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Experimental Validation for Real Time Control of DC Motor Using Novel Finite-Horizon Optimal Technique

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Abstract and Figures

—DC motors are widely used in industrial applica-tions. Controlling of DC motor is a great challenge for controlengineers. Precise equations describing DC motors are nonlinear.Accurate nonlinear control of the motion of the DC motorsis required. In this paper, a novel online technique for finite-horizon nonlinear tracking problems is presented. The idea ofthe proposed technique is the change of variables, that convertsthe nonlinear differential Riccati equation to a linear Lyapunovdifferential equation. The proposed technique is effective for awide range of operating points. Simulation results and experi-mental implementation of a DC motor are given to illustrate theeffectiveness of the proposed technique. I. I NTRODUCTION The need to improve performance in control systems re-quires more and more accurate modeling. However, if a modelis a good representation of the real system over a wide rangeof operating points, it is most often nonlinear [1]. Traditionaltechnique to control, nonlinear systems is to linearize thenonlinear system in a small region around the operating pointand then design linear controllers. These controllers withconstant gains can be expected to perform satisfactorily in theneighborhood of the operating point. However, they may notbe capable of dealing with a situation over a large range ofoperating points.The competitive era of rapid technological change hasmotivated the rapid development of nonlinear control theoryfor application to challenging, complex dynamical real-worldproblems [2]. There exist many nonlinear control designtechniques, each has benefits and weaknesses. Most of themare limited in their range of applicability, and use of certainnonlinear control technique for a specific system usuallydemands choosing between different factors, e.g. performance,robustness, optimality, and cost. Some of the well-known non-linear control techniques are feedback linearization, adaptivecontrol, nonlinear predictive control, sliding mode control,and approximating sequence of Riccati Equations. One ofthe highly promising and rapidly emerging techniques fornonlinear optimal controllers designing is the State DependentRiccati Equation (SDRE) technique [3]. The SDRE techniquecan be used for regulation or tracking of infinite-horizonnonlinear systems [4]. The SDRE-based techniques have veryimportant properties, such that applicability to a large classof nonlinear systems, and the control designer opportunity tomake tradeoffs between the state errors and the control effort[1].In this paper, we address the problem of finite-horizonpositioncontrol of a permanent magnet DC motor based on thenonlinear system dynamics. A novel technique for tracking offinite-horizon nonlinear systems is utilized. This technique isbased on the change of variable [5], that converts the nonlineardifferential Riccati equation (DRE) to a linear differentialLyapunov equation [6], which can be solved in real time ateach time step [7]. Hardware in the loop Simulation (HILS)system usinga real DC motor is used tovalidate the theoreticalanalysis.The remainder of this paper is organized as follows: thenonlinear mathematical model of the DC motor is discussed inSection II. Section IIIpresents the nonlineartrackingtechniquevia finite-horizon SDRE. In Section IV, the HILS setup ispresented. Simulation results and experimental validation arediscussed in Section V. Finally, the conclusions of this paperare given in Section VI.II. M
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Experimental Validation for Real Time Control of
DC Motor Using Novel Finite-Horizon Optimal
Technique
Ahmed Khamis
Department of Electrical Engineering
Idaho State University
Pocatello, Idaho, USA
Email: khamahme@isu.edu
D. Subbaram Naidu
Department of Electrical Engineering
Idaho State University
Pocatello, Idaho, USA
Email: naiduds@isu.edu
Abstract—DC motors are widely used in industrial applica-
tions. Controlling of DC motor is a great challenge for control
engineers. Precise equations describing DC motors are nonlinear.
Accurate nonlinear control of the motion of the DC motors
is required. In this paper, a novel online technique for finite-
horizon nonlinear tracking problems is presented. The idea of
the proposed technique is the change of variables, that converts
the nonlinear differential Riccati equation to a linear Lyapunov
differential equation. The proposed technique is effective for a
wide range of operating points. Simulation results and experi-
mental implementation of a DC motor are given to illustrate the
effectiveness of the proposed technique.
I. INT RO DU CTION
The need to improve performance in control systems re-
quires more and more accurate modeling. However, if a model
is a good representation of the real system over a wide range
of operating points, it is most often nonlinear [1]. Traditional
technique to control, nonlinear systems is to linearize the
nonlinear system in a small region around the operating point
and then design linear controllers. These controllers with
constant gains can be expected to perform satisfactorily in the
neighborhood of the operating point. However, they may not
be capable of dealing with a situation over a large range of
operating points.
