Powering a wireless sensor node with a vibration-driven piezoelectric energy harvester Elizabeth K Reilly1, Fred Burghardt2, Romy Fain3, and Paul Wright4 1
Exponent, 149 Commonwealth Drive, Menlo Park, CA 94025 Berkeley Wireless Research Center (BWRC), University of California Berkeley, Berkeley, CA 3 Mechanical Engineering, Cornell University, Ithaca, NY 4 Mechanical Engineering, University of California, Berkeley, CA 2
Email:
[email protected] Abstract This paper discusses the direct application of scavenged energy to power a wireless sensor platform. A trapezoidal piezoelectric harvester was designed for a specific machine tool application and tested for robustness and longevity as well as performance. The design focused on resonant performance and distributed strain concentrations at a given resonant frequency and acceleration. Critical issues of power coupling and conditioning between harvester and wireless platform were addressed. The wireless platform consisted of a sensor, controller, power conditioning circuitry, and a custom low power radio. The system transmitted a sensor sample once every 10 seconds in a scavenging environment of 0.25 g and 120 Hz for a system duty cycle of approximately 0.2%.
1. Introduction Interest in wireless long term monitoring of mechanical systems has been growing over the last several years. Traditionally, batteries have been used to provide energy for devices in this application space. Energy scavenging offers an attractive alternative when coupled with a rechargeable battery and/or capacitors and with recent advances in low-power wireless technology. Scavenging offers a “place and forget” system with very low installation costs and essentially zero maintenance of the sensing devices. A number of regenerative technologies for harvesting energy from the environment have been investigated, including thermoelectric, solar, electrostatic, electromechanical, and piezoelectric [1]. Since a ubiquitous energy supply is found in the form of vibrational energy, piezoelectric scavenging is a good candidate for commercial and residential environments which are rich in these sources. A recent study by the authors provides a glimpse into a number of sources that produce vibrational energy [2]. This study is summarized in table 1. Since the effectiveness of energy scavenging is highly dependent on the operating environment, a scavenger must be tailored for the target application. Table 1 illustrates the wide variation in operating frequencies of the sources; piezoelectric scavengers resonate over a fairly narrow band, so it is important to design the device for the target environment. This study describes a working prototype that operates in a machine tool environment, showing the feasibility of using energy scavenging as a source of power. The prototype device consists of a piezoelectric energy scavenger, a power management system to transform the AC signal generated by the scavenger into a DC source meeting the criteria of the load, and a load circuit composed of an accelerometer, microcontroller, and radio.
Table 1. Ambient Vibrational Energy
Frequency (Hz) Acceleration (g’s) Characterization W500 Lenovo laptop 119.0 0.199 s Milwaukee Cordless Drill 15.2 0.363 s External HD 119.3 0.014 s Washing Machine 85.0 0.314 s Rockwell Sander 59.3 0.121 s Monarch Lathe Chassis 284.0 0.144 bb Delta Drill Press 41.3 0.407 s/bb HVAC Roof 184.5 0.252 bb Driving 2002 Toyota Camry 42.8 0.022 bb Bicycle 0.2 0.091 i 15.0 0.062 s/bb Running 1.5 2.045 s/lf 5.1 0.762 s/lf Walking 1.0 0.430 s/bb/lf 3.7 0.305 s/bb/lf Refrigerator 58.7 0.018 s Electric Tea Pot 241.0 0.019 bb Characterization key is as follows: s = resonant spike, bb = broadband optimized, i = impact ( 2g
50% Power bandwidth
min ± 1 Hz @ 0.05g ± 10 Hz @ 1g
Durability
min 5 *107 @ max working excitation 1g
Size constraint
8 cm X 5 cm X 5 cm
2.1 Design The theoretical model for vibrational energy conversion has been explored in depth by others so it will only be summarized here. Williams and Yeats [4] presented the first general model of transduction based on inertial devices in sinusoidal motion. Subsequent models [5-8] have refined their basic principles and produced a more accurate result. Halvorsen [9] presented an analytical model of energy harvester responses to broadband vibrations. The equivalent circuit model for a piezoelectric vibration energy harvesting system is shown in figure 1. In this model, the harvester is an equivalent mass, Lm, spring, Ck, and damper, Rb, with the electromechanical coupling modeled as a transformer. The load is idealized as a resistor. The full derivation is explained in [10].
