Load Cell and Strain Gauge Basics

A  load cell is a type of transducer, which is a device that converts energy from one form to another. Load cells are a type of force transducer. They convert the kinetic energy of a force such as tension, compression, pressure, or torque into electrical energy; more specifically as 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

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. 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.

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.

Wheatstone Bridge

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

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      ] x  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.

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