Commonly Used Sensors Flashcards
Name the four main types of parameters that are measured by commonly used sensors
- Displacement
- Strain
- Acceleration
- Environmental parameters
What are displacement sensors used for?
Name two types of sensors
Used to measure relative movement between structural elements; commonly used in bridges, tunnels, dams to monitor deflections/expansions to the load/temperature
Linear variable differential transformers (LVDTs), Potentiometers
Describe how LVDTs work
- AC voltage applied to primary coil, generating an alternating magnetic field
- If the core moves to the left/right, this results in an output voltage which is proportional to the core displacement
What should be used for monitoring deflection and crack width?
LVDTs
What are the advantages of LVDTs?
No mechanical contact between the sensing elements
- Mechanical contact can lead to fatigue failures
- Lack of mechanical contact can allow high resolutions to be achieved
No frictional forces to distort the readings
What are the drawbacks of LVDTs
Transverse motion must be minimised to avoid internal rubbing (because sensors rely on lack of contact between core and body)
Affected by temperature and vibration
Sensitive to stray magnetic fields
Describe how potentiometers work
Change in resistance related to changes in rotational or linear position
Many different types
- e.g. spring potentiometer, with measuring cable wound around a spool
What are the advantages of potentiometers?
Unlike PVDTs, can measure displacements that are larger than the sensor itself (because the wire is wound around the spool)
- can cover bigger areas
Lightweight
What are the drawbacks of potentiometers?
They have mechanical connections
- therefore most string potentiometers have limits on frequency range, lifetime, accuracy
Tension in cable can affect the measurement for smaller structures (that are more sensitive to external loads)
What are strain gauges used for?
Name two types of strain gauge sensors
Used for direct measurements of a structure’s relative deformation under an applied load
- Can give information about a given component and the (unknown) load the structure has
Piezoresistive, Vibrating-wire
Describe how piezoresistive strain gauges work
What is the equation to relate deformations (of less than approx. 2%) to the resistance of the gauge
Piezoresistive strain gauges are simple sensors that are bonded to the structure so that any deformation causes the sensor to elongate/contract
What are the advantages of piezoresistive strain gauges?
They are small
- negligible mass loading effects on the structure
Good shock and vibration resistance
Cheap, high accuracy
What are the drawbacks of piezoresistive strain gauges?
Response is dominated by localised effects (e.g. stress concentrations)
- Sensors should be restricted to monitoring ‘hot spots’
Installation is labour-intensive and requires expertise
Sensitive to temperature
Difficult to detect slowly-varying strains due to sensor drift
How do vibrating-wire sensors work?
Alternative type of strain gauge, based on the principle that if a wire is pinned (at both ends) and put under tension, the natural frequency of the vibration of its first mode can be calculated
Using this, the strain across the length of the wire can be determined
- need to ensure the first mode of the wire is excited (typically done by exciting the wire at the middle of its span with a solenoid)
What is the vibrating-wire equation that is used to find the natural frequency of the vibration of its first mode?
L = length of wire
T = tension in wire
M = mass per unit length of wire
What are the advantages of vibrating-wire sensors?
The strain gauges are typically much larger than piezoresistive strain gauges
- often between 50 and 250mm in length
- advantage for concrete because it averages the strain over a sufficient distance; averaging out local homogeneities
For concrete, the sensors can be embedded directly in the material
What are the drawbacks of vibrating-wire sensors?
Sensitive to temperature
- however, when well-installed and used at room temperature, sensors are very stable and exhibit minimal drift
What are acceleration sensors used for?
Name three types of sensors
Acceleration sensors measure vibrations and accelerations within structures
- contain info about both the local and global characteristics of the structure
Force-balance, capacitive, piezoelectric
How do force-balance accelerometers work?
Unlike conventional accelerometers, the ‘servo type’ contains a freely suspended mass constrained by an electrical equivalent mechanical spring
Two classes: pendulous (unbalanced pivoting mass with angular displacement) and non-pendulous (mass displaced linearly)
Behaviour explained by F = ma (NII)
For the pendulous type accelerometer, what equation applies?
The polar form of Newton II
What are the advantages of force-balance accelerometers?
Very sensitive and have very good resolution at low frequency
- extensive use in applications requiring at least 0.1% accuracy
Relatively insensitive to thermal effects
Inherent sensitivity to gravity
- with certain modifications the accelerometers become very good for measuring inclination
- known as inclinometers
What are the drawbacks of force-balance accelerometers?
The control mechanism
- limits the bandwidth of the sensor to relatively low frequencies
- makes the sensor more expensive
How do capacitive accelerometers work?
They measure the displacement of a proof mass with respect to the housing of the accelerometer; determining the acceleration the sensor experiences
The motions of the proof mass are small, so the proof mass is typically suspended between the two plates
- the two capacitors (formed between the mass and top/bottom plates) utilised in a differential mode so that small drifts and interferences can be compensated for in the measurement
What are the advantages of capacitive accelerometers?
