**3. Results and discussion**

The fabricated MEMS air turbines and the multilayer ceramic magnetic circuits were evaluated. The combined electromagnetic induction-type MEMS air turbines were evaluated on the power generation.

#### **3.1. Fluid dynamic bearing system air turbine generator**

The fabricated components of the fluid dynamic bearing system air turbine and the assembled MEMS air turbine are shown in **Figure 9**. Designed dimensions and the measured dimensions are shown in **Table 1**. As a result, it is found that the error was less than 5 μm.

**Figure 10** shows the fabricated multilayer ceramic magnetic circuits. The dimensions of the step-wise shape multilayer ceramic circuit were 3.40, 3.47 and 1.88 mm, length, width and height, respectively. Inductance and DC resistance were 5.35 μH and 0.53 Ω. The dimensions of the horseshoe shape circuit were 3.25, 3.49 and 1.34 mm, length, width and height, respectively. Inductance and DC resistance were 5.85 μH and 0.94 Ω.

The result of the power generation experiment by the spindle machine is shown in **Table 2**. The load resistance of 1 Ω was connected to both magnetic circuits. The rotational speed of the spindle machine was 300,000 rpm. By the results, the maximum output power of the

**Figure 9.** Fabricated fluid dynamic bearing system air turbine components.

**Figure 8.** Schematic illustration of the combined process for the complex structure coil.

coil pattern.

180 MEMS Sensors - Design and Application

structure coil is shown in **Figure 8**.

sheet, the magnetic core is formed simultaneously. Through this process, the obtained specimen was a planar structure that had the miniature coil pattern inside the magnetic ceramic. **Figure 7** shows the schematic illustration of the fabrication process for the multilayer ceramic

**Figure 7.** Schematic illustration of fabrication process for multilayer ceramic coil pattern.

In order to combine the MEMS air turbine, more complicated structure is required. Each part was combined for the design structure. After that, the specimen was fired in the electric furnace. In the around-type coil, the pieces were firing. After that, the pieces were combined because a shrinkage process deforms the ceramic coil. Through these processes, the objective structure was completed. A schematic illustration of the combined process for the complex


**Table 1.** Dimensions of the MEMS air turbine components.

ture of the rim-type rotor blade air turbine. Dimensions were 5.24, 5.37, 4.64 mm length, width and height, respectively. These fabricated MEMS air turbine were evaluated on the rotational speed. The rotational speed was measured by a hall sensor. The comparison of the rotational speed is shown in **Figure 14**. The flow rate changed from 0 to 1.0 l/min. at pressure of 0.3 MPa. Experimental result shows that the rim-type system had a superior potential to the planar type. The flux change dependents the rotational speed because the magnet attached to the rotor. The high rotational speed air turbine is advantage for high output power. Therefore, to achieve high

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The fabricated multilayer ceramic magnetic circuit was shown in **Figure 15**. Dimensions of the circuit were 7.40, 8.47, 2.36 mm, length, width and height, respectively. The measured DC

The fabricated rim-type air turbine and the magnetic circuit were combined. **Figure 16** shows the combined electromagnetic induction-type MEMS air turbine generator. Dimensions of the generator were 7.40, 8.47, 5.82 mm, length, width and height, respectively. When the maximum

output power, the rim-type air turbine was employed the generator.

**Figure 12.** Fabricated components and assembled structure of flat-type rotor blade air turbine.

**Figure 11.** Fabricated MEMS air turbine generator and its output voltage waveform.

resistance was 2 Ω.

**Figure 10.** Fabricated multilayer ceramic magnetic circuit: (a) step-wise shape and (b) horseshoe shape.

step-wise shape was 1.47 mVA, and the output power of the horseshoe shape was 0.72 mVA. The step-wise shape magnetic circuit showed the larger output power than the horseshoe shape magnetic circuit. It is agreed from the result of the analyses. Therefore, it is found that the magnetic material surrounding the magnet improved the output power.

The fabricated MEMS air turbine and the multilayer ceramic magnetic circuit were combined. **Figure 11** shows the MEMS air turbine generator and its output voltage waveform. The step-wise shape magnetic circuit that shows the internal resistance of 1.05 Ω was used. The dimensions of the MEMS air turbine generator were 3.50, 3.47 and 3.86 mm, length, width and height, respectively. The maximum rotational speed was 30,000 rpm on the condition of 0.28 MPa. The load resistance of 1 Ω was connected to the output of the magnetic circuit. The maximum output voltage and the output power of the generator were 1.32 mV and 1.74 μVA, respectively. In the rotational motion, the fluid dynamic bearing system air turbine rotor showed the eccentric motion.

