Load Cell | Force Sensor | Force Measurement Transducer

The FUTEK load cells explainer

A foundational guide to understanding load cells/force sensors

FUTEK designs, develops, and manufactures a wide range of industry-leading load cell sensors using strain gauge technology. In this guide, you will learn the basics about these kinds of load cells; how they work, what types there are, and how to use them.

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What is a load cell?

Understanding Force Measurement

Converting mechanical force

A load cell, which is also known as a force sensor or force transducer, is a sensor that measures force by converting the input of mechanical force into the output of an electrical signal. As the force is applied to the sensor, its electrical output signal can be measured, converted, and standardized. The input force can vary between load, weight, tension, compression, or pressure, and it can only be measured by a sensor that is designed to calculate that type of force. (We will break down the different kinds of load cells/ force transducers in the section below.)

Industries that use load cells

Due to their accuracy, load cells have become an essential element in many industries. Common sectors that rely on high-precision load measurement include automotive, high-precision manufacturing, aerospace and defense, industrial automation, medical and pharmaceuticals, and robotics. The design and development of load cell sensors are continuously evolving. With the current advancements in cobots and surgical robotics, many innovative force measurement applications are emerging, requiring ever more sophisticated force measurement solutions, such as FUTEK’s miniature medical sensors for robotic surgery.

How does a load cell work?

Looking under the hood

Meet the strain gauge

The underlying physics and material science behind the force sensor working principle is tied to a component of the sensor that is called the strain gauge (or strain gage). Structurally, a load cell sensor is made of a metal body (also called flexure) to which foil strain gauges are bonded.

When stress is applied to a stationary object, the resulting deformation or displacement of its material causes strain that is captured by the strain gauge. Load cells and force sensors are designed to focus the stress (tension, compression, pressure, load) through elements where the strain gauges are located.

Anatomy of the strain gauge

Strain gauges are electrical conductors that are tightly attached to a film in a zigzag shape. When stress is transferred from the load cell flexure to the strain gauge, the resulting deformation or displacement of its material causes strain that ultimately is converted into the load cell’s measurable output. For example, when the film is pulled, it — and the conductors — stretches and elongates. When it is pushed, it is contracted and gets shorter. This change in shape causes the resistance in the electrical conductors, what we call strain gauge resistance, to also change. The strain gauge resistance increases with applied strain and diminishes with contraction. The changes are converted into an electrical signal, which can then be measured and captured using data acquisition.

Strain Gauge Diagram strain gage foil tension and compression

Understanding the Wheatstone bridge circuit

In order to measure the changes in resistance, the strain gauge must be connected to an electrical circuit that is capable of accurately responding to the changes and creating a differential voltage variation. Multiple strain gauges can be used in a divided bridge circuit that is called a Wheatstone bridge. In a Wheatstone bridge configuration, an excitation voltage is applied across the circuit, and the output voltage is measured across two points in the middle of the bridge. When there is no load acting on the load cell, the Wheatstone bridge is balanced and there is zero output voltage. Any small change in the material under the strain gauge results in a change in output.

What are the advantages of a load cell?

The case for choosing FUTEK load cell sensors

In general, metal foil strain gauge load cell sensors are a popular measurement solution because of their durability and relatively low cost. In the case of FUTEK’s load cells, however, the most favored aspects are long-term reliability, variety of sizes and sensor geometry, and—most notably—high accuracy.

What is load cell accuracy?

One of the most important qualities of strain gauge load cells is load cell sensitivity and accuracy. Sensor accuracy can be defined as the smallest amount of force that can be applied to the sensor body required to cause a linear and repeatable variation in the voltage output. The higher the load cell accuracy, the better, as it can consistently capture very small force variations.

Proprietary strain gauge capabilities

FUTEK’s load cells feature an industry-leading metal foil strain gauge technology for its sensors. A highly customized strain element has been designed to allow more strain measurement around the active sensor element, which reduces reproducibility errors from off-axis loads. It also makes the sensor less sensitive to installation mishaps. Our proprietary foil strain gauges are encapsulated and feature internal compensation, ensuring long-term stability and improved reliability.