Able to acquire measurements across a wide frequency (including static acceleration)
- can monitor large structures
Have superior stability, sensitivity, resolution
What are the drawbacks of capacitive accelerometers?
Sensitive to temperature and humidity variations
Fragile (compared to other accelerometers)
How do piezoelectric accelerometers work?
They are based upon the piezoelectric effect in which crystalline materials generate an electric change that is proportional to the net force acting on the piezoelectric material
What are the three different configurations of piezoelectric accelerometers?
Shear mode
- piezoelectric material sandwiched between a rigid post and a cylindrical proof mass
Flexural mode
- beam shaped crystal used
- best suited for low-frequency, low-amplitude applications
Compression mode
- use tensile and compressive loads to generate forces in the piezoelectric material
What are the advantages of piezoelectric accelerometers?
Versatile; can be used in a wide variety of environments
Robust and have a long service life (due to no moving components in sensor)
Can be applied across a wide frequency, possess good linearity across a large dynamic range
What are the drawbacks of piezoelectric accelerometers?
Unable to measure static accelerations
- unlike capacitive/piezoresistive accelerometers
Have a minimum frequency at which they can be used
Why are environmental sensors necessary in health monitoring systems?
Name three types of sensors
The sensors measure environmental conditions
- whilst may not directly calculate the health of the structure, essential for monitoring the loads and variations in environment
Anemometers, thermocouples & resistive thermometers, humidity sensors
What do anemometers do?
What are the two types of anemometers?
Measure wind speed
- critical for bridges (wind can significantly excite the structure) and wind turbines
Cup anemometers
- 3/4 cups
- drag-driven device that turns because drag on smooth back surface is less than that on open face of cup (imbalance)
- rotational speed of cups proportional to average wind speed
Ultrasonic anemometers
- measures time taken for ultrasonic pulse of sound to travel from north to south transducer; compares with time taken from pulse to travel from south to north transducer
- likewise between west/east transducers
What are the advantages of cup anemometers and ultrasonic anemometers?
Cup:
- low prize
- flexible design
- simple installation and operation
Ultrasonic:
- high accuracy
- rapid response time (1 sec)
- appropriate for long-term use in exposed conditions (salty air and lots of dust)
What are the drawbacks of cup anemometers and ultrasonic anemometers?
Cup:
- moving parts wear out
- don’t work well in snow and freezing rain
- don’t work well in rapidly fluctuating winds
Ultrasonic:
- not good in rain
- radio interference orientation
- fragile
- high power consumption
Why are temperature sensors important?
Temperature changes can impact the response characteristics of large & flexible structures
- can also mask changes in the structure due to damage, or result in false indications of damage
How do resistance thermometers work?
They use the principle that the resistance of metals increases with temperature
Platinum used because it has the highest possible coefficient of resistivity; slight changes in its resistance with temp can be measured using a Wheatsone bridge
How do thermocouples work?
They use the principle that if two different metal wires are connected, the voltage produced (in the vicinity of their connection) is dependent on the temperatures difference (between the connectors and other parts of those wires)
Different thermocouples exist for specific temperature ranges
Using an example, give an example of how temperature changes can impact bridges?
Freezing in bridge supports can change the natural frequencies
- In some cases, more than 10x that than changes due to damage
The first mode of the Alamosa Canyon bridge varied by approx. 5% throughout a day due to a temperature gradient across a bridge deck
What do humidity sensors do?
They measure moisture levels, which can impact materials like concrete and steel
Often used in tunnels/bridges/underground structures to monitor corrosion risk and material degradation
What are two other examples of sensors?
Tiltmeters:
- measure tilt/inclination
- used for monitoring foundations, retaining walls, structures prone to tilting (e.g. dams)
Global positioning system (GPS) sensors:
- track movement and position of structure in real time
- used in infrastructure projects to monitor settlement, deformation and displacement
What types of sensors would you install on a cable-stay bridge?
What sensors (and how many) would you install on this bridge?
For a multi-span cable-stayed bridge, where would wind sensors be installed?
- positioned outside wind barriers to avoid local turbulence
- combined with rain gauges, as combined effect may have impact on the dynamic oscillations in stay cables
For a multi-span cable-stayed bridge, where would temperature sensors be installed?
For a multi-span cable-stayed bridge, where would corrosion sensors be installed?
- located above the zone of stainless-steel reinforcement and near construction joints
For a multi-span cable-stayed bridge, where would weight sensors be installed?
- piezoelectric strips located at each end of the bridge
For a multi-span cable-stayed bridge, where would accelerometers be installed?
- at the deck, towers and stay cables
For a multi-span cable-stayed bridge, where would GPS, displacement, and tilting sensors be installed?
For a multi-span cable-stayed bridge, where would strain gauges be installed?
- strain gauges for measuring local principal strains in the concrete sections
- dynamic strain gauges for measuring local principle strains in the steel sections
- coupled with temp. meters “to allow the derivation of temperature, shrinkage, creep, elastic strains and
stresses to be identified and the global forces and bending moments at different sections to be observed”
