Strain Gauge Load Cell Basics

What is a Load Cell?

A load cell is a type of transducer, which is a device that converts energy from one form to another. Specifically, load cells are force transducers, converting the kinetic energy of a force such as tension, compression, pressure, or torque into a measurable electrical signal. The strength of the signal changes in proportion to the force applied. There are three basic load cell types based on output signal: hydraulic, pneumatic, and strain gauge. (Load Cell Central deals exclusively in strain gauge type load cells.)

The most commonly used type of load cell in industrial applications is the strain gauge load cell. This type of load cell is accurate and cost-effective. A strain gauge load cell is comprised of a solid metal body (or “spring element”) on which strain gauges have been secured. The body is usually made of aluminum, alloy steel, or stainless steel which makes it very sturdy but also minimally elastic. When a load is applied, the body of the load cell is slightly deformed, but, unless overloaded, always returns to its original shape. In response to the body shape changes, the strain gauges also change shape. This, in turn, causes a change in the electrical resistance of the strain gauge which can then be measured as a voltage change. Since this change in output is proportional to the amount of weight applied, the weight of the object can then be determined from the change in voltage.

Strain Gauges

How does a load cell work?

To answer the question "How does a load cell work?" we first need to answer the question of "How does a strain gauge work?". A strain gauge is a device that measures change in electrical resistance when a force is applied. A typical strain gauge is made up of very fine wire, or foil, set up in a grid pattern in a way that produces a linear change in resistance when strain is applied along one axis. There are numerous types of strain gauges available:

  • Linear strain gauges: The wire attached to the backing of the strain gauge runs parallel to the edges of the strain gauge. These are used to measure axial and bending strain.
  • Shear strain gauges: The wire attached to the backing of the strain gauge is laid out in a 45º orientation to the sides of the gauge. These are used to measure shear strain.

The strain gauges are often used in tandem with more strain gauges in order to increase accuracy. One active strain gauge is referred to as a quarter-bridge, two active strain gauges are a half-bridge, and four active strain gauges are a full bridge.

The strain gauge resistance changes differ in tension load cells vs. compression load cells. Tension force causes the strain gauge to get thinner and longer, increasing the resistance. Compression force causes the strain gauge to get thicker and shorter, decreasing resistance. The strain gauge is bonded to a thin backing (carrier), which is attached directly to the load cell enabling the strain of the load cell to be experienced by the strain gauge.

Wire comparison between strain gauge load cells for tension and compression force

The change in resistance measured by a single strain gauge is extremely small, around 0.12Ω. A load cell’s sensitivity increases with the number of strain gauges applied. A good way of turning these small changes into something more measurable is to interconnect them as a Wheatstone bridge.

Types of Strain Gauges

Strain gauges are laid out in different orientation patterns depending on the type of force being measured. Bending strain, shear strain, axial strain, torque, and pressure are all measured using a specific layout of strain gauges. See our blog on types of strain gauges for more information.

Wheatstone Bridge

A Wheatstone bridge is a configuration of four balanced resistors with a known excitation voltage applied as shown below:Graphic of a wheatstone bridge with four balanced resistores with a known excitation voltage applied

VEX is a known constant voltage and VO is measured. If all resistors are balanced, meaning R1/R2 = R3/R4 then VO is zero. If there is a change in the value of one of the resistors then VO will have a resulting change that can be measured and interpreted using Ohm’s law. Ohm’s law states that the current (I, measured in amperes) running through a conductor between two points is directly proportional to the voltage (V) across the two points. Resistance (R, measured in Ohms) is introduced as the constant in this relationship, independent of the current. Ohm’s law is expressed in the equation I = V/R.

When applied to the 4 legs of the Wheatstone bridge circuit, the resulting equation is:

VO   R3  –  R2     ×  VEX
R3 + R4 R1 + R2

In a load cell, these resistors are replaced by strain gauges in alternating tension and compression measurements. When a force is applied to the load cell, the resistance in each strain gauge changes and VO is measured. From the resulting data, VO can be easily determined using the equation above.

Diagram showing bridge sensors R1-R4 in action with force appliedGraphic of wheatstone bridge measuring compression & tension

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