Strain Gauges - Signal Conditioning with programmable gain

[Victor hyppolito]

Hello,

I need to make my own circuit for signal conditioning. It should
amplify a low signal (+ or -10mV max) to +-5V(gain =500). Then i need a
5V offset to make it from 0V to 10V.
Its a project that uses a full bridge configuration with strain gages.
I know after the amplification it will have a lot of noise, so, if
anybody can give me an advice to have a clean amplified signal I´d
appreciate. I looked for a instrumentation amp, and I know it rejects
the common mode noise but is there any other concern in terms of noise
or its just the common mode noise? In other words, if I use a in-amp do
I still need a filter? Analog Devices suggets an approximately 700Khz
Band-width with G=10. How do I know it´ll work for me? The last thing
is that I need a programmable gain to use it in other applications.
Please, assume that i´m new into electronics. Anything will be
helpful!
Thank You!
I appreciate!
Victor

 

[martin griffith]

Hello,
snip
Please, assume that i´m new into electronics. Anything will be
helpful!
Thank You!
I appreciate!
Victor

http://www.artofelectronics.com/


martin

 

[Rob]

Hello,

I need to make my own circuit for signal conditioning. It should
amplify a low signal (+ or -10mV max) to +-5V(gain =500). Then i need a
5V offset to make it from 0V to 10V.
Its a project that uses a full bridge configuration with strain gages.
I know after the amplification it will have a lot of noise, so, if
anybody can give me an advice to have a clean amplified signal I´d
appreciate. I looked for a instrumentation amp, and I know it rejects
the common mode noise but is there any other concern in terms of noise
or its just the common mode noise? In other words, if I use a in-amp do
I still need a filter? Analog Devices suggets an approximately 700Khz
Band-width with G=10. How do I know it´ll work for me? The last thing
is that I need a programmable gain to use it in other applications.
Please, assume that i´m new into electronics. Anything will be
helpful!
Thank You!
I appreciate!
Victor

Hello Victor
I've done designes very similar to your requirements.
I'll put something to gether on my web site, once I get it working.
I will probably have some more questions, which I will also post on the
site http://notrocketscience.mysite.wanadoo-members.co.uk as soon as I
can find some spare time.
Robert

 

[Victor hyppolito]

Thank You!
I´ll continue studying and as soon as you´ve done with your site
i´ll check this out!

 

[Rob]

Thank You!
I´ll continue studying and as soon as you´ve done with your site
i´ll check this out!

Hello Victor,
I have posted some questions on web site.
Regards
Robert

 

[Victor hyppolito]

Hello Rob,

Thanks for supporting me with my strain gage project!

I´ll try to answer your questions the best I can:

1 - The output of my strain gage assembly will be +-10mV.
2 - When the gage is not under load, the output should be zero,
shouldn´t it? I´m sorry if im being too ignorant but i´ve started
studying electronics at college about an year ago and I haven´t built
anything real, I just know theory. I know the gage might have some
voltage even if no load in applied but we intend to calibrate it to
zero.
3 - Actually, there are two projects that I want to use this signal
conditioning circuit. One will use a micro the other will just have the
signal to input a device that was bought to measure the deformation. I
think i´ll handle the nulling of the equipment with a potentiometer.
What I need is to amplify and filter it the best way and then input it
ito an A/D converter. Basically thats what I need. But I don´t know
how I can find the best frequency of my filter. Should I have a
spectrum analyser or just through the strain gage´s manual I can find
out which frequencies it gets?

Thank you again for helping me. I don´t want you to waste your time
with my stuff, so, when you have a spare time you reply me!

I appreciate!
Victor

 

[Rob]

Hello Rob,

Thanks for supporting me with my strain gage project!

I´ll try to answer your questions the best I can:

1 - The output of my strain gage assembly will be +-10mV.
2 - When the gage is not under load, the output should be zero,
shouldn´t it? I´m sorry if im being too ignorant but i´ve started
studying electronics at college about an year ago and I haven´t built
anything real, I just know theory. I know the gage might have some
voltage even if no load in applied but we intend to calibrate it to
zero.
3 - Actually, there are two projects that I want to use this signal
conditioning circuit. One will use a micro the other will just have the
signal to input a device that was bought to measure the deformation. I
think i´ll handle the nulling of the equipment with a potentiometer.
What I need is to amplify and filter it the best way and then input it
ito an A/D converter. Basically thats what I need. But I don´t know
how I can find the best frequency of my filter. Should I have a
spectrum analyser or just through the strain gage´s manual I can find
out which frequencies it gets?

Thank you again for helping me. I don´t want you to waste your time
with my stuff, so, when you have a spare time you reply me!

I appreciate!
Victor

1 - The output of my strain gage assembly will be +-10mV.
1-Answer:- OK, I will assume that you mean that when you apply your
load to the strain gauge assembly, each of it's outputs will change
by 10mV, so a 20mV change overall.

