The usefulness of load cells cannot be underestimated. These small devices permeate a whole array of equipment across hundreds of industries. Read on to learn more about load cells.
WHAT IS A LOAD CELL?
Load cells are a type of transducer: a sensor which converts a physical quantity into an electrical signal. Load cells are primarily used to measure weight. This is accomplished by converting the gravitational force acting on an object into an electrical signal. The resulting electrical signal can be measured and displayed by use of specialized analog converters, digital signal processors, and digital displays. This all sounds complicated, but in reality, the idea is straightforward.
There are many different types of load cells including strain gauge load cells, hydraulic load cells, pneumatic load cells, and capacitive load cells. Strain gauge load cells are the most popular type of load cells used today. Although the several types of load cells are different in design, all load cells convert a force into a measurable electrical signal. This article will explain the different types of load cells, but will focus on strain gauge load cells due to their overwhelming majority in modern weighting applications.
WHAT ARE LOAD CELLS USED FOR?
Generally, load cells are used for various load bearing and weight/force measuring applications. Their ability to sense force, load and weight makes them applicable in various diverse industries such as the medical and pharmaceutical industry, manufacturing industry, automotive industry, research laboratories, agricultural industry, aerospace and defense industry, robotics systems industry, marine industry, oil and gas industry etc.
Load cells are found in various devices such as weight checkers, weighing scales, medical devices, platform and industrial scales, filling and sorting machinery, rolling mill systems etc.
Load cells are also used in devices for various processes such as process weighing, parts counting, medical equipment testing, pile testing, process control and automation monitoring. In large scale applications, load cells are used in wire tension measurement, silo weight measurement and monitoring, automated container filling, robot tactile sensing and batch weighing. In reality, load cells have endless applications where force or weight is needed to be measured and quantified.
HISTORY OF WEIGHING AND SCALES
The desire to measure and quantify the world has existed since the dawn of civilization. Evidence of simple balances can be tracked back to as early as 5000 – 6000 B.C. in ancient Mesopotamia . The equal arm balance is a tool that was used in ancient Egypt, and is the most recognizable symbol of measurement in the world.
The balance consists of a vertical column connected to a pivoting horizontal bar at its top. To pans hang down from each side of the horizontal bar. These pans are equal in weight, so the balance will not move in either direction unless an imbalance is introduced to one side. Figure 1 shows an image of an equal arm balance.
Despite its popularity, the equal arm balance is unable to display measurable quantities. The need to add quantities to scales spawned the creation of the unequal arm balance, also known as the steel yard balance. This type of balance consists of a suspended metal arm with a fulcrum point off to one side.
A load hangs from one side, which creates a balanced state of suspension between it and a pan. When an object is placed in the pan, the load can be adjusted until the bar reaches a horizontal state. Markings on the bar indicate the weight of the object. Figure 2 shows an example of an ancient Roman balance.
Still, even with the ability to measure and quantify an objects weight, standardized weights had still yet to be created. The invention and adoption of the metric system has been one of the most important creations of modern civilization. Standardized measurement liberates humankind from inconsistencies and allows simplicity in every aspect of life. With the invention of the metric system came the quantization of the kilogram.
Locked deep under Paris lies the International Prototype Kilogram. This object was created in 1889 out of iridium and platinum to maintain consistency throughout the ages . It functioned, until recently, as the worlds standardized unit for the kilogram. With a standard system that can be used for measurement, the entire world does not have to worry about translation issues or discrepancies with measurements.
Figure 3 shows the International Prototype Kilogram.
As technology advanced through the industrial revolution and the discovery of electricity, more effective methods for weight measurements became possible. In 1843, English physicist Sir Charles Wheatstone invented the Wheatstone bridge. The Wheatstone bridge allows differences in voltages to be measured between two center points.
This circuit formed the basis for modern load cell technology, as the arms of the bridge circuit can be fixed with sensors that are sensitive to stress or strain. As the sensors are put under a stress or strain, a voltage difference can be detected across the two center regions. An example of this type of circuit can be seen in Figure 5.
The type of sensor that is responds to stress or strain is called the strain gauge. The strain gauge is a bonded wire device that changes its resistance when it is deformed (stretched/contracted). Strain gauge load cells are the most common form of load cells due to their simplicity. Strain gauge load cells are can be manufactured easily, and are becoming cheaper as technology continues to advance.
However, these are not the only type of load cells that exist in the modern market. Pneumatic and Hydraulic load are also used for weight measurement, but operate under different principles compared to the strain gauge load cell. They do however provide a measurable electrical signal as an output.
WHAT ARE THE DIFFERENT LOAD CELL TYPES?
Pneumatic load cells
The pneumatic load cell functions by use of a pressurized air or gas. When a force from a weighted object is placed on a scale, a specific pressure will be required to balance the scale. Air or gas can be used to balance the scale.
