Citation: Peng, Z.; Dong, K.; Wang,
Y.; Huang, X. A Fault Diagnosis
Model for Coaxial-Rotor Unit Using
Bidirectional Gate Recurrent Unit
and Highway Network. Machines
2022, 10, 313. https://doi.org/
10.3390/machines10050313
Academic Editors: Kelvin K.L. Wong,
Dhanjoo N. Ghista, Andrew W.H. Ip
and Wenjun (Chris) Zhang
Received: 2 April 2022
Accepted: 25 April 2022
Published: 27 April 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
A Fault Diagnosis Model for Coaxial-Rotor Unit Using
Bidirectional Gate Recurrent Unit and Highway Network
Zhaoqin Peng
1
, Kunyu Dong
1
, Yan Wang
2,
* and Xucong Huang
1
1
School of Automation Science and Electrical Engineering, Beihang University, Beijing 100191, China;
pengzhaoqin@buaa.edu.cn (Z.P.); dongkunyu@buaa.edu.cn (K.D.); huangxucong@buaa.edu.cn (X.H.)
2
School of Transportation Science and Engineering, Beihang University, Beijing 100191, China
* Correspondence: wybuaa@buaa.edu.cn
Abstract:
A turbojet engine is the most significant part of an Internal Combustion Engine (ICE) for
Hybrid Electric Vehicles. Specifically, the coaxial-rotor unit is the key component, whose performance
largely affects the working efficiency. Thereby, the fault diagnosis methods for coaxial-rotor units is
a main focus. In line with our test results, the bearing circlip is the most vulnerable element while
rotating. Moreover, the low-speed rotating fault diagnosis is even challenging for current methods.
Since the fault diagnosis on the bearing circlip of coaxial-rotor units is absent, this paper establishes
a test rig on a running coaxial-rotor unit under different working conditions. The three-directional
vibration signals are collected and analyzed to demonstrate the working states. On the task of bearing
circlip failure classification, a deep-learning-based model using the Bidirectional Gate Recurrent
Unit and the Highway Network is developed, which is capable of capturing hidden features and
removing unrelated information. For working performance evaluation, experiments on the data
of different rotating speeds are carried out. Among all the fault diagnosis methods, our model is
the best approach and achieves an average accuracy of 99.4%. The encouraging results reveal that
the proposed model is effective in both the high-speed and low-speed fault diagnosis of bearing
circlip malfunction.
Keywords:
turbojet engine; coaxial-rotor unit; fault diagnosis; Bidirectional Gate Recurrent Unit;
Highway Network
1. Introduction
The automobile industry faces the challenge of saving energy and eliminating environ-
mental damage, whilst delivering more goods and passengers to achieve a longer driving
range [
1
]. In line with the ongoing advancements, electric vehicles, whose primitive form
is proposed in 1900 s, have caused a revival of interest in vehicle engineering [
2
]. Based
on the developing potential of electric vehicles and batteries, pure electric vehicles will
ultimately meet the demands of high energy efficiency and low-pollutant emission. Despite
its high energy efficiency and low-pollutant emission, the pure electric vehicle is currently
a secondary choice for long-range driving restricted to battery storage capacity [
3
]. That is,
these vehicle types are not likely to be widely used within the short term. Encouragingly,
Hybrid Electric Vehicles (HEVs) bridge the gap between conventional vehicles and pure
electric vehicles, which are viable means of transportation at present [
4
]. An HEV has an
Internal Combustion Engine (ICE) as the main power source and an electric drive (i.e., a
battery and an electric motor) to improve fuel economy [
5
]. Specifically, the battery required
to power the electric motor is charged by the ICE without any external source. As such,
almost all auto makers provide a hybrid version of their popular models such as Audi A8
Hybrid, Ford Fusion, Honda Civic Hybrid, and Hyundai Sonata Hybrid [6].
Typically, a turbojet engine, integrating with an electric generator, is employed in
an ICE to supply stable and reliable power [
7
]. As long as the ICE aims to supply the
Machines 2022, 10, 313. https://doi.org/10.3390/machines10050313 https://www.mdpi.com/journal/machines
