Load Cells

Chapter 1: What is a Load Cell?

Load Cell

A load cell is a transducer that converts tensile and compressive force into measurable electrical output. The majority of load cells have a spring element with strain gauges attached and are made of steel or aluminum, which makes them sturdy with little elasticity.

The predominant form of load cell is the strain gauge type. Other types are pneumatic and hydraulic. Pneumatic load cells are used in hazardous conditions, while hydraulic ones are used in locations where there is a limited power supply. In either case, a load cell converts mechanical force into digital values that can be read and recorded.

The inner workings of load cells differ in accordance with how they will be used but are designed to measure mechanical force or the weight of objects. They are depended on for their accuracy and precision with classifications determined by how accurate they are and their accuracy.

• Strain gauge transducers
• Capacitive force transducers
• Piezoelectric transducers
• Vibrating wire transducers
• Magnetic transducers

Chapter 2: The Principle Behind Load Cells

In a load cell, areas, structures, or groups of areas are designed to be stressed when a linear force is applied. The areas of a load cell sense the force applied under pressure and provide an electrical output signal proportional to the force. The output is normally in millivolts and needs amplification in order to be read.

Common Transducers

Transducers come in several forms, with strain gauges being the most common and widely used. Additionally, capacitive, piezoelectric, vibrating, and magnetic transducers are other common ones. Each one works on a different principle to supply its data.

Strain gauge transducers work on the principle of changing electrical resistance as the conductor is being elastically deformed. Tensile stress causes the conductor‘s cross section to be narrower and its length longer, while compressive stress makes it wider and shorter. It converts, transduces, force, pressure, tension, compression, torque, and weight into electrical resistance that can be measured.

These stamp-like patterns of parallel wires are bonded onto the surface of a metal body. As the body experiences stress, the wires deform along with it and a change in resistance can be measured. This change in resistance is proportional to the stress from the applied force.

Capacitive Force Transducers change the capacitance, which is the ability of a system to maintain change when voltage is applied. In parallel plate capacitors, capacitance is proportional to the overlap in the plates and the dielectric between them but inversely proportional to the gap between them.

The basic principle of a capacitive force transducer is to take advantage of a material‘s capacitance or its electric charge storage. The main part of the instrument is the capacitor which can be flat, cylindrical, or spherical. They can also be known as capacitive strain gauges.

These stamp-like patterns of parallel wires are bonded onto the surface of a metal body. As the body experiences stress, the wires deform along with it and a change in resistance can be measured. This change in resistance is proportional to the stress from the applied force.

A simple capacitive force transducer consists of two parallel plates separated by an elastic material that also acts as a dielectric. Applying force across the plates increases the capacitance as the dielectric deforms across the plates. Capacitance depends on the area and distance between the plates.

Piezoelectric transducers, use the piezoelectric effect, which is the ability of certain materials to produce an electric charge under mechanical stress. They are an electroacoustic transducer that converts electrical charges from various types of materials. The term piezoelectric means electricity produced by pressure.

The elements of a piezoelectric transducer begin to oscillate when voltage is applied. They respond in microseconds, which makes them useful for a wide range of applications. They are small but have a wide measuring range, which makes them easy to handle and install. Piezoelectric transducers have a high frequency range that allows them to shift quickly.

Load cells that utilize the piezoelectric effect usually measure forces in one direction only. If a measurement is needed from other directions, more transducers are installed. Although they are self generating, they require a cable to a connection with an electrical interface.

Vibrating wire transducers, use the principle of natural frequency change of a tensioned wire or string. For a given length, mass, and material of a string, the higher the tension, the higher is the frequency. The tension in a string is directly proportional to the square of its frequency of vibration.

In this type of transducer design, a wire or string is used as the force sensor. An electronic oscillator circuit causes the wire to vibrate at its natural frequency. The wire is attached to a diaphragm where pressure is applied. As the pressure changes on the diaphragm, so is the tension experienced by the wire. The change in tension also changes the wire's vibration frequency. which is then measured by a sensing coil. This would then be converted into an electrical signal.

