Measurement
instrumentation
Dunnicliff Chapter 1 & 8
1
Requirements for instrumentation
• reliability
–
simplicity
–
quality
• sufficient performance
–
accuracy
–
precision
–
resolution etc.
2
Measurement instruments
Order of reliability
• Optical
• Mechanical
• Hydraulic
• Pneumatic
• Electrical
3
Electric instrumentation
• Foil strain gauge instruments
• Potentiometer (rotary and linear)
• Electrolytic level
• Geophone
• LVDT
• Vibrating wire strain gauge instruments
• Piezoelectric bender element
• Piezoelectric accelerometer
• Force balance accelerometer
Advantages
• versatile
• control over signal conditioning circuitry
• can log output
4
Electrical measurement instruments
Instrument
Type
Foil strain gauge instruments
Wheatstone bridge
Rotary potentiometer
Linear potentiometer
Electrolytic level
Variable resistance
Geophone
LVDT
Vibrating wire strain gauge instruments
Electro magnetic induction
Piezoelectric bender element
Piezoelectric accelerometer
Piezo electricity
Force balance accelerometer
Multiple
5
Potentiometer
Measures: displacement
Advantages
• low cost
• elementary circuitry
• high output voltage (no amplification required)
Limitations
• hysteresis
• not submersible
6
Rotary potentiometer
Measures: displacement
7
Linear potentiometer
Measures: displacement
8
Electrolytic level
Measures: inclination
Applications: tilt meter, inclinometer
Advantages
• low cost
• high accuracy (± 50 arc-sec)
• excellent precision (± 0.3 arc-sec)
• elementary circuitry
Limitations
• fragile
• not suitable if vibrations occur
9
Electrolytic level
10
Vibration
Displacement = A cosωt
Velocity = -ωA sinωt
Acceleration = -ω2A cosωt
11
Geophone
Measures: vibration velocity
Applications: ground vibration measurement
Spring
Coil
Magnet
Housing
Advantages
• high sensitivity (30V/m.s-1)
• no excitation voltage required
• large signal (no amplification required)
• low cost
Limitations
• frequency range (5 to 1500Hz)
12
13
14
LVDT - Linear Variable Differential Transformer
Measures: displacement
Secondary
coil
Primary
coil
Secondary
coil
Core
Shaft
Input
Output
15
LVDT - Linear Variable Differential Transformer
Advantages
• high accuracy (± 0.03µ
µm)
• no friction
• no hysteresis
• submersible
• suitable for dynamic measurements
• good long term stability
Limitations
• high cost
• non-linear output
• demodulation required
16
Soil tests
17
Vibrating wire strain gauge
Measures: strain
Advantages
• good long term stability
• measures frequency (not voltage)
• avoids: noise, cable effects, amplifier drift, etc.
Limitations
• high cost
• not suitable for dynamic measurements
18
Vibrating wire transducers
but:
where:
f: natural frequency (Hz)
L: length of the wire (m)
σ: tensile stress in the wire (Pa)
ρ: density of the wire (kg/m3)
19
Vibrating wire strain gauge
External type
Internal (embedment)
type
20
21
22
Vibrating wire pressure transducer
Pressure
transducer
Pore pressure
transducer (soil)
Porous filter
23
Piezoelectric bender element
Go = ρVs2
24
Piezoelectric bender element
Stiffness only test
2
s
Go = ρV
Go = small strain shear stiffness
Vs = shear wave velocity
ρ = mass density
25
26
Free Bending/Extending Length
27
Piezoelectric accelerometer
Measures: vibration acceleration
Applications: - vibration monitoring
- impact and explosion measurement
Housing
Mass
Piezo ceramic
Advantages
• large frequency range (5 to 10 000Hz)
• large accelerations can be measured (50g to 10 000g)
Limitations
• charge amplifier required
• high cost
28
Tilt meter (inclinometer)
29
Home made tilt meter
Hinge
Housing
Pendulum
Position
detector
Mass
30
Home made tilt meter
α
31
Home made tilt meter
32
Force balance accelerometer
Measures: acceleration or inclination
Applications: - tilt meter, inclinometer
- vibration measurement
Hinge
Advantages
• high sensitivity
