Design of driving power supply for giant magnetostrictive transducer

The rare earth giant magnetostrictive transducer is a device that converts electromagnetic energy into mechanical vibration using giant magnetostrictive materials. Compared with the widely used piezoelectric ceramic transducer, it has the advantages of wide working range, high conversion efficiency, and fast response speed. It is mainly used in the fields of underwater acoustics, ultrasound, and active vibration control. Among them, the driving power supply of the giant magnetostrictive transducer is a key factor affecting the performance of the system. In view of the digital and intelligent development of power supply control technology, this paper designs a digital inverter power supply based on DSP devices to drive the giant magnetostrictive transducer to work normally and automatically track the resonant frequency. The giant magnetostrictive transducer used in this project is mainly used in small ultrasonic cleaning machines. The main technical indicators of the driving power supply are: input AC power of 220 V, output frequency of 15-25 kHz, and output power of about 50 W. This paper first discusses the overall design of the driving power supply system, and then specifically explains it from the two aspects of hardware circuit design and software implementation. Finally, experimental tests are carried out and conclusions are given.

1

The best driving waveform of the rare earth giant magnetostrictive transducer adopted in the overall design of the system is a high-frequency sine wave, so the structure of the designed driving power system is shown in Figure 1. The




DC power supply module is composed of transformer, rectifier, filter and voltage regulator circuits, which provide DC working voltage for the high-frequency inverter circuit; the high-frequency inverter circuit adopts a half-bridge inverter circuit to amplify the SPWM wave generated by the DSP to generate an AC square wave of specified power; and the DSP signal circuit generates an SPWM wave of the corresponding frequency, which is electrically isolated from the power circuit by the optical coupling circuit, and then the power switch tube of the high-frequency inverter circuit works normally through the driving circuit; the matching filter circuit is used to filter the SPWM waveform and convert the SPWM waveform into a sine wave, and at the same time complete the impedance matching and tuning functions; the feedback circuit samples the working current of the transducer, and the over-current protection can be easily realized through the software. At the same time, the frequency is tracked according to the current value, and the software adjusts the sine wave frequency to make the transducer work in the best state.

2 Hardware circuit design and implementation

The hardware system is mainly composed of the following parts.

2.1 Inverter main circuit

The inverter main circuit includes a DC power supply module, a high-frequency inverter circuit and a matching filter circuit.

The DC power supply of the high-frequency inverter circuit adopts a voltage stabilizing circuit composed of a large-current switching power supply chip L296, with a maximum output current of 4 A and a power of 160 W. The inverter main circuit adopts a half-bridge structure as shown in Figure 2. The power field effect tube uses IRF820A, which has a rated working voltage of 500 V, a rated current of 2.5 A, and a rise (fall) time of 10 to 20 ns, which can be switched quickly. At the same time, the upper bridge arm power tube VT1 must use a suspension drive circuit to drive the gate. Here, optoelectronic isolation and independent power supply are used to realize suspension drive. The

 



 

main functions of the matching filter circuit are filtering, tuning and impedance matching. In the figure, L1 and C3 form an LC low-pass filter to filter out the high-order harmonic components in the inverter output SPWM wave, and the high-frequency transformer has the functions of electrical isolation and voltage ratio adjustment. When the giant magnetostrictive transducer is working, it is mainly driven by the alternating current wound on the coil around the giant magnetostrictive rod. Under the influence of the alternating electromagnetic field, the giant magnetostrictive rod moves along the axis. The schematic diagram is shown below. The equivalent circuit analysis of the transducer shows that the giant magnetostrictive rod is equivalent to connecting an inductor in parallel in the circuit. Therefore, the transducer as a whole is inductive impedance. Therefore, an adjustable matching capacitor C4 is connected in series in the circuit. At this time, the matching capacitor C4 and the AC coil inductance L should satisfy the series resonance relationship:, where f is the resonant frequency of the transducer.

 



 

2.2 Control circuit

The control circuit includes a DSP signal generation circuit, an isolation circuit, a drive circuit and a feedback circuit.

The DSP signal circuit generates an SPWM signal for controlling the inverter, and completes the frequency tracking and overcurrent protection functions. The DSP chip TMS320F2812 is its core component. TMS320F2812 is a 32-bit high-performance microprocessor with rich on-chip peripheral resources. The key is that the event manager module in its many peripherals can easily generate the required SPWM waveform. Each event manager has a PWM waveform generator and a programmable dead zone generator, which can generate up to eight PWM output waveforms at the same time, and provide shielded external power supply and drive protection interrupts. This improves the system's integration and reliability, and is conducive to the monitoring of system performance and status. At the same time, the 16-channel 12-bit ADC can easily sample the feedback current signal to complete the frequency tracking and overcurrent protection functions. The

isolation circuit electrically isolates the signal circuit from the power circuit, using a single-channel high-speed optocoupler 6N137. The 0.1 μF decoupling capacitor next to the power pin of the 6N137 should try to choose a capacitor with good high-frequency characteristics. Here, a tantalum capacitor is selected and is as close to the pin of the 6N137 as possible. The drive circuit uses a high-speed single-channel power field effect transistor driver chip EL7104, whose rise (fall) time during the switching process is 10 ns, and the rise (fall) delay time is 18 ns, which can fully work at a switching frequency of tens to hundreds of kilohertz. The isolation drive circuit design is shown in Figure 4 below.

