Vibration Energy Scavenging and Management for Ultra Low Power Applications * Lu Chao, Chi-Ying Tsui and Wing-Hung Ki Department of Electronic and Computer Engineering The Hong Kong University of Science and Technology Hong Kong SAR., P. R. China {eeluchao, eetsui and eeki}@ee.ust.hk ABSTRACT In this work, the design of a mechanical vibration energy scavenging and management system is presented for ultra low power applications. A new maximum power point tracking (MPPT) scheme is proposed for piezoelectric conversion. This scheme consumes very little power and is especially suitable forultra low power energy harvesting applications. This design is capable of self-starting and self-powered, thus eliminates external battery integration and significantly reduces the system volume. System modeling, analysis, and VLSI implementation were developed. Various simulations were carried out and the simulation results show that the proposed MPPT scheme can achieve an energy harvesting efficiency higher than 90%. Categories and Subject Descriptors B.7.1 [Integrated Circuits]: Types and Design Styles –Advanced technologies, Algorithms i mplemented in hardware. General TermsAlgorithms, Management, Design, Verification. KeywordsEnergy scavenging and management, MPPT, batteryless 1.INTRODUCTION Ubiquitous applications have potential to be used in many areas, where ubiquitous computing, sensing, and perception facilitate the interaction between human and the environment. Wireless sensor network is a good example. Providing the required supply voltage and power to hundreds or thousands ofdistributed sensor nodes is a challenge. The conventional solution is to use electrochemical batteries. However, battery has limited energy capacity, relatively large volume with respect to the electronic circuits, finite recharging cycle and is difficult to be replaced regularly in many cases. All the above disadvantages pose a big li mitation on the wide dep loyment of such syste ms. In some ultra low power applications (e.g. picoradio [1], smart dust [2]) that demand compact, low cost, long lifetime and high integration, eliminating the battery is much desirable. For some ubiquitous applications, the average powerconsumption can be down to the level of hundreds or even tensofmicrowatts. Power scavenged from the environment can be used as an alternative power source to provide a virtually infinite lifetime [3]. Mechanical energy conversion is one of the feasible approaches for these ultra low power applications [5][6]. Low level vibrations commonly occur in various household orindustrial environments, such as machinery or air-conditioning vibration. It is estimated that mechanical vibrations inherent in the environment can provide a power density of tens to hundreds ofmicrowatt per cubic centimeter [4], which is sufficient to sustain operations of many ubiquitous applications [5][6]. Therefore, vibration-based energy scavenging systems have drawn many attentions in the research communities [3]-[7]. Previous studies have found that for piezoelectric conversion, under a given vibration status (magnitude and frequency), there is an optimal output voltage point where maximum harvested electrical power can be obtained [9]. Vibration status is often unstable and varying, and heavily depends on the environment in which the application is located. Hence, the optimal voltage point is also changing. In order to harvest as much energy as possible, a run-time adaptive mechanism is required to track the optimal output voltage with the environmental change. At the same time, this tracking mechanism should have as low power overhead as possible since the ene rgy harvested is alrea dy very small. A vibration based self-powered wireless sensor using a fixed voltage band-band control scheme was proposed in [7]. Existing MPPT schemes for piezoelectric conversion [9][10] are not designed for low power applications because they employed complex circuit components and computation-intensive control algorithms. Hence, they are very inefficient in such low powerlevel and the power overhead could be higher than the maximum power harvested from the environment. In [9], MPPT was controlled by a DSP through comparing the harvested powerbefore and after a change in the duty cycle of a buck converter. The power overhead of the MPPT scheme was not presented. In [10], an expression for the optimal duty cycle of a buck converteroperating in discontinuous conduction mode was developed and it revealed that as the level of vibration excitation increases, the optimal duty cycle becomes relatively constant. Based on the above analysis results, a simpler and improved MPPT scheme was developed. The reported power overhead of the tracking unit is 5.74mW and is too much for ultra-low power applications. From this, we can see that in order to facilitate the use of piezoelectric conversion in ultra low power applications, it is necessary to Permission to make digital or hard copies of all or part of this work forpersonal or classroom use is g ranted without fee provi ded that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. ISLPED’07, August 27–29, 2007, Portland, Oregon, USA. Copyright 2007 ACM 978-1-59593-709-4/07/0008 …$5.00. * This work was supported in part by the Hong Kong Research Grants Council under Grant CERG 620305. 316
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develop a new MPPT method that has very little power loss and
the corresponding circuit implementation should be simple.
