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Thalester company has purchased Tektronix mts300mpeg detection system, and let its R & D department use

in the user's installation and detection. In the early stage of microcomputer application, DC sampling is widely used for parameters of power system, that is, sampling and measurement of DC after DC rectification. This "forward integration" is the keyword method here. The software design is simple and the calculation is convenient. The measured value can be obtained only by proportional transformation of the sampled value. However, there are some problems in the DC sampling method: the measurement accuracy is directly affected by the rectifier circuit; It is difficult to adjust the parameters of the rectifier circuit and is greatly affected by waveform factors. AC sampling is to sample the instantaneous value of the missing signal according to a certain law, and then use a certain numerical algorithm to obtain the measured value. AC sampling method mainly depends on two factors: measurement accuracy and measurement speed. AC sampling is equivalent to replacing a smooth sinusoidal curve with a ladder curve that only ensures the flexibility and accuracy of the pendulum and other motion mechanisms. There are two main theoretical errors: one is the error caused by using discrete data in time to approximately replace continuous data in time, which mainly depends on the conversion speed of a/d and the processing speed of CPU; The other is the quantization error caused by the quantization of continuous voltage and current, which mainly depends on the number of bits of a/d converter. With the rapid development of electronic technology, the processing speed of today's microcomputers and single-chip computers has been greatly improved. At the same time, there are a wide variety of a/d converters with good performance price ratio, such as AD574 and mac197, which have laid a solid foundation for AC sampling

I. measurement principle of AC sampling method

if the voltage effective value formula

is discretized, and a finite number of sampled voltage digital quantities in a cycle are used to replace the continuously changing voltage function values in a cycle, then

in the formula: △ TN - the time interval between two adjacent samples

un - instantaneous value of voltage sampling in the nth-1st time interval

n -- sampling points in a cycle

it is generally believed that the adjacent two times are equal, that is, △ TN is the constant △ T. considering n=t/△ t

this is the formula for calculating the effective value of the voltage signal according to the method of eliminating each sampling instant in a cycle, that is, the time value of starting the oil pump and the number of sampling points

similarly, the calculation formula of effective value of current

the discretization formula for calculating one-phase active power is:

similarly, the three-phase active power is:

apparent power:

ps=ui (7)

where in, un - is the current at the same time, and the voltage sampling value

power factor is cos θ= Pp/ps (8)

obviously, the higher the accuracy, resolution and speed of a/d conversion, the higher the sampling frequency and accuracy of the signal, and the higher the accuracy of the power parameters obtained from the discrete formula. AD574 controlled by AT89C51 can fully meet the practical requirements

II. System hardware design

the system hardware block diagram is shown in Figure 1. The hardware circuit is composed of data acquisition part, single chip microcomputer system and interface part

the three-phase voltage and current signals are added to the input of the sampling circuit, and the signal is converted into an AC voltage signal with a peak value of less than ± 5V in proportion. After impedance transformation, one of the 16 multi-channel analog switches is selected, and the lf398 sample and hold circuit is added to the input of AD574. In this design, the single chip microcomputer adopts AT89C51 of American ATMEL company, with 4K on-chip ROM, AD574 can input ± 5V or ± 10V AC voltage signal, 12 bit conversion, and the maximum conversion time is 25 μ s. Fully meet the requirements of measurement accuracy

when measuring power, the voltage and current signals must be sampled at the same time. Considering that the power frequency is generally not high (50Hz), according to the actual situation, the system uses a single chip microcomputer to alternately sample the voltage and current signals, and the current signal should be converted into a voltage signal less than ± 5V through i/u transformation, and then sent to the input terminal. This not only simplifies the hardware circuit, but also reduces the error and calibration difficulties caused by the asymmetry of the two data acquisition and processing circuits

III. system software design

according to the discretization formula, the voltage, current and active power equivalent can be calculated from the sampling values of voltage and current at different times in a cycle and the sampling points in each cycle. The standard frequency of power frequency alternating current is 50Hz and the cycle is 20ms. According to 12Mhz main frequency of 89C51 and 25 of AD574 μ S, and considering the practical accuracy requirements of power parameter analysis, the sampling period is set as 400 μ s. That is, 50 points are collected in a cycle. Practice has proved that the resolution can reach 1/4096 with 50 points per week and 12 bit a/d converter

the application software of the whole system is written in C51. Due to space constraints, only the program flow chart (Figure 2) is given

IV. conclusion

the tester uses AC sampling method to replace DC sampling, which reduces nonlinear links, reduces hardware cost, improves sampling accuracy, and can simultaneously measure a variety of power parameters. All results can be displayed by LED, or corresponding software can be installed and displayed on the computer. The tester has the characteristics of convenient and reliable use and wide testing range. It can be used in micromotor industry, energy-saving lamp industry, air conditioning industry, transformer industry, etc., providing guarantee for production line testing, and improving the automation and intelligence level of AC detection in this rich 72 hour seminar

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