A load cell is simply a transducer. That is, it is a device that converts a physical force into a measurable electrical signal. From the high-precision requirements of aerospace testing to the durability demands of agricultural weighing, load cells are the work-horse devices that make industrial control systems and data collection possible. This guide provides an overview of the five primary load cell technologies, how they function, and how to identify the right category for your specific application.
Key Takeaways
- The Basic Concept: Load cells act as the interface between the physical world (force) and the digital world (data).
- The Strain Gauge Industry Standard: Most industrial systems use strain gauge sensors since they offer the most balanced combination of precision, durability, and cost-effectiveness across nearly any application.
- Alternatives When Safety is a Concern: Use Hydraulic or Pneumatic cells in explosive or hazardous environments where electrical power is a risk.
- Precision in Specialized Applications: Choose Capacitive sensors for high-hygiene medical environments, or Piezoelectric transducers for capturing high-speed dynamic forces.
What is a Load Cell?
A load cell is a transducer or sensor that converts the kinetic energy of a force into a quantifiable output, such as an electrical signal. Without this energy conversion, a force would not be measurable. Since load and force measurements are fundamental to many industrial processes, load cells form an integral part of modern commerce.
How Do Load Cells Work?
Each load cell type exploits some physical property (electrical resistance, fluid pressure, capacitance) of its component materials to generate an output proportional to the applied force. Mechanical or electrical devices then convert the load cell’s output to a human-usable form, such as a human-readable display, stored data, or an input signal to control systems.
How Are Load Cells Used?
Load cells are most commonly used in industrial weighing applications such as scales. They are also the critical component in many automated feedback loops. In these systems, the load cell’s real-time data allows a machine to “sense” and adjust its own behavior. For example, in a cable-winding machine, a load cell constantly monitors tension. If the tension fluctuates, the sensor sends an immediate signal to the motor controller to adjust the speed. This continuous loop ensures the process remains consistent, preventing cable breakage or uneven spooling without human intervention.
Types of Load Cells
Several types of load cells exist for a wide range applications:
- Strain Gauge Load Cells
- Hydraulic Load Cells
- Pneumatic Load Cells
- Capacitive Load Cells
- Piezoelectric Transducers
We’ll cover the science behind each, their operational tradeoffs, and their ideal uses.
1. Strain Gauge Load Cells: The Standard for Precision and Versatility
Strain Gauge Engineering Principles and Components
The strain gauge load cell is a passive transducer that converts mechanical deformation into a change in electrical resistance. Its core is the “spring element” (or “elastic element”), a precision-machined metal body (usually aluminum, alloy steel, or stainless steel) designed to flex predictably under a specific load. Bonded to this element are one or more strain gauges: ultra-thin wires in back-and-forth patterns on a non-conductive substrate that stretch or compress with the metal body (Figure 1).
When the bonded gauges compress or stretch with the deformation of the loaded spring element, they change in resistance. To measure this incremental change, the gauges, wired into a Wheatstone Bridge circuit, receive an excitation voltage. The bridge imbalance caused by the resistance changes due to the load produces a differential voltage output proportional to this load. Because this setup requires an external excitation voltage to function, it is a “passive transducer”.
Since these devices come in a wide variety of shapes and mounting configurations, the possible load directions (and therefore measuring applications that use them) are numerous. The article, The Versatile Strain Gauge Load Cell explains their internal circuitry in more detail. This article also describes the main strain types of spring element geometries.
Figure 2 shows a single-end beam load cell with its components. The strain gauges at the top of the beam are in tension (stretched) under the load, while those at the bottom are in compression under the load. The spring element consists of the fixed and free ends and the machined beam in between. Both the excitation voltage and output signal travel through the cable depicted on the left.
Operational Tradeoffs and Ideal Uses of Strain Gauge Load Cells
The Advantages: Strain gauge load cells are an industry favorite for their versatility across many applications, accuracy, and cost-effectiveness compared to other load cells. They offer a high-frequency response to dynamic loads, and high-quality ones are not sensitive to temperature variations.
The Constraints: Because strain gauge load cells are passive transducers, they cannot be used where electrical power is unavailable. Likewise, they do not lend themselves to potentially explosive or flammable environments.
Additional details about strain load cells can be found in The Versatile Strain Gauge Load Cell in this Knowledge Base. Additionally, our article, Choosing the Right Load Cell For Your Job, matches specific strain gauge load cell types to their practical applications.
2. Hydraulic Load Cells: The Safety Option
Hydraulic Load Cell Principles and Components
Hydraulic load cells convert loads to hydraulic pressure. The measured load is applied to a load platform attached to a piston that sits in a closed chamber filled with fluid. When a load is applied, the piston’s pressure on the diaphragm pressurizes the liquid proportionally to the force applied by the load. A bourdon tube pressure gauge at the end of a tube attached to this fluid chamber gives the readout. Hydraulic load cells can typically measure up to 5MN and have an accuracy of about 0.25 to 1.0 percent of full-scale output. Their resolution is typically about 0.02 percent.