Magnetic Transducer

Magnetic transducers, also known as the "Pressductor" load cells, were developed by ABB. They use magneto-elastic effect or the change in permeability in a magnetic core occurring when a force is applied to the core. When exposed to mechanical force, ferromagnetic elements change the magnetic moments of its "Weiss" domains when pressure is applied, resulting in changes in the magnetic characteristics in the directions in which the mechanical forces act.

This transducer consists basically of a laminated iron core with two perpendicular windings. An alternating current through the primary winding sets up an alternating magnetic field in the core according to the "no load" pattern "no load", where no voltage is induced in the perpendicular secondary winding. When load is applied to the iron core, the change in permeability in the magnetic core causes the magnetic flux lines to change their orientation. The change in orientation will now set up a magnetic flux that passes through the secondary winding. A voltage is now induced in the secondary winding which will then be converted into a readable signal.

Hydrostatic Pressure

Another principle to measure force is hydrostatic pressure. This is different from the previous transducers since it does not need the force to be converted into an electrical signal. A hydrostatic force transducer consists of a working fluid, a piston (or a diaphragm), and a cylinder. The pressure trapped between these components is measured and converted into readable indications such as dial movement via a Bourdon tube gauge. Pneumatic and hydraulic load cells belong in this category. These are usually used in hazardous areas where an electric load cell with a high ingress protection rating or ATEX rating is economically impractical.

Strain gauge load cells will be in focus since these are the most commonly used load cells due to their simplicity and wide range of applications.

Chapter 3: The Wheatstone Bridge

Wheatstone bridge, or resistance bridge, measures unknown resistance by balancing two legs of a bridge circuit with one leg having unknown resistance. It has two known resistors, an unknown resistor; and a variable resistor, which are all connected to form the bridge. The circuit includes a galvanometer and an electromotive force (EMF) source.

The four resistors of a Wheatstone bridge are arranged as a quadrilateral ABCD. The EMF is attached between points A and B while the galvanometer is connected to points C and D. Current that flows through the galvanometer depends on the difference across it. Resistances are chosen such that the galvanometer needle does not deflect, which is called the balanced condition of the bridge.

The variable resistor of a wheatstone bridge is the sensing element or the strain gauge. If the resistance changes in the variable resistor relative to the others, the current will pass through a meter. Basically, the wheatstone bridge converts the change in resistance due to strain into a measurable electrical signal. The purpose is to measure unknown resistance.

The wheatstone bridge configuration for strain gauge load cells has four strain gauges where all four strain gauges deform and change resistance. The strain gauge load cell is balanced at around 350 ohms. It has a regulated excitation voltage with resistance remaining constant when no load is applied. With the application of force, the resistance changes for all four strain gauges and is converted to weight.

Other Wheatstone bridge designs use two variable resistors to improve the system's sensitivity, and to provide an enhanced voltage variation as a function of the changing input. When applied to a force sensor system, the bridge circuit has two fixed resistors and two variable resistors. A direct current (DC) voltage source supplies energy to the circuit. The Wheatstone bridge output is the gap voltage measured at Vout as shown. The gap voltage is proportional to the difference in the transducers‘ resistance values relative to the reference resistance in the bridge configuration.

This design allows for the measurement of very small changes and reduces the effects of noise on the gap voltage. If the input voltage fluctuates, it does not affect the gap voltage since it is related to the ratio of the resistances. Since all resistors are affected, the effect is canceled out regarding the effects of varying temperatures.

Chapter 4: Types of Elastic Element or Spring Material

Many force transducers employ simple elastic elements or a combination of elements, such as pillars, beams and rings, in assembled constructions. Force application causes deformation or deflection on these elastic elements wherein the strain gauge then senses the movement. This is then converted into measurable output by the Wheatstone bridge. The following are the types of elastic elements used for load cells.

• Tension or Compression Type

  In this type, a combination of strain gauges are placed along the side of a cylinder or a straight beam. The strain gauges are oriented either in the transverse or longitudinal direction and are connected to the Wheatstone bridge accordingly. As the column is compressed, longitudinal strain gauges contract while the transverse stretches.

