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Texas Instruments PGA460 Ultrasonic Module Hardware and Software Optimization Application notes
Application Report
SLAA732 – February 2017
PGA460 Ultrasonic Module Hardware and Software
Optimization
Akeem Whitehead
ABSTRACT
This document introduces all of the necessary design and environmental considerations when developing
and optimizing an ultrasonic sensor module using the PGA460. Ultrasonic modules behave differently
across various temperatures, transmission mediums, targets, and transducers types. Building the module,
therefore, requires an understanding of the different ultrasonic components available for pairing, and a
feasibility analysis of how external factors will impact the minimum and maximum detectable range. The
PGA460 device can accommodate a variety of use-cases, and is able to provide feedback to compensate
or retune the system for potential variation.
1
2
3
4
5
Contents
Overview ...................................................................................................................... 2
External Performance Factors ............................................................................................. 2
Component Selection ....................................................................................................... 5
PGA460 Parameters ........................................................................................................ 9
End-of-Line Calibration .................................................................................................... 12
List of Figures
1
Attenuation Characteristics of Sound Pressure by Distance ........................................................... 3
2
Sonar Configurations ........................................................................................................ 5
3
Voltage Driver Versus Sound Pressure Level
4
5
6
7
8
9
10
11
12
13
........................................................................... 6
Impedance Gain Phase Plot of Transducer Using Analyzer ........................................................... 7
Transducer and Transformer Electrical Model With Tuning Components ........................................... 8
Ringing-Decay Time Before and After Tuning of Variable Coil Transformer ........................................ 9
Echo Data Dump for Resonant Frequency Sweep of 58.5-kHz Transducer ........................................ 9
Pulse Count Increased from 2 to 20 to Determine Pulse-Count Limit .............................................. 10
Current Limit Increased from 50 mA to 500 mA ....................................................................... 10
Reliable vs Poor Examples when Using TVG and Digital Gain ...................................................... 11
Threshold Mapping Around Echo Data Dump.......................................................................... 12
Frequency Diagnostic Timing Diagram ................................................................................. 13
Sound Pressure Level as Voltage Equivalent .......................................................................... 14
List of Tables
...............................................................................
1
Acoustic Impedance Of Various Materials
2
Sonar Cross Sectional Comparison ....................................................................................... 4
3
Sonar Cross Sectional Comparison ....................................................................................... 4
4
Speed of Sound in Air Across temperature
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4
1
Overview
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Trademarks
All trademarks are the property of their respective owners.
1
Overview
The Texas Instruments PGA460 ultrasonic sensor signal conditioner acts as the driving source and
receiving amplifier for the accompanying ultrasonic transducer. An ultrasonic module, therefore, does not
perform uniformly across all application types because effectiveness of the ultrasonic module is primarily
dependent on the characteristics of the transducer and external factors. This application report discusses
how to best select the required components, including the transducer type, driving mode, and passive
tuning components. After hardware selection, this document provides the procedure to configure the
PGA460 settings based on the performance requirements of the application.
2
External Performance Factors
Several external factors determine the overall performance of the performance of the ultrasonic module.
These factors include minimum required distance, maximum required distance, target size, target material,
target speed, transducer placement, environmental noise, environmental temperature, and environmental
stability. Without considering these factors, the user may not be able to detect the intended target with the
recommended signal-to-noise (SNR) ratio of 3:1. A large SNR is required to reliably and repeatedly detect
a target when using the PGA460-based threshold mapping.
2.1
Range Requirements
Firstly, consider the minimum and maximum range requirements. A common range requirement for aircoupled ultrasonic transducer measurements is, but not limited to, object detection between 30 cm to 8 m.
Short-range measurements are a challenge for single-transducer configurations, whereby the transducer
acts as both the transmitting and receiving element. Because of the resonant behavior of transducers,
residual energy will oscillate within the transducer for a short duration immediately after excitation. This
short post-burst duration is referred to as the ringing or decay time. The decay time is based on the
equivalent model of the transducer, how long and strongly the transducer is excited, the matched or
unmatched resonance frequency of the driver components (based on secondary leakage inductance of
the transformer), and resonant frequency offset from the center-frequency of the band-pass filter.