‐1 Mass (Lm) Stiffness (Ck) Damping (Rb)
i
Cb
V R
in
Figure 1. Circuit representation of a piezoelectric generator with a resistive load.
From the circuit diagram in figure 1, Kirchhoff’s voltage and current laws can be used to obtain two Equations, (1) and (3), respectively. Substitution results in both equations in terms of strain and voltage as seen in Equations (2) and (4).
in m b k nV
(1)
cp c d y b S S m S p 31 V k1k 2 m m k1k 2 mt p k2
(2)
i iC iR
(3)
t c d 1 V p p 31 S V RC p
(4)
k1
tp 2I
(2 Lb Lm Le )
2 1 L2b ( Lb Lm ) 3 2 k2 t p (2 Lb Lm Le ) Σ N V S cp M bm
stress (analogous to voltage) transformer turns ratio voltage strain (analogous to charge) stiffness of piezoelectric layer proof mass mechanical damping coefficient
(5)
(6)
thickness of piezoelectric layer piezoelectric coefficient input vibration displacement current capacitance of the piezoelectric layer optimum load resistance dielectric constant
tp d31 Y I Cp R
The analytical expression for power transferred to a resistive load can be developed by using Equations (2) and (4) to substitute voltage for strain in order to obtain an equation for voltage, which is given in Equation (7).
j V
c p d 31t p Ainput
k 2
n2 1 2 n 2 2 n 2 j n2 (1 k 312 ) RC p RC p RC p
k31
d 312 c p
33
Ain
input acceleration magnitude
n
resonant frequency of the cantilever total damping ratio
(7)
(8)
The average power dissipated by the resistive load is P = |V|2/2R. From the above model a piezoelectric power scavenging system was designed using two main considerations: resonant performance and distributed strain concentrations. Firstly, resonant performance of the scavenger system will greatly enhance the ability of the design to meet the power performance requirements of the device electronics. To match the scavenger’s resonant frequency to that of the ambient source, the finite element analysis software tool Comsol was used to determine the natural frequencies of each candidate design. Since the target frequency was 100 Hz, a cantilever configuration was chosen. This configuration allowed for low frequency behavior and high strains due to bending moments. To increase the piezoelectric conversion from the strain and decrease the resonant frequency a proof mass was added to the free end of the cantilever. A rectangular beam is subject to extremely high strains at the corners adjacent to the mount, and can exhibit surface cracking when subject to large deflections. Such cracks inhibit charge mobility, which reduces the beams efficiency over time. Since the device must last for many years the piezoelectric must be durable. In this case durability means the ability to withstand large shocks of acceleration without significant damage to the piezoelectric. Also, to optimally utilize the piezoelectric, the strain should be distributed throughout the volume of usable active material. Based on these considerations, the shape of the scavenger which best encompasses all of the above considerations is a trapezoid (figures 2 and 3). The widened base and angled sides mitigate the high strain concentrations at the sides of the base and distributes the strain concentration more uniformly throughout the piezoelectric material (figure 4). The angled sides also protect against torsion. The wide base increases the stiffness of the device protecting it against over straining at high accelerations.
Figure 2. Geometry of scavenger
Figure 3. Photograph of scavenger
Figure 4. Strain distribution
The piezoelectric layers were 191 mm thick with a 102 mm center brass layer. The piezoelectric material used was bulk PZT-5H and the material properties are given in Table 3. Table 3. Piezoelectric Material Properties Parameter
Value
e33 @1 kHz
3800
d33 (m/V)
650 x 10-12
d31 (m/V)
-320 x 10-12
Ep (V/m)
>1.5 x 106
Ec (V/m)
8 x 105
r (kg/m3)
7800
Q
32
E3 (GPa)
50
E1 (Gpa)
62
The tungsten proof mass on the free end of the trapezoid was 5.5g and the center of mass was located at the edge of the trapezoid. 2.2 Experimental Testing of the Scavenger The piezoelectric scavenger was tested on a bench setup consisting of a “shaker table” to provide acceleration and a frequency generator to control the acceleration frequency. The piezoelectric material is bonded to a small printed circuit board (PCB) and the assembly screwed into the shaker through the PCB to avoid damage to the active material and to provide good mechanical coupling. Several tests were done to determine the performance capabilities and robustness. Tests included optimum load resistance, power bandwidth, power spectrum at increasing accelerations, and fatigue testing. This section will outline the testing procedures and discuss the outcomes.
