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What is a Load Cell? How do Load Cells Work?
Sensors are devices (usually electro-mechanical) which help us measure a physical parameter (such as temperature, pressure, force, acceleration etc.) by providing a signal that either quantitatively measures (level) that physical parameter or provides a simple binary signal that indicates a yes/no signal that tells us if something occurred or not (such as a touch sensor). Most sensors require power to be provided to a sensing element and an electrical signal is then generated after the measurement.
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 resistive load cells and capacitive 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.
Capacitive load cells work on the principle of change of capacitance which is the ability of a system to hold a certain amount of charge when a voltage is applied to it. For common parallel plate capacitors, the capacitance is directly proportional to the amount of overlap of the plates and the dielectric between the plates and inversely proportional to the gap between the plates.
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.
In this particular load cell shown to the right, there are a total of four strain gauges that are bonded to the upper and lower surfaces of the load cell.
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
Types of Resistive Load Cells
Resistive Load cells come in various shapes and sizes to meet diverse application needs. Here we show you a few common types:
|Single Point Load Cells||Button Load Cells||S-Beam Load Cells||Miniature Load Cells||Through Hole Load Cells||Pancake Load Cells|
|Generally used to build scales and in applications where space is not limited. They offer excellent off-center loading compensation.||Ideal for measuring compression forces that are applied axially. They are compact and easy to use.||Ideal for tension (pull) or Universal (push and pull) force measurement applications||Smallest miniature load cells we offer for compression force measurements only. Ideal for cramped locations.||Rugged, industrial load cells for compression and/or tension force measurements. Has a through hole with threads to attach accessories||High Capacity load cells with capacities up to 100K lbs for compression and/or tension load cell measurements.|
|RAPG:(100 g, 300 g, 500g, 1 kg, 2 kg, 3 kg)||RSB2:(25 kg, 50 kg, 100 kg, 250 kg, 500 kg, 1000 kg)||RAS1:(25 lb, 50 lb, 100 lb, 250 lb, 500 lb, 1000 lb, 2500 lb, 5000 lb, 10K lb, 20K lb, 40K lb)||RSB5:(5 lb, 10 lb, 50 lb)||RSB6:(250 kg, 500 kg, 1000 kg, 2.5K kg, 5K kg)||RAL1:(5K lb, 10K lb, 25K lb, 50K l, 100K lb)|
|RSP1:(3 kg, 5 kg, 10 kg, 20 kg, 50 kg, 150 kg)||RSB1:(2000 kg, 5000 kg)||RES2:(5 kg, 10 kg, 50 kg)||REB5: (2 kg, 10 kg, 100 kg)|
Amplifying load cell signal to get a high level voltage output (0-5 VDC)
The millivolt output is not large enough to be measured accurately by an average voltmeter or DMM. One needs to amplify the signal carefully to obtain a signal large enough to be measured with a voltmeter or to be input into a typical PLC or Micro-controller that has a Analog to Digital Converter.
In order to amplify the signal, we offer an analog load cell signal conditioner called AI-1000.
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 with all its products.
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-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.
Connecting load cell to a PC or Tablet via Wireless Connection
The DI-1000 also offers wireless output via the DI-1000ZP (ZigBEE) or DI-1000WiFi (802.11) 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.
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 remain 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 (240 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).
Complexities of Manufacturing a Resistive Load Cell
The steps involved in manufacturing strain gauge based load cells include:
- Machining of an intricate pattern to optimize stress & strain on the mechanical member
- Controlling surface roughness through surface polishing to ensure rough edges are removed prior to bonding a strain gauge
- Scoring to make alignment marks to align multiple strain gauges that need to be bonded to create the Wheatstone bridge
- Application of a bonding agent with matching temperature coefficients to the metal being used
- Applying even pressure on the strain gauge during the curing process to avoid lumpiness along the bond
- Curing at an elevated temperature in an oven to cure the bonding agent and temper the bond
- Applying a coating to protect the strain gauges and wiring from moisture and humidity and environmental effects
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.As you can see - the resistive load cells are cumbersome to build and require many manual steps during their manufacture.
