A load cell is a transducer or sensor that is used to convert a mechanical quantity proportionally into an electrical or electronic signal. A pushing action on an object or body causes it some distortions that have to be converted into measurable electronic signals. The conversion occurs in two steps:
- An action/force (or weight or torque) applied to a metallic test body induces a mechanical deformation of the metal part that is profiled in order to properly identify and measure the applied force or effort.
- This deformation is transmitted to strain gauges – as small electrical resistances – whose distortions follow that of the test body. These distortions induce a change in their electrical resistance (R) which follows the principle of ‘Pouillet’s (or Ohm’s) law’: R = αl / S
α is a coefficient dependent on the material
l is the length of the material
S is the cross-section area (which varies with the deformation).
A load cell usually consists of four strain gauges in a ‘Wheatstone bridge’ configuration. The output signal is often expressed in mVolts per Volt. To exploit the signal industrially it must be amplified through an amplifier.
The distortions are transmitted through a support film in order to convey, as accurately as possible, the deformation of the body. This support will be characterised and selected according to a set of criteria to minimise the transmission errors.
Finally, the output of the transducer is put into an algorithm to calculate the force applied to it.