 



 

The feedback circuit mainly samples the working current of the transducer and transmits it to the DSP pin. The circuit is shown in Figure 5. The Hall current sensor ACS706ELC-20 A is used to sample the current size. At this time, the current size is expressed as the size of the voltage signal. Since the digital control part can only recognize positive voltage signals, and the sampling signal is AC, it is necessary to convert the sampling signal into a signal that the DSP can fully recognize. Here, the integrated operational amplifier OP07 is used to build a boost circuit to achieve the movement of the potential and the amplification of the signal. At the same time, VD1 and VD2 are limiter circuits to ensure that the signal is between 0 and 3.3 V, while R5 and C1 are filter circuits, ADCINA is the DSP pin, and the signal is input to the DSP for digital-to-analog conversion. [page]

 



 

3 System software design

The system software design is mainly used to generate the control waveform SPWM signal, and the software realizes overcurrent protection and frequency tracking.

3.1 Overcurrent protection and frequency tracking

The overcurrent protection function of the system is realized through DSP software control. When the working current of the sampling transducer is greater than the specified rated current, the SPWM signal output of the DSP event manager is stopped to achieve the function of protecting the system. Frequency tracking can be converted into searching for the maximum value of the working current, and finally setting the working point of the transducer at the maximum current. The reason is that the transducer impedance is the smallest and the loop current is the largest in the resonant state. If the resonant frequency of the transducer is offset, the current will decrease due to system detuning. The current search program flow chart is shown in Figure 6.

 



 

3.2 Generation of SPWM waveform

SPWM wave is mainly used to control the switching state of each power field effect tube of the inverter bridge. By adjusting the SPWM wave, the frequency and amplitude of the output voltage of the inverter circuit can be changed. By analyzing the principle of SPWM wave, its waveform generation algorithm adopts the direct area equivalent method with high accuracy and moderate calculation amount, and its modulation method adopts the optimized hybrid pulse width modulation method. The hybrid pulse width modulation method is a deformation of the unipolar pulse width modulation method. It is to achieve a more ideal sinusoidal output waveform, while at the same time hoping to reduce switching losses, and the working mode is basically symmetrical. Unlike the general single-phase unipolar SPWM modulation method, it does not fix one bridge arm as a high-frequency arm and the other bridge arm as a low-frequency arm, but switches once every half modulation wave cycle, that is, the same bridge arm works at a low frequency in the first half of the cycle and at a high frequency in the second half of the cycle. This modulation method allows each bridge arm to work in a high-frequency state in turn, so that the power tube works in a balanced manner and the reliability is enhanced. For the half-bridge inverter circuit, its control waveform is shown in Figure 7.

 



 

In the program, the table lookup method is used to generate the required SPWM pulse width data table. According to different modulation intensities and the angular frequency of the sinusoidal modulation signal, the on and off moments of each switching device are calculated offline. The required data is read out by looking up the table during operation, thereby performing real-time control. The SPWM wave output program flow chart is shown in Figure 8.

 



 

4 Experimental test and analysis

In the experiment, the control signal SPWM waveform output by DSP is first tested, as shown in Figure 9(a). The two channels of the oscilloscope simultaneously display the control signals of the two switch tubes of the half-bridge inverter, which is consistent with the designed waveform. The working voltage waveform at both ends of the transducer is measured again, as shown in Figure 9(b). The frequency set here is 20 kHz. It can be clearly seen from the oscilloscope that it is a high-frequency sine wave of 20 kHz, so its output waveform has high stability and low distortion.

 



 

In the driving power efficiency test link, under the resistance characteristics (the overall impedance of the transducer after the matching network is pure resistance), the power efficiency is above 75%, and the utilization rate is high. At the same time, the frequency tracking network always makes the transducer work in the maximum current state.

5 Conclusion

In this paper, an inverter power supply system for driving a rare earth giant magnetostrictive transducer is designed based on the DSP chip TMS320F2812. The hybrid pulse width modulation method is combined to realize the SPWM waveform, and the inverter circuit, isolation drive circuit, filter matching circuit and feedback circuit are reasonably and effectively designed to ensure the driving efficiency of the drive power supply to the giant magnetostrictive transducer. At the same time, the current control frequency method is used to automatically track the resonant frequency. Experiments have shown that the output frequency of the drive circuit is stable, the waveform distortion is low, and the energy conversion efficiency is high, which has certain engineering application prospects.