Load cells - a comprehensive guide

Load cells are essential components in the world of measurement technology and form the basis for modern weighing systems. They convert a mechanical load - whether through compression, tension or bending - into an electrical signal that can then be precisely measured and analyzed. This capability makes load cells indispensable for a wide range of applications, from simple kitchen scales to complex industrial measuring systems. They play a crucial role in quality assurance, production control, construction, agriculture, medical technology and retail, to name but a few.

The technology behind load cells is based on the principle of strain gauges, which change their electrical properties under the influence of mechanical loads. These changes are translated into electronic signals which are then used for weight calculation. There are different types of load cells designed for specific applications, including bending, shear, compression and tension load cells. Each type has its own advantages and is optimized for specific environmental conditions and measurement ranges.

Thanks to their versatility and precision, load cells are indispensable for the automation of processes that require accuracy. They enable the precise dosing of ingredients in food production, ensure safety and efficiency in transportation by weighing freight and vehicles, and contribute to the accuracy of diagnostic instruments in medical technology. In research and development, load cells enable experiments and tests with precise weights, leading to innovative solutions and products.

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Load cell basics

Definition and function of a load cell

A load cell is an electromechanical sensor that is used to convert the force of a weight into an electrical signal. This conversion takes place through the physical deformation of the load cell under load, which is then measured using strain gages. The strain gauge changes its electrical resistance in proportion to the strain caused by the applied weight. This change in resistance is converted into an electrical signal, which is then amplified and converted into a digital signal for display or further processing. Thanks to this principle, load cells enable precise and reliable measurement of weights in various applications.

History of load cells: Development and progress

The history of load cells begins in the early 20th century, when the first mechanical scales were supplemented by the development of electrical measuring methods. The invention of strain gauges in the 1930s marked a turning point, as these formed the basis for modern load cells. In the following decades, load cells were continuously improved, increasing accuracy, reliability and versatility. The introduction of digital electronics in the 1970s revolutionized the design and functionality of load cells by enabling higher precision and better integration into industrial systems. Today, load cells are an indispensable tool in many sectors, from industry to medicine and commerce, and they are increasingly benefiting from advances in materials science and microelectronics.

Different types of load cells and their application areas

Load cells can be divided into different types, each of which is optimized for specific applications. Bending beam load cells are often used in floor scales and tank scales as they work well with medium to high loads. Compression load cells are suitable for use in truck scales and industrial weighing systems as they can measure high loads. Tension load cells are used in crane scales and hanging scales where weights are measured by tension. Shear beam load cells offer high precision and are ideal for silo and hopper scales. Each load cell has specific properties that predestine it for certain measuring ranges and ambient conditions.

How load cells work?

Load cells work on the principle of converting mechanical loads into electrical signals in order to measure weights precisely. This transformation takes place through the deformation of the load cell under the influence of a weight. At the core of each load cell are strain gages, which consist of thin, conductive foils. When a weight is applied to the load cell, the material of the cell is deformed and the strain gages are stretched or compressed. This physical change affects the electrical resistance of the strain gages in a way that is proportional to the applied load.

Modern load cells use the Wheatstone bridge, an electrical circuit, to measure the slight resistance changes in the strain gages. The bridge consists of four resistors (including the strain gages) arranged in such a way that a change in one resistor changes the output voltage of the circuit. This voltage change is then amplified and converted into a digital signal representing the measured weight.

Load cells can measure different types of loads, including compression, tension and shear, depending on their design and the strain gages used. The variety of load cell types and their configurations allows for a wide range of applications, from precise laboratory scales to industrial weighing systems for heavy loads. Their ability to provide accurate measurements under a wide range of conditions makes load cells an indispensable component in metrology.

Technological components of a load cell

A load cell is a complex instrument that consists of several technological components to enable accurate measurements. At the core of every load cell are the strain gages, which consist of a conductive film and are applied to the load cell's carrier material. These strain gages are the primary sensors that convert mechanical deformations into electrical signals. They change their electrical resistance in direct correlation to the strain of the material to which they are applied.

The load cell housing protects the internal components from environmental influences and carries the load. It is designed to be specifically deformed under load, allowing the strain gages to effectively measure the force.

The circuit of a Wheatstone bridge is another critical component. It integrates the strain gauges and optimizes the measurement of resistance changes by converting the changes into a measurable electrical voltage.

Amplifiers are also essential to amplify the signals generated by the Wheatstone bridge so that they can be read and analyzed effectively.

