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Introduction and Classification

Track inspection car, also known as track inspection car or track inspection instrument, abbreviated as track inspection car. The use of track inspection cars to check the geometric status of tracks is an important step in ensuring the safety of railway operations. The main contents of track deformation detection by track inspection cars include track gauge, height, track orientation, level, triangular pits, vibration acceleration, etc. The unevenness of the track is a key factor that directly restricts the increase of train speed in terms of the line. Track irregularity refers to the deviation of the geometric dimensions of two steel rails from their ideal position in the height and left-right directions. There are four types of track irregularities: 1. unevenness in the height of the track before and after. It refers to the vertical geometric deviation of the actual track centerline from the ideal track centerline along the length direction. 2. The track level is uneven. It refers to the vertical height difference between the left and right rails along the length direction. The track direction is uneven. It refers to the horizontal geometric deviation of the actual track centerline from the ideal track centerline along the length direction. The track gauge is uneven. It refers to the deviation between the actual gauge and the nominal gauge.

Figure 1 Schematic diagram of uneven track direction

Track irregularities can be divided into periodic track irregularities, random irregularities, and local irregularities. Periodic track irregularities are caused by irregularities formed by track joints with track length as the wavelength. Random unevenness is caused by errors in track laying, maintenance, and wheel rail wear, and varies with time and place. Local unevenness is caused by specific structures of the line (such as turnouts, transfer lines, sidings, transition curves, branching lines, bridges, etc.) or accidental locations (such as local defects of the line). The unevenness of the track causes significant wheel rail movement force, which not only damages the track structure itself, but also directly affects or endangers the safety of high-speed driving. The unevenness of the track is an external disturbance to the locomotive and vehicle system, and is the main source of vibration in the locomotive and vehicle system. The function description of the random variation law of track roughness is an important basic data for the dynamic analysis of locomotives and track systems. This dynamic analysis is an important means for modern locomotive and track design, maintenance, and quality evaluation.

This article mainly discusses the unevenness of track direction in track irregularities (see Figure 1). The unevenness of the track direction is caused by the positioning deviation of the track centerline during track laying and major maintenance operations, the accumulation of residual deformation in the transverse direction of the track panel, uneven wear on the side of the rail head, failure of fasteners, and inconsistent transverse elasticity of the track.

There are two main methods for detecting the smoothness of track direction:

1. Inertial reference method,

2. Short string measurement method.

The inertial reference method is an orbit measurement method based on the principle of inertial navigation to depict the trajectory of the orbit and determine the geometric state of the orbit. This method requires the use of high-precision strapdown inertial navigation systems, typically used for high-speed track inspection vehicles. Its principle is the same as that of the strapdown inertial navigation system. For details, please refer to relevant literature on strapdown inertial navigation systems. When using this system, initial alignment is usually required to determine the direction matrix between the geographic coordinate system and the carrier coordinate system, and then begin navigation calculations. The advantage of using the inertial reference method to detect orbital conditions is rapid measurement, but the disadvantage is that it is expensive.

Chord measurement method is an earlier method for measuring the smoothness of track direction, which is usually used for track inspection vehicles. Among them, the track roughness detection technology based on fiber optic gyroscope (FOG) is currently the most advanced track detection method in string measurement. The core issue of using a gyroscope to measure track direction is precise angle measurement. Based on this, a transfer function relationship between the angle change of the track detector and the track direction is established, and the track roughness information is extracted from it.

The measurement of track roughness is carried out using several chord lengths, such as 10m chord, 20m chord, 30m chord, 70m chord, 300m chord, etc. This is also known as wavelength, and each wavelength has a corresponding allowable deviation for track roughness. The deviation of short wavelengths has a significant impact on low-speed driving, while the deviation of long wavelengths has a significant impact on high-speed driving.

This article will focus on the detection of track smoothness using the fiber optic gyroscope based chord measurement method as the basic measurement method.

working principle

Principle of Fiber Optic Gyroscope Orbit Measurement

Fiber Optic Gyroscope (FOG) is a new type of all solid state gyroscope based on Sagnac effect. It is an inertial measurement element without mechanical rotating parts, with advantages such as shock resistance, high sensitivity, long lifespan, low power consumption, and reliable integration. It is an ideal inertial device in the new generation of strapdown inertial navigation systems.

Definition of Track Direction in Figure 2

This article derives the trajectory algorithm of the gyroscope angle measurement system based on the principle of using string measurement to determine the trajectory. The basic principle of the string measurement method is shown in Figure 2: there are two guide wheels 1, 3 and 4, 6 on both sides of the track detection device; A measuring wheel 2, 5. The line connecting the wheel rail contact points of the first and last wheels is taken as the chord, and the distance from the wheel rail contact point of the middle wheel to the chord is taken as the measured value in the track direction. The definition of track direction in string measurement method is as follows:

In equations (1) and (2), and respectively represent the distances between the six measurement turns and the center of the vehicle body.

Figure 3 shows a model for calculating theoretical chord measurements when the measured chord direction of a track with a curve radius of R is not smooth. From the geometric relationship, it can be inferred that:

approach

Ignore the h2 term in the equation.

