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CHOOSING LOAD CELLS FOR INDUSTRIAL WEIGHING

The overall performance of any scale or weighing system can only be as good as:

1 The original design and build procedure.

2 The quality and performance of key components.

3 The installation and commissioning procedure.

4 On going maintenance and regular calibration.

The majority of modern weighing systems rely on strain gauge load cells for the conversion of weight or load change into usable electrical output.

Although the development of modern electronics has dramatically outpaced the changes in load cell development it should not be forgotten that the overall performance of any system is still dependent on the primary transducers providing stable and accurate weight data.

Modern electronics can certainly enhance system operation and performance but the basic mechanical design of a weighing system together with the right choice of key components is of paramount importance.

Therefore, the selection of the correct load cell for a particular application is vital and should be the first consideration when designing any weighing system.

But how can design engineers be sure they make the right choice of load cell and what are the key factors that should be considered?

After this, once the correct choice of load cell has been made, what are the main factors for consideration regarding fitting, commissioning and ongoing maintenance?

LOAD CELL CHOICE

The main points to consider are:

• Basic type and mode of operation.
• Number of load cells.
• Capacity.
• Performance or accuracy level -related to system requirements.
• Method of mounting.
• Approval requirements (metrological, safety, environmental)
• Sealing level and material of construction.
• Cost.

There is wide choice of load cell types available and selecting which type of load cell to use may, at first sight, seem a daunting task. However once the size, type and mode of operation of the weighing system is determined then choosing the type of load cell becomes very straightforward.

Essentially load cells operate in two basic modes.

The weighing vessel (or similar) either sits on one or more load cells -compression mode or hangs from one or more load cells -tension mode.

Although tension applications are relatively easy to set up and gravity ensures optimum load introduction, weighing vessel design and safety considerations usually limit the practical load cell capacity range, typically to 5 tonne and below.

Once the mode of operation has been decided, system capacity usually determines which type of load cells should be used.

Overview of load cell types

Bending beams

Bending beam load cells require particular care when mounting to ensure good load introduction and to prevent damage from side or non axial forces. Fully weld-sealed stainless steel low capacity beams down to 5kg are now available offering excellent solutions to low capacity weighing in harsh environments.

Single ended shear beams

Single ended shear beams provide the ideal solution for medium capacity weighing. Their ability to withstand 100% side load without problems makes them particularly suitable for weighing systems with mixers and agitators. However these units become costly and cumbersome to mount above 5 tonnes.

Double ended shear beams

For medium to high capacity applications, the double ended shear beam offers a number of advantages over other designs. Units like the one shown opposite have built-in jacking bolts which can be used to support the weighing structure during installation thus allowing the live load cells to be fitted just prior to commissioning, avoiding damage. The load cell is designed to rock on its mounting base to accommodate misalignment in mounting surfaces whilst still being retained. The mounting arrangement also permits limited movement to allow for thermal expansion and contraction as well as lift off protection.

Canister load cells

Traditional canister load cells have stood the test of time and provide a compact and cost effective solution for many high capacity weighing systems. Some compression cells can be damaged by relatively small side loads and therefore it is advisable to use proprietary mounting hardware to ensure correct load introduction and protection from side loads. Canister load cells can be fully weld sealed and stainless steel construction is now an industry standard.

Bending Ring Load Cells

Bending ring load cells are a relatively new concept in load cell design and provide an excellent low profile solution for a wide range of weighing applications. Unlike standard compression load cells, which can readily suffer from off-axis loading problems, bending ring load cells are loaded through a centrally located annular ring. By using a central floating pin to transmit load to this ring, optimum load introduction is assured and off-axis loads up to three or four degrees can be tolerated without loss in performance. Proprietary mounting assemblies, which offer excellent versatility, are now available for both process weighing applications and platform scales. The compact design of the bending ring readily lends itself to all stainless steel welded construction.

Single point load cells

Although originally designed for platform scales these versatile load cells are now being used in a wide range of industrial applications. Improvements in sealing levels and increased capacity allows these units to be used in the harshest of environments. Single point load cells are designed so that a platform or similar can be fixed directly to the load cell they provide accurate weighing wherever the load is placed. This simplifies scale design and reduces cost. Both bending and shear versions are now available.

Smart load cells

Digital load cells are now becoming more available for industrial weighing in a number of formats. Although they offer certain distinct advantages over analogue devices, their overall performance is still dependent on sound mechanical installation procedures. Some of their advantages are:

• Robust digital output signal improves communications especially over long cable runs.
  
  
• Interfacing with standard bus systems is simplified.

• Higher capacity load cells than normal can be used in areas subject to high overload (wind, seismic).

• Calibration routines can be simplified provided systems are repeatable.



• Load cell replacement in the field under certain circumstances can be accomplished without system re-calibration.

• Built in diagnosis can help speed up fault finding.

PERFORMANCE

This complex subject needs careful consideration when designing a weighing system. Load cell performance parameters can be split into three main groups, namely those that are:

• Time dependent (creep, zero return).



• Environment dependent (temperature effects, humidity etc.).



• Mechanically dependent (inherent load cell design and build, weigh system characteristics).

In practice, these individual parameters cannot be individually isolated and the ultimate system performance will be a function of a combination of these parameters together with other weighing system effects. It can therefore be very misleading to try to calculate weighing system performance from individual load cell data.

When trying to establish expected performance criteria, the engineer should consider how the system will operate and what the worst case operating conditions may be - the widest temperature range, the smallest weight change required to be measured, the worst environmental conditions (flood, tempest, seismic activity) and the maximum overload conditions.

There is also little sense in selecting load cells with the optimum performance if the manner in which they are designed and fitted into the system is substandard.

