International Journal of Control, Automation and Systems 15(2) (2017) 575-584
http://dx.doi.org/10.1007/s12555-015-0429-9
ISSN:1598-6446 eISSN:2005-4092
http://www.springer.com/12555
Output Control of MIMO System with Unmatched Disturbance based on
High-order Unknown Input Observer
Jiancheng Zhang and Fanglai Zhu*
Abstract: In this paper, the output control for a general MIMO linear system subjected to unmatched disturbance is
considered. A smooth backstepping control law is developed under the assumption that the state and disturbance as
well as its derivatives are known firstly. Then in order to estimate the required information, a high-order unknown
input observer (HOUIO) is developed, and it is proven that based on the estimation by such the HOUIO, the designed
controller can compensate all the disturbances from output channels and asymptotically drive the output to zero.
Comparing with the traditional sliding mode control laws, the control law we design does not contain any switching
function, so it can effectively reduce the chattering. Finally, simulation results are given to show the effectiveness
of our methods.
Keywords: High-order unknown input observer, MIMO systems, smooth controller, unmatched disturbance.
1. INTRODUCTION
Uncertainties in dynamic systems, including parameter
perturbation and external disturbances, can bring adverse
effects to system performance and even cause unstable dy-
namic behaviors. Many scholars have devoted themselves
to address this practical issue, and up to now, many ef-
fective control schemes, including robust H
2
/H
∞
control
[1–4], adaptive control [5–7], sliding mode control [8–10],
and backstepping control [11] and so on, have been put
forward.
On the one hand, the methods mentioned above are usu-
ally good at dealing with systems with matching uncer-
tainty. For systems with unmatched disturbances, how-
ever, they can not compensate the uncertainty effects com-
pletely. On the other hand, the systems subjected to un-
matched disturbances are very common in engineering
application. In fact, the MAGLEV suspension vehicle
[12], the PMSM systems [13, 14] and the magnetically
suspended balance beam system [15] are examples of sys-
tems affected by unmatched uncertainties [16]. So, how to
design control law to reach the unmatched disturbance re-
jection purpose becomes one of the major considerations
recently and, of course, many control techniques, such
as Riccati approach [17], LMI-based control [18], sliding
mode control (see [19–22]), backstepping method [23–27]
and disturbance observer-based compensation strategies
[12, 28, 29], have been developed. In [23], a backstepping
control scheme is proposed to tackle the exact tracking
Manuscript received November 14, 2015; revised April 5, 2016; accepted June 5, 2016. Recommended by Associate Editor Nam H. Jo un-
der the direction of Editor Duk-Sun Shim. This work is supported by the National Nature Science Foundation of China under grant 61573256.
Jiancheng Zhang and Fanglai Zhu are with the College of Electronics and Information Engineering, Tongji University, Shanghai (201804),
China (e-mails: jcz1919@163.com, 1987jcz@tongji.edu.cn (J. Zhang), zhufanglai@tongji.edu.cn (F. Zhu)).
* Corresponding author.
issue in the presence of matching and unmatched distur-
bances for the system with block controllable form. In
[22] and [25], the control schemes for some certain vari-
ables of systems with unmatched disturbances are pro-
posed based on the exact identification of state and dis-
turbance. The former control law design is sliding mode
scheme, and the latter is a continuous one with chatter-
ing attenuation. In [30], the authors proposed a high order
augmented state observer based control such that the out-
put tracking error can be global ultimate bounded, and the
disturbance there is assumed to be in the polynomial func-
tion form. More recently, for these unmatched disturbance
rejection issues, several sliding mode control combining
with disturbance observer design methods were reported,
such as [12, 16, 28, 29, 31, 32] and so on. In particular,
based on a second-order system, [28] proposed a new con-
trol scheme such that the output y can be steered to zero
in the presence of unmatched disturbance d(t) which sat-
isfies lim
t→∞
˙
d(t) = 0 . Assumption lim
t→∞
˙
d(t) = 0 im-
plies that the d(t) is a constant or an incipient disturbance
and obviously it is too restrictive. To relax this restriction,
[31] developed a new disturbance observer-based control
method which can deal with high-order time-varying dis-
turbance signals, and then they extended the results in [28]
to n-order system. In addition, in [16] and [32], a nonsin-
gular terminal sliding mode controller and a continuous
controller are constructed, respectively, where the exact
identification of disturbance was incorporated into such
the control laws so that the influence of the disturbance
c
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