# Textile-Based Flexible Coils for Wireless Inductive Power Transmission
## 5 Experimental Procedure
### Measurement of Performance of Wireless Power Transfer System  
The printed coils with their resonant tank capacitors were connected to Qi standard compliant bqTESLA commercial driver bq500211 EVM and receiver bq51013 AEVM evaluation modules from Texas Instruments [24; 25] to evaluate the performance of the coils. The transmitter module operates between 110 kHz and 205 kHz, with the operating frequency automatically varying as a function of the load. The receiver module has an internal output voltage regulator that provides 5 V DC at up to 1 A, subject to sufficient incoming energy from the transmitter module. In the bqTESLA wireless power transfer system, a pair of rigid wound coils are provided. In this experiment, these original coils and their resonant tank capacitors were replaced by the tuned copper tape coils and printed coils. Measurements of wireless power transfer were performed on a plastic support to avoid influencing the magnetic field as shown in Figure 7. The center separation distance, misalignment, and rate of deformation of coils mounted on these supports can be adjusted. The DC output power and DC to DC efficiency were measured as follows:  
1. Mount the paired coils on the supports with the driver and receiver circuit and adjust their position to align the centers. 2. With the driver powered vary the load on the output of the receiver circuit and record the input DC current to the driver circuit and the output voltage from the receiver circuit and the corresponding load resistance. 3. Use the load resistance and output voltage values calculate the output current and power transferred. 4. Plot the output power of the receiver circuit versus its output current and locate the maximum output power point. 5. Record the input DC voltage and current to the transmitter circuit to calculate the input power for wireless power transfer and overall DC to DC efficiency. 6. Plot the overall DC to DC efficiency of the system versus the output current and locate the maximum efficiency point. 7. Adjust the separation distance between the TX and RX coils, and then repeat steps 2 to 6, plot the maximum DC to DC efficiency for each separation distance. 8. Adjust the curvature of the RX coil to simulate deformation of the wearable device while keeping the TX coil flat, and then repeat steps 2 to 7, plot the maximum DC to DC efficiency versus the deformation of flexible coil. To perform these measurements the receiver output was loaded using a resistance box to allow adjustment of the load current, load resistances between 1 k\(\Omega\) and 10 \(\Omega\) were used to give a nominal current load range of 5 mA to 500 mA at the nominal 5 V DC output level. The voltage across the load was measured using a 12-bit A/D convertor (NI USB-6008 in differential mode) to take into account any output voltage variations with load, and the output power under each different distance and rate of deformation was then calculated. The DC to DC efficiency can then be calculated as:  
\[\eta_{\mathrm{DC-DC}}=\frac{P_{Load}}{P_{\mathrm{DC}}} \tag{9}\]  
where \(P_{Load}\) is the power delivered to the load and \(P_{\mathrm{DC}}\) is the power supplied from the DC power supply. Varying the load resistance changes the output current of the receiver circuit and the input power. The output voltage of the receiver circuit is fixed at 5 V DC by its internal regulation process, providing there is sufficient energy available at its input to provide the required amount of output power. A series of system-level input and output power values can therefore be measured. The DC to DC efficiency can be calculated and graphs can then be plotted to show the output power and the DC to DC efficiency against the output current of wireless power transfer. The maximum output power and DC to DC efficiency points of the WPT can be found by locating the maximum values on the P/I and \(\eta_{\mathrm{DC-DC}}/I\) curves.