The competitive era of rapid technological change has
motivated the rapid development of nonlinear control theory
for application to challenging, complex dynamical real-world
problems [2]. There exist many nonlinear control design
techniques, each has benefits and weaknesses. Most of them
are limited in their range of applicability, and use of certain
nonlinear control technique for a specific system usually
demands choosing between different factors, e.g. performance,
robustness, optimality, and cost. Some of the well-known non-
linear control techniques are feedback linearization, adaptive
control, nonlinear predictive control, sliding mode control,
and approximating sequence of Riccati Equations. One of
the highly promising and rapidly emerging techniques for
nonlinear optimal controllers designing is the State Dependent
Riccati Equation (SDRE) technique [3]. The SDRE technique
can be used for regulation or tracking of infinite-horizon
nonlinear systems [4]. The SDRE-based techniques have very
important properties, such that applicability to a large class
of nonlinear systems, and the control designer opportunity to
make tradeoffs between the state errors and the control effort
[1].
In this paper, we address the problem of finite-horizon
position control of a permanent magnet DC motor based on the
nonlinear system dynamics. A novel technique for tracking of
finite-horizon nonlinear systems is utilized. This technique is
based on the change of variable [5], that converts the nonlinear
differential Riccati equation (DRE) to a linear differential
Lyapunov equation [6], which can be solved in real time at
each time step [7]. Hardware in the loop Simulation (HILS)
system using a real DC motor is used to validate the theoretical
analysis.
The remainder of this paper is organized as follows: the
nonlinear mathematical model of the DC motor is discussed in
Section II. Section III presents the nonlinear tracking technique
via finite-horizon SDRE. In Section IV, the HILS setup is
presented. Simulation results and experimental validation are
discussed in Section V. Finally, the conclusions of this paper
are given in Section VI.
II. MO DELIN G
The DC motor used in this paper is carbon-brush permanent
magnet 12v DC motor (see Fig. 1). The system model is shown
in Fig. 2, where Ris the armature resistance, Lis the armature
inductance, Vis the voltage applied to the motor, iis the
current through the motor, eis the back emf voltage, Jis
the moment of inertia of the load, Bis the viscous friction
coefficient,
τ
is the torque generated by the motor,
θ
is the
angular position of the motor,and
ω
is the angular velocity of
the motor.
The dynamic equations for the DC motor are:
ω
(t) = d
θ
(t)
dt ,(1)
V(t) = Ldi(t)
dt +Ri(t) + kw
ω
(t),(2)
kii(t) = Jd
ω
(t)
dt +B
ω
(t) +Csgn(
ω
),(3)
where kwis the back emf constant, kiis the torque constant of
the motor, and Cis the motor static friction.
978-1-4799-3668-7/14/$31.00 © 2014 IEEE
67
The 4th Annual IEEE International Conference on
Cyber Technology in Automation, Control and Intelligent Systems
June 4-7, 2014, Hong Kong, China
Fig. 1. Permanent Magnet DC Motor
Fig. 2. Permanent Magnet DC Motor System Model
The signum function sgn(
ω
)is defined as
sgn(
ω
) = (1 for
ω
<0
0 for
ω
=0
1 for
ω
>0
(4)
or it can be written in this form:
sgn(
ω
) = |
ω
|
ω
.(5)
The nonlinear state equations can be written in the form:
˙x1=x2,(6)
˙x2=B
JC
J|x2|x2+ki
Jx3,(7)
˙x3=kw
Lx2R
Lx3+1
Lu,(8)
y=x1,(9)
where:
θ
=x1,˙
θ
=x2,i=x3,V=u.
III. NON LI NE AR FINIT E-HO RIZ ON TR AC KI NG US ING
SDRE
Finite-horizon optimal control of nonlinear systems is a
challenging problem in the control field due to the complexity
of time-dependency of the Hamilton- Jacobi-Bellman (HJB)
differential equation. In finite-horizon optimal nonlinear con-
trol problem, the DRE can not be solved in real time by
backward integration from tfto t0because we do not know
the value of the states ahead of present time step. To overcome
the problem, an approximate analytical approach is used [8]
to convert the original nonlinear Ricatti equation to a linear
differential Lyapunov equation that can be solved in closed
format each time step.
A. Problem Formulation
The nonlinear system considered in this paper is assumed
to be in the form:
˙
x(t) = f(x) + g(x)u(t),(10)
y(t) = h(x).(11)
That nonlinear system can be expressed in a state-dependent
like linear form, as:
˙
x(t) = A(x)x(t) + B(x)u(t),(12)
y(t) = C(x)x(t),(13)
where f(x) = A(x)x(t),B(x) = g(x),h(x) = C(x)x(t).
Let z(t)be the desired output. The goal is to find a state
feedback control law that minimizes a cost function given by
[9]:
J(x,u) = 1
2e0(tf)Fe(tf)(14)
+1
2Ztf
t0e0(t)Q(x)e(t) + u0(x)R(x)u(x)dt,
where e(t) = z(t)y(t),Q(x)and Fare symmetric positive
semi-definite matrices, and R(x)is a symmetric positive def-
inite matrix. Moreover, x0(t)Q(x)x(t)is a measure of control
accuracy and u0(x)R(x)u(x)is a measure of control effort [1].
