**2. The dielectric elastomer generator**

172 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

expenditure of the person wearing the shoe.

different energy harvesting technology.

energy below:

3. A high energy density.

5. Cheap to produce and maintain.

Starner and Paradiso provide a good review of the energy available for scavenging from human movement and identify walking as a rich source of energy [5]. They highlight that 13Watts of power is available from the heel-strike of a 68Kg person if the sole of their shoe is compressed by 1cm when walking at 2 steps per second. Furthermore, a typical running shoe midsole dissipates a relatively large amount of energy as heat. Shorten's analysis of the energetics of a shoe midsole worn by a 76Kg runner suggests that a running shoe will dissipate between 2 and 10 Joules per step [6]. A well designed energy harvesting shoe could instead turn this energy into electricity, without altering the comfort or energy

The most prevalent energy harvesting technology, electromagnetic generators, have been used to harvest energy from human gait, but additional mechanisms are required to condition the mechanical energy [5, 7]. Donelan et al. developed a knee-brace generator for harvesting energy from human gait. Their generator contained auxiliary components including a gear train, bearings, and a separate input shaft to convert the mechanical energy to suit their electromagnetic generator. Their system cost on average 59W of metabolic power to carry without harvesting energy, whereas an additional 5W of metabolic power was required to produce 4.8W of electrical power [7]. Although their harvesting mechanism was extremely efficient, they could have achieved larger efficiency gains if their system's mass was reduced and the device did not alter gait patterns. This begs the question: why are auxiliary mechanical

The system by Donelan et al. was relatively heavy because it required additional componentry for it to work efficiently: electromagnetic generators produce more energy during a single rotation or stroke as velocity increases and are poorly suited for the low induced velocities associated with walking unless augmented mechanically. Thus the viability of harvesting energy from human motions could be improved by utilizing a

We propose the key characteristics of an ideal technology for harvesting biomechanical

The first two criteria eliminate the need for additional mechanisms to condition the mechanical energy when it is transferred to the generator. The first three criteria therefore provide low mass/bulk. By having good impedance matching to muscle, the generator will be comfortable to wear, reducing its effects on the person wearing it. For the mass market, it is essential that the generator is low cost because consumers are unlikely to pay a premium

The research efforts of the authors of this chapter have focused on an energy harvesting technology called dielectric elastomer generators (DEG) which have been identified as a

components required for an electromagnetic biomechanical energy harvester?

1. Efficient operation at low biomechanical "walking" speeds.

2. Ability to couple directly to large walking motions.

4. Good mechanical impedance matching to muscle.

for generators that produce power of approximately 1 Watt.

DEG are a class of variable capacitance generators that are fabricated from a rubbery dielectric material sandwiched between stretchable electrodes. They have excellent impedance matching to natural muscle [13], can be fabricated from a wide range of low cost materials (commonly acrylic or silicone membranes sandwiched between carbon-based electrodes), have demonstrated extremely high energy densities, can undergo strains in excess of 100% [14], and have the ability to work over a wide frequency range without sacrificing efficiency [15].

Mechanical energy can be converted to electrical energy by cyclically deforming a DEG and placing charges on its flexible electrodes in the deformed state. DEG are typically produced from incompressible polymers so that an area stretch results in a decreased thickness. Relaxation of the charged, deformed DEG forces the opposite charges apart and packs like charges closer together and this transfers the mechanical energy to the electrical charges. The energy flows during an ideal DEG cycle are highlighted by the red arrows in Figure 1. The system receives 1 unit each of mechanical and electrical energy and converts the unit of mechanical energy into electrical, so that it outputs two units of electrical energy.

**Figure 1.** Schematic of the DEG states during a generation cycle. The grey area represents the dielectric and the black area represents the electrodes. From top moving clockwise, mechanical energy is input to the system deforming the DEG; an electrical energy input then charges the stretched DEG; the mechanical energy is then transferred to the charges by separating opposite and compressing like charges together. The electrical energy is then extracted and the cycle repeats (From [16]).

