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An introduction to load cells; history, theory & operating principles

Force, Acceleration & Torque

The fundamental operating principles of force, acceleration, and torque instrumentation are closely allied to the piezoelectric and strain gauges devices used to measure static and dynamic pressures. It is often the specifics of configuration and signal processing that determine the measurement output. An accelerometer senses the motion of the surface on which it is mounted and produces an electrical output signal related to that motion. Acceleration is measured in feet per second squared, and the product of the acceleration and the measured mass yields the force. Torque is a twisting force, usually encountered on shafts, bars, pulleys, and similar rotational devices. It is defined as the product of the force and the radius over which it acts. It is expressed in units of weight times length, such as lb.-ft. and N-m.

Force Sensors

The most common dynamic force and acceleration detector is the piezoelectric sensor. The word piezo is of Greek origin, and it means "to squeeze." This is quite appropriate, because a piezoelectric sensor (as shown below) produces a voltage when it is "squeezed" by a force that is proportional to the force applied. The fundamental difference between these devices and static force detection devices such as strain gauges is that the electrical signal generated by the crystal decays rapidly after the application of force. This makes these devices unsuitable for the detection of static force. The high impedance electrical signal generated by the piezoelectric crystal is converted (by an amplifier) to a low impedance signal suitable for such an instrument as a digital storage oscilloscope. Digital storage of the signal is required in order to allow analysis of the signal before it decays. Depending on the application requirements, dynamic force can be measured as either compression, tensile, or torque force. Applications may include the measurement of spring or sliding friction forces, chain tensions, clutch release forces, or peel strengths of laminates, labels, and pull tabs.

A piezoelectric force sensor is almost as rigid as a comparably proportioned piece of solid steel. This stiffness and strength allows these sensors to be directly inserted into machines as part of their structure. Their rigidity provides them with a high natural frequency, and their corresponding rapid rise time makes them ideal for measuring such quick transient forces as those generated by metal-to-metal impacts and by high frequency vibrations. To ensure accurate measurement, the natural frequency of the sensing device must be substantially higher than the frequency to be measured. If the measured frequency approaches the natural frequency of the sensor, measurement errors will result.

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Impact Flowmeters

The impact flowmeter is also a force sensor. It measures the flow rate of free flowing bulk solids at the discharge of a material chute. The chute directs the material flow so that it impinges on a sensing plate (as shown below). The impact force exerted on the plate by the material is proportional to the flow rate. The construction is such that the sensing plate is allowed to move only in the horizontal plane. The impact force is measured by sensing the horizontal deflection of the plate. This deflection is measured by a linear variable differential transformer (LVDT). The voltage output of the LVDT is converted to a pulse frequency modulated signal. This signal is transmitted as the flow signal to the control system. Impact flowmeters can be used as alternatives to weighing systems to measure and control the flow of bulk solids to continuous processes as illustrated below. Here, an impact flowmeter is placed below the material chute downstream of a variable speed screw feeder. The feed rate is set in tons per hour, and the control system regulates the speed of the screw feeder to attain the desired feed rate. The control system uses a PID algorithm to adjust the speed as needed to keep the flow constant. Impact flowmeters can measure the flow rate of some bulk materials at rates from 1 to 800 tons per hour and with repeatability and linearity within 1%.

Impact Flowmeter Application
Impact Flowmeter Application
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Acceleration & Vibration

Early acceleration and vibration sensors were complex mechanical contraptions (as shown below) and were better suited for the laboratory than the plant floor. Modern accelerometers, however, have benefited from the advance of technology, their cost, accuracy, and ease of use all have improved over the years.

Early accelerometers were analogue electronic devices that were later converted into digital electronic and microprocessor-based designs. The air-bag controls of the automobile industry use hybrid micro-electromechanical systems (MEMS). These devices rely on what was once considered a flaw in semiconductor design, a "released layer" or loose piece of circuit material in the microspace above the chip surface. In a digital circuit, this loose layer interferes with the smooth flow of electrons, because it reacts with the surrounding analogue environment.

In a MEMS accelerometer, this loose layer is used as a sensor to measure acceleration. In today's autos, MEMS sensors are used in air bag and chassis control, in side-impact detection and in antilock braking systems. Auto industry acceleration sensors are available for frequencies from 0.1 to 1,500 Hz, with dynamic ranges of 1.5 to 250 G around 1 or 2 axes, and with sensitivities of 7.62 to 1333 mV/G.

Industrial applications for accelerometers include machinery vibration monitoring to diagnose, for example, out-of-balance conditions of rotating parts. An accelerometer-based vibration analyser can detect abnormal vibrations, analyse the vibration signature, and help identify its cause.

Another application is structural testing, where the presence of a structural defect, such as a crack, bad weld, or corrosion can change the vibration signature of a structure. The structure may be the casing of a motor or turbine, a reactor vessel, or a tank. The test is performed by striking the structure with a hammer, exciting the structure with a known forcing function. This generates a vibration pattern that can be recorded, analysed, and compared to a reference signature.