#### **3.2. Miniature ball bearing air turbine generator**

The fabricated components and the assembled structure of the flat-type rotor blade air turbine are shown in **Figure 12**. Dimensions of the structure were 5.23, 5.20, 4.51 mm length, width and height, respectively. **Figure 13** shows the fabricated components and the assembled struc-


**Table 2.** Power generation results of fabricated magnetic circuit using spindle machine.

**Figure 11.** Fabricated MEMS air turbine generator and its output voltage waveform.

step-wise shape was 1.47 mVA, and the output power of the horseshoe shape was 0.72 mVA. The step-wise shape magnetic circuit showed the larger output power than the horseshoe shape magnetic circuit. It is agreed from the result of the analyses. Therefore, it is found that the mag-

**Design dimension (μm) Measured dimension (μm)**

**Figure 10.** Fabricated multilayer ceramic magnetic circuit: (a) step-wise shape and (b) horseshoe shape.

The fabricated MEMS air turbine and the multilayer ceramic magnetic circuit were combined. **Figure 11** shows the MEMS air turbine generator and its output voltage waveform. The step-wise shape magnetic circuit that shows the internal resistance of 1.05 Ω was used. The dimensions of the MEMS air turbine generator were 3.50, 3.47 and 3.86 mm, length, width and height, respectively. The maximum rotational speed was 30,000 rpm on the condition of 0.28 MPa. The load resistance of 1 Ω was connected to the output of the magnetic circuit. The maximum output voltage and the output power of the generator were 1.32 mV and 1.74 μVA, respectively. In the rotational motion, the fluid dynamic bearing system air turbine rotor

The fabricated components and the assembled structure of the flat-type rotor blade air turbine are shown in **Figure 12**. Dimensions of the structure were 5.23, 5.20, 4.51 mm length, width and height, respectively. **Figure 13** shows the fabricated components and the assembled struc-

**Output voltage [mV] Output power [mVA]**

netic material surrounding the magnet improved the output power.

Rotor diameter 1580 1578.27 Stator diameter 1600 1603.32

**Table 1.** Dimensions of the MEMS air turbine components.

182 MEMS Sensors - Design and Application

showed the eccentric motion.

**3.2. Miniature ball bearing air turbine generator**

Step-wise shape 26.8 1.47 Horseshoe shape 38.4 0.72

**Table 2.** Power generation results of fabricated magnetic circuit using spindle machine.

ture of the rim-type rotor blade air turbine. Dimensions were 5.24, 5.37, 4.64 mm length, width and height, respectively. These fabricated MEMS air turbine were evaluated on the rotational speed. The rotational speed was measured by a hall sensor. The comparison of the rotational speed is shown in **Figure 14**. The flow rate changed from 0 to 1.0 l/min. at pressure of 0.3 MPa. Experimental result shows that the rim-type system had a superior potential to the planar type. The flux change dependents the rotational speed because the magnet attached to the rotor. The high rotational speed air turbine is advantage for high output power. Therefore, to achieve high output power, the rim-type air turbine was employed the generator.

The fabricated multilayer ceramic magnetic circuit was shown in **Figure 15**. Dimensions of the circuit were 7.40, 8.47, 2.36 mm, length, width and height, respectively. The measured DC resistance was 2 Ω.

The fabricated rim-type air turbine and the magnetic circuit were combined. **Figure 16** shows the combined electromagnetic induction-type MEMS air turbine generator. Dimensions of the generator were 7.40, 8.47, 5.82 mm, length, width and height, respectively. When the maximum

**Figure 12.** Fabricated components and assembled structure of flat-type rotor blade air turbine.

**Figure 13.** Fabricated components and assembled structure of rim-type rotor blade air turbine.

rotational speed was 290,135 rpm, the inlet flow was 2.4 l/min and the pressure was 0.3 MPa. The output voltage and the output power at each load resistance are shown in **Figure 16**. The maximum output power was 2.41 mVA when the load resistance was 8 Ω, and the output voltage of 139 mV was shown. The output waveform at the load resistances was 8 and 1 kΩ, as shown in **Figure 17**.

