Load Cell Terminology

Ambient Conditions		The conditions (humidity, pressure, temperature, etc.) of the medium surrounding the load cell.
Ambient Temperature		The temperature of the medium surrounding the load cell.
Angular Load Eccentric		A load applied eccentric with the primary axis at the point of application and at some angle with respect to the primary axis.
Angular Load Concentric		A load applied concentric with the Primary axis at the point of application and at some angle with respect to the Primary axis.
Axial Load		A load applied along or parallel to and concentric with the primary axis.
Calibration		The comparison of load cell outputs against standard test loads.
Calibration Curve		A record (graph) of the comparison of the load cell outputs against standard test loads.
Combined Error		(Non linearity and Hysteresis) The maximum deviation from the straight line drawn between original no-load and rated load outputs expressed as percentage of the rated output and measured on both increasing and decreasing loads.
Compensation		The utilization of supplementary devices, materials, or process to minimize known sources of error.
Creep		The change in load cell output occurring with time while under load and with all environmental conditions and other variables remaining constant.
Creep Recovery		The change in no-load output occurring with time after removal of A load which had been applied for a specific period of time. Usually measured over a specific time period immediately following removal of rated load and expressed as a percent of rated output over a specific period of time.
Deflection		The change of length along the primary axis of the load cell between no-load and rated load conditions.
Drift		A random change in output under constant load conditions.
Eccentric Load		Any load applied parallel but not concentric with the primary axis.
Error		The algebraic difference between the indicated and true value of the load being measured.
Excitation, Electrical		The voltage or current applied to the input terminals of the load Cell.
Frequency Response		The range of frequencies over which the load cell output will follow the sinusoidally varying mechanical input within specified Limits.
Hysteresis		The maximum difference between load cell output readings for the same applied load; one reading obtained by increasing the load from zero and the other by decreasing the load from rated output.
Insulation Resistance		The dc resistance measured between the load cell circuit and the load cell structure. Normally measured at fifty volts and under standard test conditions
Load		The weight or force applied to the load cell.
Load Cell		A device which produces an output signal proportional to the applied weight or force.
Natural Frequency		The frequency of free oscillations under no-load load conditions.
Nonlinearity		The maximum deviation of the calibration curve from a straight line drawn between the no-load and rated outputs; expressed as a percentage of the rated output and measured on increasing load only.
Output		The signal (voltage, current, pressure, etc.) produced by the load cell. Where the output is directly proportional to excitation, the signal must be expressed in terms of volts per volt, per ampere, etc., of excitation.
Output, Rated		The algebraic difference between the outputs at no-load an at rated load.
Overload Rating, Safe		The maximum load in percent of rated capacity which can be applied without producing a permanent shift in performance characteristics behond those specified.
Overload rating, Ultimate		The maximum load in percent of rated capacity which can be applied without producing a structural failure.
Primary Axis		The axis along which the load cell is designed to be loaded; normally its geometric centerline.
Rated Capacity (Rated Load)		The maximum axial load the load cell is designed to measure within its specifications.
Reference Standard		A force measuring device whose characteristics are precisely known in relation to a primary standard.
Repeatability		The maximum difference between load cell output readings for repeated loadings under identical loading and environmental conditions.
Resolution		The smallest change in mechanical input which produces a change in the output signal.
Sensitivity		The ratio of the change in output to the mechanical input.
Shunt Calibration		Electrical simulation of load cell output by insertion of known shunt resistors between appropriate points within the circuitry.
Shunt-To- Load Correlation		The difference in output readings obtained through electrically simulated and actual applied loads.
Side Load		Any load acting 90 degrees to the primary axis at the point of axial load application
Stabilization Period		The time required to insure that any further change in the parameter being measured is tolerable.
Standard Test Conditions		The environmental conditions under which measurements should be made when measurements under any other condition may result in disagreement between various observers at different times and places. These conditions are as follows: Temperature 23 degrees +or- 2 degrees C (73.4 degrees +or- 3.6 degrees F
Temperature Effect On Rated Output		The change in rated output due to a change in ambient temperature.
Temperature Range Compensated		The range of temperature over which the load cell is compensated to maintain rated output and zero balance within specific limits.
Temperature Range Safe		The extremes of temperature within which the load cell will operate within permanent adverse change to any of its performance characteristics.
Terminal Resistance Corner To Corner		The resistance of the load cell circuit measured at specific adjacent bridge terminals at standard temperature, with no load applied, and with the excitation and output terminals open-circuited.
Terminal Resistance Input		The resistance of the load cell circuit measured at the excitation terminals at standard temperature, with no load applied and with the output terminals open-circuited.

