Texas Instruments | How to Select and Mount Transducers in Ultrasonic Sensing for Level Sensing and (Rev. A) | Application notes | Texas Instruments How to Select and Mount Transducers in Ultrasonic Sensing for Level Sensing and (Rev. A) Application notes

Texas Instruments How to Select and Mount Transducers in Ultrasonic Sensing for Level Sensing and (Rev. A) Application notes
Application Report
SNAA266A – April 2015 – Revised November 2015
How to Select and Mount Transducers in Ultrasonic
Sensing for Level Sensing and Fluid ID
Matthew Minasi
ABSTRACT
Ultrasonic sensing utilizing Time Of Flight (TOF) measurement techniques are used in liquid level and fluid
identification sensing in the automotive, consumer and medical markets. Ultrasonic (TOF) sensing can
yield high accuracy and high reliability with low-power consumption for all of these markets. In order to
achieve the best cost-to-performance combination, care must be taken in selecting the proper transducer.
Cost-sensitive applications can be serviced by utilizing bare (unsealed) transducers making
measurements directly through container walls. This application note discusses how to select the most
appropriate transducer for liquid level sensing and fluid identification applications. In addition to sensor
selection, proper techniques for mounting transducers on the outside of tanks for rapid prototyping is also
addressed in this application note.
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2
3
4
5
Contents
Transducer Selection ........................................................................................................ 2
Tank Wall Transducer Mounting ........................................................................................... 3
Required Supplies ........................................................................................................... 7
Tank design modifications to accommodate transducer ................................................................ 7
Conclusion .................................................................................................................. 10
List of Figures
1
Test Container Preparation ................................................................................................. 4
2
Applying CYA to Test Container ........................................................................................... 5
3
Mounting Transducer to Test Container .................................................................................. 5
4
Applying Hot Glue to Transducer .......................................................................................... 6
5
Completed Assembly ........................................................................................................ 6
6
Transducer Receptacle Clearance (Front View)......................................................................... 8
7
Transducer Receptacle Clearance (Side View)
8
Transducer Receptacle (Top View) ....................................................................................... 9
9
Transducer Receptacle (Side View) ....................................................................................... 9
.........................................................................
8
List of Tables
1
Tested Transducers ......................................................................................................... 3
Trademarks
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1
Transducer Selection
1
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Transducer Selection
Selecting the right transducer starts with understanding what and how you want to make your level
sensing or fluid identification measurement. TI’s TDC1000 handles ultrasonic TOF measurements and can
work with transducers that have a resonance frequency from 30 KHz to 4 MHz. When choosing a
transducer, the following questions need to be answered: What frequency? What size? Which Piezo
material? Where does it need to be mounted? What temperature range? Each of these questions will be
discussed in more detail below.
1.1
What Frequency?
The frequency you choose depends on your application. Every transducer will resonate at multiple
frequencies depending on its piezoelectric material composition and its geometry. A deep discussion of
this topic is not within the scope of this document. As such, this application note only discusses
transducers of the disc variety working in axial or thickness mode.
1. The frequency of your transducer and the transmit pulse length can determine axial (depth) resolution.
The higher the frequency, the tighter the resolution [1]. In general a 1 MHz transducer yields better
than 1 mm resolution in most liquid level sensing applications. Proximity sensing applications on the
other hand use 40 kHz transducers, while gas flow meters use 200 kHz transducers. In general,
propagating an ultrasonic wave through liquids allows system designers to use transducers in the MHz
range while applications where the ultrasonic wave will propagate through a gas require transducers
anywhere from 40 kHz to 500 kHz depending on the distance the wave needs to travel and the
accuracy requirement of the measurement.
Axial resolution = (λ * # cycles)/2
(1)
2. The higher the transducer frequency, the tighter the beam width [2,3].
3. The smaller the transducer diameter, the wider the beam width [2,3].
Sine(ѳ) = 1.2 * V/(DF)
(2)
where:
ѳ = Beam divergence angle
V = Sound Velocity in the material (m/sec)
D = diameter of transducer (m)
F = Frequency of transducer (Hz)
1.2
What Size?
1. The smaller the transducer diameter, the less efficient the transducer is at creating an ultrasound
pulse. In addition, smaller diameters make the transducer less sensitive while trying to detect a pulse.
So in general, larger transducers improve signal to noise ratio at the expense of surface area on a
container wall and cost.
2. The size of a transducer is also influenced by its material composition.
1.3
Which Piezo Material?
There are several different materials offered by piezo manufactures and each have their tradeoffs. In
general, they differ in sensitivity (ability to convert voltage into mechanical displacement). Here is a list of
the most common materials.