Acceleration sensors also play a role in orientation and direction-finding. In such applications, miniature triaxial sensors detect changes in roll, pitch, and azimuth (angle of horizontal deviation), or X, Y, and Z axes. Such sensors can be used to track drill bits in drilling operations, determine orientation for buoys and sonar systems, serve as compasses, and replace gyroscopes in inertial navigation systems.

Mechanical accelerometers, such as the seismic mass accelerometer, velocity sensor, and the mechanical magnetic switch, detect the force imposed on a mass when acceleration occurs. The mass resists the force of acceleration and thereby causes a deflection or a physical displacement, which can be measured by proximity detectors or strain gauges (as shown below). Many of these sensors are equipped with dampening devices such as springs or magnets to prevent oscillation.

Early Mechanical Vibration Sensor
Early Mechanical Vibration Sensor

A servo accelerometer, for example, measures accelerations from 1 microG to more than 50 G. It uses a rotating mechanism that is intentionally imbalanced in its plane of rotation. When acceleration occurs, it causes an angular movement that can be sensed by a proximity detector.

Among the newer mechanical accelerometer designs is the thermal accelerometer: This sensor detects position through heat transfer. A seismic mass is positioned above a heat source. If the mass moves because of acceleration, the proximity to the heat source changes and the temperature of the mass changes. Polysilicon thermopiles are used to detect changes in temperature.

In capacitance sensing accelerometers, micromachined capacitive plates (CMOS capacitor plates only 60 microns deep) form a mass of about 50 micrograms. As acceleration deforms the plates, a measurable change in capacitance results. But piezoelectric accelerometers are perhaps the most practical devices for measuring shock and vibration. Similar to a mechanical sensor, this device includes a mass that, when accelerated, exerts an inertial force on a piezoelectric crystal.

In high temperature applications where it is difficult to install microelectronics within the sensor, high impedance devices can be used. Here, the leads from the crystal sensor are connected to a high gain amplifier. The output, which is proportional to the force of acceleration, is then read by the high gain amplifier. Where temperature is not excessive, low impedance microelectronics can be embedded in the sensor to detect the voltages generated by the crystals. Both high and low impedance designs can be mechanically connected to the structure's surface, or secured to it by adhesives or magnetic means. These piezoelectric sensors are suited for the measurement of short durations of acceleration only.

Piezoresistive and strain gauge sensors operate in a similar fashion, but strain gauge elements are temperature sensitive and require compensation. They are preferred for low frequency vibration, long-duration shock, and constant acceleration applications. Piezoresistive units are rugged, and can operate at frequencies up to 2,000 Hz.

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Torque Measurement

Torque is measured by either sensing the actual shaft deflection caused by a twisting force, or by detecting the effects of this deflection. The surface of a shaft under torque will experience compression and tension, as shown below. To measure torque, strain gauge elements usually are mounted in pairs on the shaft, one gauge measuring the increase in length (in the direction in which the surface is under tension), the other measuring the decrease in length in the other direction.

Early torque sensors consisted of mechanical structures fitted with strain gauges. Their high cost and low reliability kept them from gaining general industrial acceptance. Modern technology, however, has lowered the cost of making torque measurements, while quality controls on production have increased the need for accurate torque measurement.

Torque on a Rotating Shaft
Torque on a Rotating Shaft
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Torque Applications

Applications for torque sensors include determining the amount of power an engine, motor, turbine, or other rotating device generates or consumes. In the industrial world, ISO 9000 and other quality control specifications are now requiring companies to measure torque during manufacturing, especially when fasteners are applied. Sensors make the required torque measurements automatically on screw and assembly machines, and can be added to hand tools. In both cases, the collected data can be accumulated on dataloggers for quality control and reporting purposes.

Other industrial applications of torque sensors include measuring metal removal rates in machine tools; the calibration of torque tools and sensors; measuring peel forces, friction, and bottle cap torque; testing springs; and making biodynamic measurements.

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Sensor Configurations

Torque can be measured by rotating strain gauges as well as by stationary proximity, magnetostrictive, and magnetoelastic sensors. All are temperature sensitive. Rotary sensors must be mounted on the shaft, which may not always be possible because of space limitations.

A strain gauge can be installed directly on a shaft. Because the shaft is rotating, the torque sensor can be connected to its power source and signal conditioning electronics via a slip ring. The strain gauges also can be connected via a transformer, eliminating the need for high maintenance slip rings. The excitation voltage for the strain gauges is inductively coupled, and the strain gauge's output is converted to a modulated pulse frequency (as shown below). Maximum speed of such an arrangement is 15,000 rpm.

Inductive Coupling of Torque Sensors
Inductive Coupling of Torque Sensors

Strain gauges also can be mounted on stationary support members or on the housing itself. These "reaction" sensors measure the torque that is transferred by the shaft to the restraining elements. The resultant reading is not completely accurate, as it disregards the inertia of the motor.

Strain gauges used for torque measurements include foil, diffused semiconductor, and thin film types. These can be attached directly to the shaft by soldering or adhesives. If the centrifugal forces are not large and an out-of-balance load can be tolerated the associated electronics, including battery, amplifier, and radio frequency transmitter all can be strapped to the shaft.