Content source: transducertechniques.com

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Things to consider before buying a load cell

When your application needs to measure static or quasi-static loads, forces or weights, you would need to use a load cell. A load cell is a device that converts the applied force into an electronic signal that can be used to compute the magnitude of the force applied. Although relatively simple in concept, you’ll find that there are many complexities that might affect measurements when you actually implement your load cell system. One needs to consider a number of factors before you decide which load cell to use. These include:

1. Magnitude of force/load/weight being measured
   • This is important to know because typical load cells offer overload protections of about 1.5 times the rated load capacity. If you apply loads – even temporarily – beyond this maximum load – you can seriously damage your load cell and might need to recalibrate it to ensure accurate readings
   • If you suspect that there could be an occasional spike in loading conditions, be conservative in picking a load cell with adequate capacity to handle the highest load condition with at least 50% margin to avoid exceeding the elastic limit of the load cell
2. Accuracy needed for application
   • Accuracies of load cells are generally rated as a percentage of full scale. So a load cell with 0.1% accuracy rating for a 100 lb load cell would offer +/- 0.1 lb accuracy, but the for a 1000 lb load cell offer a +/- 1 lb accuracy. So you need to pick the right capacity load cell for your application
   • Certain test and measurement applications may need the accuracy as a percentage of reading so you may need to pick a load cell with adequate accuracy to address those applications
   • You also need to consider the accuracy of the complete system – not just the load cell. Depending on how you plan to use the load cell, the accuracy of the system could be a function of the load cell, the signal conditioning equipment, the resolution of the digitizer, the variation of the power being supplied and other environmental factors
3. Operating Temperature Conditions
   • Although most load cells offer certain degree of temperature compensation, you need to consider the operating conditions before you pick a load cell.
   • Is your application in a well controlled, room temperature or outdoor, harsh environments with changing humidity and temperature? If it is outdoor – you should look for load cells with IP68 rating
   • Even with temperature compensated load cells, there could be transient temperature conditions that affect the load cell readings. Generally speaking the compensation works best when used under constant temperature conditions
4. Duration of measurement – Short Term vs. Long Term
   • An important consideration in picking the right load cells is to understand the stability of a load cell under short term vs. long term test conditions
   • Most scales work under the scenario where someone “
   • ” the load cell and then takes a reading. Accuracies are generally very high for these applications especially under room temperature conditions
   • But when temperatures are changing or when a constant load is placed on a load cell for a long time, and one cannot “Tare” the reading periodically there could be drift or Creep in the baseline reading. If your application cannot tolerate this kind of drift then you need to purchase a load cell with good long term stability and low creep
   • Process control applications that cannot be periodically tares and still require very high resolutions and accuracy with temperature over long periods of time – are some of the most demanding applications to outfit with a load cell
5. Data Update Rate
   • How fast do you need the reading to be? 1 Hz, 10 Hz, 100 Hz or faster?
   • This is important to know because load cells are generally speaking "quasi static" devices. That is they are not meant to measure the dynamic forces acting on the sensor - rather just the static loads
   • When you need to measure forces at a very high frequency to study the dynamics of a system - you need to purchase a force sensor with those characteristics
   • The response time of a load cell is generally of the order of 3-5 milliseconds - BUT this does not take in to consideration the mass of the body attached to the load cell. Depending on what is attached to the load cell - the response time could be much slower than what you expect
