4.2 Finite Element Simulation of the DFD SAW Motor
4.2.5 Comparison study of different voltage supply of excitation
direction. Similarly, if IDTs present on the other side is activated instead of present activated IDTs, the slider will perform a translational motion in the opposite direction i.e. in the +X direction. The contact pressure at the bottom side point of the slider in contact with stator is greater than the top side point of the slider to the stator. The Figure 4. 15 shows the simulated result of contact pressure of the top and bottom points of the slider in contact with the stator.
Due to the arrangement of sandwich SAW motor accomplish miniaturisation of these devices including weight and size reduction keeping the output efficiency unharmed through the combination of the linear motor package. It gets rid of the guide rail and lubrication by providing friction drive on both sides of the moving part. This invention has applications where a light weight and the efficient motor is required to move the elements such as lenses in the camera, mirrors in precision instruments.
The wavelength of the SAW motors is very short in nature, approximate to micrometres thence the driving frequency rises to MegaHertz range because of the miniaturisation of the stator. Because of which the frequency drive condition has to be tested at high frequency in MegaHertz range and the elastic vibration amplitude i.e. in nanometres. In this case, as the device goes to micro level in terms of size and operates with small amplitude i.e. around 12 nm and high frequency about 8.28 MHz, the effect of contact pressure due to air becomes significant. So it is expected that a high contact pressure between stator and slider is required for the friction drive. The motion of the slider upon the surface of the stator is both in the forward and backwards direction of the horizontal axis.
Poisson's ratio Stator ν2 0.345
Radius of projection R 20 µm
Voltage applied V 100/125/150 V
Wavelength λ 400 µm
Mass of the ball m 0.4 µg
Static coefficient of friction s 0.45 Dynamic coefficient of friction d 0.15
The two different normal displacement of the particle motion on the surface of the stator for two different excitations is represented by Figure 4. 16. It shows the amplitude of the wave in the normal direction as 11 nm for 150 V and 7 nm for 100 V.
Figure 4. 16: Displacement of the particle present on the surface of the stator in the normal direction.
Figure 4. 17: Displacement of the particle present on the surface of the stator in the tangential direction.
The difference in the translational displacement of the particle motion on the surface of the stator for two different excitations is represented by Figure 3. 39. It shows the amplitude of
-12 -7 -2 3 8 13
0 1 2 3 4 5
Normal displacement (nm)
Time (µs)
150 V 100 V
-6 -4 -2 0 2 4 6
0 1 2 3 4 5
Translational Displacement (nm)
Time (µs)
150 V 100 V
the wave in the translational direction as 6 nm for 150 V and 5 nm for 100 V. In Figure 4. 18, it shows the simulated result of displacement of the slider in the SAW motor after a time of 10 µs. Figure 4. 18 shows how the slider has moved a distance as compared to the position of the slider at time = 0. At this point of time, the displacement of the slider in normal direction gets stabilised. As the number of the wave passes under the slider in the normal direction the oscillation of the slider reduces and picks the inertia and converts all the energy to move in a translational direction. The normal direction displacement is shown in Figure 4. 19, where the slider moves upto 3 mm/s for 150 V and 2 mm/s for 100 V of the excitation at the initial phase.
Figure 4. 18: Schematic of motion of the slider in SAW motor structure.
Figure 4. 19: Displacement of the slider of SAW motor in the normal direction.
The translational displacement of the slider observed about 350 nm and 150 nm for the excitation of 150 V and 100 V respectively for the motion for 10 µs as shown in Figure 4. 20.
Upto 2 µs, the slider used to move at the almost same scale of displacement but as the time
-6 -5 -4 -3 -2 -1 0 1 2 3 4
0 2 4 6 8 10
Normal direction velocity of the slider (mm/s)
Time (µs)
100 V 125V 150 V
IDT Projections
Displaced Slider Stator: LN
progresses the difference in the displacement of the slider for both the amplitude of the excitation changes. In both cases, the slider moves proportionally to the cycle of the SAW.
Figure 4. 20: Displacement of the slider of SAW motor in the tangential direction.
The velocity of the slider for a timed lap of 10 µs for both the excitations is carried out. It moves along with the passage of each cycle. So each cycle of the Rayleigh SAW contributes to moving the slider. As the slider is having cylindrical projections on the both sides of its contacting surfaces. Due to the friction of contact the slider used to get the force at the top and bottom stators to move with the wave. Each cycle of the SAW contributes to moving the slider in steps, Figure 4. 21 shows the enlarged plot for the motion of the slider. In the enlarged plot, it is clearly visible that the slider moves in steps.
Figure 4. 21: Graph showing an enlarged version of motion of slider for (a) 100 V (b) 150 V.
Each step contributes an approximate of 0.2 mm/s to the slider to boost and move further in its path along with the propagation of the SAW. The plot gives an idea of motion of the body, as the wave passes under the slider, the particle puts pressure on it. The pressure on the slider is generated through the top point of the wave making anti-clockwise elliptical
-50 0 50 100 150 200 250 300 350
0 2 4 6 8 10
Linear displacement of the slider (nm)
Time (µs)
100 V 125V 150 V
13 13.2 13.4 13.6 13.8 14
9.5 9.7 9.9
Speed of the slider (mm/s)
(a) Time (µs)
30 30.2 30.4 30.6 30.8 31 31.2 31.4
8.5 8.7 8.9
Speed of the slider (mm/s)
(b) Time (µs)
motion and moves ahead. But when the wave is completely passed then due to the bottom point of the wave, it drags the slider back for which a small back step motion is occurring.
Figure 4. 22: Schematic showing contact pressure generated at (a) bottom and (b) top contacting point of the SAW motor.
Figure 4. 22 shows the simulated structure of the SAW motor for the contact pressure generated by the stator due to bottom and top projections of the slider.
Figure 4. 23: Graph showing the contact pressure generated by contacting point of the SAW motor.
When slider moves on the surface of the stator due to provided excitations, the slider puts pressure on the stator. The figure shows that the points available at the ends sides carry much pressure as compared to the projections available inside. Contact pressure compares to the application of excitation is very clear, where with the application of 100 V, it generates a pressure of about 10 MPa as compared to 30 MPa in 150 V. The high amplitude
-10 -5 0 5 10 15 20 25 30 35
0 2 4 6 8 10
Contact pressure (MPa)
Time (µs)
100 V 150 V
(a) (b)
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.
4.2.6 Comparison study of different shape of projections on the surface of the