A Novel Traveling Wave Sandwich Piezoelectric Transducer with Single Phase Drive: Theoretical Modeling, Experimental Validation, and Application Investigation

: Most of t raditional traveling wave piezoelectric transducers are driven by two phase different excitation signals, leading to a complex control system and seriously limiting their applications in industry. To overcome these issues, a novel traveling wave sandwich piezoelectric transducer with a single-phase drive is proposed in this study. Traveling waves are produced in two driving rings of the transducer while the longitudinal vibration is excited in its sandwich composite beam, due to the coupling property of the combined structure. This results in the production of elliptical motions in the two driving rings to achieve the drive function. An analytical model is firstly developed using the transfer matrix method to analyze the dynamic behavior of the proposed transducer. Its vibration characteristics are measured and compared with computational results to validate the effectiveness of the proposed transfer matrix model. Besides, the driving concept of the transducer is investigated by computing the motion trajectory of surface points of the driving ring and the quality of traveling wave of the driving ring. Additionally, application example investigations on the driving effect of the proposed transducer are carried out by constructing and assembling a tracked mobile system. Experimental results indicated that 1) the assembled tracked mobile system moved in the driving frequency of 19410 Hz corresponding to its maximum mean velocity through frequency sensitivity experiments; 2) motion characteristic and traction performance measurements of the system prototype presented its maximum mean velocity with 59 mm/s and its maximum stalling traction force with 1.65 N, at the excitation voltage of 500 V RMS . These experimental results demonstrate the feasibility of the proposed traveling wave sandwich piezoelectric transducer. novel design of the piezoelectric transducer and developed its dynamical analytical model. FB guided in analyzing the operating principle of the piezoelectric transducer. VH conducted the dynamical analytical modeling of the piezoelectric transducer and experimental verification. LW and JJ carried out the experimental investigation on the case study of assembled tracked mobile system. LW and JT were in charge of the whole structure of the manuscript. All authors contributed to the preparation of the manuscript. All authors read and approved the final manuscript.


