What is a Sensor?

A sensor is usually an electro-mechanical device whose purpose is to measure a physical property such as a force, torque, pressure or temperature eletronically and send the signal to other electronics, frequently a computer processor.

What is a Load Cell?

A load cell is a sensor or a transducer that converts a load or force acting on it into an electronic signal. This electronic signal can be a voltage change, current change or frequency change depending on the type of load cell and circuitry used. There are many different kinds of load cells. We offer two kinds of load cells: resistive load cells and capacitive load cells.

Resistive Load Cells
Resistive Load Cells

Resistive load cells work on the principle of piezo-resistivity. When a load/force/stress is applied to the sensor, it changes its resistance. This change in resistance leads to a change in output voltage when a input voltage is applied.

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Capacitive Load Cells
Capacitive Load Cells

Capacitive load cells work on the principle of change of capacitance. When a force is applied, the gap or area between two capacitive plates changes its capacitance. This leads to a change in output frequency when a input voltage is applied.

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Resistive Load Cells
How Does a Resistive Load cell Work?

A load cell is made by using an elastic member (with very highly repeatable deflection pattern) to which a number of strain gauges are attached.

How Does a Load Cell Work
How Does a Load Cell Work
How Does a Load Cell Work

In this particular load cell, there are a total of four strain gauges that are bonded to the upper and lower surfaces of the load cell.

How Does a Load Cell Work

When the load is applied to the body of a resistive load cell as shown above, the elastic member, deflects as shown and creates a strain at those locations due to the stress applied. As a result, two of the strain gauges are in compression, whereas the other two are in tension.

Wheatstone Bridge Circuit

The four strain gauges are configured in a Wheatstone Bridge configuration with four separate resistors connected as shown in what is called a Wheatstone Bridge Network. An excitation voltage - usually 10V is applied to one set of corners and the voltage difference is measured between the other two corners. At equilibrium with no applied load, the voltage output is zero or very close to zero when the four resistors are closely matched in value. That is why it is referred to as a balanced bridge circuit.

When the metallic member to which the strain gauges are attached, is stressed by the application of a force, the resulting strain - leads to a change in resistance in one (or more) of the resistors. This change in resistance results in a change in output voltage. This small change in output voltage (usually about 20 mVolt of total change in response to full load) can be measured and digitized after careful amplification of the small milli-volt level signals to a higher amplitude 0-5V or 0-10V signal.

These load cells have been in use for many decades now, and can provide very accurate readings but require many tedious steps during the manufacturing process. Single point load cells and shear beam load cells incorporate multiple strain gauges to compensate for off center loading, temperature compensation and other errors that need to be addressed for commercial grade applications.

Wheatstone Bridge
Type of Resistive Load cells

Resistive Load cells come in various shapes and sizes to meet diverse application needs. Here are a few common types:

Button Load Cell

Button Load Cell

Compression Force or Push Force Measurement

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S-Beam Load Cell

S-Beam Load Cell

Tension Force or Pull Force Measurement

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Through Hole

Through Hole

Tension or Compression Force Measurement

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Pancake Load Cell

Pancake Load Cell

High Capacity, Tension or Compression Force Measurement

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Miniature Load Cell

Miniature Load Cell

Compression Force or Push Force Measurement

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Single Point Load Cell

Single Point Load Cell

Compression or Tension Force Measurement

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NTEP Load Cell

NTEP Load Cell

Load Cells for "For Trade" Applications

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Other Load Cells

Other Load Cells

Find Additional Types of Load Cells for your Applications

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Amplifying Load Cell Signal to Get a High Level Voltage Output (0-5 VDC)

When a voltage is applied to the load cell and a load is applied, at full load the change in voltage is very small. of the order of 10-40 milivolts with 1 mV/V to 4 mV/V sensitivity load cells. This millivolt output change is not big enough to be measured accurately by an average digital voltmeter or data aquisition system. One needs to amplify the signal carefully to obtain a change large enough (-5V to +5V DC, 0-5 V DC or 0-10 V DC) to be measured with a voltmeter or to be input into a typical PLC or Micro-controller that accets and analog input and has an Analog to Digital Converter (ADC). 

In order to amplify the signal, we offer an analog load cell signal conditioner called AI-1000. This amplifies the low level signal of a typical resistive load cell and enables a user to set the output to be between 0.5V to 4.5 VDC using two adjustable potentiometers.

RSB2 with AI-1000 Analog Configuration
Calibrating a Resistive Load Cell After Amplification

Once the output signal is amplified to a known 0.5 to 4.5 VDC signal, then the user can feed it into a PLC or a Data Acquisition System (DAQ) and infer the applied loads. Once the output signal has been amplified, one can calibrate the load cell by applying known loads and calculating a linear equation that best fits the curve.

Loadstar Sensors uses NIST traceable equipment to calibrate sensors and provides a calibration certificate for all its products. Just order a calibration service with your order otherwise the load cells are sold with just nominal values for a load cell without a specific calibration performed before shipping.

