How would i measure a force of around 1000 lbs?
radiohalo123
Posts: 4
I need to measure the force of a heavy object at two points to know where the most force is being applied. Dealing with very heavy loads(1000 LBS). What sensors should i use and how do i connect them with a parallax board.
Comments
Without knowing more about your application I would consider using 2 load cells. The
1000 lbs is well within range of commercial load cells. Look at e-bay under 'load cell' to
get an idea of the load range and cost. You could, with some research, make your own
load cells but it may not be worth the time, experimenting and calibration.
Normal, strain gage based, load cells, although extremely accurate and reliable, have an
full scale ratiometric output of only about 3mV/Volt. Generally the supply or bridge
voltage is around 10 Vdc. Therefore a maximum output of, say, 30 mV full scale may
have to be amplified before going into an Analog-to-Digital converter; or use a high bit
converter.. Search this forum under, say, "load cell" to see various approaches to
conditioning the output of the load cell in order to input to a Stamp.
cheers, David
www.omega.com/prodinfo/loadcells.html
I have found nothing in the search for load cells.
The parallax search function doesn't work so well. It's best to use GOOGLE's search engine and use the "advanced search" and search only the parallax.com website....
www.google.com/search?hl=en&as_q=load+cell&as_epq=&as_oq=&as_eq=&num=10&lr=&as_filetype=&ft=i&as_sitesearch=parallax.com&as_qdr=all&as_rights=&as_occt=any&cr=&as_nlo=&as_nhi=&safe=images
then you'll find things like this:
http://forums.parallax.com/showthread.php?p=682743
http://forums.parallax.com/showthread.php?p=893689
I stand corrected. The forum thread I was thinking of was related to strain gage
amplification.
My approach to using a commercial load cell is to first amplify the load cell wheatstone
bridge output of some 30mV to 5 volts; which is then inputted into a A-to-D converter.
I like the INA125 instrumentation amplifier as it seems to be designed for bridge input. It
has a selectable voltage reference also, to use for bridge supply [noparse][[/noparse]n.b. You have to use a
transistor to boost the reference. See the INA125 datasheet.]. A wheatstone bridge output is ratiometric. Therefore the bridge voltage supply has to be stable.
The A-to-D I like is the TLC2543, 11 channel, 12 bit converter. I particularily like this
one as it is well documented for use with the Stamp by Dr. Allen
[noparse][[/noparse]www.emesystems.com/OL2tlc2543.htm].
In all fairness one forum contributor suggested using a 24 bit AD converter; eliminating the need for the instrumentation amplifier. But I like to first see the amplified analog signal so I can balance the bridge and adjust the gain without Stamp involvement. You can also 'shunt' calibrate the circuit to periodically check the gain.
cheers, David
Of course I don't really know your particular situation, but wouldn't it be better to support the (rotating circular?) sandpaper at three points instead of only two? With only two support points, how will you know whether or not the (disc of?) sandpaper isn't tilted along the axis of those two support points and thus allowing a side of the sandpaper to gouge the floor? In other words, if we call your two support points Right and Left, how do you know what the Back and Front edges of the (rotating?) sandpaper are really doing?
A three-point support system would define your grinding plane more precisely. Also, if this thing is supposed to "follow" the contours of the floor automatically and always keep the "weight values" at some set value automatically, then I don't think you're going to get it done for "cheap". You'll need a control system, load cells probably, and servos to shift the weight around (or to run screw jacks to lift at the supports). That's not exactly the task for a beginner, I think. And if you think you can simply zero the weight values at the start and then grind away: ask yourself if the targeted floor is really so level that it will allow those weight values to always remain the same once they are set. I'm guessing those values will vary greatly as you move over most floors.
But, then again, I'm just guessing here since I don't know your particular set up.
Hope that helps.
Post Edited (ElectricAye) : 8/1/2010 5:05:06 AM GMT
I guess if you want to get creative and try something new(?) in the world of floor grinders, you could mount a few microphones around the sandpaper and monitor the sounds the grinder makes. Your servos would then adjust your supports so the grinding sounds are kept about the same all around the sandpaper at any given moment. Similar techniques are done using acoustic emission sensors to grind optical lenses.
http://forums.parallaxinc.com/www.llnl.gov/str/Piscotty.html
My only experience is with strain gage based load cells [noparse][[/noparse]the same as that found in
practically all digital bathroom scales] so I will talk to that approach.
Look into small 'pancake' compression load cells. I can, if you wish, smoke out some
manufacturers. They are rugged, capable of taking severe overloads and are
environmentally sealed.
As I mentioned previously the output must be amplified and converted to digital before
inputting into a Stamp microprocessor. But by using the microprocessor you can do all
kinds of processing such as, zeroing out any 'tare' loading, recalibrating, summing,
calculating differences, etc. The possibilities seem great for your application. You will
probably also want a LCD display with button switches to select functions.
If you have never worked with a Stamp and the needed circuitry there are many examples
of different applications. Look into the customer applications segment on the Parallax
website. You might also e-mail their product support folks to see if they know of
applications more pertinent to your needs. I do remember a project weighing a bin of
material and portioning out exact amounts; under Stamp control. By reading about these
applications and studying the PBasic programs ideas will come.
cheers, David