Lei Kang

The electro-mechanical behaviour of flexural ultrasonic transducers

Flexural ultrasonic transducers are capable of high electro-mechanical coupling efficiencies for the generation or detection of ultrasound in fluids. They are the most common type of ultrasonic sensor, commonly used in parking sensors, because the devices are efficient, robust, and inexpensive. The simplest design consists of a piezoelectric disc, bonded to the inner surface of a metal cap, the face of which provides a vibrating membrane for the generation or detection of ultrasonic waves in fluids. Experimental measurements demonstrate that during the excitation of the piezoelectric element by an electrical voltage, there are three characteristic regions, where the frequency of the emitted ultrasonic wave changes during the excitation, steady-state, and the final decay process. A simple mechanical analogue model is capable of describing this behaviour.

Experimental Evaluation of Three Designs of Electrodynamic Flexural Transducers

Three designs for electrodynamic flexural transducers (EDFT) for air-coupled ultrasonics are presented and compared. An all-metal housing was used for robustness, which makes the designs more suitable for industrial applications. The housing is designed such that there is a thin metal plate at the front, with a fundamental flexural vibration mode at ∼50 kHz. By using a flexural resonance mode, good coupling to the load medium was achieved without the use of matching layers. The front radiating plate is actuated electrodynamically by a spiral coil inside the transducer, which produces an induced magnetic field when an AC current is applied to it. The transducers operate without the use of piezoelectric materials, which can simplify manufacturing and prolong the lifetime of the transducers, as well as open up possibilities for high-temperature applications. The results show that different designs perform best for the generation and reception of ultrasound. All three designs produced large acoustic pressure outputs, with a recorded sound pressure level (SPL) above 120 dB at a 40 cm distance from the highest output transducer. The sensitivity of the transducers was low, however, with single shot signal-to-noise ratio (SNR)≃15 dB in transmit–receive mode, with transmitter and receiver 40 cm apart.

Nonlinearity in the Dynamic Response of the Flexural Ultrasonic Transducers

Recent studies of the electro-mechanical behavior of flexural ultrasonic transducers have shown that their response can be considered as three distinct characteristic regions, the first building towards a steady state, followed by oscillation at the driving frequency in the steady state, before an exponential decay from the steady state at the transducers dominant resonance frequency, once the driving force is removed. Despite the widespread industrial use of these transducers as ultrasonic proximity sensors, there is little published information on their vibration characteristics under different operating conditions. Flexual transducers are composed of a piezoelectric ceramic disc bonded to the inner surface of a metallic cap, the membrane of which bends in response to the high-frequency ceramic vibrations of the ceramic. Piezoelectric devices can be subject to nonlinear behavior, but there is no reported detail of the nonlinearity in flexural transducers. Experimental investigation through laser Doppler vibrometry shows strong nonlinearity in the vibration response, where resonance frequency reduces with increasing vibration amplitude.

Ultrasonic phased array for sound drift compensation in gas flow metering

We present an ultrasonic transit-time gas flow meter (UFM) that features a 40-kHz 8×8 2D-phased array for transmitting ultrasonic sound waves up- and downstream to two single element receivers. This allows us to electronically compensate for the well-known parasitic sound drift effect, resulting in larger measurement range and better signal-to-noise-ratio (SNR). The UFM fabricated consists of a pipe with an inner diameter of 146mm with a flush-mounted 2D-phased array, embedded in a 3D-printed waveguide structure. The configuration was tested in a calibrated flow rig at various gas flow velocities. The received amplitudes without sound drift compensation, i.e. the transmitted waves emitted at beam steering angles of 30° and −30° reduce by 39% and 26%, respectively, when the flow velocity was ramped up to a maximum of 41 m/s. The amplitudes only dropped by 13% in both cases at the maximum flow velocity of 41 m/s when the sound drift compensation was turned on. Our results prove that using a phased-array will significantly increase the measurement range of gas flow meters.

Design of flexural ultrasonic phased array for fluid-coupled applications

A design of a 4×4 ultrasonic phased array working in flexural mode is presented. The array consists of an elastic metal sheet, a baffle with 16 holes, a back plate and 16 piezoelectric discs. The active area of each flexural array element is defined by the diameter of the holes of the baffle and the back plate provides an additional clamped-edge-like boundary condition through the baffle for each flexural element. A finite element analysis is utilized to investigate the influence of the dimensions and the materials on the performance of the array. Optimal ratios of the radius of piezo disc to the radius of elastic element working at (0, 0) mode and (1, 0) mode are obtained. A direct comparison of the radiation patterns of the flexural transducers working at (0, 0) and (1, 0) modes proves that the (0, 0) mode is preferable as working mode for the array. The dimensions and the materials of the baffle and the back plate are chosen to effectively reduce the mechanical cross talk between the neighboring array elements. A flexural ultrasonic phased array prototype is fabricated and characterized. Experiments indicate that the ultrasonic beam of the array can be continuously steered from 0° to 50°. This proof-of-concept design demonstrates our low-cost flexural ultrasonic phased arrays design to be sufficiently robust for various fluid-coupled applications.