The major advantage that DEG hold for biomechanical energy harvesting is their ability to directly harvest low frequency motions without any gear mechanisms. To illustrate why this is true we will briefly describe their fundamental energy harvesting mechanism. As illustrated in Figure 1, DEG convert mechanical energy to electrical when the deformation of a stretched charged DEG is relaxed. During this relaxation period the thickness of the dielectric increases and the electrode area decreases, both resulting in a reduction of the capacitance. If charge is trapped on the generator during this relaxation phase, the voltage on the DEG will increase and there will be an increase in energy given by equation 1 where *Cd* and *Vd* are the capacitance and voltage of the DEG in its deformed state, and *Cr* and *Vr* are the relaxed DEG's capacitance and voltage, respectively.

$$E = 0.5\text{C}\_rV\_r^2 - 0.5\text{C}\_dV\_d^2\tag{1}$$

Since we are considering the case where the charge on the DEG is fixed during the relaxation period, and that the charge on a capacitor can be calculated from its voltage and capacitance (*Q=CV*), we can relate the voltage of the deformed DEG to the voltage of the generator in its relaxed state using equation 2. We substitute equation 2 into equation 1 to get equation 3.

$$V\_R = \frac{C\_d}{C\_r} V\_d \tag{2}$$

$$E = 0.5 \left( \frac{C\_d^2}{C\_r} - C\_d \right) V\_d^2 \tag{3}$$

These equations emphasise that along with driving voltage, *Cr* and *Cd* are the key parameters that influence the energy output of a DEG. The capacitance is dependent on the geometry of the DEG and the driving voltage is controlled by the generator's associated electronics. Neither of these parameters are dependent on the velocity at which the generator is deformed, so the fundamental mechanism of DEG is not dependent on driving velocity.

Although we have highlighted that DEG have a highly suitable mechanism and characteristics for harvesting biomechanical energy, Figure 1 highlights that DEG need an electrical circuit that will control the flow of charge onto and off of the generator at appropriate stages of the energy harvesting cycle. Such circuitry can add weight and is typically composed of stiff and bulky parts. Furthermore the requirement for the electronics to replace charge delivered to the load has traditionally reduced the portability of DEG because they have been either tethered to the grid or used batteries that need periodic replacement [10]. This chapter will focus now on recent developments that eliminate the need for a secondary power source and reduce external DEG circuitry mass and stiffness.

#### **3. Portable dielectric elastomer generator electronics**

A passive DEG circuit for controlling the charge state of the DEG appropriately as it is mechanically cycled called the Self-Priming Circuit (SPC) [17, 18] has been developed to eliminate the need for a secondary power source and thus improve DEG portability. The circuit works as a charge pump that provides energy in a higher charge form than the energy supplied to it. The self-priming circuit is configured so that it harvests energy from a DEG and then supplies that energy in a higher charge form to a load or to the DEG when it requires priming. This effectively boosts the charge of the generated energy before it is used, thus a secondary power source is not necessary because the generated energy is used to replace circuit charge losses and charge delivered to the load. A schematic of the simplest form of an SPC is given in Figure 2, showing that an SPC is simply a capacitor bank that has diodes to convert the topology of the circuit between a low capacitance when it is charged (Figure 2c, capacitors in series) and a high capacitance when it discharges (Figure 2d, capacitors in parallel). This toggling of state provides the SPC with a higher output capacitance which converts the energy to a higher charge form. This can be explained using equation 4, which provides two expressions for the energy (E) stored on a capacitor. If the energy is conserved when the capacitance (*C*) of the SPC increases, then the charge (*Q*) must increase too, and since the SPC is an adiabatic process the increased charge is accompanied by a decreased voltage (*V*). Thus the SPC outputs energy in a higher charge, lower voltage form than the input.

174 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

the relaxed DEG's capacitance and voltage, respectively.