B. Solution for Finite-Horizon SDRE Tracking
To minimize the above cost function (14), a feedback
control law can be given as
u(x) = R1(x)B0(x)[P(x)x(t)g(x)],(15)
where P(x)is a symmetric, positive-definite solution of the
state-dependent Differential Riccati Equation (SDDRE) of the
form
˙
P(x) = P(x)A(x) + A0(x)P(x)(16)
P(x)B(x)R1B0(x)P(x) + C0(x)Q(x)C(x),
with the final condition
P(x,tf) = C0(tf)FC(tf).(17)
The resulting SDRE-controlled trajectory becomes the solution
of the state-dependent closed-loop dynamics
˙x(t) = [A(x)B(x)R1(x)B0(x)P(x)]x(t)(18)
+B(x)R1(x)B0(x)g(x),
where g(x)is a solution of the state-dependent non-
homogeneous vector differential equation
˙g(x) = [A(x)B(x)R1(x)B0(x)P(x)]0g(x)(19)
C0(x)Q(x)z(x),
with the final condition
g(x,tf) = C0(tf)Fz(tf).(20)
As the SDRE function of (x,t), we do not know the value of
the states ahead of present time step. Consequently, the state
dependent coefficients cannot be calculated to solve (16) with
the final condition (17) by backward integration from tfto t0.
To overcome this problem, an approximate analytical approach
68
is used [5], [6], [7]. which converts the original nonlinear
Ricatti equation to a linear differential Lyapunov equation.
At each time step, the Lyapunov equation can be solved in
closed form. In order to solve the DRE (16), and the non-
homogeneous differential equation (19), one can follow the
following steps:
Solve Algebraic Riccati Equation (ARE) to calculate
the steady state value Pss (x)
Pss(x)A(x) + A0(x)Pss (x)
Pss(x)B(x)R1(x)B0(x)Pss(x) + Q(x) = 0.(21)
Use change of variables technique and assume that
K(x,t) = [P(x,t)Pss(x)]1.
Calculate the value of Acl (x)as
Acl(x) = A(x)B(x)R1B0(x)Pss (x).
Calculate the value of Dby solving the algebraic
Lyapunov equation [10]
AclD+DA0
cl BR1B0=0.(22)
Solve the differential Lyapunov equation
˙
K(x,t) = K(x,t)A0
cl(x) + Acl(x)K(x,t)(23)
B(x)R1(x)B0(x).
The solution of (23) is given by [11]
K(t) = eAcl(ttf)(K(x,tf)D)eAcl0(ttf)+D.(24)
Calculate the value of P(x,t)from the equation
P(x,t) = K1(x,t) + Pss(x).(25)
Calculate the steady state value gss(x)from the equa-
tion
gss(x) = [A(x)B(x)R1(x)B0(x)Pss(x)]0−1(26)
C0(x)Q(x)z(x).
Use change of variables technique and assume that
Kg(x,t) = [g(x,t)gss(x)].
Solve the differential equation
Kg(t) = e(ABR1B0P)0(ttf)[g(x,tf)gss(x)].(27)
Calculate the value of g(x,t)from the equation
g(x,t) = Kg(x,t) + gss(x).(28)
Calculate the value of the optimal control u(x,t)as
u(x,t) = R1(x)B0(x)[P(x,t)x(t)g(x,t)].(29)
Fig. 3 summarized the overview of the process of finite-
horizon SDDRE tracking technique
Note : It is easily seen that this technique with finite-horizon
SDDRE can be used for linear systems and the resulting
SDDRE becomes the standard DRE [9].
Fig. 3. Overview of The Process of Finite-Horizon SDDRE Tracking
IV. HARDWAR E INTHE LOOP SI M UL ATIO N
Traditional software-based simulation has the disadvantage
of being unable to accurately imitate real operational envi-
ronment. One way to bridge the gap between simulation and
real conditions is the HILS. Real-time HILS replaces some
simulation models of a system by one or several real hardware
subsystems, that interacts with the computer models. This
increases the realism of the simulation and provides access
to some features not accessible in software-only simulation
models [12].
The basic principle of HILS is that some subsystems are
physically embedded within a real-time simulation model.
Real-time means the simulation of each component performed
such that input and output signals show the same time depen-
dent values as in real world dynamic operation. In HILS, the
embedded system is fooled into thinking that it is operating
with real world inputs and outputs, in real-time. A computer
software with real-time simulation capabilities and communi-
cation abilities is necessary to perform HILS [13].
This section describes the HILS setup and the implemen-
tation of the finite-horizon SDRE designed in Section III to
control the DC motor given its mathematical model in Section
II.