Nonlinearity and nonrepeatability

Load cell linearity is paramount to produce accurate output. FUTEK offers high precision load cells with as low as ±0.02% nonlinearity (of Rated Output) and nonrepeatability of ±0.02% RO, which make them adequate models for rocket engine thrust test stand applications.

Material matters

The body (or flexure) of FUTEK’s load cell sensors is made of aerospace-grade 17-4 stainless steel or aluminum. The high-quality materials provide sturdiness to withstand high loads as well as elasticity to minimally deform and return to its original shape when the force is removed. The exceptional quality also makes the force sensor highly durable and reliable with high strength, high hardness, low hysteresis and creep properties, and corrosion resistance over a wide temperature range.

QLA414 Nano Force Sensor miniature micro mini force sensor load cell — QLA414 Nano Force Sensor

Strain gauge-based load cells vs. Piezo load cells

The two most common types of load cells (or force sensors/force transducers) are Strain gauge load cells and piezoelectric load cells.

As we saw in the sections above, strain gauge-based load cells measure force via strain gauges that are connected in a Wheatstone bridge circuit. Piezoelectric sensors (or Piezo sensors) apply different mechanics to measure force. The unit has two crystal disks with an electrode foil mounted in between them. When force is applied, the friction between the disks and electrode results in an electrical charge that can be measured.

Piezoelectric sensors: compact package and high stiffness

Piezo sensors have high stiffness, which provides high natural frequency and a higher dynamic response and, because of their small components, Piezo force sensors can be very compact. However, their measurements are less precise than those of strain gauge sensors, because of their higher linearity error and high drift. Hence, the applications that are best suited for Piezo sensors are dynamic applications that require fast measurements of small forces where accuracy over time is less important.

Strain gauge: Low drift and high accuracy

Strain gauge sensors have very low non-linearity and low drift, which makes them more accurate, especially for long-term measurements where the output has to stay consistent over time. The circuit that connects the strain gauges allows them to compensate for many kinds of errors (i.e. effects of temperature changes). It also enables very precise calibration. This means that strain gauge-based load cells are the optimal choice for applications that require long-term monitoring as well as mission-critical applications where failure is not an option.

LSB205 Miniature S-Beam Jr.® Load Cell 2.0

Our strain gage force measuring sensors also offer the following notable values:

Consistently high performance at up to a billion fully reversed cycles
An extensive range of geometries and customized shapes as well as flexible mounting options
Wide selection of capacities ranging from 10 grams to 100,000 pounds
Resistance to temperature variations

What are the different types of load cells?

Different Types of Load Cells load cell types of load cell

As mentioned previously, there are many different types of load cell sensors (force transducers). Within the load cell types, there are a large variety of body shapes and geometries that cater to distinct applications. Below is a selection of some of the most popular load cell sensor types that FUTEK offers, which also represent the main categories on the market.

In-line load cell

Commonly referred to as in-line load cells or a canister-style (or column) this sensor has male threads and can be used in both pushing and pulling forces applications. These models offer robust construction with a broad capacity range as well as high accuracy and high stiffness with minimal mounting clearance needed.

Typical applications include:

Robotic arm featuring QLA414 Nano Force Sensor

FUTEK has also developed a miniature Inline Load Cell for applications where size and tight environments are critical. These include micro load cells (aka micro force sensor or miniature force sensor, miniature load cell, mini load cell or milligram load cell) versions. NanoSensors such as QLA414 can be used in haptic feedback robotic surgery applications.

Load button load cell

This model has a single flat, raised surface where the compressive force is applied. Load button load cells are known for their robustness and provide exceptional dynamic performance with a frequency response of up to ~20 kHz. These small load cells have packages that range from ¼" (6mm) to 3" (76mm) OD and the most compact models, such as Miniature Load Buttons, are ideal choices for press or inline compression applications with tight and limited spaces. Many models are designed with diaphragm construction but some higher precision units feature bending beams for low capacity and shear or column design for higher capacity. For best performance, the load should be applied without the presence of any side load or torque. Mounting could be a challenge with standard load bu

Load Cells