**2. Design and concept of MEMS air turbine generator**

ratio. MEMS process is based on the integrated circuit (IC) production process, and it can form a fine pitch pattern on a planer silicon structure. Moreover, the high accuracy and the high aspect ratio pattern are realized by a Bosch process [1]. By this process, it is possible to form miniature mechanical parts and fabricate an acceleration sensor, a gyroscope sensor, and so on. The miniature sensors progress a miniaturization of an electronic device, and it supports an information society. To realize the Internet of Things (IoT) society, the miniature, an enormous amount of the sensor is required, and these are usually produced by the MEMS. One of a field of the MEMS process, the mechanical field is researched because it can form a miniature high aspect ratio pattern. Miniature MEMS actuators are researched for miniature mechanical systems [2–5]. Moreover, attention is paid to microrobots that have miniature silicon mechanical components. These robots have miniature body structure and miniature actuator [6–9]. The miniature actuators and the miniature robot can be used for the medical field. The miniature structure can work in the narrow space such as inside of a human body. For

The MEMS can realize the development of the information society and the medical field. However, a miniature power source is required for the electronic device such as the sensor, the actuator and the microrobot. Conventionally, a lithium-ion-secondary battery is used as the small- and high-power density source, but it is too large for these electronic devices. Moreover, the power density of the lithium-ion-secondary battery is approaching to the theoretical limit. Therefore, the MEMS power generators have been studied for miniature power supply system. To keep a micro scale, many researchers use a piezoelectric vibration power generator [11, 12]. It uses only a material characteristic without complex mechanical structure. However, these generators harvest the force to move the device by the environmental vibra-

On the other hand, ultra-micro gas turbine (UMGT) that used an electrostatic type was reported by the MIT group [13]. It was a remarkable generator because of its extremely high energy density in small size. The advantage of the electrostatic-type power generator is that the components are based on planar structure, and it is easy to fabricate by the MEMS process. A lot of studies on the electrostatic-type MEMS generator have been reported [14]. However, it shows charge saturation and high internal impedance; as a result, an output current of the electrostatic-type generator becomes small. For this reason, an electromagnetic induction type that is usually used a commercial size generator has been studied in the MEMS generator. The electromagnetic induction type shows low output impedance and

The conventional electromagnetic induction-type generator has a magnet, moving part such as the turbine structure, and a magnetic circuit including a magnetic core. The winding wire magnetic circuit forms a three-dimensional coil structure. In the MEMS process, the miniature moving part can realize, but it is difficult to form the three-dimensional structure coil by using the MEMS process. Therefore, the planer structures such as a spiral, a meandering or equivalent shape are employed for the electromagnetic induction-type MEMS generator. The complex three-phase coil pattern made from a copper conductor that was arranged on a plane substrate has reached milliwatt to watt class [15, 16]. However,

example, an endoscope microrobot has been researched [10].

tion, and then, its power is too small to be used for main power source.

high output power.

172 MEMS Sensors - Design and Application

#### **2.1. Electromagnetic induction-type MEMS air turbine generator**

The electromagnetic induction type is employed for developed MEMS air turbine generators. Proposed MEMS air turbine generators are combined with the MEMS mechanical parts and the ceramic electronic part. In the MEMS air turbine structure, a magnet is connected to the rotor. The rotor shows a rotational motion by the inletting fluid. This motion occurs the electromagnetic induction revolving-field.

The design concept of the air turbine is the high-speed rotational motion. The rotational structure at a bearing system and a rotor blade form are compared. In the bearing system, the fluid dynamic bearing system and a miniature ball bearing structure are discussed. The rotor blade forms influence the rotational motion. In this chapter, a flat-type rotor blade and a rim-type rotor blade are compared. Therefore, three types of air turbine structures are designed and fabricated. The design concepts of the ceramic magnetic circuit are a miniature three-dimensional structure coil and the introduction of the magnetic flux. The magnetic material designs of the circuit are analyzed, and these are compared. Moreover, the arrangement of the magnetic circuit is an important factor to the miniature electromagnetic induction-type generator. The arrangement of the magnetic circuit is discussed as the generator that combined with the air turbine and the magnetic circuit.