Introduction
Piezoelectric ceramic material utilizes its positive/converse piezoelectric effect to achieve the energy conversion between electric energy and mechanical energy, being one of commonly used smart materials in industry and research [1]. Piezoelectric transducer is the typical application of piezoelectric ceramic materials and can be employed as sensors or actuators, being widely applied to aerospace, optics, robotics, and biomedical engineering [2]. Some piezoelectric transducers utilizing the converse piezoelectric effect can convert electrical energy to micro-amplitude mechanical vibration energy, thereby they can be designed as the stator of ultrasonic motors [3]. Vibration characteristics of the piezoelectric transducers directly influence the mechanical output performance of ultrasonic motors. This is because the micro-amplitude vibration of the piezoelectric transducer is transmitted to the macro motion of slider/rotor of ultrasonic motors by friction. Elliptical motion is one of the significant evaluation indexes for the vibration characteristics of the piezoelectric transducers. Therefore, how to construct efficient elliptical motion in the driving tip of the piezoelectric transducer is the key step of ultrasonic motor design.
In general, to generate the effective elliptical motion of the driving tip, the piezoelectric transducers are expected to operate in the coupling vibration mode [4]. This is because the elliptical motion of the driving tip is used to produce the friction between the stator and the rotor/slider of ultrasonic motors for achieving macro rotary/linear motion. Thus, the generation of the elliptical motion is significant for piezoelectric transducers. However, for producing the elliptical motion, two vibration modes are generally required to be stimulated and coupled in the piezoelectric transducers, increasing the difficulties of design, manufacturing, and assembling of transducer [5,6]. Additionally, how to achieve the resonant frequency consistency of the two selected vibration modes is also a difficult issue. To avoid these problems, single-phase excited piezoelectric transducer is the preferred option [7]. This is because this type of piezoelectric transducer only needs a phase drive signal to achieve the elliptical motion of its driving tip, eliminating the problem of adjusting resonant frequencies and simplifying the control system [8].
Researchers have made some efforts in the research of single-phase excited piezoelectric transducers [9][10][11][12][13][14][15][16][17][18][19]. Kurosawa and Ueha used a small mass to attach in a ring piezoelectric transducer, realizing two orthogonal vibration modes with a slightly different resonant frequency excited in the ring transducer under a single power supply [9]. Therefore, a circular ultrasonic motor was constructed. Utilization of the structural property of rectangular piezoelectric plate and the asymmetrical resonant excitation, Vyshnevskyy et al proposed a single-mode linear ultrasonic motor [10]. For this motor operating principle, a two-dimensional extensional standing wave is stimulated in the piezoelectric plate to achieve the slope-line elliptical motion of its driving tip. Based on the internal coupling effect caused by the crystal anisotropy, Tamura et al designed a thin-plate piezoelectric transducer to be the stator of the ultrasonic motor [11]. A coupling vibration mode was excited in a LiNbO3 rectangular plate vibrator to achieve the miniature single-phase drive ultrasonic motor. He et al presented a tubular ultrasonic motor that only uses a single vibration bending mode of a piezoelectric tube to produce rotation motion [12]. The piezoelectric tube is only excited by one driving signal and no vibration mode coupling is required. Yokoyama et al developed a linear ultrasonic motor using a single resonance mode driven by a single excitation signal, in which two square plates linked by V-shaped beams are employed as the motor stator [13]. Liu et al designed a sandwich plated piezoelectric transducer with single vibration mode, producing slash elliptical motion in its driving tip to push a slider for linear ultrasonic motor [14]. Chang and Tsai proposed a screw-thread-type ultrasonic motor 3 with single-phase excitation by designing an asymmetrical piezoelectric transducer to generate two orthogonal bending vibration modes [15]. Based on the structural properties of the transducer, Ma et al [16], Dabbagh et al [17], Fan et al [18], and Zhang et al [19] developed standing wave piezoelectric transducers with single phase excitation for pushing a rotor/slider to achieve ultrasonic motors, respectively. Most aforementioned piezoelectric transducers are designed as the standing wave vibrator for ultrasonic motors. However, the wear problem is hard to avoid for standing wave ultrasonic motors due to the limited contact area between their stator and slider/rotor. It has been demonstrated that traveling wave ultrasonic motors are the cost-efficient, durable and stable actuator during industrial applications due to the large contact area between the stator and the rotor during operation. Therefore, inspired by the transducer design method in references [9][10][11][12][13][14][15][16][17][18][19], a novel traveling wave sandwich piezoelectric transducer with single phase excitation is proposed in this study. Only one excitation signal is required to apply to the transducer, and traveling waves with the same rotational direction are produced in two driving rings utilizing the structural coupling property of a beam-ring combined element. This is done in order to achieve elliptical motion of surface points of the two driving rings, largely simplifying the control system. A pre-tensioning mechanism is designed to mount four groups of complete lead zirconate titanate piezoelectric ceramic (PZT) elements by one bolt, avoiding the unequal pre-stresses issue of all PZT elements. This paper is organized as follows. Initially, the configuration and operating principle of the proposed transducer are explained in detailed. Subsequently, an analytical model is developed utilizing the transfer matrix method (TMM) to study the dynamic behavior of the proposed transducer. In addition, experimental validation is conducted in order to validate the feasibility of the developed transfer matrix model and confirm the operating principle and design of the proposed transducer.
Moreover, motion trajectory of surface points and quality of the driving ring are calculated based on the developed model to study the driving concept of the transducer. Application example of the proposed transducer is then investigated to evaluate the driving performance of the proposed transducer prototype. Finally, a conclusion is presented.

