An Autonomous Wireless Sensor Platform based on Microgenerators
1
D. Maurath1, T. Hehn1, D. Spreemann2, Y. Manoli1,2
Chair of Microelectronics, Department of Microsystems Engineering (IMTEK), University Freiburg, Germany
2
Institute for Micromachining and Information Technology, HSG-IMIT, Villingen-Schwenningen, Germany
Abstract: This paper presents a novel wireless autonomous system platform for sensor applications,
which is only based on energy scavenging power supply. This autonomous system platform needs neither any
external electrical energy supply nor any added battery or external pre-charging. The functionality comprises
high efficient voltage conversion, power management, sensor data acquisition and radio communication in
order to be able to participate in a network of distributed autonomous sensor nodes. It is presented how the
system was designed and implemented.
Key Words: micro-generator, power management, autonomous system, wireless, sensor platform
1. INTRODUCTION
By now, a vast diversity of smart autonomous
sensor systems (ASS) are developed and applied in
various scenarios and applications. Despite all the
desirable advantages, which those systems possess,
the long-term and permanent power supply relies
mainly on batteries only. An eligible and helpful
improvement of such ASS is energy harvesting with
micro-generators. However, it is quite challenging to
design an ASS, which does not have any auxiliary
power source, but only a microgenerator. Most
systems either focus especially on adjustment of the
converter duty cycle, in order to achieve maximum
conversion efficiency, or on optimized application
and radio implementation [1-3]. This novel system
presented here (Fig. 1) performs comparable
features with less energy consumption (Table 1) but
only supplied by a micro generator.
The generator for the presented system has a
tuneable frequency characteristic for excitation
frequencies in the range of 30 Hz – 60 Hz [4].
Within this frequency range the output power is
around 230 µW - 825 µW. However, the generator’s
open-circuit output voltage is not higher than 2.1 V̂
and the internal generator resistance is around
1-3 k: . Thus, the generator voltage varies
dramatically when a load current is applied. Despite
this drawback, a reliable power system is realized
with conversion efficiency from 65 % - 82 %.
The paper is sectioned in two main parts. Firstly,
it shows efficient generator interfacing, voltage
conversion and power management circuitry.
Additionally, basic design rules are introduced,
which help to optimize the power system. The
second part concerns the application electronic.
rectifier
RCB
generator
antenna
MSP430
C
boost
converter
CC2420
voltage
supervisor
Figure 1: This picture illustrates electronics for
voltage conversion, system control and radio
communication.
Subsequently, measurement results are presented
and discussed. Finally, a short outlook towards
further intended future research topics is added.
2. POWER SUPPLY CIRCUITRY
The power supply circuitry is composed of four
sections. As shown on Fig. 2, between the rectifier
and the voltage converter a reverse current barrier
and a pre-charge stage is included. The main energy
storage buffer Cb is located in-between these two
internal stages. At active periods, the boost stage is
enabled and supplies the application with VDC.
During sleep periods, the application is supplied
directly by the buffer Cb, thereby the current is
bypassed via supervisor device (TPS3619), which is
not shown in Fig.3.
281
rectifier
D1
reverse current barrier
SP1
D2
D3
Vgen
MN1 MN2
VR
+
CRCB
pre-charge stage
SP2
U1
VZ
Cb
+
D4
LBoost
VL
Vin
EN
U2
CFF
RFB1
VFB
TPS61120
CC1
VDC
Vout
RFB3
RFB2
Cout
Px.y
SN1
Figure 1: Schematic view of the rectifier, pre-charge and boost stage. The rectifier will cause only one
forward diode voltage drop and the pre-charge stage enhances start-up reliability.
2.1 Efficient Passive Rectification
Due to the relatively low generator output
voltages the diode forward-bias voltage losses of
common diode-only rectifiers are not acceptable
when micro-generator based supplies are used.
Therefore, a combination of Schottky diodes and
NMOS transistors is applied (D1, D2, MN1, MN2). On
the on hand, this gives the opportunity of only one
diode forward-bias voltage loss. On the other hand,
the reverse current is much higher compared to
diode-only rectifiers. This is caused by the
transistors, which do not completely close and by
the use of Schottky diodes, which tend intrinsically
to higher reverse currents compared to pn-diodes.
2.2 Reverse Current Barrier
These adverse reverse currents are nearly
completely blocked by the implementation of the
reverse current barrier (RCB). In order to achieve a
net power loss reduction, not only a very low power
comparator U1 is used, but also a high resistive
comparator input network. Thereby, diode D3
charges the small capacitor CRCB, which gains a
voltage higher than the resistor divider node, shortly
after the generator starts working. As soon as the
generator stops or has only an output voltage lower
then the buffer Cb, the voltage at CRCB drops,
whereby the comparator turns off switch SP1 [5].