#### **2.2. Mechanical parts of generator**

The mechanical parts of the generator are the air turbine structure. These are made from a miniature silicon parts, and the miniature structure parts are fabricated by the MEMS process. To achieve the high-speed rotational motion, two types of bearing system such as a fluiddynamic bearing system and a miniature ball bearing system are designed. The one of the bearing system, a fluid-dynamic bearing system is employed. In the miniature structure, the friction force influence to the rotational motion. Therefore, a contactless-type miniature bearing system is advantaged for the miniature silicon air turbine. However, it is difficult to rotate with stability by this method. Moreover, design of the air turbine will be complex because it requires flow passages to rotating and floating.

attached to the rotor and placed in a center of the stator. The lower layers form the passage for the fluid dynamic bearing system. The shape of the lower layers that form the fluid dynamic bearing system is bump structure. This structure realizes a short distance between the magnet and the magnetic circuit. However, the gap between the magnet and the magnetic circuit is more than 800 μm. The designed flat-type rotor has the plate structure under the rotor blade. The plate receives the air through the levitation passage, and then, the air is released from an outlet. The rotational air through the stator is introduced to the center of the rotor, and then, the air passed the same outlet. The designed dimensions of the air turbine are 3.0, 3.0 and

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Designed turbines in **Figure 2** have a difference at the arrangement of the miniature ball bearing. The flat-type rotor blade (a) has one miniature ball bearing inside the air turbine structure. The air turbine parts are made from 11 silicon layers and 3 silicon rotor parts. The compressed air flows from side inlet, and the down flow air passes to the stator and the rotor. The released air passes a top outlet. The dimensions of the miniature ball bearing are 2.0 mm (outer diameter), 0.6 mm (inner diameter) and 0.8 mm (height), respectively. It is made by a martensitic stainless steel. The rotor is putted on the ball bearing. The magnet is arranged under the bearing structure. Then, these are connected to the ball bearing through the shaft. The diameter of the shaft is 0.593 mm, the material is cemented carbide. The flat-type rotor shape is the same design with the fluid dynamic bearing system air turbine. The magnet and a magnetic yoke are combined to suppress a leaked magnetic flux. The ring-shaped magnet is neodymium magnet 2-pole radial direction. Its dimensions are outer diameter 3.0 mm, inner diameter 1.0 mm and height 0.5 mm, respectively. The ring-shaped magnet yoke is formed by a silicon steel sheet. The dimensions of the magnet yoke are outer diameter 3.0 mm, inner diameter 1.0 and height 0.38 mm, respectively. The size of the designed air turbine is 5.20 mm,

In the image (b) of **Figure 2**, two ball bearings are used for holding the rotor and the magnet. The bearings are placed above and below the rim-type rotor. The air turbine structure is constructed by seven silicon layers, rotor and magnet supporting part. The air passage for rotational motion of the rotor is formed around the rim-type rotor, and the compressed air flows from side inlet to side outlet. Thickness of the rotor blade is 750 μm, and the rotor blades are formed on the side wall of the rotor. The dimensions of the rim type are 5.20, 5.20, 4.60 mm length, width and height, respectively. The ball bearing-type air turbine can separate the rotor and the magnet because these are held by the shaft. Therefore, the gap between the magnet and the magnetic circuit is shorter than the fluid dynamic bearing-type air turbine. The gap

The fine pattern of the rotor and each layer were fabricated by the photolithography process. In this process, the miniature components were fabricated from single crystal silicon wafer. Each silicon wafer for the parts was washed, deposited with an aluminum layer by physical vapor deposition, and coated with a photoresist. The designed pattern was exposed to the resist layer and developed by soaking in the developer. The aluminum layer on the specimen was then chemically etched, leaving an imprint of the designed pattern. The patterned wafer was dry etched by high aspect ratio inductively coupled plasma etching combined with

3.0 mm, length, width and height, respectively.

