Load Cell Fundamentals

Load cells measure force, and the type of force (compression or tension) drives design, installation and application choices.

• Compression load cells measure force when the load pushes toward them causing a compressive force. This is typical in truck and industrial platform or tank scales where the load cell is mounted under a structure or platform.
  • Advantages:
    • Simple installation for static loads
    • High-capacity options are available
  • Error risks:
    • Uneven load distribution
• Tension load cells measure force when the load pulls away from the load cell causing a tensile force. Crane scales monitoring and cable tension measurement are common applications for these load cells where the load cell is in line with the load path.
  • Advantages:
    • Suited for suspended loads
    • Can measure dynamic forces in lifting systems
  • Error risks:
    • Side loading or misalignment
• Combination load cells can measure both tension and compression. These sensors are useful in testing machines or materials.
• Load cells in torque measurement are more complicated because instead of measuring a single axial force, torque sensors (rotary load cells) measure shear strain due to twisting. When bonded to a shaft at 45° angles, one gauge experiences tension while the opposite diagonal undergoes compression. In this case, the strain bridge uses opposing strains to produce a differential signal proportional to the torque.

Static measurements apply to loads that remain constant or change very slowly over time. A truck scale and a load cell that measures a stationary tank are examples. Conversely, dynamic measurements involve forces that change rapidly due to motion, vibration or impact. Applications that often measure these dynamic loads are crash testing and robotic monitoring. Distinguishing between static and dynamic conditions influences load cell selection, signal conditioning and accuracy requirements. Static applications prioritize long-term stability and minimal creep, while dynamic applications demand fast response, high natural frequency and effective noise filtering to accurately capture transient forces.

Load cells are designed to measure force, but depending on the complexity of the application, single-axis or multi-axis sensors may be required. Understanding the differences between these and knowing when to use one or the other has a big impact on implementation success.

Accuracy and precision are both essential metrics for load cell performance, but they represent different dimensions of measurement quality.

• Accuracy describes the amount of error between the measured output and the actual value of the applied load.
• Precision refers to the consistency of a number of the load cell’s measurements under the same conditions without regard to accuracy.

Example:

• If a 100 lb force is applied and the load cell reads 99.9 lbs, this would be considered high accuracy.
• Repeated measurements range from 100.1 lbs to 99.6 lbs, this would be consdiered high accuracy and precision.
• Repeated measurements range from 101.5 lbs to 98.0 lbs, this would be considered neither accurate nor precise.

It is important to consider the full operating temperature range of the load cell application. Outdoor or industrial environments often require extended ranges. Temperature variations can affect accuracy, so compensation techniques may be necessary.

Addressing corrosion resistance is important for durability and reliability of load cells. This is especially important in outdoor environments or in areas where harsh conditions occur. Hermetic sealing, IP rated enclosures, and specialty materials can often provide excellent protection. Material selection is also important to mitigate potential corrosion.

Load Cell Body: The load cell body is typically the most vulnerable element since it is often in direct contact with moisture, chemicals, salt spray, or acidic environments.

• High alloy stainless steel provides high corrosion resistance for the most demanding applications.
• Hard anodized aluminum is often used for lightweight systems that require moderate corrosion resistance.
• Protective coatings or plating can also enhance the durability of load cell body surfaces.

Strain Gauges: Exposure to moisture ingress is a serious risk factor for strain gauges. Corrosion can have a huge impact on the operation of these components.

• Hermetic sealing of the strain gauge cavity can provide a level of protection for strain gauges.
• Epoxy encapsulation can also provide ingress protection.
• Glass-to-metal seals are quite effective for this purpose as well.

Electrical Connectors: Corrosion at connection points due to humidity or chemical exposure can seriously impact sensor relability and accuracy. These risks can be mitigated using with several approaches.

• Shielded cables with waterproof jackets prevents moisture from invading the connection point.
• IP-rated connecters can be specified for sealed interfaces to minimize the corrosion risk.

 
Bonding Adhesives: Adhesives used to bond strain gauges to the body can degrade over time presenting a significant risk of corrosion. High-performance bonding adhesives rated for chemical and thermal resistance can be quite effective if their resistance targets harsh environments particular to the application.

Electromagnetic interference can cause signal drift, noise, or failure in sensitive load cell circuits. Protection from electromagnetic interference (EMI), radio frequency interference (RFI) and electrostatic discharge (ESD) are important to consider when selecting load cells. The following measures can improve signal security, extend the load cell life span and confirm compliance with EMC standards like IEC 61000, EN55032 and FCC Part 15.

• Shielded cables: Twisted-pair wires with braided or foil shielding, grounded at one end can be effective in preventing interference in signal lines.
• Faraday cage: Metal housings with conductive seals act as a Faraday cage to block external EMI/RFI.
• Filters: Ferrite beads, capacitors, and low-pass filters on signal and power lines suppress high-frequency noise and voltage spikes.
• Grounding: Proper grounding of the load cell body, cable shield and signal processing equipment provides a low-impedance path for stray currents and static discharge. 
• ESD protection circuits: TVS (Transient Voltage Suppression) diodes or ESD suppressors on IO lines can significantly reduce the risk of damage from electrostatic discharge.
• Electrical isolation: Optical or transformer-based isolation in signal conditioning modules prevents conducted interference between the load cell and data acquisition systems.
• PCB best practices: Using controlled impedence traces, ground planes and separation of analog/digital paths helps to minimize internal EMI and crosstalk.

For mobile, automotive, aerospace and industrial machinery applications, the ability to withstand mechanical shocks and vibrations without degrading load cell performance is imperative. In these applications, ruggedized designs or vibration damping features should be implemented where feasible. For example, aerospace load cells often require vibration damping mounts to maintain acceptable calibration.