Design and implementation of online tester for track circuit based on MSP430

As train speeds continue to increase, the requirements for train tracks are also getting higher and higher. The original insulated track circuit can no longer meet the needs of the development of railway transportation, so advanced industrial countries around the world have begun to develop non-insulated track circuits [1].

Among them, the UM71 non-insulated track circuit successfully developed by the French CSEE company is the most widely used. This system was introduced into my country in the late 1980s. This non-insulated track circuit uses the resonance principle to transmit low-frequency modulated FSK signals (4 carrier frequencies, 18 low frequencies) through the rails. Since the parallel rails have equivalent inductive reactance characteristics, the signal transmission distance is seriously affected. In order to prevent the output signal from being distorted, a 33μF resonant capacitor is installed every 100 meters in each block section to form a series resonant circuit with the equivalent inductance of the rail to increase the signal transmission distance. To ensure that the system works properly, railway staff must frequently check the operating parameters of the track circuit along the rails. One of the important items is the measurement of the capacitance of the parallel compensation capacitor, but the current common bridge method capacitance measurement equipment cannot perform online measurement. For trains running on the track, offline measurement of compensation capacitors poses a huge safety hazard [2]. Although the dedicated UM71 non-insulated track circuit compensation capacitance online measurement device can perform online measurement, it uses a passive measurement method, which requires a high resolution of the clamp-type current sensor, increases the complexity of the device, greatly increases the cost of the device, and the device is large and not easy to carry. In view of this, this paper designs a portable online tester.

1 Working Principle

By applying an excitation signal with a frequency higher than the signal transmitted by the track circuit itself to both ends of the capacitor to be tested, the response signal at both ends of the capacitor is collected at the same time, and the signal is sent to the single-chip microcomputer MSP430 (containing an A/D converter and a sampling and holding module) for A/D conversion after filtering out noise through a bandpass filter. Since there is a certain corresponding relationship between the capacitance value of the compensation capacitor and the converted value, according to this relationship, the capacitance value can be obtained by comparing the value after A/D conversion with the pre-calibrated value. The rationality of this measurement method has been verified by experiments. At the same time, this tester also has the function of measuring carrier frequency and frequency modulation frequency. Since the signal to be tested is a mixed signal modulated by a higher frequency carrier signal and a low frequency modulation signal, this signal is compared by a zero comparator and sent to the timer in the single-chip microcomputer to count to obtain the carrier frequency. At the same time, the signal is filtered out by a low-pass filter to remove the higher carrier signal, and then sent to the single-chip microcomputer after passing through a zero comparator to obtain the modulation signal frequency.

2 System Hardware Design

The tester adopts an active measurement method and is mainly composed of a signal generation module, a drive circuit module, a signal acquisition module, a single chip microcomputer and a display module.

2.1 Signal generation and drive module design

The first problem is the selection of the excitation signal frequency. The excitation signal frequency selected by the instrument cannot be too low. If it is too low, according to the inductive reactance formula: XL=2πfL, the rail inductive reactance will be too small, which will form a parallel effect of the compensation capacitor and make the measurement meaningless. At the same time, the frequency cannot be selected too high. Since the internal resistance of the measurement system plays a voltage divider role in the measurement, a high frequency will make the output voltage too small, affecting the resolution of the measurement system. Combining the above two points and combining the experiment, the selected excitation signal frequency is 10kHz.

The design idea of ​​this module is to divide the output of the 2MHz crystal oscillator by 200 to obtain a stable 10kHz square wave signal, and pass the signal through a Darlington transistor to improve its driving ability.

2.2 Signal acquisition module design

The signal collected by the measuring probe is relatively weak, so it must be amplified by an amplifier. Due to the harsh working environment of the railway, the collected signal is inevitably mixed with interference signals. The interference signal is also amplified when passing through the amplifier, which will seriously affect the measurement results. In order to filter out the noise signal, this tester uses the 8th-order continuous-time active filter MAX274 developed by MAXIM for filtering [3].

2.3 Design of frequency discrimination module

The collected signal is sent to the MCU's TimerA for counting after passing through the zero comparator, thereby obtaining the frequency of the higher frequency carrier signal (1700Hz, 2000Hz, 2300Hz and 2600Hz). After the signal is further filtered through a low-pass filter to remove the high-frequency signal, the frequency of the low-frequency modulation signal is obtained.

2.4 Main control chip and display module design

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Taking into account the operating environment and other factors, the high-performance, ultra-low-power 16-bit single-chip microcomputer MSP430F147[4] produced by TI was finally selected as the main control chip. The MSP430 series devices are all industrial-grade products, with an operating environment temperature of -40 to +85°C. They use a RISC structure, have a rich addressing mode and a high processing speed. The MSP430 series of single-chip microcomputers have many interrupt sources, and they can be nested arbitrarily, which is flexible and convenient to use. When the system is in a power-saving standby state, it only takes 6μs to wake it up with an interrupt request. The display part uses a low-power, beautiful LCD liquid crystal display module.

3 System Software Design

The system software design adopts efficient C language programming. According to the hardware composition of the system, the system software design mainly consists of four parts: system initialization, capacitance measurement, frequency discrimination and human-computer interaction.

4. Analysis of test results

As the capacitance of the capacitor to be measured increases, the peak-to-peak value of the response signal voltage gradually decreases. Based on this corresponding relationship, the capacitance of the capacitor can be obtained. Assuming that the first set of data is used as the calibration data, it can be seen from Figure 3 that the maximum error of the second set of data is 35.3-34.1=1.2μF. Since the accuracy of the 33μF compensation capacitor is generally ±10%. That is, the actual value is between 29.7μF and 36.3μF, it can be seen that the measurement result is relatively accurate. Railway staff can determine whether the capacitor needs to be replaced based on the measurement results. At the same time, in order to reduce the measurement error, multiple measurements can be performed and the average value can be taken as the calibration data.

UM71 compensation capacitor tester is developed to meet actual needs. It realizes the online measurement of frequency-shift track circuit parameters. It is portable and easy to use, reduces the labor intensity of employees, and eliminates the safety hazards caused by offline measurement of compensation capacitors. This instrument has the function of intuitively displaying the capacitance value and the carrier frequency and frequency modulation frequency of the transmission signal. At the same time, the tester has the advantages of strong anti-electromagnetic interference ability, small size, light weight, and good portability.