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Internal structure and working principle of weighing/tension sensor

2025-12-23

Internal structure and working principle of weighing/tension sensor

A weighing sensor is a measurement device. When subjected to external force, it generates an electrical signal at its output terminal. Strain gauge-type weighing sensors are one of the most common types, used for measuring weight. This article will introduce the working principle and specifications of weighing sensors.

What is a Weighing Sensor?

A weighing sensor is a type of force transducer. A sensor is a device that converts one form of energy into a readable electrical signal. Specifically, a weighing sensor converts force into a voltage that can be read by electronic equipment. The most commonly used weighing sensor in daily life is the strain gauge type — it exists in almost all weight-measuring devices.

How Does a Weighing Sensor Work?

Inside a weighing sensor, there is an extremely thin resistive material attached to a flexible substrate. When an external force acts on the sensor’s body, the flexible substrate deforms — forcing the resistive material to deform along with it. As the dimensions of the resistive strip change, its resistance value also changes.

However, the change in resistance is extremely small relative to the material’s original total resistance — typically on the order of 1:100. This makes directly measuring the absolute resistance value very difficult.

To solve this, the resistive material on the flexible substrate is arranged into a circuit configuration called a Wheatstone bridge (named after its popularizer, Sir Charles Wheatstone). This bridge is a diamond-shaped arrangement of resistors, with one resistor on each side (as shown in the diagram below).

How the Wheatstone Bridge Works

An excitation voltage is applied across one pair of the diamond-shaped bridge’s diagonals, and the output voltage signal is read across the other pair of diagonals:

When no external force is applied: All resistors have the same value. Each "arm" of the bridge acts as a voltage divider, so the output voltage of each arm is equal — resulting in a 0V output (since the output points are midway between the dividers).
When external force is applied: One resistor’s value changes, altering the voltage on its arm. This creates a small voltage difference at the output terminals.

How to Use a Weighing Sensor

The sensor’s output voltage is extremely small and comes from a high-impedance source (the resistor divider). To read this voltage accurately, signal conditioning is required:

The circuit needs high input impedance (to avoid load effects on the divider).
It needs high common-mode rejection (since each arm’s absolute voltage is half the excitation voltage).
It needs high differential gain (to amplify the small voltage difference).

This is typically done with an instrumentation amplifier (e.g., AD620 with a gain of 100): 1mV of input voltage becomes 100mV of output. For example, if the bridge’s full-range output is ±25mV, the amplifier outputs ±2.5V.

Digital-Signal Weighing Sensors

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