Figure 5. Testing setup
2.2.1 Load Resistance. The load resistance for the trapezoid scavenger was determined by running the trapezoid at 100 Hz and accelerations of 0.05 g, 0.5 g, and 1 g. Various load resistances were applied and
the output voltage was measured. The power was calculated using P=IV and plotted versus applied resistance (figure 6). 40
1g
35
0.5 g
30
0.1 g
25
Power (mW)
20 15 10 5 0 0
100
200
300
Load Resistance (kohm) Figure 6. Plot of load resistance at various accelerations
2.2.2 Power Bandwidth. The power bandwidth of the scavenging device was dependent on its mechanical quality factor. To determine the bandwidth and the quality factor the device was vibrated at an acceleration of 0.25 g and 91 kohm with the output collected over a 10 Hz range centered at the resonant frequency of 100 Hz (figure 7). The 50% power bandwidth of this design is approximately 3 Hz resulting in a Q of 33. 4.5 4 3.5 3 Power 2.5 (mW) 2 1.5 1 0.5 0 95
97
99
101
103
105
Frequency (Hz) Figure 7. Power bandwidth at 0.25 g
2.2.3 Power Spectrum at Increasing Accelerations. The operational range for the piezoelectric scavenger was from 0.05 g to 1.0 g. To determine the power performance at this range of accelerations the scavenger was run at 100 Hz over a load resistance of 91 kW. The output demonstrated a quadratic response as expected from the theory. The power plot is shown in figure 8.
50 40 30
Power (mW) 20 10 0 0
0.2
0.4
0.6
0.8
1
Acceleration (g) Figure 8. Power spectrum plot at increasing accelerations
2.2.4 Fatigue Testing. Since the goal of energy scavenging is to provide a long term energy solution, perhaps the most important test is fatigue under long term operation. To determine the rate at which power degrades over time, two separate trapezoids were run at 1 g for 107 cycles (figure 9). The data were collected using Labview software. From the plot below no major decrease is observed in the output power, indicating a consistant output. Tests are currently ongoing to achieve the 109 cycle goal.
55.00
Power (mW)
50.00 45.00 40.00 35.00 Trapezoid 3 Trapezoid 2 30.00 0.E+00
2.E+07
4.E+07
6.E+07
Cycles (#)
Figure 9. Power versus cycles at 1 g and 100 Hz
3. Device Electronics The functional requirements for the electronics include: take a sample, process the data, packetize the data, and transmit the packet. There is no uplink, so only a transmitter is needed. Major elements of the system include: • • • • •
Harvested energy storage Power management Sensor (in this case a vibration sensor) Microcontroller Radio
Figure 10 shows a functional block diagram. The dark blocks are core elements; those required in any sensing device in one form or another (except perhaps the level shifters). Blocks in white perform power management, a function that requires special consideration in devices relying on scavenged energy. The output from the scavenger is connected to a simple full-wave bridge followed by a large storage capacitor. The DC voltage on the capacitor is then converted via the power management subsystem into voltages expected by the sensor, microcontroller, and radio. In an ideal world, energy from a scavenger would be applied directly to the node electronics. In reality, these components often have different requirements for voltage, noise, and regulation. Radios in particular demand clean, regulated voltages and bursts of current. It is likely that the scavenger characteristics will not satisfy any of these requirements directly, so additional circuits are required to buffer and “manage” the source. 3.1 Energy Storage Candidate methods of buffering include batteries and capacitors. Depending on the design, capacitors store significant amounts of energy and can deliver power in bursts. However, the output voltage is highly variable with load and energy density is considerably less that most battery technologies; for example, 220 J/g for a NiMh battery vs. 100 J/g for a super capacitor or 2 J/g for a typical capacitor. For this application a capacitor was chosen because the intent was to completely remove power from the electronics once a packet has been sent. In this scenario, there is no power consumption during the capacitor charge cycle and issues such as battery charge level maintenance and controller sleep modes are eliminated. The design takes advantage of the fact that the main processing engine (an MSP430 microcontroller) can operate within a wide range of supply voltage, so in this case energy stored in a capacitor can in fact be applied directly to the supply pin provided the voltage is sufficiently free of ripple. Other components on the device require more stringent regulation, but since the controller is the first device to “wake up” in a charge/transmit cycle, application of regulated supplies to those components can be sequenced by the microcontroller.
