A load cell is a transducer or sensor that is used to convert a mechanical force or load into an electrical signal. It is a critical component in various applications where the measurement of force, weight, or tension is required. Load cells are commonly used in industrial, scientific, and commercial settings to obtain accurate and reliable measurements of forces or loads.
Here are the key components and functions of a load cell:
Strain Gauges: Load cells typically contain one or more strain gauges, which are thin, wire-like devices that are bonded or attached to a structural element within the load cell. Strain gauges are designed to deform when subjected to mechanical force, and this deformation results in a change in electrical resistance.
Wheatstone Bridge Circuit: Strain gauges are configured in a Wheatstone bridge circuit, which is an electrical circuit that can detect small changes in resistance. When force is applied to the load cell, the strain gauges experience deformation, causing a change in resistance. This change in resistance leads to an imbalance in the Wheatstone bridge, generating an electrical output signal.
Output Signal: The electrical output signal produced by the load cell is typically very low in magnitude, often in the millivolt (mV) range. This signal is proportional to the applied force or load and can be further processed and amplified to obtain meaningful measurements.
Calibration: Load cells must be calibrated to ensure accurate and consistent measurements. Calibration involves applying known forces to the load cell and recording the corresponding output signals. This calibration data is used to establish a relationship between the applied force and the output signal.
Types of Load Cells: There are various types of load cells, each designed for specific applications and load measurement requirements. Common types include:
Strain Gauge Load Cells: These load cells use strain gauges to measure the deformation caused by the applied force. They are suitable for a wide range of applications and are available in various configurations, including bending beam, shear beam, and single-point designs.
Compression Load Cells: Designed to measure forces that compress the load cell along its central axis, compression load cells are often used in applications like platform scales and industrial weighing.
Tension Load Cells: These load cells are designed to measure forces that pull or stretch the load cell, making them suitable for applications like crane scales and tensile testing.
Shear Beam Load Cells: Shear beam load cells are commonly used in industrial scales and conveyor systems to measure forces applied parallel to the load cell's mounting surface.
Bending Beam Load Cells: Bending beam load cells are versatile and used in applications such as floor scales and batching systems.
S-Type Load Cells: Shaped like the letter "S," these load cells can be used for both tension and compression measurements.
Load cells find applications in various industries, including manufacturing, aerospace, automotive, healthcare, agriculture, and more. They are crucial for tasks such as weight measurement, force monitoring, quality control, and process automation, contributing to increased accuracy, efficiency, and safety in numerous processes and systems..
A load cell works on the principle of converting an applied mechanical force or load into an electrical signal. It accomplishes this by utilizing one or more strain gauges, which are devices that change their electrical resistance in response to mechanical deformation. Here's a step-by-step explanation of how a load cell works:
Strain Gauges: Load cells typically incorporate one or more strain gauges, which are small sensors made of thin wires or foil elements. These strain gauges are bonded or attached to a structural element within the load cell, such as a beam or diaphragm.
Wheatstone Bridge Circuit: The strain gauges are arranged in a Wheatstone bridge circuit, which is an electrical circuit designed to measure small changes in resistance. The Wheatstone bridge consists of four resistors, with the strain gauges forming two of them. The other two resistors are typically fixed or have a known resistance value.
Zero Load State: When there is no load applied to the load cell (the "zero load" or "no force" state), the strain gauges are in their initial, undeformed condition, and their resistance values are stable.
Application of Load: When a mechanical force or load is applied to the load cell, it causes deformation or strain in the structural element to which the strain gauges are attached. This deformation results in a change in the resistance of the strain gauges.
Imbalance in the Wheatstone Bridge: The change in resistance of the strain gauges creates an imbalance in the Wheatstone bridge circuit. As a result, there is an output voltage or electrical signal generated across the bridge's output terminals. The magnitude and polarity of this output signal are proportional to the applied force or load.
Signal Amplification and Conditioning: The electrical output signal produced by the Wheatstone bridge is typically very low in magnitude, often in the millivolt (mV) range. To obtain useful measurements, the signal is amplified and conditioned using signal conditioning circuits. This may involve amplifying the signal, filtering out noise, and compensating for temperature variations.
Calibration: Load cells are calibrated to establish a linear relationship between the applied force and the output signal. During calibration, known forces are applied to the load cell, and the corresponding output signals are recorded. This calibration data is used to create calibration curves or equations for accurate force measurement.
Output Signal: The amplified and conditioned electrical signal is then available for measurement, display, recording, or control purposes. Depending on the application, load cell signals can be analog (voltage or current) or digital.
Accuracy and Resolution: The accuracy and resolution of a load cell depend on factors such as the quality of the strain gauges, the design of the load cell, the calibration process, and the signal conditioning. High-quality load cells are capable of providing accurate and precise measurements.
Load cells come in various types, including compression load cells, tension load cells, shear beam load cells, and more, each designed for specific force measurement applications. They are widely used in industries such as manufacturing, automotive, aerospace, healthcare, and research to measure forces, weights, and loads accurately and reliably.
How to choose the correct Load Cell and compatible instrument for your load or force measurement application?
Our application sales engineers do this day in and out, with years of experience so please don’t hesitate to give us a call to help you though this process, that’s what we are here for. But it all comes down to your budget, your application, what stage your application is in, and how quickly you need your load cell system.
Let me start with a few easy applications and I will follow it by a layout of things you should consider in your load cell selection process.
If you need to perform a quick one time test to verify a load or force on a component or to prove an application, you might choose an inexpensive load beam load cell with a precision power supply and a voltmeter. We can help.
If you’re designing a Test machine you would choose a load cell based upon the overall system accuracy. Is the measurement static or dynamic? Do you require a digital display or just an amplifier signal conditioner module for a data acquisition system? We can help.
