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A nondestructive continuous instrumented wheelset design is proposed based on strain gauges placing inside of the wheel web and wireless telemetry system. The signal feature analysis including frequency contents and high order harmonic ripples is also carried out. The strain gradient decoupling method for vertical and lateral force identification is proposed based on the strain distributions under respective loads. The method implements minimum crosstalk effects and insensitive to the varying contact points. The KMT telemetry system is adopted for wireless inductive powering and signal transferring. The drilling holes on the wheel and axles are avoidable to ensure the integrity and long-term using of the wheelset. Bridging and demodulating schemes for lateral and vertical force are designed respectively as they have dramatic differences at the dynamic signal features. High order harmonic ripple analysis and error estimation are gotten by independent waveforms. Based on the data form calibration test rig, it is indicated that the high order ripple amplitudes are below 10% of the demodulation amplitudes and fulfill designed requirements.

Wheel rail contact forces measurements play a critical role in the vehicle acceptance tests for derailment safety evaluations [

As shown in

Due to the wiring holes on the axle and auxiliary holes on the web, the structural integrity of the wheelset is damaged, and it is only used for acceptance tests or short-term tests for running safety. Nowadays, there is a strong need for a non-destructive instrumented wheelset in long-term tracking tests along with the wear process of the wheels. Placing the strain gauges on the inside wheel web and together with wireless signal transferring system is a feasible solution for the nondestructive instrumented wheelset. This article introduced the implementation principle of the method. The positioning method of the strain gauges based on strain gradients, and the waveform rectifying method for the vertical force were highlighted.

Expect for traction and braking states, the lateral and vertical contact forces are of more meanings to the vehicle running safety evaluations such as derailment factors and overturn ratios. This article only concentrated on the lateral and vertical force identifications through elastic deformations of the inside wheel web. It is found that wheel web has uneven deformations under the vertical and lateral loads through FEA. The critical requirement of positioning strain gauges is to get the lowest cross sensitivity to the external loads. It is implemented by algebraic operations of the individual strain gauge through bridge assembling according to the strain gradient features, respectively.

Wheel web stress distributions under vertical and lateral loads were calculated individually to get strain gradient features along the radial direction, as shown in

The most sensitive zones on the wheel web are CA1, TA1, and CA2 respective to the lateral (Q) and vertical (P) forces, as shown in

The detailed FEA results are shown in

The strain outputs at A and B under lateral and vertical loads are ε Q A , ε Q B , ε P A , and ε P B . The strain gradients are used for lateral loads identification as:

{ ( ε P A + ε Q A ) − ( ε P B + ε Q B ) = Δ ε Q ε P A = ε P B (1)

A and B are chosen at the same horizonal positions of the strain curves under P forces where the gradient is only determined by Q force Δ ε Q that means decoupling with P force.

Based on the above limitations, the position C, D, E, and F are chosen for the P force identification. As illustrated in

The strain outputs at C, D, E and F under lateral and vertical loads are ε Q C , ε Q C , ε P D , ε P D , ε Q E , ε Q E , ε P F , and ε P F . The synthetic strain outputs ε C , ε D , ε E , and ε F are computed as:

{ ( ε P E + ε Q E ) − ( ε P F + ε Q F ) = ε Q E − ε Q F = − Δ ε ′ Q ( ε P C + ε Q C ) − ( ε P D + ε Q D ) = Δ ε P + Δ ε ′ Q (2)

where Δ ε P = ( ε E − ε F ) + ( ε C − ε D ) is decoupled with Q force and the effects of varying contact points are also eliminated.

To avoid drilling wiring holes on the axle, wireless signal transmission system must be adopted. The analog signals of the bridge outputs are conditioned and digitalized firstly and then transferred out by inductive ring. The inverse D/A transform, acquisition, and digital signal processes are further processed outside of the rotating wheelset. Otherwise, power supply of the telemetry system is also implemented by resonant inductive ring. The scheme of the commercial system from KMT is shown in

Auxiliary brackets for inductive ring and coil packaging blocks are special designed for motor and trailer bogies (CRH380BG high speed trains), as shown in

The contact forces are rotating respect to the Wheatstone strain bridge when the vehicle is running. The rotation effects eliminating process using two bridges with 90 degrees delay is called signal demodulation. The principles of the common method SQ and ABS are shown in

The two-bridge demodulation method is only suitable under the condition that section angle of the stress influence zone is close to 180 degrees. The section angles of the stress influence zone under P and Q forces are shown in

As mentioned above, the position A and B are chosen to calculate strain difference which is sensitive to the Q force only. Each branch of the full bridge consists of 2 gauges for minus operation. The full bridge is comprised of 2 opposite sectors of each has 2 branches with 60 degrees angle, as shown in

The layout of the two Wheatstone bridges with 90 degrees phase delay is shown in

The demodulated signals are calculated as follow:

ε d = ε p 0 ⋅ cos θ − ε p 90 ⋅ sin θ (3)

where ε d , ε 0 , and ε 90 are demodulated signals, output signals of the phase 0 and phase 90 bridges respetivly, and θ is the rotation angle of the wheelset through the tachometer.

The output wave the single bridge has difference with the sine wave that brings high order harmonics. There are tiny 4^{th} order ripples which can be eliminated by filtering in real-time or post data process.

The output waveform of a single strain gauge at the loading condition only the P forces existing is much like a pulse with steep slope. As stress of the wheel web is less sensitive to the vertical loads, the sector angle of the influence zone is less than 60 degrees. The demodulated signals of the two-bridge scheme have large ripples with the four times frequency of the wheelset rotating, as shown in

To reduce the amplitude of the high order ripples, three-bridge scheme and demodulating algorithm were proposed. Three uniformly distributed with the section angle of 120 degrees were adopted as shown in

According to the demodulation algorithm, the single bridge outputs is computed as:

ε k = { ( ε k 12 + ε k 13 − ε k 11 k − ε k 14 ) − ( ε k 32 + ε k 33 − ε k 31 − ε k 34 ) + ( ε k 22 + ε k 23 − ε k 21 − ε k 24 ) − ( ε k 42 + ε k 43 − ε k 41 − ε k 44 ) } (4)

where k = 1 , 2 , 3 is the bridge number, ε k is the brige k strain output. The strain gauge number is ε k m n , m = 1 , 2 , 3 , 4 is bridge branch number, and n = 1 , 2 , 3 , 4 is position number represents E, C, F, and D.

The demotulation signals ε d are computed as:

ε d = Max ( | ε 1 + ε 2 | , | ε 2 + ε 3 | , | ε 3 + ε 1 | ) (5)

The calibration tests were carried out by the test rig, as shown in

The waveform outputs on the calibration rig are shown in

The nondestructive instrumented wheelset could be implemented inside wheel web strain gauge placing scheme together with wireless signal transmission system.

1) The wireless telemetry system and strain gauge inside web placing scheme keep the structural integrity of wheelset that can be used in long-term contact force monitoring.

2) Strain gradients are used for strain gauges poisoning to eliminate loads decoupling effects, and the effects of the varying contact points are also minimized.

3) The ripples amplitude ratios are 2.5% and 7.5% for lateral and vertical force identification, and the orders are 4 and 12 respecting to the wheelset rotating frequency. The high frequency demodulating disturbances could be eliminated by post filtering process.

The authors declare no conflicts of interest regarding the publication of this paper.

Wang, J.B., Li, D.D., Qu, S. and Zhang, D.F. (2021) A Nondestructive Instrumented Wheelset System for Contact Forces Measurements. Engineering, 13, 361-371. https://doi.org/10.4236/eng.2021.137026