Monday, March 9, 2020 · 0 min read
How to Measure Weight With Load Cell Sensors
In this article we will discuss how you can measure weight with load cell sensors, with enough detail so that you will:
See how load cell sensors work
Learn how weight measurements are made in science and industry
Understand how you can incorporate them into your testing
Are you ready to get started? Let’s go!
What is a Load Cell?
A load cell is essentially a force transducer or force sensor. It is used principally to measure weight. And although they can be used to measure other forces such as torque, compression, pressure, etc. we will focus on weight measuring applications in this article.
Although gravity is the weakest of the four fundamental forces that keep the universe from flying apart, it is the only one that we can directly perceive. Everything on the earth that has mass experiences it, as the force of gravity pulls it toward the iron core at the center of our planet. Weight and mass are connected by gravity.
The world standard unit of measurement of weight is the kilogram (kg). Even in the USA, where imperial measurements of weight, distance, and volume are still in wide public use, those units are referenced in the metric SI system.
So a pound (of weight) in the USA is not defined by another imperial measurement but officially as 0.45359237 kilograms. How we calculate kilogram mass was defined by the world’s scientists in 2019 to reference Planck’s constant and removed its reference to a physical specimen.
If you remove gravity an object does not literally weigh anything, but its mass remains unchanged. When you weigh something you’re really measuring that object’s mass as it relates to this planet. So in effect, a load cell measures mass. But it is only relevant in terms of what it means here on earth.
There are several basic technologies used within load cells, such as pneumatic load cells (often used in intrinsic safety applications). Hydraulic load cells do not require power and are therefore used frequently in remote, hard-to-access locations. There are also piezoelectric load cells that provide a high-level (yet non-linear) output.
But we will focus on strain-gage-based load cells they are the most prevalent type used in the world today. They are inexpensive, very reliable, and available to handle a wide range of force inputs. They are the de facto standard of the weighing industry and provide a full-scale accuracy of 0.25% or better.
You might even be using a strain gauge load cells-based machine every morning without even realizing it. It might look something like this:
While classic mechanical scales use a series of levers to distribute the applied weight and drive a spring connected to a mechanical dial, most of today’s digital models use multiple strain gauge-based load cells to calculate your weight.
When you press your foot once on the measuring plate, the microcontroller “wakes up” and performs a zero offset calibration of the strain gages. The digital display then indicates 0.0 kg (or lbs), and it’s ready for you to step on and weigh yourself.
In better digital bathroom scales, a strain gage-based load cell is placed at each of the four corners of the scale. In fact, they are often built into the “feet” of the scale. Two of the load cells are positioned in tension mode and the other two are positioned in compression mode.
When you step onto the scale, the microcontroller takes their outputs and converts them to an aggregate weight value in the selected unit of measurement, and displays that number on a gauge. Some models even employ a thermocouple to measure the ambient temperature and factor that into the equation, since strain gages are resistance-based sensors, and are affected by temperature.
OK, that was fun, but now let’s look at load cells used for scientific measuring DAQ applications.
How does a strain gage-based load cell work?
A strain gage (aka “strain gauge”) measures strain by means of a change in resistance. A metal foil pattern is mounted onto a flexible substrate, which also serves as an insulator. A current is run through the foil pattern. When the object under test is stressed (i.e., compressed or put under tension) there is a change in resistance which is proportionate to the amount of deflection.
When a conductor is stretched, its resistance increases. When it’s compressed, its resistance decreases. This change in resistance can be measured using a Wheatstone bridge, which is four strain gage sensors arranged in a pattern.
Measuring with all four sensors you get into a full-bridge configuration. In the full-bridge diagram above, the sensor's output voltage is measured at C and B, while the excitation voltage is supplied at A and D.
So when we mount a strain gage onto a mechanical housing, and then subject that housing to stress or force, such as weight, the gage will measure the relative compression or tension caused by that force. The combination of such housing with the strain gage sensor mounted onto it is a load cell.
Now let’s look at the various shapes and sizes of such load cells, and how they are used.
