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Self powered microsystem with electromechanical generator

JANÍČEK VLADIMÍR, HUSÁK MIROSLAV Department of Microelectronics

FEE CTU Prague Technická 2, 16627 Prague 6

CZECH REPUBLIC , http://micro.feld.cvut.cz

Abstract: - Growing interest in the field of micro and wireless electronic devices brings the requirement to integrate the power supply into a sensor chain and separate whole structure from the outside world. Use of alternative electrical energy sources instead of batteries has particular importance to remote sensor systems. A vibration-powered micro-generator, based on a polymer piezoelectric material, is proposed for this purpose. Key-Words: - microsystem, micro generator, piezoelectric, polymer, sensor, MEMS 1 Introduction

It is obvious that over the last years there has been a significant growing interest in miniaturizing of all kinds of electronic devices. With this demand on making everything smaller there is the same situation in the field of sensors and their wide range of applications such as process control units, medical implants, embedded sensors etc.

One specific aspect that has been always taken into account is the problem of supplying required electrical power to such a device. Conventional power supplies - such as batteries, could be used. But there are many applications, however, that require sensors to be completely embedded in the structure with no physical contact to the rest of the world. Supplying energy to such a system is difficult and the only possible solution is to make them so-called self-powered microsystems (See Fig.1). With integration of power supply unit on the same chip there are several advantages associated like noise reduction, elimination of crosstalk between power and signal lines, and reduction of power delivery control system complexity.

Energy autonomous microsystems and devices, working on the base of wireless energy and data supplying have benefit of different significant advantages [1], [2]. The whole sensor wiring is replaced by a simple parameter configuration at the source station. There has been demonstrated several automated procedures, so that sensors can be used by Plug&Play [3].

SELF POWERED MICROSYSTEM

SENSOR

SIGNAL PROCESSING

DATA TRANSCEIVER

POWER SUPPLY

Fig. 1: Energy and data flow inside the self powered microsystem

Currently, there are already several energy autonomous microsystems used especially in the medical field as implants for human beings, because of the difficult system accessibility [4].

Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp24-29)

The application of this kind of systems can be however expanded for several other applications such as: • Applications that make it impossible to be hard

wired to the base station. For example in machines [5], if there is a possibility of wires to become twisted due to the rotation of the tested object.

• Applications in contained area, such as in extreme heat, cold, humidity or chemical reactive conditions.

• Applications in which long distances are to be bridged or if there are several distributed components, like as it is in so-called smart wireless homes.

• Mobile and wireless applications, e.g. monitoring environmental conditions or positions of mobile goods.

2 Self power microsystem description

Three main units of such integrated self powered microsystem are: power supply unit with an energy generator, energy storage block and signal data processing and transmission unit.

2.1 Power supply unit with energy generator

Nowadays conventional solution to supply power is to use primary or secondary type of batteries. However, some of the drawbacks associated with such a solution are that the energy inside batteries is limited, they have short effective life in comparison with the supposed sensor system lifetime, and contain dangerous chemicals that could cause a hazard, their dimensions are not suitable for “micro world” applications and they can fail at inconvenient times. This shortens the system maintenance cycle and increases the total costs. It is therefore recommendable to make arrangements for energy generation within the microsystem. An unconventional solution is to design a micro generator to convert energy from an existing ambient energy into electrical energy. Some possible ambient energy sources, which can be converted into electrical energy, include light energy, thermal energy, volume flow energy and mechanical energy. The focus here is on applications concerning mechanical vibrations as the ambient source of interest. The transformation of mechanical vibrations into electrical power (See Fig.2) is based on piezoelectric effect. But today’s most used piezoelectric material PZT was substituted by more flexible polymer material with excellent properties applicable for typical usage fields of such kinds of sensors. The micro-mechanical generator (micro machined PVDF based device) will basically be

constructed by depositing metal based electrodes onto thin film Polyvinylidene fluoride (PVDF). Polyvinylidene fluoride possesses excellent mechanical, technological properties for this application.

Fig. 2: Piezoelectric micro generator working principle

2.2 Generated energy storage

An important question is how to store the generated energy and make it available for later usage. This is important due to several other reasons. For instants, the generated output voltage is nonlinear function of time in most cases. But the generation of ambient energy may not be present at all times or a higher start-up power may normally be required. Ideally an energy storage mechanism that can guarantee high power requirements and energy densities with the smallest size possible is required. The following candidates for energy storage mechanisms has been proposed.

• Electrochemical capacitors or so-called supercapacitors

• Secondary type (rechargeable) batteries

In general, battery technologies provide high energy density but lack of sufficient power density. On the other hand capacitor technologies provide high power density but they are limited by energy density. Therefore, there has been a third option investigated - a capacitor-battery combined system. Such electrochemical system can result a smaller and more efficient system (See Fig.3).

Generally, the microsystems should have low power consumption, in order to guarantee the system functionality as long as possible despite of the limited available power reserve. In order to reduce the stored energy, the system components and circuits should be specially selected under the consideration of low power consumption. Further significant energy economy could be reached through suitable organization of the system operating modes. The microsystem should have several operating modes, where unused system modules are turned off


depending on the active system modus and the power availability. The microsystem should be preferably operated in an alternating mode in order to benefit from the low-power consumption in the sleep operating mode.

Fig. 3: Energy autonomous sensors and microsystems structure

2.3 Output signal data processing and transmission unit

The system must be able to transmit the measured data to the outside world and it must be done while the sensor system is physically isolated. The objective is to use as small energy as possible to transmit the data. Then the power can be reduced to a low level by limiting the data rate. There are several communication media to be used. For this purpose very low power optical fiber communications can be used. Another possibility is usage of magnetic coupling resonant inductive circuits, where the transmission range is reasonably short. For longer transmission ranges, radio telemetry techniques should be investigated.

3 PVDF as piezoelectric material

Piezoelectric materials create electrical charge when mechanically stressed. The most widely used commercial piezoelectric material is various phases of lead zirconate titanate (PZT). Unfortunately, ceramic materials are fragile and it is very difficult to produce

them in large sizes. Piezoelectric polymers are increasingly considered as favorable materials for MEMS applications due to their fast response, low operating voltages and greater efficiencies of operation. The properties of polymers are so different in comparison to inorganics (Table 1) that they are uniquely qualified to fill areas where single crystals and ceramics can not be used effectively. As showen in Table 1, the polymer piezoelectric strain constant (d31) is lower than ceramic. However, piezoelectric polymers have much higher piezoelectric stress constants (g31) indicating that they are much better sensors than ceramics. Piezoelectric polymeric sensors and actuators offers the advantage of processing flexibility because they are lightweight, tough, readily manufactured into large areas, and can be cut and formed into complex shapes. Polymers also exhibit high strength and high impact resistance. Other notable features of polymers are low dielectric constant, low elastic stiffness, and low density, which result in a high voltage sensitivity (excellent sensor characteristic), and low acoustic and mechanical impedance (crucial for medical and underwater applications).

Table 1 Property comparison for standard piezoelectric polymer and ceramic materials

Property Units PVDF PZT

Density 3cmg

1.78 7.6

Relative permittivity 0ε

ε 12 1200

Elastic modulus m

N1010 0.3 4.9

Piezoelectric strain constant N

C1210−

3323

33

31

−==

dd

374

171

33

31

=−=

dd

Coupling constant Nm

CV

15.012.0

33

31

==

kk

69.034.0

33

31

==

kk

Piezoelectric stress constant N

Vm310−

330

216

33

31

−==

gg

2511

33

31

=−=

gg

Polymers also typically possess a high dielectric breakdown and high operating field strength, which means that they can resist much higher driving fields than ceramics. Polymers offer the ability to pattern electrodes on the film surface, and pole only selected


regions. Based on these features, piezoelectric polymers have their own established area for technical applications and useful device configurations.

The coupling constant shown in Table 1 represents the mechanical to electrical energy conversion efficiency of the material. The subscripts of the constants indicate the direction or mode of the mechanical and electrical interactions. "31 mode" indicates that strain is caused to axis 1 by electrical charge applied to axis 3. Conversely, strain on axis 1 will produce an electrical charge along axis 3. Bending elements, made by an expanding upper layer and a contracting bottom layer, are made to exploit this mode in industry. In practice, such bending elements have an effective coupling constant of 75 storage of mechanical energy.

The constant d33 is negative, in other words, applying an electric field in the direction of polarization (film thickness) causes the film thickness to decrease. PVDF has an anisotropic piezoelectric characteristics, where the film plane has the strongest effect in the drawing direction as opposed to the normal direction, i.e., d31 > d32 > 0. The ratio of these two d-constants can vary between 5 to 10 times.

The most efficient energy conversion, as indicated by the coupling constants in Table 1, comes from compressing PZT (d33). Even so, the amount of effective power that could be transferred by this way is minimal since compression follows the formula:

AYFHH =Δ (1)

where F is applied force, H is the unloaded height, A is the area over which the force is applied, and Y is the elastic modulus. The elastic modulus for PZT is 4.9e10 N/m2. Thus, it would take an incredible force to compress the material a small amount. Since energy is defined as force through distance, the effective energy generated by compression of PZT would be vanishingly small, even with perfect conversion.

On the other hand, bending a piece of piezoelectric material takes advantage of its 31 mode is much easier. Because it is brittle, PZT does not have much range of motion in this direction. Maximum surface strain for this material is 5e-4 N/m2. Surface strain can be defined as

2CL

xtS = (2)

where x is the deflection, t is the thickness of the beam, and LC is the cantilever length.

As a result, the maximum deflection or bending for a beam (20 cm) of a piezoceramic thin sheet (0.002 cm) before failure is

cmmeme

meetLS

x C 112

)2)(5())(( 25

2142

==== −−

−−

(3)

Thus, PZT is unsuitable for applications where flexibility is necessary.

PVDF, on the other hand, is very flexible. In addition, it is easy to handle and shape, exhibits good stability over time, and does not depolarize when subjected to very high alternating fields. The cost, however, is that PVDF's coupling constant is significantly lower than PZT's. Also, shaping PVDF can reduce the effective coupling of mechanical and electrical energies due to edge effects. In addition, the material's efficiency degrades depending on the operating temperature and the number of plies used. Fortunately, from an industry representative [6] we know a 116 cm 40 ply triangular plate PVDF based electromechanical generator with a center metal shim deflected 5 cm by 68 kg 3 times every 5 seconds results in the generation of 1.5 W of power, as developed for an application to harness energy from ocean waves [7]. 4 Layout description The proposed electromechanical microgenerator is essentially a resonant mechanical structure based on cantilever modifications. The base piezoelectric polymer polyvinylidene fluoride (PVDF) material is coated on both planar sides with a conductive metal layer. These layers act as electrodes. First generation and the simplest structure is on one side connected set of cantilever beams (See Figure 1).


FIGURE 1: Layout of the basic cantilever set Also serpentine cantilever (See Figure 2) has been designed to achieve a low resonant frequency structure as well as a low damping effect when it resonates. A small mass is attached to the free end of the beam.

FIGURE 2: Layout of the proposed serpentine cantilever with mass in the centre of the cantilever. Simulation mesh visible. There has been already published concept of microgenerators [8], which use a magnet and coil arrangement to generate the electrical power. One of the factors affecting output power delivered is the resonant frequency of the beam; the higher the frequency the more power will be available. The application areas envisaged for our system include vibrating machinery and vehicles, where the frequencies of interest do not generally exceed a few hundred Hertz, and are often restricted to below 100 Hz. As already described most important aspect is the power generation efficiency to surface ratio. To maximize the microsystem area and to optimize the energy efficiency of the layout there has been designed

a field of hexagonal shaped serpentine cantilever (See Figure 3).

FIGURE 3: Layout of the hexagonal shaped serpentine cantilever field with elongated proof mass 5 Conclusion This paper describes concept of self powered microsystem. The main problem is how to supply the energy to the system. Nowadays, there is rising demand on wireless sensor systems where no direct connection to the outside world exists. Application of integrated battery all kinds is not appropriate because of determined service life and not suitable dimensions. Autonomous micro generator based on piezoelectric polymer material with ability to change the ambient mechanical energy to the electrical energy will be appropriate. Used material shows better properties then conventional ceramic materials (PZT). Several layouts of such a micro generator were designed. The device is not optimized yet and significant improvements are envisaged in the future. References [1] M. Klein, H. Haspeklo, H. Wunderlich: “Sensor Systems using wireless Signal and Power Transmission: Applications and future requirements”, Proceedings of Sensor ’99, p. 153-156, May 18-20, 1999 [2] H. Wunderlich, G. Hettich, M. Klein, R.Schraub, J. Schrenk: “Concepts and Steps in the Development of Wireless Sensors and Actuators for Automotive


Applications”, Proceedings of Sensor ’99,p. 157-162, May 18-20, 1999 [3] M. Dunbar: “Plug-and-Play Sensors in Wireless Networks”, IEEE Instrumentation & Measurement Magazine, p. 19-23, March 2001 [4] R. Puers et al: ”A Telemetry System for the Detection of Hip Prosthesis Loosening by Vibration Analysis”, Proceedings of Eurosensors XIII, p. 757-760, September 12-15, 1999, The Hague [5] J. D. Turner, L. Austin: “ Sensors for automotive telematics”, Measurement

Science and Technology, Vol. 11 (2000), p. 58–79. [6] D. Halvorsen. Private correspondence. AMP Inc., May 1995. [7] C.B. Carroll. 5,814,921: “Frequency multiplying piezoelectric generators:, US Patent, September 29 1998. [8] P. Glynne-Jones, M. J. Tudor, S. P. Beeby, and N. M. White, „An electromagnetic, vibration-powered generator for intelligent sensor systéme“, Sensors and Actuators, A: Physical, 110(1-3):p. 344-349, February 2004.


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