For batteryless vibration based energy harvesting applications,
the feature of self-starting is very crucial since the vibration
sources can be very unstable. Sometimes, there is no vibration or
the vibration excitation is too weak to be used. Thus, no energy
can be harvested. The system will dissipate all the energy storedin the system and stop working. As there is no external battery as
the backup power source, later even the vibration excitation
becomes strong enough again; the system cannot be started up as
there is no power left in the system. Thus the capability of self-
starting is crucial for batteryless applications.
In this paper, we tackle the above design challenges. We present anew MPPT scheme and circuit implementation that have a very
low power overhead, which is in the order of microwatts. This is
about three orders of magnitude lower than the existing MPPT
approaches. Based on this MPPT scheme, a batteryless vibration
based energy scavenging and management system is presented.
The system is capable of self-starting and self-powered. It wasimplemented using a 0.35μm 5V CMOS process. Post-layout
simulations were carried out to verify the functionality and the
efficiency of the proposed MPPT scheme and the overall system.
The rest of the paper is organized as follows. Section 2
introduces the proposed MPPT scheme, and the circuit
implementations of the energy harvester and the vibrationtracking unit. Section 3 describes the proposed batteryless
vibration based energy scavenging and management system. In
addition, the circuit implementation of each functional block is
presented in detail. Various simulation results are presented in
Section 4. Finally, the conclusion is provided in Section 5.
2. The PROPOSED MPPT SCHEME
2.1 Electrical Model of Piezoelectric Material
pC p R
0 2 4 6 8 100
1
2
3
4
5
Load Resistance (M Ohms)
O u t p u t v o l t a g e ( V )
Measured
Electrical model
Figure 1 Piezoelectric film model and experimental
verification
Fig. 1(a) shows an equivalent electrical model of a
piezoelectric thin film. It consists of a sinusoidal current source
i(t)=I psin(2πft), where I p depends on the vibration magnitude, size
and material of the film, f is the vibration frequency, C p
and R p
are the internal capacitance and resistance of the film,respectively. Measurement results show that C p is almost constant
under a wide range of vibration frequencies. R p is usually very
large and can be ignored. The output voltage of a piezoelectric
film thus depends on the material’s geometry, piezoelectric
properties, the mechanical vibration strength, and the outputimpedance. We used a commercial piezoelectric thin film [8] to
conduct experiments to verify the above model. A piece of
1.3cm×2.5cm×0.2mm piezoelectric film with C p=0.5nF was
mounted on a variable vibration platform. Various resistive loads
were connected to the film and the peak-to-peak output voltages
were recorded. The measured data are marked in Fig. 1(b). The
output voltages predicted by the model are also plotted. It is clear
that the model fits well with the measurement data. In the rest of
the paper, we use this model for analysis and optimization.
2.2 Energy Harvester
The output of the piezoelectric film is an AC signal. For energyharvesting usage on most CMOS applications, we need an AC-
DC rectifier to convert the input to a DC voltage source for
powering up the circuit. Fig. 2 shows the basic structure of theenergy harvester, which consists of a rectifier and a storage
capacitor Cs. The instantaneously harvested power is small and
may not be able to sustain continuous operations of the
application. Hence, Cs is inserted to accumulate the harvested
power. From [9], it is shown that the time-averaging harvested
power <p(t)> and the output voltage Vs at the maximum input power point are given by
π
π
π
π
π
p s p s s p fC V V fC V V I t p
Δ−−>=<
842)(
2
(1)
V fC
I
V p
p
optimal s Δ−=π 4
, (2)
where ΔV is the forward voltage drop of the diode. For low level
vibration cases, the output voltage of the energy harvester is
comparable to the voltage drop ΔV. The passive diode rectifier causes a significant reduction in the output voltage of the rectifier
and hence the overall power efficiency. In [11], an active diode,
which consists of a large PMOS transistor and a comparator, was
used to reduce the voltage drop and power loss for piezoelectric
conversion. However, an external 3.3V voltage supply was stillrequired for the power supply of the comparator [11]. Thus, this
design is not suitable for batteryless applications since it is not
capable of self-starting. In this work, we propose a hybrid scheme
which uses both passive and active diodes to solve this problem.
The passive diode rectifier is used for the self-starting purpose,while the active diode rectifier kicks in to replace the passivediode when the harvester has started up and it can provide a
smaller voltage drop and higher power efficiency.
