What is the Uniformity of Transducer Amplitude?

Introduce

 

Amplitude uniformity (usually just "uniformity") is a measure of the variation of amplitude on a given surface.

 

Need a unified amplitude lever

 

For example, in an ultrasonic plastic welding application, two uniformly thick and very flexible strips are welded together. In order to weld uniformly along the joint, an equal amount of ultrasonic energy must be delivered to each point along the welded joint. Since energy transfer is a function of the amplitude of the amplitude of the amplitude rod, equal energy transfer requires equal amplitude of the amplitude rod at each position where the amplitude rod contacts the strip. If the amplitude of the amplitude rod surface is uneven, some parts of the joint may have insufficient welds, while others may have excessive welds. Therefore, the uniformity of amplitude on the surface of the amplitude lever is an important consideration in the design of the amplitude lever. (Note: Currently, this discussion will be limited to the uniformity of the horn surface. However, the amplitude uniformity of the screw surface is also important and will be discussed later.)

The precise uniformity requirements will depend on the application. Rigid plastics can tolerate uneven amplitude rods to some extent. This is because the hard plastic can transmit ultrasonic energy to adjacent joint areas, resulting in a more uniform distribution of energy along the joints. In addition, rigid plastics allow for connections (energy directors), sometimes compensating for insufficient uniformity of the amplitude rod. Finally, many plastic welding applications only require a certain average welding strength, allowing for some over welding and under welding along the joint.

 

 

When high uniformity is required:

1. Sealed. The airtight seal requires sufficient bonding strength along its entire periphery. The unified horn increases the likelihood of success.

2. Thin film applications. When welding thin films (such as synthetic fiber fabrics), the film is too flexible to transmit ultrasonic energy to adjacent areas. Furthermore, this movie cannot be designed in collaboration with an energy director. Therefore, the full responsibility for proper welding lies with the welding head (assuming the fixture has been designed correctly).

3. Composite amplitude lever. The composite amplitude rod is composed of a mother amplitude rod connected to a pointed amplitude rod. If the mother amplitude lever does not have a uniform surface amplitude, the tip amplitude lever will bend. This can lead to fatigue failure of the tip amplitude rod, connection problems between the tip and the mother amplitude rod, bending faults of the converter, false resonance problems, and poor welding.

4. High amplitude connector. When the joint operates at high amplitude, it may deteriorate due to micro movements at the contact interface. This will result in heat generation and higher power loss; If the problem is severe, the joints may get stuck. If the joint has high amplitude and the uniformity of the amplitude lever on the joint is poor, then this problem will be even worse.

 

Reasons for unevenness

Uneven amplitude is caused by Poisson coupling, so when the resonator vibrates longitudinally, it will "breathe" laterally. However, this breathing is not uniform along the length of the resonator - that is, the amount of breathing is highest at the point of highest strain (stress). Therefore, for unformed resonators, respiration will be highest at the nodes and lowest (near zero) at the output and input surfaces. This uneven breathing distribution leads to uneven facial amplitude. This effect is shown in the following figure.

 

 

Breathing volume depends on three factors

 

1. Poisson's ratio. Materials with high Poisson's ratio breathe more, resulting in reduced amplitude uniformity.

2. Fine line wavelength. Materials with short wavelengths (low wave speeds) have greater strain at a given amplitude, resulting in greater respiration.

3. Horizontal dimensions. A resonator with a larger width, thickness, or diameter will breathe more than its thinner resonator.

The first factor depends only on the material. The second factor depends on the material (wave velocity) and frequency. The third factor depends on the resonator design. The slender shape of a resonator can be defined by combining the second and third factors.

For unformed resonators, the lateral dimension is the diameter. Therefore, when the diameter of the resonator is equal to half the wavelength of the thin line, the slender length is 1.0.

Wavelength is inversely proportional to frequency. Therefore, if the lateral dimensions of the resonator remain unchanged, the resonator may appear thin at 20 kHz but may appear "robust" at 40 kHz. (There is no specific slender value to consider a resonator robust.)

 

Therefore, in order to reduce breathing and improve uniformity--

 

1. The resonator should be slender (long wavelength and small lateral dimension).

2. The resonator material should have a low Poisson's ratio.

The following table and chart show the effects of a 20 kHz unformed Ø 125 mm horn made from typical acoustic materials (aluminum, titanium, and steel) and two quite extreme materials (aluminum beryllium composite and brass).

 

Table note:

 

1. The amplitude lever is unformed and does not have screws.

2. The following table provides some additional information from the previous table. Please note that the universal material properties are used for aluminum, titanium, and steel.

 

3. The calculation of fine line wave velocity is

4. The given tuning length is only applicable to the 20 kHz unshaped Ø 125 mm horn.

5. The maximum radial amplitude occurs at the node. Its value is related to the axial amplitude at the center of the amplitude rod surface.