1. Research on Measurement Technology for Variable Frequency AC Power System (ID: 0328)
Shanshui Yang1, Li Liu1, Xi Jiang1, Li Wang1, Weifang Liu2
1Nanjing University of Aeronautics and Astronautics, Nanjing ,China
2Beijing Aeronautical Science & Technology Research Institute(BASTRI), COMAC, Beijing, China
supported by “the National Natural Science Foundation of China”( )
Large civil aircraft use variable frequency AC power system as the main power with simple structure generator, high energy conversion efficiency and high power density. The aircraft power system bring innovation at the same time, the complexity of the power system is also increasing. The test platform is used to complete the development of the aircraft power system and the measurement of the electrical parameters.
PXI test platform based on the integration of hardware and software is flexible. Star trigger structure reduces the acquisition time between the test equipment, the number of channels, the sampling rate and sampling precision is greatly improved. Variable frequency power system for large aircraft brings in challenges on the analysis of voltage steady-state parameters and transient parameters, the sampling rate, and three phase voltage data acquisition synchronization.
In this paper, the aircraft VF AC power test system based on the LabVIEW programming environment and PXI bus data acquisition board and chassis as the hardware platform is studied.
Hardware design of aircraft electric power test system
Oscilloscope
Display
PXI Chasis
Measured Signal
Conditioning circuit
Fig.1 The simplified structure of large aircraft power system
Fig.2 The structure diagram of PXI system
According to the type of signal, the test points are divided into two groups: voltage signal and current signal. In this system, the number of AC voltage detection points is 81, the number of DC voltage is 32, the number of AC current detection is 48 and the number of DC current detections is 28. The total of test points is 189, far more than the general plane detection point of test power supply system.
Software design of aircraft electric power test system
Simultaneous measurement of multiple devices
Sampling error analysis of multiplexer
one card is selected as the main card
select "ai/Start Trigger" as start trigger signal. based on the tiling sequence structure, the start task from the card is prepared before the main card.
the system main time base reference clock selects the main board to carry the 10MHz clock
from the card clock set the main card output sampling clock "ai/sampleclock"
Fig.4 The input task clock of three simulative channels
Fig.5 The equivalent model of multi-channels in measurement
Fig.3 Debugging of multi-device card

2. Root mean square algorithm error analysis and sampling rate setting
Data storage segmentation strategy
Due to the fact that the waveform of the program is converted into a two-dimensional array after the ADC conversion, the discrete points in the real domain waveform can be obtained.
Original
Transformed
The relative error
The absolute error
It can be seen that the larger the sampling point N is, the lower the relative error of the voltage effective value is.
Fig.6 Data storage strategy
Experiment
Fig.7 The test waveform of eight channels without synchronization
Fig.8 The test waveform of eight channels with synchronization
voltage waveforms can be found between the channels and have obvious phase difference, analysis of the data collected is not available for interphase parameters
the waveform can be found in each channel to achieve the simultaneous measurement of multiple devices of waveform.
Table 1 The voltage error in different sampling rate without the buffer
Table 2 The voltage error in different sampling rate with the buffer
Num
Sa / kS/s
115Vac L BUS-A/Vrms
Error/%
115Vac R BUS-A/Vrms 1 35
1.568
0.524 2 40
2.154
3.037 3 45
3.804
2.205 4 50
5.258
1.043
Num
Sa / kS/s
115Vac L BUS-A/Vrms
Error/%
115Vac R BUS-A/Vrms 1 35
0.402
0.363 2 40
0.002
0.252 3 45
0.108
0.240 4 50
0.188
0.357
Table I and table II is the steady 360Hz frequency 115Vac BUS, without buffer and with buffer A phase voltage RMS calculation results, the sampling rate gradually increased from 35kS/s. calculation accuracy was significantly improved in the 35k~70kS/s wide range of sampling rate, which is within 0.5% and the relative error is less than 0.066%.
Summary
The experimental results show that the proposed multi device synchronization measurement method to ensure the different testing equipment complete synchronization in data collection; use of unit gain buffer, avoid the test signal caused by the modulus increases the sampling rate, channel stability time reduced conversion inaccurate phenomenon, to maximize the use of acquisition speed and improve the testing board accuracy; root mean square error theory based on the method of setting the sampling rate, to avoid the error of the calculation method of influence analysis of multichannel measurement of equivalent model; using system time reference data storage section and write strategy can meet the index operation under massive data; finally, experimental verification of the system in the power system and more test channel the sampling rate increase under the condition of the test accuracy is maintained within the specified scope, and stable operation.