Ultrasound Distance Meter · Ultrasound Distance Meter. Zürcher Fachhochschule 2 Introduction...

1Zürcher FachhochschuleZürcher Fachhochschule

ETP 2011

Ultrasound Distance Meter


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



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



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


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



High Q causes slow on/off transitions

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



• Radiation Pattern is directive

• Typical Beam Width is 600


US Measuring Principles & Applications


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


Distance Measuring Application

simplest implementation


Take strongest receive pulse


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


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


Similar Approach found in FMCW Radar


Simple Pulse Circuit Example

TX

RX


Transceiver


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


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


Ultrasound Link Budget Calculation

50% air humidity


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


Received Voltage for typical 400 PT160

best 0.5 mworst 16 m


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 -


Example Layout of Transmitter Board

Ultrasound Distance Meter · Ultrasound Distance Meter. Zürcher Fachhochschule 2 Introduction...

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