5.20 and 4.50 mm, length, width and height, respectively.

dimension of the design (a) is 220 μm and (b) is gapless design.

Another one is the miniature ball bearing. It made from a mechanical process, and it can suppress an eccentric motion of the rotor because the ball bearing holds the rotor directly. The rotor and the magnet are held a shaft through the ball bearing structure. It requires the initial torque to achieve the rotational motion, but it is desired a stable rotational motion and the simple structural design. By using the miniature ball bearing, the rotational part and the magnet part can separate.

The MEMS air turbine designs are shown in **Figures 1** and **2**. **Figure 1** shows the flat-type rotor blade and the fluid dynamic bearing system air turbine. Image (a) and (b) of the **Figure 2** employs the miniature ball bearing structure for the bearing system. Image (a) uses the flattype rotor blade air turbine and (b) uses the rim-type rotor blade air turbine, respectively.

The air turbine parts of **Figure 1** are seven silicon structural layers and the rotor that has the magnet. The upper layers form the air passage structure. Through this passage, air is passed to the stator and it generates the rotational motion of the rotor. The ring-shaped magnet is

**Figure 1.** Design of MEMS air turbine structure that has flat-type rotor blade and fluid dynamic bearing system.

attached to the rotor and placed in a center of the stator. The lower layers form the passage for the fluid dynamic bearing system. The shape of the lower layers that form the fluid dynamic bearing system is bump structure. This structure realizes a short distance between the magnet and the magnetic circuit. However, the gap between the magnet and the magnetic circuit is more than 800 μm. The designed flat-type rotor has the plate structure under the rotor blade. The plate receives the air through the levitation passage, and then, the air is released from an outlet. The rotational air through the stator is introduced to the center of the rotor, and then, the air passed the same outlet. The designed dimensions of the air turbine are 3.0, 3.0 and 3.0 mm, length, width and height, respectively.

**2.2. Mechanical parts of generator**

174 MEMS Sensors - Design and Application

magnet part can separate.

requires flow passages to rotating and floating.

The mechanical parts of the generator are the air turbine structure. These are made from a miniature silicon parts, and the miniature structure parts are fabricated by the MEMS process. To achieve the high-speed rotational motion, two types of bearing system such as a fluiddynamic bearing system and a miniature ball bearing system are designed. The one of the bearing system, a fluid-dynamic bearing system is employed. In the miniature structure, the friction force influence to the rotational motion. Therefore, a contactless-type miniature bearing system is advantaged for the miniature silicon air turbine. However, it is difficult to rotate with stability by this method. Moreover, design of the air turbine will be complex because it

Another one is the miniature ball bearing. It made from a mechanical process, and it can suppress an eccentric motion of the rotor because the ball bearing holds the rotor directly. The rotor and the magnet are held a shaft through the ball bearing structure. It requires the initial torque to achieve the rotational motion, but it is desired a stable rotational motion and the simple structural design. By using the miniature ball bearing, the rotational part and the

The MEMS air turbine designs are shown in **Figures 1** and **2**. **Figure 1** shows the flat-type rotor blade and the fluid dynamic bearing system air turbine. Image (a) and (b) of the **Figure 2** employs the miniature ball bearing structure for the bearing system. Image (a) uses the flattype rotor blade air turbine and (b) uses the rim-type rotor blade air turbine, respectively.

