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Getting Started with Load Cells
Introduction
Have you ever wanted to know the weight of something? How about
knowing the change in weight over time? Do you want your project to sense
the presence of something by measuring strain or a load on some surface.
If so, you’re in the right place. This tutorial is here to help you get started in
the world of load cells and their variants.
One of many kinds of load cells.
Suggested readings:
Before jumping into load cells and all of their awesomeness, we suggest
you familiarize yourself with some basic concepts if you haven’t already:
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Voltage, Current, Resistance, and Ohm’s Law
Series and Parallel Circuits
Resistors
Voltage Dividers
How to Read a Schematic
Load Cell Basics
Types of Load Cells
A load cell is a physical element (or transducer if you want to be technical)
that can translate pressure (force) into an electrical signal.
So what does that mean? There are three main ways a load cell can
translate an applied force into a measurable reading.
Hydraulic Load Cells
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Hydraulic load cells use a conventional piston and cylinder arrangement to
convey a change in pressure by the movement of the piston and a
diaphragm arrangement which produces a change in the pressure on a
Bourdon tube connected with the load cells.
Diagram of a Hydraulic Load Cell from Nikka’s Rocketry
Pneumatic Load Cells
Pneumatic load cells use air pressure applied to one end of a diaphragm,
and it escapes through the nozzle placed at the bottom of the load cell,
which has a pressure gauge inside of the cell.
Diagram of a pneumatic load cell from Instrumentation Today
Strain Gauge Load Cells
And lastly (though there are many other less common load cell set ups),
there is a strain gauge load cell, which is a mechanical element of which
the force is being sensed by the deformation of a (or several) strain gauge
(s) on the element.
Strain gauge load cell diagram from Scalenet.com
In bar strain gauge load cells, the cell is set up in a “Z” formations so that
torque is applied to the bar and the four strain gauges on the cell will
measure the bending distortion, two measuring compression and two
tension. When these four strain gauges are set up in a wheatctone bridge
formation, it is easy to accurately measure the small changes in resistance
from the strain gauges.
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More in depth diagram of strain gauges on bar load cells when force is
applied
In this tutorial we will be focusing on strain gauge load cells like the ones
SparkFun carries:
Load Cell - 200kg, Disc
(TAS606)
SEN- 13332
Load Cell - 10kg, Straight Bar
(TAL220)
SEN- 13329
Load Cell - 50kg, Disc
(TAS606)
SEN- 13331
Load Cell - 10kg, Wide Bar
(TAL201)
SEN- 13330
Most strain gauge load cells work in very similar ways, but may vary in size,
material, and mechanical setup, which can lead to each cell having different
max loads and sensitivities that they can handle.
Strain Gauge Basics
A strain gauge is a device that measures electrical resistance changes in
response to, and proportional of, strain (or pressure or force or whatever
you so desire to call it) applied to the device. The most common strain
gauge is made up of very fine wire, or foil, set up in a grid pattern in such a
way that there is a linear change in electrical resistance when strain is
applied in one specific direction, most commonly found with a base
resistance of 120Ω, 350Ω, and 1,000Ω.
Each strain gauge has a different sensitivity to strain, which is expressed
quantitatively as the gauge factor (GF). The gauge factor is defined as the
ratio of fractional change in electrical resistance to the fractional change in
length (strain).
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(The gauge factor for metallic strain gauges is typically around 2.)
We set up a stain gauge load cell and measure that change in resistance
and all is good, right? Not so fast. Strain measurements rarely involve
quantities larger than a few millistrain
(fancy units for strain, but
still very small). So lets take an example: suppose you put a strain of
500me. A strain gauge with a gage factor of 2 will have a change in
electrical resistance of only
For a 120Ω gauge, this is a change of only 0.12Ω.
0.12Ω is a very small change, and, for most devices, couldn’t actually be
detected, let alone detected accurately. So we are going to need another
device that can either accurately measure super small changes in
resistance (spoiler: they are very expensive) or a device that can take that
very small change in resistance and turn it into something that we can
measure accurately.
This is where an amplifier, such as the HX711 comes in handy.
SparkFun’s HX711 Amplifier breakout board
A good way of taking small changes in resistance and turning it into
something more measurable is using a wheatstone bridge. A wheatstone
bridge is a configuration of four resistors with a known voltage applied like
this:
where Vin is a known constant voltage, and the resulting Vout is measured.
If
then Vout is 0, but if there is a change to the value
of one of the resistors, Vout will have a resulting change that can be
measured and is governed by the following equation using ohms law:
By replacing one of the resistors in a wheatstone bridge with a strain
gauge, we can easily measure the change in Vout and use that to assess
the force applied.
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Bar load cell wheatstone bridge example From All About Circuits
Combinator Basics
Now that you have a load cell with a strain gauges hooked up to an
amplifier, you can now measure force applied to your cell. For more
information about how to hook up strain gauges, load cells, and amplifiers
go to our hookup guide.
Bathroom scale using the Load Sensor Combinator to combine twelve
wires into one wheatstone bridge
But what happens when you don’t have a load cell with four strain gauges?
Or you want to measure something really heavy on something scale like?
You can combine four single strain gauge load cells (sometimes referred to
as Load sensors)! Using the same wheatstone bridge principle, you can
use a combinator to combine the single strain gauge load cells into a
wheatstone bridge configuration where the force applied to all four single
strain gauge load cells is added to give you a higher maximum load, and
better accuracy than just one, and then the combinator can be hooked up to
the same amplifier for easier measuring.
For more information on hooking up load sensors go to our hookup guide.
This is the same layout that you would find in say your home scale. There
would be four strain gauge load cells hooked up to a combinator and an
amplifier to give you your weight reading.
Resources and Going Further
For more information about setting up load cells and how to integrate them
into your next project, check out our HX711 hook up guide:
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