The major advantage that DEG hold for biomechanical energy harvesting is their ability to directly harvest low frequency motions without any gear mechanisms. To illustrate why this is true we will briefly describe their fundamental energy harvesting mechanism. As illustrated in Figure 1, DEG convert mechanical energy to electrical when the deformation of a stretched charged DEG is relaxed. During this relaxation period the thickness of the dielectric increases and the electrode area decreases, both resulting in a reduction of the capacitance. If charge is trapped on the generator during this relaxation phase, the voltage on the DEG will increase and there will be an increase in energy given by equation 1 where *Cd* and *Vd* are the capacitance and voltage of the DEG in its deformed state, and *Cr* and *Vr* are

Since we are considering the case where the charge on the DEG is fixed during the relaxation period, and that the charge on a capacitor can be calculated from its voltage and capacitance (*Q=CV*), we can relate the voltage of the deformed DEG to the voltage of the generator in its relaxed state using equation 2. We substitute equation 2 into equation 1 to get equation 3.

> *d R d r <sup>C</sup> V V*

2 <sup>2</sup> 0.5 *<sup>d</sup>*

*r <sup>C</sup> E CV C* = − 

These equations emphasise that along with driving voltage, *Cr* and *Cd* are the key parameters that influence the energy output of a DEG. The capacitance is dependent on the geometry of the DEG and the driving voltage is controlled by the generator's associated electronics. Neither of these parameters are dependent on the velocity at which the generator is deformed, so the fundamental mechanism of DEG is not dependent on driving velocity.

Although we have highlighted that DEG have a highly suitable mechanism and characteristics for harvesting biomechanical energy, Figure 1 highlights that DEG need an electrical circuit that will control the flow of charge onto and off of the generator at appropriate stages of the energy harvesting cycle. Such circuitry can add weight and is typically composed of stiff and bulky parts. Furthermore the requirement for the electronics to replace charge delivered to the load has traditionally reduced the portability of DEG because they have been either tethered to the grid or used batteries that need periodic replacement [10]. This chapter will focus now on recent developments that eliminate the need for a secondary power source and reduce external DEG circuitry mass and stiffness.

A passive DEG circuit for controlling the charge state of the DEG appropriately as it is mechanically cycled called the Self-Priming Circuit (SPC) [17, 18] has been developed to

**3. Portable dielectric elastomer generator electronics** 

*d d*

2 2 0.5 0.5 *rr dd E CV CV* = − (1)

*<sup>C</sup>* <sup>=</sup> (2)

(3)

$$E = \frac{Q^2}{2C} = \frac{CV^2}{2} \tag{4}$$

**Figure 2.** Schematic of a self priming circuit (a) which connects to a DEG in parallel (b), the diodes control current flow so that the SPC capacitor bank takes on the form given in (c) when energy is transferred to it from the DEG, when energy is transferred off the SPC the diodes configure it to take on the higher charge form given in (d).

The SPC has additional benefits for portable DEG. First, the SPC is passive and requires no active switching or control, so it does not require any power to drive it. Second, the SPC accumulates charge from cycle to cycle, this means that the system voltage climbs (see Figure 3). The ability to boost its own voltage is highly desirable because as demonstrated in equation 3, generated energy climbs with priming voltage. This high voltage has

traditionally been supplied by a high voltage power supply or converter, so the SPC eliminates the need for these typically high cost components.

**Figure 3.** The output voltage waveform of a self-priming generator mechanically deformed at 3 Hz. The voltage climbs from cycle to cycle because the generated energy accumulates in the form of extra charge stored on the generator and priming circuit (From [19]).

Although the SPC is low cost, low complexity, and autonomous it still adds considerable mass and stiffness to the DEG system. The SPC consists of diodes and capacitors. The function of the capacitors is to store priming charges and the diodes control the transfer of charge to and from the DEG, so that an appropriate generation cycle is achieved. We will now discuss how these functions have been integrated onto the DEG membrane to produce a generator that can be fabricated entirely from soft elastomers.