Structure Design
The three-dimensional view of the proposed traveling wave sandwich piezoelectric transducer excited by single phase signal is illustrated in Figure 1. It is composed of a sandwich composite beam and two driving rings. The sandwich composite beam is comprised of a variable cross-section beam, a pre-tensioning mechanism, and four groups of PZT elements. The pre-tensioning mechanism consists of two isosceles trapezoid blocks, four right trapezoid blocks, and a bolt. Each PZT element is polarized along its thickness direction, and two pieces of complete and rectangular PZT elements with opposite polarization directions form a group of excitation unit. One isosceles trapezoid block and two right trapezoid blocks form a rectangular block. Therefore, the pre-tensioning mechanism is comprised of two rectangular blocks and a bolt. The middle of the variable cross-section beam is designed with two rectangular grooves at its upper and lower surfaces. A rectangular block and two groups of PZT elements placed at its both ends are combined together to fill into one rectangular groove. Therefore, the two isosceles trapezoid blocks are connected by the bolt, ensuring the pre-tensioning mechanism and the four groups of PZT elements are mounted in the variable cross-section beam. This ensures that all the PZT elements are applied with equal pre-stress. The two driving rings are placed at both ends of the sandwich composite beam.  Figure 1. Three-dimensional view of the proposed traveling wave sandwich piezoelectric transducer.

Operating Principle
In order to achieve the drive function, elliptical motion is required to be produced at surface points of the two driving rings. As an electrical signal is simultaneously applied to all the PZT elements, the sandwich composite beam is excited to vibrate at the first order longitudinal vibration, and two driving rings are stimulated to vibrate at a third order in-plane bending vibration, as shown in Figure 2(a).
Therefore, a periodical movement of each surface point of the driving ring along its radial direction is achieved. It should be pointed out that the two third order in-plane bending vibrations of the two driving rings have a spatial phase difference of π. Due to the structural coupling between the sandwich composite beam and two driving rings, the sixth order bending vibration is induced in the sandwich composite beam, and another third order in-plane bending vibration is produced in the driving ring along the tangential direction as well, as shown in Figure 2(b). The two third order in-plane bending vibrations produced in each driving ring along the radial and tangential directions, respectively, have a spatial phase difference of π/2, leading to the generation of traveling wave in each driving ring. For each surface point, its movements are occurred in the radial and tangential directions of the driving ring in the same time. This results in the generation of elliptical motion of the driving rings. As the two induced third order in-plane bending vibrations of the two driving rings along the tangential direction have a spatial phase difference of π, the traveling waves produced in the two driving rings rotate the same direction. Therefore, surface points of the driving rings move in elliptical motion with the same direction, as shown in Figure 2(c).

First order longitudinal vibration
Third order in-plane bending vibration along the radial direction

Analytical Modelling
To further understand the operating principle and analyze the dynamic behavior of the proposed transducer, an analytical model is developed using the TMM in the section. Compared to other analytical solutions, the TMM minimizes the computational effort and provides a precise solution.
Using the TMM, a complex system can be divided into many simple and continuous elements, such as beams, rods, or plates [20][21][22]. Then the independent transfer matrix model can be described for each element according to the basic wave motion equations and the arbitrary harmonic boundary conditions [23]. Through transfer conditions, the overall transfer matrix model of the entire system can be calculated by combining the independent transfer matrices of all discrete elements. Applying boundary conditions of the system, a linear equation set can be obtained and solved to describe the dynamic behavior of the complex system. For the PZT element, the mechanical and electrical properties can be simultaneously considered in its transfer matrix model [24], making the TMM suitable for the analysis of piezoelectric transducers.
The discrete model of the proposed sandwich piezoelectric transducer is shown in Figure 3. The transducer is divided into eleven elements, including two curved elastic beams, seven elastic beams, and two sandwich beams. The sandwich beam is composed of an elastic beam and two groups of PZT elements located on the upper and lower surfaces of the elastic beam, respectively. As the four groups 6 of PZT elements are excited to vibrate at the longitudinal vibration, the seven elastic beams and two sandwich beams are subjected to this vibration and the bending vibration induced by the structural coupling of the sandwich composite beam and driving ring. Therefore, the longitudinal and bending vibration transfer matrix models should be created for these beam elements. As the two curved beams are excited at the in-plane bending vibration, the corresponding transfer matrix model should be carried out. In the following sections, the aforementioned transfer matrix model of each discrete element will be given and explained in detail.

