1 Introduction
The purpose of this paper is to illustrate the design of the ultrasonic flow transducer, including its composition, structure, packaging and technical requirements. The designed ultrasonic transducer is applied to the ultrasonic heat meter and the flow measurement of the ultrasonic water meter.
2. composition
The ultrasonic transducer consists of piezoelectric sheet, conductive adhesive, sound absorbing material, conductive column, junction plate, junction plate, baffle and metal shell, as shown in Figure 2-1.

3. Component description
3.1 The shell size of the shell transducer is shown in Fig. 3-1. The whole shape is a convex cylindrical body, which is divided into upper and lower parts. The upper part is an open cylindrical body. Its shape is made of external threads and clamps, which are used to support and fix the whole shell during installation. The lower part is a closed cylinder whose protruding surface acts as the acoustic radiation surface. There are two chambers with different diameters inside the shell. The lower chamber is called piezoelectric chamber, which is used to install piezoelectric sheets and fill sound absorbing materials. The upper chamber is used to install wiring plates and baffles, which is called wiring chamber. The two chambers are connected and coaxially concentric. The following points should be paid attention to when manufacturing transducer housing:

Material Selection: Lead-free Secret Phosphorus Brass Bundle Car Processing, such as C3604, C3771 or HPB59-1. Piezoelectric sheet sound absorbing material conductive pillar high temperature conductive adhesive junction plate baffle ring of the above labeled material Figure 2-1 ultrasonic flow transducer constitutes external threaded metal shell of signal cable radiation surface
The speed of sound is about 4270-4400 m/s.
Machining accuracy: The cleanliness of the inner and outer surfaces of the lower chamber should meet the requirement above_6, with a thickness of 1 mm and a tolerance of 0.05 mm.
The end of the inner chamber should be burr-free and smooth.
3.2 piezoelectric chip
The piezoelectric ultrasonic transducer works on the principle of piezoelectric effect of piezoelectric patches. When the transducer transmits the electric pulse signal, the mechanical vibration of the transducer generates the ultrasonic signal, so it can propagate in solid and fluid. The frequency of the ultrasonic wave depends on the inherent resonance frequency of the oscillator. The performance of ultrasonic transducer depends on the technical performance and reasonable structure design of piezoelectric wafer. Table 3-1 gives the basic parameters of the ultrasonic flow transducer.
Table 3-1

The physical properties, size and shape of piezoelectric materials are closely related to the characteristics of sensors. This design chooses ANN-P5F-1001 piezoelectric chip, its main technical parameters are as follows:
Piezoelectric material: lead zirconate titanate (PbTi03-PbZr03) Fig. 3-1 transducer housing size 3
Thickness Frequency: 2.0MHz + 5%
- Electrostatic capacitance: 1350 pf + 12.5%
Coupling factor (Kt): 0.44
Coupling coefficient (Kp): 0.64
Piezoelectric charge constant (d33): 520 x 10-12 C/N
Maximum operating temperature: 260 C

3.3 High Temperature Conductive Adhesive
High temperature conductive adhesives have the following functions:
- Acoustic Coupling Agents for Two Different Interfaces (Piezoelectric Piezoelectric and Copper Material);
Fixed the piezoelectric sheet by pasting.
Electrical connection between piezoelectric sheet and copper shell is realized by conductive adhesive bonding, and the shell is used as a large area grounding electrode to effectively improve the electromagnetic compatibility performance.
DB5015 is chosen as the high temperature conductive adhesive. Its main characteristics are as follows:
Solvent-free conductive adhesive made of epoxy resin, silver powder and curing agent has excellent conductivity.
It is suitable for bonding between metal and metal, metal and non-metal with conductive requirements, especially for inconvenient soldering occasions. For example, copper, iron, aluminium, gold, silver and other metals are connected with piezoelectric wafers, conductive ceramics and other components.
Volume resistivity 10-3~10-4_/cm
Working temperature range: - 40-120 C
Packing specifications: DB5015 is 100g/set, two-component, inorganic SILICATE-ALUMINATE material, conductive material is silver powder, can withstand high temperature up to 1200 C, can also choose DB5016 high-strength silver powder conductive adhesive: packaging specifications: 100g/set, two-component
Curing time: 12 hours at room temperature and 2 hours at 80 C (see instructions).
3.4 Sound Absorbing Materials
The sound absorption material is filled on the back of the piezoelectric oscillator to absorb and suppress the reverse vibration wave of the piezoelectric oscillator, so as to obtain a pure single pulse emission waveform. Absorbing material is a two-component S 2116 high elastic electronic encapsulating adhesive. It has good fluidity during encapsulation and sound absorption after curing. Its curing properties are as follows:
Volumetric resistance coefficient, _/cm: > 1014
Permittivity: 2.7-3.3
Breakdown voltage KV/mm: > 15
Hardness: A16
Tensile strength Mpa: 0.4
Elongation: 150% 4
Absorption coefficient: > 0.6 (frequency 2MHz)
3.5 conducting post
Conductive column is used for the transition connection between piezoelectric sheet and junction plate. One end of piezoelectric sheet is bonded or welded with conductive adhesive, and the other end of junction plate is pressed. DB5015 is used for bonding and low temperature welding material is used for welding. The shape and size of conductive column are shown in Figure 3-2. When bonding or welding the conductive column, the piezoelectric sheet should be fixed on the fixture, and the conductive column and the piezoelectric sheet should be kept perpendicular and concentric, with a center deviation of <0.1mm. Figure 3-2 Conductive Column Size

