Load Cell Guide: Difference between revisions

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==Introduction==
==Introduction==
[[Image:loadcells.jpg|link=|500px|right]]
Do you need to build a system that can measure weight? Using load cells and load cell interfaces from Phidgets Inc. makes this extremely simple! Watch the video below, or keep reading for more information.
A load cell is a force sensing module - a carefully designed metal structure, with small elements called strain gauges mounted in precise locations on the structure.  Load cells are designed to measure a specific force, and ignore other forces being applied.  The electrical signal output by the load cell is very small and requires specialized amplification. Fortunately, the [{{SERVER}}/products.php?product_id=DAQ1500 DAQ1500 - Wheatstone Bridge Phidget] will perform all the amplification and measurement of the electrical output.
<br><br>
<center>{{#ev:youtube|jCoPfQKMksg|||||rel=0}}</center>
<br><br><br>
[[Image:Strain_gauge_load_cells2.png|400px|link=https://wwwdev.int.phidgets.com/docs/images/5/59/Strain_gauge_load_cells2.png|thumb|<center>Common types of strain gauge load cells.</center>]]
==What Is a Load Cell?==
A load cell is a device that converts mechanical force into a signal that can be measured.  


Some load cells are designed to measure force in one direction on one axis (e.g. down, but not up), while others are more versatile. Be sure to keep this in mind when selecting a load cell for your application.


==How they work==
There are different types of load cells, such as pneumatic, hydraulic, capacitive, etc. but '''strain gauge load cells''' are the most common.
[[image:wheatstone.png|thumb|250px|Generic Wheatstone bridge configuration]]
<br><br><br><br><br><br>
Strain-gauge load cells convert the load acting on them into electrical signals. The measuring is done with very small resistor patterns called strain gauges - effectively small, flexible circuit boards. The gauges are bonded onto a beam or structural member that deforms when weight is applied, in turn deforming the strain-gauge.  As the strain gauge is deformed, it’s electrical resistance changes in proportion to the load.
===Strain Gauge Load Cells===
Strain gauge load cells use a small resistive sensor called a ''strain gauge'' to convert mechanical force into an electrical signal. <br><br>
There are many different types of strain gauge load cells (some can be seen in the image above). Each type of load cell is designed for a particular application, however, they all have the same basic components:<br><br>
{| style="margin:auto;width:75%;" class="table-no-border"
| [[File:1_green.png|25px]] || An inflexible frame or body, generally made of aluminum or a steel alloy.
|-
| [[File:2_green.png|25px]] || One or more strain gauges.
|-
| [[File:3_green.png|25px]] || Wires for power, ground, and signals.
|}
<br><br>
[[File:Singlepoint_straingauge_exposed.png|center|500px|link=https://wwwdev.int.phidgets.com/docs/images/a/ac/Singlepoint_straingauge_exposed.png]]
<center>''Strain gauges are typically covered in a protective coating. Here, the coating has been removed from a single-point load cell to expose the strain gauges underneath.''</center>


The main result is that when measured by a [{{SERVER}}/products.php?product_id=DAQ1500 DAQ1500 - Wheatstone Bridge Phidget], they produce a signal that is directly proportional to the force applied, once a baseline offset (''zero'', ''tare'') is accounted for.
<br clear=all>


==Common Load Cell Types==
====Strain Gauge Basics====
===Single-Point Load Cells===
A strain gauge consists of a thin metal wire that is arranged in a specific pattern and attached to a flexible, non-conductive backing material.
[[File:singlecell.jpg|link=|thumb|300px|A single-point load cell. Applying force to one end creates stress in the bar, causing the strain gauge encased in the white plastic to flex.]]
[[File:Strain_gauge_single.png|link=https://wwwdev.int.phidgets.com/docs/images/c/c5/Strain_gauge_single.png|center|500px]]
<center>''A strain gauge.''</center>


This Single Point Load Cell is used in small jewelry scales and kitchen scales.  It’s mounted by bolting down the end of the load cell where the wires are attached, and applying force on the other end in the direction of the arrow. This type of load cell is designed such that the effect of the specific placement of the force is minimized. If you mount a small platform on the load cell, as would be done in a small scale, this load cell provides consistent readings regardless of the position of the load on the platform.


