A sandwich structure SAW motor can be made using a flat plane slider[99] as shown in Figure 4. 36. The arrangement of the DFD SAW motor is done as like previous section motor.
In this type of motor, the material of stator changes from Single crystal lithium niobate LiNbO3 to chemically reduced LiNbO3. Due to thelarge piezoelectric coefficient, the single crystal lithium niobate is used widely for developing surface acoustic wave (SAW) devices.
Low electric conductivity, which is on the order of 10−15 (Ω.cm)−1[100], coupled with the intrinsic pyroelectricity of lithium niobate generates pyroelectric surface charges with temperature changes. The surface charging effect causes contact electrification when lithium niobate is used as a contacting material. The surface charging and due to this sparking occurs which may damage electrodes and crack the crystals during device production processes [101].
Figure 4. 36: Schematic of DFD SAW motor with the flat plane slider.
Table 4. 5: Parameters taken for simulation of the device
Parameter symbol value Units
Frequency applied f 8.28e6 [Hz]
Preload Fn 2.63e-3 [N]
Young's modulus slider E1 1.69e11 [Pa]
Young's modulus stator E2 1.72e11 [Pa]
Poisson's ratio slider ν1 0.3 Poisson's ratio Stator ν2 0.345
Voltage applied V 150 [V]
Wave length λ 400e-6 [m]
length ln 340e-6 [m]
width wd 20e-6 [m]
Mass of the slider m 0.4e-6 [kg]
Static coefficient of friction s 0.45 Dynamic coefficient of friction d 0.15
The surface charging is predicted to increase water adsorption on the surface in an atmospheric environment due to the diffuse electric double layer effect [16]; the film
Propagation of SAW
Damping material
Piezoelectric stator
z x
Motion of slider
Glass Substrate
Piezoelectric stator Glass Substrate
IDT
Damping material
IDT Spacer
Spacer Slider
Preload
thickness of water on lithium niobate increases with relative humidity up to more than 15 nm [17]. The wetting water produces capillary condensation around surface sites such as cracks and pores or asperity contact sites when contacting rough surfaces [102]. The adhesion force between slider and stator at contacting surfaces increases the relative static coefficient of friction [103].The surface charging effect can be reduced by increasing of electric conductivity while keeping SAW propagation characteristics unchanged [14].
Hence, the post-growth chemical reduction method for lithium niobate is studied [14], [20], because the electric conductivity of dielectric crystals increases when they are reduced [20].
Contact electrification is considered to be the cause of sticking phenomenon in the SAW motor in the present study [21], [22]. The use of chemically reduced lithium niobate, hence, would eliminate the undesirable sticking phenomenon through reduced surface charging and enhance the stability of the SAW motor.
Figure 4. 37: Schematic model of the of DFD SAW motor in COMSOL Multiphysics after meshed.
Figure 4. 38: Simulated surface profile of the DFD SAW linear motor showing the motion of the flat plane cuboid slider.
IDT Stator: LN
Slider at t = 0
Stator: LN
Displaced slider IDT
Stator: LN
Slider at t
= 0
Stator: LN
The SAW motor needs threshold voltage; that is, it expands its dead zone, in a humid environment, which is probably due to the increase of meniscus force and the resulting static frictional force. Because the adsorbed water film thickness increases with the surface charge [16], reducing the surface charge would diminish the film thickness and the meniscus force.
Thus, it could be expected that the use of chemically reduced lithium niobate would reduce the expansion of the dead zone and make the SAW motor less sensitive to humidity. The Figure 4. 37 shows the Schematic model of the of DFD SAW motor in COMSOL Multiphysics after meshing is carried out for the simulation. The surface profile of the displaced slider is shown in Figure 4. 38. The displacement amplitude of the point on stator is7 nm in the normal direction and 7 nm in translational directions.
Figure 4. 39: Normal displacements of the slider of simulated SAW motor.
Figure 4. 39 shows normal displacement of slider indicating the highest oscillation at the initial phase of excitation.
Figure 4. 40: Translational displacement of slider of simulated SAW motor.
