DEG Control Requirements: Review and Analysis

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Chapter 3 Electronics for DEGs: Review and Analysis

DEGs require charging with electrical energy when stretched and recovery of electrical energy when relaxed to generate electrical power. The charging and discharging of the DEG would be achieved using electronic circuits. In typical closed-loop control, the electronics will make measurements (sensors), determine what to do base on this information (controller), and make changes to the system accordingly (converter). Controller electronics often come in the form of micro-processors and error amplifiers. These are standard implementations of a controller and will not be discussed in much detail here. Textbooks on power electronics and control provide a good guide for controller design [42], [51]. In this chapter, we will explore a range of established sensor and converter techniques which can be used in a DEG control system. We shall find that these conventional techniques can be too large, complex, and/or power hungry to be useful for small milliwatt DEGs.

DEG circuit requirements

For DEGs to generate electrical power, step-up converters are required to charge the DEG to a high voltage, and step-down converters are required to convert the high voltage energy into low voltage for common devices to use. Electronic sensors and a controller can control the converters to allocate the electrical energy appropriately. The circuit requirements for the charge, voltage, and electric field control methods are summarised in Table 3-1. Sensors are required to determine the DEG’s mechanical and electrical state. Controllers and converters are required to charge and discharge the DEG at the appropriate time by converting energy between low voltage (e.g. 5V) and high voltage (e.g. 2kV).

Review of high voltage energy sources

There has been work on variable capacitance generators other than DEGs. These MEMS variable capacitance generators consist of electrodes with micro-structures that increase the effective area contributing to its capacitance [41]. Small displacements of one electrode relative to the other causes a change in capacitance which can be used to generate electrical power. Unlike DEGs, they use a gas as a dielectric which has a much lower breakdown strength than the elastomer. Hence, they have commonly been used to generate power only up to a couple hundreds of volts, while DEGs generate power at a few kilovolts. The lower voltage requirements close to that of mains voltage make designing a suitable converter an easier problem due to the abundance of components designed to operate at a few hundred volts. However, even with the lower voltage requirements, there still lack an efficient circuit for controlling the charging, discharging, and voltage conversions of low-power MEMS generators that generate microwatts of power.
Piezoelectrics can produce high voltage energy in the kilovolts when a high pressure is applied to the material [52]. The high pressure can be achieved by striking the piezoelectric. Little work has been done to develop high-voltage piezoelectric pulse generators. Possible reasons for why high-voltage piezoelectric generators have not been investigated include the difficulty in exerting such a large pressure, the small amount of energy generated, and the difficulty in using the kilovolt energy. Low-voltage piezoelectric pulse generators have received more interest, including an investigation on using the generated power to operate RF transmitters [53]. With the generated power being AC in nature, a transformer can be used to step down the voltage before rectifying the power into DC form. A more common piezoelectric generator harness energy from vibrations. These piezoelectric vibration energy harvesters are commercially available [9]. They can harness energy from vibrations up to a few hundred hertz and produce voltages up to approximately 10V.
One application involving the conversion of high-voltage DC energy to a more useful form is the conversion of AC power into high-voltage direct current (HVDC) to reduce the transmission losses over long distances. The HVDC power is converted back to AC form before being distributed for consumption. These HVDC converters work with AC and DC voltages of approximately 500kV, and converts gigawatts of power [54]. The converters use high-voltage IGBT transistor and thyristor switches with high current capabilities. Although variants of these systems may be used with DEGs producing similarly large amounts of power, issues such as charge leakage and overhead power losses make them unsuitable for small, low-power DEGs.
Another source of high voltage energy is static electricity, also known as triboelectricity. Friction between objects can cause a transfer of charge between them, resulting in an accumulation of electrical charge [55]. This energy commonly exists in either very large or very small amounts. A large quantity of electrical energy accumulates in the atmosphere as a result of the triboelectric effect, cosmic rays, and other effects. When this energy grows too large, it discharges to the Earth’s surface in the form of lightning. There have been brief studies in both the gradual tapping of the stored electrical energy in the atmosphere and the capture of energy from lightning strokes [56]–[58]. The studies have done modelling and small-scale experiments. The atmosphere is estimated to store 100GJ of electrical energy at more than 100MV [56]. These energy harvesters will need to capture an enormous amount of energy, and therefore have different requirements compared to small, milliwatt DEGs.
The accumulation of smaller amounts of triboelectric energy is often undesirable. The charge accumulation arising from a person’s movement in clothing or a vehicle moving through air causes discomfort to people and damage to devices when discharged. Hence, engineers often aim to minimise the triboelectric effect in their devices and products. However, one can also harness this energy. Generation of 130V energy from a triboelectric generator has been reported but the average power generated was not measured [59]. Ways of making use of the relatively high voltage energy have not been investigated.
In summary, DEG energy harvesting is not the only problem requiring the conversion of energy from high DC voltages to readily usable forms. However, low-power DEG energy harvesting requires special electronics because of the combination of the small amount of energy that needs converting and the kilovolt potential of the energy (e.g. 20mJ for 10nF DEG at 2kV). The solutions for the problems discussed above cannot be applied directly to DEGs. Nevertheless, these examples give us ideas and help us gain a better understanding of what can be achieved with modern technology.

