack user from Ulladulla.

Thanks for all the info. If you dont mind I would like to email you
and ask a few more questions.

 

[jlwilson]

Thanks for the replies and the links guys. I have read most of them
and understand what to do.

I really do appreciate it.

John

 

[jlwilson]

Update:

Again I thank you guys for your help and direction. I made my own
inst. amp and the sensor seems to be working wonderful. I used a POT
to set the null offset and another to set the gain. I have tested
quite a lot and all seems good.

JWilson