How load cells work?
Mass is among the seven fundamental physical quantities. In everyday applications, mass is often realized in the form of the object’s weight. The weight is the force of gravity acting on an object due to its mass.
How is an object’s weight determined? It is done by detecting the pressure that is being applied by it under the influence of gravity. A load cell is a device that determines weight by detecting the pressure it exerts. Microcontrollers and microcomputers may directly interact with strain gauge load cells, which are widely used. We will talk about various gauge load cells and types of load cells in this article.
What is a load cell?
A load cell transducer converts pressure or mechanical force into a measurable electrical signal. There are many forms of load cells that differ in size and shape, as well as the working principles. Load cells are often used both in industrial and domestic applications. Some of the primary applications of load cells in domestic settings include personal weighing machines, kitchen scales, bathroom scales, pallet weighing, and luggage weighing machines. Other industries, such as geotechnical equipment, medical equipment, transport scales, hopper scales, and truck scales, benefit from load cells. Some industrial applications of load cells are geotechnical devices, medical equipment, belt scales, hopper scales, track scales, and onboard weighing machines.
Various load cell types
Load cells come in a wide range of sizes, shapes, uses, and operational principles. The operating concept is the most straightforward method for classifying load cells. As a result, the vast majority of load cells fit into one of the three categories listed below.
- Hydraulic load cells
- Pneumatic load cells
- Strain gauge load cells
Hydraulic load cells detect fluid thrust to determine the magnitude of mechanical force. Their piston and cylinder setup is standard, with the fluid being held between two pistons. A fixed and immovable piston is one of the pistons. When pressure or thrust force is applied, the other piston moves. A pressure gauge detects the change in pressure that the piston’s movement creates inside a Bourdon tube. These load cells are often analog in nature.
Pneumatic load cells detect air or gaseous pressure to quantify mechanical force. A moving piston is positioned at the top of the load cell, which also has a cylindrical arrangement and is filled with air or gas. Air escapes from a nozzle at the bottom of the cylinder when pressure is applied to the piston, which causes the air pressure inside the cylinder to vary. A pressure gauge located inside the load cell measures the pressure that the escaping air or gas is exerting.
Strain gauge load cells use deformation in one or more strain gauges inside the load cell to detect mechanical force. Strain gauge load cells are the most common type of load cell. These come in a wide variety of shapes and configurations. For instance, a bar-type load cell has four strain gauges—two on either end of the bar. The load cell is positioned in Z-formation between two platforms on a weighing scale. The load cell bends when weight is applied to the bar because of the applied force (top platform). Two strain gauges in the bar track compression, and two more gauge tension to determine the bending distortion that results. Bar load cells and bending beam load cells are interchangeable terms.
In a typical weighing machine, four load cells are stacked in a Wheatstone bridge arrangement for maximum sensitivity.
strain gauge load cell types
According to the type of the cells, different strain gauge types may be divided into two categories (based on shape). Some of the predominantly available types (based on shape) and their setup are as follows.
These are also called bending beam load cells. The load cell is a straight bar with holes on each end of the bar. These holes fasten screws to make a Z-formation with two platforms. One platform remains fixed and stationary, while the other can bend down and place weight onto it. An example of a bar load cell is shown above.
There could be one strain gauge or several strain gauges on a single-bar load cell. Only one strain gauge is present between the two ends of the bar in a single strain gauge load cell. The strain gauge experiences tension or compression depending on how it is arranged when mechanical force is applied. The strain gauge can be used to measure tension or compression by measuring how much stretching power it can withstand.
These are load cells with multiple strain gauges that are the most common. These load cells have two strain gauges connected in Wheatstone formation. There are two sets of strain gauges at both ends of this bar. The pair of strain gauges at both ends measures tension and compression in equal and opposite directions, while the four strain gauges act as pressure-dependent variable resistors. When one gauge is destroyed, the resistance of such a bar assumes its capacity value. When a weight is placed in the loading location of the load cell sensor, it undergoes a bending distortion. The bending distortion is typically between 0.1 mm to 1 mm for full-scale load capacity.
Bending distortion causes the strain gauge pairs to have to bend tension and compression from end to end, as shown in the previous image.
The strain gauges are placed over the bar where they can experience maximum strain on the bending distortion of the bar. Internally, the strain gauges are placed in the Wheatstone formation, as shown in the image.
When under tension, a strain gauge becomes longer and thinner. Its resistance subsequently grows. The compression causes a strain gauge to become thicker and shorter. Its resistivity is consequently reduced. All strain gauges have the same resistance when everything is operating normally, and there is no voltage differential at the Wheatstone bridge’s output. Strain gauges R1 and R3 experience tension, and increasing resistance, while strain gauges R2 and R4 experience compression, decreasing resistance when weight is applied to the sensor. The voltage at the Wheatstone bridge’s output changes as a result. The output voltage is passed on to an amplifier circuit and is in the mV range.
The Wheatstone calibrating bar has at least four wires. There are two wires connected to the excitation wire of the suspension bridge, i.e., supply voltage and ground cables. The other two wires are present in the difference voltage of the Wheatstone bridge.
The bar load cells can be a single point, single-ended shear beam, double-ended shear beam, or wire-rope load cell.
These load cells are known as S-type load cells and are used to measure suspended weights. The load cell is an S-shaped device with holes at either end for rod ends or suspension bolts. The strain gauges at either end are under tension when weight is suspended from the load cell. Compression of the strain gauges at both ends occurs when the weight is placed over the load cell. A similar concept underlies tension link type load cells as well.
Load cells with a single strain gauge: These load cells have a single strain gauge. Depending on its setting, the strain gauge experiences tension or compression. An amplifier circuit detects the change in output resistance caused by tension or compression. These load cells are offered as load pins, load buttons, and planer load cells. To measure weight, four of these load cells are typically positioned beneath a platform at equal intervals.
Canister load cells are like disc load cells. These load cells are used for high-capacity measurements such as truck scales, railway car weighing, hopper scales, and agriculture scales. These load cells are used to measure only compression.
These torque force detectors are known as load cells, and torque is the force that acts on a piece of equipment causing it to rotate. An example of such a torque load cell is shown below.
Load cell output
The output voltage of the load cell is expressed to the excitation voltage, regardless of whether it is a single strain gauge load cell or a Wheatstone load cell. For full-load capacity, this is presented as a mV/V standard. For instance, the specification sheet for a load cell specifies 4mV/V as the output voltage. The output voltage of the load cell will be 40 mV at full load capacity if the excitation voltage is 10V. The output voltage of a load cell can be calculated using the equation below:
Vout = Vexcitation * Z mV/V
Where, Z is the mV/V specification.
Conclusion
Weight evaluation is done with the use of load cells, which are offered in a variety of shapes, sizes, circumstances, and functionality and are employed in various applications. These load cells include strain gauge load cells that are bending beam load cells, button disc type, S-type, torque load cells, or canister load cells. In accordance with its shape and setup, strain gauge load cells can be bending beam load cells, button disc type, S-type, torque load cells, or canister load cells.