Problem
Does an electronic kitchen scale use an inductive sensor to weigh objects? Which physical principle is actually exploited? Discuss the difference between a magnetic scale (for ferromagnetic metals) and one with load cells.
Solution
No: a kitchen scale does not weigh by induction. Almost all electronic scales use load cells with strain gauges. The pan rests on an elastic element that deforms under the weight; a strain gauge is bonded to it, and its resistance changes with the deformation. A Wheatstone bridge converts the change in resistance into a voltage proportional to the weight force. The principle is purely mechanical-electrical (deformation resistance) and has nothing to do with flux variation.
What an inductive sensor does instead. An inductive sensor contains a coil carrying alternating current. When a conductor or ferromagnetic material approaches, the eddy currents induced in the object (and/or the change in permeability) alter the coil’s inductance . The sensor detects the presence of the metal, not its mass.
Key difference.
- Load cell: measures a force (universal, works with any object — plastic, food, metal).
- Magnetic/inductive sensor: only responds to conducting or ferromagnetic materials and measures proximity/amount of metal, not weight.
So an inductive sensor can act as a metal detector or a limit switch, but weighing your shopping requires a load cell.
Collegamenti
Argomenti: Electromagnetic induction Concetti: Inductance and self-induction · Eddy currents