Micro-Electro-Mechanical System (MEMS) is a multi purpose platform which integrates several different fields such as mechanical, electronic, control, optical, chemical, and biomedical technology. Through combining different types of subsystems on a common substrate, the most significant advantage is reduced cost due to miniaturization. Quality, performance, and durability may also be improved dramatically in many different cases. The concept of systems on chip (SOC) is made possible by the integration of these subsystems. This state-of-the-art technology has already been applied in numerous occasions. There are many common applications such as sensors and actuators, micro optical systems, biomedical systems, and aerospace and defense systems. The best known MEMS device is the accelerometer mounted on the hand controller of video gaming machine.
Due to the advance of manufacturing technology and the requirements from consumers, portable electronic devices have received increasing interests in recent years. However, conventional power storage devices have limited energy capacity, causing power supply a primary concern [1]. Generally, energy is stored in storage devices such as traditional batteries [2], micro-batteries [3], micro-fuel cells [4], ultra capacitors [5], micro heat engine [6], and radioactive materials [7]. High efficiency and low power loss are necessary to prolong the active time of these storage devices.
Researchers have attempted to increase the energy density in those storage devices, but still, finite lifetime and high maintenance cost remain problems.
Thanks to the breakthrough of low power CMOS VLSI (Very Large Scale Integrated circuit) technology and the low duty cycle characteristics, it is feasible to
systems. These devices have reduced power consumption in the level of tens to hundreds of microwatts [8]. Scavenging ambient energy from the environment to power these sensor nodes become a possible method. One can design a self-sustainable or self-renewable energy device by scavenging the environmental energy to supply part of or all of the consumed energy.
1.1 Literature review
Different technologies, such as light exposure, thermal gradients, human power, air flow, and vibration [9], can be used to scavenge or harvest energy from the environment. The environment is a sustainable energy supply compared with the common storage devices like batteries or fuel cells. Various approaches to convert energy from the environment to electrical energy to drive low power electronics are reviewed in this chapter. Due to the inexhaustible nature of scavenged energy, the performance of the energy devices is characterized by their power density, instead of energy density used for traditional storage devices.
1.1.1 Light exposure
Light exposure is a popular and mature method to scavenge energy. Solar or photovoltaic cells are the leading technology to convert solar energy directly to electricity with high efficiency. Solar cells can be manufactured by IC-compatible technologies with high quality, therefore causing its popularity. Such devices can supply low cost and pollution free energy.
Photovoltaic cells function by the photovoltaic effect [10]. When the cell is exposed to light, a light-induced voltage is generated. The photons of the incident solar radiation excite the electrons in the semiconductor, allowing the electrons to
move freely and thus cause an electric current flow through a load. The operation is shown in Fig. 1.1. For single crystal silicon, the device has efficiency ranging from 12% to 25%. The thin film polysilicon and amorphous silicon cost less than single crystal silicon cells but have lower efficiency [11].
In total, photovoltaic energy conversion can provide sufficient power and with a mature IC-compatible technology. However, the output power of photovoltaic devices depends heavily on the environmental circumstances. For example, the photovoltaic cells offer adequate power density up to 15 mW/cm2 if the device is placed outdoor and operated primarily during daytime. However, in regular indoor office lighting conditions, the same photovoltaic cell will merely produce about 10 μW/cm2 [10].
Because of the environmentally dependent characteristics, photovoltaic cells are limited to certain applications.
Fig. 1.1 Photovoltaic energy conversion [10]
Photon incident
Photon absorbed
Photon absorbed Photon
Reflected
Electron excited
1.1.2 Thermoelectric effect
Temperature gradient is basically a power source which can be converted into electric power. This thermal-to-electric behavior is described by the Seebeck effect, or the thermoelectric effect [12]. Once two different metals are connected in a closed loop, a temperature variation in the loop will cause electrons to move and a voltage potential is built up between the two metals or semiconductor junctions.
According to the Seebeck effect, the developed voltage is proportional to the temperature difference between the high temperature and low temperature ends, and to the Seebeck coefficients of the two materials. Large Seebeck coefficients and high electrical conductivity can increase conversion efficiency and decrease power losses and, therefore, is beneficial to the thermal-to-electric energy conversion.
Materials typically used for thermoelectric energy conversion, including Sb Te , 2 3
2 3
Bi Te , Bi-Sb, PbTe, Si-Ge, polysilicon, BiSbTeSe compounds, and InSbTe, are not completely compatible to the IC process. In [13], annealing conditions have tremendous influence on the electrical resistivity of Bi-Sb and the thermoelectric generator performances as a consequence. An output power density of 140 μW/cm 3 for a 100 ˚C temperature difference is obtained but the temperature difference of this level is not commonly seen in a micro system [14]. So the output power is limited without large thermal gradients.
