In recent years, there has been a proliferation of load cell designs based on piezo-resistance strain gauges bonded to a variety of shaped tension and/or compression bodies to suit loadings from a few newtons (N) to hundreds and even thousands of kilo-newtons (kN). The reliability and accuracy of these designs has been proven in many applications, some of which we will be touching on in this article to illustrate the range of applications.
Typical metal foil strain gauge element
First, a word or two on the piezo-resistive effect as nearly all modern load sensors rely on this effect. The strain gauge is usually a very thin silicone/metal foil bonded sandwich with the metal foil arranged as shown in the illustration. They are bonded to the steel body with either cyanoacrylic glue or more usually a form of epoxy cement for long lasting applications. The elongation in the long direction causes a change in the electrical resistance of the array. The low temperature coefficient of expansion of the alloy avoids much of the problems of resistance changes with temperature. Nearly all such stain gauges are arranged in a diamond with 2 points arranged through the axis of the force to be measured. The gauges are wired into in a resistance (Wheatstone) bridge to ensure that the arrangement has self-temperature compensation (STC) across a wide temperature range usually -20°C to +80°C. This simple arrangement is repeated depending on how many axes are to be sensed in the tension/compression body.
The button load cell is a particular variant that is used for compressive loads and usually it can be found at the base of weighing vessels, foundations, compressive testing machines for construction materials and weigh bridges at the larger end and in weigh scales and other lab test machines at the lower end. They are capable of weighing compressive loads between 0-250kN to a massive 0 – 30 tonnes and they in common with most force sensors of this type, can follow dynamic loading variations to a maximum of some 50kHz.
Other variants of this type is the cannister load cell for high accuracy applications and the pancake load cell. Yet another form is the donut or low profile load cell that is used typically as a washer replacement in structural elements including post-tensioning anchors and mast guy anchor blocks.
S Type Force Sensor
The body of the S-Beam or Z-Beam force sensor is, as it’s name suggests, thus shaped and is used for both tension and compression loading with threaded holes for in line fixing as shown. These are often used to measure tension forces in suspended hoppers or as a load link in a tensioned cable assembly for lifting operations. They are also found in materials testing machines for both compressive and tension forces with ranges of less than 50kg or greater than 5000kg, our covers load ranges from 0-1kg (10N) up to 0-30,000kg (300kN) with typical accuracies of 0.03% FS .
We have already covered torsion load cells on this blog with the torque sensors page and these are used particularly for monitoring rotating shafts for the dynamic type and static torque on various testing and industrial applications.
Another type that is quite distinct is the shear and loading beam load cell type in which a cantilever body is deformed under load. They are specifically designed for the measurement of tensile and comprehensive forces where they form the bridge between the system framework and the live load as can be found in foundations for measuring vessel charge loads.
Finally, there are the multi axis force sensor products that measure 2 or 3-axes that are usually custom built to measure both torque and tension in the case of the 2-axes and torque, compression and side force for such applications as continuous low-stir friction welding and load and torque monitoring for wind turbines, in soil mechanics and medical industries applications.