Proximity and displacement sensors also can detect torque by measuring the angular displacement between a shaft's two ends. By fixing two identical toothed wheels to the shaft at some distance apart, the angular displacement caused by the torque can be measured. Proximity sensors or photocells located at each toothed wheel produce output voltages whose phase difference increases as the torque twists the shaft.

Another approach is to aim a single photocell through both sets of toothed wheels. As torque rises and causes one wheel to overlap the other, the amount of light reaching the photocell is reduced. Displacements caused by torque can also be detected by other optical, inductive, capacitive, and potentiometric sensors. For example, a capacitance-type torque sensor can measure the change in capacitance that occurs when torque causes the gap between two capacitance plates to vary.

The ability of a shaft material to concentrate magnetic flux, magnetic permeability also varies with torque and can be measured using a magnetostrictive sensor. When the shaft has no loading, its permeability is uniform. Under torsion, permeability and the number of flux lines increase in proportion to torque. This type of sensor can be mounted to the side of the shaft using two primary and two secondary windings. Alternatively, it can be arranged with many primary and secondary windings on a ring around the shaft.

A magnetoelastic torque sensor detects changes in permeability by measuring changes in its own magnetic field. One magnetoelastic sensor is constructed as a thin ring of steel tightly coupled to a stainless steel shaft. This assembly acts as a permanent magnet whose magnetic field is proportional to the torque applied to the shaft. The shaft is connected between a drive motor and the driven device, such as a screw machine. A magnetometer converts the generated magnetic field into an electrical output signal that is proportional to the torque being applied.

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Load Cell Designs

Before strain gauge based load cells became the method of choice for industrial weighing applications, mechanical lever scales were widely used. Mechanical scales can weigh everything from pills to railroad cars and can do so accurately and reliably if they are properly calibrated and maintained. The method of operation can involve either the use of a weight balancing mechanism or the detection of the force developed by mechanical levers. The earliest, pre-strain gauges force sensors included hydraulic and pneumatic designs.

In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances. The Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gauges. Although the first bonded resistance wire strain gauge was developed in the 1940s, it was not until modern electronics caught up that the new technology became technically and economically feasible. Since that time, however, strain gauges have proliferated both as mechanical scale components and in stand-alone load cells.

Today, except for certain laboratories where precision mechanical balances are still used, strain gauge load cells dominate the weighing industry. Pneumatic load cells are sometimes used where intrinsic safety and hygiene are desired, and hydraulic load cells are considered in remote locations, as they do not require a power supply. Strain gauge load cells offer accuracies from within 0.03% to 0.25% full scale and are suitable for almost all industrial applications.

In applications not requiring great accuracy, such as in bulk material handling and truck weighing mechanical platform scales are still widely used. However, even in these applications, the forces transmitted by mechanical levers often are detected by load cells because of their inherent compatibility with digital, computer-based instrumentation.

The features and capabilities of the various load cell designs are summarised in the table below.

Load Cell Performance Comparison
Type Weight Range Accurracy (FS) Apps Strength Weakness
 Mechanical Load Cells
Hydraulic Load Cells Up to 5000 tonnes 0.25% Tanks, bins and hoppers, hazardous areas Takes high impacts, insensitive to temperature Expensive, complex
Pneumatic Load Cells Wide High Food industry, hazardous areas Intrinsically safe, contains no fluids Slow response, requires clean, dry air
Bending Beam Load Cells 5 to 2500 Kg 0.03% Tanks, platform scales Low cost, simple construction Strain gauges are exposed, require protection
 Strain Gauge Load Cells
Bending Beam Load Cells 5 to 2500 Kg 0.03% Tanks, platform scales Low cost, simple construction Strain gauges are exposed, require protection
Shear Beam Load Cells 5 to 2500 Kg 0.03% Tanks, platform scales, off centre loads High side load rejection, better sealing and protection  
Canister Load Cells to 250 tonnes 0.05% Truck, tank, track, and hopper scales Handles load movements No horizontal load protection
Ring and Pancake Load Cells to 250 tonnes   Tanks, bins, scales All stainless steel No load movement allowed
Button and washer Load Cells 0 to 25 tonnes / 0 to 100 Kg typical 1% Small scales Small, inexpensive Loads must be centred, no load movement permitted
 Other Load Cells
Helical 0 to 20 tonnes 0.2% Platform, forklift, wheel load, automotive seat weight Handles off-axis loads, overloads, shocks  
Fibre Optic   0.1% Electrical transmission cables, stud or bolt mounts Immune to RFI/EMI and high temps, intrinsically safe  
Piezo-resistive   0.03%   Extremely sensitive, high signal output level High cost, nonlinear output
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Operating Principles

Load cell designs can be distinguished according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.).

Hydraulic load cells are force-balance devices, measuring weight as a change in pressure of the internal filling fluid. In a rolling diaphragm type hydraulic load cell, a load or force acting on a loading head is transferred to a piston that in turn compresses a filling fluid confined within an elastomeric diaphragm chamber. As force increases, the pressure of the hydraulic fluid rises. This pressure can be locally indicated or transmitted for remote indication or control. Output is linear and relatively unaffected by the amount of the filling fluid or by its temperature. If the load cells have been properly installed and calibrated, accuracy can be within 0.25% full scale or better, acceptable for most process weighing applications. Because this sensor has no electric components, it is ideal for use in hazardous areas.