1. Barium titanate (older technology)
2. Lead zirconate titanate or PZT is most commonly used today. For more information, the following is a
good website for information on the different types of PZTs: http://www.bjultrasonic.com/ultrasonictechnical-info/piezoelectric-ceramic-technical/
It is common for a piezo manufacturer to not include information about material in their datasheets. If that
is the case, it is recommended to ask the manufacturer in order to make sure that the performance would
be similar to the PZT sensors that are recommended in application notes SNIA020 (Water Flow Meter)
and SNAA265 (Fluid Identification and Contamination).
2
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Transducer Selection
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1.4
1 MHz Transducers for Liquid Level Sensing and Fluid Identification
For the scope of this Application Note, 1 MHz transducers will be the primary focus. In the table below are
the transducers that have been tested with the TDC1000 for level sensing and liquid identification. All of
the transducers are approximately 2 mm thick. This is due to the requirement of axial (thickness)
resonance mode of 1 MHz for these applications. Some of these are listed on their websites referencing
their radial resonance frequencies. It’s important to understand they are both correct and that care needs
to be taken when selecting a transducer.
Table 1. Tested Transducers
FREQ
(THICKNESS
MODE)
THICKNESS
(mm)
RADIUS
(mm)
SEALED
1 MHz
2
10
N
1 MHz
2
15
N
1 MHz
2
7
N
1 MHz
2
10
N
1 MHz
2
15
N
1 MHz
2
15
N
1 MHz
2
15
Y
1 MHz
2
14
N
MODEL #
VENDOR
WEBSITE
SMD10T2R111WL
https://www.steminc.com/PZT/en/piezoceramic-disc-10x2mm-r-215-khz-wireleads-smd10t2r111wl
SMD15T21R111WL
https://www.steminc.com/PZT/en/piezoceramic-disc-1-mhz
BSU-P7-1000B-W200
http://www.bestartech.com/piezoelement-dia.-thickness-p-435-l-en.html
BSU-P10-1000B-W200
http://www.bestartech.com/piezoelement-dia.-thickness-p-436-l-en.html
BSU-P15-1000B-W200
http://www.bestartech.com/piezoelement-dia.-14.85-thickness-p-437-len.html
AW5Y15.5200AL145
http://www.audiowell.com/en/productdetail.aspx?id=53
T/R975-US0014L353-01
http://www.audiowell.com/en/productdetail.aspx?id=58
AW5Y14200A
http://www.audiowell.com/en/productdetail.aspx?id=54
2
Tank Wall Transducer Mounting
2.1
Where Does the Transducer Get Mounted?
1. Inside or outside the tank? Inside or outside the tank will greatly affect the cost of the transducer.
Applications that require transducers to be mounted on the inside of a tank need sealed transducers
that are mechanically more complex as they need to withstand chemical (liquid) exposure. On the
other hand, applications that allow transducers to be mounted on the outside of a container can utilize
bare, unsealed transducers which are much lower cost. These applications also have the added
benefit of being non-invasive and can be retrofitted to existing tanks. In general, if an application can
tolerate external mounting, total system cost is reduced and the system benefits from higher reliability
and accuracy.
2. Top or bottom of the tank? Top or bottom will also greatly affect the cost of the transducer and the
resonance frequency. If the transducer is mounted on the bottom of the tank on the outer wall, the bare
transducer type can be utilized. However, if the transducer is mounted on the top in the inner wall, for
example to measure level, the conducting medium for the ultrasonic wave is now a gas, and thus the
resonance frequency of choice needs to be 40 kHz to 200 kHz depending on the level accuracy
requirement and level height that is being monitored. For fluid identification applications, transducers
are typically mounted on the outside of the outer wall. Since the transducers will not come in contact
with the liquid medium inside the container, the less expensive, bare transducers can be utilized.
3. Temperature range? Most PZT should not be used above 80°C as it degrades the piezoelectric
capability rendering it ineffective. For high temperature applications, appropriate transducers are
available such as Modified lead metaniobate which is available from the following vendors:
http://www.trstechnologies.com/Materials/High-Temperature-Piezoelectric-Ceramics and
http://www.piezotechnologies.com/piezoceramics
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Tank Wall Transducer Mounting
2.2
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Transmitting Through a Tank Wall
It is possible to transmit through most tank walls because the ultrasonic waves travel through tank walls
with no issues depending on the measurement requirements and mounting techniques. Soft tank
materials, such as automotive HDPE (High Density Polyethylene) and Stainless Steel, have been tested
with good results. The process has two key features to maximize success:
1. Good mechanical connection between the transducer and the tank wall. This is achieved using
Cyanoacrylate adhesive as it cures quickly with low compressibility and high adhesion to most
materials.