Hydraulic load cells have the following components (depicted in Figure 3):
- An elastic diaphragm
- A piston connected to a load platform
- Hydraulic fluid which is usually oil or sometimes water
- Pressure gauge or gauges
- A tube connecting the chamber to the pressure gauge
- Steel housing for the assembly
Features, Benefits and Disadvantages of Hydraulic Load Cells
The Advantages: Because the hydraulic load cell contains no electrical components, it is essential in environments where explosion safety is a concern or where connecting to an outside power source may be difficult.
The Constraints: Hydraulic load cells tend to be more expensive than other types, making them cost-prohibitive for certain applications. They are sensitive to ambient pressure and temperature. Therefore, their readout must be reset to zero before each use.
3. Pneumatic Load Cells
Pneumatic load cells function similarly to their hydraulic counterparts in that they convert fluid pressure into a load measurement. However, the pressurized fluid in a pneumatic load cell is a type of gas, oftentimes air.
The force to be measured is applied to a loading platform on one side of a diaphragm, and a pressure supply regulator introduces a pressurized gas to a chamber on the opposite side of the diaphragm to balance out the force. A nozzle connected to a pressure gauge allows some of the pressurized gas to escape the chamber. The system then measures the pressure of the gas flowing through this nozzle. This pressure is proportional to the force applied.
Pneumatic Load Cell Components
Pneumatic load cells have the following components:
- A loading platform to apply the force
- A steel chamber filled with pressurized gas or air
- An elastic diaphragm connected to the loading platform that seals the chamber
- An air supply regulator
- Nozzle (bleed valve)
- Pressure gauge
Features, Benefits and Disadvantages of Pneumatic Load Cells
Like their hydraulic counterparts, pneumatic load cells are explosion resistant and are generally used in applications with intrinsic safety concerns. The pneumatic load cell is also tolerant of temperature changes. Finally, this type of load cell is sensitive to small loads. This makes them practical for systems requiring real-time accuracy with the lightest of loads, such as dispensing IV fluids.
4. Capacitive Load Cells
Capacitive load cells operate on the ability of a material or system to store a charge. They consist of two parallel plates with a gap between them. An electric current is supplied to the plates until a stable charge forms on each: one with a positive charge and the other negative. When a load is applied to one of the plates, the gap narrows causing a stored charge (or capacitance) between the plates. This charge creates the output of the load cell, which is then translated to a load measurement.
Capacitive Load Cell Components
Capacitive load cells consist of:
- A loading platform external to the housing to apply the force
- An insulated housing containing a free moving and a fixed plate
- A dielectric material between the plates (which may be air)
- Electrical wires to the plates
- A rigid rod or connector between the loading platform and the free moving plate in the housing
Features, Advantages and Disadvantages of Capacitive Load Cells
Capacitive load cells are highly sensitive and accurate over a wide range of forces, large and small. They are also rather simple in design, making them more cost-effective than other load cell types. Their ability to be hermetically sealed without compromising their operation makes them a good choice for food and medical weighing applications where hygiene is an issue.
Because capacitive load cells operate using an electric charge, they may not be a good choice in flammable environments. Also, some dielectric materials are sensitive to temperature, which can affect the load cell’s accuracy.
5. Piezoelectric Transducers
Piezoelectric sensors operate based on the piezoelectric effect. The piezoelectric effect is a natural property of materials such as quartz crystal and other ceramics.
Piezoelectricity is produced when this polarized crystalline material is stressed or deformed. The stress then causes a shift in the orientation of the internal dipoles of the material. It is similar to di-electricity, which occurs when a charge develops from a shift of electrons in an insulator. Piezoelectric sensors can quantify force, pressure, and displacement. Metallic electrodes bonded to the surface of the material form a measurable net charge. For proper function, the design must place these electrodes perpendicular to the applied force.
Both compression and tension forces create this piezoelectric effect. Compression forces create an opposite polarity to tension forces. The output voltage is directly proportional to the applied force.
Piezoelectric Transducer Components
Piezoelectric load cells consist of:
- A loading platform or system to apply the force
- Metallic electrodes bonded to the piezoelectric material
- The piezoelectric material
- Output wires to measure a change in voltage caused by the change in charge
Features, Benefits and Disadvantages of Piezoelectric Transducers
A piezoelectric transducer is an active transducer, meaning it does not require an external power source to generate an output signal. This characteristic makes this device desirable in applications where an external power source is inconvenient. However, its output signal does require amplification as it is very small.
The piezoelectric effect happens for dynamic forces. Once a force becomes static, the output of the sensor returns to zero. Therefore these transducers lend themselves to applications requiring the measurement of a transient force.
Piezoelectric transducers are more durable than other load cells, and have a high frequency response.
Conclusion
Load cells are extremely accurate measuring devices used in a variety of applications. Various types exist to suit these applications. Tacuna Systems offers a wide product line of quality strain gauge load cells to meet these applications’ requirements. Ask about what innovative solutions we can provide today.