A. Hardware In The Loop Simulation Setup
The experimental setup used in this paper, as shown in Fig.
4, consists of a Hilink microcontroller board manufactured by
Zeltom Educational and Industrial Control System Company
[14], a corresponding Simulink library for Matlab/Simulink,
DC motor with encoder, and current sensor. The real-time
control board is based on a dsPIC30F2012 digital signal
controller. It has a total number of 8x16 bit inputs and 8x16
69
Fig. 4. Hardware In The Loop Experimental Setup
bit outputs capability. The board is interfaced to the main
computer that runs Matlab through a serial port. The HILINK
platform offers a seamless interface between physical plants
and Matlab/Simulink for implementation of hardware-in-the-
loop real-time control systems and it is fully integrated into
Matlab/Simulink.
V. SIMULATIO N AN D EXPERIME NTAL RESULTS
In this section simulation results in Matlab/Simulink are
presented and compared with the real-time hardware exper-
imental results via experimental setup illustrated in Section
IV. Thus, the verification of our proposed method is more
effective.
A. Simulation Results
The system nonlinear state equations of the DC motor (6-8)
can be rewritten in state dependent form:
"˙x1
˙x2
˙x3#=
0 1 0
0B
JC
J|x2|ki
J
0kw
LR
L
"x1
x2
x3#+
0
0
1
L
u,(30)
where:
θ
=x1,˙
θ
=x2,i=x3,V=u.
Let the reference angle is
z(t) = 90o,(31)
and the selected weighted matrices are
Q=diag(60,0,0),R=0.7,F=diag(1,1,1).(32)
The simulations are performed for final time of 10 seconds
and the resulting angle trajectories is shown in Fig. 5, where
the dash-dot line denotes the reference angle trajectory, and the
solid line denotes the actual trajectory. The optimal control is
shown in Fig. 6.
Comparing these trajectories in Fig. 5, it’s clear that the
propose algorithm gives very good results as the actual optimal
angle is making a very good tracking to the reference angle
with standard deviation error of 0.02o, which is acceptable in
that system.
012345678910
−10
0
10
20
30
40
50
60
70
80
90
100
Time [sec]
Optimal Angles [deg]
Actual Angle
Desired Angle
Fig. 5. Optimal Position Tracking for The Simulated DC Motor
012345678910
−25
−20
−15
−10
−5
0
5
10
15
20
25
Time [sec]
Optimal Control [V]
Fig. 6. Optimal Control Voltage for The Simulated DC Motor
B. Experimental Results
Once the algorithms have been developed and tested in
software, the next step is to bridge the gap between software
simulation and real world applications. Here, the method of
HILS is applied by using the experimental setup introduced in
Section IV.
A schematic diagram of the experimental setup is given
in Fig. 7. The experiment is performed for final time of 10
seconds and the resulting angle trajectories is shown in Fig. 8,
where the dash-dot line denotes the reference angle trajectory,
and the solid line denotes the actual trajectory. Comparing
these trajectories in Fig. 8, it’s clear that the propose algorithm
gives very good results as the actual optimal angle is making
a very good tracking to the reference angle with standard
deviation error of 0.025o.
For further demonstration, the performance of the DC
motor with the finite-horizon SDRE controller to track a multi
step reference is shown in Fig 9. It can be seen that the
controller gives almost the same performance regardless the
value of the step reference.
70
Fig. 7. Hardware In The Loop Simulation Schematic Diagram
Fig. 8. Optimal Position Tracking for The HILS System
VI. CON CL US IO N
The paper presented a new finite-horizon tracking tech-
nique for nonlinear systems. This technique based on change
of variables that converts the nonlinear differential Riccati
equation to a linear Lyapunov equation.The Lyapunov equation
is solved in a closed form at the given time step. The consis-
tency of the experimental results with the simulation results
demonstrates the effectiveness of the developed technique.
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Fig. 9. Multi Step Position Reference Tracking With The Finite-Horizon
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... A backward trajectory can be used for backward integration in time with a controller to provide the state information [64,100]. Numerous usages of the SDRE tracking controller considering the differential equations reveal the popularity of this method [101][102][103][104][105][106][107][108][109][110][111][112][113]. ...
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In the process of designing digital controls for power electronics, hardware-in-the-loop testing is increasingly recognized as an effective approach. A high performance simulation environment is necessary to obtain high-fidelity results. Many experiments have been proposed in the literature already. However in most cases, the environments are single platform, single solver, or non real-time. In this paper, we present the implementation and testing procedure of the real-time virtual test bed, a multi-platform, multi-solver, hard real-time simulation environment for power electronics controls. By using the signal extension resistive companion solver, RTVTB enables natural coupling between hardware and software, and thus makes power exchange with closed-loop hardware possible. As an application example, a boost converter state space feedback control system is studied and analyzed.