<br clear=all>
As the strain gauge experiences tensile or compressives forces, the metal wire deforms which causes the resistance of the strain gauge to change. This change in resistance causes a voltage change which can be measured. To learn more about strain gauges and how they work, visit our [https://www.phidgets.com/docs/Strain_Gauge_Primer strain gauge primer].<br><br>


===Button Load Cells===
==Which Load Cell Should I Use?==
[[File:buttoncell.jpg|link=|thumb|300px|A button load cell. Applying force to the bump in the middle of the cell causes a change in the output.]]
There are many different types of strain gauge load cells to choose from. Phidgets Inc. carries the following types:


This Button Load Cell is used in applications that require a thin form factor.  The bottom of the load cell is bolted to a surface, and force is applied to the button on the top.  By loading only the button, which is slightly rounded, the load cell is less sensitive to errors resulting from the load not being perfectly aligned with the middle of the load cell.


{| style="margin:auto;width:85%;"
|-
! Type !! Description !! Capacity !!
|-
| [[#Single-Point Load Cells|Single-Point]] || Used for small to medium-capacity weighing scales (e.g. kitchen scale). || 50kg || [[File:Singlepoint_illustration.png|200px]]
|-
| [[#S-Type Load Cells|S-Type]] || Used to measure tension (e.g. hanging loads), and, less commonly, compressive forces. || 500kg || [[File:Stype_illustration.png|200px]]
|-
| [[#Button Load Cells|Button]] || Used for weighing scales, especially where space is limited. || 1000kg ||[[File:Button_illustration.png|200px]]
|}


[[File:button_cell_diagram.jpg|link=]]


<br clear=all>


===S-Type Load Cells===
===Single-Point Load Cells===
[[File:scell.jpg|link=|thumb|300px|An S-type load cell. By pulling on both ends or pushing on both ends, the stress in the S changes and the cell's output changes.]]
Single-point load cells, sometimes referred to as ''Platform'' or ''Binocular'' load cells, are easy-to-use load cells that are commonly seen in small weighing scales (e.g. kitchen scale).


This S-Type load cell can be operated in compression or tension, as illustrated below.


====Form Factor====
[[File:Measurement_axis.png|thumb|link=https://wwwdev.int.phidgets.com/docs/images/9/9a/Measurement_axis.png|<center>A single-point load cell with a capacity of 780g</center>]]
These load cells are beam shaped and typically have two circular holes machined out of the middle (hence, 'binocular load cells'). These holes encourage bending along the measurement axis. They also have mounting holes (generally 1-2) at both ends for simple installation.


[[File:s_cell_diagram.jpg|link=]]
====Use Cases====
<br clear=all>
Single point load cells are primarily used to create platform scales, like the [https://www.phidgets.com/?tier=3&catid=9&pcid=7&prodid=1196 Weighing Scale Kit]. They are ideal for this task, because they support off-axis loading. This means that users can place their object anywhere on the surface of the scale and the results will still be accurate.


==Calibration==
====Installation====
Single-point load cells typically have an arrow showing which direction the load should be applied. One end of the load cell is secured to the base of the platform, while the other is secured to the top plate.
[[File:Singlepoint_scale_installed.jpg|center|600px|link=https://wwwdev.int.phidgets.com/docs/images/8/81/Singlepoint_scale_installed.jpg]]
<center>''A 5kg single-point load cell installed in a small weighing platform.''</center>


Load cells have enough variation through manufacturing differences that we strongly recommend each one be calibrated once it is built into its final application. Thankfully this is a fairly easy process, and you can find instructions and an example of this on our page for [[Calibrating_Load_Cells|Calibrating Load Cells]].
===S-Type Load Cells===
S-type load cells, sometimes referred to as ''Z-type'' or ''S-beam'' load cells, are easy-to-use load cells that allow you to measure both tensile and compressive forces.  


