Load Cells

Chapter 1: What is a Load Cell and The Principles Behind It?

Load Cell

A load cell is a transducer which converts mechanical energy (tensile and compressive forces) into electrical signals. There are different transducer operating principles that can be utilized to convert forces. The most common are the following:

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

Strain Gauge Transducers

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.

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

Capacitive force transducers take advantage of a material’s capacitance or the amount of electric charge storage. The main part of the instrument is the capacitor which can be configured as flat, cylindrical or spherical. Sometimes, capacitive force transducers may also be regarded as capacitive strain gauges.

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

Piezoelectric transducers, as the name suggests, use the piezoelectric effect. The piezoelectric effect is the ability of certain materials to produce an electric charge under mechanical stress. This electric charge is proportional to the applied force.

Load cells that utilize piezoelectric effect usually measure forces in one direction only. If measurement is needed from other directions, more transducers are installed.

Vibrating Wire Transducers

Vibrating wire transducers, on the other hand, utilizes the principle of the changing natural frequency 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 vibration frequency of the wire which is then measured by a sensing coil. This would then be converted into an electrical signal.

Magnetic Transducer

Last on this list is the magnetic transducer. Also known as the “Pressductor” load cell developed by ABB, this type utilizes the 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 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 its 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 by 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 are 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 2: The Wheatstone Bridge

A Wheatstone bridge is a four-armed bridge of resistors, usually having three fixed resistors and one variable resistor. The variable resistor is the sensing element or the strain gauge. If the resistance changes in the variable resistor relative to the others, current will pass through a meter. Basically, the Wheatstone bridge converts the change in resistance due to strain into a measurable electrical signal.

Other Wheatstone bridge designs use two variable resistors to improve the sensitivity of the system, 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 from the ratio of the resistances. Regarding the effects of varying temperatures, since all resistors are affected, the effect is cancelled out.

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Chapter 3: 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 movement is then sensed by the strain gauge. 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 at 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 which creates 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, aluminium and stainless steel. Alloy steels are the most widely used because of its 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.

Aluminium is used in single-point, low capacity applications. The main advantage of using aluminium elastic elements is its cost; aluminium is the cheapest among the three materials mentioned. Common aluminium 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. 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 4: Different Load Cell Shapes

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

• Canister Type

  The canister load cell is one of the earliest designs of load cells. 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 its inability to normally withstand side loads.

  

  
  

• Bending Beam

  Bending beam load cells, at the first glance, may be thought of 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 described above. 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 centre of the load cell.

  

  
  

• Load Pins or Bolts

  Load measuring pins or bolts are used to measure tension. These are actually short, thick-walled tubes with strain gauges bonded on each side of the centre section at 45⁰ angles with respect to the tube axis. Load measuring pins or bolts are typically installed into machines in place of normal shafts wherein it acts as 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 centre of the disc. Shear strain gauges are located within these holes at 45⁰ angles with respect to the loading axis.

  

  
  

• 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.

  

  
  

• 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.

  

  
  

Chapter 5: 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 gases. For these reasons, load cells are constructed either of the following.

• 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 which is 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 gases, these gases 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 6: 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 which can be applied to a load cell without causing 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 7: Load Cell Applications

All industries that require weighing use load cells due to its compact profile as compared to mechanical scales. Load cell application is not limited to weighing; load cells are used in automation and structure monitoring as well. 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

A 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 Machineries 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.

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 the hermetically sealed load cell.

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