   • Furthermore, the response time is generally indicated as that of the load cells' analog output - and not that of the complete digitized system. You need to consider the data rate of the data acquisition system, the PC or PLC being used etc. to get very high data rates
6. Direction of loading
   • Load cells generally are designed for measurement in one direction - either in Tension or in Compression
   • The reason for this is that -calibration systems and calibration processes are designed for one direction.
   • If your application needs forces to be measured in both directions i.e. Universal loads - then the accuracy of the system may be lower than it is in just one direction.
   • Some load cells such as the S-Beam load cells are better for such universal applications and if your application permits the special mechanical configuration of the S-Beam load cell - then you should consider using that kind
   • Manufacturers of load cells may charge you an extra fee for calibrating the load cells in both directions
7. Mounting Options
   • This may be one of the most often overlooked aspect of load cells! Designers, engineers or users often pick a load cell and then discover they have to spend hundreds of dollars more to design fixtures or other accessories to actually use it in practice!
   • Pay special attention to the mounting options offered by load cells on the top as well as bottom of the load cell. Threaded holes are easier to work with and are generally lower in profile and less expensive to use than if you have to drill a hole and thread it on your end!
   • Also pay special attention to off-center loading conditions when you design your application. Some load cells such as pancake, button style and other circular load cells are more accurate when used axially along the center of the load cells. When off center loading conditions exist - then certain load cells such as cantilevered single point load cells offer better performance than others.
8. Output Required (Analog or Digital)
   • This is one of the most important factors to consider when picking a load cell since the type of output you want for your application is one of the biggest determining factors in the final cost of your solution
   • The most common output available from conventional load cells is mV/V. When you apply 10V DC power - you get about 20 milliVolt for full scale outputs. This is a low level analog signal which cannot be input into general purpose DAQs or PLCs and requires amplification before it can be used in that mode. Look for load cells with amplified outputs to reduce the headache associated with this step and the additional cost incurred
   • If the distance between the load cell and the DAQ/PLC is large - you may need to use the 4-20 mA type of analog output.
   • Better still - if your application works with a PC or a DAQ/PLC with digital inputs then look for load cells with Serial/RS-232/RS-485/USB or Wireless outputs
9. Total cost of Ownership
   • The cost of the solution depends on many factors:
     • Cost of load cell
     • Cost of signal conditioning equipment needed
     • Cost of digitizers/control systems
     • Cost of Programming or applying calibration to get readings in lb/Kg/N
     • Cost of periodic recalibration
   • You need to consider the total cost of solution before making a decision about which load cell to purchase
   • In addition, there could be significant unit to unit variation in performance and accuracy - so you need to consider the cost of calibrating the system - if you are an OEM designing a system to be used by an average consumer
10. Certifications Needed
    • If your application is going to be used commercially to transact business on the basis of weight, there are rules and regulations that you'll need to follow
    • For example if you are selling coffee beans packaged in 1lb bags, then you need to use NTEP certified load cells for measuring the coffee bags during packaging
    • In Europe and other areas this certification is called "OIML".
    • In general R&D, Education and Test and measurement applications do not require such NTEP/OIML certifications, but there might be industry guidelines and recommendations you need follow

