Principle structure and selection standard of ultrasonic transducer

Update time:18-06-2021

In contemporary society, various plastic products have […]

In contemporary society, various plastic products have penetrated into all areas of people's daily lives. Traditional processing technology can no longer meet the development needs of modern plastics industry. When welding plastic products, the ultrasonic plastic welding machine neither needs to add any adhesive, filler or solvent, nor consumes a large amount of heat source. It has the advantages of simple operation, fast welding speed, high welding strength and high production efficiency. Therefore, ultrasonic welding technology is more and more widely used. The ultrasonic transducer system usually includes an ultrasonic transducer and an ultrasonic horn, which are the basic components for ultrasonic welding processing, and a good ultrasonic transducer is a prerequisite for ensuring ultrasonic welding processing.

1. Working principle of ultrasonic transducer

"1.1" Ultrasonic transducer is a component that converts electrical energy into mechanical vibration and amplifies the amplitude. It mainly includes ultrasonic transducer, ultrasonic horn and ultrasonic welding head. The working principle of the ultrasonic transducer on the ultrasonic plastic welding machine is to use the inverse piezoelectric effect of the piezoelectric ceramic material to generate vibration. Placing a piezoelectric crystal in an external electric field, under the action of the electric field, causes the center of gravity of the positive and negative charges inside the crystal to move, and this polarization displacement causes the crystal to deform, which is called the inverse piezoelectric effect.

   Ultrasonic horn is one of the important components of the ultrasonic vibration system in the ultrasonic processing equipment. In the working process of the ultrasonic vibration system, the vibration amplitude generated by the radiating surface of the ultrasonic transducer is small. When the working frequency is within 20 kHz, the amplitude of the radiating surface of the ultrasonic transducer is only a few microns, while in ultrasonic welding The required amplitude is approximately tens to hundreds of microns. Therefore, it is necessary to use the horn to amplify the displacement and movement speed of the mechanical vibrating mass point, and concentrate the ultrasonic energy on a smaller area to produce an energy gathering effect. The ultrasonic horn can also be used as a mechanical impedance converter to build a bridge between the transducer and the load for impedance matching, so that the ultrasonic energy can be transmitted from the transducer to the load more effectively.

1.2 Selection of ultrasonic transducer:

   Ultrasonic transducer is an energy conversion device. Its function is to convert the input electrical power into mechanical power (ultrasound) and then transmit it, and it consumes a small part of the power (less than 10%). Therefore, the most important issue to consider when using an ultrasonic transducer is the matching with the input and output ends, followed by mechanical installation and matching dimensions. There are many types of ultrasonic machines on the market, and customers must provide accurate and reliable indicators to ensure that the ultrasonic transducer products provided by the company can be well matched with your company's ultrasonic machines and achieve the best performance.

1.3 The selection of ultrasonic transducer and ultrasonic vibrator needs to pay attention to the following parameters:

① Resonance frequency: f, unit: KHz

   The frequency refers to the frequency measured by the transmission line method with a frequency generator, millivoltmeter, etc., or the frequency measured with an impedance characteristic analyzer or similar instruments. Generally known as small signal frequency. The opposite is the machine frequency, that is, the actual operating frequency measured when the customer connects the transducer to the drive power supply through a cable and is energized when there is no load or load. Because the matching circuits of customers are different, the frequency of the same transducer with different driving power supply is different, and such frequency cannot be used as the basis for ordering.

② Transducer capacity: CT, unit: PF

   is the free capacitance of the transducer, which can generally be measured with a capacitance bridge at a frequency of 400Hz-1000Hz, or an impedance characteristic analyzer similar to the instrument. No matter how simple it is, measuring with a general portable capacitance meter can also meet the requirements.

③ Working mode of transducer

Due to different processing methods and requirements, the working mode of the transducer can be roughly divided into continuous work (lace machine, CD set machine, zipper machine, metal welding, etc.) and pulse work (such as plastic welding machine). The requirements of the transducer are different. Generally speaking, continuous work has almost no pause time, but the working current is not very large, and the pulse work is intermittent, with pauses, but the instantaneous current is very large. On average, the power in both states is large.

④ Transducer type and maximum power

  The manufacturer of the complete machine may have different regulations on the nominal power of the machine for different purposes and purposes. In other words, the nominal power of the same transducer used on different machines may be different. To avoid ambiguity, customers should specify the structure of the transducer, such as column type, inverted horn type, etc., as well as the diameter and number of piezoelectric ceramic wafers.

⑤ Installation and matching dimensions

   mainly include ultrasonic horn material, surface treatment method and shape. The ultrasonic transducer is connected to the ultrasonic horn with the thread, the ultrasonic horn is connected with the ultrasonic mold, and the diameter, thickness, number and position of the gap or screw hole at the flange of the ultrasonic horn. What structure of the ultrasonic horn and the structure of the transducer are connected and matched, and they cannot be connected at will. The output amplitude in the form of the transducer and the structure and amplitude of the horn must be considered together. Improper connection and use will cause the system to work abnormally.

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