  

  
  

• Roberval-type, Double/Parallel Beam, or Bending Type

  A Roberval mechanism is a scale in which two horizontal beams, one over the other, are attached to a vertical beam at both ends. This configuration is applied to strain gauges load cells having four thin portions two at each of the upper and lower beams. One end of the double beam is fixed in a cantilever manner, while the other end carries the applied load. The strain gauge is attached to the top and bottom surfaces adjacent to the thin portions.

  

  
  

  This configuration is well suited for high-precision load cells since all four arms of the Wheatstone bridge are active, creating higher sensitivity. When a load is applied, this elastic element deflects as shown in the figure above, where two strain gauges are in tension and the other two are in compression.

• Shear Type

  For shearing strained elastic elements, the strain gauges are oriented at 45⁰ angles with respect to the direction of the applied load. In this configuration, it is possible to measure loads accurately independent of where the load is applied. This is because of the cancellation of the bending strain experienced by the strain gauges. Half of each strain gauge will experience some bending strain, while the other half will experience the same magnitude, but in the opposite direction.

  The shearing strained elastic elements can be further classified as:

  • "I"-profile or beam
  • Pin or bolt
  • Hollow disc
  • Z or S-type
  • Helix

  These different profiles of shapes will be discussed further in the next section.

These elastic elements can be constructed from different materials. The most common are alloy steel, aluminum, and stainless steel. Alloy steels are the most widely used because of their cost efficiency. A popular alloy steel used in load cells is AISI 4330, a medium carbon, low-alloy steel consisting of chromium, nickel, and molybdenum. This alloy steel has good hardenability, high transverse strength and toughness.

Aluminum is used in single-point, low capacity applications. The main advantage of using aluminum elastic elements is its cost; aluminum is the cheapest among the three materials mentioned. The common aluminum alloy used is 2023 because of its low creep and hysteresis characteristics.

Stainless steel is the material of choice for wet or corrosive applications. Stainless steels are generally more expensive than alloy steels. The popular stainless steel used for load cell construction is alloy 17-4PH, or AISI 630, which is a martensitic chromium-nickel stainless steel stabilized with niobium. Aside from corrosion resistance, these have high strength, and toughness.

Chapter 5: Different Load Cell Shapes

Load cells may also be classified based on their outer shape. Different shapes have their own specific applications which use one or a combination of elastic elements.

• Canister Type

  The canister load cell is one of the earliest designs. Its elastic element is a single or multiple stretched or compressed column hermetically sealed to protect the strain gauge. Canister load cells can have a capacity from 100lbs to 500,000lbs, depending on the number of columns. They can measure either tension or compression. A limitation of canister load cells is their inability to withstand side loads.

  

  
  

• Bending Beam

  Bending beam load cells, at first glance, may be thought of as being similar to the shear beam. However, the two have different elastic elements. The bending beam does not have a reduced cross-section for the strain gauges, rather this beam is machined all the way through. The strain gauges are bonded as shown in the Roberval-type load cell.

  

  
  

• Single-Ended Shear Beam

  These load cells use the concept of the "I"-profile shear beam. In this design, the elastic element of spring material has a reduced cross section where the strain gauges are bonded. One end of the shear beam contains the mounting holes, while the other end is where the load is applied. These are commonly used in low profile scale applications.

  

  
  

• Double-Ended Shear Beam

  The double-ended shear beam is similar to the single-ended. Instead of being secured only at one end with the load applied to the other end, the double-ended shear beam is secured at both ends with the load applied to the center of the load cell.

  

  
  

• Load Pins or Bolts

  Load measuring pins or bolts are used to measure tension and have rod shaped elements. These are actually short, thick-walled tubes with strain gauges bonded on each side of the center section at 45⁰ angles with respect to the tube axis and have a spring element that applies perpendicular force to the horizontal axis. Load measuring pins or bolts are typically installed into machines in place of normal shafts wherein it acts like a normal piece of the assembly.

  

  
  

• Pancake Type

  These are hollow disc load cells that have a circular array of holes located about half of the radius from the center of the disc. Shear strain gauges are located within these holes at 45⁰ angles with respect to the loading axis. They have a very low profile and can be mounted between components for compression or used in tension. Force is applied to the center of the cell where the beam arms meet.