RSB2 with AI-1000 Analog Configuration
Connecting Load Cell to a PC or Tablet via USB

In many cases, you might need to not only display the data, but also "capture" the data for further analysis or record keeping needs. In those instances, it would be very convenient to use a PC/Tablet to capture the data. We make it really easy to do that using our DI-100U or DI-1000U USB Load Cell Interfaces.

The DI-100U is a single channgel low cost 16 bit UART/USB load cell interface with a 16 bit ADC and about 40 Hz data output rate. The higher speed verson of it DI100UHS can output data at 250 hz. These devices are good enough to get +/- 0.1% accuracy from load cells that offer that kind of accuracy or better. For four channel, use the integrated four channel version of the DI-100U/DI-100UHS - the DI-400U/DI-400UHS.

RSB2 with AI-1000 Analog Configuration

The DI-1000U has a 24-bit ADC in addition to the regulator and amplifier and supports sensors up to +/- 2 V via a variable gain setting. This interface can be also used with sensors that provide a 0-5 VDC output by using the DI-1000U-5V version.

RSB2 with AI-1000 Analog Configuration
Connecting Load Cell to a PC or Tablet via Wireless Connection

The DI-1000 also offers wireless output via the DI-1000ZP (XBee), DI-1000BLE (Bluetooth Low Energy)  or DI-1000WiFi (802.11) devices and a rechargeable battery that enables cordless operation to a distance of about 1000 feet. The DI-1000ZP comes with an external receiver dongle that you plug into the USB port of a PC. Alternatively, the DI-1000WiFi has a WiFi transmitter that you can use with your Windows Tablet or Notebooks internal WiFi module or a desktop's external WiFi adapter. And the DI-1000BLE works with the BLE protocol on your iOS devices such as an iPhone or iPad.

RSB2 with AI-1000 Analog Configuration
Measuring Fast Changing Forces Using a High Speed Load Cell Interface

There is huge difference between static forces/loads vs. dynamic forces/loads. Most weighing applications are static in nature, where the measured value remains the same after a brief settling period. For such applications most of our standard DI-100U or DI-1000U models with about 40 Hz and 60 hz data update rate are sufficient to gather enough data and average them to get a good reading.

For cases where one needs to measure dynamic forces, one may want to take a look at the DI-100UHS (250 Hz max data update rate) or the DI-1000UHS (500 Hz max data update rate) or the DI-1000UHS-1K (1000 Hz max data update rate) or the DI-1000UHS-10K (with 10 KHz to 50 KHz max data update rate). The faster the data update rate, the better the likelihood of 'catching' the peak force on a sharp spike in values.

RSB2 with AI-1000 Analog Configuration

Complexities of Manufacturing a Resistive Load Cell

Machine a Load Cell

Machining of an intricate pattern to optimize stress & strain on the mechanical member

Machine a Load Cell

Control Surface Roughness

Controlling surface roughness through surface polishing to ensure rough edges are removed prior to bonding a strain gauge

Align Multiple Strain Guages

Scoring to make alignment marks to align multiple strain gauges that need to be bonded to create the Wheatstone bridge

Align Multiple Strain Guages

Application of a Bonding Agent

Applying a bonding agent with matching temperature coefficients to the metal being used

Application of Even Pressure

Applying even pressure on the strain gauge during the curing process to avoid lumpiness along the bond

Application of Even Pressure

Curation at Elevated Temperature

Curing at an elevated temperature in an oven to cure the bonding agent and temper the bond

Application of Protective Coating

Applying a coating to protect the strain gauges and wiring from moisture and humidity and environmental effects

Application of Protective Coating

Testing at Different Temperatures

Once the load cells are built, they need to be tested at different temperatures in order to ascertain their drift with temperature. Thermistors are installed to reduce the temperature effect at zero loads and then at full scale loads prior to final test and calibration.

Cumbersome Building Process

Resistive load cells are cumbersome to build and require many manual steps during their manufacture

Cumbersome Building Process

Capacitive Load Cells
What is a Capacitive Load Cell? How Does a Capacitive Load Cell Work?

Capacitance is the ability of a system to store a charge when a voltage differnce is applied between two conductive plates. If a capacitor is built using the classic parallel plate approach then its ability to store a charge is directly proportional to the area between the two plates and inversely proportional to the gap between the plates. Capacitive load cells work on the principal of a change in capacitance when a force is applied to the load cell. 

Capacitive Load Cell

When a force or load or pressure is applied this gap between the plates changes due to the deflection of the housing and results in a disproportionate change in capacitance making capacitive sensors extremely sensitive.

Capacitive Load Cell

The construction of a capacitive sensor is much simpler than constructing a resistive load cell. You can read more about it by visiting this page that describes "Why Capacitive Sensors". In addition, by optimizing the starting gaps and/or overlap areas of the two plates, once can vary the sensitivity and the output signal to optimize it for various applications.

Capacitive techniques can be used to measure gaps (proximity), humidity, tilt, force, torque, acceleration, fluid quality and many other physical parameters! It is a very versatile parameter that offers tremendous sensitivities in small packages.