Learn more about the strain gages and strain measurement:
Load cell measurement applications
There is virtually an unlimited number of possible applications for load cells, large and small. Here are just a few that come to mind:
Materials testing - measuring the weights of parts as they are manufactured for consistency
Aerospace - testing the thrust of jet engines, measuring the load on wheels and undercarriages
Marine - monitoring the tensions on mooring lines as they stretch under the load of the ship moving with the water
Transportation - torque measurements on electric, gasoline and diesel engines, axle load monitoring, train and truck wheel loads, measuring cargo weights at highway weighing stations
Industrial - Force measurements on gearboxes and pumps, tension measurements in undersea pipe-laying applications, tension and force measurements in the paper mill and steel mill applications, Hopper and vessel weighing
Medical / Healthcare - Hospital beds that weigh the patient, precision Infant and infant incubator scales, load measurements on physical therapy and exercise equipment.
Construction - Cable forces in elevators, and forces on scaffolding according to international standards. For an interesting application note.
Entertainment - S-type load cells are installed in the middle of cables used to hoist acrobats and actors, ensuring that the forces do not exceed prescribed levels
Petrochemical - measuring the forces on oil and gas drilling tools
Farming and Ranching - Measuring the weight of livestock passing through chutes, measuring forces on cables and hosting equipment, hopper, tankard and silo weighing
Household / Consumer - digital bathroom scales
When we consider the sheer variety represented by just a dozen or so applications, it is clear that force and weight measurements are similar to temperature measurements in that they are fundamental to tens of thousands of applications across virtually every industry.
Considerations when using load cells
Maximum load ratings
It is important to choose a load cell that not only matches the kind or direction of load that you want to measure but one which is also rated to handle the maximum anticipated load.
For example, load cells are rated in terms of their maximum kg (or lbs.). To measure up to 1000 kg you’d obviously want a load cell that was rated at least to 1.5 to 2 times that weight.
Two-point calibration and zeroing
The calibration sheet that each load cell is shipped with will show you the preferred excitation voltage and sensitivity of the strain gages built into it. You will need these in order to set up the software to accurately measure the output in the desired unit of measurement, usually kg or lbs.
In DewesoftX software it is quite easy to perform a ZERO calibration of any sensor, for example, remove all loads and then press ZERO in the software (just like your bathroom scale does automatically). You can also perform a 2-point calibration using a known reference load, ensuring that your load cell is outputting correct, linear values between zero and the reference load that you apply.
If the load cell will be used in an environment that contains corrosive gases, water, steam, etc, a load cell that is rated as corrosion-resistant must be used. Stainless steel load cells that are sealed (look for high IP ratings like IP65 and above), are made specifically for these unfriendly environments. And of course, any sensor or system that will be used in an explosive atmosphere must be certified as intrinsically safe and/or explosion-proof depending on regulatory requirements.
Types of load cells
There are load cells made in various shapes and sizes, for a variety of applications. Here are the main ones available today:
Bar / Bending Beam Load Cells
Canister Load Cells
S-Beam Load Cells
Through-Hole / Donut Load Cells
Pancake Load Cells
|Type||Bar / Bending Beam||Canister||S-Beam||Through- Hole / Donut|
|Application(s)||Application(s)Used for measuring weight. Great at making off-center measurements||Used to measure compression loads||It can be used for both compression and tension. Can be attached in the middle of a steel cable to||Most rugged type for high compression and tension. The hole(s) allow the structure to pass through the load cell.|
|Can Measure||Tension and compression||Compression||Tension and compression||Compression only OR tension and compression|
Bar/bending beam load cells
There is also a distinction between single-point load cells and heavy-duty compression models that distribute the load across four or more strain gage sensors for enhanced accuracy and/or capacity. The above represents just some of the major load cell types. There are many others, as well as several speciality types engineered for special applications and environments. There are low-profile, miniature and ultra-miniature load cells that can be used in very small places. On the other end of the spectrum, there are load cells used to measure hundreds of tons, which are quite large for obvious reasons.
A bar or bending beam (aka binocular beam) load cells are commonly used for industrial weighing applications. One end of the bar is fixed to a structure, while a force is applied to the free end of the sensor (see F in the graphic above).
This force causes the four strain gauges that are built into top and bottom and each end of the load cell to elongate or compress depending on how much application or removal of the force stresses the load cell structure. These tiny changes in potential from the strain gages are easily converted to weight within our Dewesoft X data acquisition software.