Modern load cells also often contain digital interfaces and microprocessors for signal processing to enable direct communication with external devices and offer greater precision and flexibility in data evaluation. These components work together to deliver the precise and reliable measurements that industry, commerce and research rely on.

Differences between analog and digital load cells

Analog and digital load cells are both important components in the world of measuring and weighing technology, but they differ significantly in their functionality and data evaluation.

Analog load cells convert the mechanical deformation caused by a weight directly into an electrical signal. The strength of this signal depends proportionally on the load. Signal processing takes place outside the load cell, often in a separate transmitter. This makes analog systems susceptible to interference such as electromagnetic influences or temperature fluctuations, which can affect the signal during transmission.

Digital load cells, on the other hand, integrate signal processing directly into the load cell. They contain a microprocessor that not only converts the raw signal generated by the strain gauges, but also compensates and calibrates it. This leads to greater accuracy and reliability of the measurement results, as digital load cells are better protected against external interference. In addition, digital load cells enable easier integration into networks and systems thanks to standardized interfaces, which simplifies remote monitoring and diagnostics.

The main difference therefore lies in the processing and transmission of the measurement data: Analog load cells offer a simple, direct signal, while digital load cells offer a processed, more stable and often more precise data transmission.

How do you evaluate a load cell?

The evaluation of a load cell, i.e. the conversion of the signals it generates into precise weight data, requires an understanding of how it works and the electronics involved. First, the electrical signal generated by the load cell, which is proportional to the load, is amplified by an amplifier to make it measurable. With analog load cells, this is done externally, while digital load cells do this internally.

The amplified signal is then forwarded to an evaluation unit or an analog-to-digital converter (ADC), which converts the analog signal into a digital one. This digital information can now be processed by microprocessors, which are able not only to calculate the weight, but also to perform calibration, compensation and diagnostic functions. Modern load cells can use internal algorithms to correct for influences such as temperature fluctuations, air pressure and humidity, thereby increasing the accuracy of the measurement.

The evaluation also often includes an interface to a user display or a network to make the data accessible. Advanced systems enable remote monitoring, data logging and integration into larger automation systems. The evaluation of a load cell is therefore a combination of hardware and software that aims to provide accurate and reliable weight information.

Types of load cells

Single point load cells

Single point load cells are designed to distribute weight evenly over a large area and are ideal for small to medium sized platform scales such as kitchen, mail or medical scales. Their compact design allows easy integration into scales, with only one load cell required for precise measurements.

Bending bar load cells

Bending bar load cells measure weight by the deformation of a metal bar under load. This deformation is detected by strain gauges attached to the surface of the rod. They are particularly suitable for tank and silo scales as well as for industrial applications where reliability and precision are required under harsh conditions.

Compression load cells

Compression load cells are designed to determine weights by measuring compressive forces. Typically used in floor scales, truck scales and industrial weighing systems, they are characterized by their ability to accurately measure high loads. Their robust construction allows them to be used in demanding environments where accuracy and reliability are required.

Tension load cells

Tension load cells are specially designed to determine weights by measuring tensile forces. They are primarily used in crane scales, hanging scales and force measurement systems. These load cells enable precise measurements in applications where loads are suspended or pulled and are known for their reliability and accuracy in a wide range of applications.

Shear beam load cells

Shear beam load cells measure the load by detecting the shear force generated when a material or component is loaded perpendicular to the main axis. They are particularly suitable for industrial weighing systems, silo and hopper scales, as they offer high accuracy and reliability even under high loads and in demanding environments.

Special load cells

Special load cells are designed for unique and specific measurement requirements that go beyond standard applications. These include load cells for extreme load ranges, miniaturized versions for laboratory applications or models specially designed for extreme environmental conditions such as high temperatures or corrosive environments. Their development enables precise measurements in specialized or demanding areas of application.

Selection criteria

How to choose the right load cell?

Selecting the right load cell requires consideration of several factors to ensure that the load cell meets the specific requirements. First, consider the maximum load to be measured to select a load cell with the appropriate capacity. The environmental conditions, such as humidity, temperature and the presence of chemicals or dust, influence the material selection and protection class of the load cell. The accuracy requirements of the application determine the selection of the load cell type and its precision class. In addition, mounting options and physical size should be considered to ensure compatible and efficient integration into the overall system.

