# Acoustic Backscatter Communication and Power Transfer for Batterless Wireless Sensors
## 3 Materials and Methods
### Energy Harvesting  
The energy harvesting circuit collects energy from the tag's transducer and provides a regulated supply voltage sufficient to run the MCU. Figure 3 provides a schematic overview of the circuit. In contrast to most existing studies, we use the same transducer for harvesting and backscattering, which saves space and costs. As we test the limitations of WPT in metal channels, we want to design a harvester that functions in a broad range of scenarios. That poses three challenges, namely  
1. The voltage on the receiving transducer is likely smaller than the required supply voltage for the MCU and potential sensors,
2. The maximum power that can be extracted from the piezo may not be sufficient to power the MCU continuously,
3. The transducer's output impedance may vary strongly. Therefore, the harvester needs to work efficiently with a broad range of impedances. The first challenge can be overcome using a voltage booster circuit, the second requires harvesting energy in a storage capacitor and duty-cycle of the MCU, and the third requires a load adaption mechanism, e.g., MPPT. A flexible solution to all three problems is to use a commercially available PMU IC. We decided to use the ADP5091 from Analog Devices because it has an inbuilt boost regulator that can boost input voltages as low as 80 mV. However, in cold start, a minimum voltage of 300 mV is required. To fulfill this cold start threshold voltage, we first amplify and rectify the AC voltage from the transducer with a voltage quadrupler circuit. The PMU also supplies duty cycling through an output pin that signals if the voltage on the charge element has reached a configurable threshold. Furthermore, it can efficiently manage stored energy and prevent damage to the storage element by only charging it up to a configurable voltage and not discharging it below a certain voltage. Additionally, the PMU performs MPPT with a configurable voltage threshold. MPPT is a technique to adapt the resistive load of the harvester dynamically. The PMU realizes MPPT by regularly stopping harvesting, sampling the open circuit voltage at the input  
Figure 3: The harvesting circuit connected to the piezoelectric transducer. The voltage quadrupler is adapted from [7]. Figure 2: The general architecture of the hardware prototype. The backscatter modulator and the energy harvester work on the same piezoelectric transducer. capacitor, and then only extracting energy from the input capacitor \(C_{\text{IN}}\) to the storage capacitor \(C_{\text{ST}}\) when the voltage exceeds the configurable MPPT threshold. We investigate the optimal MPPT threshold in Section 4.1.1. In general, the piezoelectric source has a complex-valued internal impedance, and for maximum power transfer, and the harvester's load impedance has to be matched with a complex matching circuit between the transducer and harvester. However, the complex impedance strongly depends on the structure, carrier frequency, and exact position of the transducer on the structure. Therefore, designing exact impedance matching circuits is a task that has to be manually repeated for every sensor node [17], which is highly impractical. Instead, we approximate the impedance only by adapting the resistive load. Once the storage capacitor exceeds a terminal voltage \(V_{\text{th+}}\), the PMU provides a turn-on signal, which is called PGOOD. A hysteresis is programmed so that PGOOD stays high until the system voltage drops below a second threshold \(V_{\text{th-}}\). We use this signal to enable a low-power LDO regulator that provides the regulated supply voltage for the MCU. Setting the size of \(C_{\text{ST}}\), and the threshold voltages, the harvester can be configured to supply a wide range of applications with different power requirements independent of the input power provided by the transducer through duty cycling.