Source: http://www.loadstarsensors.com/
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Types of load cells

Types based on working principle-

• Cantilever/Bending beam
• Compression
• Tensile or In line
• Universal (tensile/compression)
• Shear
• Torque

Types based on design

• Bending beam
• Binocular Beam
• Column (Single column/Multi-column)
• Shear beam (Single ended/Double ended)
  
• S-type
  
• Load button
• Load pin
• Pancake

Types based on output signal

• Analog
  
• Digital

Types based on electrical properties

• Resistive (Analog)
• Piezoelectric (Analog)
• Capacitance (Digital)

Types based on connectivity

• Wired
• Wireless

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Load Cell manufacturers - few international brands

Vishay BLH

Transducer Techniques


Strainsert


Sensortronics


Precision Transducers


Omega


Mettler Toledo


Loadstar Sensors


Interface


Honeywell


HBM


Haris Sensor


Futek


Eilersen


ADI ARTECH


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How does a load cell work?

Load Cell is a device that converts force into electrical signal.

A strain gauge based load cell is made of a specially engineered mechanical component called as an element. Four strain gauges are positioned precisely and bonded on to the element using a special adhesive. (strain gauge is a device that changes resistance when it is deformed or stressed).


The force-electrical signal conversion happens in two stages-

• force applied deforms the element which in turn deforms the strain gauges
• strain gauges converts the deformation to electrical signals

The strain gauges are wired in Wheatstone bridge configuration. The bridge is connected to a power supply unit and a signal conditioner through a 4-core cable. When an input voltage (10VDC) is applied to the bridge, the output is a voltage in the range of few millivolts.


The load cell output is proportional to the load applied to the element.


Basic specifications of a load cell-

• Capacity (kg)
• Full scale Output (mV)
• Sensitivity (mV/V)
• Excitation Voltage (VDC)
  
• Input resistance (ohms)
• Output resistance (ohms)

Below are the commonly used standard values-

• Excitation voltage: 10V or 12V DC
• Full scale output: 10mV, 20mV and 30mV
• Load cell sensitivity: 1mV/V, 2mV/V and 3mV/V
• Input resistance: 350Ω and 700Ω
• Output resistance: 350Ω and 700Ω

Sensitivity = full scale output/excitation voltage
Example: 20mV/10V = 2mV/V

Two articles to you can read-
Introduction to Load Cells
How Does a Load Cell Work?

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How does a Digital Weigh Scale work?

Imagine you pick an apple and try to guess it's weight. You would place it in your palm and toss it lightly to feel the weight. After 3 or 4 tosses you would say "200 grams". Your hand sensed the weight and sent a signal to your brain which after some calculations estimated the weight. Right? Similarly, when you place the same apple on a digital weigh scale, the load cell senses the weight and sends the signal to the indicator. The signal conditioner processes the load cell signal and displays weight.

Basically all weigh scales consist of-

• a structure
• one or more load cells
• signal conditioner

Structure is the load bearing part. It transfers the weight to the load cell(s). In low capacity scales (table-top/bench/platform) the structure houses the load cell and electronic components. In higher capacity scales (floor/dormant/weigh bridge) the structure rests on load cells. The weight indicator

Load Cell is basically a transducer which converts force into electrical signal. Every digital weigh scale has one or more load cells. Table-top/Bench/Platform scales are single load cell weighing machines. Floor/Dormant scales have four load cells. Truck scales or Weigh bridges have 4 to 8 load cells depending on the rated capacity and length of the bridge. For all scales load cell is the part which determines the scales rated capacity. Of course the mechanical structure is designed to withstand loads well beyond the rated capacity. Load cells capacities range from 500g to 100T.

Signal Conditioner is the electronics part, it powers the load cell, receives signal from the load cell(s) and converts that signal to readable numbers. The numbers- digital output- can be made to display in desired unit of weight. Signal conditioners have extended features like RS232 or 4-20MA output which can be used for data acquisition or control other electrical equipment. An electronic weigh scale hooked up to a computer can automate many aspects of business operations like materials management or process control or billing automation.

Compare a mechanical scale to an electronic scale. Mechanical scale just shows the weight but an electronic scale does much more than that- it helps automate.

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Introduction

I'm an Industrial Engineer who worked for a load cells & electronic weighing systems manufacturing industry. The objective of this blog is to explain load cells and electronic weighing systems in layman terms. I do not claim to be an expert, there's still so much to learn. I grew up seeing load cells & electronic weighing machines and then worked for almost a decade & half manufacturing and building weighing systems for different industries and applications.

In my opinion, weighing is the best method of measuring the quantity of matter in any form- solid, liquid or gaseous. Liquids can be measured by volume but volume changes with temperature. Some material's length change with temperature. But nothing can affect mass or weight.

Mass and weight are not same.

Mass is how heavy an object is without gravity or how much matter an object has. SI unit for mass is kilograms.

Weight is the force of gravity on the object or the product of mass and acceleration of gravity.

Weight = mass x gravity
SI unit for weight is Newton.

1 kilogram (mass) weighs 9.8 Newtons under standard conditions on the Earth's surface. However, kilogram is the most commonly used unit for weight for commercial purposes.

1000 grams = 1 Kilogram
100 kilograms = 1 Quintal
1000 kilograms = 1 Tonne

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