Section 3.4 presents the techniques on external matching network-compensation design for improved
short-range performance. The matching network consists of inductive, capacitive, and resistive
components, which can be optimized to reduce decay time and improve the minimum distance that can be
measured using an ultrasonic sensor. Long-range measurements are less of a concern because the
decay profile has typically subsided to the same level as the noise floor at the time of object detection.
When using a dual-transducer (bi-static) configuration, which includes a separate transducer dedicated
exclusively to transmitting, and another transducer dedicated to receiving, the decay time becomes
irrelevant because the receiving transducer is only excited by the returning ultrasonic echo. Dualtransducer configurations are recommended for very short object detection (at nearly 0 cm).
Long-range detection must account for the attenuation of ultrasonic energy as it attenuates through air.
The rate of attenuation is primarily dependent on frequency. The relationship of transducer frequency to
maximum detectable distance is provided as the following:
↑ Frequency :: ↑ Resolution :: ↑ Narrower Directivity :: ↑ Attenuation :: ↓ Distance
Ultrasonic energy does not decay linearly across distance. Figure 1 shows the attenuation of sound
pressure by distance and frequency.
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0
-10
Attenuation (dB)
-20
-30
-40
-50
-60
-70
-80
-90
200 kHz
80 kHz
40 kHz
20 kHz
-100
0.1
1
Distance (m)
10
D008
Figure 1. Attenuation Characteristics of Sound Pressure by Distance
The benefits of high-frequency transducers include an increase to resolution and focused directivity
(forward facing beam pattern), but the disadvantage is the increase to attenuation. The rate at which the
ultrasonic energy experiences scattering and absorption while propagating through the medium of air
increases with frequency and therefore the decrease in maximum detectable distance.
2.2
Detectable Target and Objects
The type of target from which the ultrasonic echo reflects from will impact the returning echo strength. For
example, a large, flat steel wall provides a greater return echo compared to a narrow tree. This difference
is because a combination of the acoustic impedance, surface coarseness, orientation, and maximum
cross section of the target.
Acoustic impedance is based on the density and acoustic velocity of a given material, and is important to
determine the amount of reflection that occurs at the boundary of two materials having different acoustic
impedances. The acoustic impedance of air is four orders of magnitude less than that of most liquids or
solids; therefore, the majority of ultrasonic energy is reflected to the transducer based on the difference in
reflection coefficients. However, lighter materials with low densities or significant amount of air gaps, such
as sponge, foams, and loosely woven fabrics, tend to absorb more ultrasonic energy. Table 1 shows an
example listing of characteristics of various material types as they relate to air-coupled ultrasonic
absorption.
Table 1. Acoustic Impedance Of Various Materials
Material
Density (kgm–3)
Speed of Sound (ms–1)
Acoustic Impedance
(kgm–2s–1 x 106)
Air
1.3
330
0.000429
Sponge
100
750
0.075
Fat
925
1450
1.38
Water
1000
1450
1.45
Soft tissue
1050
1500
1.58
Muscle
1075
1590
1.70
Aluminum
2700
6320
17.1
Steel
7800
5900
46.02
Iron
7700
5900
45.43
Gold
19320
3240
62.6
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External Performance Factors
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A flat or smoother surface results in the strongest reflections, while a coarse or ridged surface causes the
ultrasonic echo to scatter in multiple directions, reducing the return strength in the direction of the
transducer. The amount of surface area at a right angle to the transducer provides maximum returns. This
surface area is defined as the maximum cross section (σ), which measures of the ability of the target to
reflect sonar signals in direction of the sonar receiver, in m2, and applies to both ultrasonic sonar and
radar applications. Table 2 provides a description of how the sonar cross section of certain targets impacts
performance.