2. Transducer backside acoustic dampening (and wire strain relief). This is achieved using “Hot glue” as it
has high adhesion to acrylic and moderate flexibility allowing for both acoustic dampening and good
wire strain relief. Commonly available silicon RTV can be used instead but requires a longer (24 hr)
cure time.
3. The tank wall is homogeneous (has no air gaps and/or is not porous in nature). Air gaps cause
reflections and can attenuate the ultrasonic waves, thus prohibiting them from passing through the
wall.
2.3
Assembly Steps
1. Roughen 1 cm square area on lower side of plastic container with sandpaper.
Figure 1. Test Container Preparation
2. De-grease with alcohol.
3. Dry area with air gun.
4. Apply CYA glue to 1 cm square area.
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Tank Wall Transducer Mounting
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Figure 2. Applying CYA to Test Container
5. Attach transducer to glue spot with leads out to side of container.
Figure 3. Mounting Transducer to Test Container
6.
7.
8.
9.
While applying gentle pressure to transducer to hold it in place apply “CYA Accelerator” to transducer.
Wait approximately 15 sec for glue to cure. Blow off excess Accelerator.
Apply alcohol to clean area. Dry area with air gun.
Apply hot glue around the circumference of the transducer.
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Tank Wall Transducer Mounting
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Figure 4. Applying Hot Glue to Transducer
10. Continue covering transducer until it is completely covered in hot glue.
11. Use air to gently cool glue.
Figure 5. Completed Assembly
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Required Supplies
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3
Required Supplies
•
•
•
•
•
•
•
•
4
CYA Glue with Latex
This CYA has latex in it which makes it a good compromise for incompressibility (CYA) and flexibility
(latex) to ensure the transducer will not shear off the flexible tank wall.
CYA Accelerator
Both of the above are from this vendor which is available on amazon.com: http://www.bsiinc.com/Pages/hobby/ca.html
Plastic “test cell” container
400 grit sandpaper
Alcohol
Hot glue gun
Ultrasonic transducers w/attached wires
Spray duster/ air gun
Tank design modifications to accommodate transducer
The previous section discussed the process of mounting a transducer externally to a tank for prototyping
purposes. This section discusses some considerations to take into account when modifying a tank to
accommodate an unsealed transducer. The tank design modification consists of creating a receptacle or
mounting "well" where the transducer can be protected from external damage and allow for acoustic
dampening material to increase short range performance. Below are some mechanical drawings to
consider when modifying a tank design. For the following discussion I'll be utilizing a 2.1mm thick
transducer. The adhesive considerations are the same as discussed in the above sections.
The first consideration is to leave enough space radially around the transducer to prevent radial vibrations
from propagating into the tank wall in the direction opposite from what's required for a level measurement.
As is shown below a 2mm clearance should easily achieve this and allow an acoustic dampening material
(in this case RTV silicone) to enter the gap and help dissipate radial vibrations. The next consideration is
to recess the transducer into the tank wall deeply enough to allow for a minimum of 1x the transducer
thickness of acoustic dampening material (RTV etc.). For the example below the transducer receptacle is
depressed approximately 4.1mm in from the tank's outer wall. Deeper recession may help reduce the
transducer ring-down depending on the dampening material and tank construction.
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Tank design modifications to accommodate transducer
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Figure 6. Transducer Receptacle Clearance (Front View)
Figure 7. Transducer Receptacle Clearance (Side View)
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Sensing and Fluid ID
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Tank design modifications to accommodate transducer
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2 mm clearance
Figure 8. Transducer Receptacle (Top View)
Figure 9. Transducer Receptacle (Side View)
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Conclusion
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Conclusion
Ultrasonic sensing utilizing Time Of Flight (TOF) measurement techniques is used in liquid level and fluid
identification sensing applications. In order to create a solution that is optimized for size, cost, and
performance, one must first choose the right transducer for the application. Determining if the application
can tolerate the transducer being mounted on the outside of the tank is the first start. Once this is known,
a system designer can select a transducer with the appropriate package requirements and resonance
frequency. With the transducer selected, the designer can now start to prepare the tank surface for proper
transducer mounting. Having properly mounted the transducer, Texas Instruments TDC1000 can be
connected to the transducer and application specific measurements can begin.
Texas Instruments offers a wide variety of tools in order to help a system designer create their ultrasonic
sensing solution for liquid level sensing, liquid identification, and flow sensing using the TDC1000. Visit
http://www.ti.com/ultrasonic for more details on the tools and collateral available.
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