ECE5320 MechatronicsLiterature Survey on Sensors

Topic: Time Domain Reflectometry Sensor

Prepared by:

Ricardo Estevez

Dept. of Electrical and Computer Engineering Utah State University

E: restevez@cc.usu.eduT: (435)238-0193

3/7/2008

Outline– Reference list– To probe further– Applications– Basic working principle– Sample Configuration– Major Specifications– Limitations– Alternatives and Cost

Reference List– http://www.campbellsci.co.uk/index.cfm?id=117– http://soilphysics.usu.edu/SCOTT/2005_JHYDROL_

Blonquist-etal_TDR.pdf– http://www.sowacs.com/feature/streats/aquaflex2.html– http://www.home.agilent.com/agilent/product.jspx?nid

=-536900317.536883000.00&cc=US&lc=eng– http://www.home.agilent.com/agilent/product.jspx?nid

=-35186.536883565.00&cc=US&lc=eng– http://www.sandia.gov/Subsurface/factshts/ert/fiberopt

.pdf

Reference List

• https://engineering.purdue.edu/TDR/Papers/33_Paper.pdf• http://ieeexplore.ieee.org/iel5/10686/33799/01610137.pdf• http://www.campbellsci.com/documents/manuals/tdr100.pdf• http://www.cosis.net/abstracts/EGU2008/08460/EGU2008-A-08460-

3.pdf?PHPSESSID=

• http://www.acclima.com/08products/sensors.html• http://www.msi-sensing.com/pubs/msi_dcb_jcm_2006.pdf• http://cp.literature.agilent.com/litweb/pdf/5966-4855E.pdf

To Explore Future• For more information about TDR sensor and its

applications please read the following research papers:

• Standardizing Characterization of Electromagnetic Water Content Sensors:

http://www.acclima.com/08images/research/1EMSensorMethodologyVZJV04-0140Final.pdf

• Application of Time Domain Reflectometry to Soilless Substrate Moisture Measurement :

http://asae.frymulti.com/request.asp?JID=5&AID=14114&CID=lnv2003&T=2

• Calibration & Testing of On-demand Irrigation Control Systems

http://www.fngla.org/reports/166/report1.pdf

Major ApplicationsTime Domain Sensors (TDR) are used in a variety of applications such as:

– Apparent permittivity– Soil Volumetric Water Content– Soil Electrical Conductivity– Rock Mass Deformation– Solution electrical conductivity– Water level detection

Major Applications

– Time domain reflection waveform – Find single and multiple cable faults:

– Connectors– Terminations– Locate faults anywhere along a cable

– Determining impedance– Cable integrity monitoring

Basic Working Principle

• The travel time if a electromagnetic pulse traveling along a waveguide depends on the velocity of the signal and the length of the waveguide.

• However, the velocity of the signal depends on the dielectric constant of the material surrounding the waveguide.

• This relationship can be expressed as:

But

Where:Ka is the apparent dielectric constant

c is the velocity of a EM signal in free space

∆t is the travel time

L is the waveguide length

La is the apparent Probe length

Basic Working Principle The apparent dielectric constant can be expressed as the ratio of the

apparent probe length to the real probe length (L), that is:

The amplitude of the reflected voltage is dependent on electrical conduction of the applied signal between the rods.

Where:

σ is the bulk electricalK is probe constant

Zc is the cable impedance ( 50 ohm)

Basic Working Principle• Probe designs have evolved

from a coaxial cell to multi-rod probes

• Fewer conductors reduce disturbance to the porous medium.

• Concentration of EM field lines near conductors emphasize material permittivity in this region.

• Multiple probes can be connected via multiplexer and the measurement cycle can be automated.

Basic Working Principle

Output waveform of a typical TDR sensorDifferent types of TDR sensor

Acclima

TDR

CS 616

ECH2O

Hydra

Theta

Acclima

TDR

CS 616

ECH2O

Hydra

Theta

Basic Working PrincipleIn order to the TDR sensor works some extra equipment are needed:

– One that generates a very fast rise time pulse that its sent to the connecting cable and probe.

– Another one that sample and digitalize the reflection signal over a specified length of transmission line.

– Finally a signal processing software that analyze the data is also needed.

The figure on the right shows the main part of a acclima’s TDR sensor

Sample Configuration

Three-wire coaxial probes and the MUT

Tektronix 1502b cable tester and WINTDR99 analysis software

Sample Configuration

TDR Cable Tester(Tektronix 1502B)

3-RodProbe

BNCConnector

Waveform

Three-wire coaxial probes and the MUT

The Acclima Digital TDR Moisture sensor.http://www.acclima.com/08products/sensors.html

Major specificationsThese are the specification for two TDR sensor manufactured by Acclima and Campbell Scientific, Inc.

– Acclima Digital TDT® Moisture sensor• Accurate to within 1% permittivity resulting in Stable VWC Readings over the following

conditions: – 0 to 50 degrees C (non frozen soil) – 0 to 4 dS/m in-soil EC (up to 35 dS/m saturated paste extraction from coarser soils)

• Large 100 ml sampling volume (decreases in high EC soils) • Robust, long life materials and construction • Industry standard SDI Interface available • Connection to a PC through an RS-232 serial port • Low Power Operation:

– 6-12 volts DC – 10 ua typical standby mode – 80 ma moisture read mode (requires 0.5 seconds)

Major specifications– TDR100: Campbell Scientific, Inc.

• Pulse generator output: 250 mV into 50 ohms • Output impedance: 50 ohms ±1% • Timing resolution: 12.2 picoseconds • Waveform averaging: 1 to 128 • Temperature range: -40° to 55°C • Dimensions: 21 cm x 11 cm x 5.5 cm (8.3” x 4.3” x 2.2”) • Weight: 700 g (1.5 lbs)

Even though the operation theory for TDR sensors is the same, does not matter what company built them. The specification parameter will change from one company to another because the hardware that they used to process the information.

Limitations• Complex electronics and expensive equipment required for

"pure" TDR.

• In soils with appreciable salinity, conventional TDR becomes progressively inaccurate due to signal attenuation to the point of failure

• TDR sensor are limited to the low operation frequency.

• TDR sensors are either costly with high accuracy or have a low cost and are inaccurate.

Limitations

• The opposite dielectric response to temperature step changes in soils with high surface area (large amount of bound water) vs. free water behavior can be seen in the travel distance measured by TDR.

• For the series of waveforms measured in water the travel distance becomes smaller as temperature increases.

• For the sand-clay the travel distance becomes longer (in the same sample).

-0.8

-0.6

-0.4

-0.2

0

0.2

Refl

ecti

on

Co

eff

icie

nt

0 0.5 1 1.5 2 2.5 Distance (m)

Water

50oC

5oC

-0.8

-0.6

-0.4

-0.2

0

0.2

Refl

ecti

on

Co

eff

icie

nt

0.05 0.25 0.45 0.65 0.85 Distance (m)

18% Montmorillonite, 8% Water5

50

40

30

60

70

10

20

Principal Manufacturers• Campbell Scientific Inc. • Aquaflex SE200 Soil Moisture Meter• Soilmoisture Equipment Corp. • Automata Inc. • E.S.I. Environmental Sensors Inc.• Dynamax, Inc. • GeoTDR Inc.• Spectrum Technologies Inc.

Alternatives and Cost

200 ps (1.8 GHz)

0.3 V

300 ps (1.2 GHz)

125 ps (2.8 GHz)

1.6 V

270 ps (1.3 GHz)

0.25 V

Price range from $3700 to $12,000.

Different TDR Measurement Devices