Force measurement using load cell


Force and weight relationship

The weight of an object is defined as the force of gravity on the object and may be calculated as the mass times the acceleration of gravity, w = m*g. Since the weight is a force, its SI unit is the newton.

For an object in free fall, so that gravity is the only force acting on it, then the expression for weight follows from Newton's second law.

Force measurement using a load cell

A Load cell is a force transducer intended for weight measurement that generates an electrical signal whose magnitude is directly proportional to the force being measured.


Commonly types of load cells

There are four common types of load cells.  

  1. Pneumatic 
  2. Hydraulic
  3. Capacitive
  4. Strain gauge 

Pneumatic load cells: 

Let us now understand how a pneumatic load cell works. Since it is a pneumatic load cell, we know that it will deal with air pressure.

Construction: 

A pneumatic load cell consists of an 

  • elastic diaphragm attached to a platform surface where the weight will be placed

  • An air regulator, to limit the flow of air pressure through the system and 

  • A pressure gauge. 

Working: 

  • When the load is placed on a platform of the pneumatic load cell, the elastic diaphragm deflects.

  • The pneumatic load cell then uses pressurized air or gas to balance out the weight of the object.

  • The air required to balance out the weight will determine how heavy the object weights. 

  • The pressure gauge can convert the air pressure reading into an electrical signal.


Hydraulic load cell:

Next, let’s talk about a hydraulic load cell. In the case of pneumatic load cell where compressed regulated air was used, a hydraulic load cell, on the contrary, will use compressed fluids such as oil or water.


Construction: 

A typical hydraulic load cell consists of:

  • An elastic diaphragm
  • A piston with a loading platform on top of the diaphragm
  • The fluid inside the piston
  • A bourdon tube pressure gauge
Working: 

  • When a load is placed on the loading platform the piston applies pressure to the fluid contained inside it. 
  • The pressure increase of the liquid is proportional to the applied force or weight. 
  • After calibrating the pressure, you can accurately measure the force or weight applied to the hydraulic load cell.

Capacitive load cell:

Capacitive load cells are built on the principle of a change in capacitance when a force is applied to the load cell. Capacitance is the ability of a system to store a charge. If a capacitor is built using the classic parallel plate approach then its ability to store a charge is directly proportional to the area between the two plates and inversely proportional to the gap between the plates.
 

Construction: 

  • The capacitive load cell is made up of two flat plates parallel to each other. 
  • One of the plates is coupled to the platform. 
  • Current is applied to the plates and once the charge is stable, it gets stored between the plates.

 Working:

  • When a force or load or pressure is applied this gap between the plates changes due to the deflection of the housing and results in a disproportionate change in capacitance making capacitive sensors extremely sensitive.

Strain gauge load cell:

The most common and popular style of load cells is a strain gauge. A strain gauge load cell is a transducer that produces a change in electrical resistance when under stress or strain. 


Construction: 

  • A strain gauge load cell is made up of four strain gauges connected in a Wheatstone bridge configuration. 
  • A Wheatstone bridge is an electrical circuit that measures unknown electrical resistance by balancing two legs of the bridge circuit. 
  • One of the legs contains the unknown component. 
  • The Wheatstone bridge circuit provides incredibly accurate measurements. 
  • The strain gauges in the Wheatstone bridge are bonded on to a beam which deforms when weight is applied. 

 Working:

  • The electrical resistance produced by the strain gauge is proportional to the strain or stress placed on the cell, making it easy to calibrate into an accurate measurement. The electrical resistance from the strain gauge is linear and therefore can be converted into a force and weight if needed.

Strain measurement using strain gauge load cell
  • In practice, the strain measurements rarely involve quantities larger than a few mill strain ( ε × 10-3). Therefore, to measure the strain requires accurate measurement of very small changes in resistance. 
  • For example, suppose a test specimen undergoes a substantial strain of 500 με. A strain gauge with a gauge factor GF = 2 will exhibit a change in electrical resistance of only 2×(500 ×10-6) = 0.001Ω = 0.1%. For a 120 Ω gauge, this is a change of only 0.12 Ω.
  • To measure such small changes in resistance, and compensate for the temperature sensitivity, strain gauges are almost always used in a bridge configuration with a voltage or current excitation source. 
  • The general Wheatstone bridge illustrated below consists of four resistive arms with an excitation voltage, Vin, that is applied across the bridge.

The output voltage of the bridge, Vout, will be equal to:

  • Under the balanced condition, the bridge will produce zero output voltage. Any change in resistance in any arm of the bridge will result in a nonzero output voltage.
  • Therefore, if we replace resistance R4 in the above figure with an active strain gauge, any changes in the strain gauge resistance will unbalance the bridge and produce a nonzero output voltage. 
  • If the nominal resistance of the strain gauge is designated as RG, then the strain-induced change in resistance, ∆R, can be expressed as ∆R = RG×GF×ε (strain).
  • Assuming that R1 = R2 and R3 = RG, the bridge equation above can be rewritten to express Vout/Vin as a function of strain as shown below:

  • Alternatively, you can double the sensitivity of the bridge to strain by making both gauges active, although in different directions. 
  • This can be achieved using a half-bridge configuration, whose circuit diagram is also illustrated in the figure below, yields an output voltage that is linear, and approximately twice the output of the quarter-bridge circuit.


  • Finally, you can further increase the sensitivity of the circuit by making all four of the arms of the bridge active strain gauges, and mounting two gauges in tension and two gauges in compression. 
  • The full-bridge circuit is shown in the figure below. The output voltage of the full-wave configuration is linear and directly proportional to the strain. 

  • The equations given here for the Wheatstone bridge circuits assume an initially balanced bridge that generates zero output when no strain is applied. 
  • In practice, however, resistance tolerances and strain induced by gauge application will generate some initial offset voltage. This initial offset voltage is typically handled in two ways. 
  1. First, you can use a special offset-nulling, or balancing, the circuit to adjust the resistance in the bridge to rebalance the bridge to zero output.
  2. Alternatively, you can measure the initial unstrained output of the circuit and compensate it in the software. 

Choosing a load cell for an application

Choosing a load cell for the required application scenario is dependent upon the accuracy and sensitivity requirement of that application.
  • A capacitive load cell is highly accurate and sensitive as compared to other load cells, followed by the most popular style of the load cell, the strain gauge load cell.
  • Pneumatic and hydraulic load cells are less accurate and sensitive as well as costly and complex. Yet they are used in certain applications such as weight measurement of tanks, trucks, or air-planes.


Applications of load cell

Load cells are widely used in day to day life. However, few industrial applications of load cell are: 
  • Weight measurement of tanks, bins and hoppers in hazardous areas
  • Weighing and packaging section in the food industry, pharma industry
  • Scales for weight measurements of trucks, airplanes, etc..
  • Door panel press machines
You can find out a few industrial applications of load cell in the food industry




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