Types of load cell and principle

load cell instrument

1. LOAD CELL Mostafa Ragab

2. LOAD CELL: Definition: A load cell is a type of transducer , specifically a force transducer. It converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. As the force applied to the load cell increases, the electrical signal changes proportionally. The most common types of load cell used are hydraulic, pneumatic, and strain gauge . Mostafa Ragab

3. Types of load cells Strain gauge load cells Capacitive load cells Vibrating wire load cells Piezoelectric load cells Hydraulic load cells Pneumatic load cells Mostafa Ragab

4. Resistive Load Cells It is the most common type of load cell used widely in various industries for a large variety of load cell applications. Mostafa Ragab

5. Principle of operation  The strain gauge measures the deformation ( strain ) as a change in electrical resistance, which is a measure of the strain and hence the applied forces  Working Principle  Where R: Initial resistance of the strain gauge  ΔR: Resistance change caused by elongation or contraction  K: Proportional constant (called the “gauge factor”)  ε: Strain R1 / R2 = R3 / Rx Rx = (R2 * R3) / R1 Mostafa Ragab

6. Types Mostafa Ragab

7. 2. Capacitive Load Cell:  Capacitive load cells measure the deformation of strain Gauge by their capacitance  Working Principle: Capacitive load cells are built on the principal of a change in capacitance when a force is applied on the load cell Mostafa Ragab

8. Types of Capacitive Load Sensors 1 lbs Pound = 0.45359237 kg Mostafa Ragab

9. 3. Vibrating wire Load Cells :  Vibrating load cells are used for measurement of load in Structural geometry Mostafa Ragab

10. Vibrating wire Load Cells: Advantages and Limitations:  Advantages:  An efficient way to monitor the load in structural elements  High accuracy and Long-term performance  Multiple gage system allowing to account for eccentric loading effects Limitations:  Not suitable for dynamic loading  Special manufacturing technologies required to minimize the zero drift  Need to lightning protection should be evaluated  Zero drift  Gradual change in the scale zero of a measuring instrument. Mostafa Ragab

11. 4. Piezoelectric Load Cell:  A piezoelectric load cell converts an applied force into an electric charge by a piezoelectric ceramic or crystal. Piezoelectric load Cells consist of piezoelectric material. Working: When e force is applied to piezoelectric materials they Create Electric Voltage proportional to force Mostafa Ragab

12. Piezoelectric Load Cell:  Advantages and Limitations: Advantages  No need of external force.  Easy to handle and use as it has small dimensions.  High frequency response it means the parameters change very rapidly. Limitations :  It is not suitable for measurement in static condition.  It is affected by temperatures.  Output is low so some external circuit is attached to it.  It is very difficult to give desired shape to this material and also desired strength. Mostafa Ragab

13. 5. Hydraulic Load Cell: Construction : This particular type of load cell uses conventional piston and cylinder arrangement. The piston is placed in a thin elastic diaphragm. The piston doesn't actually come in contact with the load cell. The load cell is completely filled with oil . Working principle: When the load is applied on the piston, the movement of the piston and the diaphragm results in an increase of oil pressure which in turn produces a change in the pressure on a Bourdon tube connected with the load cells. Mostafa Ragab

14. Hydraulic Load Cell: Limitations:  This technology is more expensive than other types of load cells. It is a more costly technology and thus cannot effectively compete on a cost of purchase basis This hydraulic load cell have an accuracy of the order of 0.1 percent of its scale and can measure loads up to 2.5*10^5 Kg Mostafa Ragab

15. 6. Pneumatic load cell:  The Load cell is designed to automatically regulate the balancing pressure. Air pressure is applied to one end of the diaphragm and it escapes through the nozzle placed at the bottom of the load cell. Working Principle:  The force is applied to one side of a diaphragm of flexible material and balanced by pneumatic pressure on the other side. The counter-acting pressure is proportional to force and is displayed on a pressure dial , As the flapper closes the nozzle opening, a back pressure results underneath the diagram. This back pressure acts on the diaphragm producing an upward force Mostafa Ragab

16. Pneumatic load cell: Advantages and Limitations: Advantages :  The advantages of this type is that they are explosion proof.  They are not sensitive to temperature variations.  They contain no fluids like other type that might contaminate the process if diaphragm destroyed. Limitations :  They are relatively slow speed of response.  They need for clean, dry, regulated air or Mostafa Ragab

17. Excitation, Rated output and hysteresis The bridge is excited with stabilized voltage (usually 10V, but can be 20V, 5V, or less for battery powered instrumentation). The difference voltage proportional to the load then appears on the signal outputs. The cell output is rated in millivolts per volt (mV/V) of the difference voltage at full rated mechanical load. So a 2.96 mV/V load cell will provide 29.6 millivolt signal at full load when excited with 10 volts. Typical sensitivity values are 1 to 3 mV/V. Typical maximum excitation voltage is around 15 volts. Mostafa Ragab

18. Calibration Annual re-calibration is considered best practice by many load cell users for ensuring the most accurate measurements. Standard calibration tests will use linearity and repeatability as a calibration guideline as these are both used to determine accuracy. Calibration is conducted incrementally starting working in ascending or descending order. For example, in the case of a 60 tonne load cell, then specific test weights that measure in 5, 10, 20, 40 and 60 tonne increments may be used – A five step calibration process is usually sufficient for ensuring a device is accurately calibrated. Repeating this five-step calibration procedure 2-3 times is recommended for consistent results Zero calibration & Span calibration Mostafa Ragab

19. The End Mostafa Ragab