Longitudinal Vibration Modelling
Longitudinal vibration is the major vibration stimulated in the sandwich composite beam, therefore, the transfer matrix model of each discrete element in this beam must first be developed. In our previous work [23], [25][26], the longitudinal vibration transfer matrix model of the elastic beam has been presented as: ,11 ,12 ,21 ,22 (1) where vil and vir are the left and right velocities of the elastic beam i, respectively, Fil and Fir are the left and right forces of the elastic beam i, respectively, and TiL is the longitudinal vibration transfer matrix with following components: is the wave velocity, ω is the angular frequency, Ai is the cross-section area, Ei and ρi are the Young's modulus and density of the elastic beam i, respectively, and li is the length of the elastic beam i (The subscript i is the number of the discrete element, in which i⸦ [1 11] (i.e., i belongs to the interval 1 to 11)).
Due to the fact that each group of PZT elements operates at their d33 vibration mode to produce the longitudinal vibration, its transfer matrix model (see in figure 4) must simultaneously include the mechanical and electrical coupling properties [23][24][25][26], as written: where ()pl and ()pr are the parameters (force and velocity) of the two ends of the group of PZT elements, respectively, U and I are the input voltage and induced current of the group of PZT elements, respectively, and TpL denotes the transfer matrix of the group of PZT elements operating in the d33 vibration mode, in which its matrix components are expressed as follows:  For the sandwich beam, its transfer matrix model in longitudinal vibration can be described by combining the transfer matrices of the elastic beam and the two group of PZT elements and considering the boundary conditions, as shown in Figure 5. In our previous work [26], the coupling longitudinal vibration transfer matrix model was developed and expressed as:

Bending Vibration Modelling
Due to the structural coupling of the sandwich composite beam and driving rings, all discrete elements in the sandwich composite beam are induced to present bending vibration. According to the geometrical property of the elastic beam, its bending vibration can be formulated using the Timoshenko beam theory by considering the rotary inertia and shear deformation, which has been reported by Pestel and Leckie [20], written as: where shear velocity w, angular velocity  , bending moment M, and shear force Q are the mechanical parameters to describe the bending vibration of the elastic beam, and TiB is the bending vibration transfer matrix of the elastic beam, in which its matrix components can be found in the Appendix B.
For the sandwich beam, the two groups of PZT elements are subjected with the same bending vibration as elastic beams, meaning that the electric parameters do not involve in the bending vibration.
Therefore, the expression of the bending vibration transfer matrix model of the sandwich beam should be described the same as that of the elastic beam: where TsB is the bending vibration transfer matrix of the sandwich beam with the substitutions of where c is the curved beam number 1 or 11, R=(R1+R2)/2 is the radius in the neutral axis of the curved beam while its outer and inner radii are denoted as R1 and R2, respectively, the quantities k 2 , 2 1 k and 2 2 k are the dimensionless parameters, and 2 cc Therefore, the transfer matrix model of the curved beam with the in-plane bending vibration can be formulated as: where

Damping Characteristic
The damping effect of the real material has a substantial influence on the output performance of the proposed transducer and should therefore be considered in the transfer matrix model developed here.
As reported in reference [28], the damping effect can be described by the complex Young's modulus for general materials, and the complex compliance coefficient 33 with the dissipation factors tan(δ) and tan(η) for the dielectric loss and piezoelectric loss, respectively.

Transfer Matrix Model of Combined Vibrations
As the longitudinal and bending vibrations are simultaneously generated in the sandwich composite beam, a total transfer matrix model used to describe these two vibrations together should be developed to simplify the final analytical model. The combined transfer matrix model for the elastic beam can be formulated as:

Transfer Conditions
In order to combine all the discrete elements, transfer conditions between two neighbouring elements should be analyzed by following the rules of force balance and equal velocity at the connecting interface. In our proposed transducer, five kinds of transfer conditions exist. The first transfer condition is defined that the vibration is transferred from the left curved beam 1 to the neighbouring elastic beam 2, which can be formulated as: is the input state vector of the elastic beam 2.
The second transfer condition is used to describe the connection between two neighbouring elastic beams i and i+1, which can be written as: where the transfer condition matrix Cu is a unit matrix with the dimension of 6×6, where C4 is the transfer condition matrix with the dimension of 7×7.
The fourth kind of transfer condition is defined as the vibration that is transferred from the sandwich beam to the neighboring elastic beam i. In this case, the transfer condition should be described as: is the input state vector of the elastic beam i.
The last transfer condition is that the vibration transfers from the elastic beam 10 to the right curved beam, which is written as:  Using the aforementioned five kinds of transfer conditions, all the discrete elements can be physically combined together.

