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LOAD CELLS

LOAD CELLS. VISHAL VASISTHA ASSISTANT PROFESSOR (ME) DRONACHARYA COLLEGE, GURGAON. INTRODUCTION. Transducer that is used to convert a force/pressure into an electrical signal

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LOAD CELLS

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  1. LOAD CELLS VISHAL VASISTHA ASSISTANT PROFESSOR (ME) DRONACHARYA COLLEGE, GURGAON

  2. INTRODUCTION • Transducer that is used to convert a force/pressure into an electrical signal • Electrical output is in the order of a few millivolts and requires amplification by an instrumentation amplifier for further use • Load Cells are also called Load Transducer or Load Sensors • Force is measured in terms of deflection or strain of elastic member (strain gauge type), increase in fluid pressure (hydraulic type).

  3. TYPES OF LOAD CELLS • Hydraulic load cells • Pneumatic load cells • Strain gauge load cells

  4. HYDRAULIC LOAD CELLS

  5. DIAGRAM

  6. Hydraulic Load Cells • Hydraulic load cells are force-balance devices, measuring weight as a change in pressure of the internal filling fluid. The liquid(usually oil) has a preload pressure. Application of the force to the loading member increases the fluid pressure which is measured by a pressure transducer or displayed on a pressure gauge dial via Bourdon tube.

  7. Pressure, Force or Load Diaphragm d • Diaphragms are thin circular plates broadly used for the measurement of both low and high values of pressure, force or load. The principle is based on deflection.

  8. Pressure, Force or Load Pressure, Force or Load Corrugated diaphragm Corrugated diaphragm Deflection, d Deflection, d Corrugated diaphragm (a) Capsule (b) Bellows • In order to improve the sensitivity, corrugated diaphragms, are designed. These are called capsules. The materials used for diaphragms are nickel, phosphor and stainless steel.

  9. Pneumatic Load Cells • Pneumatic load cells also operate on force-balance principle. The force is applied to one side of a diaphragm of flexible material and balanced by pneumatic pressure on the other side. The counteracting pressure is proportional to the force and is displayed on a pressure dial.

  10. Working Principle • A load cell is made by bonding strain gauges to an elastic material. For accurate measurement, strain gauges are bonded to the position on the Elastic material where the impact of the stress will be the largest. The strain gauge utilizes this principle and detects a strain by changes in electrical resistance.

  11. Advantages • Rugged and compact construction • No moving parts • Highly Accurate • Wide range of measurement, can be used for static and dynamic loading

  12. Disadvantages • Mounting is difficult • Calibration is a tedious procedure

  13. Strain Gauge Load Cells • This is the most common type of load cell. Such a load cell is used for measuring compressive forces. • The length of the load sensing element is made short so that it does not buckle under the maximum allowable load.

  14. Different Shapes of Strain Gauges

  15. Selection Criteria • Capacity • Basic type and mode of operation • Number of load cells • Method of mounting • Performance or accuracy level-related to system requirements • Sealing level and material of construction • Approval requirements (metrological, safety, environmental) • Cost

  16. APPLICATIONS

  17. Dual Tank Level Controller

  18. Bag Filling Machine

  19. Tank / Silo / Hopper

  20. Food Packaging

  21. Load Cells • Leann Armbrust • Travis Wyatt SRJC, Engr 45 Fall 2010

  22. Strain Gauge What Is It? • A Strain Gauge is a device used to measure the strain of an object. • The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern.

  23. Strain Gauge l ρ R = A • As Strain Gauge under tension, Resistance goes up. • As Strain Gauge under compression, Resistance goes down.

  24. Strain Gauge • The gauge is attached to the object by a suitable adhesive. • As the object is deformed, the foil is deformed, causing its electrical resistance to change. • The resistance change is commonly measured using a Wheatstone bridge.

  25. Wheatstone Bridge R1 R4 + - V0 VEX R2 R3

  26. Load Cells Types Button Canister S Type Beam Shear

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