Weight sensors, using Load Cells as weighting systems

In the airline industry, weight is money. Every additional pound requires more fuel to lift, so making sure there’s enough gas in the tanks means knowing what the aircraft weighs. Weight distribution is another factor. Too much at the back and the aircraft flies nose-up, altering the angle of attack and consuming more fuel.

Weighing something as large as an aircraft—even a Boeing 747—is not particularly difficult. The way it’s done is using load cells as weight sensors. This White Paper from OMEGA Engineering explains how load cells are used to weigh very large objects.

The designs of the earliest weighing systems were based on the work of Archimedes and Leonardo Da Vinci. They used the positioning of calibrated counterweights on a mechanical lever to balance and thereby determine the size of unknown weights. A variation of this device uses multiple levers, each of a different length and balanced with a single standard weight. Later, calibrated springs replaced standard weights, and improvements in fabrication and materials have made these scales accurate and reliable.

But the introduction of hydraulic and electronic (usually strain gage-based) load cells represented the first major design change in weighing technology. In today's processing plants, electronic load cells are preferred in most applications, although mechanical lever weight sensors are still used if the operation is manual and the operating and maintenance personnel prefer their simplicity.

Mechanical lever scales also are used for a number of applications such as motor truck scales, railroad track scales, hopper scales, tank scales, platform scales and crane scales. The zero and span shifts they experience due to gradual temperature changes can be corrected by manual adjustment or the application of correction factors. Compensation for rapid or uneven temperature changes is much more difficult, and they often cannot be corrected. Because of the accuracy and reliability of well maintained and calibrated mechanical scales, they are used as standards for trade and are acceptable to government authorities.

Spring-balance weight sensors also are simple, and, if made of high-grade alloys (having a modulus of elasticity unaffected by temperature variations), they can be quite accurate if properly calibrated and maintained. They are inexpensive and are best suited for light loads.

The function of any weighing system is to obtain information on gross, net, or bulk weight, or some combination of these. Obtaining the net weight of a vessel's contents requires two measurements: the total weight and the weight of the unloaded container. Net weight is obtained by subtracting one from the other.

Bulk weighing involves the weighing of large quantities. The total weight is often obtained by making incremental measurements and adding up these incremental weights to arrive at the total. This allows a reduction in the size of the weighing system, reducing the cost and sometimes increasing accuracy.

Belts can also be used for bulk weighing. This is a less accurate method, whereby the total bulk weight is obtained by integrating the product of the belt speed and the belt loading over some time period.

Batch weighing systems satisfy the requirements of industrial recipes by accurately dispensing a number of materials into a common receiving vessel for blending or reaction.

A load cell is a device that converts a force (mass multiplied by gravity) to an electrical signal. This is commonly done through either the piezo-electric effect or with strain gauges. Piezo materials are those which output a small electric signal as they are compressed. While piezocrystals are the best known, there other similar materials that do the same, such as piezoceramics.

A strain gauge is an electrical device made from a material whose resistance changes with strain, usually manifested as deformation. These are used in load cells designed to deflect in response to a load. Most load cells are designed with a beam configuration that bends under load, although some use the expansion in cross-section resulting from longitudinal or axial compression. These generally give a less linear output than the bending configurations, making calibration a consideration.

When a load is applied to the center line of a cylindrical load cell, it causes tension, or compression. When applied to a beam, it causes shear, or bending. Beams can be installed in either single-ended or double-ended configurations. Factors in making the decision between the two options include structural and stabilization requirements and the associated considerations of cost, complexity, and maintenance. The selected load cell should always be suitable for the operating environment in terms of its corrosion resistance, electrical safety (intrinsically safe designs are available), hose-down requirements, etc.

The first step in selecting load cells is to determine the total weight to be supported (gross weight). This is the sum of the net weight of the tank contents, the weight of the vessel and attached equipment--including relief valves, instruments, mixers and their motors, ladders, heating jackets and their contents--and any weight that might be imposed on the tank by piping or conduits. If the tare weight of the vessel is excessive compared with the contents, the accuracy of the measurement will be reduced.

Pressurized vessels and vessels with vapor phase heating jackets require additional compensation because the weight of the vapors will vary as temperature and pressure change. Even if the tank contains only air, a 5,000-gallon vessel will gain 45 lbs. if the pressure is increased by one atmosphere at ambient temperature.

When evaluating load cells for an application, consideration should be given to the following:

Other potential issues to watch for are: the possibility of shock loading, off-center loading, and the need for environmental protection. An example of shock loading would be when a load is dropped onto the load cell. Impact-absorbing materials can reduce the impact of such loads. Off-center loads will produce misleading results and can damage the load cell. Load cells intended for outdoor environments should be specified to meet appropriate IP standards.

Aircraft, from two-seat civilian planes to the largest passenger and freight aircraft, are weighed regularly for two reasons. First, it’s a requirement that the operator knows the weight of the aircraft, and as weight can change over time, periodic re-weighing is mandated. And second, a pilot may wish to know the weight of cargo or passengers and luggage taken on-board to determine both the fuel needed and the weight distribution (on small turboprop planes it’s not uncommon for passengers to be moved to even out weight distribution).

Truck weighing is another big application of load cells. Heavy vehicles cause significant damage to roadways and especially bridges, so states have limits on the maximum permissible load that may be carried. Enforcement of these limits is performed at roadside weigh stations where all trucks are required to stop for weighing.

Trains too need weighing. As with roads, excessive loads accelerate 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 essential to know the load in each rail car or wagon.