It is important to remember that the method of calibration will also determine the optimum accuracy which can be achieved. The uncertainty of measurement of the calibration method should be three times better than the required system accuracy. For high capacity weighing systems this may limit the system accuracy to 0.75% at best.

LOAD CELL CAPACITY

There are a number of recognised ways of calculating the required load cell capacity for particular application.

Essentially the load cells must be capable of supporting:

• The weight of the weighing structure (dead load)
• The maximum live load which can be applied (including any static overload)
• Additional overload arising from external factors such as wind loading or seismic activity.
  
  Significant overturning forces can be generated as a result of wind on vessels installed in exposed areas. The forces generated are proportional to the square of the wind speed which can be significantly increased by adjacent buildings, local topography and altitude. Such overturning forces will significantly increase the loading on the load cells and this must be calculated when deciding on the required capacity. Any loading assemblies must also meet necessary safety and integrity requirements. In severe cases, it may be necessary to fit load cells with at least twice the capacity which would be required for the same system fitted indoors.

One important point, often over looked, which has a major impact on overall system performance is "load cell output per unit load change". It is vitally important that the engineer fully understands this when designing any system.

It is very straightforward to calculate the output per unit load change provided the engineer understands how any weigh system will be operated. In other words, consideration should be given to the minimum load change that has to be measured and relate this back to load cell output and the ability of the electronics to discriminate effectively changes of this magnitude.

The key phrase here is 'minimum load change'. Often, for a number of practical reasons, weighing systems have a significantly larger capacity then their actual operating capacity. In these situations the load cells are chosen to accommodate the overall capacity and are then required to provide weight data over a much smaller range. As an example consider a 90 kg vessel supported on four 30 kg load cells which is used to deliver batches of material of 10 kg anywhere over the 90 kg range. If we assume the load cells have 2 mV/V rated output and are connected to electronics supplying 10 volts, then :

Full scale system output (at 90 kg) =90x2x10/4x30 = 15 mV or 15000uV

Actual output over 10 kg range =10x15000/90 = 1667 uV

This is the output that the electronics has to process and the resolution of the weighing system must be related back to this figure.

If the electronics has a minimum requirement of 1V per scale increment, then the

best resolution that could be expected is 10x1000/1667= 6 gm

Practically, the scale increment must be a multiple of 10 (ie 1, 2 or 5, or decimals thereof) and so the best working resolution in this example would be 10 gm giving a working resolution of 1 part in 1000.

It should be noted that this calculation is based on the minimum signal level required by the electronics. Consideration must be given to the performance of the load cells within the working conditions of the system to try to relate this resolution to overall accuracy or uncertainty of measurement, Relating this figure of 10 gm back to the load cells themselves gives a required resolution of :

10/30x4000x4 or 1 part in 12,000

Note that we are talking about resolution here and this should not be confused with load cell or system accuracy.

OVERLOAD

Load cells are usually specified as having a safe overload of 50% of rated capacity This overload capability should be used as a safety valve, never as part of the normal operating range.

Although there are no moving parts within a strain gauge load cell, fatigue can cause failure and it is important to understand the limitations especially in applications where high frequency operations or shock loading is expected.

Failure can occur in the metallic element of the load cell, the bonding of the strain gauge or in the materials of the strain gauge itself.

LOAD INTRODUCTION

The key to reliable and high performance weighing is to ensure optimum load introduction even under adverse conditions. Load cells are designed and tested to measure load through their primary axis. Any irregular loading which introduces off axis forces as a result of poor mounting disciplines will almost certainly introduce unwanted errors and can cause permanent mechanical damage.

The way to minimize these effects is to use proprietary mounting hardware, designed specifically for a particular load cell.

Traditionally, only lower capacity load cells have been available with complete mounting hardware assemblies, relying on engineers to design their own fittings for high capacity systems -a situation which often resulted in cumbersome arrangements and poor weighing performance.

Fortunately weighing companies have recognised these problems and loading assemblies specifically designed for higher capacity systems are now readily available.

Such units usually incorporate both side and lift-off restraints and are designed to accommodate limited vessel movement resulting from thermal expansion and contraction. In parallel they are designed to resist the effects of agitators and mixers whilst still permitting accurate weighing.

OVERLOAD PROTECTION

In applications where loading situations can exceed the rated capacity of the load cells, overload protection should be designed into the system. Load cell deflections are very small and therefore direct mechanical overload stops can be difficult to set up with sufficient accuracy and can also be a cause of problems in dirty industrial environments.

An alternative method is to use `relative motion' overload stops. Here a stiff elastic material or special springs are used between the load cell and the weighing system to provide additional deflection thus allowing easier setting up of the stops. Note that this elastic material should be between the load cell(s) and the weighing structure, not between the load cell(s) and the support structure or ground.

However, any problems arising from this additional deflection must be considered . As well as providing over load stops in the normal direction it is prudent to provide `lift off ' protection in certain applications especially on outdoor systems where wind can be an important factor.

ENVIRONMENTAL CONSIDERATIONS

It is important to ensure that the load cells chosen for a particular application will meet the required sealing levels to prevent premature failure. Consideration should be given to:

• The material of construction
• The method of sealing (welded, potted, open)
• Cable entry integrity

Although IP ratings are used by load cell manufacturers, such ratings do not fully define environmental compatibility. Fully welded stainless steel load cells usually provide the best protection but remember that the stainless steel used is not 316 and can corrode under certain conditions especially if chlorine is present.

Note

If weighing systems are to be installed in designated Hazardous Areas, then expert advice must be sought. Intrinsically safe load cells are readily available which meet CENELEC requirements for use in hazardous areas.

If care is taken in the choice of the right load cell for a particular application, then you are well on the way to achieving a reliable and accurate weighing system. However correct installation and commissioning procedures are vital to ensure a high performance weighing system.