ultrasound distance meter · ultrasound distance meter. zürcher fachhochschule 2 introduction...
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1Zürcher FachhochschuleZürcher Fachhochschule
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