If your application is on a race car suspension system recording real time data while testing the car on an actual race track, like the Cornell racing team in one of our featured videos, you would need a load cell that could perform in a dynamic application and be able to handle extraneous loads like our MLP series load cell did. We double the normal load capacity of the load cell (which is a good rule of thumb in dynamic loading applications) and still exceeded the accuracy they required. We supplied our TM0-1 amplifier signal conditioner module that sent high speed data to their on board data acquisition system. We also designed a custom load pin load cell to measure the torque on the drive train. In some applications going with a custom load cell makes more sense.
National Geographic had an application on one of their super human strength episodes where they needed to measure the force of a strong man pulling a Semi tractor trailer (featured in our video gallery) that they were filming the next day. We choose the HSW-20k load cell with the smart plug and play option (also known as our Cal-Teds option) because of the accuracy and the ease of placing it in the application. We supplied our smart plug and play DPM-3 load cell meter with an analog output option that sent real time data to their data acquisition system that they were pushing in a cart alongside of the test. Having the Cal-Teds option on the load cell allowed the load cell and the smart plug and play DPM-3 meter to automatically perform a system calibration by just plugging them together (meter reads the EPPROM chip in the Cal-Teds option that populates an IEEE 1451.4 template 33 calibrating the system instantly). We generally don’t recommend pushing a cart alongside of a test, but this was already taking place for other equipment. This was a very inexpensive load cell system that allowed them to film the next day as planned.
Below is a list of things to consider when selecting a load cell for your load / force measurement requirement:
Step 1
Price and availability are normally the first two things to consider. If you’re on a limited budget or need something the next day this may narrow your selection. However Transducer Techniques does offer educational discounts and we stock our standard products ready for next day shipping.
Step 2
Define how you want to conduct the measurement and how you will apply the load. Will your loading be static or dynamic? A dynamic application usually requires a load cell with a higher frequency response (a good rule of thumb is the lower the load cell deflection the high the frequency response). Will you be loading the load cell in-line or require a load cell that can handle extraneous loads? Will you be measuring bending, tension or compression or both, multi-axis such as thrust and torque?
Step 3
Define how this load cell will be mounted in your application. Do you require male or female threads or a flange mount? Do you need a through hole or compression load washer load cell that allows a structure to pass through the load cell?
Step 4
Define the environment in which the load cell is intended to be used in (laboratory, warehouse, outdoors, underwater). For extended outdoor use or in a marine environment or for underwater use, we recommend a hermetically seal load cell.
Step 5
Define the overall accuracy, output, bridge resistance, nonlinearity, hysteresis, nonrepeatability, frequency response.
Step 6
Define if there are any special options required, such as, connectors, addition cable lengths, high temperature,or if Cal-Teds (plug and play smart load cell option) is required.
Step 7
Define if load cell instrumentation is required. From a precision power supply, to an amplifier signal conditioner module, to a digital display with alarms, analog output or data logging, we have economical solutions for your load cell instrument requirements.
In summary, for someone using a load cell for the first time this can be overwhelming. But this is what we enjoy doing and we are always here for you. Please allow us to help you select the right load cell from installation, to setup, we will be here to assist you every step of the way.
SWP Series Load Cell Applications.
The Transducer Techniques SWP Series low-profile shear web load cells are specialized force sensors designed for applications that demand ultra stiffness and resistance to extraneous bending and side load forces. These load cells, manufactured from heat-treated 17-4 PH stainless steel, offer excellent accuracy and are suitable for both tension and compression force measurement applications. Here are some specific applications where the SWP Series load cells are commonly used:
1. Aerospace Testing: The aerospace industry relies on SWP Series load cells for structural testing of aircraft components. These load cells offer stiffness and precision, making them ideal for ensuring the safety and reliability of aerospace structures.
2. Automotive Testing: Automotive manufacturers and testing facilities use SWP Series load cells for quality control and testing of vehicle components, including suspension systems and chassis. These load cells contribute to evaluating the performance and safety of automotive parts.
3. Materials Testing: SWP Series load cells are employed in materials testing laboratories for conducting a wide range of tests, including tensile, compressive, and fatigue tests on various materials. Their stiffness and resistance to extraneous forces make them valuable tools for accurate material property determination.
4. Industrial Automation: In industrial automation systems, SWP Series load cells are integrated into machinery and equipment to monitor and control forces in manufacturing and assembly processes. They are particularly valuable in applications where stiffness and precision are critical.
5. Quality Control: Industries with stringent quality control requirements, such as aerospace and automotive manufacturing, use SWP Series load cells to verify that products meet specific force and load specifications, ensuring product quality and safety.
6. Research and Development: Engineers and researchers use SWP Series load cells during product development and prototyping to evaluate the performance, durability, and structural integrity of new designs and components.
7. Custom Machinery and Equipment: Manufacturers and research facilities integrate SWP Series load cells into custom-built machinery and equipment designed for specialized testing and manufacturing processes that require ultra-stiff and precise force measurement.
8. Tension and Compression Force Measurement: SWP Series load cells are versatile and can be used for both tension and compression force measurement applications. They are often used with load buttons or load-bearing compression-only applications.
9. In-Line Tension Applications: For in-line tension applications, the SWP Series load cell can be combined with a tension base (-TB option) to provide hardness, flatness, stiffness, and alignment for optimal performance.
The Transducer Techniques SWP Series shear web load cells are known for their stiffness, precision, and resistance to extraneous forces, making them valuable tools for force measurement applications where accuracy and reliability are paramount. Their versatility and durability contribute to improved product quality, safety, and process efficiency.
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