Internal structure and working principle of weighing/tension sensor

Internal structure and working principle of weighing/tension sensor

A weighing sensor is a device used to measure force. When subjected to an external force, it generates an electrical signal at its output. The strain gauge weighing sensor is one of the most common types used for measuring weight. This article will introduce the working principle of weighing sensors and their various specifications.

**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, which is present in almost all devices that measure weight.

**How Does a Weighing Sensor Work?**

A weighing sensor contains extremely thin resistive material attached to a flexible substrate. When an external force acts on the body of the weighing sensor, the flexible substrate deforms, forcing the resistive material to deform as well. As the dimensions of the resistive traces change, their electrical resistance also changes.

However, the change in resistance is very small compared to the original total resistance of the material—typically on the order of 1:100. This makes directly measuring the absolute resistance very difficult. Therefore, the resistive material on the flexible substrate is arranged into a circuit structure called a Wheatstone bridge. The Wheatstone bridge (named after its promoter, Charles Wheatstone) is a diamond-shaped arrangement of resistors, with one resistor on each side, as shown in the figure below:

An excitation voltage is applied across one pair of opposite corners of the diamond shape, and the output voltage signal can be read from the other pair. Typically, when no external force is applied, all resistors have the same value. Each "leg" of the Wheatstone bridge acts as a resistive voltage divider. Since all resistances are equal, the output voltage from each leg is also equal – meaning the differential output is 0V, as the output points are at the midpoint of each divider. When an external force is applied, the resistance of one of the resistors changes, altering the voltage across that leg. This results in a small voltage difference appearing at the output.

**How to Use a Weighing Sensor?**

The output voltage of a weighing sensor is very small and comes from a high-impedance source (i.e., the resistive dividers). Therefore, to read the voltage accurately, signal conditioning is essential. The signal conditioning circuit must have a high input impedance to avoid loading the dividers, a high common-mode rejection ratio because the absolute voltage on each leg is half the excitation voltage, and also a high differential gain. This requires the use of an instrumentation amplifier, which meets all the above criteria. The figure below shows an example circuit using an AD620 instrumentation amplifier with a gain of 100, meaning for every 1mV of input voltage, the output will be 100mV. For instance, if the bridge's full-scale output is ±25mV, the amplifier's output will be ±2.5V.

**Digital Weighing Sensors**

To overcome the technical challenges associated with using analog weighing sensors, digital weighing sensors with interfaces like UART and RS485 can be selected. These sensors support the standard MODBUS-RTU communication protocol, enabling easy connection to devices such as PLCs, computers, display modules, and wireless data transmission units.