Type of Capacitive Load Cells

Capacitive Load cells come in various capacities and sizes to meet diverse application needs. Here are a few common types:

iLoad Mini With Threaded Stud

iLoad Mini With Threaded Stud

Frequency Output
Compression & Tension

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iLoad Mini with Dome

iLoad Mini with Dome

Frequency Output
Compression Only

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iLoad Mini Pro with Threaded Stud

iLoad Mini Pro with Threaded Stud

Frequency Output
Tension or Compression

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iLoad Mini Pro

iLoad Mini Pro

Frequency Output
Compression Force Only

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iLoad Pro

iLoad Pro

UART/USB Output
Compression & Tension

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iLoad TR

iLoad TR

UART/USB Output
Compression & Tension

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iLoad Low Profile

iLoad Low Profile

UART/USB Output
Compression Force Measurement

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Other Load Cells

Other Load Cells

Find Additional Load Cells for Your Applications

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iLoad Mini Configurations

The iLoad Mini Load Cells need a separate interface to work with a PC which converts the frequency output from the load cell into a calibrated value and connects to a PC via USB or can be used to connect to a microcontroller via USART or UART (Serial TTL) protocol.

RSB2 with AI-1000 Analog Configuration

The mini load cell comes in two versions. One has a simple dome shaped top and can be used only for compression applications. The other has a threaded stud and can be used for tension or universal (both tension and compression) applications. They are both meant to be used for quick tests where one can zero out initial value and take a reading under steady state controlled temperature conditions.

RSB2 with AI-1000 Analog Configuration
How to Connect to Arduino

If you wish to input force values from an iLoad mini load cell or any of the iLoad Series capacitive load cells into an Arduino micro controller, you can simply remove the S2U serial TTL to USB adapter and access the Tx, Rx pins via an HX-100 hybrid adapter as shown below. You can then send ASCII commands and receive force data back grom the sensor into the micro-controller.

USBA Dongle
Arduino HX-100
Arduino Wiring

Remove the S2U Serial to USB Adapter from the output side of the cable

Connect the cable to the HX-100 Adapter to access the Tx, Rx pins

Connect the Tx from the load cell/interface to the Rx pin on the Arduino and the Rx from load cell/interface to the Tx pin on the Arduino

Load Sensors Available in Digital and Analog Designs

The iLoad Series Load cells built on our capacitance technology come built in with both analog- and digital outputs! Using a simple hybrid adapter, one can simultaneously read the digital output on a PC while measuring the analog output on a voltmeter or data acquisition system!

This unique hybrid output feature enables developers to monitor the sensor output on a PC while debugging their embedded application which samples the analog output.

iLoad, iLoad Pro or iLoad TR load cells can be used to obtain USB and/or Analog output as shown below:

RSB2 with AI-1000 Analog Configuration

The iLoad Mini Load cell with a DQ-1000A can connect to a PLC or Data Acquisition system as show below:

RSB2 with AI-1000 Analog Configuration

These capacitive load cells are very versatile and offer a quick solution to measure, display, log and plot data on a Windows PC. One can easily connect the sensors/interfaces to a micro-controller like an Arduino, Raspberry Pi, Edison or other popular ones by using a HX-100 adapter which provides access directly to the Tx, Rx pins of the UART.

Multiple Load Cell Configurations

If your application needs multiple load cells to bear the loads under a large platform, you can use a configuration as shown below. The DQ-4000 is a four channel mini load cell interface with USB output or optional wireless (XBee, WiFi, BLE) output. The DS-4000 is the same as the DQ-4000 but has an LCD Display built in to show up to four force values and the sum of all four.

iLoad MIni with DQ-4000 and SensorVUE Software
iLoad Mini with DS-4000 and SensorVUE Software

The main difference between the iLoad Mini/Mini Pro and the iLoad/iLoad TR/iLoad Pro series is that the latter ones have all the electronics built into the sensors themselves, so do not need any interfaces. If the PC has four USB ports you can connect directly to it. If not, you can use a USB Hub to do so. A four load cell iLoad Pro USB Load Cell Kit is shown below:

RSB2 with AI-1000 Analog Configuration

Load Cell Selection Criteria

The above section showed you different types of load cells. Here are some important factors to consider when you pick a load cell for your application.

01

WaterProof Load Cells

If you are going to be using the load cells in a wet environment, then you need to get load cells that are IP60 or higher rated. We offer hermetically sealed IP68 rated waterproof sensors - the RRS3 and RRS4 load cells.
02

NTEP Rated Load Cells

If you are selling something by weight (known as "for trade") then you must use an NTEP rated load cell for your application. We offer several NTEP rated load cells as part of our offering.
03

Operating Temperature Conditions

Well controlled, room temperature or outdoor, harsh environments with changing humidity and temperature
04

Max Range & Accuracy

What is the maximum force or weight you need to measure? How accurately do you need to measure it? Accuracy is specified generally as a % of full scale in the load cell industry.
05

Size Constraints

What are the dimensions of the load cell that can fit into the available space. How are you going to mount and fasten it into your application?
06

Off center loading

If you are building a scale, what is the size of the platform and the base? You must pick a special load cell like the Single Point Load Cell that offers off center loading compensation.
Looking for the best load cell for your application? Allow us to be of help!