Canister style load cells
Also known as a “canister load cell,” a compression load cell is the most fundamental type of load cell. It is used primarily as an industrial scale to measure the weight of a load that is placed on it. It gets its name from its canister-like shape.
S-Beam load cells
S-Beam load cells are also suitable for both pushing/pulling applications. They get their name from their distinctive shape. They are typically connected in the centre of a steel cable. Imagine a cable being used to hoist a heavy load.
Through-hole/donut load cells
Some of these types are made only for compression, while others are available for both compression and tension. The centre hole allows part of the structure to pass through the load cell if needed. Some models have additional through-holes arranged around the centre hole for additional mechanical connections.
Pancake load cells
Pancake load cells are designed for applications where both tension and compression (pulling and pushing) will occur.
Dewesoft load cell compatible data acquisition systems
SIRIUS DAQ systems
SIRIUS DAQ systems offer DualCoreADC® technology which boosts dual 24-bit delta-sigma ADC converters with an anti-aliasing filter on each channel, achieving an astonishing 160 dB of dynamic range in the time and frequency domains, with up to 200 kS/s/ch sampling rate per channel.
SIRIUS slices can hold to 8 or 16 analog input channels. Every SIRIUS DAQ system includes award-winning Dewesoft X data acquisition software for complete system setup, operation, display, storage, analysis and report generation.
The list of SIRIUS amplifiers that are compatible with load cell sensors:
SIRIUS STG (8 channels), 200 kS/s, Supports all strain types, high input range
SIRIUS STGM (8 channels) 200 kS/s, Low power, sensor and amp balance
SIRIUS-LV (8 channels), 200 kS/s, Supports full bridge connection
SIRIUS-HD STGS (16 channels) 200 kS/s, Low power, sensor and amplifier balance
SIRIUS-HS STG (8 channels) 1 MS/s, Supports all strain types, high input range
SIRIUS-HS-LV (8 channels), 1 MS/s, Supports full bridge connection
KRYPTON DAQ systems
KRYPTON DAQ family offers ruggedized strain gage modules with 1, 3 or 6 channels per module. Many other KRYPTON and KRYPTON ONE modules are available, for other sensor and signal types. KRYPTON DAQ modules offer an IP67 degree of protection and are made for the toughest operating environments - shock, vibration and temperature extremes. Every KRYPTON system includes award-winning Dewesoft X data acquisition software for complete system setup, operation, display, storage, analysis and report generation.
The list of KRYPTON DAQ modules that are compatible with load cell sensors:
KRYPTON-3xSTG and KRYPTON-6xSTG: 3 or 6 channels DAQ module that supports all strain gage types and high input range.
KRYPTONi-1xSTG: 1 channel isolated DAQ module that supports all strain types and high input range.
IOLITE DAQ and control systems
IOLITE DAQ and control system combines powerful data acquisition with real-time control via dual EtherCAT interfaces. Note that the IOLITE-6xSTG also accepts DSI modules that adapt it to handle a wide variety of other sensors. Available in a 19” rack model as well as a benchtop model. Every IOLITE system includes award-winning DEWESoft X software for complete system setup, operation, display, storage, analysis and report generation.
The list of IOLITE DAQ modules that are compatible with load cell sensors:
IOLITE-6xSTG (6 channels) Supports all strain types, high input range
DEWE-43A DAQ system
The DEWE-43A is a handheld 8-channel DAQ module that features eight full-bridge / low voltage inputs on DB-9 connectors, plus 8 counter/encoder inputs and two high-speed CAN bus interfaces.
It connects to any Windows computer via a locking USB connector. The analog inputs can natively be used for full-bridge connections, or you can use DSI adapters for quarter-bridge and half-bridge hookups.
Other DSI adapters allow each input to handle other signals, such as IEPE, charge, LVDT, 200V, amps, and milliamps, etc. Every DEWE-43A system includes award-winning DEWESoft X software for complete system setup, operation, display, storage, analysis and report generation.
DEWE-43A analog input (8 channels) 200 kS/s. Supports full-bridge (others via DSI adapters)