2.3 Voltage Conversion, Start-up and Reliability
Inductive switch-mode converters exhibit high
efficiency but less reliability at start-up. In order to
ensure a reliable usage of the inductive converter
even though critical start-up performance, a precharge stage is used. In addition to this, stable loadtransition regulation has to be performed as well.
Therefore, a feed-forward capacitor CFF is added.
282
Due to correspondence of power efficiency and
application supply voltage, the converter output
voltage is re-programmed within the active time.
Thereby, during data acquisition the output voltage
is around Vout = 3.3 V and during radio transmission
the voltage is decreased to Vout = 2.2 V. This is
realized by a switchable third feedback resistor, as
shown in Fig. 2.
2.4 Pre-Charging Circuitry
The inclusion of the pre-charging stage (PCS) has
two main intentions. Firstly, the boost stage needs
especially at start-up relatively large currents, which
exceeds the ability of the generator. Secondly, due
to large internal generator resistance, an increased
current load lowers Vgen. This may result in start-up
failure of the boost stage, as well as reduced
rectification efficiency is more probable. Thus, a
buffer has to be sufficiently charged before the boost
stage starts conversion. As the buffer voltage
initially rises from the uncharged state, voltages VZ
and VR rise as well. However, once the Zener
voltage is crossed, diode D4 keeps the voltage VZ
constant, while VR rises further. So, the comparator
switches and turns on SP2.
2.5 Design Methodology
In order to implement the system concept
according to a given generator and an intended
application, the buffer capacitance, the trigger levels
of the PCS and the cycle times have to be
dimensioned reasonably. The minimum size for the
buffer
is
calculated
by
the
equation
Cb t
Qactive
'Vb
Qactive
l
V gen VµC ,min
(1)
where Qactive is the average application load current
times the active time Tactive, and VµC,min is the
minimum required operation voltage. This voltage is
equal to the turn-off voltage of the PCS. Whereas,
the PCS turn-on voltage is set to
Von t VµC ,min Qstart
Cb
(2)
where Qstart is the average load current times the
initialization time. A further issue is the fact, that the
converter needs a certain input-power during startup time. In order to achieve a large current
conduction angle and rather better impedance
matching a small buffer capacitance is preferable.
However, if the generator size and thus, its output
energy, is reduced, also the PCS turn-on level as
well as the buffer capacitance has to be increased.
This is because Cb has to provide the energy
difference between the generator output and the
converter input energy. Otherwise, the buffer
voltage will drop below the turn-off voltage more
likely in case of smaller generators.
Additionally, for high efficient conversion an
inductance of only 15 µH is applied even though
small load currents. By reducing inductance one has
to take care about the ccm/ dcm mode crossing as
the inductor current drops below I L, rms d 2'I L , where
'I L is the inductor ripple voltage. However, a
smaller inductor causes less resistive losses. Hence,
due to a reduced inductor resistance a notably
increased efficiency is achieved, as Fig. 3 illustrates.
3. POWER AND SYSTEM MANAGEMENT
The efficient and reliable operation of measurements
and communication can be notably improved by
reasonable system management and parameter
selection. Therefore, the management concepts are
explained in a quite general manner, whereas the
realized design values are mentioned in Tab. 1.
3.1 Dynamic Processing Adaptation
In practical usage generator excitation may vary
and so the average available power is non-constant.
This requires the ability of dynamic processing
adaptation. Therefore, certain system parameters,
like sleep time Tsleep, communication activity as well
as sensor resolution are dynamically adapted with
respect to the available power.
3.2 Sensor Data Acquisition and Computing
Since the efficiency of boost converters are load
current dependent, during the active time Tactive,
preferably all task are executed simultaneously.
Consequently,
the
sensors
are
triggered
simultaneously. The controller is programmed in an
event-driven style. Thus, good modularity and
efficient operation is possible. All events are
triggered by an interrupt, which allows setting the
controller in low power mode after an accomplished
operation. The next event will re-wake the controller
by an interrupt. Moreover, modularity is very
helpful in combination with the usage of different
sensors or RF modules, which are connected with
SPI or I²C interfaces. So, different sensors can be
plugged easily to the platform.