3.6 wiring board
It is very important that wiring plates are used as buffer devices between piezoelectric patches and cables to protect piezoelectric patches from the influence of cable welding stress. The wiring board is made of double-sided copper clad sheet with thickness of 1 mm, and is processed by drawing and wiring. The central hole is required to be pressed with the conductive column, and the connection plate is required to be porous and gold-plated, as shown in Figure 3-3.

3.7 retaining ring
The baffle is used to clamp the wiring plate. The baffle ring adopts standard parts. Its outer diameter should be matched with the screw movement of the inner wall of the upper chamber of the shell, so that it can be screwed in and installed. Special calipers are required for installation. Refer to the attached table for specifications of hole retaining rings. Darkening is recommended.
3.8 After assembling and testing the above components, the lead-out cable is welded and sealed with electronic glue, which has reached the IP68 protection standard. The cable is made of multi-strand flexible high temperature resistant cables with a diameter of 3-3.5 mm and a bending radius of more than 6 mm.
4. Pre-assembly work
4.1 Assembly Requirements
1. Before assembling, all parts must be inspected. All unqualified parts are forbidden to go online.
2. Because of the small size of each component and the inconvenience of manual operation, some fixtures need to be made to ensure the smooth assembly.
3. Prepare necessary test instruments to test the product performance of each component and the transducer as a whole.
4. The assembler should wear protective gloves to prevent sweat from contaminating the components.
5. Be extra careful in assembling piezoelectric sheets. Do not clamp components with sharp hard objects to prevent scratches or damages on the silver-plated surface of components.
4.2 Component Appearance Inspection
All components shall be inspected before assembly, and the appearance shall meet the requirements of Table 4-1.

4.3 Basic Performance Test
By testing the basic performance of piezoelectric sheet, we can check whether the quality of piezoelectric sheet meets the application requirements.
4.3.1 Testing Insulation Resistance
The insulation resistance should be infinite when the multimeter resistance R * 100K block is used.
4.3.2 Testing Electrostatic Capacitance
When the capacitance of multimeter is 2nF, the normal value of electrostatic capacitance of piezoelectric chip should be 1100-1600pF (1.1-1.6nF). If the capacitance is very small, the dielectric constant of internal material is not qualified.
4.3.3 Testing electromechanical characteristics
Placing one side of the piezoelectric sheet on a flat thin copper sheet as a contact electrode of the piezoelectric sheet, the thickness of the copper sheet is about 1 mm.  The output voltage of the piezoelectric chip is measured by the 2.0V DC voltage barrier of the multimeter to confirm the electromechanical characteristics. Specific methods are as follows: the red pen is connected with copper sheet, the black pen is connected with the other electrode surface of piezoelectric sheet, and then the piezoelectric sheet is pressed slightly with an insulating holder, and then released immediately. Thus, two voltage signals with opposite polarity will be generated at both ends of the piezoelectric ceramic sheet. The number displayed at first is positive, then returns to zero, and then the number displayed is negative, and the change amplitude is about +0.1-0.2V. The larger the amplitude, the higher the sensitivity. If the multimeter is stationary, the leakage or damage inside the piezoelectric chip will be explained.
Through the above checks, we can basically confirm whether the piezoelectric chip is available or not, but it can not guarantee that the piezoelectric chip is on-line completely, and other parameters need to be tested.
4.4 piezoelectric parameter measurement
The main piezoelectric parameters include:
Testing the resonant and anti-resonant frequencies of piezoelectric plates fr, fa
Measuring resonant impedance Zm
4.4.1 Test Principle
The resonant characteristics of piezoelectric ceramics are used as piezoelectric transducers because piezoelectric plates are an elastic body and have inherent resonant frequencies. When the frequency of external action equals the resonant frequency, piezoelectric plates will produce mechanical resonance. The larger the amplitude of resonance, the greater the mechanical energy produced. Therefore, based on the piezoelectric effect of the piezoelectric sheet, the piezoelectric sheet is mechanically vibrated by using the inverse piezoelectric effect and the piezoelectric effect is used to output the electrical signal.
4.4.2 Test Circuit
Figure 4-1 shows the complete test circuit. It consists of a sine wave signal generator, a high frequency AC voltmeter, a variable resistor and a switching circuit. The circuit can test the resonant frequency, anti-resonant frequency, resonant impedance and quality factor of piezoelectric patches, and obtain the electromechanical coupling coefficient through calculation.