One drawback is that the elastomeric diaphragm limits the maximum force that can be exerted on the piston to about 1,000 psig. All-metal load cells also are available and can accommodate much higher pressures. Special metal diaphragm load cells have been constructed to detect weights up to 5000 tonnes.

Typical hydraulic load cell applications include tank, bin, and hopper weighing. For maximum accuracy, the weight of the tank should be obtained by locating one load cell at each point of support and summing their outputs. As three points define a plane, the ideal number of support points is three. The outputs of the cells can be sent to a hydraulic totaliser that sums the load cell signals and generates an output representing their sum. Electronic totalisers can also be used.

Pneumatic load cells also operate on the force-balance principle. These devices use multiple dampener chambers to provide higher accuracy than can a hydraulic device. In some designs, the first dampener chamber is used as a tare weight chamber. Pneumatic load cells are often used to measure relatively small weights in industries where cleanliness and safety are of prime concern.

The advantages of this type of load cell include their being inherently explosion proof and insensitive to temperature variations. Additionally, they contain no fluids that might contaminate the process if the diaphragm ruptures. Disadvantages include relatively slow speed of response and the need for clean, dry, regulated air or nitrogen.

Strain Gauge load cells convert the load acting on them into electrical signals. The gauges themselves are bonded onto a beam or structural member that deforms when weight is applied. In most cases, four strain gauges are used to obtain maximum sensitivity and temperature compensation. Two of the gauges are usually in tension, and two in compression, and are wired with compensation adjustments (as shown below, left). When weight is applied, the strain changes the electrical resistance of the gauges in proportion to the load.

Wheatstone Circuit with Compensation
Wheatstone Circuit with Compensation
Button style compression load cells
Button style compression load cells
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New Sensor Developments

In the area of new sensor developments, fibre optic load cells are gaining attention because of their immunity to electromagnetic and radio frequency interference (EMI/RFI), suitability for use at elevated temperatures, and intrinsically safe nature. Work continues on the development of optical load sensors. Two techniques are showing promise: measuring the micro-bending loss effect of single-mode optical fibre and measuring forces using the fibre Bragg Grating (FBG) effect. Optical sensors based on both technologies are undergoing field trials in Hokkaido, Japan, where they are being used to measure snow loads on electrical transmission lines.

"S" Beam load cells for compression or tension applications
"S" Beam load cells for compression or tension applications

A few fibre optic load sensors are commercially available. One fibre optic strain gauge can be installed by drilling a 0.5 mm diameter hole into a stud or bolt, and then inserting the strain gauge into it. Such a sensor is completely insensitive to off-axis and torsion loads.

Micromachined silicon load cells have not yet arrived, but their development is underway. At the Universiteit Twente in the Netherlands, work is progressing on a packaged monolithic load cell using micromachining techniques, and it is possible that silicon load cells might dominate the industry in the future.

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Strain Gauge Configurations

The spring elements in a load cell (also called the "beam") can respond to direct stress, bending, or shear. They are usually called by names such as bending beam, shear beam, column, canister, helical, etc. (as shown below). The two most popular designs for industrial weighing applications are the bending beam and the shear beam cells.

Load Cell Spring Elements
Load Cell Spring Elements

The bending beam sensor is one of the most popular load cell designs because of its simplicity and relatively low cost. It consists of a straight beam attached to a base at one end and loaded at the other. Its shape can be that of a cantilever beam, a "binocular" design (as shown above figure (A)) or a "ring" design (as shown above figure (B)). Strain gauges are mounted on the top and bottom to measure tension and compression forces. Because the strain gauges are vulnerable to damage, they are typically covered by a rubber bellows. The beam itself often is made of rugged alloy steel and protected by nickel plating.

In medical instrumentation, robotics, or similar low-load applications, smaller mini-beam sensors are available for measuring loads of up to about 40 pounds (18 kg). For loads up to 230 grams, the beam is made of beryllium copper, and for larger loads stainless steel is used. In this design, strain gauges typically are protected by a urethane coating.

Ring or Pancake designs are round and flat bending beam sensors consisting of bonded foil strain gauges encapsulated in a stainless steel housing. The entire package resembles a flat pancake (as shown above figure (B)). Compression-only sensors can be mounted in a protective, self-aligning assembly that limits load movement and directs the load toward the centre of the pancake. Compression-tension designs have a threaded hole running completely through the centre of the sensor. Stabilising diaphragms are welded to the sensing load button.

Shear beam sensors measure the shear caused by a load. A bending beam sensor cannot measure shear, because shear stresses change across the cross section of the cell. In a shear sensor, the I-beam construction produces a uniform shear that can be accurately measured by strain gauges. A shear beam sensor (as shown above figure (C)) is provided with a pair of strain gauges installed on each side of the I-beam, with grid lines oriented along the principal axes. Advantages of a shear beam sensor over a bending beam include better handling of side loads and dynamic forces, as well as a faster return to zero.