State of DEG electronics

Some of the first DEG electronics were designed with the sole purpose of achieving and demonstrating power generation with DEGs. The circuit shown in Figure 3-1 was often used in these studies [12]. This circuit charges the DEG to a high voltage, and measures the increase in voltage when the DEG generates power. Finding ways to use the generated energy was not considered in these early studies.
Step-up converters suitable for charging DEGs with high voltage energy do exist in the market. These converters can convert low-voltage power (~5V) into high voltages (~6kV) at powers up to a couple of watts [49], [60]. However, our focus is on low-power converters for small DEGs that produce power in the order of 1mW. Although high-power converters can have efficiencies exceeding 98% [48], low-power converters are much less efficient. For example, the H25B series of high voltage converters that can produce 0.3W of power at 3kV have conversion efficiencies of 25% [49]. They can be used to drive and control DE actuators as well. A number of researchers have also built their own step-up converters for their lab experiments [61], [62].
A low-power and efficient DEG discharge converter which steps-down large DC voltages (2kV) to lower, more useful voltages (5V) is more difficult to achieve. The main reasons are the higher switch requirements and the relatively large losses incurred by high-voltage components. These will be discussed further in Section 3.4.3. Achieving efficient DC-DC step-down is the main challenge when designing control electronics for DEGs.
One of the first attempts to efficiently convert the high-voltage power from a DEG into lower voltages was described in a publication by Due et al. in 2010 [35]. Figure 3-2 shows their circuit which uses the simplest types of converters. A boost converter charged the DEG with high-voltage energy using a low-voltage source, and a buck converter discharged the DEG and converted the energy to low voltage. The circuit converted the energy between 300V and 2kV. MOSFETs each rated to 1.5kV were stacked in series to increase the maximum blocking voltage to 3kV. The DEG capacitance swung between 116nF and 154nF and was operated up to 2kV. They reported 26.7mJ of energy generated per DEG oscillation in their experiments. However, the efficiency of the converters were not measured and the power consumed by the control electronics was not accounted for. Size, cost, and power loss of such a circuit would likely be too large for use with small milliwatt DEGs. In one study, a DEG smaller than 1cm3 generated 1.8mW [63]. The authors have produced no further publications on this topic since their study in 2010.
The only significant and on-going study that developed electronics for converting high-voltage power from DEGs into low voltages was conducted by Eitzen et al. [64]. They began publishing their work on developing converters and control systems for DEGs in 2011. They have investigated the use of bidirectional converters for charging and discharging the DEG with a low-voltage source. The topologies investigated were modular H-bridge converters and flyback converters as shown in Figure 3-3. By stacking multiple converters in series, Eitzen aimed to achieve a large voltage conversion without the use of high-voltage switches. High-voltage switches lose out on other desirable properties to achieve their high voltage rating. These properties are discussed further in Section 3.4.3.
Eitzen has modelled the circuits and some preliminary results on the circuit’s performance have been reported. The bidirectional flyback converter was able to convert power between 60V and 600V [65]. The study looks promising. The circuit has not yet been tested with a DEG though. The efficiency and the power consumed by the control electronics have not been measured. The component choices were not disclosed. The size, cost, and power losses of these circuits can be relatively large for small milliwatt DEGs and may only be suited for large DEG systems. Use of these circuits in small DEG energy harvesters may result in power consumption rather than power generations.
Another study by McKay et al. developed a self-priming circuit for DEGs as shown in Figure 3-4
The circuit can passively charge and discharge the DEG at the correct times without any sensors or controllers. However, the circuit only partially discharges the DEG and thus the amount of energy generated per oscillation is much lower than the control methods discussed earlier in Section 2-3. An analysis in Section 5.2.5 compares the amount of power generated by the self-priming circuit with the constant-variable control methods. A DEG using the self-priming circuit generated approximately 5 times less power than a DEG utilising constant charge control. However, the self-priming circuit do not have power consuming electronics (such as sensors) nor does it use loss-incurring converters to charge and discharge the DEG every oscillation. If these losses are large relative to the amount of power generated, it is possible that more power is obtained with a self-priming circuit than constant charge control. Hence, the self-priming circuit can potentially offer a viable solution for harnessing power with low-power DEGs below 1mW. However, there needs to be a low-loss method to convert the energy generated into a low voltage for use.
Very few publications have divulged ways in which one can get useful low-voltage power from DEGs. This may be because such information is commercially sensitive. Companies attempting to commercialise DEG technology may keep their methods as trade secrets. For example, the only known commercial DE devices which use high voltage electronics are those from ViviTouch [13]. In 2011, they began selling products which use DE actuators to create vibrations. Their line of products include vibrators for smart phones and headsets. ViviTouch have not disclosed any information on how their DE actuators are driven, nor has anyone else published detailed solutions to powering and controlling DE actuators or generators. Other companies may also be keeping their DE developments secret.

