Designing Novel MEMS Cantilevers for Marine Sensing Robots Using COMSOL Modeling and Different Piezoelectric Materials ^†

Abstract

The present work presents an innovative marine sensing robotics device based on piezoelectric cantilever-integrated micro-electro-mechanical systems (MEMSs) modeled on fish lateral lines. The device comprises 12 cantilevers of different shapes and sizes in a cross-shaped configuration, embedded between molybdenum (Mo) electrodes in a piezoelectric thin film (PbTiO₃, GaPO₄). It has the advantage of a directional response due to the unique design of the circular cantilevers. In COMSOL software 5.5, we designed, modeled, and simulated a piezoelectric device based on a comparative study of these piezoelectric materials. Simulations were performed on cantilever microstructures ranging in length from 100 µm to 500 µm. These materials perform best when lead titanate (PbTiO₃) is used. A maximum voltage of 4.9 mV was obtained with the PbTiO₃-material cantilever with a displacement of 37 µm. A laser Doppler vibrometer was used to measure the resonance frequency mode and displacement. Our simulations and experiments were in good agreement. Its performance and compactness allow us to envision its employment in underwater acoustics for monitoring marine cetaceans and ultrasound communications. In conclusion, MEMS piezoelectric transducers can be used as hydrophones to sense underwater acoustic pulses.

1. Introduction

Human scientific advancements have always been inspired by nature. The mechanisms by which living organisms sense the presence of other species have been uncovered and applied to various sectors by humans [1,2]. Animals obtain information from their environments through mechanoreceptors and translate it into significant biological signals for survival [3,4]. As an example, the lateral line system of a fish aids it in recognizing external stimuli and responding accordingly. Artificial hair-like sensors in water, such as hydrophones, can be developed by mimicking these natural cilia. Researchers have been investigating biomimetic cilia-based devices using micro-electro-mechanical systems (MEMSs). A piezoelectric hydrophone is an acoustic device that detects underwater sounds and signals; therefore, it is essential in marine resource exploration and sonar systems [5,6,7,8].

MEMS cantilevers for underwater acoustic sensors have been created for marine sensing robotics [9]. Our MEMS cantilever forms a directional hydrophone that detects the direction of travel [10,11,12,13,14,15].

Many researchers have been interested in microelectromechanical systems (MEMSs) over the past two decades, and particularly in microsensors and actuators. Pressure sensors are one of the most essential MEMS [16,17]. Nanoscale and microscale devices can be developed efficiently with piezoelectric thin films because of their electromechanical couplings and micromachinability [18,19]. PZT, BaTiO3, ZnO, and PZT thin films are used as surface acoustic wave (SAW) filters, bulk acoustic wave (BAW) resonators, and actuators in MEMS/NEMS systems [20,21]. As an acoustic device, the piezoelectric hydrophone detects underwater noises and signals and is helpful in marine sensors and robotics [22,23]. MEMS flow sensors based on biomimetic mechanoreceptors have been developed [24,25].

COMSOL was used to study the displacement and voltage response of MEMS cantilevers made from PbTiO₃ and GaPO₄, two piezoelectric materials. Miniaturization, sensitivity, and bandwidth are all critical components of the proposed research.

2. Principles of Bionics and Vibration Sensing

The lateral line in fish contains neuromasts and cilia-based mechanoreceptors. Each of these cilia is covered by a jelly-like cupola and located along the body canals or on the surface of the fish. Figure 1a–c show representations of fish lateral lines, while Figure 1d shows the schematics of its sensing mechanism.

3. Device Design and Modeling

Simulations were carried out using COMSOL Multiphysics FEM software to implement the constitutive equations of piezoelectricity. Electrical and mechanical properties can be coupled through piezoelectricity. Piezoelectric materials produce electrical charges upon mechanical deformation and vice versa. The stress-charge form of these piezoelectric constitutive equations, also known as “coupled equations”, can be found below [10,11].

T = s^E S − e^T E

(1)

D = e S + ε E

(2)

where S is the strain tensor, s^E is the elasticity matrix, T is the stress tensor, e is the piezoelectric coupling matrix, D is the tensor of the electric displacement, ε is the electrical permittivity, and E is the electric field.

PbTiO₃ and GaPO₄ were used to simulate and compare different piezoelectric materials to determine the best material to use for MEMS cantilevers. We simulated cantilever microstructures between 100 and 500 µm in length in order to investigate how length affects their displacement and voltage response. The behavior of these microcantilevers was studied using solid mechanics, electrostatics, and pressure acoustics. Additionally, one end of the cantilever was constrained and the other was free. Static equilibrium existed in each cantilever layer. Each cantilever had a fixed width of 50 µm. We used metal electrodes with a thickness of 200 nm and a piezoelectric thin film of 1 µm thickness on the microcantilevers. Figure 2 represents the schematic of MEMs cantilevers with their stacks of layers.

4. Results

We used COMSOL Multiphysics to determine the displacements and potential voltage responses of microcantilevers of different lengths between 100 µm and 500 µm and the results are presented in Figure 3. Our simulated results showed that microcantilevers made of PbTiO₃ had the largest displacements, whereas those made of GaPO₄ showed the smallest displacements. Further, the microcantilevers that showed the most significant potential voltage responses were made from PbTiO₃.

Figure 4 illustrates the measured and simulated frequency responses of the MEMS cantilevers. It shows that the microcantilevers fabricated with both materials have similar frequency responses.

5. Conclusions

This work presents the design and modeling of MEMS cantilevers using COMSOL (Multiphysics). In COMSOL, the MEMS piezoelectric cantilevers were analyzed using built-in material property, thickness, and motion equations. The setup and parameters of the simulation were defined. Based on these simulations, PbTiO₃ performs best out of these piezoelectric materials. The design and optimization of piezoelectric micro-cantilever pressure sensors can be guided by comparative analyses. The amplitude and direction of underwater acoustic pulses can be measured using MEMS piezoelectric cantilevers. In addition to cantilever lengths, cross-configurations are useful for identifying the directions of acoustic waves.