4.4.3 Test resonant frequency
The piezoelectric chip is connected to the test circuit. The switch K1 is placed in position A and the switch K2 is placed in position B1 or B2. The output voltage of the signal generator is 1V, and the voltage frequency is gradually changed from low to high. When the signal frequency is adjusted to close to 2MHz, the piezoelectric wafer corresponding to the frequency should produce resonance, and the frequency should be further fine-tuned to minimize the resonance impedance and maximize the output voltage. This point is the frequency when the piezoelectric wafer has the minimum impedance. Conversely, when the signal frequency continues to increase to a certain frequency, the wafer impedance is the largest and the output voltage is the smallest to represent the frequency of the maximum impedance. The test results show that the frequency at the minimum impedance is regarded as resonant frequency, and the frequency at the maximum impedance is regarded as anti-resonant frequency.  Design requirements: resonant frequency should meet 2 MHz (+15%) and anti-resonant frequency should be 3.414 MHz (+15%).
4.4.4 Test resonant impedance
Put the switch K1 to position C, that is to say, use the inductance-free resistor instead of the piezoelectric chip. The position B1 or B2 of the switch K2 is dialed to test the piezoelectric chip, keeping the signal generator at the resonant frequency, changing the resistance value of the resistor, so that the indication value of the voltmeter is the same as that of the piezoelectric chip. When the switch K1 is dialed to position A, the voltage indication value remains unchanged. At this time, the resistance value of the measuring resistor is the resonant impedance Zm of the piezoelectric chip. There is a deviation in the resonant impedance of different piezoelectric plates at the same resonant frequency, but the deviation is within (+10%).
5. assembly
5.1 Assembly Process
The whole assembly work is carried out according to the flow chart shown in Fig. 5-1.  Assembly is divided into 10 steps:
Step 1: Clean the shell, conductive column and piezoelectric wafer
Step 2: Bond or weld the conductive column on the piezoelectric chip
Step 3: Paste Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezo
Step 4: Filling Sound Absorbent
Step 5: Install wiring board and retaining ring
Step 6: Test the AC Voltmeter of the Solidified Piezoelectric Signal Generator
Step 7: Weld the lead-out cable after passing the test