  

  
  

• Z or S-type

  These load cells have an elastic element deformed by tension or compression at both ends. Its strain gauges are usually configured in an "X"-pattern. These strain gauges are compact, inexpensive, monolithic and easy to install. They are commonly used in tension applications and are found in hanging scales or other suspended weighing applications. S-type load cells can be suspended from shackles or mounted between items using eye bolts at their top and bottom.

  

  
  

• Helix Load Cells

  These are based on the principle of a wire wound spring. The helix works by converting the applied load into a torsional moment in the wire. This configuration is insensitive to off-axis loading due to how the torsional moment propagates through the helix. The orientation of the strain gauge is unimportant.

• Button Load Cells

  The elastic element is usually a bent ring, bent membrane, or a diaphragm depending on the manufacturer‘s design, but it ultimately utilizes the principle of bending beam elastic elements. These load cells have a low profile enabling them to be used in a variety of applications. However, they can only measure compression. A smaller profile of button load cells is called miniature load cell.

  

  
  

  Tension Linkload cells are a versatile form of load cell that can be customized for low, medium, and high load applications. They are used in crane and hoist scales, pull testing, and tensile strength measurement. Tension link load cells have a figure 8 shape so that they can be linked between cables, ropes, or chains. Their loading axis is vertical with the strain gauges placed parallel to each other to measure loading strain.

  

  
  

Chapter 6: Load Cell Air Tightness

The strain gauge is made up of thin wires which are prone to the effects of the environment. Varying temperatures can expand or contract a strain gauge creating noise and inaccurate measurements. Corrosion can also creep into the elastic element, which will shorten the life of the device. Aside from the possible deterioration, the load cell itself may cause safety problems to the environment. For applications in an industrial plant such as a refinery or chemical plant, there is a risk of igniting flammable liquids and gasses. For these reasons, load cells are constructed in either of the following.

• Ingress Protection (IP) Rating:The ingress protection rating is a measures the protection a load cell has against solid objects and liquids. The IP rating consists of two numbers with the first indicating the protection against solid objects while the second number refers to protection against liquids. An IP rating of IP67 means that a device is dust tight and can be immersed. The image of the scale below has the definitions of the first numbers or solid objects on the left and the numbers for liquids on the right. It is part of the International Electrotechnical Commission's standard 60529 and serves as a general rule for all load cell manufacturers.

  

  
  

  Hermetically Sealed

  Hermetically sealed load cells offer the best protection available. This is achieved by welding, epoxy sealing, or glass-to-metal bonding. The inside cavity is filled with a pressurized inert gas. Hermetically sealed load cells are air and water tight characterized by Ingress Protection (IP) rating.

  

  
  

• Open

  These are designed in normal environments in indoor or protected outdoor applications. Environment protection of open load cells is through soft resin or rubber covering. This type of protection makes the strain gauge vulnerable to moisture and temperature fluctuations.

• Explosion proof

  Explosion proof means the load cell will contain or prevent an explosion that may originate within the device. For any device or equipment with internal cavities exposed to flammable gasses, these gasses will eventually creep inside, filling the cavity with an explosive mixture. Sparks from the load cell will ignite this mixture, causing an explosion. An explosion proof rating can be achieved by a combination of containment, energy limitation. and segregation.

Chapter 7: Load Cell Specifications

When designing a system using load cells, it is important to consider the following.

• Rated Capacity or rated load is the maximum capacity or weight that a load cell can measure. When designing a system to measure a certain weight, the load cell rated capacity must be greater than the weight.

• Overload Rating (Safe) is the maximum load that can be applied to a load cell without causing a permanent change in measuring characteristics or performance.

• Overload Rating (Ultimate), on the other hand, is the maximum load which can be applied without causing damage to the load cell.

• Rated Output is the electric output signal per strength of the excitation voltage expressed in mV/V.

• Zero Balance is the electric output signal with rated excitation voltage when no load is applied.