Figure 10. Block diagram
3.2 Power Management A typical approach to power management is to use either an up or down converting DC-DC switching regulator or a linear regulator. Most currently available devices are not designed for extreme low power and consequently have quiescent currents that are far higher than can be tolerated. For linears, there is and additional problem of losses due to heat produced as a by-product of regulation. Switching supplies require relatively large discrete inductors. The circuit in Figure 10 requires three power supplies: 1.8 to 3.3 V for the microcontroller and sensor, 1.0 V regulated for the radio digital logic, and 0.65 V tightly regulated with low noise for the radio RF section. The controller and sensor operate reasonably well over the voltage range, so regulation is unnecessary as long as the voltage limits are observed. Because the piezoelectric scavenger is capable of producing voltages higher than any of the electronic components can tolerate, power is applied and removed through a solid state switch controlled by a comparator. The comparator samples the storage capacitor voltage, and when that voltage reaches Vinmax of the controller, the power switch is closed.
When the capacitor voltage drops below the minimum operating voltage of the controller the switch is opened. Duty cycle is thus determined by the high and low setpoints of the comparator. The remaining supplies can be disabled when not in use. Since the device operates at a very low duty cycle, losses during the short “on” time can be tolerated so in this case efficiency is less important than physical size. Many commercial off-the-shelf (COTS) regulators have shutdown pins for low power operation, but even in shutdown they tend to draw significant quiescent current (for a circuit powered by scavenged energy, at least: on the order of 10-100uA). A better option is to simply gate power at the input of the unneeded regulators. To do this, two methods were employed. Firstly the 1 V supply was used for the radio digital interface, which includes configuration registers and the transmit bit stream. Tight regulation was not needed and power requirements were small, so the drive of a microcontroller general purpose I/O pin via a simple shunt regulator was sufficient. Secondly, the 0.65 V RF supply required good regulation, low noise, and up to 15 mA of burst current. This supply was on for even less time than the 1 V supply, so it was clear that sacrificing power efficiency for a clean, stable voltage was advisable. A linear regulator was chosen because this device class has two advantages over switching DC-DC converters: low noise and small footprint (no inductors). The storage capacitance was gated to the input through a solid state switch, controlled by a microcontroller GPIO pin. Recent work has integrated power management onto a single die [11]. A chip such as this would replace the diode bridge, all of the regulators, and all of the switches. 3.3. The Radio The radio is a product of research at the Berkeley Wireless Research Center (BWRC). It uses a transmitter based on Film Bulk Acoustic Resonator (FBAR) technology for RF carrier generation [12]. The FBAR is a MEMs device that behaves like a capacitor except at resonance, where it has Q > 1000. There are two transmitters on the radio die, each using one FBAR. The FBARs are separate die, wire bonded to the transmitter. Baseband data is modulated onto the carrier using on-off keying (OOK) by power cycling the FBAR oscillator and the low power amplifier. Transmitter properties include 1.863 and 1.916GHz channels. Efficiency is 46% at 1.2mw transmit power with a 650mv supply. The die is 1.2x0.8mm in 0.13um CMOS. With 50% OOK, total power consumption is approximately 1.35mW at data rates up to 330kbps. 3.4 The Circuit Board Figure 11 shows the top of the electronics board. The large square device at center right is a LIS3L02AQ three axis linear accelerometer from ST Microelectronics. The TI MSP4301222RHBT microcontroller is the smaller square device adjacent to the white connector, and the radio (including FBARs) is under the translucent material at the top center, next to the antenna. The antenna (large gold structure at left) was designed specifically for the radio e.g. it is “tuned” to the characteristics of that particular radio. The remaining components on the board are the rectifier, storage capacitance, and circuitry required for power management.
