Sensor (application to measurement)

1. Sensor (application to measurement) Lecture 3 (Chapter 2)

2. Sensors Electrical Input Physical parameter Electrical Output Physical Output Sensor is a Transducer:What is a transducer ? An electromechanical device that converts a mechanical change into a change in an electrical signal e.g. Piezoelectric: Force -> voltage Actuators SME3252: Mechatronics Lecture 3

3. Output Input Measured valueof variables True valueof variables Essential Elements Instrumentation System SensingElement ConditioningElement ProcessingElement DisplayingElement Chapter 2 Chapter 3 Chapter 4,5,6 SME3252: Mechatronics Lecture 3

4. Example of Instrumentation System SME3252: Mechatronics Lecture 3

5. 2.1: Sensor • Measurement in mechanical engineering: • Displacement, position and proximity • Velocity and motion • Force • Pressure • Fluid flow • Liquid level • Temperature • Light intensity

6. 2.2 Performance terminology • Range and span • Error • Accuracy • Sensitivity • Hysteresis error • Nonlinearity error • Repeatability / reproducibility • Stability – zero drift • Resolution • Output impedence

7. Examples - • Ranges: 70 to 1000kPa, 2000 to 70000 kPa • Supply voltage: 10 V d.c or a.c. rms • Full range output: 40 mV • Nonlinearity and hysteresis:±0.5% full range output • Temperature range: -540C to +1200C when operating • Thermal zero shift: 0.030% full range output /0C

8. 2.3: Displacement, position and proximity • Displacement – how much the object has been moved • Position – position of an object with a reference point • Proximity – position sensor to detect when an object has moved • 2 basic types of displacement / position sensor – contact and noncontact

9. Displacement measurement • Examples of application: • Location & position of object on a conveyor • Orientation of steel plate in a rolling mill • Liquid/solid level measurement • Location or position of work piece in milling operation

10. Location & position of object on a conveyor Application : Bottle sensing. Type           : Photoelectric Sensor

11. Rolling of steel plate

12. Application : Liquid level indicator. Type           : Capacitive Sensor

13. Milling process

14. 2.3.1: Potentiometer Translational and Rotational Potentiometers Translational or angular displacement is proportional to resistance. Taken from www.fyslab.hut.fi/kurssit/Tfy-3.441/ luennot/Luento3.pdf

15. 2.3.2: Strain-gage high alternating stresses that cause damage to blade Strain gages allow monitoring of dynamic torsional motions

16. 2.3.3: Capacitive sensor

17. How does it works: Capacitive A capacitive touchscreen consists of a glass panel with a capacitive (charge storing) material coating its surface. Circuits located at the corners of the screen measure the capacitance of a person touching the overlay. Frequency changes are measured to determine the X and Y coordinates of the touch command. The touchscreen controller sends data via serial port to the host computer and emulates a mouse.

18. 2.3.4: LVDT

19. Inductive Sensors - LVDT An LVDT is used as a sensitive displacement sensor: for example, in a cardiac assist device or a basic research project to study displacement produced by a contracting muscle. LVDT Linear Variable Differential Transformer Taken from http://www.pages.drexel.edu/~pyo22/mem351-2004/lecture04/pp062-073lvdt.pdf

20. 2.3.6: Inductive sensor

21. 2.3.7: Optical encoder • Provide digital output from linear and angular displacement

22. A limit switch A float switch 2.3.9: Proximity switch • simplest form of digital displacement sensor • many forms: lever or push-rod operated microswitches; float switches; pressure switches; etc.

23. Lim Kim Fung, PSM 2006/2007

24. 2.4: Velocity sensor • A) Linear velocity measurement • Linear velocity transducer or LVT • Based on inductive transducer principle • Magnetic field associated with velocity to be measured moves w.r.t fixed conductor

25. B) Angular velocity measurement • Many common machine have rotating shafts – angular velocity or shaft speed • Stroboscopic – flashing light, mark on pulley, rpm • Photoelectric – used photodetector

26. P R3 , R1 eT R2 , R4 eL P 2.5: Force measurement - Load Cell Performance Load range: 5 to 250 lbs Non-Linearity: 0.05% F.S. Hysteresis: 0.03% F.S. Non-Repeatability: 0.03% F.S. Output: 3 mV/V Resolution: Infinite Environmental Temp. operating: 0 to 130 °F Temp. compensated: 30 to 130 °F Mechanical Static overload: 50% over capacity Full Scale

27. 2.6: Fluid pressure • hydraulic pressure is used to measure force applied to diaphragm • when force F applied, pressure is developed in fluid (normally oil),device to measure normally Bourdon gage

28. 2.6.1: Piezoelectric Sensors What is piezoelectricity ? Strain causes a redistribution of charges and results in a net electric dipole (a dipole is kind of a battery!) A piezoelectric material produces voltage by distributing charge (under mechanical strain/stress) Discovered in 1880 by Pierre Curie in quartz crystals. The greek word “piezein”, which means “to press” Examples --- Quartz, Barium titanate, tourmaline

29. 2.7: Liquid flow The most common principals for fluid flow metering are:  • Differential pressure flowmeter • Velocity flowmeter • Positive displacement flowmeter • Open channel flowmeter

30. The most common types of differential pressure flowmeters are: • Orifice Plates • Flow Nozzles • Venturi Tubes • Variable Area - Rotameters

31. 2.9: Temperature • RTD • Thermistor • Thermocouple

32. 2.10: Photoelectric sensor • Photodiode • Phototransistor • Photocell • Solar cell

33. Presence of object Samsul Tongaji, PSM 2006/2007

34. Exercises (Textbook) Identify / suggest suitable sensors for the following applications: • Control system for a furnace to monitor rate of heating oil flows along a pipe line • Control system to determine difference of liquid levels in two container • Control system to control thickness of rolled sheet that emerges from rollers

35. End of Lecture 3