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Closed loop control of the magnetic flux in a magnetic circuit

Stefan Neureiter

Today the field of quality assurance is getting more and more popular, particularly in the steel industry. In order to evaluate the quality of the steel sheet, the principle of a magnetic leakage flux testing is used. To find deficiencies in the steel sheet the magnetic leakage flux, which is induced by an electromagnet, is captured with help of a giant magneto-resistive (GMR) sensor and with this received signal a conclusion can be drawn about the quality of the steel sheet. To guarantee constant measurement conditions, the magnetic flux density must be controlled at a constant level independent of the air cap between the electromagnet and the steel sheet. Figure 1 shows the magnetic circuit with some field lines.

Figure 1: Magnetic circuit with some field lines

Figure 1: Magnetic circuit with some field lines

The aim, to get a constant sensor voltage respectively a constant magnetic flux density, was reached by developing a closed loop control circuit which controls the current of the coil of the electromagnet correspond to a change of the air gap between the electromagnet and the steel sheet. The schematic concept of the closed loop control circuit is shown in Figure 2.

The closed loop control circuit consists of a discrete PI-controller, which is realized by an ATmega8-USB-Module. This module has integrated A/D- and D/A-converters on board. The D/A-converter has a pulse width modulation output in combination with a low pass filter first order, to provide the set point signal. In the next step the sensors output signal lead to the U/I-amplifier which supplies the coil of the electromagnet. The magneto-resistive sensor in combination with an instrumentation amplifier captures the magnetic leakage flux density under the steel sheet and gives it out in a form of a proportional voltage. The output voltage of the sensor is fed back to the A/D-converter of the discrete PI-controller, which calculates the new actuating variable to counteract the disturbance influence.

Figure 2: Schematic concept of the closed loop control circuit. The transferfunction G(s) represents the dynamic behaviour of the magnetic circuit, whereas the discrete PI-controller is described by R(s). The U/I-amplifier and the magneto-resistive sensor are illustrated by the constant factors V und k. The disturbance variable dstör correspond to a change of the air gap between the electromagnet and the steel sheet.

Figure 2: Schematic concept of the closed loop control circuit. The transferfunction G(s) represents the dynamic behaviour of the magnetic circuit, whereas the discrete PI-controller is described by R(s). The U/I-amplifier and the magneto-resistive sensor are illustrated by the constant factors V und k. The disturbance variable dstör correspond to a change of the air gap between the electromagnet and the steel sheet.

Keywords: PI-controller, magnetic leakage flux density, closed loop control circuit

August 16, 2010