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I would rather be driving
 
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Make your own corner balance scales?

Has anyone made their own corner balance scales. I think that $1K for a set of scales is crazy. I thought about making my own.

All you need are four button style compression load cells (used for ~$50 each 1K#s) A few small voltage amplifiers (home built) and a digital MM to read the V out. Heck, even buy 4 of these for $10 each. I think your home-built price would be about $300.

the load cells typically have a calibration curve to convert voltage out to load (mass). I think it should be easy enough to put the load cell between some plates to drive the car onto.

What do you think? Discuss amongst yourselves.
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Old 04-27-2007, 10:08 AM
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subscribed. No idea what a compression load cell is, but i'd like to meet one.
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Old 04-27-2007, 11:16 AM
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Yes, if you include force platforms to measure the pressure distribution of running mammals & hopping birds. Piezo-electric cells were used way back in the 1970s - search on Thom McMahon & Nike to see some cool graphs they did on running shoes...

So you are on the right track... do you have a bunch of 'free' graduate students to work up the design?
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Old 04-27-2007, 11:25 AM
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I would rather be driving
 
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I wish I had free grad students. It would be payback for my days in school.

I do have access to many electronics techs. None are familiar with load cells - yet. I guess I need to learn and then teach them.

I will see what I can dig up.
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Old 04-27-2007, 11:42 AM
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I agree, should be easy to come up with a USB interface so we can use a laptop instead of readout... Would anyone be interested in a group build if we could design it ?
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Old 04-27-2007, 11:47 AM
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Some of you guys are way too smart.
I think I'll just read along from the side lines.
Fasinating stuff though
Steve
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Old 04-27-2007, 11:56 AM
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How about these?

http://cgi.ebay.com/New-550-lb-Talking-Digital-Bathroom-Weight-Scale-550lb_W0QQitemZ160110276580QQihZ006QQcategoryZ7812 2QQrdZ1QQcmdZViewItem

One for each front wheel, and two for each rear, with a steel plate between them
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Old 04-27-2007, 12:24 PM
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That would be great, Jamie.

And, yes, I'd be interested - but it all depends on price, doesn't it?

The cells need to be rel. accurate at the expected load and they need to be able to handle that load. 3rd is the issue of linearizing the output (unless units can be obtained that already have a linear output). 4th is things like susceptibility of the circuits to temperature, etc.

My guess is that this will be a whole lot easier these days than it used to be.
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Old 04-27-2007, 12:56 PM
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I am very interested. I thin DIY is always the best way, specially when you have the resources of a forums like this which is plenty of such smart people....ME? I am only a freerider in this case, but will pay for a set or even build one if you come up with some down to earth instruction..:-)

Keep working on the idea!! Tx
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Old 04-27-2007, 01:22 PM
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For a low-tech look at a Myth Buster application of load cells, find the episode that tries to confirm or bust if the weight of a container with pigeons changes if they were to fly inside.

The flock of pigeons weighed 11 lbs. and they were able to precisely tell any difference in the weight of the container.

You're welcome,
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Old 04-27-2007, 01:38 PM
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I've always wondered if a plumb-bob could be rigged over a graph to show an accurate balance. I'm thinking of a tripod attached to the car with the plumb-bob hanging over a flat graph attached inside the legs. If you could calibrate it with with a known center, subsequent measurements would at least show the changes. Using thin greased plates under the the tires will allow adjustment with the suspension under load, and settled. Could be the poor-man's solution to self corner balancing.
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Old 04-27-2007, 01:58 PM
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Sounds like a perfect application for the Arduino. The Arduino is an open source microcontroller platform. Its based off the Atmega168 microcontroller which has a built in A/D converter. For $35 you can get one of these mcirocontrollers on a board, with USB interface, etc. The Arduino programming environment (a version of C) has built in routines for serial port communications. There are a number of cheap LCDs available now that could be used for displaying the weights.