The output voltage *V = R*L*I* is given by the following equation: (1) when the load resistance *RL* is connected to the generator and the current *I* flows through the circuit. In this equation, it is necessary to consider the influence of the voltage drops by the self-inductance *L* and the internal resistance *r* of the connected magnetic circuit.

$$V = R\_\text{L}I = N \, d\varphi / dt - i\omega LI - rI \tag{1}$$

The magnetic flux passing the magnetic circuit is ϕ. N is the turn number of coil. L is the measured value at the equivalent frequency of 290,135 rpm (impedance analyzer: Agilent 4294A), and it was 241 μH. When the *R*L was 1 kΩ, the voltage drop due to self-inductance and the internal resistance become small because the current carrying the circuit is sufficiently small. Therefore, the output voltage is approximated by *dϕ/dt*. The surface magnetic flux density of the permanent magnet and the magnitude of the magnetic flux on the magnet surface are 0.159 T and 0.375 μWb, respectively. Theoretical value if the all flux enters in the circuit, the output voltage was calculated to be 570 mV. Compared with the output voltage that connected 1 kΩ, 36% of the maximum magnetic flux contributes to the actual power generation. On the other hand, when *R*L of 8 Ω was connected, the reactance of *ωL* is calculated 7.3 Ω. As a result, values of *ωL* and *r* are close to *R*L, and their influence appears. The absolute value of

]

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<sup>1</sup>/<sup>2</sup> (2)

the output voltage is estimated as Eq. (2).

**Figure 16.** Output voltage and output power at each load resistance.

**Figure 15.** Fabricated multilayer ceramic magnetic circuit.

∣ *V* ∣ = (*N dϕ*/*dt R*L)/[(*R*<sup>L</sup> + *r*)<sup>2</sup> + *ω*<sup>2</sup> *L*<sup>2</sup>

**Figure 14.** Comparison of rotational speed.

**Figure 15.** Fabricated multilayer ceramic magnetic circuit.

rotational speed was 290,135 rpm, the inlet flow was 2.4 l/min and the pressure was 0.3 MPa. The output voltage and the output power at each load resistance are shown in **Figure 16**. The maximum output power was 2.41 mVA when the load resistance was 8 Ω, and the output voltage of 139 mV was shown. The output waveform at the load resistances was 8 and 1 kΩ, as

**Figure 13.** Fabricated components and assembled structure of rim-type rotor blade air turbine.

The output voltage *V = R*L*I* is given by the following equation: (1) when the load resistance

*V* = *R*<sup>L</sup> *I* = *N dϕ*/*dt* − *iωLI* − *rI* (1)

 is connected to the generator and the current *I* flows through the circuit. In this equation, it is necessary to consider the influence of the voltage drops by the self-inductance *L* and the

shown in **Figure 17**.

184 MEMS Sensors - Design and Application

**Figure 14.** Comparison of rotational speed.

internal resistance *r* of the connected magnetic circuit.

*RL*

**Figure 16.** Output voltage and output power at each load resistance.

The magnetic flux passing the magnetic circuit is ϕ. N is the turn number of coil. L is the measured value at the equivalent frequency of 290,135 rpm (impedance analyzer: Agilent 4294A), and it was 241 μH. When the *R*L was 1 kΩ, the voltage drop due to self-inductance and the internal resistance become small because the current carrying the circuit is sufficiently small. Therefore, the output voltage is approximated by *dϕ/dt*. The surface magnetic flux density of the permanent magnet and the magnitude of the magnetic flux on the magnet surface are 0.159 T and 0.375 μWb, respectively. Theoretical value if the all flux enters in the circuit, the output voltage was calculated to be 570 mV. Compared with the output voltage that connected 1 kΩ, 36% of the maximum magnetic flux contributes to the actual power generation. On the other hand, when *R*L of 8 Ω was connected, the reactance of *ωL* is calculated 7.3 Ω. As a result, values of *ωL* and *r* are close to *R*L, and their influence appears. The absolute value of the output voltage is estimated as Eq. (2).

$$\mid \mid V \mid = \left( \text{N } d\varphi/dt \, R\_{\text{L}} \right) / \left[ (R\_{\text{L}} + r)^2 + \omega^2 \, L^2 \right]^{1/2} \tag{2}$$

**Figure 17.** Output waveform at the load resistances were (a) 8 Ω and (b) 1 kΩ.

The calculated result of Eq. (2), |V| becomes 367 mV. If the contribution rate 36 % of the magnetic flux to this result, |V| become 132.12 mV. It almost coincides with output voltage at 8 ohm (**Figure 17(a)**).

In the electromagnetic induction type, the magnetic circuit occurs a braking torque to the permanent magnet. However, the rotational speed of **Figure 17(a)** and **(b)** shows almost equal. Therefore, the braking torque by current is sufficiently small in this turbine structure. The magnetic flux density from the magnetic circuit can be calculated at Eq. (3).

