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What Is A Load Cell?Load cell is a generic term applied to a transducer which converts an applied load to an electrical signal output. The majority of today's industrial weighing equipment contains a load cell of one kind or another, this device has revolutionised weighing equipment from the days of mechanical scales with levers, bearings, knife edge pivots and balance weights.
Load cells are available in a range of different shapes, sizes, material type and capacities although they all consist of a carefully machined steel block with four strain gauges arranged in a wheatstone bridge and bonded to the surface of the metal to measure the strain in the load cell. The major different types and their typical applications is detailed below.
Although the load cell has greatly simplified the design of weighing equipment, the laws of physics have not changed and it is as important as ever to design and manufacture the scale structure and load cell mounts correctly.
Mechanical Installation Considerations
Load cells that use strain gauges are sensitive enough to detect very small changes in weight. The trick is to make sure that they react only to the weight you want to measure, not to any other forces. To get accurate weight readings, you must carefully control how and where weight is applied to a load cell. Ideally, it should be installed so that the load is applied vertically throughout the entire weight range.
To attain that ideal weight loading, the weigh platform and load cell support would need to be level, parallel and infinitely rigid. When a scale and its structural support are designed and installed carefully, it is possible for the scale to approach an ideal loading application. When a scale is not installed properly, there are several types of forces that can affect its accuracy. These include angular loading, eccentric loading, side and end loading, and torsional loading.
To put some perspective on what we expect from a load cell and surrounding structures just consider the following example of a typical airport baggage scale with conveyor.
A conveyor scale typically uses four shear beam load cells of 250kg capacity each, making a total load cell capacity of 1,000 kg.
A baggage scale is typically calibrated to 150kg by 0.1kg divisions. This divides the weighing range (span) into 1,500 steps (counts or divisions), it can also be expressed as a resolution of one part in 1,500.
The above scale capacity means we are using only 15% (150/1,000) of the full load cell capacity. That is the "live load" (measuring range) is 15% of load cell capacity.
The maximum deflection of a typical shear beam load cell is 1.0mm at full load. The above example uses 15% of this range, this means our scale capacity of 150kg results in a load cell deflection of only 0.15mm to the load cell.
Since our scale must measure down to one part in 1,500, our system must be capable of accurately measuring a load cell deflection of one fifteen hundredth of 0.15mm. This is 0.0001mm deflection for each 0.1kg reading on the scale!
As you can see from the above example, we are measuring very small deflections, today's weighing components can easily achieve this if they are of the correct quality and are properly installed into a correctly designed structure. Unwanted deflections in the platform or support structures can easily swamp the very measurement we are trying to make.
This example is also typical for any given cargo scale application - the principles are the same.
Since infinite rigidity of a support structure and platform is not possible, consideration must also be given to the method used to apply the measurement force to the load cell. The objective is to minimise any unwanted forces being applied to the load cell and ensure only the vertical component due to the weight of the object being measured is applied to the load cell, through a single load path that is consistent throughout the entire weighing range.
In summary, the three major mechanical factors in determining load cell performance are load cell selection, the surrounding structure and the method of applying the load to the load cell.
Electrical Installation Considerations
We must not only be concerned with the mechanical details but also with the principles of good electrical installation to ensure best performance from our weighing system. If we consider the above example of a baggage scale but in respect to the load cell signal outputs, we can see that equal consideration must be given to electrical installation.
A load cell typically has a full scale signal output of 2 milliVolts for each Volt of excitation applied (from the digital readout). That is if a load equal to the load cell capacity is applied, we get a 2mV output signal for every Volt of excitation we supply to the load cell.
Digital readouts of today supply either 5 Volt or 10 Volts excitation. At 10 Volts excitation the load cell has a signal output of 20 milliVolts (2mV x 10v) at full load.
If our scale installation uses only 15% of the load cell full capacity, this gives a signal output of 3 milliVolts (20mV x15%) at 150kg of load.
Therefore every 0.1kg division on the scale is equal to only 2 microVolts of signal output from the load cells (3mV¸1500 divisions). That is only two millionths of a Volt for the scale to change reading by 0.1kg!
As you can see from above, the scale is working with not only very small deflections but very low signal voltages as well. It is essential that the load cell signal cable that connects the load cells to the digital readout is a quality screened cable and is routed to avoid other electrical cables that may cause interference.
The weighing equipment must be supplied with "clean" electrical power, the correct components must be properly "earthed" and the load cells, cabling and read outs should be kept away from unwanted stray electrical signals (electrical noise).
Common Load Cell Types
Single Point
Shear Beam
Double Ended Shear Beam
Tension Cell
Canister/Compression Cell
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