5 Common Types of Load Cells
Recently updated on August 21st, 2024 at 07:55 am
Load cells come in various types, each designed to suit specific applications and measurement requirements. Understanding the different types of load cells can help in selecting the right one for your needs. Here, we’ll explore the most common types of load cells, their working principles, applications, and advantages.
Several types of load cells exist for varying applications:
- Strain Gauge Load Cells
- Hydraulic Load Cells
- Pneumatic Load Cells
- Capacitive Load Cells
- Piezoelectric Load Cells
1. Strain Gauge Load Cells
Strain gauge load cells are the most widely used type of load cells in industrial settings due to their high level of accuracy and durability in harsh conditions. Their simple yet effective design facilitates integration into a broad range of equipment, from high-precision laboratory scales to heavy-duty industrial weighing systems.
1.1 How Strain Gauge Load Cells Work
Strain gauge load cells operate on the principle of piezoresistance. Inside each load cell, at least one strain gauge device is used. A strain gauge consists of a thin wire etched in a back-and-forth pattern onto a non-conductive substrate material, with connectors at each end of the wire. When force is applied to the load cell, it causes the strain gauge to deform. The deformation alters the length and cross-sectional area of the wire, which changes its electrical resistance. In tension, the wire’s resistance increases, while in compression, the resistance decreases.
The load cell converts this change in resistance to a measurable electrical signal, proportional to the applied force, using a Wheatstone bridge circuit. Systems using strain gauge load cells direct the measured force to cause a deflection (either tension or compression) in the strain gauges. These devices come in various shapes and mounting configurations, allowing for numerous possible load directions and diverse measuring applications, from high-precision laboratory scales to heavy-duty industrial weighing systems.
1.2 Strain Gauge Load Cell Components
Strain gauge load cells consist of a loading platform that applies force to one or more strain gauges, which are typically bonded to a body made of high-grade alloy steel, stainless steel, or aluminum. These strain gauges detect deformation caused by the applied force, changing their electrical resistance in response. An excitation voltage source provides a stable voltage to the strain gauges, arranged in a Wheatstone bridge circuit. Output wires then measure the voltage change, which is proportional to the applied force, allowing for accurate force measurement.
1.3 Advantages and Applications of Strain Gauge Load Cells
Strain gauge load cells offer high accuracy and sensitivity, making them ideal for industrial weighing systems, automotive testing, aerospace applications, and medical devices. Their versatile design supports a wide measurement range and suitability for various applications, from high-precision laboratory scales to heavy-duty industrial weighing systems.
2. Hydraulic Load Cells
Hydraulic load cells are devices used to measure force or weight through the use of a liquid-filled chamber and pressure measurement. They are robust and can operate in harsh environments.
2.1 How Hydraulic Load Cells Work
Hydraulic load cells work by converting a load into hydraulic pressure. The process begins with the load being applied to a platform connected to a piston. This piston is situated in a closed chamber filled with fluid, typically oil or water. When the load is applied, the piston presses against an elastic diaphragm, causing the fluid within the chamber to be pressurized. The increase in fluid pressure is directly proportional to the force exerted by the load. This change in pressure is then measured by a pressure gauge, such as a Bourdon tube pressure gauge, which provides a readable output corresponding to the applied load.
2.2 Hydraulic Load Cell Components
Hydraulic load cells have the following components:
- An elastic diaphragm
- A piston connected to a load platform
- Hydraulic fluid, typically oil or sometimes water
- A pressure gauge or gauges
- A tube connecting the chamber to the pressure gauge
- Steel housing for the assembly
2.3 Advantages and Applications of Hydraulic Load Cells
Hydraulic load cells are durable and suitable for hazardous environments due to the absence of electrical components. They offer high accuracy (0.25-1.0% of full-scale) and resolution (0.02%). However, they are expensive, require strict sealing to prevent leaks, and need zero-resetting due to ambient pressure sensitivity. Their complexity and size may limit use in compact spaces, but they excel in steady-state force measurement for industrial applications.
3. Pneumatic Load Cells
The function of pneumatic load cells is similar to that of hydraulic load cells. They convert fluid pressure into load measurements. However, the pressurized fluid in pneumatic load cells is a gas, usually air.
3.1 How Pneumatic Load Cells Work
Pneumatic load cells measure force by balancing it with pressurized gas. When a force is applied to a diaphragm, a pressure regulator introduces gas into a chamber on the opposite side. Some gas escapes through a nozzle connected to a pressure gauge, and the pressure of this escaping gas is measured. This pressure is directly proportional to the applied force, providing an accurate load reading.
3.2 Pneumatic Load Cell Components
Pneumatic load cells have the following components:
- Loading platform: Applies the force to be measured.
- Steel chamber: Filled with pressurized gas or air.
- Elastic diaphragm: Connected to the loading platform, seals the chamber.
- Air supply regulator: Controls the introduction of pressurized gas into the chamber.
- Nozzle (bleed valve): Allows some of the pressurized gas to escape the chamber.
