1 Principle of automatic test system for signal equipment
The automatic test system for signal equipment is mainly designed for the transmitter and receiver of the ZPW2000A type non-insulated track circuit equipment. Its hardware structure is shown in 1.
As can be seen from 1, the system is mainly composed of 6 parts: data acquisition card, digital I / O card, signal conditioning module, relay array, signal generator, and computer. Among them, the data acquisition card is the core device of the system, collects all signals of the detection object, and transmits the signals to the memory through the PCI bus using the DMA method for analysis and processing by the application program. The digital I / O card is the key to realize the automatic test system, through which it generates TTL compatible levels and controls the relay array. The relay array is a device used to perform actions. Through its on / off, different test items are selected to complete the detection of various indicators of the device under test, thereby realizing the function of automatic testing. For the measurement of the voltage index of the track circuit, due to the relatively large amplitude of the voltage and the measurement of various currents, they must be converted into appropriate voltage amplitudes. The current sensor and signal conditioning module convert the signal into a signal that can be measured by the data acquisition card. The signal generator provides standard railway equipment signals for the detected objects. The computer provides a common platform.
2 System error analysis
Test system errors mainly include random errors and system errors. Since random errors can be eliminated by eliminating bad values ​​and calculating the arithmetic mean value of the measured values, it has little effect on the results, so focus on analyzing the transmission and synthesis of system errors in the test system.
After the measured signal enters the virtual test instrument as an input signal, it is transformed into an output signal after a series of transformations, and also includes the following two errors: one is the input signal error caused by the conversion of the system transfer function; the other is The error of the virtual test instrument itself is introduced. The system error transmission process is as shown.
Each part of the automatic test system will introduce different errors in the test process, and these errors will form the total error of the system through certain transmission. If the error transfer coefficients of the sensor, signal conditioning circuit module, data acquisition card, and computer are a1, a2, a3, and a4, because these links are connected in series, the multiplication of these error transfer coefficients is the entire system The error transfer coefficient a, so that the following formula can be used to represent the transfer process of systematic errors.
y = a (1 e) x
Among them: x is the error of the signal under test when it enters the test system; e is the error of the test system itself; a is the error transfer coefficient of the test system; y is the error of the output signal.
As can be seen from the above formula, the error of the virtual test instrument appears in the form of an error transfer coefficient, which is the result of the combined effect of the system transfer function and its own error factor, which can be corrected by the correction factor.
3 Error handling methods
Because the characteristic of the virtual instrument itself is software, that is, the error transfer coefficient of the virtual instrument test system can be obtained through a series of calibration tests, and then the test system can be compensated and corrected to obtain the required high-precision test results. The schematic diagram of the calibration principle of the test system is shown below. First, provide a standard signal source V1 and use it as the calibration standard, then design a calibration procedure, test the standard source V1 to obtain the test value V2, take k = V1 / V2 and store k in the calibration configuration file. When the test system performs the test, it first reads the calibration coefficient k from the configuration file, then measures the measured signal, and the measured value is V4. Finally, the coefficient k is used for calibration, and the output result is V = kV4.
3 Schematic diagram of test system calibration The following uses ZPW2000A type non-insulated track circuit equipment transmitter power supply voltage calibration test as an example. Since the true value of the measured value cannot be obtained in practice, the 34401A612 digital multimeter of Agilent and the actual value of the measured signal source are used in the test. The calibration process is as follows.
First, use calibration to detect the output voltage of the power supply, and at the same time monitor the output of the power supply with the above multimeter, record and compare the data of the two, as shown in 1.
1 Power supply voltage data measured before system calibration Data measured by the test system V1 / V multimeter Tested data V2 / VV2 / V1 Relative error / 2223242526
-057-053-0502 Average the 5 data of V2 / V1 to obtain the correction coefficient 10057 for this project, and then save the coefficient to the calibration coefficient configuration file. After calibration with this coefficient, the measured data of the automatic test system is shown in 2.
2Measured power supply voltage data after system calibration System measured data after calibration / V System measured data after calibration / Relative error after V calibration / 22232425262202323008240062499725983-0104-0035-002500120065 As can be seen, after system calibration, the maximum relative error is 0104 Compared with 065 before calibration, the test accuracy is greatly improved, and exceeds the relative error of 1 required by the test outline of the device under test, which meets the test requirements.
4 Summary
The core concept of virtual instruments is that the software is the instrument, that is to say, with the help of the powerful processing power of the computer, some hardware can be replaced with software. As far as the error of the test system is concerned, it is necessary to consider which hardware may cause the error when constructing the test system. It should be implemented as much as possible, otherwise the system can be calibrated with reference to the above examples to obtain high-precision measurement results. The automatic test system for the transmitter and receiver of the ZPW2000A type non-insulated track circuit equipment has been put into trial use. The error correction method is simple and practical, which greatly improves the measurement accuracy and reaches the expected goal.
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