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Assessment of sensor and converter techniques

As discussed in Section 3.1, achieving power generation with DEGs will require appropriate sensors and converters. The remainder of this chapter will explore a range of sensor and converter techniques that can be used to achieve the control required for generating power with DEGs (summarised in Table 3-1) and convert that power into low voltages.

Sensors: Measuring DEG stretch

The DEG must be charged when stretched and discharged when relaxed. Charging and discharging need to occur based on the mechanical state of the DEG to maximise power generation. Sensors are required to monitor the stretch of the DEG. Voltage control and field control both require real-time knowledge of the state of stretch so the voltage or electric field can be controlled accordingly. Knowledge of the electrode separation is also required for controlling the electric field. Changes in the electrode separation can be inferred from the amount of stretch for an incompressible DEG. There are a number of sensor techniques that can be used to determine the DEG’s mechanical state. These include simpler sensors that are triggered when displacement exceeds a certain threshold, and real-time sensors that continuously measure displacement.

Proximity switches

Proximity switches can detect when the DEG stretch exceeds an upper threshold and reduces below a lower threshold. Simple examples include limit switches, magnetic reed switches, and configurations where two conducting surfaces come into contact, while more complex examples include the use of optical switches.
However, proximity switches can only be used when there is repeatable relative movement between two objects. If the DEG is fixed at one end while the other end moves freely, then there is nothing that can trigger the switch. An example of such a configuration is a DEG oscillating like a flag in flowing fluid. Reliably attaching proximity switches to or around the DEG may also be difficult for some configurations.

External sensors

Sensors can be placed near the DEG to continuously measure its state of stretch. Proper use of this information can increase the amount of electrical energy generated by operating the DEG closer to dielectric breakdown. Cameras have been used in laboratory experiments to capture images of the DEG and determine stretch based on the size of the electrode [30]. The use of cameras is not very elegant, requiring a lot of power and image processing. Another option is to use distance sensors such as laser displacement sensors or Hall Effect sensors. However, the former would be expensive if high resolution is required, while the latter is non-linear, generally used for close distance measurements, and susceptible to noise.
The use of strain sensors would be a cheap and simple solution, but such sensors must be capable of measuring large strains. Highly elastic sensors made from the same material as DEGs have shown promise in measuring large strains of over 100% [14], [15]. The capacitance of these sensors changes when stretched, and the capacitance measurements are used to infer stretch. However, the maximum sampling rate of these sensors can be very slow depending on the material used and the size of the sensor [67]. Elastomer strain sensors utilising a material’s piezoresistive effect have also been demonstrated [68], [69]. With these sensors, their resistance changes with stretch. However, long term reliability and repeatability of both capacitive and resistive elastomer strain sensors must be achieved before they can be used in many engineering problems. Recent research in the electrical properties of spider silk also show promise as ultra-thin, highly-stretchable, and resilient strain sensors [70].
Similar to the proximity switches, cameras and distance sensors must be placed at a reference point and would not be suitable for all DEG configurations. The elastomer strain sensors can be attached directly on to the DEG and would be suitable in many DEG configurations, including DEGs configured to flutter like a flag.

Table of Contents
Chapter 1 Introduction 
1.1. Established generator technologies
1.2. Dielectric elastomer generators (DEGs)
1.3. Research objective
1.4. Thesis outline and contribution
1.5. Chapter summary
Chapter 2 DEG Control Requirements: Review and Analysis
2.1. Generating power with DEGs
2.2. Basic DEG control schemes
2.3. Analysing the three control schemes
2.4. Advanced control methods
2.5. Chapter Summary
Chapter 3 Electronics for DEGs: Review and Analysis
3.1. DEG circuit requirements
3.2. Review of high voltage energy sources
3.3. State of DEG electronics
3.4. Assessment of sensor and converter techniques
3.5. Chapter summary
Chapter 4 ‘Passive’ Electronics for DEGs
4.1. Passive self-priming circuit
4.2. Self-switching converter
4.3. Issues with self-switching converter
4.4. Alternatives to RC buffer
4.5. Self-powered converter
4.6. Chapter summary
Chapter 5  Analysis of the Self-priming Circuit (SPC) 
5.1. Operating principle
5.2. Design considerations
5.3. Comparison with constant-variable control methods
5.4. Experimental validation
5.5. Chapter summary
Chapter 6 Analytical Design of the Self-powered Converter 
6.1. Main converter
6.2. Self-switching converter
6.3. Pulse generator
6.4. Output load and output voltage
6.5. Chapter summary
Chapter 7 Experimental Results of the Self-powered Converter  
7.1. Analysing circuit operation
7.2. Effect of varying circuit parameters
7.3. Chapter summary
Chapter 8 Temporary High Voltage Storage 
8.1. High-voltage energy transfer circuit
8.2. Experiment
8.3. Design considerations
8.4. Chapter summary
Chapter 9 Conclusions 
9.1. Thesis summary
9.2. Thesis contributions
9.3. Publications and achievements
9.4. Future prospects
Converters for Milliwatt Dielectric Elastomer Generators

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