Capitolo 0. INTRODUCTION 1.1
Mathematical models of physical systems
Modeling a physical system is always a compromise between the simplicity of the model and the accuracy of the model.
Example. Let us consider the following electric network:
Iu V
I
R C L
IR I1
IC
Vu
The system dynamics can be described using the following block scheme:
Iu
V
- -
1 R
6
6
IR
I1 -
1 Cs
?
?
V
-
IC
-
1 Ls
6
6 -
I
Vu VL
The dynamic model of this system can be obtained by choosing a state variable for each dynamic element of the system, i.e. for each physical element which stores energy. In this case Q is the electric charge and Φ is the magnetic flux:
Q = C V, Φ = LI
The dynamic equations of the system are:
dQ
dt = C ˙V = Iu − I − V R dΦ
dt = L ˙I = VL = V − Vu
Zanasi Roberto - System Theory. A.A. 2015/2016
Capitolo 1. SYSTEM THEORY 1.2
Let x denote the state vector:
x = V I
→ ˙x =
V˙
˙I
, u = Iu Vu
The dynamic equations of the system can be written as follows:
˙x(t) =
"
− 1
CR −1
C 1
L 0
#
x(t) +
1
C 0
0 −L1
u(t) y(t) =
1 0 x(t)
In this case the system is linear and time-invariant.
Let us suppose that we want to take into account in the model of the fact that the resistance varies with the temperature: R = R(θ). In this case we have to add to the mathematical description of the system the following differential equation which describes the thermal dynamics of the resistance:
d[cθ]
dt = V2
R −G(θ − θe) → ˙θ = V2
cR − G
c θ + G c θe The parameters have the following meaning:
cθ heat stored in the resistance θ resistance temperature
θe external temperature
c thermal capacitance of the resistance G thermal conductance of the resistance
The nonlinear dynamic equations of the system are now the following:
V˙ = Iu
C − V
C R(θ) − I C
˙I = V
L − Vu
L
˙θ = V2
c R(θ) − G
c θ+ G cθe
Denoting with x and u the stator an output vectors:
x =
V
I θ
, → u =
Iu
Vu
θe
the system can be described in compact form as follows:
˙x = f(x, u)
Zanasi Roberto - System Theory. A.A. 2015/2016
Capitolo 1. SYSTEM THEORY 1.3
Matlab file “Second Order Step Trajectory.m”:
--- Volt=1; Amp=1; Ohm=Volt/Amp; sec = 1;
Henry=Volt*sec/Amp; Farad=1;
C=0.01*Farad;
L=0.01*Henry;
R=1*Ohm;
A=[-1/(C*R) -1/(C); 1/L 0];
B=[-1/(C*R) 0 ; 0 -1/L];
C=[ 1 0]; D=0;
Sys=ss(A,B(:,1),C,D);
pole(Sys)
[yt,t,x]=step(Sys);
%%%%
figure(1); clf
plot(x(:,1),x(:,2),’Linewidth’,1.5); % Plot hold on; axis square; grid on;
xlabel(’Voltage V’) % Label along axis x ylabel(’Current I’) % Label along axis y title(’State Space trajectories’) % Title
%%%%
figure(2); clf
plot(t,x(:,1),’Linewidth’,1.5); % Plot grid on;
xlabel(’Time [s]’) % Label along axis x
ylabel(’Voltage V’) % Label along axis y title(’Voltage step response’) % Title
%%%%
figure(3); clf
plot(t,x(:,2),’Linewidth’,1.5); % Plot grid on;
xlabel(’Time [s]’) % Label along axis x
ylabel(’Current I’) % Label along axis y title(’Current step response’) % Title
---
−0.10 0 0.1 0.2 0.3 0.4 0.5 0.6
0.2 0.4 0.6 0.8 1 1.2 1.4
Voltage V
Current I
State Space trajectories
Zanasi Roberto - System Theory. A.A. 2015/2016
Capitolo 1. SYSTEM THEORY 1.4
Step response Iu = 1 A, Vu = 0 V: Voltage V
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
−0.1 0 0.1 0.2 0.3 0.4 0.5 0.6
Time [s]
Voltage V
Voltage step response
Step response Iu = 1 A, Vu = 0 V: Current I
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Time [s]
Current I
Current step response
Zanasi Roberto - System Theory. A.A. 2015/2016
