Strain gauge loadcell ppt

Strain Gauge Load Cell

1. STRAIN GAUGE load cell MANEEB MASOOD(M.Tech.15152010) Mining E ngineering Department IIT(BHU), Varanasi

2. INTRODUCTION A load cell is a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured. The various types of load cells include hydraulic load cells, pneumatic load cells and strain gauge load cells. The most common type is a strain gauge load cell.

3. Typical load cell

4. Compression Load Cell

5. Foil Strain gauge

6. Working Principle The working principle is based on the strain/resistance relationship of electrical conductors. Any electrical conductor changes its resistance with mechanical stress, e.g. through tension or compression forces. The resistance change is partially due to the conductor's deformation and partially due to the change in the resistivity of the conductor material as a result of microstructural changes. Operating Principle: Welded Sensor utilizes bonded strain gages connected in Wheatstone bridge circuit. The output is derived from imbalance in the bridge circuit as load is sensed by sensor.

7. Design & Construction The design should be such that it ensures a uniform strain distribution over the gauge area with the gauges mounted at the maximum strain locations. This is to ensure the highest possible output. Strain level induced in the gauge(s) at maximum rated load, usually design for a range in the gauge area. This maintains high gauge linearity and fatigue life. Monolithic construction to improve repeatability and minimize hysteresis( time-based dependence of system’s output on present and past inputs).

8. Design Details(GEOKON) In the field of Rock Mechanics, the load cells are basically used are primarily annular design. They are majorly used on tiebacks and rockbolts . They can also be used during pile load tests and monitoring loads in tunnel supports. Load cells are made from an annulus of high strength steel or aluminium. Electrical resistance strain gauges are cemented around the outside of the annulus and connected to a Wheatstone bridge. Half the gauges measure vertical strains, half gauges circumferential strain.

9. An outer shell protects the gauges from damage and rings on the either side of the gauges ensure that the load cell is water proof. The cable is attached to the cell through a waterproof gland. A strain relief, in the form of a Kellem’s grip, prevets the cable from coming out. Cables have thick PVC jackets .

10. Resistive Strain Gage Type Anchor Bolt Load Cell

11. Resistive Strain Gage Type Anchor Bolt Load Cell

12. Resistive Strain Gage Type Compression Load Cell

13. High Capacity Compression Load Cell

14. Design Details( Roctest ) The load sensing element is a spool of high strength heat-treated steel or aluminum that withstands rough handling and loading. Electrical resistance strain gauges are bonded to the periphery of the spool. The gauges are mounted in a full bridge configuration that compensates for unevenly distributed loads. High resistance strain gauges are used to minimize cable effects. The load cells are compensated for temperature variations encountered during normal operations. A steel housing with O-ring seals covers the spool and protects the strain gauges from mechanical damage and water infiltration.

15. A plain PVC cable is wired directly to the cell or is connected via a detachable multi-pin connector. On large cells, the cable exit is parallel to the surface of the steel housing to give better clearance.

16. When force is applied to any metallic wire its length increases due to the strain. The more is the applied force, more is the strain and more is the increase in length of the wire. If L 1  is the initial length of the wire and L 2  is the final length after application of the force, the strain is given as: ε =(L 2 -L 1 )/L 1 As the object is deformed, the foil is deformed, causing its electrical resistance to change. Further, as the length increases, diameter decreases and hence, the resistance decreases. The input and output relationship of the strain gauges can be expressed by the term gauge factor or gauge gradient, which is defined as the change in resistance R for the given value of applied strain ε. The resistance change is commonly measured using a Wheatstone bridge.

17. Strain Gauge Mechanical Force Electrical Signal Signal Conditioning Calibration Readout Electrical Optical Mechanical Voltage Current Potential Divider Wheatstone Bridge Change in P roperty

18. Measurement can be done using a single wire also but we use one or more strain gauges in a Wheatstone’s bridge. WHY?

19. What’s the Wheatstone Bridge? Wheatstone bridge is an electric circuit suitable for detection of minute resistance changes, therefore used to measure resistance changes of a strain gage The bridge is configured by combining four resistors as shown in Fig . Initially R1=R2=R3=R4, in this condition no output voltage is there, e=0 When one of the Resistances is replaced by strain Gauge attached to the object whose strain is to be measured and load is applied, then there is small change in the resistance of gauge, hence some output voltage is there which can be related to strain as From this, strain can be easily determined using the relation

20. Full Bridge Configuration To further enhance the sensitivity , all 4 resistances are replaced by strain gauges. While this system is rarely used for strain measurement, it is frequently applied to strain-gage transducers. When the gages at the four sides have their resistance changed to R1 + ΔR1, R2 + ΔR2, R3 + ΔR3 and R4 + ΔR4, respectively, the bridge output voltage, e, is Or Where K is the Gauge Factor.

21. Half Bridge Configuration To increase the sensitivity of measurement, two strain gauges are connected in the bridge, this type of configuration is called as Half bridge as shown in fig. and the output voltage and strain can be related as When gauges are connected to adjacent arms and When gauges are connected to opposite arms

22. Wheatstone Bridge Calculations

23. Application in the field

24. Specifications of geokon model 3000 load cell

25. Read-out unit

26. Amplification and Digitization of Output

27. Sensitivity and accuracy Sensitivity of load cells is the reciprocal of the calibration factor(c) Where λ is gauge factor E is the modulus of elasticity v is the Poisson’s Ratio

28. POSSIBLE SOURCES OF ERROR IN STRAIN GAUGE LOAD CELL SIGNALS Improper Loading and Orientation Wrapping of bearing plates Friction between bearing plate and load cell Cross-sensitivity Bonding faults Hysteresis Effects of moisture Temperature change

29. Load Cells Compensation for Error Hysteresis Effects reduced by material selection. Creep Adhesive and geometry of gauge. Temperature Wheatstone bridge, additional temperature sensitive resistors in series with the bridge, with a dummy.

30. References Geokon Manual( Model 3000 Load Cell) Websites: Geokon , Roctest

31. Thank you