2 - When the gage is not under load, the output should be zero,
shouldn't it? I'm sorry if I'm being too ignorant but I've
started
studying electronics at college about an year ago and I haven't built

anything real, I just know theory. I know the gage might have some
voltage even if no load in applied but we intend to calibrate it to
zero.

2-Answer:- In an ideal world, the output is zero for zero load. In
practice, it never is. So all I really need to know is how you intend
performing this null. It sounds like you intend performing this
manually, so I will amend the circuit accordingly. I will however need
to know what the approximate error is likely to be. This should be
specified as part of the specification of the assembly.

3 - Actually, there are two projects that I want to use this signal
conditioning circuit. One will use a micro the other will just have the

signal to input a device that was bought to measure the deformation. I
think I'll handle the nulling of the equipment with a potentiometer.
What I need is to amplify and filter it the best way and then input it
into an A/D converter. Basically that's what I need. But I don't
know
how I can find the best frequency of my filter. Should I have a
spectrum analyser or just through the strain gage's manual I can find

out which frequencies it gets?

3-Answer:- OK, that should be fine, see answer 2 for nulling. Regarding
the filter, what is the maximum rate of change of load that you wish
the system to respond to? In other words for example, you might apply
the load and read the result 1 second later or 5 seconds etc.

Thank you again for helping me. I don't want you to waste your time
with my stuff, so, when you have a spare time you reply me!

No problem, this is much more interesting than the crap I have to do
for a living!!

 

[Victor hyppolito]

Hello Rob,

What I mean by +-10mV is that when we compress the object we should
read a negative value, when we pull the object we should read a
positive value. But the overall is 20mV as you mentioned.
Unfortunatelly I won´t be able to tell you an approximate error. It
should be small but i know "small" depends on the application. I´ll
find out as soon as i can and I´ll tell you.
What I´m most worried about is the filter part. One of the projects,
we apply the load and read the result 1 second later. The other one,
the whole thing lasts miliseconds, so, we should have to read 1 result
and then read the other one 10microseconds(100 Khz frequency) later. As
I need to input the signal in a A/D I need to project an anti-aliasing
filtering. 40khz LP filter should be good for anti-aliasing, but I
would still have lots of noise because of the high frequency, wouldn´t
I?
As time goes by things seem to get more complicated instead of getting
easier! That´s because I´m learning new concepts which are
important for the project!
Thats it, thanks a lot again and I hope things get more exciting at
your job!

Victor

 

[Rob]

What I mean by +-10mV is that when we compress the object we should
read a negative value, when we pull the object we should read a
positive value. But the overall is 20mV as you mentioned.

Answer:- This is an important bit of additional information.
You want the output to be 0-10V. But as you are indeed applying force
in both directions to your gauge assembly, the output with no load will
be +5V. At full positive load the output will be +10V and at full
negative load the output will be 0V.
Or do you want the output to go negative for negative applied loads? I
don't think you do, as your ADC is probably not able to accept this.

Unfortunately I won't be able to tell you an approximate error. It
should be small but I know "small" depends on the application. I'll
find out as soon as I can and I'll tell you.

Answer:- In my circuit I have assumed that the bridge supply is 10V (a
common supply voltage). The bridge output is commonly specified as so
many volts per volt of bridge supply. So if your assembly has an output
of +/-10mV with a 10V bridge supply, it will have an output of 1mV/V.
The 'Null error' is needed so that I can set the amount of null
adjustment needed. I will allow a reasonable amount of adjustment (say
20% (4mV null error) of the full scale output. Then we can refine it
later.

What I'm most worried about is the filter part. One of the projects,
we apply the load and read the result 1 second later. The other one,
the whole thing lasts milliseconds, so, we should have to read 1 result

and then read the other one 10microseconds (100 kHz frequency) later.
As I need to input the signal in a A/D I need to project an
anti-aliasing
filtering. 40khz LP filter should be good for anti-aliasing, but I
would still have lots of noise because of the high frequency,
wouldn't I?

Answer:- For the application requiring a 1 second response time,
filtering should not pose a problem. Much more than 4 samples a second
and the human eye/brain is not capable of following what's going on
anyway.
Your other application is more of a challenge. If the equipment is
properly built, the noise should not really be as much of a problem as
you anticipate. But in order to capture a fast moving event (the
breaking of a rod under tension?), your ADC needs to be able to capture
samples at the rate you mention, and ideally much faster than 10uS.
Then you can do software averaging on a number of separate samples.
I will configure the filter as a 40kHz low pass.

As time goes by things seem to get more complicated instead of getting
easier! That's because I'm learning new concepts which are
important for the project!

As the saying goes, "the devil is in the detail"
Robert

 

[Victor hyppolito]

Hello Rob,

Sorry for getting too long to reply. The last days were too confusing
here.
You´re right, i don´t want the output to go negative for negative
applied loads.
Since I´ve got a 5V bridge supply, and my assembly output is +-10mV I
have an output of 2mV/V.
The fast moving event that I reffered to is a pendulum that hits a big
iron bar. It lasts milliseconds.
I have a question. Do you know any article on the internet or something
you have at your cpu that explains there kind of stuff? If you can
recommend something I´d appreciate!
Thanks again!