- Pressure gauge: Measures the pressure of the escaping gas, which is proportional to the applied force.
3.3 Advantages and Applications of Pneumatic Load Cells
Pneumatic load cells are explosion-resistant and ideal for environments with intrinsic safety concerns. They tolerate temperature changes well and are highly sensitive to small loads, making them suitable for precise, real-time measurements in applications like dispensing IV fluids. These load cells are commonly used in industries prioritizing cleanliness and safety, such as food processing and pharmaceuticals. Their advantages include temperature stability and the absence of contaminating fluids.
However, pneumatic load cells respond relatively slowly and require a clean, dry, regulated air supply. They are less precise than strain gauge load cells and need regular maintenance to ensure accurate measurements. Additionally, their bulkiness can limit their use in compact spaces.
4. Capacitive Load Cells
Capacitive load cells are devices that measure force or weight by detecting changes in capacitance, which is the ability of a system to store an electric charge. These load cells use the principle of capacitance change between two parallel plates.
4.1 How Capacitive Load Cells Work
Capacitive load cells measure force by detecting changes in capacitance. When a force is applied to the load cell’s sensing element, it causes a physical deformation, altering the gap between two parallel plates or changing the dielectric material’s properties between them. One plate stores a positive charge and the other a negative charge. This change in capacitance, proportional to the applied force, is measured by an electronic circuit that converts it into a readable electrical signal.
4.2 Capacitive Load Cell Components
Capacitive load cells have the following components:
- Loading platform: External to the housing, applies the force to be measured.
- Insulated housing: Contains the internal components, including a free-moving plate and a fixed plate.
- Parallel plates: One fixed and one free-moving, with a gap between them.
- Dielectric material: Positioned between the plates, can be air or another insulating material.
- Electrical wires: Connected to the plates to measure the change in capacitance.
- Rigid rod or connector: Links the loading platform to the free-moving plate inside the housing.
4.3 Advantages and Applications of Capacitive Load Cells
Capacitive load cells are known for their high sensitivity and accuracy over a wide range of forces, both large and small. Their simple design makes them more cost-effective compared to other load cell types. Additionally, they can be hermetically sealed without compromising their operation, making them suitable for food and medical weighing applications where hygiene is crucial. These advantages make capacitive load cells an excellent choice for environments requiring precise and reliable measurements.
However, capacitive load cells have some drawbacks. They operate using an electric charge, which makes them unsuitable for flammable environments due to the risk of sparks. Additionally, certain dielectric materials used in these load cells are sensitive to temperature changes, which can affect their accuracy. This sensitivity to environmental conditions can limit their use in harsh or variable environments, necessitating regular calibration to maintain precision.
5. Piezoelectric Load Cells
Piezoelectric load cells are devices used to measure force or weight by utilizing the piezoelectric effect. This effect occurs when certain materials, such as quartz or ceramic crystals, generate an electric charge in response to mechanical stress.
5.1 How Piezoelectric Load Cells Work
Piezoelectric load cells convert mechanical force into an electric signal through the piezoelectric effect. When force is applied, it exerts pressure on the piezoelectric material, causing it to deform and generate an electric charge proportional to the force’s magnitude. This charge is captured by electrodes and converted into a voltage signal using a charge amplifier. The resulting signal is processed to provide a readable output corresponding to the applied force, making these load cells suitable for precise and dynamic force measurements.
Both compression and tension forces can create the piezoelectric effect in piezoelectric materials. Compression forces generate an electric charge with an opposite polarity compared to tension forces. The output voltage produced by the piezoelectric material is directly proportional to the magnitude of the applied force, whether it is compressive or tensile.
5.2 Piezoelectric Load Cell Components
Piezoelectric load cells have the following components:
- Loading platform: Applies the force to be measured.
- Piezoelectric material: Generates an electric charge when deformed by the applied force.
- Metallic electrodes: Bonded to the piezoelectric material, capture the electric charge produced.
- Output wires: Measure the change in voltage caused by the change in charge.
- Charge amplifier: (Optional) Converts the small electric charge into a usable voltage signal.
- Housing: Protects the internal components and ensures stable operation.
5.3 Advantages and Applications of Piezoelectric Load Cells
Piezoelectric transducers offer high sensitivity and accuracy, making them ideal for dynamic force measurements and applications requiring real-time data. Their compact design, high frequency response, and durability ensure long operational life and suitability in various environments, including industrial automation and medical devices.
However, they are sensitive to temperature changes, require charge amplifiers due to their small output signal, and are not suitable for static force measurements. Additionally, their integration can be complex and costly.
How to Choose a Load Cell for Your Application
Choosing the right load cell depends on the required sensitivity and accuracy of your application.
Capacitive load cells offer the highest sensitivity and accuracy, making them ideal for precise measurements.
Strain gauge load cells are next in line, providing reliable accuracy and sensitivity suitable for most industrial applications.
Pneumatic and hydraulic load cells, while still useful in specific scenarios, tend to be less sensitive and accurate compared to capacitive and strain gauge types, making them better suited for applications where extreme precision is not as critical.
