Weight translates to money when it comes to the airline industry. Every additional pound requires more fuel to lift, so making sure there is sufficient gas in the tanks means knowing what the aircraft weighs. Another factor to be considered is weight distribution. Too much at the back and the aircraft flies nose-up, changing the angle of attack and consuming additional fuel.
Load Cells to Weigh Aircraft
Something as large as an aircraft—even a Boeing 747— can be weighed effortlessly with load cells. This article from OMEGA Engineering explains how load cells are employed to weigh extremely large objects.
The Importance of Weight in Transportation
Low Profile Compression Load Cell
Aircraft, from two-seat civilian planes to the biggest freight and passenger aircraft, are weighed commonly for two reasons. Firstly, it’s an FAA requirement that the operator knows the weight of the aircraft, and as weight can change eventually, periodic re-weighing is made necessary. Secondly, a pilot may wish to know the weight of passengers or cargo and luggage taken on-board to determine both the fuel required and the weight distribution (on small turboprop planes it is not uncommon for passengers to be moved to even out weight distribution).
Another big application of load cells is truck weighing. Heavy vehicles cause considerable damage to roadways and particularly bridges, so states have limits on the maximum permissible load that can be carried. Enforcement of these limits is carried out at roadside weigh stations where all trucks are required to stop for weighing.
In addition, trains also need weighing. Similar to roads, excessive loads speed up track wear, and as with aircraft, uneven weight distribution can result in stability issues. Freight moved by rail is sometimes priced on the basis of weight, making it important to know the load in each wagon or rail car.
Load Cells Weighing Technology
For centuries, everything was weighed using scales that used either the extension of a balance or a spring that compared one load with another. In recent years though, for most commercial and industrial uses, these methods have been supplanted by load cells when it is necessary to measure heavy weight.
Load Cell Basics
A load cell is a device that transforms a force (mass multiplied by gravity) to an electrical signal. This is usually done through either the strain gages or piezo-electric effect. Piezo materials are those which output a minute electric signal as they are compressed. While piezocrystals are famous, piezoceramics are other similar materials that do the same.
A strain gage is an electrical device made from a material whose resistance alters with strain, often manifested as deformation. These are employed in load cells designed to deflect in response to a load. Most load cells are exclusively developed with a beam configuration that bends under load, even though some use the expansion in cross-section resulting from axial or longitudinal compression. Generally, these provide a less linear output than the bending configurations, making calibration a concern.
The ‘S’ beam load cell is a typical bending configuration. In profile, it looks like the letter ‘S’ and has four strain gages fitted to the horizontal sections. On applying a load vertically downwards on the top of the ‘S’, the bending puts two of the gages in compression and the other two in tension. Coupling these gages in a Wheatstone bridge arrangement allows the minute changes in resistance to generate a measurable electrical signal.
The load must be applied in the direction of operation for a strain gage load cell to deliver useful measurements. Side loads will lead to inaccurate readings and may damage the device. Piezo sensor systems are more robust in this regard but are less accurate on the whole. Furthermore, the output from many piezo materials is rather temperature-dependent.
Load Cell Selection
The following factors should be considered when evaluating load cells for an application:
- Measuring range
- Safe load limit (the highest load that can be used without causing a permanent shift in readings)
- Ultimate overload (the load that would break the load cell)
- Safe side load (the highest lateral load the load cell can use without causing a permanent shift in readings)
Other possible issues to watch for are: the possibility of off-center loading, shock loading and the need for environmental protection. An instance of shock loading would be when a load is dropped onto the load cell. Impact-absorbing materials can lower the impact of such loads.
Off-center loads will generate misleading results and can break the load cell. Load cells meant for outdoor environments should be specified to meet suitable NEMA and IP standards.
Truck Weighing Systems
The majority of weigh stations employ either strain gage or piezo-based load cells. These are embedded into the road surface and the load produced by each axle measured. A latest innovation is so-called Weigh-in-Motion (WIM) technology, where the truck can be accurately weighed without having to stop it. These systems use a combination of load cells and inductive loops that discover vehicle presence. They are accurate and fast and most importantly, remove the need for each truck to stop to be weighed. This overcomes the problems of traffic backups experienced at busy times, which usually forces the temporary closure of the weigh station.
Requirements for the highway WIM systems are defined in ASTM E1318-02.
Train Weighing Systems
Low Profile Compression Load Cell
Similar to trucks, systems are also available for both WIM and static measurement. These can determine individual bogey loads, axle loads and even the weight of an entire locomotive or wagon. These systems use load cells that have accuracies of ±1% or better.
Aircraft Weighing Systems
Aircraft are weighed with platform scales integrating load cells. Usually, the aircraft is pulled forward in such a way that all the wheels are on platforms. The overall weight is then the sum of the readings from each platform. Differences and distances between platform readings are employed to compute weigh distribution.
Precise and Robust
Periodic weighing is required for trains, aircraft and trucks. This is done with the help of load cells. A load cell uses either piezo materials that generate electrical charge when under pressure or strain gages where a measurable change in resistance occurs as the material is deformed. These signals are amplified and can produce readings accurate to ±1%.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.