• Excitation Voltage is the voltage supplied to the transducer circuit.

• Non-linearity is the load cell‘s calibration curve deviation from a straight line, starting from zero load up to the cell‘s maximum rated capacity. This is the weighing error over its entire operating range.

• Hysteresis is the difference between two load cell output readings for the same applied load. One reading is obtained by increasing the load from zero, while the other by decreasing the load from the load cell‘s maximum rated capacity.

• Combined Error is the combination of non-linearity and hysteresis.

• Repeatability is the maximum difference between load cell output readings for repeated loads under identical loading conditions.

• Temperature Effect on Rated Output is the deviation in output readings caused by temperature changes.

• Temperature Effect on Zero is the deviation of the zero balance caused by temperature changes.

• Input and Output Resistance is the resistance of the load cell‘s circuit measured at the positive to negative and negative to positive excitation leads, respectively.

• Insulation Resistance is the resistance of the load cell measured between the load cell circuit and the load cell housing.

Chapter 8: Load Cell Applications

All industries that require weighing use load cells due to their compact profile as compared to mechanical scales. The load cell application is not limited to weighing; load cells are also used in automation and structure monitoring. Listed below are the general applications of load cells.

Industrial and Manufacturing Process Weighing

Industrial and manufacturing plants use load cells in measuring quantities of their raw materials and finished products. Moreover, along the process line, there is a need to know how much material is present so process parameters can be adjusted accordingly. Load cells are used on industrial equipment such as hoppers, silos, tanks, conveyors, packaging machines, etc.

Testing and Laboratory

Popular equipment that uses load cells under this category is the universal testing machine. Laboratory scales also use load cells with very high precision. However, other force transducer principles are used other than the strain gauge.

Automation and Process Control

Output signals from load cells can be used as feedback signals by process lines for automation. Load cells are used for automatic packaging, distribution, and sorting.

Weighing Machinery and Scales

Load cells are also used in truck platforms, weighbridges, cranes, railways, etc. Load cells in this category are mainly used for measuring goods loaded onto trucks and containers.

Chapter 9: Load Cell Installation

The best way to ensure the proper performance of a load cell is to follow the manufacturer's installation guide. All manufacturers are available to supply guidance and perspectives regarding the use of their load cells in a project.

Frame and Fixture

The correct frame and fixture are one of the most critical factors related to load cell installation. They maximize stiffness while minimizing weight and costs. The frame should be capable of supporting the measuring device and rigid enough to be able to withstand deformation and flexing.

Vibration Factors

Several factors that can cause vibrations that affect the frame such as compressors, pumps, actuators, and engines. In certain areas, seismic activity can interfere with measurements. Having the frame set on a hard flat surface can significantly reduce the effects of vibrations.

Thermal Expansion

The frame should be designed to allow for thermal expansion and contraction clearance, which can damage the load cell and its frame.

Rotational Movement

The load cell frame should be designed to limit that its rotation and movement. This will prevent damage to the hardware of the frame.

Electrical Interference

When the frame is assembled, it should be done out of the presence of the load cell since stray currents from welding can damage it.

Proper Directionality

All loads and forces should be sent through the load cell. If they are not, the measuring system will not accumulate correct readings. The axial direction is marked on the load cell and is perpendicular to the loading surface.

Conclusion:

• There are several types of force transducer principles that have been developed such as resistive, capacitive, piezoelectric, vibrating wire and magnetic. Among these, resistive transducers are the most common. These transducers are also known as strain gauges.
• The wheatstone bridge is the circuit that can convert the change in resistance of a strain gauge into a measurable electric output.
• Elastic elements or spring material is where the strain gauge is bonded or attached. The elastic element experiences deformation which is then transferred to the strain gauge. These are classified according to the direction of load: tensile or compressive, bending, and shearing.
• There are different shapes and profiles of strain gauges which are characterized by the elastic element within. Common load cell shapes are canister type, bending beam, shear beam, load pins, and so forth.
• Strain gauges consist of thin, stamp-like circuits which are prone to damage and noise. There are several constructions available depending on the application. The most airtight construction is then hermetically sealed load cell.