Quality is one of the seven fundamental physical quantities. In practical applications, mass is often manifested in the form of object weight. Weight is the force acting on an object due to its mass.
How is the weight of an object measured? It is accomplished by sensing the pressure exerted under the influence of gravity. The device that measures weight by sensing the applied pressure is called a weighing sensor. Strain gauge weighing sensors are very common, directly connected to microcontrollers and microcomputers. In this article, we will discuss different types of weighing sensors and instrument weighing sensors.
A weighing sensor converts pressure or mechanical force into a measurable electrical signal. Weighing sensors come in various variants, differing in size, shape, and operating principles. Weighing sensors are commonly used in household and industrial applications. Some domestic applications of weighing sensors include personal scales, kitchen scales, bathroom scales, tray scales, and luggage scales. Some industrial applications of weighing sensors include geotechnical engineering equipment, medical devices, belt scales, hopper scales, rail scales, and vehicle weighing machines.
There is a significant variation in the size, shape, application, and operating principles of weighing sensors. The simplest way to classify weighing sensors is based on their operating principles. Based on this, almost all weighing sensors belong to one of the following three categories.
Hydraulic Weighing Sensors
Pneumatic Weighing Sensors
Strain Gauge Weighing Sensors
Hydraulic weighing sensors measure mechanical force by sensing the thrust applied to the fluid. They have a typical piston and cylinder configuration, where fluid is stored between two pistons. One piston is fixed, and the other piston can move under the action of pressure or thrust. The movement of the piston causes a change in pressure inside the Bourdon tube, and a pressure gauge detects this change. These types of weighing sensors are typically analog.
Pneumatic weighing sensors measure mechanical force by sensing air or gas pressure. It has a similar cylindrical configuration, where the cylinder is filled with air or gas, and a movable piston is placed on top of the weighing sensor. When pressure is applied to the piston, the pressure inside the cylinder changes, causing air to escape from the nozzle at the bottom of the cylinder. The pressure applied by the escaping air/gas is measured by the pressure gauge inside the weighing sensor.
Strain gauge weighing sensors measure mechanical force by sensing the deformation of one or more strain gauges inside the weighing sensor. Strain gauge weighing sensors are the most common type. They come in various shapes and configurations. For example, bar-type weighing sensors have four strain gauges; two are placed at the ends of the bar. The strain gauges are arranged in a Z-shape between two platforms on the weighing scale. When some weight is placed on the bar (top platform), the weighing sensor bends due to the applied force. Two strain gauges in the bar measure tension, and two measure compression to detect the resulting bending deformation. Bar-type weighing sensors are also called bending beam weighing sensors.
Four weighing sensors are arranged in a Wheatstone bridge form to achieve maximum sensitivity in typical weighing machines.
Strain gauges, placed along the bar, bear the maximum strain on the bending deformation of the bar. Internally, strain gauges are arranged in a Wheatstone bridge structure, as shown in the figure below.
The strain gauge under tension becomes thinner and longer. As a result, its resistance increases. The compressed strain gauge becomes thicker and shorter. As a result, its resistance decreases. In normal conditions, the resistance values of all strain gauges are the same, and the voltage difference at the output of the Wheatstone bridge is zero. When weight is placed on the sensor, strain gauges R1 and R3 are subjected to tension, causing an increase in resistance, while strain gauges R2 and R4 are subjected to compression, causing a decrease in resistance. This creates a different voltage at the output of the Wheatstone bridge. The output voltage is in the mV range and is transmitted to the amplifier circuit.
Wheatstone bar-type weighing sensors have at least four wires. Two wires are for the excitation of the strain gauges (power voltage and ground), and the other two wires measure the differential voltage of the Wheatstone bridge.
Bar-type weighing sensors can be single point, single-ended shear beam, double-ended shear beam, or wire rope weighing sensors.
S-type weighing sensors: These weighing sensors are used to measure suspended weights. The weighing sensor is S-shaped, with holes at both ends for fixing hanging bolts or rod ends. When a weight is suspended on the weighing sensor, both strain gauges at the ends experience tension. If the weight is placed on the weighing sensor, both strain gauges at the ends experience compression. Rod-type weighing sensors also work on the same principle.
Single strain gauge weighing sensors: These weighing sensors consist of a single strain gauge. The strain gauge bears tension or compression depending on its configuration. Under tension or compression, its output resistance changes, detected by the amplification circuit. These weighing sensors can be used as weighing buttons, flat weighing sensors, and load pins. Typically, four such weighing sensors are placed at equal distances below the platform to measure weight.
Tank-type weighing sensors: Tank-type weighing sensors are similar to disc-type weighing sensors. These weighing sensors are used for high-capacity measurements, such as vehicle scales, railway vehicle weighing, hopper scales, and agricultural scales. These weighing sensors are designed to measure only pressure.
Torque weighing sensors: These weighing sensors are used to measure torque force. Torque is the force that causes an object to rotate. The figure below shows an example of a torque weighing sensor.
Weighing sensor output
Whether it's a single strain gauge weighing sensor or a Wheatstone weighing sensor, the output voltage of the weighing sensor is expressed as excitation voltage. This is provided as the mV/V specification for full load capacity. For example, the output voltage of the weighing sensor is specified as 4mV/V in the specification table. If the excitation voltage is 10V, the output voltage of the weighing sensor at full load is 40mV. The following formula gives the output voltage of the weighing sensor –
V_output = V_excitation * Z mV/V
Where Z is the mV/V specification.
Weight measurement is carried out using weighing sensors, which come in various shapes, sizes, use cases, and operating principles, suitable for various applications. These weighing sensors can be single strain gauges or Wheatstone weighing sensors. Depending on their shape and configuration, strain gauge weighing sensors can be bending beam weighing sensors, button/disc-type, S-type, torque weighing sensors, or tank-type weighing sensors. Even single strain gauge weighing sensors are often used in Wheatstone configurations to achieve maximum sensitivity."
