1. Mechanical Sensors

2. Mechanical Sensors Class of sensors to measure mechanical phenomena
For example
Displacement, location, position sensors
Strain sensors
Motion sensors
Pressure sensors
Flow sensors

3. Displacement, location, position sensors
In industrial process, it is sometimes required to measure position
location of object on conveyor system
orientation of steel plates in a rolling mill
liquid or solid level

4. Potentiometer sensors
Potentiometer consists of wire wound around a rod with fixed resistor R
Movement of wiper change the resistance of the potentiometer

5. Example
A potentiometric displacement sensor is to be used to measure work-piece motion from 0 to 10 cm. The resistance changes linearly over this range from 0 to 1 k. Develop signal conditioning to provide a linear, 0- to 10- V output

6. Capacitive and Inductive Sensors
The displacement measurement due to the change in capacitive or inductive
Capacitive

7. There are 3 ways to change capacity
variation of plate separation
variation of plate area
variation of dielectric constant

8. Inductive a motion of permeable core changes the inductance
AC bridge can be used to detect the change in inductance

9. Variable Reluctance Sensor
A motion of coil varies the magnetic flux coupling between the two coils
Used in measure translation and rotational motion
called linear variable differential transformer (LVDT)

11. LVDT LVDT consists of 3 coils
The primary is connected to AC source, thus inducing ac voltage in coils 2 and 3
coil 2 and 3 are connected in series with opposition direction
when core is in middle, zero voltage output
when core is move to one side, the net voltage will be increase

12. The circuit for LVDT is shown below

13. Level Sensors Level sensors measure the level of solid and liquid
Example
Mechanical
Electrical
Ultrasonic

15. Example
The level of ethyl alcohol is to be measured from 0 to 5 m using a capacitive system. The following specifications define the system:
for ethyl alcohol: K = 26 (for air, K = 1)
cylinder separation: d = 0.5 cm
plate area: A = 2RL
where
R = 5.75 cm = average radius
L = distance along cylinder axis
Find the range of capacity variation as the alcohol level varies from 0 to 5 m.

16. Strain Sensors
The sensor to measure the deformation of solid object due to stress
Stress can be categorized in 3 types
Tensile stress
Compressional stress
Shear stress

18. Strain
Strain is defined as the fractional change in length of the sample due to stress

19. Stress-Strain Curve
In linear region, the slope is constant which is called modulus of elasticity, Young’s Modulus

20. Modulus of Elasticity Material Modulus (N/m2) Aluminum 6.89x1010
Copper
11.73x1010
Steel
20.70x1010
Polyethylene
3.45x108

21. Example
Find the strain that results from a tensile force of 1000 N applied to a 10 m aluminum beam having a 4x10-4 m2 cross-sectional area

22. Strain Gauge Principles
The resistance of sample material is
After the application of stress
The change in resistance due to stress is

23. Example
Find the approximate change in a metal wire of resistance 120  that results from a strain of 1000 m/m

24. Measurement Principles
The strain gauge (SG) is glued to the element whose strain is to be measured
When stress is applied, the element and the SG is deformed at the same manner

25. Metal Strain Gauges The device used to measured strain
Gauge factor is defined as
Indicates how easy the strain can be measured
Large GF means large change in resistance for a given strain

26. Construction Wire or foil
Design to make it long in order to give large resistance change
Sufficiently fine, not to resist the deformation
Unidirectional
Nominal value is 60, 120, 240, 350, 500 and 1000 

27. Signal Conditioning
The bridge circuit is used to detect the change in resistance of the SG

28. The resistor value is set equal to nominal resistance of unstrained SG
The resistance of active SG is
The offset voltage is then

29. Finally, or

30. Example
A strain gauge with GF = 2.03 and R = 350  is used in the bridge. The bridge resistors are R1 = R2 = 350  and the dummy gauge has R = 350 . If a tensile strain of 1450 m/m is applied, find the bridge offset voltage if Vs = 10.0 V. Find the relation between bridge off-null voltage and strain. How much voltage results from a strain of 1 micro

31. Load Cells
The direct application of SG is the measurement of force or weight
The load cell measure the deformation produces from the fore or weight
A beam or yoke assembly is used that has several SGs mounted

32. Example
The load cell consists of an aluminum post of cm radius with a detector and compensation strain gauges. The 120  strain gauges are used in the bridge with V = 2 V, R1 = R2 = RD = , and GF = Find the variation of bridge offset voltage for a load of 0 to 5000 lb.

33. Motion Sensors
Motion sensors are designed to measure the rate of change of position, location, or displacement
The primary form of motion sensor is the accelerometer
The speed, and position can be found from integration

34. Principle of Accelerometer
In industrial application, the design of accelerometer is based on Newton’s Law and Hooke’s Law of Spring

35. Natural Frequency
A spring and attached mass always exhibits oscillations at some characteristic natural frequency.
This natural frequency is given by

36. Damping
The friction that eventually brings the mass to rest is defined by a damping coefficient , which has the units of s-1

37. If the spring-mass system is exposed to a vibration, then the resultant acceleration of the base is given by

38. Example
An accelerometer has a seismic mass of 0.05 kg and a spring constant of 3.0x103 N/m. Maximum mass displacement is ±0.02 m. Calculate (a) the maximum measurable acceleration in g, and (b) the natural frequency

39. Type of Accelerometers
Potentiometer
LVDT
Variable Reluctance
Piezoelectric

40. Potentiometer
Attaching the spring mass to the wiper arm of a potentiometer
The displacement is converted to resistance

41. LVDT In these instruments, the LVDT core itself is the seismic mass.
Displacements of the core are converted directly into a linearly proportional ac voltage.