Typical high-capacity canister load cell
Typical high-capacity canister load cell

Direct Stress (or column/canister) load cells are essentially bending beam sensors mounted in a column inside a rugged, round container (as shown above figure (D)). The beam sensor is mounted upright, with two of the four strain gauges mounted in the longitudinal direction. The other two are oriented transversely. The column may be square, circular, or circular with flats machined on the sides to accommodate the strain gauges.

If provided with a rocker assembly or with self-aligning strut bearings, a canister load cell can tolerate a certain amount of tank movement and is relatively insensitive to the point of loading. Also, the canister protects the strain gauges from physical and environmental damage. Canister cells range in size from 38 mm. diameter "studs" with 50-250 Kg. capacity to 165 mm. diameter compression cells suitable for weighing trucks, tanks, and hoppers up to 250,000 kg.

Helical load cells are better able to handle off-axis loading than are canister-type compression cells (as shown above (E)). The operation of a helical load cell is based on that of a spring. A spring balances a load force by its own torsional moment. The torsional reaction travels from the top of the helix to the bottom. By measuring this torsional moment with strain gauges mounted on the spring, a helical load cell can provide reasonably accurate load measurement without the need for expensive mounting structures. Forces caused by asymmetrical or off-axis loading have little effect on the spring, and the strain gauge sensors can measure both tension and compression forces.

A helical load cell can be mounted on rough surfaces, even where the upper and lower surfaces are not parallel, and total error can still remain within 0.5%. The helical load cell also is resistant to shock and overload (it can handle a thousandfold overload), making it ideal for force or load measurements on vehicle axles, seats, or in forklift applications.

Button and flat washer bonded strain gauge load cells are available in sizes from 6 to 38mm. diameter. The smallest sensors are available only in compression styles, but some of the larger cells have threaded holes for also measuring tension. While most of the tiny sensors handle up to about 100 Kg, some are capable of measuring up to 25,000 Kg. Because these little cells have no fixtures or flexures, off-axis loading and shifting loads cannot be tolerated. On the other hand, button and flat washer load cells are extremely convenient and easy to use. Even the smallest sensor is built of stainless steel, has a built in, full four-arm Wheatstone bridge, and can measure up to 100 Kg. at temperatures up to 800°C.

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Load Cell Variations

Subminiature and Miniature Compression Load Cells

Subminiature and Miniature Compression Load Cells

Designed for applications with minimum space and high capacity loads, rugged all stainless steel construction and high performance strain gauges assure superior linearity and stability.

Subminiature and Miniature Tension/Compression Load Cells

Subminiature and Miniature Tension/Compression Load Cells

Designed for the demanding environment of industrial automation and robotics, ideal where the load may go from tension to compression and vice versa.

"S" Type Load Cells

"S" Type Load Cells

Receive tension or compression output readings. Applications include tank weighing, hoppers, suspended loads and truck scales. "S" beam load cells provide superior performance in an compact, versital package. Install using rod ends or load buttons for best performance.

Tension Link Load Cells

Tension Link Load Cells

Ideal for measuring in-line tension forces. Applications vary from cables on cranes, support rods in architectural designs, engine test beds, to industrial weighing applications.

Low Force Sensors

Low Force Sensors

Designed for laboratory force measurement to permit measurement of small forces.

Beam, Shear Beam and Double Beam Load Cells

Beam, Shear Beam and Double Beam Load Cells

Used in multiple load cell applications, tank weighing and industrial process control.

Tank and Bin Weighing Hardware and Summing Accessories

Tank and Bin Weighing Hardware and Summing Accessories

A variety of weighing assemblies, systems and summing boxes.

Single Point and Platform Load Cells

Single Point and Platform Load Cells

Ideal for commercial and industrial weighing systems, they provide accurate readings regardless of the position of the load on the platform.

Load Washers and Through-Hole Load Cells

Load Washers and Through-Hole Load Cells

Extremely low profile and compact design, this load cell is ideal for applications requiring a through-hole design, such as clamping forces, bolt loading forces and other compressive loads.

Canister Style Load Cells

Canister Style Load Cells

Used for single and multi-weighing applications, many feature an all stainless steel design and are hermetically sealed for washdown and wet areas.

Hydrostatically Compensated Load Cells

Hydrostatically Compensated Load Cells

Designed for operation underwater. Applications include dry docks, marine weighing, submerged platforms, and industrial weighing where pit flooding occurs.

Load Bolts

Load Bolts

A unique alternative to conventional load cells, ideally suited for studying joint forces, separations, and design.

Reaction and Rotary Torque Cells

Reaction and Rotary Torque Cells

Designed to be mounted to the torque producing element, to measure the starting torque of motors.

Load Cell Accessories and Options

Load Cell Accessories and Options

Get the most out of your load cells, load buttons, summing boxes and more.

Platform and Industrial Scales

Platform and Industrial Scales

Industrial platform scales in a variety of sizes for heavy weighing applications.

Low Profile Load Cells

Low Profile Load Cells

Compression and tension / compression load cells, used frequently in weighing research and in-line force monitoring.

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Weighing Applications

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 gauge-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 scales 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 scales 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.

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Weighing System Design

When a load is applied to the centre 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 stabilisation 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, 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.

