Bode

I am new to Matlab. I need to use the Bode command. When I run my code, I get this error: "Undefined function or method 'bode' for input arguments of type 'double'."

[Ruben Ubeda]

i have the same problem!

Answer 1

[Oleg Komarov]

You may not have the Control System Toolbox. To check that type ver in the command window.

Answer 2

[Paulo Silva]

Besides Oleg suggestion, bode doesn't take double arguments, you must provide one transfer function instead.

%simple example from the bode function documentation
g = tf([1 0.1 7.5],[1 0.12 9 0 0]);
bode(g)

Reference: bode function

Here's a way to do the bode plot without the control toolbox:

sys=@(s) (s.^2+0.1*s+7.5)./(s.^4+0.12.*s.^3+9*s.^2)
w=1:0.01:40;
amp=10*log10(abs(sys(w*i)));
ang=angle(sys(w*i))*180/pi;
subplot(211)
plot(w,amp)
title(char(sys))
ylabel('Magnitude (DB)')
set(gca,'xscale','log')
subplot(212)
plot(w,ang)
set(gca,'xscale','log')
xlabel('frequency (rad/sec)')
ylabel('phase (deg)')

[Seng Tak Goh]

Hi Paulo,

That was amazing! Do you mind to explain how sys=@(s) actually does the trick?

Many thanks in advance!

[Hamed Pouria]

It has some promlems.

in line 3 you must write :

amp=20*log10(abs(sys(w*i)));

and also if the number of poles - number of zeros of the system will be bigger than 2 (for example 3) it does not work correctly because of the strict of the function angle(); It will return the phase angles only within the interval [-pi to pi].

Answer 3

[Diego Aguilera]

I'm guessing it's not installed...

-------------------------------------------------------------------------------------
MATLAB Version 7.11.0.584 (R2010b)
MATLAB License Number: ###########[SCd Censor]
Operating System: Mac OS X  Version: 10.6.7 Build: 10J869 
Java VM Version: Java 1.6.0_24-b07-334-10M3326 with Apple Inc. Java HotSpot(TM) 64-Bit Server VM mixed mode
-------------------------------------------------------------------------------------
MATLAB                                                Version 7.11       (R2010b)
Simulink                                              Version 7.6        (R2010b)
Curve Fitting Toolbox                                 Version 3.0        (R2010b)
SimPowerSystems                                       Version 5.3        (R2010b)

[Oleg Komarov]

No guess here.

[Diego Aguilera]

Ha. I'm upgrading to the newest version. I believe the it has the Control System Toolbox. I'll find out in a few hours. Thanks.