==Multiple Load Cells Measuring the Same Platform==
====Form Factor====
Some applications (such as bathroom scales) use multiple load cells in concert to get a better reading of a weight on a platform, regardless of positioning. In these applications, it is possible to connect all the load cells together in parallel on a single channel of a [{{SERVER}}/products.php?product_id=DAQ1500 DAQ1500 - Wheatstone Bridge Phidget], and calibrate the entire system at once. However, we only recommend this type of solution in applications that can be quickly re-zeroed (''tared'') like the previously mentioned bathroom scale. For all other applications, we recommend measuring and calibrating each load cell individually.
[[File:S type mounting.jpg|thumb|link=https://wwwdev.int.phidgets.com/docs/images/2/26/S_type_mounting.jpg|<center>A S-Type load cell with a capacity of 500kg</center>]]
These load cells are S-shaped, with two mounting holes located at the top and bottom.


==Load Cell Specification Glossary==
====Use Cases====
When comparing load cells, there are a number of specifications to consider. This section lists and defines the properties commonly listed on load cells.
S-type load cells are primarily used to measure the weight of suspended loads, the strength/durability of wires, cables, ropes, etc., and any other tensile application. They can also be used in compression, though it is less common.


===Capacity===
====Installation====
The maximum load the load cell is designed to measure within its specifications.
S-type load cells are generally used with eye bolts or rod end bearings which provide an easy connection point to the rest of a system.


===Creep===
[[File:S_type_eye_bolt.jpg|center|500px|link=https://wwwdev.int.phidgets.com/docs/images/6/66/S_type_eye_bolt.jpg]]
The change in sensor output occurring over 30 minutes, while under load at or near capacity and with all environmental conditions and other variables remaining constant.  
<center>''A 100kg S-Type load cell with eye bolts attached. ''</center>


===Full Scale or FS===
===Button Load Cells===
Used to qualify error - FULL SCALE is the change in output when the sensor is fully loaded.  If a particular error (for example, Non-Linearity) is expressed as 0.1% F.S., and the sensor's maximum range is 0.1V/V, the maximum non-linearity that will be seen over the operating range of the sensor will be 0.0001V/V.  An important distinction is that this error doesn’t have to only occur at the maximum load.  If you are operating the sensor at a maximum of 10% of capacity, for this example, the non-linearity would still be 0.0001V/V, or 1% of the operating range that you are actually using.
Button load cells, sometimes called ''load buttons'', are low-profile load cells that allow you to measure compressive forces.


===Hysteresis===
====Form Factor====
If a force equal to 50% of capacity is applied to a load cell which has been at no load, a given output will be measured. The same load cell is at full capacity, and some of the force is removed, resulting in the load cell operating at 50% capacity. The difference in output between the two test scenarios is called hysteresis.
[[File:Loadcell_button.gif|thumb|link=https://wwwdev.int.phidgets.com/docs/images/4/4e/Loadcell_button_big.gif|<center>A button load cell with a capacity of 50kg</center>]]
These load cells are small and cylindrical. They have a small button protruding from the top where mechanical force is intended to be focused. They also have mounting holes (generally 3-4) on the bottom.  


===Excitation Voltage===
====Use Cases====
Specifies the voltage that can be applied to the power/ground terminals on the load cell. In practice, if you are using the load cell with a Phidget, you don’t have to worry about this spec.
Button load cells are primarily used for weighing applications where space is limited. Multiple button load cells can be used together to increase weighing capacity.


===Input Impedance===
====Installation====
Determines the power that will be consumed by the load cell.  The lower this number is, the more current will be required, and the more heating will occur when the load cell is powered. In very noisy environments, a lower input impedance will reduce the effect of Electromagnetic interference on long wires between the load cell and Phidget.
The bottom of the load cell can be easily secured to a base using the mounting holes. A top plate can be added, though precise cutouts for the protruding buttons are required.


===Insulation Resistance===
[[File:Button_installed.jpg|center|500px|link=https://wwwdev.int.phidgets.com/docs/images/4/42/Button_installed.jpg]]
The electrical resistance measured between the metal structure of the load cell, and the wiring.  The practical result
<center>''Three 50kg button load cells installed in a weighing platform (top plate not shown).''</center>
of this is the metal structure of the load cells should not be energized with a voltage, particularly higher voltages, as
<br><br>
it can arc into the Phidget.  Commonly the load cell and the metal framework it is part of will be grounded to  
==How Do I Get Data From My Load Cell?==
earth or to your system ground.
The output from a load cell is a very small analog voltage. Amplification is required in order to obtain useful data. Luckily, the load cell interfaces from Phidgets Inc. do this for you, and provide a clean, stable digital value to work with.


