Mass is one of the seven basic physical quantities. In real-world applications, mass is often perceived in the form of the object's weight. Weight is the force of gravity that acts on a body due to its mass.
How is the weight of an object measured? This is done by detecting the pressure exerted by it under the influence of gravitational force. A device that measures weight by detecting the pressure exerted by it is called a load cell. Strain gauge load cells are very common and have direct interface with microcontrollers and microcomputers. In this article, we will discuss different types of load cells and measuring load cells.
What is a load cell
A load cell transducer converts pressure or mechanical force into a measurable electrical signal. There are many variations of load cells that vary in size, shape and operating principle. Load cells are often used in domestic and industrial applications. Some of the domestic load cell applications include personal scales, kitchen scales, bathroom scales, pallet weighing, and luggage scales. Some industrial applications of load cells include geotechnical devices, medical equipment, belt scales, hopper scales, conveyor scales, and integrated weighing machines.
Types of load cells
Load cells vary significantly in size, shape, application and operating principle. The easiest way to categorize load cells is by their operating principle. Based on this, almost all load cells fall into one of the following three categories.
- Hydraulic load cells
- Pneumatic load cells
- Extensometer Load Cells
Hydraulic load cells measure mechanical force by sensing the thrust applied to a fluid. They have a typical piston and cylinder configuration in which a fluid is stored between two pistons. One of the pistons is fixed and immobile. The other piston is movable on application of pressure or thrust force. The movement of the piston causes a change in pressure within a Bourdon tube, which is detected by a pressure gauge. These types of load cells are usually analog.
Pneumatic load cells 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 load cell. When pressure is applied to the piston, the air pressure inside the cylinder changes, causing air to escape from a nozzle at the bottom of the cylinder. The pressure exerted by the escaping air/gas is measured by a pressure gauge inside the load cell.
Extensometer load cells measure mechanical force by detecting deformation on one or multiple extensometers within the load cell. Strain gauge load cells are the most common type of load cells. They come in a variety of shapes and configurations. For example, a bar load cell has four strain gauges; two placed at each end of the bar. The load cell is arranged in a Z formation between two platforms on a scale. The load cell is bent due to the force applied when some weight is placed on the bar (top platform). Two strain gauges on the bar measure tension and two measure compression to detect the resulting bending distortion. Bar load cells are also called bending beam load cells.
Four load cells are arranged in a Wheatstone bridge formation for maximum sensitivity in a typical scale.
Types of strain gauge load cells
Strain gauge load cells come in a variety of shapes and configurations. Following are some of the commonly available types (based on shape) and their configuration.
Bar load cells: These are also called bending beam load cells. The load cell is a straight bar with holes at each end of the bar. These holes hold screws to make a Z-formation with two platforms. One platform remains fixed and stationary, while the other can bend and put weight on it. An example of a bar load cell is shown in the image above.
A bar load cell can have a single extensometer or multiple extensometers. In a single extensometer load cell, only one extensometer is present between the two ends of the bar. When mechanical force is applied, the extensometer undergoes tension or compression depending on its configuration. Tension or compression changes the resistance value of the strain gauge, which is detected by an amplifier circuit. A single strain gauge bar load cell comes with just three wires, where two wires are for excitation (supply voltage and ground) and the third wire is used for analog output.
Bar load cells with multiple strain gauges are more common. These load cells usually have four strain gauges connected in a Wheatstone formation. There are two strain gauges at both ends of the bar. The pair of strain gauges at both ends measure tension and compression in equal and opposite directions – the four strain gauges act as pressure-dependent variable resistors. When a weight is placed at the load cell sensor load point, it experiences bending distortion. Bending distortion is generally between 0.1mm and 1mm for full-scale load capacity.
As a result of bending distortion, strain gauge pairs experience tension and compression at both ends, as shown in the image below.
Strain gauges are placed along the bar where they can experience maximum stress in the bending distortion of the bar. Internally, the strain gauges are arranged in a Wheatstone formation, as shown in the image below.
An extensometer under tension becomes thinner and longer. As a result, your resistance increases. An extensometer under compression becomes thicker and shorter. As a result, your resistance decreases. Under normal conditions, all strain gauges have the same resistance and the voltage difference at the output of the Wheatstone bridge is zero. When weight is placed on the sensor, extensometers R1 and R3 undergo tension, causing an increase in resistance, and extensometers R2 and R4 undergo compression, causing a reduction in resistance. This generates a different voltage at the output of the Wheatstone bridge. The output voltage is in the mV range and is passed to an amplifier circuit.
The Wheatstone bar load cell comes with at least four wires. There are two wires for excitation of the extensometer, i.e. supply voltage and ground. The other two wires measure the voltage difference across the Wheatstone bridge.
Bar load cells can be single-point, single-ended shear beam, double-ended shear beam, or wire rope load cell.
Type S Load Cell: These load cells are used to measure suspended weights. The load cell comes in an S shape with holes on both ends for attaching suspension bolts or rod ends. When a weight is suspended from the load cell, the strain gauges at both ends experience tension. If weight is placed on the load cell, the strain gauges at both ends will experience compression. Tension link type load cells also work on the same principle.
Single extensometer load cells: These load cells consist of a single extensometer. The extensometer undergoes tension or compression depending on its configuration. Under tension or compression, its output resistance changes, which is detected by an amplifier circuit. These load cells are available as load buttons, flat load cells and load pins. Typically, four of these load cells are placed under a platform at equal distances to measure weight.
Canister load cells: Canister load cells are similar to disc load cells. These load cells are used for high capacity measurements such as truck scales, railway car weighing, hopper scales and agricultural scales. These load cells are designed to measure compression only.
Torque Load Cells: These load cells are used to measure torque force. Torque is the force that acts on an object causing it to rotate. An example of a torque load cell is shown in the image below.
Load cell output
Whether it is a single strain gauge load cell or a Wheatstone load cell, the output voltage of the load cell is expressed as the excitation voltage. This is provided as a mV/V specification for full load capacity. For example, the output voltage of a load cell is specified as 4mV/V in the specification sheet. If the excitation voltage is 10 V, at full load capacity, the load cell output voltage will be 40 mV. The following equation gives the output voltage of a load cell –
V out =V excitation * Z mV/V
Where Z is the mV/V specification.
Conclusion
Weight measurement is done using load cells, which come in various shapes, sizes, use cases and operating principles and in a variety of applications. These load cells can be simple strain gauges or Wheatstone load cells. Based on their shape and configuration, strain gauge load cells can be flexible beam load cells, button/disk type, S-type, torque load cells, or canister load cells. Even single strain gauge load cells are often used in a Wheatstone configuration for maximum sensitivity.