Step 8: Encapsulation of electronic glue
Step 9: Repeat Step 6
Step 10: Marking and warehousing of finished products
5.2 operation
5.2.1 Cleaning Parts
Before assembly, piezoelectric wafers, conductive columns and shells must be cleaned to remove oil, impurities and dust. The cleaning material is KX-106 HA aluminium alloy phosphating agent. This product is a colorless transparent liquid with a specific gravity of 1.15 and a PH value of 2.
The specific operation methods are as follows:
1. Take appropriate amount of KX-106 HA and inject clean water in proportion of 1:19, i.e. 1 kg of cleaning liquid, 19 kg of clean water, and put it into non-metallic tank for reserve.
2. Soak the cleaning workpiece in solution for about 10 minutes, then take it out to dry.
3. If the cleaning effect is not good, the cleaning time can be prolonged appropriately.
Attention: If splashed into eyes, please rinse immediately with clear water.
5.2.2 Bonding or Welding Conductive Columns
Conductive column and piezoelectric sheet can be bonded or welded. The difference between the two is that the bonding takes a longer curing time, but the advantage is that the piezoelectric sheet will not be heated and the performance will be reduced; the welding process is relatively simple, but the welding speed is required to be fast, so as to avoid the damage of the electrode caused by the piezoelectric sheet overheating. In addition, lead welding can also be used. Welding Conductive Column
Describe the operation process:
1. Place the piezoelectric sheet in a special fixture to fix it. Its function is to prevent the piezoelectric sheet from moving during welding.
2. Place the conductive column in another fixture and fix it. The fixture should cooperate with the piezoelectric patch fixture to form a combination of upper and lower parts and disassemble the fixture.
3. The upper and lower clamps are combined to keep the conductive column and the piezoelectric sheet in coaxial and close contact.
4. Grind the soldering head into a sharp shape, and adjust the soldering head to the appropriate welding temperature.
5. Four solder joints are evenly distributed at the edge of the contact surface between the conductive column and the piezoelectric sheet. The diameter of the solder joints is less than 1.5mm.
6. After cooling, loosen the clamp gently, pinch the conductive column with tweezers, remove the piezoelectric film, and prepare for the next process.
5.2.3 Paste Piezoelectric Piezoelectric Pieces
The cleaned shell is mounted on the shell clamp, and then the piezoelectric sheet is prepared to be pasted. The specific steps are as follows:
1. Use a small amount of high temperature conductive adhesive dipped in a small regular wool pen (mixed preparation) to evenly coat the bottom of the piezoelectric cavity. The thickness of the coating is about 0.1-0.2 mm, and the coating area is slightly smaller than the diameter of the piezoelectric chip.
2. Hold one end of the conductive column with a suitable tool, place the piezoelectric sheet at the bottom of the piezoelectric cavity and rotate for a week to make the conductive adhesive uniformly distributed, eliminate the bubbles, and then press the tool tightly and fix it temporarily.
3. Conductive adhesives need to be cured for 12 hours at room temperature and for 2 hours at 80 C.
4. The contact resistance of conductive adhesives should be tested after curing. The DC contact resistance between shell and piezoelectric sheet is tested by m_meter. The qualified value should be less than 10m_.
5.2.4 Filled Sound Absorbent
When the conductive adhesive is completely cured, it is ready to be filled with sound absorbing material.
1. Prepare sound absorbent in prescribed proportion.
2. Drop the absorbent into the piezoelectric cavity and fill it with a thickness of about 3 mm, leaving about 1 mm space on it.
3. It takes 24 hours to complete curing at room temperature and 4 hours at 80 C.
5.2.4 Installation wiring board
After the sound absorbent is completely cured, the wiring board is installed.
1. Place the wiring board in the wiring cavity and expose the central hole of the wiring board at the top of the conductive column.
2. Clamp the two forceps of the retaining ring with special forceps, press inward to make the retaining ring shrink to the appropriate position, then put it on the wiring board, loosen the retaining ring slightly, and rotate the retaining ring clockwise for one week until the wiring plate is pressed.

3. Test J1 and J2 of two contacts on the wiring board with Multimeter or m_meter to check whether the wiring resistance meets the requirements of Table 5-1.
5.2.4 Test the cured piezoelectric sheet
After the wiring board is installed, the assembly of the transducer is basically completed. The remaining wiring work of the cable will be carried out after the transducer is qualified and tested according to the sixth step.
5.2.5 Welded Outlet Cable
Take a 250m long cable, peel off the ends and tangxi, and then weld one section to two solder joints of the junction board. The solder joint J1 is the signal end, J2 is the ground end, and J3 is the shielding end. The cable is connected to J1 by red core, J2 by black core and J3 by shielding sleeve. The cable wiring is shown in Fig. 5-2.

The last step of 5.2.6 sealing glue is sealing glue to ensure that the whole transducer reaches IP67 protection level.
1. The piezoelectric wafer is mounted in the shell by electrode lead welding. Usually, the bottom of the wafer is grounded and the signal is transmitted on it. Electrode leads are usually welded with 0.2mm thin copper wires by internal heat electric soldering iron. When welding, the soldering head should be grinded into a sharp cone shape to reduce the wafer heating area and small solder joints. Before welding, absolute ethanol should be used to clean the welding area and rosin should be used as flux (no solder paste containing corrosive ingredients is allowed). Specific operation steps:
(1) First polish with metallographic mortar pulp lead and eat tin, then weld.
(2) The welding speed should be fast, the solder joints should be small, and the welding should be firm, so that the false welding and false welding can not be carried out. In addition, the position of solder joint is close to the edge of wafer.
(3) After welding, the excess flux is cleaned with anhydrous ethanol, and there is no short circuit or circuit break between the two poles.
(4) In order to reduce the thermal effect, special low-temperature soldering tin with a melting point of about 80 can be used. Specific formula:
Tin-weight ratio 16;
Bismuth-weight ratio 38;
Lead-to-weight ratio 39;
Cadmium-weight ratio
7. Put the matched metals in a corundum crucible and melt them together. Cool them and they will be ready.
2. Bonding of piezoelectric wafer to protective film
(1) Fast adhesive bonding with 502
(2) Use the following formula of adhesives:
618 epoxy resin 100g
Diethylene ethylene by 8 grams
10 g dibutyl phthalate
(3) When metal rings are used for grounding electrodes, conductive adhesives are used for bonding wafers to metal rings (DB5015, DB2011)