)2sin()( ft I t i p π =
Figure 2 Energy Harvester
)2sin()( ft I t i p π =
Figure 3 Schematic of the proposed energy harvester
Fig. 3 shows the schematic of the proposed rectifier [12]. Itintegrates the passive and active diode structures together. During
3.1 Tracking Pulse GeneratorSince the vibration frequency varies with the environment, so it
is a challenge to generate a tracking signal Φ with a fixed duty
cycle, while at the same time consuming very low power. In thisdesign, we directly use the AC signals from the piezoelectric film
to generate the tracking signal. The tracking pulse generator is
shown in Fig. 7. We connect one end of the piezoelectric film to a
comparator with a DC bias voltage that is generated by a bandgap
reference and a resistor divider. A clock-like signal, which has thesame frequency as the environmental vibration, is then generatedat the output of the comparator. A counter and combinational
logic gates are used to generate a tracking pulse with a fixed duty
cycle of 1/64, i.e., around 1.56%. In addition, the tracking pulse
width is generated to be 2T to satisfy the minimum requirement of
1.5T for normal tracking.
)2sin()( ft I t i p π =
Figure 7 Block diagram of the tracking pulse generator
3.2 Refreshing UnitIn Fig. 6, there is a refreshing unit located between the tracking
unit and the control unit. It is used to periodically refresh the
stored reference voltage Vref and provide a correctly updatedvoltage V4 to the control unit. As shown in Fig. 4, Vref is kept and
maintained by a capacitor Cr and Vref is updated through an active
diode D5. When the vibration becomes weaker, the active diode is
cut off; and the Vref cannot decrease to reflect the variations of the
vibration. Therefore, we need to refresh the value V ref so that the
tracking unit can generate a correct value of V4 to the control unit.
Fig. 8(a) shows the proposed refreshing unit, which consist of
five MOS switches and an internal capacitor C0. Fig. 8(b) shows
the sequence of control signals that turn on/off the switches.When a new tracking starts, MN2 is on and charge sharing occurs
between Cr and C0. The value of C0 is chosen to be much less than
that of Cr and hence the voltage at C0 is very close to Vref . Note
that the control signal for MN2 is driven by Vs which is about 4
times higher than Vref and hence there is no threshold drop across
the NMOS switch. Before the charge sharing action, C0 isdischarged to 0 by the switch MN1 driven by Con1. After the
charge sharing is completed, the output of the refreshing unit is
connected to C0 through MN4. Cr is cut off from the output. A
Con3 signal is generated to turn on MN3 to discharge Cr . After it
is discharged, it is ready for another tracking update. The tracking
pulse width is 2T and the above action occurs in the first 0.5T.
This leaves enough time for Cr to be charged up to a new V ref .
When the tracking process finishes, i.e., tracking pulse Φ returns
to zero, Vref is already updated and kept by Cr . The output of therefreshing unit is connected back to Cr through MP1 and
disconnected from C0 by turning off MN4.
Figure 8 Schematic of the refreshing circuit and control
signals
3.3 Control UnitThe control unit implements a band-band control strategy to
maintain the output voltage Vs of the energy harvester at theoptimal value. It mainly consists of a Schmitt trigger and a
voltage comparator, as shown in Fig. 9(a). Resistors R 1~R 6 of Fig.
6 form 3 voltage dividers to provide V1= (1/3.95)Vs, V2=
(1/4.05)Vs and V3= (1/3.96)Vs to the control unit. Note that the
refreshing unit provides a reference voltage V4 = (1/4)Vs, optimal tothe control unit as well. Two control signals “con” and “enable”
are generated to turn on/off the switch S1 to track the output
voltage of energy harvester to be optimal and to enable the
application, respectively. When Cs is charged up to a value higher
than 1.0125Vs, optimal, “con” becomes low and turns on S1. It will
trigger the application to wake up from sleep mode and starts an
atomic operation. Power is then transferred to the load through the buck converter. When Vs is lower than 0.99Vs, optimal, “enable”
becomes low to disable the application, which should store the
operation state for next operation before going to sleep mode
again. When Vs is lower than 0.9875Vs, optimal, “con” becomeshigh and turns off S1. Power transfer to the buck converter is
stopped and one operation round is finished. Once the harvested
power charges up Cs and Vs reaches 1.0125Vs, optimal, the operation
cycle repeats again. The control unit maintains Vs between
0.9875~1.0125Vs,optimal.
Figure 9 (a) Control unit diagram (b) Comparator
architecture
The speed of the control unit does not need to be very high and
low power consumption is the most important design factor. Thecomparators are operated in subthreshold region [11], and its
schematic is shown in Fig. 9(b). It contains a two-stage