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ETP 2011

Ultrasound Distance Meter

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Introduction Ultrasound

1482

5960

Fresh Water

Steel

1450Fat

331Air (00C)

Velocity of Sound (m/s)Material

f = 20 kHz…20 MHzf

c=λ Example: 40 kHz λλλλ = 8.275 mm

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Introduction Ultrasound

C2731

sm3.331c

0air

ϑ+⋅= with ϑ in degree Celsiusor precisely

331.3 m/s is the 0o speed

Keep in mind: nearly 2 0/00 error per deg

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Introduction Ultrasound

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Introduction Ultrasound

[ ] Decade/dB20dBLoss =In air, for 20…40 kHz:

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Introduction Ultrasound

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Note: 1 Pascal = 10µBar

Transducer Parameters

• High Q-Factor Devices: Q = B/fo = 20...40

• Transmitter: Pressure in dB @ 30 cm

Reference 0 dB = 20 µPa/10 Vrms

• Receiver: Sensitivity in dB

Reference 0 dB = 10 V/Pa respectively 1 V/µBar

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Transducer Parameters

High Q causes slow on/off transitions

Keep in mind: 1 ms error in time causes 33 cm error in distance

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Transducer Parameters

• Radiation Pattern is directive

• Typical Beam Width is 600

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US Measuring Principles & Applications

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Distance measurement by Pulse-Echo method

• TOF = Time of Flight Measurement

• Easy Implementation

• Limited by Pulse Width which has to be approx.

0.5 ms due to high Q of Transducers

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Distance Measuring Application

simplest implementation

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Take strongest receive pulse

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Velocity measurement by Pulse-Doppler method.

• Usage of a long pulse duration leads to famous CW Doppler Radar

• Velocity measurement only, e.g. Traffic Speed or Intrusion Alarm

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LPM (Linear-period modulated)

• The received signal, which includes the reflected echo, is correlated with the LPM signal.

• Cross-correlation operation is the method for effective improvement of the resolution of the TOF.

• The cross-correlation function of the reflected echo (2 continuous LPM signals) and the LPM signal has 2 peaks.

• The first peak of the cross-correlation function shows the Doppler-shifted TOF.

• The second peak shows the length of the LPM signal, which is also Doppler-shifted in proportion to the velocity.

• Therefore, the velocity can be calculated from the Doppler-shifted length.

• Finally from this the corrected TOF can be calculated.

ADC

Distance measurement by Linear Sweep method

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Similar Approach found in FMCW Radar

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Simple Pulse Circuit Example

TX

RX

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Transceiver

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Mode TX: operated in

Series Resonance

(driven by voltage source)

√√√√

Mode RX: operated in

Parallel Resonance

(feeding high ohmic load)

nok

Transducer Operation

40 kHz TX Device:

Z, ∠∠∠∠

Re, Im

Use Transceiver or

dedicated TX and RX decives

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Model Development

Cd = Static Capacity

Cs = Equivalent Capacity

Ls = Equivalent Inductance

Rs = Real Part of Imp @ fs

• Read from Graph: fs @ Zmin, fp @ Zmax

• Read from Graph: Z and ϕ value at fs

• Read: Cd from Datasheet

s

2

s

2

2

s

2

p

f@)cos(

ZRs

Csf4

1Ls

1f

fCdCs

ϕ=

⋅⋅π⋅=

−

⋅=

PT400: Cd = 2400pF, Z = 850 Ω, ϕ =500

fs = 39.5kHz, fp = 40.5kHz

Rs = 1320 Ω Cs = 122 pF Ls = 132 mH

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Ultrasound Link Budget Calculation

50% air humidity

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Our case with 2d = 16 m:

SPL = 120 dB, S = -60 dB,

Calculate: x1 = 35 dB, x2 = 20 dB, assume: x3 = 10 dB 100uV

Goal: min. Vdet = 100 mV Min. Receiver Gain = 1000

Adjustable/programmable attenuator will be used for smaller gain

Calculating Received Voltage

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Received Voltage for typical 400 PT160

best 0.5 mworst 16 m

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Common Transmitter Circuit

© Design hhrt

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Transmitter Circuit Signals

T

Time (ms)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

ITrafo (A)

-2

0

2

T11

0

5

T12

0

5

T13

0

5

T14

0

5

UTrafo (V)

-2.5

0.0

2.5

acoustic pressure

-12

0

12

supply limit (V)

4.5

5.0

transducer (V)

-25

0

25

receive - receive - receive damp - damp - damp - send - send - send - send - send - send - send -

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Example Layout of Transmitter Board