ACFM Test System Design And Experimental Study Based On TMR Sensor

ACFM (Alternating Current Field Measurement) is an emerging non-destructivetesting technology. It can produce alternating current on the surface of test specimenbased on the principle of electro-magnetic induction. The flow of the current willdeflect when flaws are examined, which will affect the distribution ofelectromagnetic field on the surface of test specimen. ACFM can not only apply forqualitative analysis of flaws, but also for accurate quantitative analysis. All of thesebenefit from accurate mathematical model of ACFM. Firstly, a detailed analysis hasbeen made on ACFM and its future directions by the domestic and foreign scholarsand experts. According to the basic principles of ACFM, Maxwell equations worksout a mathematical model of ACFM while ACFM instrument is designed because of it.This detecting instrument are made up of hardware and software components, and thehardware part consists of probe and signal processing circuit,while the software partconsists of User Interface on computer and software programming of ATmega128.Actually, probe is an important part for ACFM detecting instrument and itsperformance will directly affect the accuracy of detection of the whole device. Thedesign of probe include the choose of sensor，probe skeleton and turns per coil。Wecompared with various kinds of magneto-dependent sensor and its advantages anddisadvantages in the article, finally TMR sensor was chosen as the sensor of ACFMdetecting instrument’s probe. The skeleton of probe adopt U magnet yoke forproducing Magnetic field gathered effect, which made current flow more orderly.Then we can confirm the turns of probe through a large number of experiments. Incircuit design of front end, we firstly make sure power supply programs of the wholedevice. Considering the expansion of function in later time and the requirements forportability, we employ the power modules made by MORNSUN to produce±12Vpower supply which amplifier with high precision needed. And amplifier with highprecision employ ina2128of TI Company to amplify signals which sensor collected.In addition, circuit design of front end also cover the design of filter circuit and TrueRMS conversion circuit Probe coil’s excitation source of this ACFM detectinginstrument adopt DDS chip to produce accurate sine wave signals and the control forDDS chip is realized through ATmega128SCM. As DDS signal source fails to driveprobe coil, so we need to amplify signal source’s loading capacity with first level ofpower. Considering the extended function of device in later time, we adopt OPA548amplifier with large current of TI Company. AD sampling module employATmega128SCM’s own ADC module which accuracy is10and the accuracy andspeed of this module which completely meets the needs of detecting instrument. ADdata sending function is achieved through USART module of ATmega128SCM. Thismodule serves as a bridge between detecting instrument and computer. Finally, we didpractical test on assembled equipment and made estimation on measurement accuracyof flaws with artificial test block and pressure vessel welding line which is normallyused in field test. Also, the accuracy of detecting equipment is verified by ultrasonictesting and Penetrant Testing.