Design and implementation of the most accurate sin

2022-07-29
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Design and implementation of high precision sine automatic excitation signal source

1 Introduction

in many engineering measurements, a fixed frequency sinusoidal signal is required as the excitation source. For example, the output of analog sensors will be used to verify the functions of the system and detection unit in the Asia Pacific region for about 1/4 of the global R & D activities of the developed monitoring system in the future, or to calibrate the acquisition range. In these cases, directly using a signal generator with superior performance can certainly meet the work requirements, but it also brings new problems. On the one hand, the signal generator is an external instrument, which increases the cost of the system, on the other hand, it is not convenient for automatic measurement. Using d/a converter and high-order filter can also achieve the above functional requirements, but under the windows operating platform, it puts forward higher requirements for software technology. This paper happens to encounter such a problem in the research work of the scientific research project. In the signal detection and calibration work, a 120Hz high-precision sinusoidal excitation signal with adjustable peak value from 0.01V to 10V and distortion less than 1% is required. In this paper, a conventional circuit is used to realize this function

2. Brief introduction of principle and implementation process

the undergraduate research project is a certain type of aircraft engine parameter detection system based on PC-104, which is constantly improving the international influence of China's plastic machinery industry. The system requires an excitation signal setting unit for aircraft vibration calibration. After careful analysis of the requirements of technical indicators, the unit needs an AC signal source with adjustable amplitude from 0.01 V to 10 V, stable amplitude, small waveform distortion and 120Hz frequency, and the amplitude is given at an interval of 0.01 v. If we use the principle of brick weighing, we can quickly complete this function. Obviously, the signal excitation only needs two digits after the decimal point, that is, the peak value of the sinusoidal signal varies from 10mV to 10V, which has one integer digit and two decimal digits. If we centrally implement a 120Hz high-precision sine wave oscillator, and then take 5V, 4V, 2V, and 1V "brick code" signals from it, we can form any peak value between 1V and 10V through electronic switch combination and adder. Similarly, 0.5V, 0.4V, 0.2V, and 0.1V "brick code" signals can form 0.1V to 0.9V sinusoidal signals, and 0.05v, 0.04V The "weight" signals of 0.02v and 0.01V can form sinusoidal signals from 0.01V to 0.09v. When these three groups of "weight" signals are combined, they can give sinusoidal excitation signals with any peak value from 0.01V to 10V and amplitude change step of 0.01V, which can fully meet the needs of the project

according to the above analysis, we designed the hardware block diagram as shown in Figure 1. In Figure 1, MAX038 chip is selected as the sine wave signal source, and its output sine wave frequency can be adjusted in a wide range. The internal structure design of the chip can ensure that the sine signal with distortion less than 1% can be provided externally; In order to improve the proportional accuracy of the signal, all the partial voltage resistors are customized, and the resistance accuracy can reach one thousandth; The low drift operational amplifier lm124 is selected as the operational amplifier; The high-performance max4536 is selected as the electronic switch to analyze the iron content in the lubricating oil. The 4-way single pole single throw switch is used when the friction surface continuously supplies lubricating oil; In addition, considering the voltage drop of tens of ohms after the electronic switch is turned on, in order to reduce its influence, the feedback resistance and accumulation resistance in the adder are selected to be about tens of kiloohms, further weakening the influence of the electronic switch on resistance in the proportional adder. Because of the above measures, the performance of the circuit in practical use can be greatly improved

in the circuit shown in Figure 1, the electronic switch is controlled after decoding, and one bit control code controls one circuit of switch. Therefore, the control of the electronic switch requires a total of 12 digital output interfaces, which is not allowed in the embedded system used by the author because there are not so many resources. In order to further meet the requirements of the system, the single parallel conversion technology is adopted, and three 4-bit shift registers ct1194 are connected in series to form a 12 bit shift register, The block diagram is shown in Figure 2

in Figure 1, there are 4096 combinations of 12 electronic switches, and each combination corresponds to a sinusoidal AC signal of a specific size. Although 12 i/o ports are required for the control of these electronic switches, as long as the serial in and parallel out shift register in Figure 2 is used, we can send any of 4096 combinations to Q1 to Q12 through the two output ports of data port data1 and clock port CLK The drive system of plastic tensile testing machine is checked, and the control of 12 channel electronic switch is realized with two i/o ports. In the i/o card of PC-104 used by the author, the external expansion i/o port is realized by 8255. Since the C port of 8255 has a bit control function (position bit or bit reset), any two bits from the C port are taken as the data port and clock port of the shift register. Under the action of 12 pulse rising edges, any 12 bit binary number can be sent to ports Q1 to Q12, so as to complete the desired control of the electronic switch, Obtain the constant frequency sine wave with the desired amplitude at Vout in Figure L

3. Implementation process

in order to obtain the amplitude required by the excitation signal, this unit uses the position control function of the C port of the i/o module of pc.104 to control the electronic switch. Firstly, the amplitude required by the excitation signal is given on the control panel, and then the value is encoded by using the 5421 code sequence. The so-called 5421 code means that the weights of the corresponding bits of the code system are 5, 4, 2 and 1 respectively, that is, the decimal values represented by the corresponding bits are 5, 4, 2 and 1 respectively. The specific coding rules are shown in Table 1. After encoding the amplitude required for excitation, the binary code obtained is input into the shift register from low to high. The code is output to the control end of the electronic switch in parallel by the register to control the opening and closing of the switch, so as to control the output result of the adder and obtain the sine excitation signal of the required amplitude. The flow chart of the control process is shown in Figure 3. In order to introduce the implementation process of this process in more detail, an example is given below

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