===Maximum Overload===
===Selecting a Load Cell Interface===
The maximum load which can be applied without producing a structural failure.
Phidgets Inc. offers two load cells interfaces:
{| style="margin:auto;width:75%;" class="table-no-border"
|-
| [[File:DAQ1500_square.jpg|link=https://wwwdev.int.phidgets.com/docs/images/0/08/DAQ1500_square.jpg|250px]] || [[File:1046_square.jpg|link=https://wwwdev.int.phidgets.com/docs/images/c/ce/1046_square.jpg|250px]]
|}
<br><br>
====Relevant Specifications====
{| style="width: 85%;
|-
! !! [https://www.phidgets.com/?tier=3&catid=64&pcid=57&prodid=957 Wheatstone Bridge Phidget (DAQ1500_0)]!! [https://www.phidgets.com/?tier=3&catid=98&pcid=78&prodid=1027 PhidgetBridge 4-Input (1046_0B)]
|-
| Controlled By || VINT || USB
|-
| Load Cell Inputs|| 2 || 4
|-
| Maximum Sample Rate (One Input Used) || 50Hz|| 122Hz
|-
| Maximum Sample Rate (All Inputs Used)|| 10Hz || 122Hz
|-
| ADC Resolution|| 24 bits - 59.9nV/V || 24 bits - 59.9nV/V
|-
| Avg System Cost || $60USD || $90USD
|}


===Non-Linearity===
===Connecting Your Load Cell to Your Load Cell Interface===
Ideally, the output of the sensor will be perfectly linear, and a simple 2-point calibration will exactly describe the behaviour of the sensor at other loads. In practice, the sensor is not perfect, and Non-linearity describes the maximum deviation from the linear curve.  Theoretically, if a more complex calibration is used, some of the non-linearity can be calibrated out, but this will require a very high accuracy calibration with multiple points.
After selecting a load cell interface, you can easily connect your load cell to it using a 2mm screwdriver.
[[File:1046_side_wires.jpg|center|500px|link=https://wwwdev.int.phidgets.com/docs/images/b/b7/1046_side_wires.jpg]]
<center>''Load cell connected to PhidgetBridge 4-Input</center>
<br><br>
Be sure to check the data sheet for your specific load cell before connecting.
{| style="width:75%;"
|-
! Wire Color !! Function !! Load Cell Interface Port
|-
|-
| Red || VCC || 5V
|-
| Green || + || +
|-
| White || - || -
|-
| Black || GND || G
|}


===Non-Repeatability===
==How Do I Convert My Load Cell Output To Pounds, Grams, Newtons, etc.==
The maximum difference the sensor will report when exactly the same weight is applied, at the same temperature, over multiple test runs.
The output from a load cell is normally reported with units of volts per volt (V/V) or milliVolt per volt (mV/V).


===Operating Temperature===
The extremes of ambient temperature within which the load cell will operate without permanent adverse change to any of its performance characteristics.


===Output Impedance===
[[File:Bridge_controlpanel_1_arr.png|500px|center|link=https://wwwdev.int.phidgets.com/docs/images/4/4d/Bridge_controlpanel_1_arr.png]]
Roughly corresponds to the input impedance. If the Output Impedance is very high, measuring the bridge will distort the results. The Phidget carefully buffers the signals coming from the load cell, so in practice this is not a concern.


===Rated Output===
Is the difference in the output of the sensor between when it is fully loaded to its rated capacity, and when it’s unloaded.  Effectively, it’s how sensitive the sensor is, and corresponds to the gain calculated when calibrating the sensor.  More expensive sensors have an exact rated output based on an individual calibration done at the factory.