Pressurised vessels and vessels with vapour phase heating jackets require additional compensation because the weight of the vapours will vary as temperature and pressure change. Even if the tank contains only air, a 20,000 litre vessel will gain 25 Kgs. if the pressure is increased by one atmosphere at ambient temperature.

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Performance Considerations

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 gauge load cell circuits. For operation outside the typical temperature limits of -20 to 70°C, 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 260°C. The bonding substances used as backings on strain gauges 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 aluminium oxide is sprayed on the strain sensing grid to hold it in place. Such installations can tolerate short-term operations up to 540°C.

Vibration influences can be minimised 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.

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Vessel Support Structure

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 equalised 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 scale. 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 3 mm; 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 centreline of the web of the supporting steel. This will prevent twisting of the beams. Gussets should be provided at the support locations.

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Vessel Stablisation

To provide unrestricted vertical motion while allowing for horizontal thermal expansion, stay rods and check rods are used. They are made from threaded rods and nuts and serve to provide lateral restraint. Their nuts are adjusted snug to the gusset of the vessel support bracket and to a rigid bracket on the structure. Nuts should be finger tight and then secured with jam nuts (as shown below).

Tank Staying Arrangement
Tank Staying Arrangement

Rods must be level and installed perpendicular to the direction of thermal expansion of the vessel. This allows unrestricted vertical movement without producing a side load. Stay rods should be installed as close as possible to the plane of vessel support. On long, round, horizontal tanks, the axial vessel stay rod connection should be near the centre of the vessel, and lateral restraints should be located near the ends. This helps to avoid large axial thermal expansion.

Check rods are identical to stay rods except that their fit is made loose by providing a 3 mm gap at the nut and oversised rod holes. Check rods may be mounted above or below the support plane or vertically to prevent vessel overturn. On suspended vessels, check rods also serve as back-up hangers.

To determine the required size and location for stabilisation systems, external forces (seismic, agitator, etc.) must be evaluated. The most stable support plane is at the centre of gravity of the tank when it is full. Suspended vessels require check or stay rods only when horizontal vessel movement can be caused by external forces. For minor forces, bumpers may be sufficient.

Thermal expansion of vessels relative to their supports can cause undesirable side loads on the load cells. Some load-cell designs provide for horizontal vessel movement to relieve side loading. Load cell rods suspending a vessel must remain plumb to within 0.5°. Single and double-ended shear beam cell designs can eliminate or minimize the need for stay or check rods (as shown below), while cylindrical cells always require both.

Cantilever Load Cells Reduce Staying Requirements
Cantilever Load Cells Reduce Staying Requirements

In terms of allowing horizontal movement, load cell designs can be "fixed" (allowing no movement), "linear" (allowing linear movement), or "full" (allowing any horizontal tank movement). Fixed and linear cells are mounted in support positions that are farthest apart, with the linear movement being allowed in a line that intersects the fixed cell.

Load cell adaptors are used in vehicle scales where large horizontal forces occur due to the deceleration or acceleration of the vehicles on the scale. The adaptor suspends the weighing platform from the top of the load cell through swivel links connected to the lower plate and the platform. The load cell is supported by a base plate that absorbs heavy side loads when the horizontal deflection exceeds the clearances around the base plate. Similar designs are available for double-ended shear beams (as shown below).

End-Loaded Shear-Beam Installation
End-Loaded Shear-Beam Installation

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Piping Connections

If a pipe is attached to a weighed vessel, it will introduce vertical and horizontal forces. The total vertical force (V) generated by all piping connected to a weighed vessel should be less than 30 times the system accuracy (A) multiplied by the maximum live load (L):


The forces imposed by the pipe supports, the pipe, and the pipe contents, plus the spring forces resulting from pipe movement due to thermal expansion must all be included in V, and in the evaluation of horizontal forces. The horizontal forces acting on the vessel should be zero.

Following are some general rules to assist in obtaining an acceptable design:

    Piping must align with the vessel connection without requiring any force.
    The length of pipe between the vessel and the first pipe support should be long enough to provide vertical flexure, but not so long that the pipe will sag and add weight to the vessel.
    Load cell supports should also support the first two pipe supports.
    The up and down motion of the pipe supports must be limited.
    When possible, use a lighter schedule pipe because it will provide more flexibility. For example, schedule 10S is more flexible than 40S.
    The transmission of horizontal forces should be eliminated by the use of expansion joints and by piping designs having 90° turns in two planes.
    Flexible fittings, universal joints, and hose may only be used when making horizontal connections and must align normal to the tank connection, without force. Braid-jacketed hose should not be used. Flexible rubber boots are acceptable for making vertical connections.
    When a hopper and its hood are independently supported and sealed with a boot, weighing error can occur due to the pressure change caused by in-rushing or out-flowing material. Hood venting (and, therefore, vacuum breaking) is required to eliminate this error source.
    Hose should not be used to make turns.
    Do not use rigid insulation on flexible joints.
    On horizontal round tanks, the best location for the pipe entrance is near the "fixed" load cell.
    The electrical devices on the tank (including load cells) should be wired using flexible conduit that is "looped."