3.3 Radio Interface and Communication
Radio communication is realized by a CC2420
RF transceiver chip. This chip has very short startup times and offers efficient operation with little
protocol overhead. For low power consumption
requirements packet payload length and power
amplifier gain is adapted with respect to current
operating conditions. Hence, the transceiver is set in
power down mode if no radio communication is
required. Thus, data reception is only possible at
short timeslots after a data packet was transmitted.
4. SYSTEM SET-UP AND RESULTS
The specific parameters of the actually built system
are shown in Tables 1 and 2. Especially the duty
cycle D and the averaged consumption results in
Tab. 2 and Fig. 3 are maximum values, measured
with an ideally excited generator. Thereby, the
transceiver is only used after the power supply
system has steady-state condition. For less generator
power, the duty cycle is decreased, the transceiver
power amplifier gain is reduced and the sensor
performance is minimized by reducing measurement
time and current. As described above, the
conversion efficiency not only depends on the load
current, but also depends strongly on the inductance
value and inductor resistance. Nevertheless, the
presented efficiencies are strongly related to
Vgen
Ri
Cb
Lboost
4.1 Vpp
2.98 kȍ
470 µF
15 µH
Tactive
Tsleep
D
rms
6I sleep
190 ms
810 ms
< 0.2
< 5.5 µA
CFF
20 nF
rms
6I active
1.6 mA
Von
1.9 V
6Prms
497 µW
Table 1: Power supply parameters of the built
energy harvesting based sensor platform.
283
state
PV
[µW]
Transceiver sleep
0.04
CC2440
active 16800
µC
sleep
0.2
MSP430
active 1980
Sensors +
sleep
4,1
Peripheries active 2700
EV,total
time
[ms]
995.5
4.5
876
124
830
170
EV [µJ]
0.0398
75.6
0.18
245.5
3.32
175
499.6
Table 2: Average power consumption for one
measurement cycle and transmission of 1000 bits
every one second is about 500 µW.
efficiency
67.1k, 15H
0.6
0.4
3k, 2.2mH
0.2
67.1k, 2.2mH
0.0
1.8
2.2
2.6
3.0
Vbuffer
3.4 [V]
Figure 3: Measured conversion efficiency over the
entire input voltage range as well as with different
resistive loads and inductances.
appropriate usage of the converter. That means the
converter is only used in active periods and never
during sleep periods. The mentioned resistor values
correspond to representative currents, at active
period only.
Fig.4 illustrates the general transient power
consumption profile. The transmission power and
sensor power, PTx and Psens respectively, are highly
related to the used parameter settings.
5. CONCLUSION
Altogether, it is shown that the presented system
works with reasonable parameter settings. Hence,
efficient and reliable operation of a microgeneratoronly driven autonomous system can be obtained
with reasonable effort. The possibility to drive the
system with smaller generators by adapting
operating parameters and circuit dimensions is also
given. The most valuable results of the work are the
284
Tx
PTx
sensors
Psens
Pcalc
P sleep
µC
sleep
t sens
t sleep
t calc tTx
sleep
t
t cycle
Figure 4: Averaged power profile of the wireless
sensor during one measurement/ transmit cycle.
portability to other designs and the developed and
proved building blocks for the power management
and conversion circuitry. Also additional
implementation techniques and flexibility is shown,
with regards to the sensor attachment and high
performance data acquisition possibilities as well as
for the controller programming and self-controlled
operation.
3k, 15H
0.8
P
ACKNOWLEDGEMENT
This work is supported by the German Research
Foundation (Deutsche Forschungsgesellschaft –
DFG) under Grant Number GR 1322.
REFERENCES
[1] A. Chandrakasan, R. Min, M. Bhardwaj, S. Cho,
A. Wang, Power aware wireless microsensor
systems, European Solid State Conference 2002,
24-26 Sept. 2002, pp 47 - 54
[2] H. Dubois-Ferrière, L. Fabre, R. Meier, P.
Metrailler, TinyNode: a comprehensive platform
for wireless sensor network applications, 5th
international conference on information
processing in sensor networks, 2006, pp. 358 –
365
[3] J. Polastre, R. Szewczyk, D. Culler Article,
Telos: enabling ultra-low power wireless
research, IPSN 2005, No. 48, pp. 364-369
[4] D. Spreemann, B. Folkmer, D. Maurath and Y.
Manoli, Tunable transducer for low frequency
vibrational energy harvesting, 20. EuroSensors
2006, 17-20. Sep.
[5] T. Hehn, Energy Harvesting Based Sensor
System, diploma thesis, University of Freiburg,
IMTEK, 2006