Table 2. Sonar Cross Sectional Comparison
Target
Maximum Sonar Cross Section
Advantage
Disadvantage
Sphere
σmax = π × r2
Nonspecular
Lowest RCS for size; radiates
isotopically
Cylinder
σmax = (2 × π × r × h2) / λ
Nonspecular along radial
axis
Low RCS for size; specular
along axis
Flat rectangular plate
σmax = (4 × π × l2 × w2) / λ2
Largest RCS for size
Specular along both axes;
difficult to align
Depending on the target, the sonar cross section can be averaged based on size and orientation to
determine the reflected portion of incident power in units of sound pressure. Table 3 lists example targets
in relation to sonar cross section as they equate to point-like targets to show the effects of target strength.
Table 3. Sonar Cross Sectional Comparison
2.3
Target
Sonar Cross Section (dB)
Rodent
–20
Human
0
Automobile
20
Truck
25
Corner reflector
40
Ambient Environment
Changes to temperature, humidity, and air pressure influence the speed of sound and the transmission
impedance characteristics of the transducer just as a variable parallel load at the transducer would.
Temperature has the greatest impact on the performance of ultrasonic sensors. Sound and heat are both
forms of kinetic energy, whereby an increase to temperature yields an increase to the rate of molecular
vibration. Because of the fluctuation in molecular vibration, sound waves are able to travel from 300 to 400
m/s. Use Equation 1 to calculate the speed of sound in air (v) as a dependency to temperature (T).
v = 331 m/s + 0.6 m/s/°C × T
(1)
Table 4 shows the speed of sound across temperature.
Table 4. Speed of Sound in Air Across temperature
4
Temperature (°C)
Speed of Sound (m/s)
–40
307
–30
313
–20
319
–10
325
0
331
10
337
20
343
30
349
40
355
50
361
60
367
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Table 4. Speed of Sound in Air Across
temperature (continued)
Temperature (°C)
Speed of Sound (m/s)
70
373
80
379
90
385
100
391
110
397
120
403
When converting the round-trip time of an ultrasonic time-of-flight based echo, the speed of sound must
be considered in order to prevent ±15cm of error to the distance equivalent of the target.
The resonant frequency of the transducer decreases as temperature increases. Therefore, to compensate
for the point at which the phase change will occur, the transducer must be driven at an offset frequency, or
external passive components must be introduced beyond a certain temperature to retune the resonance
towards the nominal frequency. The PGA460 device offers a temperature decoupling mode to introduce
additional passives in parallel to the transducer beyond a user-specified temperature.
3
Component Selection
When the environmental considerations have been accounted for, selection of the sonar configuration,
ultrasonic transducer type, transducer frequency, and driver mode is required.
3.1
Sonar Configuration
Air-coupled ultrasonic transducers can be used in a wide variety of applications, from automotive park
assist and autonomous robotics, to paper counting and room occupancy detection. The most basic
approach to ultrasonic measurements is to use a mono-static configuration for linear time-of-flight ranging.
This measurement requires a single transducer to serve as both the transmitter and receiver. The monostatic configuration has limitations to the minimum detectable distance because the ringing-decay time,
and limitations to the maximum detectable distance because of the loading-resonant effects of the
transformer or driver circuit.
For improvements to both the minimum and maximum range requirement, a bi-static configuration is
required to separate the transmit and receive functions to two independent transducers. The bi-static
option allows for near 0-cm detection, especially when the receiving transducer is recessed in comparison
to the transmitting transducer. For angular orientation, tracking, and triangulation, three or more ultrasonic
transducers are required, whereby each transducer is paired with an independent PGA460 device. A
single PGA460 device can support the mono-static or bi-static configuration for standalone purposes.
Figure 2 shows an example of the mono-static and bi-static configurations.