Fig. 1.2 illustrates the simplest thermoelectric generator comprising a p-type and a n-type thermoelements connected electrically in series and thermally in parallel.
Heat is pumped into one side of the couple and rejected from the opposite side. An electrical current is produced, proportional to the temperature difference between the hot and cold junctions.
In general, connecting several thermocouple elements in series can achieve better performance. However, large series resistance increases ohmic power loss and thus
reduces the overall power conversion efficiency.
Fig. 1.2 Thermoelectric energy converter [12]
1.1.3 Human body movement
Human power is known as one of the most conventional energy sources.
However, human-movement-to-electric power conversion has not been studied until recent years. The conversion principle is to transfer power from human activities to electric power. The activities include walking, breathing, body heat and so on. It is possible to power portable devices by harvesting energy from the human movement.
In recent years, needs of wearable electronic devices [15-17] have grown significantly. Many researchers focus their efforts on walking since this process seems a more practical energy source for wearable electronic devices. For example, a field scientist or explorer carrying heavy load can use a specially-designed power harvesting back pack to generate electric energy for his instruments such as GPS or notebook computers [16]. An average 7.37 W power output was measured from 6 participants who walked with speed ranging from of 4.0 to 6.4 km/hour and carried 20-, 29-, and 38 kg loads in addition to the fixed frame weighing 5.6 kg as shown in
Fig. 1.3 [16]. During walking, a person moves like an inverted pendulum, as shown in Fig. 1.4, causing the hip to move up and down by 4 to 7 cm, a considerable amount of mechanical energy must be transferred if the load is heavy.
Fig.1.3 Components of suspended-load backpack [16]
Fig.1.4 Inverted pendulum model of human walking [16]
“Heel-strike” devices are another walking power harvester [17]. However, the energy level of generation is relatively small (10 to 20 mW). This energy could be used in a variety of low-power applications, such as health monitors, self-powered emergency receivers, and radio frequency identification tags. The application is limited by the piezoelectrics and IC integration issues as well as power delivery issues.
The piezoelectric shoe inserts offer a good solution for specific requirement such as RFID tags or other wireless devices worn on the foot.
1.1.4 Wind
Wind power is a renewable power generation technology to become a mainstream alternative for generation capacity expansion in the twenty-first century.
This idea is to convert wind energy into a useful form like electricity by wind turbines or windmill. Due to the rotating characteristic of wind blades, the majority type of wind energy converter is electromagnetic conversion. Wind generated energy is also environmentally dependent which is similar to solar energy.
The power from wind is related to the air velocity. With slow wind at 3 m/s velocity, the average power is about 80 μW/cm3. The maximum average power density of 1060 μW/cm3 at 12 m/s air velocity was produced from a strong wind [18].
This indicates more usable power can be generated from high-velocity wind. However, wind power generation should be at large scale in order to obtain large amount of energy. Wind energy generators should be placed at locations where a sustainable and stable air flow is present. Therefore, wind power is a suitable energy source for wireless sensors for where a suitable wind source exists.
Few small-scale air flow harvesters have been proposed to date. One device based on MEMS technology is shown in Fig. 1.5 [19]. It comprises a 12-mm-diameter
output power of 1 mW could be delivered at a volume flow of 35 l/min and a pressure drop of 8.4 mbar. For operation in a free stream, the same output power would be expected at a flow speed of around 40 m/s which is rarely seen in practical use.
Fig. 1.5 MEMS air flow harvester, with 10 pence coin for scale [19]
1.1.5 Ambient Vibration
Vibration can also serve as an energy source. Ambient vibration can be observed in many environments. Most sources of vibrations are at low frequencies ranging from 60 to 200 Hz [9]. Different levels of mechanical vibration occur in exterior windows, aircraft, automobile, industrial equipments, and many small household appliances. Generally, the maximum power is extracted at resonance with the ambient vibration source. Theory and experiments show that the power density of more than 300 μW/cm3 can be generated [20]. A more detailed discussion of this harvesting method is presented in Chapter 2.
1.1.6 Summary of power sources
Table 1.1 shows the comparison of the several power sources for portable devices. The values in this table are estimates taken from literatures or analysis based on the survey in the previous sections. Vibration is chosen as the source of energy scavenging in this study because of its ubiquity and sufficient power density.
Table 1.1 Comparison of power sources
Power sources Power density Commercially
available?