Most weigh stations use either piezo-based or strain gauge load cells. These are embedded into the road surface and the load created by each axle measured. A recent innovation is so-called Weigh-in-Motion (WIM) technology where the truck can be weighed accurately without needing to stop. These systems use a combination of load cells and inductive loops that detect vehicle presence. They are fast and accurate, and most importantly, eliminate the need for each truck to stop to be weighed. This overcomes the problems of traffic backups experienced at busy times, which often forces the temporary closure of the weigh station.

Requirements for the WIM systems for highway use are defined in ASTM E1318-02.

As with trucks, systems are available for both static and WIM measurement. These can determine individual axle loads, bogey loads and even the weight of an entire wagon or locomotive. Load cells are used in these systems and have accuracies of ±1% or better.

Aircraft are weighed with platform scales incorporating load cells. Typically the aircraft is pulled forward so all the wheels are on platforms. The total weight is then the sum of the readings from each platform. Distances and differences between platform readings are used to compute weigh distribution.

The next step in the design process is the selection of the required structural supports for the tank. Tension support can only be used to weigh small vessels because of the limited weight ranges of tension cells. In tension-type installations, one to four cells are used (usually one), while in compression-type installations usually three or more are used. When accuracy is not critical (0.5% full scale or less) and the tank contains a liquid, costs can be reduced by replacing load cells with dummy cells or with flexure beams. Vertical round tanks are typically supported off three, while four are used for square or horizontal round vessels. It is preferable that all load cells in the system be of the same capacity.

Vessels that are very large, have unbalanced loads, contain hazardous materials, or are at risk of overturning might use more cells. If wind shielding is not provided for the vessel, cell capacity must be increased to also provide for the uplift and downthrust caused by the worst case of wind-induced tipping.

Three cells are best for accurate weighing because three points define a plane and therefore the load will be equalized naturally. Four or more cells require load adjustments. The minimum load cell range (size) is obtained by dividing the gross weight by the number of support points. One usually selects the next standard cell which exceeds the calculated requirement. Some application engineers will add a safety factor of 25% to the gross weight before making the above calculation. Others will also add a dynamic loading factor if, prior to weighing, the load is dropped onto the weight sensor. It also is preferable that all load cells in the system be of the same capacity. The vessel support structure must be rigid and stable, while leaving the tank completely free to move in the vertical. Each weighing system structure should be independent of structures supporting other vessels or vehicular traffic.

The combined deflection of the structure supporting the cells and the structure supported by the cells, when going from unloaded to fully loaded (including vessel wall flexure), should not exceed 1/1,200th of the distance between any two cells. This corresponds to an angle of 0.5*. Some shear beam mounting yokes allow a little more.

Support leg bowing also adds torque to the support beam. Uneven loading due to wind shear, uplift, and download must also be considered in order for the structural design to meet structure performance specifications. A wind shield is essential, if without it any one of the load cells could be totally unloaded. For most cells, wind effect without shields will cause errors under 0.1% full scale.

The support structure should be level to within 1/8 in.; otherwise, shims should be placed under the cell(s) to provide a level loading plane. In both compression and tension applications, the vessel load must be transferred through the load cell to the centerline of the web of the supporting steel. This will prevent twisting of the beams. Gussets should be provided at the support locations.

Weighing system performance is affected by many factors including: temperature, vibration, structural movement, environment, and maintenance. Temperature compensation is usually provided for most systems and its range should always exceed the expected range of ambient and operating temperature variations. When the process vessel is hot (or cold), tank-to-cell temperature isolation pads can be provided.

Temperature compensation adjustments for zero and span are built into most high quality strain gage load cell circuits. For operation outside the typical temperature limits of -4 to 160*F, added correction is needed, or the temperature around the load cell should be controlled. The load cell should also be protected from strong radiant heat, particularly if it reaches only one side of the cell.

In the metal processing industry, load cells must be able to operate continuously at temperatures as high as 500*F. The bonding substances used as backings on strain gages typically limit their application for high temperatures. For high temperature applications, strain sensing wire alloys can be installed with inorganic (ceramic) bonding cement. Alternatively, a flame spray technique can be used, where molten aluminum oxide is sprayed on the strain sensing grid to hold it in place. Such installations can tolerate short-term operations up to 1000*F.

Vibration influences can be minimized by isolating the weighing system supports from structures or concrete foundations that support motors or other vibrating equipment or are affected by nearby vehicular traffic. Vibration absorption pads are available to isolate the load cells from the vibration of the tank, but performance will be best if isolation pads are used at the vibration source. Similarly, weight transmitters can be provided with filtering for the removal of noise caused by vibration, but it is best if vibration does not exist in the first place. During weighing, it is desirable to stop all in-and outflows and to turn off all motors and mixers that are attached to the weighed tank, if at all possible. In agitated vessels, baffle plates should be added to reduce surging and gyration of the contents.

The load cell environment is a dynamic one and therefore requires periodic checking. This should include an attempt to keep the cell(s), cable, and associated junction box clear of debris, ice, or standing water (or other liquids), and shielded from heat, direct sunlight, and wind. Cells should also be protected from lightning and electrical surges. Maintenance should include checking the load cell environment, structures, wiring and junction boxes (for moisture and to tighten terminals), adjustment of stay and check rods, and periodic calibration and checking to make sure that the load is balanced.

Load cells can withstand up to 200% of their capacity in side loads. If a vessel is bumped by a vehicle or is otherwise disturbed, the cells should be checked for damage and be recalibrated. Maintenance related checking should be performed with the vessel both loaded and unloaded, and at all possible vessel/structure temperatures.