XDCR RX
XDCR TX+RX
XDCR RX
Mono-Static Configuration
Bi-Static Configuration
Figure 2. Sonar Configurations
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Component Selection
3.2
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Transducer Selection
Transducer selection initially requires consideration to the operating environment. If the transducer module
will be exposed to the outdoors, positioned in an active warehouse or production floor, or is highly mobile,
such that water droplets, dirt, or airborne debris are present, a closed-top or closed-face transducer is
recommended. Closed-top transducers are typically hermetically sealed to prevent the piezoelectric
membrane from being damaged by environmental debris or alien particles, and are able to tolerate a wider
temperature range. As a result of the additional protective overhead from closed-top transducers, the
piezoelectric membrane must be excited with a sinusoidal voltage averaging 100 VPP. If the protective
overhead is not required, and the transducer will be operating in a controlled, indoor environment, opentop transducers are available as an alternative. Open-top transducers offer an increase to driver and
receiver sensitivity since the piezoelectric membrane is directly exposed to air, and less acoustic
impedance mismatch exists at the face of the transducer. Open-top transducers typically require ten times
less in their driving voltage requirement, averaging 10 VPP.
3.3
Driver Selection
Percentage of Maximum Sound Pressure
Level Transmitted (%)
Transducers require a sinusoidal or square wave voltage driver to properly excite the piezoelectric
membrane for oscillation at the specified resonant frequency. Because the wide variety of air-coupled
transducers of the open and closed-top types, maximum drive voltage specifications typically range
between 5 VPP to 200 VPP. The driving voltage specification is important to consider when wanting to
maximize the amount of sound pressure level (SPL) generated for long-range measurements. SPL is
defined as the logarithmic measure of the effective or RMS sound pressure of a sound relative to the
threshold of hearing reference value, measured in decibels (dB). At the maximum driving voltage
specification, the amount of SPL a transducer is able to generate is saturated, such that driving a
transducer beyond the maximum driving specification will not yield in any additional gains. Figure 3 shows
the typically relationship between driving voltage and transmittable SPL.
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
Percentage of Maximum Specified Voltage Drive (%)
100
D003
Figure 3. Voltage Driver Versus Sound Pressure Level
To generate a large driving voltage averaging 100 VPP for closed-top transducers, a single-ended or
center-tap transformer is typically paired with the transducer, such that the primary-to-secondary turns
ratio acts as a times ten multiplier. This ratio is a common turns ratio assuming a PGA460 supply voltage
of 6 to 18 V DC. The transformer driver mode enables a low-voltage DC reference to be amplified at the
secondary as a sinusoidal waveform. If a smaller driving voltage averaging 10 VPP is required for open-top
transducers, the transformer can be replaced with a direct driver using either a half-bridge or full-bridge
driver configuration. The direct-driver mode allows the PGA460 device and transducer to reference the
same supply voltage without the need for any boost circuitry to excite the transducer. The PGA460 device
can only use the mono-static configuration in half-bridge mode. The full-bridge mode is only compatible in
the bi-static configuration when using the PGA460 device. Closed-top transducers can be direct driven for
short to mid-range applications but will not generate the maximum amount of transmittable SPL for longrange applications.
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3.4
Passive Tuning
Transducer and transformer modeling must be considered when optimizing the ultrasonic module for
short-range measurements to minimize the ringing-decay time of mono-static configurations.
3.4.1
Impedance Gain-Phase Analyzer
An impedance gain-phase analyzer is an instrument that allows a frequency of the transducer to be swept
and plotted against impedance (Ω) and phase (°). An example instrument is the HP 4194A impedance
gain-phase analyzer. The equivalent circuit of the transducer can be extracted using these plots when
fitted with a Butterworth-Van Dyke (BVD) model. BVD parameter fitting is a built-in function on some
analyzers, or can be fitted using a numerical computing environment such as MATLAB.
