基于谐波抑制的多频信号的仿真与实现
Simulation and Realization of Multi-Frequency Signals Based on Harmonic Suppression

金属检测设备主要利用平衡线圈的原理，实现微弱金属信号的检测和金属杂质的剔除，广泛应用于矿业、安防、食品等行业。目前国内食品金属检测机厂商普遍使用单频信号作为发射信号，频率多为50~300 kHz，由于不同种类的金属异物检测灵敏度随频率变化不一致，且含水含盐量高的产品效应类似于铁磁性物质，因此难以实现单个频率下较好的带产品检测精度。为了弥补单频信号的不足，本文旨在实现多频发射信号的产生。首先分析选择性谐波抑制脉宽调制技术的原理，建立固定频率分量时的数学模型，通过MATLAB求解优化问题，求出输出波形导通角序列。建立Simulink电路仿真模型，通过生成的导通角，对该方法进行仿真验证，仿真表明，可以通过该方法在线圈负载上产生预期的多频信号。最后通过FPGA控制全桥D类功率放大电路驱动发射线圈，对硬件系统测试的结果显示，能够在金属探头的发射线圈上实现需要的PWM波形，实现同步多频的金属发射信号的产生。通过这样的方法产生多频信号，弥补单频金属检测机的不足。
Metal detection equipment mainly uses the principle of balanced coil to realize the detection of weak metal signals as well as the rejection of metal impurities, which is widely used in mining, se-curity, foodstuffs and other industries. At present, domestic metal detector manufacturers of food-stuffs industry generally use a single-frequency signal as the transmitting signal, the frequency is mostly 50~300 kHz, due to the different types of metal foreign object, there will be an inconsistent trend of sensitivity with frequency change, and high water and salt content of the product generates a signal which is similar to the ferromagnetic material, so it is difficult to achieve effective distinc-tion between food and metal with single frequency. In order to make up for the deficiency of sin-gle-frequency signal, this paper aims to realize the generation of multi-frequency transmitter sig-nals. Firstly, we analyze the principle of selective harmonic suppression by pulse width modulation technology, establish the mathematical model at fixed frequency components, and solve the opti-mization problem through MATLAB to find the output waveform conduction angles sequence. A Simulink circuit simulation model is established and the method is validated by using of those conduction angles, which shows that the desired multi-frequency signal can be generated on the coil load by this method. Finally, the transmitter coil is driven by a full-bridge Class D power amplifier circuit controlled by an FPGA development board. Testing the hardware system, the results show that by this means, we can achieve the desired PWM waveforms on the transmitter coil of the metal probe to realize the generation of synchronized multi-frequency metal transmitter signals. As a re-sult, the shortcomings of single-frequency metal detectors are compensated.