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Installation & Calibration

To check if the transducers and load cells are functioning properly, the following should be evaluated: Does the weight indication return to zero when the system is empty or unloaded? Does the indicated weight double when the weight is doubled? Does the indicated weight remain the same when the location of the load changes (uneven loading)? If the answers are yes, the cells and transducers are probably in good condition and need no attention.

Before calibration, the mechanical system should be examined and the cell installation checked for the following:

Inspect the load cell cables, and coil and protect any excess. The load should be equally distributed among multiple load cells of multiple load cell installations. If they differ by more than 10%, the load should be rebalanced and adjusted with shims.

When calibrating, installing, or removing a cell, the vessel should be lifted without unloading or overloading the other cells. The design of the system should provide for jacking and the horizontal removal of the cell.

Dummy load cells should be used in place of operational ones until all construction and welding are completed.

The calibration of the vessel requires hangers or shelves to support the calibration weights. And these should be added when the vessel is fabricated. Calibration to an accuracy of 0.25% full scale or better is usually performed with dead weights and is the only calibration method recognised by weights and measures agencies. All calibration starts by zeroing the system:

During deadweight calibration, the vessel is evenly loaded to 10% of the live load capacity using standard weights. The weight indication is recorded and the weights are removed. Next, process material is added to the vessel until the weight indicator registers the same (10%) weight as it did with the calibration weights. Now the calibration weights are loaded on the vessel again and the reading (now about 20%) is recorded. These steps are repeated until 100% of capacity is reached.

Live weight calibration is a novel and faster method, which uses pre-weighed people instead of calibration weights. The procedure is identical to deadweight calibration. This method should not be used if there is a risk of injury.

The "material transfer" method of calibration uses some other scale to verify weight. This method is limited by the accuracy of the reference scale and risks some error due to possible loss of material in transfer.

A master cell can also be used for calibration as long as the master is about three times more accurate than the accuracy expected from the calibrated system. The calibration procedure involves incremental loading and the evaluation at each step of the output signals of both the calibrated weighbridge and of the master load cell ( as shown below). The number of divisions used and the method of applying the force (hydraulic or servomotor) is up to the user.

If a load cell system is causing problems, four tests can be conducted:

Mechanical Inspection: Check the load cell for physical damage. If it has been physically deformed, bent, stretched or compressed relative to its original shape, it is not repairable and must be replaced. Look for distortion or cracks on all metal surfaces. Flexure surfaces must be parallel to each other and perpendicular to both end surfaces. Check all cables along their entire length. Nicked or abraded cables can short out a load cell.

Zero Balance (No Load): Shifts in the zero balance are usually caused by residual stress in the sensing area. Residual stresses result from overloading the cell or from repeated operation cycles. With a voltmeter, measure the load cell's output when there is no weight on the cell. It should be within 0.1% of the specified zero output signal. If the output is outside the zero balance tolerance band, the cell is damaged but perhaps correctable.

Bridge Resistance: Measure the resistance across each pair of input and output leads. Compare these readings against the specification of the load cell. Out-of-tolerance readings are usually caused by the failure of one or more elements, typically the result of electrical transients or lightning strikes.

Insulation Resistance: Connect all the input, output, sense and common leads together, and measure the resistance between the load cell body and the leads with an ohmmeter. The reading should be at least 5,000 megaohms. If the load cell fails this test, repeat the test without the common wire. If it still fails, the load cell requires repair. If it passes, the problem may be in the load-cell cable. It is usually the infiltration of moisture that causes short circuiting (current flow) between the load cell's electronics and the cell body.

Master Load Cell for Weighbridge Calibration
Master Load Cell for Weighbridge Calibration
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Specialised Installations

Leg-mounted load cells measure stress changes in the vessel's support structure and can determine tank weights to between 0.1% and 0.5% full-scale accuracy. These cells can be installed on existing tank supports, and several can be mounted or bolted to the legs of a vessel. The legs can be made of I-beams, pipes, concrete-filled pipes, or angle iron.

These load cells are available in single and double-axis designs. Double-axis cells are able to provide perpendicular strain monitoring of thermal or other (interfering) stresses, which can eliminate errors from the primary signal. If single-axis cells are used, a second cell can be installed perpendicular to the first to measure and eliminate errors caused by thermal stresses.

These cells are very temperature sensitive and therefore require sun and wind shielding and insulation. Locating the cell on an I-beam web will minimise temperature error. The base metal of single-axis cells must exactly match the vessel leg material, or errors will be introduced. If dual-axis cells are used, they compensate for material differences and this will not be a concern. The best design is to mount a dual-axis cell at the centre of the I-beam web. The next best is to install two single-axis cells mounted opposite each other on the face of the flange where the flange is joined to the web.

Treadle scales eliminate the complexity of building vehicle scales from individual load cells, weigh- bridges, and stabilisation hardware, and therefore are less expensive (as shown below). A treadle scale is a self-contained unit that can be readily lowered into a shallow pit. In addition to being accurate, directional strain gauges are provided to sense vehicle motion.

