Automatica 79 (2017) 290–295
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Automatica
journal homepage: www.elsevier.com/locate/automatica
Brief paper
A novel sliding mode observer for state and fault estimation in
systems not satisfying matching and minimum phase conditions
✩
Xianghua Wang
a
, Chee Pin Tan
b
, Donghua Zhou
a
a
College of Electrical Engineering and Automation, Shandong University of Science and Technology, Qingdao, 266590, China
b
School of Engineering and Advanced Engineering Platform, Monash University Malaysia, 46150 Selangor, Malaysia
a r t i c l e i n f o
Article history:
Received 10 December 2015
Received in revised form
25 October 2016
Accepted 6 January 2017
Keywords:
Fault estimation
Sliding mode observer
Matching condition
Non-minimum phase
a b s t r a c t
In this paper, a novel sliding mode observer is proposed for systems that do not satisfy common
conditions required for fault estimation, in particular the so-called matching condition and minimum
phase condition. Other works attempt to circumvent the matching condition by using multiple observers
in cascade, or higher order sliding modes, whilst the minimum phase condition can be relaxed at the cost
of some faults and the states being not estimated, or the fault estimation being corrupted by the fault time
derivative (or other dynamics). For the proposed scheme in this paper, only one observer is required, the
minimum phase condition is relaxed to detectability, and the matching condition is greatly simplified to
a condition related to the system dimensions which makes the scheme also applicable to the case where
there are less outputs than faults. Finally, a simulation example is conducted to verify the effectiveness of
the proposed observer.
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Fault detection and isolation (FDI) which can be used to increase
safety, reliability and maintainability, has received considerable
attention in the past few decades, both in research and application.
The main function of an FDI scheme is to generate an alarm when
a fault occurs (detection), and to determine its location (isolation)
(Liu, Laghrouche, & Wack, 2013). However, information such as
magnitude and shape which are very useful for fault-tolerant
control cannot be provided by the FDI scheme (Zhang, Swain, &
Nguang, 2014), hence a fault reconstruction and estimation (FRE)
scheme is required.
The sliding mode observer (SMO) has been effectively used for
FRE in many systems (Edwards, Spurgeon, & Patton, 2000; Walcott,
1990; Walcott & Zak, 1987). This is due to not only its robustness
✩
This work was supported by the National Nature Science Foundation of China
under Grants 61210012, 61290324, 61490701, 61603222, 61273197, 61503224
and China Postdoctoral Science Foundation 2016M592217 and Research Fund for
the Taishan Scholar Project of Shandong Province of China LZB2015-162 and the
Ministry of Education Malaysia (FRGS/2/2013/SG04/MUSM/02/1). The material in
this paper was not presented at any conference. This paper was recommended
for publication in revised form by Associate Editor Michele Basseville under the
direction of Editor Torsten Söderström.
E-mail addresses: xianghuaw@pku.edu.cn (X. Wang),
tan.chee.pin@monash.edu (C.P. Tan), zdh@mail.tsinghua.edu.cn (D. Zhou).
and insensitivity to the so-called matched uncertainty but also its
capability of estimating unknown inputs based on the equivalent
injection input concept (Zhou, Soh, & Shen, 2014). The necessary
and sufficient conditions for the existence of SMOs are that the
invariant zeroes of the system must be stable (minimum phase),
and the matching condition is satisfied (Defoort, Veluvolu, Rath, &
Djemai, 2015); these conditions could be stringent and limit the
applicability of this technique.
Two methods have been developed to relax the matching
condition. The first uses high-order sliding mode differentiator
(Bejarano & Fridman, 2010; Bejarano, Fridman, & Poznyak, 2007;
Floquet & Barbot, 2006; Floquet, Edwards, & Spurgeon, 2007;
Fridman, Levant, & Davila, 2007, 2008; Kalsi, Lian, Hui, & Zak,
2010; Yang & Zhu, 2013; Yang, Zhu, & Sun, 2013) to generate
the derivatives of the outputs which are added to the original
system to form a ‘new system’ which satisfies the matching
condition. The second one uses multiple SMOs in cascade (Tan,
Crusca, & Aldeen, 2008; Tan & Edwards, 2010), where signals from
an observer are used as the output of a fictitious system whose
input is the fault; another observer is then implemented for the
fictitious system and this process is repeated until the fictitious
system satisfies the matching condition. Although both methods
are effective, the structure of the FRE scheme is complicated and
large approximation errors could occur. In addition, the system
must be minimum phase. In Alwi, Edwards, and Tan (2009), that
condition was relaxed but at the cost of the fault estimate being
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