Process models usually exhibit some form of non-linearity due to the multiplication of an extensive flowrate and an intensive thermodynamic state variable, chemical kinetics, or various types of transport phenomenon. Near a steady-state, however, an approximate linear models often provide a 'good-enough' dynamical model for control design and analysis. Here we show how a linear approximation is constructed using a Taylor series expansion.
As we will learn later in this course, linear models are amenable to control design and analysis. For process systems that will be operated near a steady-state, linear models provide a useful framework for control design.
We start with a process model that consists of a single first-order ordinary differential equation of the form
$$\frac{dh}{dt}=f(h,q)$$where $h$ is the state variable and $q$ is an input variable. Choose a nominal value for the process input $q$, we'll call this nominal value $\bar{q}$. The following procedure will produce a linear approximate valid near the steady-state.
Find the steady-state value of $h$ by solving
$$0=f(\bar{h},\bar{q})$$where $\bar{q}$ is a nominal (i.e, typical, or desired value) of a manipulated process variable.
Compute the first terms in the Taylor series and evaluate at steady-state. The higher-order terms are not needed provided the lower-order terms are non-zero and the deviations are small. $$ f(\bar{h}+x,\bar{q}+u)\approx f(\bar{h},\bar{q})+\left.\frac{\partial f}{\partial h}\right|_{\bar{h},\bar{q}}x+\left.\frac{\partial f}{\partial q}\right|_{\bar{h},\bar{q}}u+\cdots $$
The linear approximation is $$ \frac{dx}{dt}=\underbrace{\left.\frac{\partial f}{\partial h}\right|_{\bar{h},\bar{q}}}_{a}x+\underbrace{\left.\frac{\partial f}{\partial q}\right|_{\bar{h},\bar{q}}}_{b}u $$
A simple model for the liquid height in a gravity-drained tank with cross-sectional area $A$ is
$$A\frac{dh}{dt}=q_{in}-C\sqrt{h}$$where $q_{in}$ is a volumetric inflow and $C$ is a constant associated with the drain. This is a non-linear process model that can be written
$$\frac{dh}{dt}=f(h,q_{in})$$where
$$f(h,q_{in})=\frac{1}{A}\left(q_{in}-C\sqrt{h}\right)$$Given a nominal inlet flowrate $\bar{q}_{in}$, the steady state value of $h$, that is $\bar{h}$, is found by solving the steady state equation
$$0=f(\bar{h},\bar{q}_{in})=\frac{1}{A}\left(\bar{q}_{in}-C\sqrt{\bar{h}}\right)$$which gives
$$\bar{h}=\frac{\bar{q}_{in}^{2}}{C^{2}}$$It's interesting to note the steady-state height of the liquid in a gravity-drained tank is proportional to the square of the nominal flowrate. A 50\% increase in flowrate more than doubles the liquid height.
Let $x$ and $u$ represent the deviations from steady-state
\begin{eqnarray*} x & = & h-\bar{h}\\ u & = & q-\bar{q} \end{eqnarray*}Then
\begin{eqnarray*} \frac{d(\bar{h}+x)}{dt} & = & \frac{1}{A}\left(\bar{q}_{in}+u-C\sqrt{\bar{h}+x}\right) \end{eqnarray*}The Taylor series expansion
$$f(\bar{h}+x,\bar{q}_{in}+u)\approx f(\bar{h},\bar{q}_{in})+\left.\frac{\partial f}{\partial h}\right|_{\bar{h},\bar{q}}x+\left.\frac{\partial f}{\partial q}\right|_{\bar{h},\bar{q}}u+\frac{1}{2}\left.\frac{\partial^{2}f}{\partial h^{2}}\right|_{\bar{h}}x^{2}\cdots$$For this example
$$\frac{dx}{dt}=\left(-\frac{C}{2A\sqrt{\bar{h}}}\right)x+\left(\frac{1}{A}\right)u$$An alternative form of the model is found by substituting the solution for $\bar{h}$. While these have somewhat different analytical expressions for a given application they will yield identical numerical results.
$$\frac{dx}{dt}=\left(-\frac{C^{2}}{2A\bar{q}_{in}}\right)x+\left(\frac{1}{A}\right)u$$How well does this approximation work?
This question can be answered by comparing the results of two simulations. In the first case the simulation consists of integrating
$$A\frac{dh}{dt}=q_{in}-C\sqrt{h}$$as shown in the graph below.
where $A=1$, $C=2$, initial condition $h(0)=0$, and a constant input $q_{in}=\bar{q}_{in}=1$. For these parameter values, the approximate linear model for the deviation from steady-state is given by
\begin{eqnarray*} \frac{dx}{dt} & = & \left(-\frac{C^{2}}{2A\bar{q}_{in}}\right)x+\left(\frac{1}{A}\right)u\\ \\ & = & -2\,x+u \end{eqnarray*}In terms of deviations from steady state, the input $u=q_{in}-\bar{q}_{in}=0$ and the initial conditionis $x(0)=h(0)-\bar{h}=-\bar{h}$. Plotting $h(t)$ and $x(t)+\bar{h}$ on the same axis produces the results shown in Figure \ref{fig:LinearApproximation}.
#Simulation of a Gravity-Drained Tank
%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np
from scipy.integrate import odeint
# parameter values
qin = 1
A = 1
C = 2
# steady state
hbar = (qin/C)**2
# nonlinear simulation
def hdot(h,t):
return (qin - C*np.sqrt(h))/A
t = np.linspace(0,5)
h = odeint(hdot,[0],t)
# linear approximation
a = -C**2/2/A/qin
b = 1/A
def xdot(x,t):
return a*x + b*u
u = 0
x = odeint(xdot,[-hbar],t)
# visualization
plt.plot(t,h)
plt.plot(t,x+hbar)
plt.legend(['Nonlinear','Linear Approximation'],loc='lower right')
plt.xlabel('Time [min]')
plt.ylabel('Height [m]')
plt.title('Gravity-drained tank: Nonlinear model vs. linear approximation')
plt.grid()
1. Suppose you have a tank with an cross-sectional area of 1 square meter, a steady inlet flow $\bar{q}_{in}=10$ liters/min, and observe a liquid height of 0.5 meters. What is the constant $C$? What is the characteristic time constant for the tank?
2. You have an elementary reaction
$$2{\cal A}\longrightarrow Products$$carried out under isothermal conditions in a stirred tank of volume $V$. The reaction rate is given by the expression
$$R_{A}=k_{A}C_{A}^{2}$$and the inlet concentration to the tank is $C_{A,in}$ at flowrate $q$. Construct a linear approximation for the dynamics of this process in the neighborhood of a steady state.