The air turbine parts of **Figure 1** are seven silicon structural layers and the rotor that has the magnet. The upper layers form the air passage structure. Through this passage, air is passed to the stator and it generates the rotational motion of the rotor. The ring-shaped magnet is

**Figure 1.** Design of MEMS air turbine structure that has flat-type rotor blade and fluid dynamic bearing system.

Designed turbines in **Figure 2** have a difference at the arrangement of the miniature ball bearing. The flat-type rotor blade (a) has one miniature ball bearing inside the air turbine structure. The air turbine parts are made from 11 silicon layers and 3 silicon rotor parts. The compressed air flows from side inlet, and the down flow air passes to the stator and the rotor. The released air passes a top outlet. The dimensions of the miniature ball bearing are 2.0 mm (outer diameter), 0.6 mm (inner diameter) and 0.8 mm (height), respectively. It is made by a martensitic stainless steel. The rotor is putted on the ball bearing. The magnet is arranged under the bearing structure. Then, these are connected to the ball bearing through the shaft. The diameter of the shaft is 0.593 mm, the material is cemented carbide. The flat-type rotor shape is the same design with the fluid dynamic bearing system air turbine. The magnet and a magnetic yoke are combined to suppress a leaked magnetic flux. The ring-shaped magnet is neodymium magnet 2-pole radial direction. Its dimensions are outer diameter 3.0 mm, inner diameter 1.0 mm and height 0.5 mm, respectively. The ring-shaped magnet yoke is formed by a silicon steel sheet. The dimensions of the magnet yoke are outer diameter 3.0 mm, inner diameter 1.0 and height 0.38 mm, respectively. The size of the designed air turbine is 5.20 mm, 5.20 and 4.50 mm, length, width and height, respectively.

In the image (b) of **Figure 2**, two ball bearings are used for holding the rotor and the magnet. The bearings are placed above and below the rim-type rotor. The air turbine structure is constructed by seven silicon layers, rotor and magnet supporting part. The air passage for rotational motion of the rotor is formed around the rim-type rotor, and the compressed air flows from side inlet to side outlet. Thickness of the rotor blade is 750 μm, and the rotor blades are formed on the side wall of the rotor. The dimensions of the rim type are 5.20, 5.20, 4.60 mm length, width and height, respectively. The ball bearing-type air turbine can separate the rotor and the magnet because these are held by the shaft. Therefore, the gap between the magnet and the magnetic circuit is shorter than the fluid dynamic bearing-type air turbine. The gap dimension of the design (a) is 220 μm and (b) is gapless design.

The fine pattern of the rotor and each layer were fabricated by the photolithography process. In this process, the miniature components were fabricated from single crystal silicon wafer. Each silicon wafer for the parts was washed, deposited with an aluminum layer by physical vapor deposition, and coated with a photoresist. The designed pattern was exposed to the resist layer and developed by soaking in the developer. The aluminum layer on the specimen was then chemically etched, leaving an imprint of the designed pattern. The patterned wafer was dry etched by high aspect ratio inductively coupled plasma etching combined with

a Bosch process [1]. The parts were achieved after removing the aluminum and washing. Through these processes, the silicon structural components were fabricated. The obtained components were assembled by hand and alignment pin. **Figure 3** shows the schematic illus-

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**Figure 3.** Fabrication process for miniature components of designed MEMS air turbine.

The electronic part for the electromagnetic induction-type MEMS air turbine generator is the magnetic circuit. To achieve the miniature generator, the miniature magnetic circuit that has a three-dimensional structure coil and a magnetic core for introducing a magnetic flux is required. Moreover, low internal resistance is an important factor to achieve high output power. Therefore, the three-dimensional coil pattern is required. The multilayer ceramic technology is used for the magnetic circuit. Optimization of the magnetic circuit design and the

Designs and analyzed results of the magnetic circuit for the miniature generators are shown in **Figures 4** and **5**. **Figure 4** shows designs for the fluid dynamic bearing system-type air turbine generator. The shape of (a) is a step-wise shape, and (b) is a horseshoe-shaped structure. These magnetic circuits have two-pole coil structures. Each pole has 12 turn coil structure, and they are connected at the connecting layer. Therefore, the designed magnetic circuits have 24

tration of the fabrication process.

**2.3. Electronic parts of generator**

fabrication process are explained.

**Figure 2.** Design of MEMS air turbine structures (a) flat-type rotor blade and miniature ball bearing and (b) rim-type rotor blade and miniature ball bearing.