Treadle Scale Design
Treadle Scale Design

Monorail weighing transducers measure "live" loads using integrated load cell and flexure assemblies built into a single self-supporting module (as shown below). The strain gauge arrangement in this module detects the correct weight independent of load position. The sloping arrangement on the top of the module decouples the load from the "pusher" during weight measurement and thereby eliminates these forces.

Monorail Weighing Transducer
Monorail Weighing Transducer

Belt weighing systems are used on flat or trough belts. Flat belts are more accurate, but also tend to spill more material. This type of weighing system consists of load cells supporting a set of rollers, including three idler rollers on either side that stabilise and support the belt and its contents as they move over the scale. Delivered weight is determined by integrating the product of weight and belt speed signals.

The weighing system should be located away from the material loading impact and spread area, and on the opposite end from the drive pulley to avoid high belt tension. Belts should be single-ply, flexible, and should track without lateral movement. The belt tension should be maintained by weight-and-pulley to minimise jamming or resistance to movement. Belt tension should be adjusted after monitoring the system's response with more or less tension. A loose belt causes side load error because of belt slap or wrap, while a tight belt can cause the load cell to measure belt tension instead of load. Load cells are widely used in applications requiring precision weighing of solid and liquid materials. Depending on whether the receiver or dispenser is being weighed, these applications are referred to as gain-in-weight or loss-in-weight configurations (as shown below).

Load Cell Configurations for Solids Batching
Load Cell Configurations for Solids Batching

Loss-in-weight scales measure the rate at which the total weight in the dispensing tank changes. They are used to control small mass flow rates into a process. These scales consist of a small load cell system, a differentiating measurement and control system and a variable speed dispenser. Normally, the speed of the dispenser is adjusted to maintain the mass flow rate into the process; during the refill cycle, it is held constant at its last setting.

The scale hopper is weighed by load cells connected via a summing box to a weight transmitter. The control system runs the screw feeder at a high rate of speed (bulk rate) until the total target weight is approached. At that point the control system slows the screw feeder down to a "dribble rate". The screw feeder continues charging at the dribble rate for a short period of time, stopping just before the target weight is attained.

The difference between the target weight and the weight at which the screw feeder is stopped is called the "pre-act" weight. This pre-act difference setting allows the control system to consider the in-flight material that is still falling from the screw feeder into the scale hopper. The pre-act weight can be adjusted either manually or automatically, and its correct setting is critical for high accuracy applications.

In the case of loss-in-weight batching, a feeder is provided with an on-off valve at its inlet and with a variable speed screw feeder at its outlet. The entire feeder, including the inlet hopper and the screw feeder, is mounted on load cells. When the feeder inlet valve is closed, the slope at which the total weight is dropping indicates the continuous discharge from the feeder. This slope is controlled by "loss-in-weight" controls, which calculate the rate at which the total weight is changing. The feed rate is set in pounds per hour, and the control system regulates the speed of the screw feeder to maintain this desired discharge feed rate.

The control system speeds up the screw feeder when the feed rate is below setpoint, and slows it down when it is above setpoint. When the feeder is nearly empty, the control system switches the feeder into its refill mode. In this mode, the inlet valve is opened and it is kept open until the desired full weight is reached.

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References & Further Reading

Omegadyne Pressure, Force, Load, Torque Databook, Omegadyne, Inc., 1996.
The Pressure, Strain, and Force Handbook, Omega Press LLC, 1996.
Industrial Control Handbook, E. A. Parr, Butterworth, 1995.
Instrument Engineers' Handbook, Bela Liptak, CRC Press LLC, 1995.
Instrumentation Reference Book, 2nd Edition, B.E. Noltingk, Butterworth-Heinemann, 1995.
"Overcoming the High Cost of Torque Sensing in Industrial Applications," Darrell Williams, Eaton Corp., 1998.
Process/Industrial Instruments and Controls Handbook, 4th Edition, Douglas M. Considine, McGraw-Hill, 1993.
Sensor and Analyzer Handbook, H.N. Norton, Prentice Hall, 1982.
"Sensors: The Next Wave of Infotech Innovation," Paul Saffo, Institute for the Future,1998.
Van Nostrand's Scientific Encyclopedia, Douglas M. Considine and Glenn D. Considine, Van Nostrand, 1997.
Vibration Analysis for Electronic Equipment, 2nd Edition, Dave S. Steinberg, Wiley, 1988.

Elements of Electronic Instrumentation and Measurements, 3rd Edition, Joseph J. Carr, Prentice Hall, 1996.
Weighing and Force Measurement in the '90s, T. Kemeny, IMEKO TC Series, 1991.

"How to troubleshoot your electronic scale," Brent Yeager, Powder and Bulk Engineering, September, 1995
"Ten rules for installing a belt scale", Steve Becker, Powder and Bulk Engineering, September, 1996.
"Flat-belt weigh feeder accuracy: How to achieve it, maintain it, and verify it", Pete Cadou and Chuck Homer, Powder and Bulk Engineering, September, 1997.
Electronic Weigh Systems Handbook, BLH Electronics, 1986.
Marks' Standard Handbook for Mechanical Engineers, 10th Edition, Eugene A. Avallone, and Theodore Baumeister, McGraw-Hill, 1996.
McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill, 1998.

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