Capacity and accuracy requirements

The capacity of a load cell refers to the maximum load it can accurately measure and must cover the highest load to be weighed plus a safety margin. Accuracy requirements are critical as they determine the quality of the measurement results. They are often expressed as a percentage of full scale or in parts per million (ppm). When selecting a load cell, it is important to choose one with the required capacity and accuracy to meet the specific needs of the application to avoid overloading and ensure reliable measurement data. However, higher accuracy can mean higher costs, which requires balancing costs and benefits.

Environmental conditions and materials

Environmental conditions play a decisive role in the selection of a load cell. Factors such as temperature, humidity, dust and aggressive chemicals can affect the function and longevity of a load cell. It is important to choose a load cell with a suitable degree of protection (IP rating) that protects it against the specific environmental conditions. Materials such as stainless steel or special alloys offer corrosion protection and are suitable for use in harsh or hygienic environments. Taking these aspects into account ensures that the load cell provides reliable measurements under the given conditions and has a long service life.

Installation und Montage

The correct installation and mounting of a load cell are crucial for accurate measurement results. First of all, the installation location should be stable and free from vibrations to minimize measurement errors. The mounting surface must be level and firm to ensure even load distribution. When mounting, make sure that all load cells of a scale are aligned identically to avoid systematic errors. Special mounting kits can make installation easier and protect the load cell from unwanted forces such as side loads or torque, which can affect measurement accuracy. Careful planning and execution of the installation is essential for the long-term stability and precision of the weighing systems.

Maintenance and calibration

Regular maintenance and calibration are essential to ensure the accuracy and reliability of load cells throughout their lifetime. Maintenance includes cleaning the load cell and its surroundings to avoid accumulation of dust or debris that could affect measurement results. It is also important to carry out regular inspections to identify mechanical damage or corrosion. Calibration should be performed at set intervals or after each repair to ensure that the load cell is providing accurate readings. Professional calibration services use known weights to check and adjust the accuracy of the load cell, which is essential for precise measurements.

FAQ - Things to know about load cells

How do dynamic load cells differ from static load cells?

Dynamic load cells are designed to measure weights in motion, such as on conveyor belt scales or truck scales in drive-through operation. Static load cells, on the other hand, measure stationary loads, such as on a platform scale. The main distinction lies in the ability to perform precise measurements in motion or under static conditions.

What factors influence the accuracy of a load cell?

The accuracy of a load cell is influenced by several factors: environmental conditions such as temperature and humidity, mechanical influences such as vibrations, correct installation and alignment, electrical interference, as well as the quality of the load cell itself, including its sensitivity and calibration. Each of these factors can significantly influence the measurement result.

Can load cells be used in potentially explosive atmospheres?

Yes, load cells can be used in hazardous areas, provided they are specifically designed and certified for these environments. These load cells meet strict safety standards and are equipped with protection mechanisms to prevent sparking or overheating, making them suitable for use in areas with a high risk of explosion.

How does the alignment of the load cell influence the measuring accuracy?

The alignment of the load cell is crucial for measurement accuracy, as misalignment can lead to uneven load distribution and therefore incorrect measurements. Correct alignment ensures that the forces acting on the load cell are transmitted directly and without lateral displacement or twisting, which enables precise and repeatable measurement results.

What steps need to be taken when installing a load cell for the first time?

The following steps should be observed when installing a load cell for the first time: Selecting a suitable location, ensuring a stable and level mounting surface, correctly aligning the load cell to take up the load, wiring it correctly and connecting it to the evaluation electronics, carrying out an initial calibration and function test to ensure optimum performance and accuracy.

How can a load cell be protected against overload?

To protect a load cell from overload, mechanical limiters or overload protection devices can be used to limit the maximum load. Selecting a load cell with a higher capacity than required and implementing warning systems for load limits in the software are also effective measures to prevent damage caused by overloading.

What role does temperature compensation play in load cells?

Temperature compensation plays a crucial role in load cells, as temperature fluctuations can affect the electrical resistance of the strain gauges and therefore the measurement accuracy. Built-in temperature compensation corrects these effects by automatically compensating for temperature-related changes in the signal, resulting in stable and accurate measurements over a wide temperature range.

Are there load cells that are suitable for use under water?

Yes, there are specially designed load cells for use underwater or in wet environments. These load cells are waterproof and corrosion-resistant, often with a high degree of protection (e.g. IP68) so that they can be permanently immersed in water. They are suitable for marine applications, aquaculture and other wet environments.

