Tham khảo tài liệu 'data acquisition part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Data Acquisition Systems for Magnetic Shield Characterization 241 In Fig. 10 a basic measurement scheme for characterization of soft magnetic materials is reported. Fig. 10. Measurement scheme for characterization of soft magnetic materials. Fig. 11. Major loop obtained for a commercial soft ferrite. Coercive field Hc and induction remanence Br are indicated by arrows. 242 Data Acquisition An arbitrary function generator is connected to the primary winding via a power amplifier. The primary current i1 is measured via the voltage drop across a calibrated resistance RH. The secondary open circuit voltage v2 is measured by means of a high-impedance differential amplifier and a data acquisition system. The data acquisition system must perform synchronous acquisitions between the two channels a couple of identical DC-coupled variable-gain low-noise amplifiers is generally interposed between the H t and dB dt signal sources and the acquisition device Fiorillo 2004 . The mutual inductance Ma in the scheme of Fig. 10 is used to automatically compensate the air flux linked with the secondary winding. The presented scheme can be used to impose a prescribed time dependence often sinusoidal of the magnetization . the secondary voltage v2 t for example by means of a digitally controlled recursive technique. In Fig. 11 the hysteresis loop obtained measuring data on a commercial ferrite toroid is reported. Although a complete model of magnetic hysteresis is very complex the coercive field Hc and the induction remanence Br are two key paramaters that together to the saturation Hat Bsat values define in a first approximation the material magnetic behavior. The remanence Br represents the induction value obtained after applying a large field to the specimen and then removing it while the coercive field is the field needed to bring the induction field from Br to zero. On the basis of Hc and Br values magnetic materials are commonly classified into soft and hardmagnetic materials.