$$\mathbf{B} = \,\mu\_0 \,\mu\_r \,\text{NI} \tag{3}$$

output power of 1.74 μVA, when the rotational speed was 30,000 rpm, the output voltage was 1.32 mV and the load resistance of 1 Ω was connected. However, it showed eccentric motion because it was not supported by structurally. Therefore, another one of the air turbines used the miniature ball bearing system. The developed ball bearing air turbines were compared with the rotational speed between the different rotor blades. As a result, the rim-type rotor blade showed high rotational speed than the flat-type rotor. Moreover, the ball bearing-type air turbine could separate the magnet from the rotor. Therefore, the short distance between the magnet and the magnetic circuit was realized. The shape of the magnetic circuit was around type that had the magnetic flux induction parts. To evaluate the power generation, the rim-type air turbine and the around-type multilayer ceramic magnetic circuit were combined. The maximum rotational speed was 290,135 rpm. The output power of fabricated MEMS air turbine generator was 2.41 mVA when the load resistance was 8 Ω and the output voltage of 139 mV was shown. By these results, the milliwatt-level MEMS air turbine generator was realized by the high-speed rotational motion structure that had the rim-type rotor blade and the

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The sample of this study was fabricated by the facility at the Research Center for Micro Functional Devices, Nihon University. Part of this study was supported by the CST research

Department of Precision Machinery Engineering, College of Science and Technology,

[1] Bhardwaj Jy K, Ashraf H. Advanced silicon etching using high-density plasmas. In: Proceedings of the SPIE Micromachining and Microfabrication Process Technology; 19

[2] Long-Sheng F, Yu-Chong T, Muller RS. IC-processed electrostatic micro-motors. In: Proceedings of the Int. Electron Devices Meeting (IEDM '88). Technical Digest; 11-14

[3] Zhang W, Zou Y, Lin T, Chau FS, Zhou G. Development of miniature camera module integrated with solid tunable lens driven by MEMS-thermal actuator. Journal of Microelectromechanical Systems. 2017;**26**:84-94. DOI: 10.1109/JMEMS.2016.2602382

miniature ball bearing system, and by introduction of the magnetic flux.

project of Nihon University and by JSPS KAKENHI (16 K18055).

\*Address all correspondence to: takato@eme.cst.nihon-u.ac.jp

September 1995; Austin, TX, United States. pp. 224-233

December 1988; San Francisco, CA, United States. pp. 666-669

Minami Kaneko\*, Ken Saito and Fumio Uchikoba

**Acknowledgements**

**Author details**

**References**

Nihon University, Chiba, Japan

The vacuum permeability and the ferrite relative permeability are expressed in *μ*<sup>0</sup> and *μ*<sup>r</sup> . As a result, B is calculated as 1.96 mT. The maximum braking torque occurs when the magnetization of the permanent magnet and the magnetic flux density of the magnetic circuit cross perpendicular. If all the magnetic flux contributes, the braking torque is 42.9 pNm. This value is considered as sufficiently small value.

#### **4. Conclusions**

The electromagnetic induction-type MEMS air turbine generator was proposed. In this chapter, three types of MEMS air turbine generators that included the different bearing systems, shape of the rotor blades and shape of the magnetic circuits were discussed to achieve the high output power. In the MEMS air turbine, the purpose was achieving high-speed rotational motion. The magnetic circuit required the miniature structure that had the three-dimensional coil, magnetic core and magnetic flux introduction design. Therefore, the multilayer ceramic technology and the ferrite ceramic were used. One of the developed air turbines employed the fluid dynamic bearing system and flat-type rotor. In the miniature structure, the contactless-type miniature bearing system is advantaged because the friction force is impact issue. Moreover, two types of magnetic circuits for the fluid dynamic bearing turbine generator were compared with the magnetic flux loss. By the power generation experiment, the stepwise shape circuit that had the magnetic material introducing the magnetic flux from the magnet was suitable to the generator. The fabricated MEMS air turbine generator showed the output power of 1.74 μVA, when the rotational speed was 30,000 rpm, the output voltage was 1.32 mV and the load resistance of 1 Ω was connected. However, it showed eccentric motion because it was not supported by structurally. Therefore, another one of the air turbines used the miniature ball bearing system. The developed ball bearing air turbines were compared with the rotational speed between the different rotor blades. As a result, the rim-type rotor blade showed high rotational speed than the flat-type rotor. Moreover, the ball bearing-type air turbine could separate the magnet from the rotor. Therefore, the short distance between the magnet and the magnetic circuit was realized. The shape of the magnetic circuit was around type that had the magnetic flux induction parts. To evaluate the power generation, the rim-type air turbine and the around-type multilayer ceramic magnetic circuit were combined. The maximum rotational speed was 290,135 rpm. The output power of fabricated MEMS air turbine generator was 2.41 mVA when the load resistance was 8 Ω and the output voltage of 139 mV was shown. By these results, the milliwatt-level MEMS air turbine generator was realized by the high-speed rotational motion structure that had the rim-type rotor blade and the miniature ball bearing system, and by introduction of the magnetic flux.