42. Variable Reluctance The test mass is usually a permanent magnet.
The measurement is made from the voltage induced in a surrounding coil as the magnetic mass moves under the influence of an acceleration.

43. Piezoelectric
The piezoelectric accelerometer is based on a property exhibited by certain crystals where a voltage is generated across the crystal when stressed.

44. Example
An accelerometer outputs 14 mV per g. Design a signal-conditioning system that provides a velocity signal at 0.25 V for every m/s, and determine the gain of the system and the feedback resistance ratio

45. Pressure Sensor
Pressure sensor is use to measure the pressure of fluid and gas in industrial application

46. Pressure Principles
Pressure is force per area that fluid exerts on its surroundings
Static Pressure
The fluid is not moving
Dynamic Pressure
The fluid is moving

47. Units for Pressure Gauge pressure SI unit: N/m2 (Pa)
English unit: lb/in2 (psi)
Atmosphere unit (1 atm = 14.7 psi = kPa )
Gauge pressure
Describe a pressure in a relative sense, compared to atmospheric pressure

49. Head Pressure Used to describe pressure for liquid in tank or pipe
The pressure that are produced by the weight of liquid above that measured point

50. Example
A tank holds water with a depth of 7.0 ft. What is the pressure at the tank bottom in psi and Pa (density = 103 kg/m3)

51. Pressure Sensors (p > 1 atm)
A device measures pressure more than 1 atm
Normally, convert pressure into physical displacement which is then converted to electrical signal

52. Diaphragm A thin, flexible piece of metal as shown in the picture
If a pressure p1 exists on one side, p2 on the other side, the net force is then

53. Bellows
Convert pressure into physical displacement
The displacement is converted to voltage using LVDT

54. Bourdon Tube A hard metal tube is flattened and one end is closed
The other end is attached to header by which pressure is introduced

56. Electronic conversions
Convert mechanical displacement to electrical signal by mechanical linkage
Connect to potentiometer
Connect to strain gauge
Connect to LVDT

57. Diaphragm with Feedback

58. Solid State Pressure Sensor
Integrated circuit sensor
Range from 0 to 100 kPa (0 to 14.7 psi)
The silicon wafer acts like a diaphragm that deflects in response to a pressure difference

60. Pressure Sensors (p < 1 atm)
Measurement of pressure less than 1 atm are done using purely electronic method
The device is based on the rate at which heat is conducted and radiated away from the heated filament
Heat loss is proportional to number of gas molecule per unit volume (Pressure)

61. Pirani Gauge
Determine the filament temperature thought a measurement of filament resistance
Pirani gauge is used with bridge
High pressure causes gas molecules to collide with filament and gas molecule absorb energy from the filament, resulting in cooling of the filament

62. Use to measure a very low pressures about 10-3 to 10-13 atm
Ionization Gauge
Use to measure a very low pressures about 10-3 to atm
A regulated electron current (typically 10 mA) is emitted from a heated filament. The electrons are attracted to the helical grid by a dc potential of about +150 volts
Some electrons collide with gas molecules in the gauge envelope causing a fraction of them to be ionized
The gas ions are attracted to the central ion collector wire by the negative voltage on the collector (typically a minus 30 volts). Ion currents are on the order of 1 mA/Pa. This current is amplified and displayed by a high gain differential amplifier
+150 V
-30 V

63. Flow Sensors
Manufacturing processes involve the movement of raw materials, product, and waste throughout the process
Automobile parts through assembly line
Methyl chloride through pipe
Flow can be categorized into solid, liquid, and gas

64. Solid-Flow Measurement
Such as crushed material or powder carried on conveyer
The flow rate is measured in mass or weight per unit of time
Kg/min
lb/min

65. Flow rate is measured by weighting the amount of material on a platform of length L

66. The flow sensor to measure weight passing through the platform
Load cell
LVDT

67. Liquid Flow The unit of flow are
Volume flow rate (gals/min, m3/h, ft3/h)
Flow velocity: express as the distance the liquid travels in the carrier per unit of time (m/min, ft/min)

68. Mass or weight flow rate: expressed as mass or weight flowing per unit of time

69. Example
Water is pumped through a 1.5 in diameter pipe with a flow velocity of 2.5 ft/s. Find the volume flow rate (ft3/min) and weight flow rate (lb/min). The weight density is 62.4 lb/ft3

70. Restriction Flow Sensor
Introduce a restriction in the pipe.
Measure the pressure drop across the pipe
With restriction, velocity of fluid increases but pressure decreases

73. Example
Flow is to be controlled from 20 to 150 gal/min. The flow is measured using an orifice plate system. The orifice plate is described by K =119.5 (gal/min)/psi A bellow measures the pressure with an LVDT so that the output is 1.8 V/psi. Find the range of voltages that results from the given flow range.

74. Obstruction Flow Sensor
Operates by the effect of obstruction placed in the flow stream
Rotameter flow meter
Moving vane flow meter
Turbine flow meter

76. Magnetic Flow Meter
Charged particle move across the magnetic field, potential is established across the flow