 

[Rob]

Hello Rob,

Sorry for getting too long to reply. The last days were too confusing
here.
You´re right, i don´t want the output to go negative for negative
applied loads.
Since I´ve got a 5V bridge supply, and my assembly output is +-10mV I
have an output of 2mV/V.
The fast moving event that I reffered to is a pendulum that hits a big
iron bar. It lasts milliseconds.
I have a question. Do you know any article on the internet or something
you have at your cpu that explains there kind of stuff? If you can
recommend something I´d appreciate!
Thanks again!

Hello Victor,
I will reconfigure the circuit on the web to use a 5V bridge supply.
I will configure two circuits:- one for use with a DVM or similar and
to have a low frequency response and have zero load as 0V, as a DVM
will indicate both positive and negative strain outputs. The second
will have a 40kHz LP filter and an output voltage of 5V for zero load.
Thus positive full scale loads will be +10V and full scale negative
load will be 0V.
Regarding good sources of information. I can't think of anything better
than the book "The Art Of Electronics" co-authored by Win Hill, a
regular contributor on this board. Then there is always 'Googling'.
I will have a quick go and let you know anything I find.
Regards
Robert

 

[Joseph2k]

Hello Rob,

Thanks for supporting me with my strain gage project!

I´ll try to answer your questions the best I can:

1 - The output of my strain gage assembly will be +-10mV.
2 - When the gage is not under load, the output should be zero,
shouldn´t it? I´m sorry if im being too ignorant but i´ve started
studying electronics at college about an year ago and I haven´t built
anything real, I just know theory. I know the gage might have some
voltage even if no load in applied but we intend to calibrate it to
zero.
3 - Actually, there are two projects that I want to use this signal
conditioning circuit. One will use a micro the other will just have the
signal to input a device that was bought to measure the deformation. I
think i´ll handle the nulling of the equipment with a potentiometer.
What I need is to amplify and filter it the best way and then input it
ito an A/D converter. Basically thats what I need. But I don´t know
how I can find the best frequency of my filter. Should I have a
spectrum analyser or just through the strain gage´s manual I can find
out which frequencies it gets?

Thank you again for helping me. I don´t want you to waste your time
with my stuff, so, when you have a spare time you reply me!

I appreciate!
Victor

While you are at it look up load cell, i suspect you are trying to make
something like one. Besides, it should improve your understanding of
strain gauges a bit.

 

[Victor hyppolito]

Hello Rob!

I saw you´ve updated your site! Thats awesome!
The configuration you are going to make is perfect and I think it´ll
be very useful for me!
Regarding the book, I´ll order it soon!
Thanks a lot!!!
Victor

 

[Rob]

Hello Victor,
I've updated the first schematic and added a second.
I hope they do what you want. In order to get the best out of them, you
need to pay attention to the physical construction, e.g. common mode
rejection, etc
Regards
Robert

 

[Victor hyppolito]

Hello Rob,

I checked the site you´ve updated! That´s really nice you´re helping
me with this. I´ve already ordered the book you(and almost everyone)
recommends - The Art of Electronics. As I´m from Brazil, it may take
about 3 weeks to get here.I told you I studied electronics the last
months but I was using Sedra and Smith book. It´s good but there´s
too much theory in it. I know what that book teaches but I can´t apply
it to anything real.
I also put the schematic on Pspice to see how it works. It seems
perfect. I know in the computer is much different than in real life,
because of many things including the physical construction as you said.
The "full load" condition went to 10,005V(something like this) and the
no load condition to 4.9995V and the negative one to 1.XXmV.
I feel a little embarrased to ask you what I´m going to ask but, since
i´m still learning electronics, I think It would be good for me if you
just explain a little bit of what each part of the circuit does. I
don´t want you to explain everything because I know it might be too
long and you could spend too much time on it. I know that the in-amp
have a x50 gain and as its supposed to do, it must reject the common
mode noise. I also know that the in amp gain is set through the
resistor. The last op-amp is the filter part. What about the other
op-amps? Another question,(i promise it will be the last one , in
case I change the strain gage assembly so that its output is +- 20mV,
is it too hard to modify the circuit? In which parts I have to focus?
Thanks a lot again! I don´t know how to repay this! It was awesome! If
you send me your addess I´ll send you something from Brazil, like a
caipirinha, don´t know if you know it, its a traditional drink from
Brazil.
Anyways, I don´t even know if you drink, so if you dont, never mind
that!
Thank you!!!
Victor

 

[Rob]

Hello Victor,
No problem, I will break down the circuit down into parts and explain
the operation of each one. Also how to make changes so that you can
change the gain and so on.
AofE is good in that it explains a wide number of design areas in a
simple, easy to understand way, with the least amount of mathematics I
always have my copy to hand, it's my bible, as far as electronics is
concerned.
Thanks very much for offer of traditional Brazilian drink, that would
me very appreciated. I will post my address on the web site.
Regards
Robert