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**Figure 3.** Fabrication process for miniature components of designed MEMS air turbine.

a Bosch process [1]. The parts were achieved after removing the aluminum and washing. Through these processes, the silicon structural components were fabricated. The obtained components were assembled by hand and alignment pin. **Figure 3** shows the schematic illustration of the fabrication process.

#### **2.3. Electronic parts of generator**

**Figure 2.** Design of MEMS air turbine structures (a) flat-type rotor blade and miniature ball bearing and (b) rim-type

rotor blade and miniature ball bearing.

176 MEMS Sensors - Design and Application

The electronic part for the electromagnetic induction-type MEMS air turbine generator is the magnetic circuit. To achieve the miniature generator, the miniature magnetic circuit that has a three-dimensional structure coil and a magnetic core for introducing a magnetic flux is required. Moreover, low internal resistance is an important factor to achieve high output power. Therefore, the three-dimensional coil pattern is required. The multilayer ceramic technology is used for the magnetic circuit. Optimization of the magnetic circuit design and the fabrication process are explained.

Designs and analyzed results of the magnetic circuit for the miniature generators are shown in **Figures 4** and **5**. **Figure 4** shows designs for the fluid dynamic bearing system-type air turbine generator. The shape of (a) is a step-wise shape, and (b) is a horseshoe-shaped structure. These magnetic circuits have two-pole coil structures. Each pole has 12 turn coil structure, and they are connected at the connecting layer. Therefore, the designed magnetic circuits have 24

**Figure 4.** Design of the magnetic circuits for fluid dynamic bearing system generator: (a) step-wise shape and (b) horseshoe shape.

turn coil. The horseshoe-shaped circuit has only the coil layer and the connecting layer, and the step-wise shape circuit has the magnetic material layer. When the magnetic circuit and the air turbine are combined, the magnet inside the air turbine is placed between the magnetic material layers. The magnetic flux loss is compared between the magnetic circuits. The results are shown in **Figure 5**. The magnetic flux is introducing from the magnet to magnetic circuit in (a). On the other hand, (b) shows the magnetic flux loss more than (a). In the both structures, the dimensions are 3.5 and 3.5 mm, length and width, respectively. The heights are 2.0 mm in the step-wise shape and 1.2 mm in the horseshoe shape.

The output powers of these designs magnetic circuit are compared. In this evaluation, a spindle machine is used for power evaluations.

are arranged in side parts, and these have 50 turns each other. The total number of coil turns is 100 turns; these are bonded with the magnetic parts for introducing the magnetic flux. By this process, the closed magnetic circuit is obtained. The magnetic parts are introducing the magnetic flux from the magnet. In **Figure 6**, the analyzed result is shown in (b). By this result, the magnetic flux through the magnetic parts were observed. Dimensions of the designed

**Figure 6.** Design of around-type magnetic circuit for ball bearing-type air turbine (a) schematic illustration of circuit

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The magnetic ceramic material is low-temperature co-fired Ni-Cu-Zn ferrite with the perme-

ferrite can be fired at around 900°C. Therefore, it is possible to use the low-resistance silver

The fabrication process for the miniature magnetic circuits is the green sheet process, that is, the multilayer ceramic technology. In this process, the ceramic slurry was made for forming a sheet structure. This sheet is called the green sheet. The slurry in our fabrication was made of the mixture of the ferrite ceramic powder, binder, dispersing agent, plasticizer and organic materials. The through hole for connecting the under layer was machined, and then the coil pattern was printed on the ferrite green sheet by the screen printing technology. The material of the coil pattern was silver paste as a conductor paste. The multiple sheets were stacked, and the specimen was diced into the designed part. By using the magnetic ceramic for the green

O3

–8.8 NiO-10 CuO-32ZnO. The Ni-Cu-Zn

circuit are 7.40, 8.50, 2.40 mm, length, width and height, respectively.

ability of 900. The compositional ratio is 49.2 Fe2

conductive material.

design, (b) analyzed result.

