Fitting strain gages to a bending beam
rogerdw
Posts: 5
Hi all
I came across this forum while searching for info on fitting strain gages to a bending beam to create a load cell.
I was impressed with some of the responses to other posts and the general helpfulness and respect shown.
Ive done a lot of research and understand the basics, but now that Im ready to actually buy my strain gages, there are decisions to make about what type and their exact positioning.
To provide some background I am building a device that will measure the actual amount of weight or force that is applied to a bar by a weightlifter.
The device is adjustable so that the weightlifter can get into the various positions that enable them to apply maximum force for all the basic exercises and therefore muscle groups so records can be taken and progress monitored.
The hardware is nearly complete and now I need to make the load measuring part of the machine.
I will attempt later to attach a diagram of the concept but if you can imagine a substantial metal post with a collar on it that can be slid up or down and locked in place and the collar has a 30mm square solid bar (spring steel) sticking out at right angles approx 450mm long.
At the end of that bar, a cross member is attached that is equivalent to a normal weightlifting bar minus the weights and immovable.
The 450mm long, 30mm square bar is the bending beam, and that is the device that requires the strain gages to be fitted.
My initial questions are
How do I work out exactly where to locate the strain gages?
Exactly what ones to purchase to fit?
I assume I should be fitting 4 gages in a bridge configuration similar to Fig C on the first page of this article (referred to by dre in one of his posts on this forum .. thanks) http://www.omega.ca/techref/pdf/Positioning_Strain_Gages.pdf
Thanks for any help anyone can offer.
Regards, Roger
I was impressed with some of the responses to other posts and the general helpfulness and respect shown.
Ive done a lot of research and understand the basics, but now that Im ready to actually buy my strain gages, there are decisions to make about what type and their exact positioning.
To provide some background I am building a device that will measure the actual amount of weight or force that is applied to a bar by a weightlifter.
The device is adjustable so that the weightlifter can get into the various positions that enable them to apply maximum force for all the basic exercises and therefore muscle groups so records can be taken and progress monitored.
The hardware is nearly complete and now I need to make the load measuring part of the machine.
I will attempt later to attach a diagram of the concept but if you can imagine a substantial metal post with a collar on it that can be slid up or down and locked in place and the collar has a 30mm square solid bar (spring steel) sticking out at right angles approx 450mm long.
At the end of that bar, a cross member is attached that is equivalent to a normal weightlifting bar minus the weights and immovable.
The 450mm long, 30mm square bar is the bending beam, and that is the device that requires the strain gages to be fitted.
My initial questions are
How do I work out exactly where to locate the strain gages?
Exactly what ones to purchase to fit?
I assume I should be fitting 4 gages in a bridge configuration similar to Fig C on the first page of this article (referred to by dre in one of his posts on this forum .. thanks) http://www.omega.ca/techref/pdf/Positioning_Strain_Gages.pdf
Thanks for any help anyone can offer.
Regards, Roger
Comments
Sorry but I can't visualize your weightlifting device. So I will have to reply in a general
sense.
You want to mount the gages at the location of maximum stress; which will also be the
location of maximum strain. Metals do not exhibit much strain so it is paramount to find
the best placement. You may need to seek the help of a structural engineer in
determining the best location for the gages. Do you have access to a university with an
engineering department? I would think that mechanical or civil engineering students or
their profs would love to help.
You are correct in fitting all 4 arms of the bridge with gages and not to use ordinary
precision resistors to complete the wheatstone bridge. Using gages of the largest size
practical means that you can use a bridge power supply of greatest voltage which also
gives the greatest output. Here is a link to excellent information about the availability of
gages and of their use:
http://www.vishaypg.com/micro-measurements/
I would use the 'CEA' gages of which there is a great selection and which are already
protected. I have also seen these gages offered on e-bay.
Also read the information on gage installation, as proper installation is important.
There is so much more to discuss. Let me know if this helps.
cheers,
David
Thanks for your input. Ill attach a rough sketch of the device to make it easier to understand.
Ive had a good read of the source you suggested and will keep studying.
Your suggestion to use a larger gage size .. enabling a higher excitation voltage .. which should give a greater output, makes sense especially seeing that there will not be much deflection in the beam.
Thanks for your suggestion to use the CEA gages too narrows down the field a fair bit.
Im in Australia and have not been able to find any helpful Aust suppliers so far though I did get an email back from a co. that sells Vishay stuff but their freight charges were crazy. I may as well buy them off ebay!!
I already have an APCS strain gage transmitter (WT127) so that part is covered
http://www.apcs.net.au/products/premier/ds127711.pdf
I started this project years ago (2004) and then mothballed it and have only recently started again. I had a friend do some figures back then, but Im embarrassed to say I couldnt really make sense of them as there were too many variables that he couldnt decide on so I had no chance. :blank:
Maybe Ill ask him again or see if he knows anyone at uni who may be able to help.
Thanks again for any advice.
Regards, Roger
Sorry it isn't practical to get MM gages there. You might contact the structural test
department of an aircraft experimental facility to see where they get gages. They would
also be an excellent source for gage application.
Your sketch was great. The gages are simply mounted on a 'cantilever' beam. Therefore
the 2 gages on the top will be in tension for an applied download. These gages will
increase in resistance for the download and therefore one should be wired between the
[+] power supply and the [-] output and the other between the [-] supply and the [+]
output. The gages cemented on the bottom should then be wired in the remaining arms of
the bridge.