Overall Transfer Matrix Model
Combining all independent transfer matrices of the divided elements, transfer conditions, and boundary conditions, the overall transfer matrix model of the proposed transducer is formulated as: In this overall transfer matrix model, Sm is the systematic matrix, including total physical transfer matrix Tpm, total transfer condition matrix Cm, and total boundary condition extraction matrix Bm, Zm is the state vector containing all input and output parameters, and Am is the total electrical excitation conditions, as the entire transducer is a closed system without any mechanical boundary conditions. In the total boundary condition extraction matrix Bm, the matrix B with the dimension of 1×7 is used to extract the excitation voltage UsL applied to the sandwich beam. Therefore, the corresponding value of the extracted excitation voltage is expressed by the matrix A with the dimension of 1×1. It can be noted that the total transfer matrix model the proposed transducer is a linear equation set, which can be easily solved using the Matlab software. Thus, the dynamic behavior of the transducer can be calculated by solving the developed transfer matrix model.

Experimental Validation
To validate the effectiveness of the developed transfer matrix model and to confirm the feasibility of the design and operating principle of the proposed transducer, we first manufactured and assembled the prototype of the transducer and then conducted vibration measurements. The prototype of the proposed transducer with a weight of 225 g is depicted in Figure 6, and its geometrical sizes are listed in Table 1.
All elastic beams and curved elastic beams were made of stainless steel, and the PZT element was made of PIC-181 type piezoelectric ceramic (PI Company, Germany). The material parameters of the transducer prototype are listed in Table 2.

Frequency Response Characteristic
The frequency response characteristic of the transducer prototype was initially measured to obtain the resonant frequency of the operating vibration mode. The measured frequency was swept from 19 kHz to 20 kHz with an interval of 1 Hz while the excitation voltage was 1 V. The connection position between the sandwich composite beam and left driving ring was tested to obtain the frequency response characteristic of the transducer prototype and the result is plotted in Figure 8,

Vibration Shapes
At the measured resonant frequency, the vibration shapes of the left driving ring and the sandwich composite beam were measured with the excitation voltage of 2 V using the aforementioned experimental system, as illustrated in Figure 9. To further validate the developed transfer matrix model, the vibration shapes of the transducer were computed and compared to the experimental results in Figure 9. The calculated vibration shapes were computed at the calculated resonant frequency. The measured and computed first order longitudinal vibration shapes of the sandwich composite beam presented in Figure 9 (a) showed good agreement with each other. Due to the structural coupling between the sandwich composite beam and the left driving ring, the sixth order bending vibration was induced in the composited beam, as illustrated in Figure 9 (b). It can be seen that the computed bending vibration shape matches well with the experimental result. In Figure 9 (c), the radial and tangential displacements of each point in the left driving ring are calculated and exhibit the in-plane bending vibration shapes of the left driving ring in the radial and tangential directions. The third order in-plane bending vibration shapes in radial and tangential directions were observed, respectively, in which the 0° position is the connection place between the sandwich composite beam and left driving ring. The measured third order bending vibration shapes in tangential and radial directions matched well with the calculation results. The measured and calculated results indicated that the two third order in-plane bending vibration shapes in tangential and radial directions have a spatial phase difference of π/2. This is the reason why the traveling wave can be generated in each driving ring at a single phase excitation.
Certainly, the maximum amplitude of the left driving ring in the radial direction is larger than that in the tangential direction, indicating that the standing wave ratio (SWR) is larger than traveling wave. Therefore, energy dissipation is inevitable as the metal track is driven by the transducer prototype.
Comparisons between the experimental and simulated results adequately proved that the operating principle of the proposed transducer is feasible and the developed transfer matrix model is effective.

