Introduction and Principle of Transducers
Introduction to Ultrasonic Transducers:
A transducer is an electronic device that converts energy from one form to another. The process of converting energy from one form to another is called transduction.
Ultrasonic transducers convert the electrical output of a power source into vibration output. This electromechanical conversion can be achieved through piezoelectric ceramics (as shown in the figure below) or magnetostrictive materials. Piezoelectric ceramics are the core of transducers.
The sensor requirements will depend on the application. Many requirements will conflict with each other and be given different priorities. Therefore, there is no set of guidelines that can cover all requirements, and there are many different methods to achieve the same goal.
From an analytical perspective, the performance of the transducer can only be predicted in a general way. This is because the characteristics of piezoelectric ceramics are often highly dependent on working conditions, including temperature, electric field strength, static compressive prestress, dynamic stress, number of load cycles, and time. These operating conditions can affect each other, and the impact of these conditions is usually nonlinear. In addition, many characteristics of piezoelectric ceramics are orthotropic and can vary between individual piezoelectric ceramics and between piezoelectric ceramic batches. In addition, the interactions at various component interfaces (such as at threads) may be difficult to characterize and the convective heat transfer coefficient used for air cooling can only be approximately estimated. Therefore, most design processes involve experimental testing.
Working principle of ultrasonic transducer:
Ultrasonic transducers can be divided into transmitters, receivers, and transceivers according to their functions, as shown in Figure 1. Taking the operation of a transducer at 40 kHz as an example, the resonant frequency (fr) of the transmitter is designed to be applied to electrical signals at frequencies close to the operating frequency, as shown in Figure 2, in order to optimize the transmission efficiency. On the contrary, the anti resonant frequency (fa) of the receiver is designed to be close to the received ultrasonic frequency, as shown in Figure 3, in order to optimize the receiving efficiency. The operating frequency of the transceiver is designed between the resonant frequency (fr) and the anti resonant frequency (fa) of the transceiver, as shown in Figure 4. The higher the operating frequency of the transducer, the better the resolution, but the shorter the detection range.
In order for the generated ultrasonic waves to be effectively transmitted from piezoelectric ceramics to objects or fluids (such as in air or water), the acoustic impedance between piezoelectric ceramics and objects or fluids must be matched through an acoustic matching layer. The sound velocity and acoustic impedance characteristics of common substances are as follows:
Taking ultrasonic air transducers as an example, the acoustic impedance of piezoelectric ceramics is about 35 MRayl (106 kg/m2 ∙ s), while the acoustic impedance of air is as low as around 414 Rayl (kg/m2 ∙ s). Therefore, the acoustic matching layer becomes an essential component of ultrasonic transducers, placed between piezoelectric ceramics and air to match their acoustic impedance and effectively transmit ultrasonic energy into the air.
The ideal acoustic impedance value for the matching layer of an ultrasonic air transducer is Rayl, approximately 0.122 MRayl, but it is difficult to find materials with acoustic impedance lower than 1 MRayl and durability. At present, the commonly used acoustic matching layer material is a composite material made of polymer matrix and hollow powder to achieve lower acoustic impedance and reasonable reliability. According to the application, ultrasonic transducers can be used in either a transmit receive mode or a transmit receive mode. It should be noted that ultrasound transducers inherently have ringing characteristics. When designing ultrasonic transducers for proximity measurement, ringing limits the minimum detection distance. Generally, a damping layer is used to quickly restore the ultrasonic transducer to a static state and reduce ringing.
Structure of Ultrasonic Transducers:
The ultrasonic piezoelectric transducer comprises a housing: a matching layer or sound window, piezoelectric ceramic, backing, and outgoing cable, including a cymbal array receiver, consisting of outgoing wires, 8-16 cymbal transducers, metal rings, and rubber gaskets; The cymbal array receiver is located above the disc piezoelectric transducer 3; Using piezoelectric ceramic disc transducer as the basic ultrasonic piezoelectric transducer, transmitting and receiving ultrasonic signals; The Cymbal array receiver is located on the disc piezoelectric transducer and serves as an ultrasonic receiver for receiving Doppler echo signals outside the frequency band of the disc transducer. Suitable for ultrasonic equipment.
How to choose a transducer:
1. The transducer should have high input impedance and low output impedance to avoid load effects.
2. The transducer should be highly sensitive to the desired signal and insensitive to unwanted signals.
3. The transducer should be able to operate in corrosive environments.
4. The transducer circuit should have overload protection to withstand overload.
