# calibrating strain gauge

Discussion in 'Physics' started by arivalagan13, Feb 6, 2010.

1. ### arivalagan13 Thread Starter Member

Apr 30, 2009
29
0
Hi group,

First of all thanks to the group for doing unmeasurable help..

I'm about to calibrate the strain gauge..
For that I have a cantilever beam whose dimensions are 127 x 13 x 3.7mm(l x b x h)..

What is the procedure and precautions I need to follow while doing the calibration...

Questions like end I need to place the strain gauge (fixed or free, I want to measure the tensile stress) and so..and answer to any basic questions are welcome..

I designed the bridge circuit and Instrumentation amplifier(gain=500 adjustable up to 10000)..

Thanks & regards,
Arivu

2. ### jans123 New Member

Jan 30, 2010
20
0
I need more information to help you; how many gages in the bridge, how are they oriented, the gage factor (you can find it on the package of the gages), the gage resistance and material in the beam, then it is quite simple...
On the other hand, if you know the load, the you can calculate the tension of the surface of the beam.
By the way, if you ad a simple drawing of your design and explain what you want to do with it then it will be a little easier.

-can't check the forum every day, so please be patient...

3. ### arivalagan13 Thread Starter Member

Apr 30, 2009
29
0

Hi..

I'm using a quarter-bridge circuit,so I'm mounting only one gauge in the bridge.The gauge factor is 2(constantan), 350Ω,the beam is made up of some kind of cast iron(it seems)...I'm doing the usual calibration procedure...

I attached the diagram..In that u can see a cantilever beam surrounded with a sphere

The final product should work like this:
The force I'm applying to the sphere(solid) should be transferred to the strain gauge(one on each side of the beam)...

This device will be connected to the computer to control the cursor movement according to the direction of the force applied(in which direction)..

I used four gauges(one for each direction.I divided the screen in to x,-x,y,-y axes)...

for this purpose I need to calibrate the gauge..

This interface can act as a mouse for children affected with cerebral paralysis...The bigger structure is designed for them to average out their involuntary(unwanted) muscle movements..
once this device is being developed they find much application to such children...

Am I clear?

Thanks & regards,
Arivu

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4. ### arivalagan13 Thread Starter Member

Apr 30, 2009
29
0

The dimensions of the structure:
beam:20 x 1 x 1cm(l x b x b)
sphere:20cm

5. ### jans123 New Member

Jan 30, 2010
20
0
I do not like to make someone disappointed, but the mechanics will probably not work that simple.
1. The positioning of the gages are critical as you need to put them where the strain gives you the largest signal as we are talking about stretching or compressing the material under the gage only a few nanometres.
2. It is possible to separate the tension in a beam into four signals, but I advice you not to do it. It is much easier to put t the gages in half bridges. Half bridges will solve some of the problems with temperature drift you can't avoid in a quarter bridge. Two half bridges can give you the signals you need and you save the cost of two amplifiers.

Calibration can be done in two ways:
1. Normally to determine strain the gage under calibration will be paralleled with a shunt resistor to calibrate the circuit gain. (I can send info, but not now, I'm on a journey. By the way: I am working on a "Hands on guide" for experimenting newbies and will put on my homepage during the spring. It is dedicated to the electronics and mechanics amateurs interested in using strain gages. It has lots of drawings, lots of words but not much "rocket science" theory.)
2. A usual way to calibrate the design is simply to put a known load on the mechanical structure. Actually, most test engineers use both methods in their setups, the first to calibrate the electronics the second to calibrate the mechanics.

I think your biggest problem will be the mechanics not the electronics.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Strain gauges measure strain, not stress.

I asked in your previous thread if you understood the difference and I think it is clear you do not, although you did not answer my question.

In a cantilever, stresses are at a maximum at the fixed and and zero at the free end.
Stresses are also at a maximum on the surface of a cantilever. This is why we can use strain gauges at all.
There are also twisting or torsional stresses. You need your gauge arrangement to avoid these.

Jans is doing a pretty good job of helping you but so far he is doing most of the work, you need to try to understand the machine you are building a little better or your results may be meaningless.

7. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
I don't claim any expertise in strain gauges, though I got to build some to spec.

What is the shape of the metal base you will be using?

The one we had measured force in one axis. The thin part of the metal the engineer referred to as webs.

It is symmetrical, there are resistor stickers glued on the other side. This was the part that required the greatest care, the stickers had to be completely bonded to the metal.

I redrew this image to reflect the thinness of the metal. A nice piece of machining. I had to make new ones periodically because if the machine crashed it would bend on the weak points, trashing that strain gauge.

I uses 30 gauge wire and connected the gauge together using the schematic shown. I showed the solder pads on the resistance stickers.

Calibration consisted of hanging known weights off the end and recording the voltages, and putting them in a table in the software.

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8. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
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Generally, when implementing strain gauges, the structure is engineered such that the measureable strains are funneled through controlled components, producing single axis reactives. That's not to say that strain gauges can't successfully be applied to devices not initially designed for thier application.

Calibrating strain gauges are more about the design of the structure that supports the gauge, than the gauge itself.

9. ### jans123 New Member

Jan 30, 2010
20
0
Well, if you are designing a load-cell or a pressure-transducer, then it is possible to do a lot by doing something about the construction, but in my case that goes for one gage out of fifty. A lot can also be done by analysing a structure in a finite-element model to find possitions and directions most suitable for positioning gages for mesuring forces in certain directions, torque and other parameters. In some cases gages are put in or near a "hot-spot" where high strain are expected. I use to get stress data (yes, I record strain but analyse stress in dynamic environments) to calculate expected life of structures.

Now to Mr arivalagan13's problem. I belive he is trying to build a load-cell that works like a joystick. Not an ordinary joystick but one intended in a medical environment so it might be possible that Mr arivalagan13 actually want to get information of muscular stress. Can we help him to sort things out about what he actually wants? Are the problems (the medical and the mechanical) defined enough? I see a barrel of possibilitie and problems involved here, almost enough for a forum it selves...

10. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Only with his co operation.

I was going to post some diagrams/explanation for the stresses in a cantilever and the proper positioning of gauges. However I am holding back until I find out if he knows the difference between Force, Stress and Strain.

It seems to me designing the calculating element before estimating the calculation range is back to front.

I suggested rough figures could be obtained from a simple weighing scale, available very cheaply these days, but received no answer.

Last edited: Feb 7, 2010
11. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Like I said, I have very little expertise in this area, but slapping resistor stickers on the side of a rod doesn't seem like it would do it to me. If you were to drill two holes offset to create webs seems like a logical first start, each hole being part of a X,Y axis. The idea, as I understand it, is to concentrate the stress/strain in the metal for maximum response.

12. ### arivalagan13 Thread Starter Member

Apr 30, 2009
29
0
sorry..I was out of station n not able to reply for the posts..

Thanks a lot for the very valuable responses from jans123 and studiot..
now I understood about force,stress and strain..I spent a lot of time understanding that..
I'd done some projects on electronics but very very new to mechanics.
thats why I'm struggling a lot with mechanical design..

studiot has replied that he is having diagrams/explanations for the stresses in a cantilever and the proper positioning of gauges. Kindly give me those because I'm now in a bad position as my deadline is nearing...

As u had suggested in ur earlier posts I measured the force due to human press(on the mouse) using weighing scale and got the range of force(but forgotten to reply u,sorry) and I'm working only with that data...

Kindly help me...

Thanks & regards,
Arivu