1. Introduction
Estimating the sinusoidal signal is a significant problem for the control system. It is essential to identify the parameters of unknown periodical excitations in tracking and rejection control [1-2]. For example, estimating the frequencies of unknown disturbances in practice is difficult because the waves acting on a ship are nonstationary and unknown in advance [
1]. Under maneuvering conditions, it could hardly measure the exact amounts of time-varying disturbances for a ship, such as waves, winds, currents, ice-covered waters, green waters, etc. To realize the safe voyage of a nonlinear vessel in unexpected sea situations, this paper investigates the parameter estimation of unknown disturbances and the suppression of chaotic roll motions with its prediction.
It is known that a periodic excitation consists of the sum of frequency, amplitude, bias (offset), and phase (randomness). The word periodic is still careful since it is close to approximately periodic, including perfect condition [
3]. As for the real-time processing of chaotic motion in nonlinear systems, a potential solution based on Fourier analysis is deemed one of the unwelcome methods owing to the maximization of the periodogram [
4]. Beyond the perspective of signal processing only, further study is needed to converge the parameter estimation related to the tracking performance of adaptive mechanisms [
5]. The effect of nonlinear plants on parameter convergence is well explained in [
6].
A similar work [
4] identified the full parameters using a fifth-order estimator, showing the complexity and computational cost. The frequency and other parameter estimation techniques are separated in the present paper. Other parameter estimations of amplitude, bias, and phase are treated using the simple update law without any observers, as in [2, 7]. To design the disturbance rejection control, precise frequency estimation will be guaranteed with finite-time convergence like [
1]. As for the problem of periodic disturbance cancellation, readers may refer to [
8].
To achieve the stability and robustness of a nonlinear system [
5], this paper implements linear second-order filters and parameter estimation errors to converge the global parameter estimation without a higher-order estimator. Such a filter operation [
9] overcomes the infinitely increasing auxiliary vector [
10]. Then, a backstepping control will be designed to suppress the chaotic roll motions of the nonlinear system under regular disturbances.
Chaos is aperiodic, long-term motion in a deterministic system [
11]. Even slight initial conditions (IC) changes result in various outcomes [
12]. From a positive viewpoint, the sensitiveness of a chaotic dynamical system has merit because, without the whole reconstruction of a system, it shows a different periodic orbit using a light adjustment of parameters [
13]. However, controlling the nonperiodic behaviors of a chaotic system is not a trivial issue in the real world.
Recently, a simple or complex system under a veil of chaos has been studied with machine learning (ML) techniques, which contribute to predicting dynamic behaviors [
14]. Notably, echo state networks (ESN, [
15]), which are termed reservoir computing (RC, [
16]), are efficient and easy to apply to black box modeling of dynamical systems [
17]. As it is known, RC is a recurrent neural network (RNN)-based framework [
18] that enables the readout to extract the desired output using a linear mapping [
19]. The sensitivity of a chaotic system challenges long-term prediction [
12], which only works if the initial uncertainty is not quickly multiplied by the evolution law [
20]. However, RC is preferable for long-term prediction because it remembers past values and handles external disturbances, where all the past elements are implicitly contained in a state vector [
18]. Moreover, this paper briefly starts to predict the chaotic roll motions before their manipulation and employs the Lyapunov exponents and the Poincaré map.
The remainder of the paper is organized as follows: A prediction scheme with RC, control synthesis for chaotic roll regulation using backstepping, estimation of frequency, and other parameters will be studied in
Section 2. Some numerical simulations verify the proposed schemes in
Section 3. The dynamical theory will be used to explore the uncontrolled chaotic roll responses using the bifurcation diagram, Poincaré map, and Lyapunov exponents (LEs). Finally, final remarks are given along with the following research directions in
Section 4.
2. Materials and Methods
2.1. Prediction of chaotic roll motions using RC
One may experience walking around Lotus Pond, where the green leaves are naturally situated in a reservoir. In contrast to conventional RNN, only the readout weight is trained; input weight (
), feedback weight (
), and adjacency matrix (
) are fixed and chosen randomly. In some simple applications where feedback response is not required,
can be omitted [
21]. Based on similar effects on reservoirs,
, and
are primarily constructed similarly. Both input and feedback responses can be used for generating output [
22]. For a reservoir with
neurons, the structure of a general ESN, having
reservoir states
,
I inputs
, and
O outputs
, is illustrated in
Figure 1 [
21]. The linear mapping input-output at a perceptron is presented in
Figure 1.