Figure 11. The electronics
3.5 Description of Operation A charge/transmit cycle is as follows: (refer to figure 10): 1. The scavenger charges the storage capacitor to a threshold voltage determined by the comparator, at which time the 1.8-3.3 V supply for the microcontroller and sensor is enabled by the first switch. 2. The microcontroller boots and takes a sample from the sensor and assembles a packet. 3. The microcontroller then enables the 1.0 V radio digital supply, and programs the radio configuration registers. 4. The microcontroller enables the 0.65 V radio power amplifier supply by closing the second switch, which applies the main power to a linear regulator. 5. The 48 bit packet is then transmitted using OOK modulation until the storage capacitor voltage falls below a low threshold voltage, at which point the comparator opens the first switch and voltage to all electronics is removed. The operational duty cycle of the device was determined by several factors, including size of the scavenging system, amount of storage capacitance, acceleration frequency and amplitude applied to the scavenger, and power demands of the electronics. For demonstration purposes a set of parameters was used that mimicked the vibrational characteristics of a typical machine tool while producing a reasonably short duty cycle and at least one transmitted packet: a vibration table was used to apply an acceleration of 0.25g at 120Hz. The piezoelectric energy harvester charged 500uF of ceramic capacitance. The resultant duty cycle was approximately 10 seconds, and one full packet was generated each cycle with about onehalf packet residual. Power demand from the electronics varied significantly depending on supply voltage and operational mode. At microcontroller boot time when the radio is off and the controller supply voltage was maximum the system consumed approximately 6 mW, but since the voltage consistently decreased, power consumption was significantly lower by the time the packet was transmitted. The radio contributed about 2.5 mW when the packet bit was ‘1’ (~0 mW when the bit was 0). Because the input impedance of the circuit from the viewpoint of the scavenger was non-optimal (much lower than 91kohm), in this application the scavenger was operating close to the Y axis in figures 6.
Figure 11. Radio board and sensors on top of the piezoelectric energy scavenger encased in an enclosure built using a Fused Deposition Machine (FDM).
4. Conclusions This paper described the design and testing of a piezoelectric energy scavenger and the accompanying electronics required to create a wireless sensor node powered by ambient vibration. The trapezoidal scavenging device was also tested in fatigue; showing no deterioration in performance when driven at 1 g for 107 cycles. The scavenger was then paired with an electronic subsystem to buffer the incoming AC power, convert it to DC, and sequence a sensor, microcontroller, and radio. The duty cycle of the radio was approximately 0.2% (10s charge, 20ms compute/transmit) in a scavenging environment of 0.25 g and 120 Hz with good repeatability. At least one packet containing a sensor sample was transmitted each duty cycle. 5. Acknowledgements The authors wish to acknowledge the contributions of the students, faculty and sponsors of the Berkeley Wireless Research Center. We also thank Avago Technologies and ST Microelectronics for the FBAR resonator and radio CMOS fabrication respectively. This research was funded in part by DARPA (Grant No. N66001-01-1-8967). References [1] Cook-Chennault K A, Thambi N, and Sastry A M, 2008, Powering MEMS portable devices— A review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems Smart Materials and Structures17 043001 [2] Reilly E K, Miller L, Fain R, and Wright P K, 2009A Study of Ambient Vibrations for Piezoelectric Energy Scavenging International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (POWERMEMS) Washington DC, USA 312-315 [3] Mitcheson P D, Reilly E K, Toh T, Wright P K, and Yeatmen, EM 2007 Performance limits of the three MEMS inertial energy generator transduction types J. Micromech. Microeng. 17 S211-216 [4] Williams C B and Yates R B, 1996 Analysis of a micro-electric generator for microsystems, Sensors and Acutators A, 52 8-11 [5] Shu Y C and Lien I C, 2006 Analysis of power output for piezoelectric energy harvesting systems, Smart Mater. Struct. 15 1499–1512 [6] Erturk A and Inman D J, 2009 An experimentally validated bimorph cantilever model for piezoelectric
energy harvesting from base excitations Smart Materials and Structures 18 1-18. [7] N. G. Stephen, (2006) On energy harvesting from ambient vibration ,Journal of Sound and Vibration 293 409-25 [8] duToit N E, Wardle B L, and Kim S-G, 2005 Design Considerations for MEMS-scale Piezoelectric Mechanical Vibration Energy Harvesters, Integrated Ferroelectrics, 71 121-60 [9] E. Halvorsen 2008 Energy Harvesters Driven by Broadband Random Vibrations, Journal of Microelectromechanical Systems 17 1061-71 [10] Roundy S and Wright P K 2004 A Piezoelectric Vibration based Generator for Wireless Electronics, Smart Materials and Structures 13 1131-1142 [11] Seeman M, Sanders S, and Rabaey J An Ultra-Low-Power Power Management IC for Wireless Sensor Nodes, IEEE CICC, Sept. 2007. [12] Chee Y.H, Niknejad A, and Rabaey J,2006 A 46% Efficient 0.8dBm Transmitter for Wireless Sensor Networks, 2006 Symposium on VLSI Circuits Digest of Technical Papers