So to build a standalone unit, from an electronics perspective all you'd need is an arduino board ($35), a serial enabled LCD ($25), a small protoboard with some opamps ($10 ?), and a 5 volt wall-wart supply. With the microcontroller you could write code to take the raw data from each load cell amplifier and do a calibration table lookup.

links: http://www.arduino.cc/ (arduino site)
http://www.sparkfun.com/ (place to buy parts)

-Tony
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Old 04-27-2007, 02:12 PM
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You might say 'it's not rocket science...', Actually it is. Here's a link to a 'Rocket Scientist' who made all you would need quite cheaply.

http://www.nakka-rocketry.net/strainlc.html
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Old 04-28-2007, 04:55 AM
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I thought of this way back when, but then I thought again..

By the time I spend time and money to make scales the way I want it, I'd ended up burning more time and money than what the scales I have cost me. Now don't jump all over me like the guys over in the SSI discussion, I'm very much for the thought of engineering a solution that is low cost and more simple to achieve the same end.

I'd have to fabricate plates that will hold the load cells, remain relatively flat when the weight of a tires on it, show no weight influence by the fabrication of the plates, tie all the cells together to a nice digital display that will do percenttages, cross jacking, left, right, front, rear....

And I'd be better off spending all that time driving If someone does come with with a cheap way to do it, I'd be happy to give them a thumbs up
Old 04-28-2007, 05:13 AM
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It can be cheap (something home-made). It can be easy (pay someone to do it, or go visit someone that has built/bought one).

I doubt it can be both.
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Old 04-28-2007, 05:18 AM
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Quote:
Originally posted by rvanderpyl
How about these?

http://cgi.ebay.com/New-550-lb-Talking-Digital-Bathroom-Weight-Scale-550lb_W0QQitemZ160110276580QQihZ006QQcategoryZ7812 2QQrdZ1QQcmdZViewItem

One for each front wheel, and two for each rear, with a steel plate between them
Buy one and take it apart. Maybe it can be modified to work.

Cheap bathroom scales work with springs, if you can change the springs and re-calibrate them in pairs you're ready to go.

Or just wait. From what I've seen at the Mall 1,000 lb scales will be at walmart any day now.
Old 04-28-2007, 05:22 AM
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I have a set of longacre scales. They are not cheap, but Souk is right.

Although you could fab something up for a material cost of significantly less than $1k, you will burn a lot of hours making setup anywhere near as functional as the longacres.

The pads that the tires rest on are very sturdy (and need to be). This alone is a challenging design and fabrication task. Then you have the electronics.

Like so many DIY projects, do it for fun / challenge / enjoyment. But the hours that will go into it would make it impractical...unless you place no value on your time.
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Old 04-28-2007, 06:57 AM
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You guys just need better friends - I just borrow scales when I need them.
-Chris
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Old 04-28-2007, 07:09 AM
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uhm gee Chris we were going to give you a set after we engineered them ....
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Old 04-28-2007, 07:29 AM
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This could be done with compression loadcells. Typically the accuracy specifications will be written something like " combined error =.2% of F.S." or something close to that. That means that the sum of all errors caused by non-linearity, hysteresis, non-repeatability, temp drift, etc. will not exceed .2% of whatever the rated load is. The tighter that error number is, the better, and typically more expensive, they are. also. look into the overload ratings. Very important, especially in an application where the load shifting, shear loads, etc. may be high while driving onto the scale, for example.

I used to be a Transducer engineer/designer and designed many strain-gage based loadcells/systems. (now I just play one in the movies.)

Regards,
Al

P.S. I have many projects going on, but I would be glad to review your loadcell selection and approach. One of the electronics engineers who worked for me, designing these weighing systems, works for me still and I'm pretty sure that he would take a look at your controller selection and make reccomendations.

P.S.S. Check out Omega catalog or website for loadcell and simple controller ideas. They are too expensive thru them, but good for ideas and then head to ebay.
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Old 04-28-2007, 07:59 AM
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