I'm helping my dad customize some power wheelchairs we bought from - u guessed it - Ebay! There are serious weight constraints to be considered, and I find myself in need of a 4 by 4 foot platform scale to, on occasion, weigh from 50 to 800 lbs. I went to Google first, but the models I saw were professional scales costing around $2K. Yeow!
I discovered that there are things called BIA Body Fat Scales that allege to measure body fat and water content: "Electrodes built into the Chrome Foot Pads send a safe electronic signal through the body. Weight, body fat percentage, and body water percentage are calculated automatically! * * If you're wearin' a PACEMAKER, don't be usin' a Body Fat Scale! * * Is this a hoax? There seem to hundreds of them listed there.
It's always surprising to see what's for sale on Ebay. Sorry, I digressed. I finally found some cheap, serviceable step-on type bathroom scales by the caselot.
My question is this: If I took four identical bathroom scales, a 4x4 sheet or two of 3/4" plywood and attached wood block "feet" at each corner that would sit on the bathrooms scales, would their readings (assuming each scale was zeroed out to start) total up to the weight of the object? After my knockout gaffe today, I'm looking for a sanity check just to make sure I haven't overlooked the obvious. I think it should work and since I have the plywood, the only cost would be the four scales.
I'm really looking for a load measurement to the nearest 10 and maybe even
20lbs. Better resolution would be nice, but it's not necessary. I don't making walking around to add up the numbers. If I got scales that had digital readouts, I might even be able to network them together by tapping into the display circuitry. The reality is that the need is so infrequent that simple visual summation of all four scales isn't a problem.
Good. I'll make the feet out of scrap of doubled-up 2x4 scrap pieces all miter cut identically and mounted across the corners as you've suggested to give a fairly broad, central footprint on each scale. Then I'll have a wheelchair scale and four bathroom scales, one for each bathroom and then some. For another $50 I can aim pinhole B&W board cameras -- they're down to $12 each with cabling:
at each readout and display them thru my new MUX box on the video feed. Gotta love COTS solutions.
I'm not sure. Right now I have only one bathroom scale so I can't test the concept. What I would expect is if the scales are zeroed out and a 50 pound weight were placed equidistant from each scale on that board that they would both read 25 pounds or that one would read 25 and the other 24. Anything else would seem to be creating or destroying mass. Cold fusion Phase II.
When the scales come, I have some large barbell weights I am going to use to calibrate it. Until then I have only the queasy memory of college physics and levers from so long ago, the neurons have been recycled to hold old episodes of Seinfeld. Maybe someone with two bathrooms scales will straddle them and report back before then.
Even if the basic premise of simple totalling is wrong, there will probably be a function revealed through a number of test runs that will tell me how to process all four numbers to get the proper result.
But Steve, where's the *fun* in that? :-)
Abledata has a lot of info about mobility devices that they assembled under some sort of government contract but I'd be dubious about its accuracy. I just ran a CarFax report on my on VIN. It's pretty inaccurate. The Feds can't have done much better.
I'm probably going to try building a robotic "tender" that follows the main chair around with a selection of tools, chilled drinks, cordless phones and what not. Non-standard enough that I'd like to know the aggregate weight although I could weigh each piece separately before construction. Damn, I had my heart set on a big, roll-on platform scale.
So far, I haven't popped open any of the wheelchair motors. They look a little intimidating compared to a standard electric motor. I assume that's because they need to be well-sealed, traveling low to the ground the way they do. I also think I've not cracked them yet because the gearbox housing is bolted down like the power steering pump that once blew up in my face, literally.
My wife ran into the garage after the loud bang and the cloud of PS fluid that hit the manifold turned into thick, black smoke. She turned the corner and saw me emerging from the smoke cloue with my face covered with brigh red PS fluid and she fainted dead away. Thank God for goggles.
I'll have to research the proper torque settings and techniques. I know that there isn't 2000PSI building up in the wheelchair gear housing but traumatic events stay with you!
Right now I am de-gunking, re-painting, cleaning contacts, inspecting welds, tires and controls and generally doing light stuff. On the powerchair I'm adding a separate 12V electric system to power various accessories like running lights, a radio and turn signals.
Most of the manuals I have seen are user manuals and are nearly worthless in doing a motor or gearbox or brake rebuild. Fortunately, almost all of them seem to come from one manufacturer or the other. The motors on very different brands of scooter lifts are often identical and look much like the motors that activate automatic chair swivels. I assume once you get the hang of the basic model, the rest comes easy. If you know of any sources for Merits, Shoprider or Everest & Jennings manuals, please share! I alway buy detailed manuals for cars I own, but they're a little more complex.
A car manual can save you immense trouble, especially when it comes to knowing the "magic techniques" to remove plastic covers, reach hidden screws, etc. I'm not sure a scooter or wheelchair manual will reveal much. Do you have one for your chair? What did it cost?
Thanks! You've given me a number of things to think about.
How do you add algebraically v. directly? I know only one way to add.
How does the whole equal more than the sum of it's parts?
Try this. Take a one hundred lb wheelchair. Cut it into 4 equal pieces and weigh them. They should each be 25 lbs. The mass doesnt' change. The force of gravity doesn't change. Therefore the weight (which is the force of gravity acting on the mass) doesn't change.
If you had TWO scales, and a board bridging them, and put a known weight on them, would each scale weigh half the tare (weight minus board) weight?
I think you have to add these algebraically, not directly. Each scale is seeing more than the 1/4 load.
Simple enough to test the concept. Weigh something like concrete blocks, or a stack of phone books, on several scales and take an average so you have a 'standard'.
Then set up your 4 scales and see what you get.
Or take two scales, weigh yourself, then straddle both and see what you see.
I am an electric wheelchair user, and the specs for anything in the last 20 years are available on the web. That might save you the effort. There may be some minor differences due to accessories like baskets, cane or oxygen bottle holder, etc. but all well within your tolerance requirement.
As wheelchairs must all be approved by some or another letter agency, you may be able to find a database with all needed info in one place.
Don't forget dead batts will weigh 5-10 pounds less than good ones. And some have chargers onboard which may be missing, so look for that and consider the weight.
Tires are high wear items and generally are replaced as standard every year. Batts may last a year or two, rarely longer, depending on how they're used. Brake linings need replacement periodically. Bearings need lubrication esp if the thing is used outside or they will need periodic replacement.
Each manufacturer will have a maintenance schedule for his chair, and a service manual. You'll start seeing more than one of the same type as there are models distinctly popular and unpopular. Manuals may be worth the expense.
Good luck with your project ... Steve
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Zero without the platform and then weigh the platform alone to establish the tare. If using mechanical scales, the adjustment range may be less than the weight of the platform. And adjusting one corner at a time while weighted might have you running around in circles.