Speaker
Description
One of the practical applications of pulsed magnetic fields (PMF) is in magnetotherapy systems. These systems often generate low-frequency PMF for non-invasive therapy of muscles or bones. Some PMF system manufacturers use commercial teslameters with a Hall probe to measure the generated amplitude of the magnetic flux density $B_m$. However, the traceability and accuracy measurements of $B_m$ of PMF using a Hall probe is an issue. There are papers describing methods using various types of search coils or pick-up coils with a digital oscilloscope or a DAQ device [1] for this kind of measurement, but they offer no stated traceability or detailed uncertainty analysis. Integration of the induced voltage pulse using an integrator is needed in order to calculate the measured value of Bm. A method using a special PCB search coil [2] with a digital oscilloscope has been developed at CMI. The method has been tested on pulse magnetic field signals (Fig. 1) with repeating frequencies of 40 Hz and 140 Hz (generated $B_m$ value of about 2 mT and about 12 mT). This method can be used without using an integrator, and the $B_m$ value is determined directly from the sampled pulse parameters. However, the expanded uncertainty value of the measurement is quite high (about 40 $\%$ in the worst case for a $B_m$ value of about 2 mT using a DSO-X 4024A oscilloscope). A comparison was performed between the use of an 8-bit (DSO-X 4024A) and a 16-bit (PicoScope 5444D) digital oscilloscope. Further, the method was improved using a 3458A digital multimeter as a digitizer, which significantly reduced the expanded uncertainty of the measurement (about 3 $\%$ for a $B_m$ value of about 2 mT). The measurement results obtained using the 3458A multimeter were compared to the results obtained using digital oscilloscopes and also with measurements obtained by various teslameters with Hall probes. A description will be presented of the method, the results and a detailed uncertainty analysis using digital oscilloscopes and a 3458A multimeter.
Fig. 1 Example of the PMF waveform - induced voltage from the PCB search coil at 40 Hz.
References
[1] B. Zubrzak, P. Bieńkowski, “50 Hz pulse magnetic field measurements - metrological possibilities of magnetic field meters, ” Przegląd Elektrotechniczny, R. 94, No. 1, pp. 121-124, 2018. doi: 10.15199/48.2018.01.31
[2] M. Ulvr, “Design of PCB search coils for AC magnetic flux density measurement,” AIP Advances, vol. 8, no. 4. AIP Publishing, Oct. 17, 2017. doi: 10.1063/1.4991643.
