Design of excitation source for AC electromagnetic field detector based on FPGA

AlternaTIngCurrentFieldMeasurement (ACFM) is an active electromagnetic non-destructive testing technology based on the principle of electromagnetic induction. It has unique advantages in detecting the surface and near surface cracks of conductive workpieces. The AC electromagnetic field detection needs to induce an alternating current on the surface of the workpiece to be tested. When there is no defect, the induced current is evenly distributed. When there is a defect, the defect disturbs the induced current, and the length of the defect is determined by extracting the induced magnetic field to change the length of the defect. And depth and other information.

Field Programmable Gate Array (FPGA) is a new type of highly integrated digital device in recent years. It has been widely concerned and widely welcomed by electronic engineering designers all over the world, and can meet the design of most special detection systems. In this paper, FPGA is used as the development platform to design the excitation source module with adjustable frequency. The excitation source is realized by direct digital frequency. The synthesized frequency is suitable for different materials to be tested, which can meet the requirements of the AC electromagnetic field detection system and reduce the work of the system. Consumption and cost. Moreover, the FPGA has abundant pins and large storage area, which is especially suitable for the real-time detection system on site, which is convenient for field and high-altitude operation of the detection instrument.

DDS or DDFS (DirectDigital Frequency Systhesis) is the direct digital frequency synthesis. The principle is to normalize the complete cycle of the signal, sample enough points to store, and output the amplitude corresponding to the storage table address through the clock frequency to output to the high-speed DA. The converter outputs an analog signal by a low pass filter. The typical DDS principle is shown in Figure 1. The waveform storage table can be set to a sine wave) square wave) triangle wave and other waveforms.

It is assumed that one cycle of the sine wave signal is stored in 2N samples, the fclk clock frequency fn is the synthesized frequency, and T0 is the composite signal period. And assuming that the number of bits of the accumulator is the same as the number of address bits, the control synthesis frequency has two methods of full sampling and frequency control word sampling.

The clock frequency finds each address of the waveform storage table, and the amplitude corresponding to the output address. Then, a complete cycle of the synthetic excitation source requires 2n lookup tables, and the relationship between the synthesis frequency and the clock frequency is as shown in equation (1):

Where: fn is the composite frequency, Tn is the composite signal period, fclk is the clock frequency, and 2n is the number of samples. It can be seen from equation (1) that when all sampling points are searched for output amplitude, signals of different frequencies can be synthesized by changing the clock frequency fclk .

When the sampling clock frequency is constant, the length of the known waveform storage table is unchanged. By controlling the size of the accumulated frequency control word each time, the number of times of the lookup table is changed, and signals of different frequencies are synthesized.

Incremental linear encoders

Incremental Encoder is commonly used, and Absolute Encoder is used if there are strict requirements on position and zero position. Servo system should be analyzed in detail, depending on the application situation. Commonly used incremental encoder for speed measurement, which can be used for infinite accumulation measurement; Absolute encoder is used for position measurement, and the position is unique (single or multiple turns). Finally, it depends on the application situation and the purpose and requirements to be realized.

Incremental Linear Encoders,Linear Optical Encoder,Linear Position Encoder,Encoder Bearing Tester

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