You can easily calibrate the output in terms of force but, of course, the output needs to be
amplified before the signal can be inputted into the Stamp for further processing. I
assume your transmitter does this.
Read all you can about gage installation prior to the actual cementing. You will probably
still need a balancing circuit as even the best installation results in minor initial
unbalance. Again, I assume your transmitter takes care of balancing. If not I can talk to
this if you want. And 'exercising' the gages by applying loads before actual usage helps. There
is an 'eraser' test for gage cement integrity. With the bridge hooked up and the amplified
output displayed, push on the surface of the gages with a pencil eraser. If the reading
returns to near the initial reading upon pencil removal the installation is good. Especially
with such low bridge output, repeatability is paramount. Nonlinearity [shouldn't be much]
can be taken care of with the Stamp.
Enough for now,
cheers, David
Thanks for all your additional info.
I made a few more calls today but didnt really find anyone who could provide much info even getting prices was like pulling hens teeth.
Im pretty sure the strain gage tx I have will address all thats needed its still choosing the actual gages and fitting them that is my main issue.
Its probably time I just buy some (any type) from the limited types available over here fit them and see what happens.
Ill let you know what happens once I do that.
Regards, Roger
I apologize in that I did not read the specs on your transmitter prior to answering your
previous thread. I now see that the tare adjustment can be set from -50 to 100% of span.
This is in addition to the offset adjustment of +/-20%. This is great for your usage in that
if you want to output both an up & a down force you can initially set the tare to 50% of
span and the output would read in the positive range for either force direction.
As to excitation voltage: Does your model have the adjustable voltage? Even if it doesn't
and uses only the 10 VDC bridge supply then you can use most any moderate size 350 ohm gages,
say, 1/4 inch gage length - which are offered in several models & relatively easy to apply.
If you find it impossible to get gages please let me know. I don't know the shipping costs
from the US but the gage package is small & light. And, again, e-bay often has packages
of MM CEA gages for sale. Let me know if you need help decoding any MM gage
package offered. For example: one number pertains to temperature compensation. By
using all 4 gages in the configuration shown you don't need to be concerned about
compensation as temperature effects are cancelled out.
One possible problem I can think of is the application of uneven forces at the ends of the
cross bar. Such uneven force would cause a twisting force on the instrumented bar. This
may interfere with calibration. Would such torsional force be minimal?
Again, enough for now. But let me know if I can be of further help.
cheers,
David
Thanks again for your help.
Yes, the transmitter has an adjustable excitation voltage from 4 17V dc, so is pretty versatile.
Regarding the actual gages, I can see my best option is as you say to find some on ebay and I have found some that look ideal (from my limited understanding anyway) and the price is certainly ok. I would actually buy 2 or 3 sets so I can make different length bars ... and the postage shouldn't be to bad considering the size.
http://www.ebay.com/itm/MICRO-MEASUREMENTS-STRAIN-GAGE-CEA-06-500UW-350-PKG-5-/290699537027?pt=LH_DefaultDomain_0&hash=item43af0aea83#ht_500wt_950
Unfortunately the seller only deals within the US and is not contactable via ebay first time Ive ever seen that so I may need to pay a little more.
In the time I have been looking, a few more items have come up for sale so I just need to be patient.
The issue you raised regarding torsional forces has come to mind before and Im sure if the crossbar is not forced evenly, it will mess up the reading.
The user will have full view of the display, so the hope is that they will self adjust to get the highest output which can only occur (I assume) when they are pressing evenly.
As you can tell theres a lot of wishful thinking here but the easiest way for me to find out is to actually finish building the thing and experiment.
Again, I appreciate the support its keeping me motivated to finish this thing off. 8 years is too long !!!
Regards, Roger
The gages you mentioned can be decoded as follows:
CEA-06-500UW-350 PKG/5
CEA - Encalulated [sp?] 'ea' alloy foil [I forgot what 'ea' stands for but is the alloy of choice for
general purpose usage. It produces a gage factor of around 2. Gage factor is the constant
in the relationship: strain x G.F. = delta gage resistance/initial gage resistance. The exact
G.F. is given on the information sheet in the package.
06 - Is the temperature compensation for, I believe, steel. So if,say, a single gage is mounted on
steel, temperature effects are minimized. For your 4 active arm configuration temperature
effect is automatically cancelled out.
500 - The gage length in inches x 10^3. So 500 = 1/2 inch.
UW - The gage configuration; which you have to look up on the Micromeasurements
site. I think UW is a standard wide gage; which is good at taking care of heating due to the
voltage drop across the gage. This ability also can be found in the application notes at
MM. Use the general stress analysis accuracy for finding the max allowable bridge
supply voltage.
350 - The initial, unstressed, resistance of the gage in ohms.
PKG/5 - There are 5 gages in each package. Why 5? Got me! Lost in history but maybe
one spare; just in case.
In summary this is a good selection for your purpose.
Again, let me know if I can be of further help.
Wishing you success on finding gages, especially at
such a reasonable price;
cheers,
David
Well the strain gages I spotted are gone that will teach me to point them out !!!
Ive sent off an email to another seller to see if they will break their own rules and sell some into Australia. Will have to wait and see but 3 times the price
Ill keep watching and waiting.
Regards, Roger