According to [
22,
23], the complete form of the update equation for reservoir state vector
r(n) is defined as follows:
where
is the bias of the reservoir’s input;
is the input fed to the reservoir at the sample
;
is the input weight matrix from input to the reservoir;
is the adjacency matrix describing the connection of the nodes in the reservoir;
is the feedback weight matrix from the output back to the reservoir,
is the leaking rate (
);
is the activation function. The weighted sum of the input states is then fed through an activation function to give the final output. The most basic activation function is the step function. However, smooth (sigmoid) functions are mostly preferred, such as hyperbolic tangent functions
. Equations (1) and (2) indicate that the reservoir state
will be updated based on the current input
and the feedback from the previous sample
. The feedback term can be omitted in some tasks where the feedback state is unnecessary. The output state
of the reservoir at the sample is achieved from a linear relationship of the reservoir state and input state as below [
22,
23]
where
is the weight matrix from the reservoir to the output. In the training procedure, the input data is the reference data (teacher data). The actual output of the reservoir would be replaced by the desired output [
22]. Within a training duration of
samples, all input and output data will be collected into matrices
and
, by concatenating
T columns
and
. Regarding equation (3), the linear relation between
and
is defined as
At the end of the training phase, the trained weight matrix
can be computed analytically using Ridge regression.
where
is a parameter added to avoid overfitting. After the training phase, the output weight
is computed and can be used for continuous computation. The actual output of the iteration can be reapplied as input for the next iteration. The teacher data is now unnecessary because the reservoir computer can keep on generating prediction data. As presented in equation (3), the actual output of the reservoir can be obtained.
2.2. Control synthesis for chaotic roll suppression using backstepping algorithm.
The idea of backstepping is to recursively design a controller by considering some of the state variables as “virtual controls” and creating intermediate control laws for them [
24]. This method is one of the proper nonlinear controllers for regulating the desired ship motions [
25,
26]. By adding the actuation input
to a ship model [
27,
28], the complete control system represents a forced rolling system with active control input by
where the periodic excitation
is given as a time-varying disturbance
. In fact, an active controller is essential to achieve a satisfactory anti-rolling effect because roll motions may result in the phenomenon of resonance or parametric instability [
29,
30]. With selecting the state variables as
and
, the governing equation (6) can be rewritten into the state-space representation as follows:
where state vector (
), system matrices (
), and nonlinear term (
) are described by
The two state variables
and
are rewritten in the state-space representation form:
From the first equation in (9),
is considered a virtual control input for
. To make
exponentially converge to zero, the desired value for
is chosen at
where
is a positive constant. Consequently,
would yield the solution
. Declare
as tracking error of state
and define a Positive Definite (P.D, [
31]) Lyapunov function as]
Then the derivative of
is given as
As
should be asymptotically stable,
is expected to be a Negative Definite (N.D) function. In case the disturbance
is well-defined, the control input
can be chosen as
where
is a positive constant, resulting in a P.D function
. However, the amplitude and frequency of disturbance are hardly recognized, meaning that the control input
cannot be defined as (12). In fact,
is dependent on the estimated value
instead of
, so the controller in (12) should be rewritten as
which would yield
The critical problem is to make an estimation that could eliminate the term . In general, four crucial features must be determined to completely define a sinusoidal signal, including offset, amplitude, frequency, and phase. Without loss of generality, assuming that where is the estimate offset, is the estimate frequency, is the estimate amplitude and is the estimate phase, the following subsections will present an adaptive mechanism to update those components.
2.3. Frequency estimation
Let us introduce a second-order filter for disturbance
as shown below
where
are positive constants that make
a Hurwitz polynomial [
2]. Neglecting the IC, it is simple to obtain the relation:
By choosing the updated law [
1]
With a positive constant
, the estimate error
is guaranteed to converge to zero as explained below
With a P.D Lyapunov function
where
is a symmetric P.D matrix, using the result in (18) the following can be obtained
It is clear from (19) that is a non-increasing function and hence is bounded. According to Barbalat’s Lemma, as , which also leads to . Consequently, the updated law in (17) is proven to estimate the frequency for the sinusoidal signal.
2.4. Estimation of offset, amplitudes, and phase
To estimate the rest of the parameters, the disturbance
will be reformed as below
where
is vector of unknown constants and
is the regression vector. Replacing into (14) gives
where
is the estimated error. With a P.D Lyapunov function
where
is a symmetric P.D matrix, the derivative
is given as
To make
an N.D function, the update law should be chosen as
Finally, with the chosen update law, is a non-increasing function. and are bounded, hence and as . To sum up, the necessary parameters for estimating sinusoidal disturbances and controllers have been explained. In the next section, some simulation results will be illustrated to show the system's dynamic behavior under backstepping control with adaptive mechanisms as well as the estimation process to formulate the external disturbance.
Author Contributions
Conceptualization, S.S.Y.; methodology, L.N.B.L.; software, L.N.B.L.; validation, S.S.Y.; formal analysis, L.N.B.L.; investigation, B.D.H.P.; resources, S.D.L.; data curation, B.D.H.P.; Visualization, B.D.H.P.; writing—original draft preparation, S.D.L; writing—review and editing, H.S.K.; supervision, S.S.Y.; project administration, H.S.K.; Funding acquisition, H.S.K.; All authors have read and agreed to the published version of the manuscript.