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VIBRATING WIRERoger C. Ellis
ECE 5320 - MechatronicsAssignment #1
Outline
Reference list Major applications Working principle Example in field Specifications Limitations Where to find
Reference list
Campbell Scientific Inc. Durham Geo Slope Indicator Geokon Gauge Technique Roctest
Web references
http://www.slopeindicator.com/instruments/piezo-vw.html
http://controls.engin.umich.edu/wiki/index.php/PressureSensors
http://www.gage-technique.demon.co.uk/instruments/sg-intro.html
http://www.canterbury.gov.uk/buildpage.php?id=705
Major applications
Monitoring water pressure in and out of a tank.
Pore pressure monitoring on land fills and tailing dams.
Structural monitoring of strain and/or load.
Any other application that requires a monitoring of pressure, strain, load or water depth.
Basic Working PrinciplePressure
Figure from:http://www.canterbury.gov.uk/buildpage.php?id=705
Basic Working PrinciplePressure
Bore holes are drilled into sensor to allow pressure into main chamber. Usually passing through a filter in order to rid the system of particles.
Fluids or gases then push on the diaphram Pressures on the diaphragm cause changes on
the tension & frequency of the wire. Wire is excited by the electromagnetic coils Changes in these parameters are then
transmitted through the coils to a read out device or a datalogger to save measurement details
Sample Configuration 1
Pressure in fluid tank• A vibrating wire piezometer is inserted into a
tank.• Calibration is typically not required depending
on model • Once inserted into tank a measurement is
taken. Giving the user the pressure at the bottom of the tank.
• Any changes in fluid level will result in a new measurement shown by the sensor.
Basic Working PrincipleStrain
Figure from:http://cee.engr.ucdavis.edu/faculty/chai/Research/Benicia
%20Martinez/vibrating-wire-strain-gage.jpg
Basic Working PrincipleStrain
Connect across the axis of interest• Usually involves soldering of the sensor to the
structure Strains to the load or structure will cause
the resistive element inside the sensor to expand or contract.
Measuring the total resistance of the element will tell the amount of strain the structure or load.
Sample Configuration 2
Strain• Solder sensor to desired axis of measurement
on bridge• Connect sensor to a voltage source as well as
a data logger to keep track of the measurements and data
• When load is under stress and experiences a strain the sensor will interpret that with a change in resistance and thus a change in measurement will occur.
Major Specifications
Pressure: different models range from 0 – 1000+ psi.
Pressure accuracy: ±0.1% of full scale output
Temperature: Operating range -40o C to 230o C Extremes not available in a single model
Resolution: 0.025%FS Filter: 50-micron sintered stainless steel
• Some models also have ceramic filters Mass: anywhere from .10kg to 1.5kg
Major Specifications
Diaphragm displacement: < .001 cm3
Linearity: < 0.5% FS Sensitivity: 1*10^5 Mpa Thermal Drift: < 0.1% F.S. / °C Input signal, sinusoidal: 150 to 170 mV
rms Output signal, sinusoidal: 10 to 30 mV
rms
Choosing a sensor
What type of material is your vibrating wire senor going to be located in? Some environmental factors require the that
sensor have a non-corrosive exterior Higher pressure applications require a
stronger exterior Sensor data must be transmitted to a source
via a cable. Choosing the cable sheath is also important, different applications call for different requirements.
Choosing a sensor
How are you going to extract the data given by the sensor? Handheld devices are available to read out the
current data Data loggers are mini computers that are
programmable and can be used to read, process, and save data from the vibrating wire sensors. These are typically used if you want to monitor a system for an extended period of time.
Limitations
High rate of change in pressures (depending on make and model)
Casing – different applications require different housing
Various models don’t come with a thermistor for temperature acquisition
Cheaper models cover smaller pressure spectrum
7.5 MPa is highest pressure I’ve found Thermal operating range – higher temps
usually require more expensive models
Retailors and Costs
Geokon: $395 - ~$1300 Gauge Technique Roctest Slope Indicator (VWP series): $350