This system operates very similarly to the ancient Roman steel yard balance, except through used of a pressurized system. In Figure 3, if a weight is placed on the top of the scale (W) the force from the weight will cause a change in pressure. This change in pressure can be measured, and expressed as a function of weight.
A pneumatic load cell has the following parts; an elastic diaphragm attached with a platform surface to which force can be applied, an air supply regulator, a nozzle and a pressure gauge. These gauges are usually made of stainless steel. Figures 6 and 7 depict typical pneumatic load cells.
Hydraulic load cells
Hydraulic load cells are quite similar to pneumatic load cells. Instead of air or gas, a piston-cylinder is filled with a pressurized liquid. When a force is applied to the piston, a measurable pressure change can be detected in the piston. The increase in pressure is proportional to the strength of the applied force.
The output can be a classic analog gauge or an electric signal from a pressure sensor. The main components are; an elastic diaphragm, a piston with a loading platform placed on top of the diaphragm, a liquid medium and a bourdon tube pressure gauge. The metal body parts are always made from stainless steel. Figures 8 and 9 depict typical hydraulic load cells.
Strain gauge load cells
A strain gauge sensor is a transducer that changes in electrical resistance when under a stress or strain. This change is proportional to the stress or strain placed on the cell. When an object is placed on a load cell, it causes a deformation in the dimensions of the underlying strain gauge.
The strain sensors are electrically built into the load cell using a Wheatstone bridge configuration. When powered by a voltage source, typically between 10-15 V, the Wheatstone bridge strain gauge configuration will respond to stress and strain from a weighted object. This change in voltage is typically very small, and must be amplified in order for a digital system to display accurate force or weight.
How does a Strain Gauge Load Cell Work?
As was briefly described above, strain gauge load cells make use of strain sensors. Strain sensors respond to stress or strain with a proportional change in resistivity. Since the change in resistivity is linear, this can be converted back into a force and subsequently a weight.
Making use of strain gauge sensors is most easily done with a Wheatstone bridge configuration. There are many different ways of doing this, but all result with a difference in voltage between two points. This difference is typically very small, since the change in resistivity is also quite small. An output signal from a strain gauge Wheatstone bridge can be in the microvolt range. This signal must be amplified by use of instrumentation or differential amplifiers.
These types of amplifiers can boost signals ranging from 10 to 100,000, sometimes even more. Once the signal is readable, it can be analyzed by a digital signal processor, and translated into a readable force or weight. Load cells are given full scale outputs, which can be translated back into the typical changes in resistivity due to the stress or strain by a force. Generally, the number of active elements legs in the Wheatstone bridge determines the type of bridge configuration.
The strain gauge is a bonded resistive foil sensor which is capable of stretching and contracting. When a sensor is stressed, its resistivity is changed. There are some limitations to the linear region in which a strain gauge sensor can be flexed, but these considerations are considered when designed for application. Strain gauges are mounted in areas that exhibit a strain in compression or tension. Most strain gauges are made from a copper-nickel (constantan) alloys, the alloy metal is cut into zigzag foils to form the strain gauge.
WHAT IS A TENSION LOAD CELL?
Tension load cells make use of strain gauges and are designed to measure tensile loads. A tensile load constitutes a tensile pulling force. A tensile force is one which pulls away from the object which it acts on. A voltage must be applied to the load cell in order to measure an output from a change in force applied to the load cell.
Tension load cells are of great use for wound or unwound processes e.g. in the production film and ribbons. If the tensile force is too much, the film can break, rendering it useless and creating a lot of down time as the machine will need to be re-threaded. Also, if the tensile force is too low when the film is winding, then the rolls are too loose, leaving them prone to exposure or causing other machines to jam. Tension load cells are also used for more than just film, spring coils are also wound with tension load cells to keep the coils from snapping as the metal is bent.
The types of tension load cells include S-beam load cells, columns, and low profile, shackle and link type load cells.
THE FUTURE OF LOAD CELLS
As technology continues to advance, better load cell technology becomes possible. Load cells are typically given a range of weights that are weighable using that specific cell. As technology improves, load cell accuracy improves as well. A single load cell might be able to measure weights from a gram up to tons accurately. Typical load cells today have limitations in the range of weights that can be measurable.
Load cells will also find more applications in microsystems, hence resulting into something called Miniaturized Load Cells. These load cells will be produced as MEMS (Micro Electro Mechanical Systems) devices, which are becoming more and more popular due to improvements in technology. MEMS are micro-sized silicon structures etched in the form of beams, diaphragms or plates that can function as sensors within a load cell. MEMS are fabricated using the bulk and surface micromachining. MEMS can then be mass produced, because thousands of sensor elements can be fabricated on single wafer with integrated supporting circuits.
Although millions of sensors can be mass produced at a very low price (as low as a few dollars), their applications are still limited compared with foil strain gauges. The advantages of these load cells will be higher precision, high portability, low power requirements, and even lower costs. Clearly the future of load cell technology is bright; new technologies are creating creative solutions for today’s and tomorrows challenges.
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