-0.008 -0.006 -0.004 -0.002 -2E-17 0.002 0.004 0.006
0 2 4 6 8 10
Normal velocity of the slider (m/s)
Time (µs)
100 V 150 V 180 V
-0.05 3E-16 0.05 0.1 0.15 0.2 0.25 0.3 0.35
0 2 4 6 8 10
Displacement of the slider (µm)
Time (µs)
100 V 150 V 180 V
The motion of the slider is stabilised in the normal direction and moves in the translational direction. The translational displacement of the slider in the SAW motor is simulated for three different voltage signals as shown in Figure 4. 40. It is found that the slider achieves a distance of 0.105 µm with the application of 100 V at the end of 10 µs. While 0.267 µm and 0.35 µm are achieved at the end of 10 µs for the same slider with the application of 150 V and 180 V respectively.
The contact pressure on the surface of the stator in the SAW motor is simulated for three different voltage signals as shown in Figure 4. 41. It is found that the contact pressure of 10 MPa, 20 MPa, and 25 Mpaon the surface of the stator is measured with the application of 100 V, 150 V, and 180 V respectively.
Figure 4. 41: Contact pressure of simulated SAW motor.
Figure 4. 42: Force applied on the slider of simulated SAW motor.
The force on the contacting surface of the slider is simulated for the SAW motor for three different voltage signals as shown in Figure 4. 42. This simulation is carried out without any phase difference of the applied excitation to the top and bottom stator with a Voltage supply of 100 V, 150 V, and 180 V.
-10 0 10 20 30
0 1 2 3 4 5
Contact pressure (MPa)
Time (µs)
100 V 150 V 180 V
-1 0 1 2 3
0 0.5 1 1.5 2
Force on the slider (mN)
Time (µs)
100 V 150 V 180 V
4.5.1 Comparative study of different shape of sliders
The comparison among normal motions of 3 types of the slider is represented in Figure 4.
43 of the SAW motor made. From the plot for normal displacement of the slider, it shows all are achieving their stabilisation condition after the slider is run for some time.
Figure 4. 43: Normal direction displacement of the slider.
The force generated on the various types of the slider through the stator is shown in Figure 4. 44. The plot shows that the slider having no projections gets the high force of about 0.25 mN to be driven by the wave. The slider having cylindrical projections gets a force of 0.05 mN, which very less as compare to other two types of the slider.
Figure 4. 44: Force acting on the slider due to Rayleigh wave propagation on the surface of the stator.
The translational displacement of the slider observed about 25 nm, 22 nm and 11 nm for projection, square and cylindrical cross-sectional projections respectively for the excitation of 100 V in 1 µs as shown in Figure 4. 45. The initial phase of transition the slider having cylindrical and no projections started making motion whereas the slider having square
-8 -6 -4 -2 0 2 4 6
0 0.2 0.4 0.6 0.8 1
Normal displacement of slider (nm)
Time (µs)
Cube Without Cylindrical
-0.2 -0.1 0 0.1 0.2 0.3 0.4
0 0.2 0.4 0.6 0.8 1
Force on slider (mN)
Time (µs)
Cylindrical Cube Without
section projections starts with a less time but overtakes the cylindrical type projection of slider. In all the cases the slider moves proportionally to the cycle of the Rayleigh wave.
Figure 4. 45: The translational motion of slider.
Comparison plot for contact pressure is showing that the slider having no projections generate less pressure as compare to other two as shown in Figure 4. 46. The slider with cylindrical and square projections generate almost equivalent contact pressure on the stator.
Figure 4. 46: Contact pressure between slider and stator at contacting interface.
Compare to the application of excitation is very clear, where it generates a pressure of about 12 MPa in both cylindrical and square whereas 5 MPa in the case of the slider having no projections. The high amplitude results in a high-pressure generation at the contacting point of the slider and stator. This shows that the high pressure is generated due to high contact, and the high contact means a high amount of force will be applied to the contacting projections, which makes high-speed motion as compared to the low excitation.
-5 0 5 10 15 20 25
0 0.2 0.4 0.6 0.8 1
Displacement of the slider (nm)
Time (µs)
Cylindrical Cube Without
-10 -5 0 5 10 15
0 0.2 0.4 0.6 0.8 1
Contact pressure on stator (MPa)
Time (µs)
Cylindrical Cube Without