Solar (outdoors) [10] 15, 000 μW/cm2 Yes
Solar (indoors) [10] 10 μW/cm2 Yes
Temperature gradient [13] 140 μW/cm3 at 100˚C gradient Soon
Human power [15, 16] 330 μW/cm2 No
Wind energy [18] 1060 μW/cm3 at 12 m/s velocity No
Vibration [20] 375μW/cm3 at 120Hz, 2.5m/s2 Yes
1.2 Ambient vibration energy conversion
Ambient vibration can be converted into electricity based on the overview described in previous sections. Vibration-driven harvesters will be introduced and discussed in this section. Three types of methods can be utilized to generate electricity from vibration sources.
In conventional macroscale engineering, electrical generators are based on electromagnetic transduction. In small-scale energy harvesting, two main techniques are added. Piezoelectric transduction is generally impractical for rotating systems but is well suited to the reciprocating nature of the motions typically used for harvesting.
Electrostatic transduction, which is both impractical and inefficient for large machines,
becomes much more practical at small size scales and is well suited for MEMS implementation.
1.2.1 Electromagnetic energy conversion
As described by Faraday’s low of induction, a change of magnetic flux linkage with a coil induces a voltage in the coil, driving a current in the circuit. The combined force on the moving charges in the magnetic field acts to oppose the relative motion, as described by Lenz’s low. The mechanical work done against the opposing force is converted to energy in the magnetic field associated with the circuit inductance. A typical electromagnetic energy converter is shown in Fig. 1.6 [21]. Mechanical acceleration is produced by vibrations that cause the mass to oscillate. A coil is attached to the mass and moves through a magnetic field built by a permanent magnet.
The induced voltage was produced by the change of magnetic flux. Thus, the output power is proportional to the magnetic field and coil number.
Fig. 1.6 Electromagnetic energy converter [21]
Microscale electromagnetic generation technologies can be broadly classified into three categories: rotational, oscillatory, and hybrid devices [22], as shown in Fig.
1.7. Rotational generators imitate the operation of macroscale motor/generators and have been designed to operate using rotational power from miniature turbines or heat engines. They are designed for continuous rotational motion under a steady driving torque. In contrast, oscillatory generators operate in a resonance mode, usually relying on relatively small displacements between a permanent magnet and coil to acquire power from environmental vibrations. Lastly, hybrid devices rely on vibrations, but convert linear motion into rotational motion using an imbalanced rotor.
Fig1.7 Three types of permanent magnet generation technologies [22]
The most common issue for electromagnetic energy conversion is the relatively low induced voltage. Methods to increase induced voltage include increasing the number of turns of the coil or increasing the permanent magnetic field. However, it is difficult to fabricate large number of coils with planer thin film processes. Thus the power density of electromagnet converter is lower than other types of device.
The first microscale implementation of this type of vibration-driven harvester was reported in 1995 by Williams et al. [23]. The 25mm device demonstrated a 3 peak power of 0.3 μW for a 0.5μmvibrations at 4400Hz. Ching et al. used a small NdFeB magnet supported by a laser-micromachined Cu spring structure [24]. A
coil
and the devices successfully powered low data rate infrared and RF wireless communication modules.
There are also commercial products that utilize resonant magnetic power generation schemes. Perpetuum Co. Ltd. markets a 130 cm vibration energy 3 harvester tuned to 100 or 120 Hz vibrations that delivers 3.5 mW for 0.1 g vibrations or 40 mW at 1 g [25]. Ferro Solutions offers a similar 87cm product that can produce 3 10.8 mW for 0.1 g vibrations at 60 Hz [26].
(a) (b) Fig. 1.8(a) PMG-17 energy harvester from Perpetuum Co. [25]
and (b) VEH-3 energy harvest from Ferro solutions[26]
1.2.2 Piezoelectric energy conversion
Piezoelectric effect is a phenomenon whereby a strain in a material generates an electric field in that material, and inversely an applied electric field generates a mechanical strain [27]. The former can be used to convert energy. When an external force is applied, some of the work done is stored as elastic strain energy, and some in the electric field associated with the induced polarization of the material. If there is an external conduction route through a load, a current is generated to neutralize the net
charge produced as a result. Piezoelectric materials with high electromechanical coupling coefficients are generally ceramics, with lead zirconate titanate being the most common. The most common geometry is to place the piezoelectric material as a thin layer on a cantilever beam from which the proof mass is suspended.
Fig.1.9 A two-layer cantilever beam piezoelectric energy converter [28]
The first reported piezoelectric microgenerators appear in the patent literature.