• good precision (± 50 arc-sec)
Limitations
• slow response (0 to 100Hz)
• zero drift, if sensor can not be reversed
• high cost
+
_
33
34
35
Measurement instruments
Order of reliability
• Optical
• Mechanical
• Hydraulic
• Pneumatic
• Electrical
36
Non-electric instrumentation
• Optical
• Mechanical instruments
• Hydraulic instruments
• Pneumatic instruments
Advantages
• more reliable than electric instruments
• no electronic circuitry required
Limitations
• can not log output automatically
• requires operator to record readings
37
Dial gauge (indicator)
Type: Mechanical
Measures: displacement
Advantages
• low cost
Limitations
• accuracy (2.5 - 25µ
µm)
• susceptible to environmental effects
• hysteresis
38
Human hair
Cloud water droplet
10µ
µm
0.1mm = 100µ
µm
Can see with the naked eye
Can not see with the naked eye
39
40
Vernier caliper
Type: Mechanical
Measures: thickness, width , depth
Advantages
• low cost
Limitations
• accuracy (50 - 100µ
µm)
• susceptible to environmental effects
41
42
Micrometer
Type: Mechanical
Measures: thickness
Advantages
• low cost
Limitations
• accuracy (10µ
µm)
• susceptible to environmental effects
43
Order of reliability of instrumentation
• Optical
• Mechanical
• Hydraulic
• Pneumatic
• Electrical
44
Bourdon tube gauge
45
Bourdon tube gauge
46
Bourdon tube gauge
Type: Hydraulic
Measures: Fluid pressure
Typical accuracy:
ranges from 2% of FS to 0.2% of FS
Advantages
• cost effective
Limitations
• requires large volume change (soft system)
• some hysteresis (slack in mechanical linkages)
47
Piezometers
Landslides
Retaining Walls
Embankment dam
Tailings dam
48
Open standpipe hydraulic piezometer
Type: Hydraulic
Measures: ground water pressure
Borehole
Advantages
• low cost
• high reliability (no moving parts)
Limitations
• slow response to change in pressure (in clays)
• susceptible to damage by construction equipment
(piezometer tip)
49
Open standpipe piezometer tip
Slotted
openings
Porous filter
50
Dipmeter
51
Twin tube hydraulic piezometer
Type: Hydraulic
Measures: ground water pressure
Ground surface
(piezometer tip)
Advantages
• fast response to change in pressure (in clays)
• high reliability (no moving parts)
• flushable
Limitations
• requires de-aired water and regular flushing
52
Twin tube hydraulic piezometer tip
Porous filter
53
Normally closed pneumatic piezometer
Type: Pneumatic
Measures: ground water pressure
Advantages
• low maintenance
• flushable
Porous filter
(water)
54
Pneumatic piezometer tip
Casing
Porous filter
55
Order of reliability of instrumentation
• Optical
• Mechanical
• Hydraulic
• Pneumatic
• Electrical
56
Strain and temperature measurement with optical fibres
Two classes of fibre optic instrumentation are used:
• Discrete strain and temperature measurement
• Distributed strain and temperature measurement
Light source
1.
2.
3.
4.
Glass core 8 - 9 µm
Cladding 125 µm
Buffer 250 µm
Jacket 900 µm 57
Discrete strain and temperature measurement
• Discrete strain measurement uses fibre Bragg gratings (FGBs).
FBGs are optical strain gauges “burned” into the optical fibre.
• When white light is shone into the optical fibre, light with the wave
length of the Bragg grating is reflected back.
58
Discrete strain and temperature measurement
• Discrete strain measurement uses fibre Bragg gratings (FGBs).
FBGs are optical strain gauges “burned” into the optical fibre.
• When white light is shone into the optical fibre, light with the wave
length of the Bragg grating is reflected back.
59
Gautrain fibre Bragg grating (FGBs) installation
60
Distributed strain and temperature measurement
Transmitted light
1. Rayleigh (amplitude) – fibre condition
2. Brillouin (phase shift) - strain
3. Raman (amplitude) - temperature
Backscattered light
61
Applications of strain and temperature measurement with optical fibres
62