Convert model from continuous to discrete time
sysd = c2d(sys,Ts)
sysd = c2d(sys,Ts,method)
sysd = c2d(sys,Ts,opts)
[sysd,G]
= c2d(sys,Ts,method)
[sysd,G]
= c2d(sys,Ts,opts)
discretizes
the continuoustime dynamic
system model sysd
= c2d(sys
,Ts
)sys
using zeroorder
hold on the inputs and a sample time of Ts
seconds.
discretizes sysd
= c2d(sys
,Ts
,method
)sys
using
the specified discretization method method
.
discretizes sysd
= c2d(sys
,Ts
,opts
)sys
using
the option set opts
, specified using the c2dOptions
command.
[
returns
a matrix, sysd
,G
]
= c2d(sys
,Ts
,method
)G
that maps the continuous initial conditions x_{0} and u_{0} of
the statespace model sys
to the discretetime
initial state vector x [0]. method
is
optional. To specify additional discretization options, use [
.sysd
,G
]
= c2d(sys
,Ts
,opts
)

Continuoustime dynamic
system model (except frequency response data models).
The following identified linear systems cannot be discretized directly:
For the syntax 

Sample time. 

String specifying a discretization method:
For more information about discretization methods, see ContinuousDiscrete Conversion Methods. 

Discretization options. Create 

Discretetime model of the same type as the input system When


Matrix relating continuoustime initial conditions x_{0} and u_{0} of
the statespace model $$x\left[\text{\hspace{0.05em}}0\right]=G\cdot \left[\begin{array}{c}{x}_{0}\\ {u}_{0}\end{array}\right]$$ For statespace models
with time delays, 
Discretize the following continuoustime transfer function:
This system has an input delay of 0.3 s. Discretize the system using the triangle (firstorderhold) approximation with sample time Ts
= 0.1 s.
H = tf([1 1],[1 4 5],'InputDelay', 0.3); Hd = c2d(H,0.1,'foh');
Compare the step responses of the continuoustime and discretized systems.
step(H,'',Hd,'')
Discretize the following delayed transfer function using zeroorder hold on the input, and a 10Hz sampling rate.
h = tf(10,[1 3 10],'IODelay',0.25);
hd = c2d(h,0.1)
hd = 0.01187 z^2 + 0.06408 z + 0.009721 z^(3) *  z^2  1.655 z + 0.7408 Sample time: 0.1 seconds Discretetime transfer function.
In this example, the discretized model hd
has a delay of three sampling periods. The discretization algorithm absorbs the residual halfperiod delay into the coefficients of hd
.
Compare the step responses of the continuoustime and discretized models.
step(h,'',hd,'')
Discretize a statespace model with time delay, using a Thiran filter to model fractional delays:
sys = ss(tf([1, 2], [1, 4, 2])); % create a statespace model sys.InputDelay = 2.7 % add input delay
a = x1 x2 x1 4 2 x2 1 0 b = u1 x1 2 x2 0 c = x1 x2 y1 0.5 1 d = u1 y1 0 Input delays (listed by channel): 2.7 Continuoustime model.
Use c2dOptions
to create a set of discretization
options, and discretize the model. This example uses the Tustin discretization
method.
opt = c2dOptions('Method','tustin','FractDelayApproxOrder',3); sysd1 = c2d(sys,1,opt) % 1s sample time
a = x1 x2 x3 x4 x5 x1 0.4286 0.5714 0.00265 0.06954 2.286 x2 0.2857 0.7143 0.001325 0.03477 1.143 x3 0 0 0.2432 0.1449 0.1153 x4 0 0 0.25 0 0 x5 0 0 0 0.125 0 b = u1 x1 0.002058 x2 0.001029 x3 8 x4 0 x5 0 c = x1 x2 x3 x4 x5 y1 0.2857 0.7143 0.001325 0.03477 1.143 d = u1 y1 0.001029 Sample time: 1 Discretetime model.
x3
, x4
,
and x5
corresponding to a thirdorder Thiran filter.
Since the time delay divided by the sample time is 2.7, the thirdorder
Thiran filter (FractDelayApproxOrder
= 3) can approximate
the entire time delay. Discretize an identified, continuoustime transfer function and compare its performance against a directly estimated discretetime model
Estimate a continuoustime transfer function and discretize it.
load iddata1 sys1c = tfest(z1,2); sys1d = c2d(sys1c,0.1,'zoh');
Estimate a second order discretetime transfer function.
sys2d = tfest(z1,2,'Ts',0.1);
Compare the two models.
compare(z1,sys1d,sys2d)
The two systems are virtually identical.
Discretize an identified statespace model to build a onestep ahead predictor of its response.
load iddata2 sysc = ssest(z2,4); sysd = c2d(sysc,0.1,'zoh'); [A,B,C,D,K] = idssdata(sysd); Predictor = ss(AK*C,[K BK*D],C,[0 D],0.1);
The Predictor
is a two input model which
uses the measured output and input signals ([z1.y z1.u])
to
compute the 1step
predicted response of sysc
.
c2dOptions
 d2c
 d2d
 thiran
 translatecov