How often does a load cell need to be calibrated?

The frequency of calibration of a load cell depends on the application, the required accuracy and the operating conditions. Generally, annual calibration is recommended, but shorter intervals may be necessary in high-precision or critical applications. A regular check of the measurement accuracy can help to determine the need for calibration.

What are the symptoms of a defective load cell?

Symptoms of a faulty load cell may include inaccurate or unstable readings, drift in the zero point display or the complete absence of measurement signals. Visible damage to the housing or connections can also indicate problems. Such signs indicate that the load cell needs to be checked, repaired or replaced.

What to do if a load cell provides inaccurate measurements?

  1. Checking the installation: Make sure that the load cell is installed correctly, without mechanical stresses or deformations that could affect the measurements.

  2. Check calibration: Check that the load cell is correctly calibrated. Recalibration may be necessary, especially if the load cell has been exposed to large temperature fluctuations or mechanical loads since the last calibration.

  3. Evaluate environmental conditions: Check whether external factors such as vibrations, temperature fluctuations or electromagnetic interference affect the measurements.

  4. Check electrical connections: Check all electrical connections for corrosion, looseness or damage. Defective cables or connections can lead to signal interference.

  5. Carry out maintenance: Perform thorough maintenance, including cleaning the load cell and checking for visible damage such as cracks or deformation.

  6. Check software settings: Check the settings of the connected evaluation devices or software to ensure that there are no incorrect configurations.

  7. Seek expert help: If the above steps do not resolve the problem, a specialist or the manufacturer should be contacted to check the load cell and repair or replace it if necessary.

Is it possible to combine several load cells in one system?

Yes, several load cells can be combined in one system to distribute the load evenly and increase the overall capacity. This is common with large scales, such as truck scales or silo scales. The signals from the individual load cells are combined to determine an accurate total weight. Correct calibration and alignment are crucial here.

How does electromagnetic interference affect load cells?

Electromagnetic interference (EMI) can affect the measurement accuracy of load cells by interfering with the electrical signals generated by the strain gages. This can lead to inaccurate or fluctuating readings. To avoid this, load cells should be used in environments with low EMI or protected by suitable shielding and filters.

Are there differences in the maintenance of analog and digital load cells?

Yes, there are differences in the care of analog and digital load cells. Analog load cells require regular calibration and inspection of mechanical connections to ensure accuracy. Digital load cells, on the other hand, require less physical maintenance as they often have self-calibrating features and enhanced diagnostic capabilities, which simplifies troubleshooting and maintenance. Both types benefit from clean, dry and low-vibration operating conditions.

What are IP protection classes and how do you choose the right one for a load cell?

IP protection classes define how well electrical devices are protected against foreign bodies and moisture. The choice for a load cell depends on the environment. Dusty or humid areas require higher IP ratings such as IP67 or IP68, which offer complete protection against dust and immersion in water.

Can load cells be adapted to customer-specific electronics?

Yes, load cells can be adapted to customized electronics to meet specific requirements. By adapting the interfaces, output signals and power supply, load cells can be integrated into various electronic systems to enable precise measurements in customized applications such as industrial processes or embedded systems.

What data transmission options do digital load cells offer?

Digital load cells offer various data transmission options, including wired interfaces such as USB, RS232, Ethernet for direct connections and wireless technologies such as Bluetooth and WLAN for flexible, cable-free solutions. This variety enables easy integration into networks, remote monitoring systems and IoT applications, increasing efficiency and connectivity in various applications.

How is the long-term stability of a load cell ensured?

The long-term stability of a load cell is ensured by high-quality materials, precise manufacturing processes and a robust design. Preventive measures such as temperature compensation, moisture protection and mechanical protection against overload and shock contribute to this. Regular calibration and maintenance are also crucial to ensure measurement accuracy and reliability over long periods of time.

Can load cells be retrofitted into existing systems?

Yes, load cells can be retrofitted into existing systems to extend their functionality or improve accuracy. Seamless integration can be achieved by selecting compatible models and adapting the interfaces and mounting points. Professional planning and installation are essential to ensure the performance and reliability of the overall system.

Which legal regulations and standards must be observed when using load cells?

When using load cells, legal regulations and standards such as the OIML guidelines, the EN 45501 standard for measuring instruments and the respective national verification laws must be observed. These specifications define requirements for accuracy, reliability and safety in order to ensure fair trade, consumer protection and compliance with technical specifications.