If two strings of different lengths are measured, let l1=nl2 (n is an integer), and h1=n2h2 (5)

The international upper chord measurement method commonly uses the transfer function of small measurement to large measurement as the daily railway formula, and its basic content is: when l1=nl2 (n=2,4,8,16...),

Among them, Vl1 is the chord measurement of l1 chord length, and Vi is the chord measurement of l2 chord length at the i-th point.

In engineering, a measurement scheme with l1=10m and l2=1.25m, i.e. n=8, is commonly used. At l2/2 measurement intervals, the 1.25m chord measurement values of each measurement point are measured in sequence. Then, by using a small to large transfer function, the track irregularities of the 1.25m chord are converted into track irregularities of the 10m chord. The formula for measuring the 10m string is as follows:

V10m=V1+2V2+3V3+4V4+5V5+6V6+7V7+8V8+7V9+6V10+5V11+4V12+3V13+2V14+V15   (7)

The measurement process is shown in Figure 4:

Figure 4 Schematic diagram of deriving 10m string measurement values from 1.25m string measurement values

Trajectory transfer function of gyroscope angle measurement

The core of the gyroscope based trajectory measurement method is to use gyroscopes for angle measurement. The angle measurement system measures the direction of the orbit as chordless detection. When the gyroscope measures the angle, the system collects the angle rotated by the gyroscope every l2/2. If the angle collected for the i-th time is A (i+1), then the angle rotated by each adjacent point is A (i+1) - A (i), as shown in Figure 5.

Due to geometric relationships, it can be obtained

B=1/2[A(i+1)-A(i)]   (8)

Then:

The radius R of a typical railway track is calculated in kilometers. Therefore, between two measurement points with a step size of l2/2, the gyroscope rotation angle is extremely small (to improve accuracy, the step size l2 can be reduced). The chord measurement value of the l2 chord length corresponding to the angle A (i+1) - A (i) rotated by the chord l2 is:

Substituting (7) yields:

In the formula, l2=1.25m. This transfer function can collect angles every l2/2 and then calculate the chord measurement value of 10m chord, which is the trajectory calculation formula.

Figure 5 Principle of chord measurement derived from angle

The Influence of Gyroscope Angle Drift on Orbit Direction

What is the maximum measurement error of a 10m string caused by angle drift when the gyroscope is stationary. Firstly, assume:

1. The gyroscope runs on a straight track with a theoretical angular velocity of zero

2. The angular velocity drifts uniformly in one direction

3. The running speed of the track inspection car is 1.25/s

According to the assumed conditions, the car travels 1.25 meters per second. If the angle of gyroscope drift per second is known, the 1.25 meter chord measurement drift can be calculated according to the formula. The car running along the ideal straight rail can be understood as the car being stationary.

The angle random walk coefficient of AgileLight fiber optic gyroscope is

The bias instability is

S Ω (f) is the noise power spectral density function. The angle deviation caused by random angle walk within 1 second is:

The angle deviation caused by bias instability is

According to the formula, the error in string measurement per second caused by gyroscope drift is

Among them, it represents the angular drift of the gyroscope within one second.

So we obtained a measurement value of 10m string, which is much smaller than the specified error value of 1mm.

Based on the above theoretical analysis, the impact of gyroscope angle drift on string measurement is minimal. In fact, angle drift has both positive and negative effects, which largely cancel each other out. The probability of drift exceeding 0.135mm on a 10 meter string measurement is almost zero. In fact, the randomness of angle drift causes positive and negative errors between the measured values and the actual values of string measurements, with a general error of less than 0.1mm. Therefore, the accuracy index of AgileLight gyroscope can fully meet the accuracy requirements of track inspection vehicles.

Advantages of using AgileLight series fiber optic gyroscope in rail cars

The system applied to track inspection vehicles is generally a strapdown inertial navigation system. Compared to small orbit inspection systems, strapdown inertial navigation systems face two insurmountable obstacles: 1) volume issue. Although the strapdown system is much smaller than the platform system, its size is still too large for the orbit detector to tolerate; 2) Price issue. The cost of a strapdown inertial system is calculated in millions of yuan, while the market positioning of an orbit gauge is calculated in tens of thousands of yuan, and there is an order of magnitude relationship between the two. So the gyroscope used in orbit inspection instruments should be relatively cheap, and our AgileLight happens to have a significant price advantage.

Compared to the commonly used Russian VG951 in the market, VG951 is an analog output that requires additional digital circuits to be configured for use. AgileLight is a digital output that can directly obtain angle measurement data. Moreover, the drift and random walk indicators are smaller, the angle measurement is more accurate, and the fault free usage time is 55000 hours, which is approximately three times the usage time of VG951 gyroscope of 20000 hours. Therefore, AgileLight has a clear cost-effectiveness advantage.

conclusion

The AgileLight series fiber optic gyroscope produced by Wuxi Huilian Information Technology Co., Ltd. has excellent performance, can resist impact and various harsh environments, and is particularly suitable for use in track inspection instruments.