However, when built properly from a trusted source, resistive load cells built with bonded foil strain gauges, offer accuracies as good as +/- 0.02% of full scale like our RSP1 load cells! And when properly compensated for temperature variations they can be used between -10C to 40C generally with very good resistance to temperature variations. Finally, these load cells offer excellent off-center load compensation and hence are very useful for building scales where the weight to be measured could be placed randomly anywhere on the loading platform.
Capacitive load cells are built on the principal of a change in capacitance when a force is applied on the load cell. Capacitance is the ability of a system to store a charge. 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.
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.
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 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 a small package.
Types of Capacitive Load Sensors Offered
Ideal for controlled room temperature applications ~25C for short durations
|iLoad Mini with Threaded Stud||iLoad Mini with Dome||iLoad Mini Pro with Threaded Stud||iLoad Mini Pro||iLoad Pro||iLoad TR||iLoad Low Profile|
(10 lb, 50 lb, 100 lb, 200 lb)
(10 lb, 50 lb, 100 lb, 200 lb)
|iLoad Mini Pro (1K, 2.5K, 5K, 10K lbs)||iLoad Mini Pro(1K, 2.5K, 5K, 10K lbs)||iLoad Pro (50 lb, 100 lb, 250 lb, 500 lb, 1K, 2.5K, 5K, 10K*)||iLoad TR (10 lb, 50 lb, 100 lb)||iLoad (50 lb, 100 lb, 250 lb, 500 lb)|
|Compression and/or Tension Available||Compression Only||Compression and/or Tension Available||Compression Only||Compression and/or Tension Available||Compression and/or Tension Available||Compression Only|
DQ-1000U single channel, Optional Analog Output via DQ-1000A
DQ-4000U, DS-4000U four channel for USB Output
DQ-4000ZP, DS-4000ZP four channel for Wireless XBee Output
DQ-4000WiFi, DS-4000WiFi four channel for Wireless WiFi Output
DS-4000 offers a LCD screen and Tare, Units button and can serve as a display without need for PC/Tablet
|No need for additional interface|
|Optional Analog Output (0-5 VDC) via HX-100|
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.
|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:
The iLoad Mini Load cell with a DQ-1000A can connect to a PLC or Data Acquisition system as show below:
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 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:
Load Cell Selection Guidelines
When you measure forces or loads, in addition to the capacity and size of the load cell, a number of application requirements need to be carefully considered. These include:
- Operating Temperature Conditions (Well controlled, room temperature or outdoor, harsh environments with changing humidity and temperature)
- Duration of measurement (Short term measurement with a Tare or longer term measurements where Tare is not feasible)
- Absolute Accuracy Required (as a percentage of full scale output or as a percentage of reading)
- Direction of loading (tension or compression or both)
- Certifications Needed (for trade applications needing NTEP or OIML certifications or test and measurement needing ASTM E-74 OR R & D)
- Mounting Options (Fasten sensor on both sides or no need for fastening at all)
- Output required (Analog mV/V, 0-5V, 4-20mA, Digital USB, Digital Wireless, Digital RS-232/485)
- Measurement Speed (1Hz or up to 100Hz or faster than 100Hz)
- Total cost of operation (small quantity or large volumes)
In general, our capacitive load cells are an excellent integrated solution for measuring forces/loads for R & D environments. They offer high sensitivities in a very rugged package with easy mounting options to incorporate into your application. Our sensors offer direct connectivity to PCs and make it effortless to write software applications. These are suitable for a wide variety of general purpose applications where accuracies required are in the range of +/- 0.25% to +/- 1% of full scale loads and will be used at steady state room temperature conditions around 25C such as what is found in temperature controlled labs, schools, R&D facilities etc.
When you need very high accuracies, NTEP/OIML Certifications and need long term stability, for those applications we recommend you use our resistive load cells.