The example in Figure 4 shows the analyzer plot of a transducer swept from 35 kHz to 70 kHz. The peak
in the phase angle (red) indicates the resonant center-frequency of the transducer. The impedance
(purple) corresponds to the reactive components or the inductive and capacitive properties of the
transducer. At resonance, the current and voltage are in phase, resulting in a 0° phase angle which is
observed as the mid-point of the rising impedance slope.
Figure 4. Impedance Gain Phase Plot of Transducer Using Analyzer
3.4.2
Tuning Capacitor
When using the transformer driven mode, the equivalent circuit of the transformer introduces additional
parasitics. The parasitic characteristic with the greatest performance-impact is the secondary-side leakage
inductance (LSEC) of the transformer. The transducer resonates most efficiently at a single frequency. For
instance, a 40-kHz transducer cannot be driven at 20, 30, or 50 kHZ; any drift from the resonant frequency
yields a loss in SPL. When the series inductance is introduced to the transducer, the driving frequency,
equivalent BVD model of the transducer, and effective versus expected receiving frequency will be at a
mismatch. To match the secondary inductance of the transformer to the resonant frequency of the
transducer, a tuning capacitor (CTUNE) is added in parallel to the transducer (see Figure 5).
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Transducer BVD Model
LSEC
RT
RDAMP
CTUNE
CPT
LT
CT
Figure 5. Transducer and Transformer Electrical Model With Tuning Components
Use Equation 2 to calculate the value of CTUNE.
CT u L T
CTUNE
CPT
LSEC
(2)
If the tuning capacitor is too large, the attenuation factors increases significantly. Typical values for tuning
capacitance ranges from 100 pF to 2000 pF. When driving the transducer in half-bridge configuration and
full-bridge, configuration resonance is primarily dependent on the transducer and therefore a tuning
capacitor is not required.
3.4.3
Damping Resistor
The damping resistor (RDAMP) is a resistor added in parallel to the transducer to help reduce the ringingdecay time without jeopardizing the driver strength to maximize long-range measurements. A damping
resistor can benefit both the transformer driven and bridge driven modes as a bleed-out resistor
immediately at post-excitation. The damping resistor has minute-loading effects on the transducer during
the bursting and receive segments and therefore a damping resistor is recommended for any mono-static
configuration. Because of the complexity and number of components at the transducer, optimizing the
value of RDAMP is currently an arbitrary process of monitoring the decay profile by trial and error. Given that
the value of RDAMP ranges from 500 Ω to 25 kΩ, TI recommends to use a potentiometer to sweep and finetune the value for the specific sensor, driver, and component combination.
3.4.4
Tunable Transformer
In addition to the appended tuning capacitor, variable coil transformers offer the ability to further tune the
secondary-side inductance of the transformer. The tunable transformer can be adjusted by the top notch
of the screw-type transformer, which is especially useful for systems that require short-range optimization.
To observe the effects of tuning the transformer, the ringing-decay profile or low-noise amplifier output
must be monitored. Figure 6 shows the ringing-decay profile of a transducer before and after the
transformer is tuned for a –600-µs (+10 cm) improvement.
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Figure 6. Ringing-Decay Time Before and After Tuning of Variable Coil Transformer
4
PGA460 Parameters
Optimization is the most cost efficient and least time consuming when all parameters are controlled by and
verified in software. The integration of all key operating parameters of the PGA460 device enables
software based performance sweeps and automated module characterization. This section lists the
PGA460 registers and parameters from greatest importance to least importance. The ultrasonic module
example used for this section assumes the use of a Murata MA58MF14-7N closed-top transducer and
EPCOS B78416A2232A003 fixed center-tap transformer at a voltage reference of 12 V DC.
4.1
Center Frequency
Pulse generation is achieved by a burst-control logic circuit with a pulse frequency that can be configured
from 30 kHz to 80 kHz in 251 steps, or from 180 kHz to 480 kHz, and is configured by the FREQ bit in the
PGA460 EEPROM. Given the 200-Hz step resolution, the optimal resonance frequency can be located if
the transducer is swept within its specified range of tolerance.