===Resolution===
<center>''The raw output from a load cell measured using the [https://www.phidgets.com/docs/Phidget_Control_Panel Phidget Control Panel].''</center>
Resolution is somewhat misleading when it comes to load cells.  People often ask why we don't specify the resolution for our load cells.  The fact of the matter is, resolution isn't really a property of the load cells.  Rather, it is based on the equipment you are using to measure from the load cell instead.  For example, the [{{SERVER}}/products.php?product_id=DAQ1500 DAQ1500] has a 24 bit ADC on board. This means that the resolution of a load cell connected to it is 24 bits.  Since the excitation voltage for the DAQ1500 is 5V, your expected resolution is:


<center>
:<math>\text{Resolution} = \frac{5}{2^{24}}</math>
</center>


This is affected by the chosen gain as well.  In general, noise means you won't be able to achieve this reliably but the point is that resolution is not a useful spec for a load cell to have.
In order to convert the units into something more meaningful like grams, pounds, newtons, etc. a simple calibration is required. For more information, visit our [https://www.phidgets.com/docs/Calibrating_Load_Cells load cell calibration primer].


===Safe Overload===
The maximum axial load which can be applied without producing a permanent shift in performance characteristics beyond those specified.


===Compensated Temperature===
[[File:Bridge_controlpanel_2_arr.png|500px|center|link=https://wwwdev.int.phidgets.com/docs/images/0/0e/Bridge_controlpanel_2_arr.png]]
The range of temperature over which the load cell is compensated to maintain output and zero balance within specified limits.


===Temperature Effect on Span===
Span is also called rated output. This value is the change in output due to a 1-degree Celsius change in ambient temperature.


===Temperature Effect on Zero===
<center>''The calibrated output from a load cell. Here, weight is being measured in grams.''</center>
The change in zero balance due to a 1-degree Celsius change in ambient temperature.
<br><br>


===Zero Balance===
==Load Cells - Related Topics==
Zero Balance defines the maximum difference between the +/- output wires when no load is applied. Realistically, each sensor will be individually calibrated, at least for the output when no load is applied. Zero Balance is more of a concern if the load cell is being interfaced to an amplification circuit - the Phidget can easily handle enormous differences between +/-.  If the difference is very large, the Phidget will not be able to use the higher Gain settings.
* [https://www.phidgets.com/docs/Calibrating_Load_Cells Calibrating Load Cells]

Revision as of 18:50, 17 April 2023

Introduction

Do you need to build a system that can measure weight? Using load cells and load cell interfaces from Phidgets Inc. makes this extremely simple! Watch the video below, or keep reading for more information.




Common types of strain gauge load cells.

What Is a Load Cell?

A load cell is a device that converts mechanical force into a signal that can be measured.


There are different types of load cells, such as pneumatic, hydraulic, capacitive, etc. but strain gauge load cells are the most common.





Strain Gauge Load Cells

Strain gauge load cells use a small resistive sensor called a strain gauge to convert mechanical force into an electrical signal.

There are many different types of strain gauge load cells (some can be seen in the image above). Each type of load cell is designed for a particular application, however, they all have the same basic components:

1 green.png An inflexible frame or body, generally made of aluminum or a steel alloy.
2 green.png One or more strain gauges.
3 green.png Wires for power, ground, and signals.



Singlepoint straingauge exposed.png
Strain gauges are typically covered in a protective coating. Here, the coating has been removed from a single-point load cell to expose the strain gauges underneath.


Strain Gauge Basics

A strain gauge consists of a thin metal wire that is arranged in a specific pattern and attached to a flexible, non-conductive backing material.

Strain gauge single.png
A strain gauge.


As the strain gauge experiences tensile or compressives forces, the metal wire deforms which causes the resistance of the strain gauge to change. This change in resistance causes a voltage change which can be measured. To learn more about strain gauges and how they work, visit our strain gauge primer.

Which Load Cell Should I Use?

There are many different types of strain gauge load cells to choose from. Phidgets Inc. carries the following types:


Type Description Capacity
Single-Point Used for small to medium-capacity weighing scales (e.g. kitchen scale). 50kg Singlepoint illustration.png
S-Type Used to measure tension (e.g. hanging loads), and, less commonly, compressive forces. 500kg Stype illustration.png
Button Used for weighing scales, especially where space is limited. 1000kg Button illustration.png


Single-Point Load Cells

Single-point load cells, sometimes referred to as Platform or Binocular load cells, are easy-to-use load cells that are commonly seen in small weighing scales (e.g. kitchen scale).


