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Self-powered Signal Processing Using Vibration-based Power Generation - Solid-State Circuits, IEEE Journal of IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 33, NO. 5, MAY 1998
687
Self-Powered Signal Processing Using
Vibration-Based Power Generation
Rajeevan Amirtharajah and Anantha P. Chandrakasan,
Member, IEEE
Abstract Low power design trends raise the possibility of
using ambient energy to power future digital systems. A chip
has been designed and tested to demonstrate the feasibility of
operating a digital system from power generated by vibrations
in its environment. A moving coil electromagnetic transducer
was used as a power generator. Calculations show that power on
the order of 400

W can be generated. The test chip integrates
an ultra-low power controller to regulate the generator voltage
using delay feedback techniques, and a low power subband lter
DSP load circuit. Tests verify 500 kHz self-powered operation
of the subband lter, a level of performance suitable for sensor
applications. The entire system, including the DSP load, consumes
18

W of power. The chip is implemented in a standard 0.8 m
CMOS process. A single generator excitation produced 23 ms of
valid DSP operation at a 500 kHz clock frequency, corresponding
to 11 700 cycles.
Index TermsDC/DC conversion, low power, self-powered.
I. I
NTRODUCTION
T
HERE has been much interest in recent years in low
power very large scale integration (VLSI) design stem-
ming from the demands of long battery life in portable systems
and heat removal in larger, nonportable ones. Voltage scaling
coupled with other algorithmic and architectural optimizations
have allowed dramatic scaling of power consumption for a
wide variety of low to medium throughput DSP applications
[1]. Fig. 1 shows the current power consumption for a variety
of such DSPs, including programmable [2], custom [3],
and sensor related systems [4], [5]. Projecting current power
scaling trends into the future (based on deep voltage scaling
and other power management techniques), we expect the
power consumption to be reduced to tens of
W to hundreds
of
W. At these low power levels, an interesting question
arises: can we use ambient energy sources to power electronic
systems? Ambient energy is energy that is in the environment
of the system and is not stored explicitly, for example, in
a battery. Portable systems that depend on batteries have a
limited operating life and can fail at inconvenient times, while
a circuit powered by ambient sources has a potentially innite
lifetime. In long-lived systems where battery replacement is
difcult, generating power from ambient sources becomes
imperative. For example, in a smart structure where sensors
Manuscript received September 1997; revised November 26, 1997. This
work was supported by the ARL Advanced Sensors Federated Lab program
under Contract DAAL01-96-2-0001. The work of R. Amirtharajah was
supported by an NSF Fellowship.
The authors are with the Massachusetts Institute of Technology, Cambridge,
MA 02139 USA.
Publisher Item Identier S 0018-9200(98)02231-8.
Fig. 1.
Trends in power consumption for low to medium throughput DSP.
and actuators are embedded in a bulk material to modify its
properties, access to the electronics is greatly reduced.
In this paper, we explore the feasibility of operating a DSP
system on power generated by external means. Since ambient
energy sources are by denition uncontrolled, we require a
mechanism for converting the energy to a form usable by
digital logic. We propose a system as in Fig. 2, consisting
of a generator to create a voltage
, which can vary rapidly
depending on the energy environment of the system, a voltage
regulator to set the voltage to a desired level,
, and a DSP
load circuit which performs some computation. The desired
voltage is set using delay feedback instead of voltage feedback
[6][8]. Some measure of the performance of the DSP,
,
based on its critical path delay is compared to a desired
performance
, and
is adjusted until that performance
constraint is met. In a conventional xed supply voltage
scheme, the supply is at a level high enough to meet the most
demanding performance required from the load circuit under
worst case process and temperature conditions. However, the
supply voltage is often higher than necessary under nominal
operating conditions, and the circuit is then idle for some
portion of the cycle. The delay (or performance) feedback
scheme, on the other hand, compensates for temperature,
process, and computational workload variations. It also allows
a simple, all digital implementation of the control loop.
In addition to a generator, the self-powered system requires
a backup power source providing voltage
. This is neces-
sary since at startup the voltage regulator must derive its power
00189200/98$10.00
�
1998 IEEE 688
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 33, NO. 5, MAY 1998
Fig. 2.
System block diagram.
from some source and the generator output is too uncontrolled
to be used. The source could be a very small battery or a
previously charged large capacitor, but it need not provide
much energy since it is only used during the startup transient
of the system. Another key difference is that the generator
output voltage
varies rapidly with time, in contrast to the
slowly drooping battery voltages of conventional systems.
The rest of the paper describes in detail the design, imple-
mentation, and test of a self-powered DSP system. Section II
describes the particular approach to power generation taken
in this study and evaluates its potential for power output.
Section III discusses the design of the voltage regulation
scheme. Test results for the overall system are shown in
Section IV. Finally, Section V concludes and discusses the
potential for future work in this area.
II. P
OWER
G
ENERATION FROM
M
ECHANICAL
V
IBRATION
Our particular approach to using ambient energy sources
for power involves transduction of mechanical vibration to
electrical energy. We also performed some side experiments
on using incident sound as another power source.
A. Sources of Ambient Energy
Various schemes have been proposed to eliminate the need
for batteries in a portable digital system [9]. The most familiar
ambient energy source is solar power, but other examples
include electromagnetic elds (used in RF powered ID tags
[10], inductively powered smart cards [11], or noninvasive
pacemaker battery recharging [12]), thermal gradients, uid
ow, energy produced by the human body [13], and the action
of gravitational elds [14]. A generator based on transducing
mechanical vibrations can be enclosed to protect it from a
harsh environment, it functions in a constant temperature eld,
and a person can activate it by shaking it. However, its mov-
ing parts imply less long-term reliability and more complex
mechanical design. Applications include sensors mounted on
vibrating machinery or worn on the body. Ambient acoustic
energy can also be used, but as Section IV-A will show, the
high eld intensities required make this approach very difcult.
B. Vibrational Power Transducer
An inertial electromechanical generator has been proposed
[9]. A mechanical drawing of this generator appears in Fig. 3.
The device consists of a mass
connected to a spring . The
other end of the spring is attached to a rigid housing. As the
housing is vibrated, the mass moves relative to the housing
and energy is stored in the mass-spring system. A wire coil
Fig. 3.
Generator mechanical schematic.
Fig. 4.
Block diagram of traditional low-frequency moving-coil transducer
model.
is attached to the mass and moves through the eld of a
permanent magnet
as the mass vibrates. The moving coil
cuts a varying amount of magnetic ux, which in turn induces
a voltage on the coil in accordance with Faradays Law. This
voltage is the electrical output of the generator and is the input
to the voltage regulator of Fig. 2.
We have developed a linearized model of the generator by
adapting traditional linear models for loudspeakers [15], [16].
Moving coil speakers use a very similar electromechanical
transducer to the one proposed for the generator. Fig. 4 shows
this model in block diagram form. A mechanical input force
feeds into a second-order mechanical system, the spring mass
of the generator plus a dashpot with damping coefcient
corresponding to the mechanical losses due to friction. The
output of the mechanical system is the position of the mass,
which feeds into an electrical system corresponding to the wire
coil loaded with a resistor
. The electrical system looks like
a rst-order
circuit, with the inductance
in series with
the load resistance and the parasitic resistance of the coil
.
The voltage is proportional to the derivative of coil position,
so we have a zero at the origin in the system tra