The example in Figure 7 shows how much the returning peak amplitude can vary when execute a burst
and listening cycle at each frequency value within a ±2-kHz single-increment sweep for the nominally
58.5-kHz transducer. In this example, a frequency of 58.6 kHz produced the best peak result, without
extending the decay time.
275
Freq 58
Freq 58.2
Freq 58.4
Freq 58.6
Freq 58.8
Freq 59
Freq 59.2
Freq 59.4
Freq 59.6
Freq 60
Freq 61
250
225
Amplitude (8-bit)
200
175
150
125
100
75
50
25
0
0
1
2
3
4
5
Time (ms)
6
7
8
D006
Figure 7. Echo Data Dump for Resonant Frequency Sweep of 58.5-kHz Transducer
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PGA460 Parameters
4.2
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Pulse Count
A trade-off exists between a large pulse count and short ringing-decay period. The larger the pulse count
value, the longer the transducer excitation length, the more energy is required, and the more time the
transducer spends ringing upon release. As a result, detecting short-range objects becomes difficult.
However, if short-range detection is not a concern, then optimizing the point of SPL saturation of the
transducer will help to preserve energy in the long term. The transducer itself cannot infinitely generate
more sound pressure level by exceeding the driving voltage or pulse-count specification. Instead, if the
transducer is over-supplied and over-excited, the transducer characteristics can be change and life-cycle
can be reduced.
The example in Figure 8 shows at which point the SPL of the transducer becomes saturated because of
pulse count. For this particular transducer in the example, the peak amplitude did not improve beyond 20
pulses. This transducer should not be pulsed more than twenty times per burst cycle as specified in the
data sheet of the transducer.
275
2 pulses
4 pulses
8 pulses
10 pulses
14 pulses
16 pulses
18 pulses
19 pulses
20 pulses
250
225
Amplitude (8-bit)
200
175
150
125
100
75
50
25
0
0
1
2
3
4
5
Time (ms)
6
7
8
D007
Figure 8. Pulse Count Increased from 2 to 20 to Determine Pulse-Count Limit
4.3
Current Limit
A current limit is most relevant for transformer driven modules because a transformer-driven solution
typically requires higher drive currents through the primary windings as compared to the bridge-driven
mode. However, depending on the transducer paired with the bridge driven solution, the current limit can
still have an impact on maximum sound pressure level generated.
The example in Figure 9 shows that the transformer-driven solution is very sensitive to the current limit
and approaches saturation near the 450 to 500-mA maximum limit offered by the PGA460 device. A
smaller current limit also yields a shorter decay time and therefore has the benefit of a median current limit
for short-to-mid range evaluation.
275
50 mA
100 mA
200 mA
300 mA
400 mA
450 mA
500 mA
250
225
Amplitude (8-bit)
200
175
150
125
100
75
50
25
0
0
1
2
3
4
5
Time (ms)
6
7
8
D008
Figure 9. Current Limit Increased from 50 mA to 500 mA
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4.4
Time-Varying Gain and Digital Gain
The gain features should be implemented in a manner that allows the peak echo to nearly be saturated
without truncating the peak. This implementation ensures that the maximum SNR is captured in order to
set the threshold timing and levels with the most amount of granularity. The gain features do not
necessarily enhance the SNR but rather scale the echo data dump result to size.
The example in Figure 10 shows a reliable echo data dump output versus two less favorable outputs.
275
Reliable
Poor-A
Poor-B
250
225
Amplitude (8-bit)
200
175
150
125
100
75
50
25
0
0
1
2
3
4
5
Time (ms)
6
7
8
D009
RELIABLE — Peak-echo amplitude nearly saturated. Properly scaled SNR of 3:1.
POOR-A — Echo amplitude too low because time-varying gain is too low and no digital gain is applied.
POOR-B — Saturated peak clamps the amplitude and effectively reduces the SNR. Also increases decay time.
