Strain Gauge / Load Cell

A strain gauge is a sensor whose electrical resistance changes when it is mechanically stretched or compressed. The sensing element is a thin metallic foil (usually constantan alloy) bonded to a backing material. When the surface it is bonded to deforms, the foil stretches or compresses, changing its length and cross-section and therefore its resistance.

A single strain gauge produces only a tiny resistance change — a few ohms at full load. To convert this into a useful signal, four gauges are wired in a Wheatstone bridge configuration. Two gauges are in tension and two in compression, which doubles sensitivity and cancels out temperature errors. The bridge output is typically 2-3 mV per volt of excitation voltage.

A load cell is a complete force-measurement device: it consists of a precisely machined metal element (the spring element) with strain gauges bonded at positions of maximum stress. Common geometries include the S-beam (tension/compression), bending beam (platform scales), and compression canister (silo weighing).

The signal conditioner amplifies the millivolt bridge output to the standard 4-20 mA or 0-10 V range that the PLC analog input card accepts. High-quality conditioners include features like zero calibration, span adjustment, and digital filtering to remove mechanical vibration noise.

PLC scaling follows the same linear scaling formula as any other analog sensor: engineering_value = (raw_count / max_count) * (max_EU - min_EU) + min_EU.