I saw a fancy digital kitchen scale with a cantilevered surface for holding the load. I assume it has a load cell, but why does it read the same no matter where the load is placed? I would expect a larger reading if the load is farther from the fulcrum.
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LOAD SURFACE........ ______________ SUPPORT............. | LEVER............... *____________________________ FULCRUM............. | CRITICAL DIMENSION.. ^.................^
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
So what happens whan I place a load off-center on the platform? Why doesn't it tip over?
I've done this...
----T----T--------------------- platform | | | | | | | | load | | cell | | | | --T----T-- / / / / / / / / /
where T is bolts, and the platform is a simple sheet of aluminum, and the load cell acts electrically as a single bridge.
Given two weights, I could weigh them individually, anywhere on the platform, and then put them both down, anywhere, and have the sum always match the components to 50 PPM or so.
Cheap scales may use some sort of mechanical linkage to eliminate position error, but good weighing cells take care of it themselves.
John
I don't currently own one, but I've seen platforms with suspensions such that force can only go straight down... indeed a kitchen scale, so not terribly accurate below about 1/8 oz.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
Because the actual design of such a device includes a balancing weight suspended below the load surface such that, for loads below the capacity of the device, the CG of the load surface and support remains below the flexible joint.
Sometimes this includes a cleverly designed flexible joint such that the apparent axis of rotation lies above the load surface.
In other words, they are insensitive to torque. The actual construction is probably more involved than this:
The load cell is split in half and each half is attached to one of the two vertical supports (in your diagram) from the base to the platform. The signal from each is summed such that the difference due to torque on the platform is canceled.
-- Paul Hovnanian paul@hovnanian.com ----------------------------------------------------------------------- Have gnu, will travel.
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