Figure 10. Reliable vs Poor Examples when Using TVG and Digital Gain
The time-varying gain should increase and ramp more aggressively over time to compensate for the
attenuation of sound. The digital gain multiplier is intended to help scale mid-to-long range echoes.
4.5
Threshold
Setting the threshold is the most important feature to optimize, such that no false positives or noise
transients trigger the device to calculate distance, amplitude, and width of unwanted signals, but also
ensure enough margin is provided to ensure worst-case (weak) reflections from targeted objects can be
recognized. By default, and for initial evaluation, TI recommends settings the threshold at 50% of the
averaged peak of the return echo.
The example in Figure 11 shows how the threshold was set for a reliable echo data dump. With the noise
floor at a maximum value of 24, and an echo peak at 236, the 50% segment is at 130. The ultrasonic
measurement result corresponds to the actual distance calculated and observed on the echo data dump
profile.
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End-of-Line Calibration
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275
Echo Data Dump
Mid-Code Thr
Optimized Thr
250
225
Amplitude (8-bit)
200
175
150
125
100
75
50
25
0
0
2
4
6
8
Time (ms)
10
12
14
D010
Figure 11. Threshold Mapping Around Echo Data Dump
The closer the threshold is set to the base of the echo, the more stable and accurate the result will be;
however, this also increases the risk for false positives, unless the noise is known to be steady, controlled,
or repeatable.
5
End-of-Line Calibration
The combination of the PGA460, an ultrasonic transducer, and a transformer can vary the performance of
the sensor module because of the independent range of tolerance of each element. As a result, the
transmitting sound pressure level and receiving sensitivity of each module may not be identical, and
performance losses in the detectable minimum and maximum distances is likely to result. To avoid such
performance losses, and identify defective modules, functional tests and tuning procedures can be applied
to each element.
5.1
Transducer Parameters
For this discussion, calibration of a single transducer will be used as an example, although the techniques
also apply to the bi-static configuration.
5.1.1
Optimal Frequency and Sound Pressure Level Measurements
The resonant frequency of the transducer typically has a tolerance of ±5% or ±2 kHz from the nominal
frequency at a given temperature. To measure the resonance frequency of the transducer, two methods
are available: PGA460 frequency diagnostics and external microphone measurements.
5.1.1.1
Frequency Diagnostic Feature of PGA460
The PGA460 device offers a feature to measure the ringing-decay frequency of the transducer. The user
has the ability to set the start time (FDIAG_START) and window length (FDIAG_LEN) of the frequency
measurement to validate the performance and proper tuning of the transducer. In addition, a frequency
error feature is implemented in the PGA460 device to signify that the measured transducer frequency is
outside of the limits set by the FDIAG_ERR_TH threshold parameter. Both the measured frequency and
error status can be read through any of the interface options.
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OUTA
Analog Front End Input (INP ± INN)
OUTB
BURST
ECHO DETECTION
DECAY
1
2
Digital Data-Path Output
VPP
tr = 1 / fr
3
tdecay
SAT_TH
time
Figure 12. Frequency Diagnostic Timing Diagram
5.1.1.2
External Microphone
To monitor both the emitted frequency and SPL of the transducer in amplitude, an external microphone
must be used, such as the G.R.A.S. 46BF Free Field Microphone, with an oscilloscope. To convert the
peak-to-peak SPL from voltage to dB, use Equation 3 and Equation 4.
SPL Pa = VMeasured mVRMS / 3.4 mV
SPL dB = 20 × log10 (SPL Pa / PO)
(3)
where
•
PO is reference sound pressure of 20 µPa
(4)
In the example in Figure 13, the green waveform represents the driving voltage across the transducer and
the purple waveform represents the ultrasonic echo captured by the external microphone at 30 cm. Both
the frequency and dB equivalent can be monitored using this method.
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13
End-of-Line Calibration
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Figure 13. Sound Pressure Level as Voltage Equivalent
14
PGA460 Ultrasonic Module Hardware and Software Optimization
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SLAA732 – February 2017
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