RF energy harvesters for wireless sensors, state of the art, future prospects and challenges: a review

The power consumption of portable gadgets, implantable medical devices (IMDs) and wireless sensor nodes (WSNs) has reduced significantly with the ongoing progression in low-power electronics and the swift advancement in nano and microfabrication. Energy harvesting techniques that extract and convert ambient energy into electrical power have been favored to operate such low-power devices as an alternative to batteries. Due to the expanded availability of radio frequency (RF) energy residue in the surroundings, radio frequency energy harvesters (RFEHs) for low-power devices have garnered notable attention in recent times. This work establishes a review study of RFEHs developed for the utilization of low-power devices. From the modest single band to the complex multiband circuitry, the work reviews state of the art of required circuitry for RFEH that contains a receiving antenna, impedance matching circuit, and an AC-DC rectifier. Furthermore, the advantages and disadvantages associated with various circuit architectures are comprehensively discussed. Moreover, the reported receiving antenna, impedance matching circuit, and an AC-DC rectifier are also compared to draw conclusions towards their implementations in RFEHs for sensors and biomedical devices applications

Optimization Algorithm for Antenna Impedance Matching in Digitally Tunable Network

In this work, we explore different methods to tune the antenna impedance in mobile devices. Mismatch from antenna impedance can cause undesirable effects such as spurious emissions, channel leakages, increased noise floor, degraded receiver sensitivity and so on. With the advancement in technology, digitally tunable reactive components are now available. Thus, a feedback system with tunable circuitry and the aperture tuning method where the component is directly embedded in the antenna design are some of the popular choices of solution. The ‘look-up table’ method is currently widely adopted in wireless industry. The hardware component chain (RF chain) contains the set-up to measure Γ_in(ratio of reflected signal to transmitted signal in dB) and a circuit to be tuned according to the values found in the look-up table. The look-up table is a pre-defined calibration chart provided by the manufacturer. It is saved in the memory of the device for permanent use during its lifespan. In this thesis, in the effort to eliminate the process of creating this look-up table and also to free up large space of memory, we approach an analytical solution to predict the exact values of the component in the tunable circuit – hence, making the procedure a one-time measurement, so called the open-loop configuration. In Chapter 3, a thorough mathematical analysis has been developed to integrate the Q factors of each component into a sample pi-circuit. In such setup, the system is expected to calculate ZL (or the antenna, load) with measured Γ_in and then compute the three capacitance values that yield the best transducer gain by conjugate matching method. However, due to many non-ideal characteristics of the components, calibrating the setup and incorporating the calibration data into analytical solution becomes very challenging. Therefore, the closed-loop configuration is more useful. It collects the empirical data of Γ_in, apply the optimization algorithm and then tune the circuitry in feedback manner, until the lowest desired Γ_in is reached. (Note that there is no difference between the closed and open loop configuration in the physical set-up. ) The purpose of this thesis is to develop the optimization algorithm used in closed-loop configuration. It involves three degrees of freedom using three Digitally Tunable Capacitors (DTCs). Accordingly, the challenge of this research points to inventing a 3D-unconstrained optimization technique that is simple enough to be implemented in a microprocessor without employing complex equation-solving libraries. In Chapter 4, the Hill-Climbing algorithm is investigated to see if it provides a suitable approach for finding the global minimum Γ_in in the 3D space gradient defined by 3 variables or DTCs. The Hill Climbing method, however, will limit its solution to finding only the local minimum within the gradient. This means that the location of the solution will change with the resolution of the gradient and the search step-size. Therefore,it is expected that Hill Climbing algorithm yields different solutions depending on the increment size and the starting location of the search. Chapter 5 develops a new algorithm that is based on Grid Searching. The main idea is to grasp the picture of the entire gradient of 3D space and zoom-in closer to the global point by iteration. The challenge lies in defining the boundary of zoom-in region without leaking the global point and leaving it behind. Also, the scanning of the reduced region in each iteration must not be too rigorous – meaning requiring too many data points. All different pi-network will have its limited coverage region on Smith Chart, of which the load impedance can be matched with. Therefore, selecting the reactive component with the suitable range of capacitance is also an important step, in order to fully utilize the work of this thesis. Apart from that, the algorithm does not require any information about the antenna, frequency of operation nor the configuration of the DTCs. Overcoming these challenges will guarantee the device to have the best optimized state of impedance match, at a specific frequency. Given that the algorithm is a 3D optimization technique, the work of this research does not only apply to tuning a pi-tuner. The three DTCs can be also integrated in the aperture tuning system. Thesis Supervisor: Professor Safieddin Safavi-Naeini.

Excitation Method for Thermosonic Non-destructive Testing

Thermosonics is a non-destructive testing method in which cracks in an object are made visible through the local generation of heat caused by friction and/or stress concentration. The heat is generated through the dissipation of mechanical energy at the crack interfaces by vibration. The temperature rise around the area close to the crack is measured by a high-sensitivity infrared imaging camera whose field of view covers a large area. The method therefore covers a large area from a single excitation position so it can provide a rapid and convenient inspection technique for structures with complex geometry and small and closed cracks. An ultrasonic horn, originally designed for welding, has generally been used for thermosonic testing. However, it is diffcult to obtain reproducible and controllable excitation with the existing horn system because of non-linearity in the coupling; surface damage can also be produced by chattering caused by loss of contact between the tip of the horn and the structure. Therefore, the general aim of the study was to develop a reliable and convenient excitation method that should excite sufficient vibration for the detection of the defects of interest at all relevant positions in the structure and must also avoid surface damage. In this thesis, a numerical and experimental study for the development of the ex- citation method for reliable thermosonic testing is presented. Successful excitation methods for the detection of delaminations in composites and cracks in metal struc- tures are described. A simple, small wax-coupled PZT exciter is introduced as a con- venient, reliable thermosonic test system in applications where relatively low strain levels are required for damage detection such as composite plates. A reproducible vibration exciter may be su cient for thermosonic testing in some metal structures such as a thin plates. However, higher strain levels are often required in metal structures, though the required strain level is dependent on the crack size. This level of strain is not easily achieved within the reproducible vibration range because of non-linearity in the contact between the exciter and the structure. Therefore, studies are conducted with an acoustic horn with high power capability to investi- gate the characteristics of the vibration produced in a real structure with complex geometry and to develop a excitation method for achieving reliable excitation in the non-linear vibration range for thermosonic testing. An excitation method for a complicated metallic structure such as a turbine blade is also investigated and the in uence of the clamping method and the excitation signal that is input to the horn on the vibration characteristics generated in the testpiece is presented. As a result, a fast narrow band sweep test with a general purpose amplifier and stud coupling is proposed as an excitation method for thermosonic testing. This method can be ap- plied to different types of turbine blades and also to other components. One typical characteristic of a thermosonic test using non-linear vibration is the lack of repeata- bility in the amplitude and the frequency characteristic of the vibration. Therefore, vibration monitoring is necessary for reliable thermosonic testing and a Heating In- dex(HI) has been proposed as a criterion indicating whether su cient vibration is achieved in a tested structure or not. The HI is calculated from different vibration records measured by different sensors and these results are compared in this thesis. A microphone can provide a cheaper and more convenient non-contacting vibration monitoring device than a laser or strain gauge and the heating index calculated by a microphone signal shows similar characteristics to that calculated from the other sensors