Form Factor

A single-point load cell with a capacity of 780g

These load cells are beam shaped and typically have two circular holes machined out of the middle (hence, 'binocular load cells'). These holes encourage bending along the measurement axis. They also have mounting holes (generally 1-2) at both ends for simple installation.

Use Cases

Single point load cells are primarily used to create platform scales, like the Weighing Scale Kit. They are ideal for this task, because they support off-axis loading. This means that users can place their object anywhere on the surface of the scale and the results will still be accurate.

Installation

Single-point load cells typically have an arrow showing which direction the load should be applied. One end of the load cell is secured to the base of the platform, while the other is secured to the top plate.

Singlepoint scale installed.jpg
A 5kg single-point load cell installed in a small weighing platform.

S-Type Load Cells

S-type load cells, sometimes referred to as Z-type or S-beam load cells, are easy-to-use load cells that allow you to measure both tensile and compressive forces.

Form Factor

A S-Type load cell with a capacity of 500kg

These load cells are S-shaped, with two mounting holes located at the top and bottom.

Use Cases

S-type load cells are primarily used to measure the weight of suspended loads, the strength/durability of wires, cables, ropes, etc., and any other tensile application. They can also be used in compression, though it is less common.

Installation

S-type load cells are generally used with eye bolts or rod end bearings which provide an easy connection point to the rest of a system.

S type eye bolt.jpg
A 100kg S-Type load cell with eye bolts attached.

Button Load Cells

Button load cells, sometimes called load buttons, are low-profile load cells that allow you to measure compressive forces.

Form Factor

A button load cell with a capacity of 50kg

These load cells are small and cylindrical. They have a small button protruding from the top where mechanical force is intended to be focused. They also have mounting holes (generally 3-4) on the bottom.

Use Cases

Button load cells are primarily used for weighing applications where space is limited. Multiple button load cells can be used together to increase weighing capacity.

Installation

The bottom of the load cell can be easily secured to a base using the mounting holes. A top plate can be added, though precise cutouts for the protruding buttons are required.

Button installed.jpg
Three 50kg button load cells installed in a weighing platform (top plate not shown).



How Do I Get Data From My Load Cell?

The output from a load cell is a very small analog voltage. Amplification is required in order to obtain useful data. Luckily, the load cell interfaces from Phidgets Inc. do this for you, and provide a clean, stable digital value to work with.

Selecting a Load Cell Interface

Phidgets Inc. offers two load cells interfaces:

DAQ1500 square.jpg 1046 square.jpg



Relevant Specifications

Wheatstone Bridge Phidget (DAQ1500_0) PhidgetBridge 4-Input (1046_0B)
Controlled By VINT USB
Load Cell Inputs 2 4
Maximum Sample Rate (One Input Used) 50Hz 122Hz
Maximum Sample Rate (All Inputs Used) 10Hz 122Hz
ADC Resolution 24 bits - 59.9nV/V 24 bits - 59.9nV/V
Avg System Cost $60USD $90USD

Connecting Your Load Cell to Your Load Cell Interface

After selecting a load cell interface, you can easily connect your load cell to it using a 2mm screwdriver.

1046 side wires.jpg
Load cell connected to PhidgetBridge 4-Input



Be sure to check the data sheet for your specific load cell before connecting.

Wire Color Function Load Cell Interface Port
Red VCC 5V
Green + +
White - -
Black GND G

How Do I Convert My Load Cell Output To Pounds, Grams, Newtons, etc.

The output from a load cell is normally reported with units of volts per volt (V/V) or milliVolt per volt (mV/V).


Bridge controlpanel 1 arr.png


The raw output from a load cell measured using the Phidget Control Panel.


In order to convert the units into something more meaningful like grams, pounds, newtons, etc. a simple calibration is required. For more information, visit our load cell calibration primer.


Bridge controlpanel 2 arr.png


The calibrated output from a load cell. Here, weight is being measured in grams.



Load Cells - Related Topics

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