Investigation on the Driving Concept
Since the proposed transducer is excited to produce the traveling wave in each driving ring by a single phase signal, therefore, to investigate its driving concept is very important for further understanding of the operating principle of the transducer. Two aspects can be used to describe the driving concept clearly. One is the motion trajectory of surface points of the driving ring, and another one is the quality of the traveling wave of the driving rings. In this section, the investigations on the driving concept of the transducer are carried out.

Motion Trajectory
At first, we experimentally and theoretically investigated the motion trajectory of surface points of the driving ring. Three points (points A, B, and C) on the central line of the left driving ring were selected to study their motion trajectories, as shown in Figure 10 (a), in which the surface region from point A to point C is used to drive the metal track and point B is located on the middle of this region.
The excitation conditions are the same as in the last section, and the motion trajectories of the selected three points are measured and plotted in Figure 10

Quality of Traveling Wave
To study the influence of geometrical dimensions on the quality of the traveling wave in the driving rings, the model-based investigation was conducted. The quality of the traveling wave in each driving ring directly reflects the driving effect of the transducer. To quantitatively investigate the quality of the traveling wave in the driving ring, the SWR is utilized as the figure of merit. This is because the vibration of the driving ring is composed of standing wave and traveling wave at the single phase excitation. The SWR is defined as the maximum amplitude divided by the minimum amplitude. A pure traveling wave occurs when the SWR of the driving ring is equal to 1, while a pure standing wave exists as the SWR of the driving ring is infinite. Once the SWR of the driving ring is greater than 1, it indicates that the generated traveling wave is not pure. To calculate the SWRs of the driving ring in the radial and tangential directions, the whole driving ring is first divided into 360 elements and then amplitudes of all element interfaces in both directions are computed, respectively.
Due to the fact that the tangential displacement of surface points of the driving ring is induced by the structural coupling between the sandwich composite beam and driving ring, it means that the generated traveling wave would be disturbed by standing wave. To investigate the influence of geometrical dimensions on the SWR of the driving ring, we conducted calculations. At first, we studied the influence of the inner radius (R1) and outer radius (R2) of the driving ring individually. When R1 was varied from 15 mm to 16 mm in steps of 0.1 mm or R2 was changed from 19 mm to 20 mm in steps of 0.1 mm, the minimum SWR of the left driving ring is more than 100 in both directions, indicating that these two geometrical dimensions have a small influence on the SWR. Then we investigated the influence of the length (l2) of the elastic beam element connecting with the driving ring. The computed SWRs of the left driving ring in the radial and tangential directions are plotted in Figure 11

Application Example Investigation
To verify the driving effect of the proposed traveling wave sandwich piezoelectric transducer, an application example is presented in this section. In our previous work [29,30], tracked mobile systems driven by traveling wave piezoelectric transducers with two phases excitation signals have been investigated. A metal track is adopted as the rotor to be driven by a piezoelectric transducer via friction.
Compared to traditional driving forms utilizing electromagnetic motors and retarding mechanisms, the piezoelectric actuated method greatly simplifies the configuration of the tracked mechanism, eliminating the problem of lubricant volatilization and deflation within operating in vacuum environments, and is free from electromagnetic interference. Therefore, the approach utilizing piezoelectric transducers to drive track presents potential application for planetary explorations.
Referring to our previous designs, the application example of this proposed transducer is still to build a tracked mobile system, as shown in Figure 12.