**Figure 6** shows the around-type magnetic circuit. It is used for the miniature ball bearing-type air turbine generator. Designed magnetic circuit is constructed by four pieces. The coil layers

**Figure 5.** Analyzed results of the magnetic circuits: (a) step-wise shape and (b) horseshoe shape.

**Figure 6.** Design of around-type magnetic circuit for ball bearing-type air turbine (a) schematic illustration of circuit design, (b) analyzed result.

turn coil. The horseshoe-shaped circuit has only the coil layer and the connecting layer, and the step-wise shape circuit has the magnetic material layer. When the magnetic circuit and the air turbine are combined, the magnet inside the air turbine is placed between the magnetic material layers. The magnetic flux loss is compared between the magnetic circuits. The results are shown in **Figure 5**. The magnetic flux is introducing from the magnet to magnetic circuit in (a). On the other hand, (b) shows the magnetic flux loss more than (a). In the both structures, the dimensions are 3.5 and 3.5 mm, length and width, respectively. The heights are

**Figure 4.** Design of the magnetic circuits for fluid dynamic bearing system generator: (a) step-wise shape and (b)

The output powers of these designs magnetic circuit are compared. In this evaluation, a spin-

**Figure 6** shows the around-type magnetic circuit. It is used for the miniature ball bearing-type air turbine generator. Designed magnetic circuit is constructed by four pieces. The coil layers

2.0 mm in the step-wise shape and 1.2 mm in the horseshoe shape.

**Figure 5.** Analyzed results of the magnetic circuits: (a) step-wise shape and (b) horseshoe shape.

dle machine is used for power evaluations.

horseshoe shape.

178 MEMS Sensors - Design and Application

are arranged in side parts, and these have 50 turns each other. The total number of coil turns is 100 turns; these are bonded with the magnetic parts for introducing the magnetic flux. By this process, the closed magnetic circuit is obtained. The magnetic parts are introducing the magnetic flux from the magnet. In **Figure 6**, the analyzed result is shown in (b). By this result, the magnetic flux through the magnetic parts were observed. Dimensions of the designed circuit are 7.40, 8.50, 2.40 mm, length, width and height, respectively.

The magnetic ceramic material is low-temperature co-fired Ni-Cu-Zn ferrite with the permeability of 900. The compositional ratio is 49.2 Fe2 O3 –8.8 NiO-10 CuO-32ZnO. The Ni-Cu-Zn ferrite can be fired at around 900°C. Therefore, it is possible to use the low-resistance silver conductive material.

The fabrication process for the miniature magnetic circuits is the green sheet process, that is, the multilayer ceramic technology. In this process, the ceramic slurry was made for forming a sheet structure. This sheet is called the green sheet. The slurry in our fabrication was made of the mixture of the ferrite ceramic powder, binder, dispersing agent, plasticizer and organic materials. The through hole for connecting the under layer was machined, and then the coil pattern was printed on the ferrite green sheet by the screen printing technology. The material of the coil pattern was silver paste as a conductor paste. The multiple sheets were stacked, and the specimen was diced into the designed part. By using the magnetic ceramic for the green

**2.4. Experimental procedure**

**3. Results and discussion**

power generation.

waveform was measured by an oscilloscope.

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

tively. Inductance and DC resistance were 5.85 μH and 0.94 Ω.

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

The combined MEMS generator was evaluated on the output power. The compressed nitrogen gas was injected to the MEMS generator. The rotational speed and the output voltage were measured. Load resistances were connected to the ceramic magnetic circuit. The output

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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

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

**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, respec-

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

are shown in **Table 1**. As a result, it is found that the error was less than 5 μm.

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

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

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

#### **2.4. Experimental procedure**

The combined MEMS generator was evaluated on the output power. The compressed nitrogen gas was injected to the MEMS generator. The rotational speed and the output voltage were measured. Load resistances were connected to the ceramic magnetic circuit. The output waveform was measured by an oscilloscope.
