How would a cantilevered scale work?

Question

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.

Jim Thompson · Accepted Answer

LOAD SURFACE........  ______________SUPPORT.............         |LEVER...............         *____________________________FULCRUM.............                           |CRITICAL DIMENSION..         ^.................^* = Flexible joint so torque can't be transferred                                        ...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  |    1962     |              I love to cook with wine     Sometimes I even put it in the food

John Larkin · Answer

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

Jim Thompson · Answer

I don't currently own one, but I've seen platforms with suspensionssuch 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  |    1962     |              I love to cook with wine     Sometimes I even put it in the food

Paul Hovnanian P.E. · Answer

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.

How would a cantilevered scale work?

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