The Dynamic Performance of Flexural Ultrasonic Transducers

Flexural ultrasonic transducers are principally used as proximity sensors and for industrial metrology. Their operation relies on a piezoelectric ceramic to generate a flexing of a metallic membrane, which delivers the ultrasound signal. The performance of flexural ultrasonic transducers has been largely limited to excitation through a short voltage burst signal at a designated mechanical resonance frequency. However, a steady-state amplitude response is not generated instantaneously in a flexural ultrasonic transducer from a drive excitation signal, and differences in the drive characteristics between transmitting and receiving transducers can affect the measured response. This research investigates the dynamic performance of flexural ultrasonic transducers using acoustic microphone measurements and laser Doppler vibrometry, supported by a detailed mechanical analog model, in a process which has not before been applied to the flexural ultrasonic transducer. These techniques are employed to gain insights into the physics of their vibration behaviour, vital for the optimisation of industrial ultrasound systems.

Two-dimensional flexural ultrasonic phased array for flow measurement

The arrival time detection probability and the measurement range of transit-time ultrasonic flow meters are undermined by the sound drift effect. One solution to this problem is utilizing a phased-array beam steering technique to compensate the bend of the ultrasonic beams. The design, the fabrication and the characterization of two-dimensional flexural ultrasonic phased arrays is investigated in this paper. A meter body with an inner diameter of 146 mmis machined to accommodate the arrays, and flow tests are carried out at different flow rates ranging from 0 to 2500 m3/h. Experimental results indicate that, with the increase of flow rate, the optimum steering angle of arrays increases from 30° to 40.5° when ultrasonic beams travel upstream and decreases from 30° to 22.5° when ultrasonic beams travel downstream. This proof-of-concept design demonstrates the potential of the flexural ultrasonic phased array as an accurate, economic, efficient, and robust solution for gas flow measurement.

Flow measurement based on two-dimensional flexural ultrasonic phased arrays

Transit-time flow measurement is a technology which has been increasingly utilized in recent years, in industries such as petrochemical, water, and gas. In general, this method of flow measurement employs two ultrasonic transducers, one situated upstream, and the other downstream. The fluid flow is then characterized via transmission and detection of ultrasound using the transducers. However, there are notable limitations of the transit-time method, including drift of the propagation direction of the ultrasonic beam. This is termed the sound drift effect. This paper reports on the latest developments of ultrasonic phased arrays, which are a potentially robust and economic solution to compensating for this sound drift effect. The design and fabrication of phased arrays is discussed, and experimental flow measurement results are reported, utilizing flow rates from 0 to 2500 m3/h. The results show that the compensation of the sound drift effect has been achieved, demonstrating the feasibility of phased array...

Dynamic characteristics of flexural ultrasonic transducers

The flexural ultrasonic transducer is a robust and inexpensive device which can be used as either a transmitter or receiver of ultrasound, commonly used as proximity sensors or in industrial metrology systems. Their simple construction comprises a piezoelectric disc bonded to a metal cap, which is a membrane that can be considered as a constrained plate. Flexural transducers tend to be driven with a short voltage burst of several cycles at a nominal resonant frequency, in one of two vibration modes. The physics of their vibration response has not been thoroughly reported, and yet an understanding of their operation is essential to optimise application. The vibration behaviour of a flexural transducer can be discretised into three principal zones, comprising a build-up to steady-state, steady-state, and a natural decay, or ring-down. This discretisation can be used to develop mathematical interpretations of the flexural transducer response. Through a combination of experimental methods including laser Doppler vibrometry, and the development of a mechanical analog model, the response mechanisms of flexural transducers are investigated.The flexural ultrasonic transducer is a robust and inexpensive device which can be used as either a transmitter or receiver of ultrasound, commonly used as proximity sensors or in industrial metrology systems. Their simple construction comprises a piezoelectric disc bonded to a metal cap, which is a membrane that can be considered as a constrained plate. Flexural transducers tend to be driven with a short voltage burst of several cycles at a nominal resonant frequency, in one of two vibration modes. The physics of their vibration response has not been thoroughly reported, and yet an understanding of their operation is essential to optimise application. The vibration behaviour of a flexural transducer can be discretised into three principal zones, comprising a build-up to steady-state, steady-state, and a natural decay, or ring-down. This discretisation can be used to develop mathematical interpretations of the flexural transducer response. Through a combination of experimental methods including laser Dopp...

HiFFUTs for high temperature ultrasound

Flexural ultrasonic transducers have been widely used as proximity sensors and as part of industrial metrology systems. However, there is demand from industry for these transducers to have the capability to operate in both liquid and gas, at temperatures of 100-200°C and higher, significantly greater than those tolerated by current flexural transducers. Furthermore, flexural transducers tend to be designed for operation up to around 50 kHz, and the ability to operate at higher frequencies will open up new application and research areas. A limitation of current flexural transducers is the electromechanical driving element, usually a lead zirconate titanate piezoelectric ceramic, which experiences significantly reduced performance as temperature is increased. This investigation proposes a new type of flexural transducer, the HiFFUT, a high frequency flexural ultrasonic transducer, comprising a bismuth titanate ceramic for operation at high temperatures, that could be replaced by another suitable high Curie ...