Frequency Sensitivity Measurement
Although the resonant frequency of the operating principle of the transducer prototype has been determined, the assembly of the metal track changes its boundary conditions. Therefore, we conducted the frequency sensitivity measurement to find an optimal driving frequency with the maximum mean velocity for the tracked mobile system prototype. The experimental results are plotted in Figure 15. The excitation voltage applied to the system prototype was 300 VRMS. The driving frequency varied from 19360 Hz to 19460 Hz with a step of 10 Hz. The results showed that the mean velocity of the system prototype first increased and then decreased when the driving frequency increased, leaving an optimal driving frequency of 19410 Hz corresponding to the maximum mean velocity of the system prototype of 45.6 mm/s. Compared to the measured resonant frequency of the transducer prototype in the last section, the obtained driving frequency of the assembled system was shifted by 18 Hz. This indicates that the installation of the metal track changed the resonant frequency of the transducer prototype, due to the fact that its vibration property was primarily influenced by the contact stiffness between the transducer and the metal track.

Motion Characteristic
Based on the measured optimal driving frequency of 19410 Hz, motion characteristics of the system prototype were tested under different voltages by using the aforementioned experimental setup ( Figure   14), as illustrated in Figure 16. The motion characteristic curve showed that the mean velocity of the system prototype rose with the increase of the excitation voltage. It can be found that a specific mean velocity of the system prototype occurred in the excitation voltage of 250 VRMS, making that the motion characteristic curve varied non-linear. This is caused by the accumulation of experimental errors due to the fact that each measured mean velocity was calculated in terms of the experimental results of 5 times.
The maximum mean velocity of 59 mm/s was obtained when the excitation voltage was 500 VRMS. The results demonstrated that the transducer prototype can drive the metal track while it was excited with a single-phase voltage varied from 150 VRMS to 500 VRMS.

Traction Performance
The traction force is generally adopted as an important index for evaluating the performance of the mobile system. For our assembled tracked mobile system, we built an experimental setup to test its traction performance, as illustrated in Figure 17 (a). To avoid slide phenomenon during measurements of the assembled system, the surface of the vibration isolation platform was covered with abrasive papers with the roughness of P100 (Germany standard). The measured optimal frequency of 19410 Hz was still used as the driving frequency for traction performance measurements of the prototype.
First, the maximum stalling traction performance was measured under different excitation voltages, and the results were plotted in Figure 17 (b). The maximum stalling traction force increased linearly when the excitation voltage increased. When the voltage was 500 VRMS, the maximum stalling traction force reached up to 1.65 N. For real applications, the traction performance of the tracked mobile system should be improved.
Additionally, the loading characteristic of the system prototype was measured using the same experimental setup when the excitation voltage was 300 VRMS, as shown in Figure 17

Comparisons
The measured performances of the single phase excited tracked mobile system are compared to some developed traveling wave tracked mobile systems under the two excitation voltages, as listed in Table 3.
The maximum mean velocity and loading weight of the tracked mobile system driven by a surface-bonded type piezoelectric transducer with four driving rings [29] are 57 mm/s under two excitation voltages of 460 VRMS and 240 g under two excitation voltages of 460 VRMS, respectively. The tracked mobile system driven by a sandwich piezoelectric transducer with two driving rings [30] achieved a maximum mean velocity of 72 mm/s under two excitation voltages of 500 VRMS and a maximum loading weight of 1440 g two excitation voltages of 300 VRMS. Although the maximum mean velocity of the proposed tracked mobile system is a little lower than the aforementioned two tracked mobile systems, it only required a single-phase excitation voltage, simplifying the control system. It should be pointed out that the loading weight for the tracked mobile system reported by references [29,30] indicates the actual loading while our measured loading weight in the last section represents the traction force for the assembled system prototype, thus, the comparison is not given for these three tracked mobile systems.

Conclusion
A novel traveling wave sandwich piezoelectric transducer with single phase excitation was proposed, theoretically analysed, experimentally validated and investigated in this study. Using the structural coupling property between the sandwich composite beam and driving rings, the proposed piezoelectric transducer can produce traveling waves in the driving rings under a single-phase excitation signal, greatly simplifying the control system compared to the piezoelectric transducers with two phase different excitation signals [29,30].
An overall transfer matrix model was first created to describe the dynamic behavior of the proposed

Appendix A
The components of the longitudinal vibration transfer matrix TsL of the sandwich beam are expressed as follows: