Keywords

1 Introduction

The idea of widely interconnected, dynamic and rapid sensing and computing networks has been developed for many decades. The combination of sensing and wireless communication has led to the development of WSNs. The basic building blocks a sensor node has are (i) sensing subsystems to read or acquire the data, (ii) a subsystem to process the data, (iii) a wireless subsystem for communication, also the power supply; usually a battery is used to power the subsystems. Generally, it is very difficult or sometimes impossible to recharge the batteries depending on the terrain or geography where the sensors are installed, and they will be very large in number. Wherever the site of installation be, the purpose of continuous functioning must be met; to fulfill this requirement needs periodic onsite checkup and maintenance act, which may take a long time to process and face a delay in the duty assigned with the networks which is not a good sign. This difficulty must be overcome by any methods, which can extend the lifetime of battery without exceeding the budget of the overall system. Among the three subsystems described, the communication subsystems consume more energy than remaining two. It is seen that the supply voltage of using majority of sensor platforms is in the range from 2.7 to 3.3 V, so the task of designer of harvesting system is to have a voltage of 3.6 V, to meet the requirements. The challenges to be dealt with the battery-powered networks are the current leakages that consume the battery even if it is not in use, the weather conditions to which the batteries are subjected to and resultant chemical leakages that can cause environmental problems [1]. Limited energy density of batteries is also a limiting factor in these operations. What happens as the consequence of these limiting factors is that the battery drains much before its approximated life span depleting the efficiency of the node.

2 Literature Review

Vibration-based energy harvesters have been a serious topic of research for powering the wireless sensors for over a decade. Even if the methodology adopted or ways travelled differ by each research, the prime goal is to utilize the vibration energy from the ambient environment and power the low power electronic loads such as wireless sensor nodes. The typical discussion on energy harvesting covers the means such as electromagnetic, electrostatic and piezoelectric based [2]. Each of these techniques have their own advantages and disadvantages; of these three, the one special with piezomaterials are that they do not want a separate power source to operate, and they are basically economic, less complex in structure and improved power density [3,4,5]. Such materials when stressed by a mechanical force generate an alternating voltage, and this is called the direct piezoelectric effect; the reverse also happens where an electric field tends to deform the material [6]. Several literature so is available in this area of research, and all of them have the common subsystems for the piezo-system; they are piezoelectric transducer, rectifier, power converter and the load. The transducer converts the vibrational energy into electrical energy which is harvested by the converter to drive the load. The load may be discontinuous or continuous with low duty cycle. There are piezoelectric ceramics and films; the selection of material depends upon the application and medium of application; for example, when we use the system in a moist atmosphere, there is a chance of deterioration of the material and loses its piezoelectric property. Lead zirconium titanate materials are used widely, because the efficiency with which it generates a voltage with proportion to the mechanical force is higher than other available materials. The operation of piezomaterial is characterized by certain constants, the charge constant and the voltage mode constants. Two modes of operation are denoted by d31 and d33 mode. If the voltage output and the responsible force are perpendicular to each other, it is the d31 mode. Coupling factor of d33 mode is generally higher than that of d31 mode for all piezomaterials, which shows a higher efficient conversion [8]. It is mentioned that the material generates and ac voltage, requires rectification and conditioning. The diode rectification and further DC-DC converter stage are there. The disadvantages of them being, losses due to high input currents and forward voltage drops which could not be eliminated; these two predominantly causes inefficient power conversion [9,10,11]. A dual polarity boost converter is reported in a direct AC to DC converter [11]. This system has the drawback of having two larger capacitors and inductors, where the discharge of the capacitors result in large voltage drops resulting in the converter response to be slower [12]. The author explains this circuit following the assumption that the periodic excitation and the speed of the mass are in phase. In case of frequency deviation, the output power reduces. Overcoming this issue requires additional circuitry which again reduces the output power. In case of nonlinear loads, the rectifier circuits are unsuitable for low-voltage applications. Utmost care must be taken while deciding the harvesters, which implies the specificity of the power management circuit to be used.

3 Energy Harvester Description

The power generated by the piezoelectric material cannot be directly given to the load under consideration; they have to be properly conditioned. Upon varying vibrations, the piezomaterial generates proportional voltage thereby the power; this is allotted to the power conditioning system. A boost rectifier is used along a buck boost converter for rectifying and boosting the level of voltage from the source, the final output being stored in a battery, viz. Li-ion, a super capacitor, etc. We cannot completely eliminate the dependence on rechargeable battery owing to the reason that the sensor node is not always in high or active state. This method may be thought as a harvest store–use method instead of direct harvest and use method. The intermittency of vibrations or variation in amplitude of vibrations some way drives us to depend on such a method. There we need mechanisms like the maximum power point tracking and power electronic circuits with lower switching losses. In instances we use the battery, in order to protect the battery from the damage due to overcharging and over discharging a battery control unit is needed. It is followed by a regulator, highly efficient in providing the regulated voltage (Fig. 1).

Fig. 1
figure 1

Block diagram of the energy harvester

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A simple selection of piezomaterials is not enough for satisfying the goal of generating the electrical power to drive the load. The kind of piezoelectric material configuration selected depends upon the medium of application, power budget, the cost and size considerations. Like any materials, the vandalism and corrosion stands limiting factors for piezomaterials too. However, the basic and foremost index of selection is the charge and voltage constants. In this work, we focus our work on piezoelectric cantilever beam and piezoelectric disks, both available off the shelf. While using the cantilever, any one of the ends of the beam is fixed and the next end kept free. In this configuration, when the force is distributed uniformly, the free end starts oscillation and experiences a certain displacement. The property of piezoelectric materials is such that any mechanical impact on it produces a proportional output voltage. Any such changes in physical dimensions of the beam material have direct effect on harvested power. The piezoelectric materials will not produce an output power upon constant application of constant value of pressure; there should always be a gradient of power on the surface of the material and definitely depends on the material selected and the direction of application of pressure. The equivalent structure is shown in Fig. 2.

Fig. 2
figure 2

Representation of cantilever beam

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The equivalent circuit is drawn with a resistive load in megaohms, while the capacitor range is in microfarad. The piezoelectric transducers can be modeled as a voltage source in series with a capacitor and resistor or charge source parallel with resistor and capacitor.

Figure 3 shows the equivalent circuit of a basic piezoelectric sensor which when excited by a force displaces from its mean position. The two current sources are used to simulate the up and down motions of the sensor used. The power waveform from each of the resistors over a range is shown; the maximum power is ~160 mW from one sensor, but in practical scenario, we cannot expect this value of power (Fig. 4).

Fig. 3
figure 3

Equivalent circuit simulated in LTSpice

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Fig. 4
figure 4

Power output from various resistors connected to a single piezosensor

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3.1 Power Conditioning Circuit

The output power from the vibration energy harvester is comparatively low, which compels us to use a power conditioning circuit. This circuit must efficiently transfer the energy, and the accumulation must be managed well. Figure 5 shows the schematic diagram of the proposed power conditioning circuit, which includes AC-DC boost converter. Some assumptions are made while designing the circuit, main assumption being that the input to the conditioning circuit is a sinusoidal voltage. The input signal frequency is much less compared to the switching frequency of the converter. During the boost operation, voltage across the switch is expressed as

Fig. 5
figure 5

Power conditioning circuit

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$$ V_{1} \left( t \right) = \left( {1 - d_{1} - d_{2} } \right)V_{\text{in}} \left( t \right) + d_{2} V_{o} \left( t \right) $$
(1)

\( V_{1} \) is the voltage across the switch, \( V_{\text{in}} \) is the input voltage, \( V_{o} \) is the output voltage,\( d_{1} \) and \( d_{2} \) are the duty cycles of the switches 1 and 2, respectively

$$ L = \frac{{0.2T_{s} \left( {V_{o} - V_{\text{in}} } \right)}}{{i_{\text{in}} }} $$
(2)

Lithium-ion battery is chosen by considering advantages such as long battery life, low battery cost, very low self-discharge, lightweight, high-energy density of 3.7 V, 150 mAh. The LTC1761 buck regulator is used to provide a regulated voltage of 3.3 V. It draws a supply current of only 20 μA during the operation and <0.1 μA under the shutdown mode.

3.2 Results

A 50 kHz switching frequency is selected for the operations and an inductor of 4.7 μH. The AC output is fed to the interface circuit to have the rectified output voltage meet the required load. The maximum power can be extracted when the load impedance is close to the harvester’s internal impedance, by the maximum power transfer theorem. Also, piezos produce most power when they operate at approximately half the open-circuit voltage for a given vibration intensity. The resonance frequency is found to be 21.5 which lies in acceptable range of the ambient vibrations. The harvester output is directly connected to the condition circuit (Fig. 6).

Fig. 6
figure 6

Conditioned output voltage at 3.3 V

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4 Conclusion

Wireless sensor networks are gaining significant importance in these times as the world becomes smart in every aspect of living. But powering up the wireless systems have always been an interesting issue. In this paper, a method utilizing the vibration energy on the piezoelectric materials is discussed along a power conditioning system. The circuit was expected to perform well powering up the sensor node without outage; a constant voltage of 3.3 V at a lower resonance frequency of 21.5 Hz from PZT-5H is obtained which is clearly in the admissible range for the system of low-power devices. The function of efficient rectification, maximum power extraction and voltage regulation is also met in the system.