Snyder described the use of a piezoelectric generator mounted on the wheels of a car to power the tire pressure sensors [29, 30]. The device would be powered from the wheel vibration during driving. If an abnormal tire pressure is detected, the signal could be reported to the driver via a low-power radio link. This kind of device is mounted on all newly produced cars in the United States due to the government regulation.
Roundy et al. described an RF beacon powered by both a solar cell and an optimized piezoelectric generator [28]. A power level of 375 Wμ was generated from an acceleration of 2.25m / s at 60Hz, corresponding to amplitude of 16 m2 μ .
Fig. 1.10 A typical piezoelectric generator [28]
The difficulty of using piezoelectric material is the incompatibility with MEMS and CMOS processes. Another drawback of the piezoelectric converter is the requirement of additional circuitry to rectify the AC current. The supplementary circuitry has power losses and decreases the efficiency of the conversion. Most researches so far utilize bulk materials, which is still not suitable for integration with microsystem technology.
1.2.3 Electrostatic energy conversion
Electrostatic energy converters mainly use the change of capacitance of a mechanically driven variable capacitor as shown in Fig. 1.11. Mechanical forces from vibration are utilized to do work against the attraction of opposite charged parts. For a typical electrostatic energy converter, the variable capacitor is initially biased from a voltage source and disconnected instantly after the capacitor is fully charged. In the
constant charge process, when the capacitance decreases due to vibration, the voltage on the variable capacitor increases (Qconstant=CV) and thus the mechanical kinetic energy is transfered into electrical potential energy.
Fig. 1.11 (a) Gap-closing and (b) overlap in-plane variable capacitors [21]
MEMS variable capacitor can be fabricated through mature silicon-based micromachining process such as deep reactive ion etching. Therefore, it is relatively suitable for IC processes as mentioned earlier in this thesis. The converter also provides relatively high output voltage levels and adequate power density compared with electromagnetical counterparts. However, the disadvantage of the converter is the necessity of an auxiliary voltage source Vin used to initiate the conversion cycle.
The lifetime of the voltage source is unfortunately limited. One solution proposed by Bernard et al. [32] was to use an inductive flyback circuitry to constantly feedbacks the temporary storage energy back to the external voltage supply for further usage.
Another solution was proposed by Ingo Kuehne et al. [33] where the build-in voltage caused by the work function difference between two conductors was utilized as a bias source and thus no outer bias source is needed. In addition, one can also employ a moving layer of permanently embedded charge, or electret, to carry out electric energy harvesting [34], although such devices currently suffered from low power
(a) (b)
device is AC signal which needs to be rectified and thus the power loss is inevitable.
A capacitive energy converter was implemented by Meninger et al. [35]. The comb finger structure in MEMS technology was used with silicon on insulator (SOI) wafers. The simulation showed an output power of 8.6 μW with a device size of 1.5 cm × 0.5 cm × 1 mm from the vibration of 2.52 kHz. Another design was proposed by Roundy [9], which could achieve an output power density of 110 μW/cm3 under vibration input 2.25 m/s2 and frequency of 120 Hz.
In electrostatic capacitive energy conversion, the switch timing control should be controlled accurately to achieve maximum conversion efficiency. A prototype circuitry with the two ideal diodes was proposed by Roundy [9]. The experiment showed excessive power reduction due to the far from ideal operation of the diodes.
The power consumption by the electronics or parasitic capacitive and resistive coupling still existed. Therefore, improved switch design is critical for better energy conversion efficiency.
1.2.4 Comparison of vibrational energy conversion technologies
From the above literature overview, the three types of vibrational energy converters are listed in Table 1.2 [21]. According to the characteristic comparison of these energy converters, electrostatic capacitive vibration-to-electric energy conversion is suitable for scavenging ambient energy because of its ubiquity in the environment and sufficient output power density. The fabrication technologies for electrostatic converter are very mature in MEMS system. The materials and process are compatible with IC process.
Table 1.2 Comparisons of three types of vibrational energy converter [21]
Converter Power density Advantages Disadvantages
Piezoelectric ~200 μW/cm 3
1.3 Thesis objectives and organization
Most electrostatic energy converters use switching devices such as diodes, MOSFET, or integrated mechanical switches for the control of conversion cycles.
This kind of operation results in a nonlinear system, especially in the movement of mechanically variable capacitor. Many researchers [33, 36, 37] used simulation tools to model their devices. However, it is difficult to perform systematic analysis by using
This kind of operation results in a nonlinear system, especially in the movement of mechanically variable capacitor. Many researchers [33, 36, 37] used simulation tools to model their devices. However, it is difficult to perform systematic analysis by using