How to change the default x-axis unit in a Bode diagram to Hertz?

Rad/s is nice for mechanical people, but I'm an EE, and I much prefer Hertz. It should be something simple, but I can't find it in the help.

[dpb]

Don't believe there is a way in the base routine; it's built entirely around rad/timeunits per the system object.

You could write a wrapper routine to convert frequency units and update the plot labels.

Answer 1

[Star Strider]

Use bodeplot (link) instead of bode.

It gives you that option, and the documentation specifically mentions that.

[marcel hendrix]

Thanks, this is exactly what I needed -- no fiddling with options or handles.

[Star Strider]

My pleasure.

The bodeplot function is useful, since bode does not allow easy adjustment of its properties.

A simple work-around is probably to get the outputs of bode and plot them. Since rad/s=2*pi*f (with ‘f’ in Hz), it is probably easiest to simply divide the ‘wout’ output by (2*pi):

Fs = 1000;                                                  
H = tf([-0.1,-2.4,-181,-1950],[1,3.3,990,2600]);
[mag,phase,wout] = bode(H);
wrange = [min(wout); max(wout)];                            % Units: rad/sec
frange = wrange/(2*pi);                                     % Units: Hz

figure
subplot(2,1,1)
plot(wout, squeeze(mag))
grid
subplot(2,1,2)
plot(wout, squeeze(phase))
grid
xlabel('Frequency (rad/s)')

figure
subplot(2,1,1)
plot(wout/(2*pi), squeeze(mag))
grid
subplot(2,1,2)
plot(wout/(2*pi), squeeze(phase))
grid
xlabel('Frequency (Hz)')

The two range vectors are not necessary for the code. I used them to check to be certain everything looked correct.

[marcel hendrix]

I've put the below function in my userpath . It'll do for now.

   % a new bode() command that has Hz as default
   function h = bodef(x)
   P = bodeoptions; P.FreqUnits = 'Hz';
   h = bodeplot(x,P);

[Star Strider]

That is what I would do.

I defer to bodeplot because it allows some customization. I use and plot the bode outputs only if I want other options.

EDIT —

I add that this was my original recommendation!

Answer 2

[Dimitris Kalogiros]

clc;close all; clc

% test system 
s=tf('s');
H=(s-1)/((s-3)*(s-2))

% bode
[mag,phase,wout] = bode(H);

%plot results, with frequency expressed at Hz
figure;
subplot(2,1,1);
semilogx(wout(:,1)/(2*pi), 20*log10(squeeze(mag)), '-b'); zoom on; grid on; 
title('magnitude'); xlabel('Frequency (Hz)'); ylabel('Magnitude (dB)');
subplot(2,1,2);
semilogx(wout(:,1)/(2*pi), squeeze(phase), '-r'); zoom on; grid on; 
title('Phase'); xlabel('Frequecy (Hz)'); ylabel('Phase (deg)');

[Charl]

This is the correct answer. Some of the others simply change the label to Hz without rescaling.

Answer 3

[Tjeerd Ickenroth]

Type 'ltiview' in your command window. The Linear System Analyzer will pop up. Click on: File --> Toolbox Preferences... --> Units --> Frequency: Hz

[marcel hendrix]

Then I must interact with the GUI for every plot. Ideally, everything is explicitly programmable.

[Tjeerd Ickenroth]

You need to change it once in the GUI and you always obtain bode plots in Hz. The setting remains even when you restart Matlab.

[marcel hendrix]

And what when I share my function/script with others? Or use different (older) versions of MATLAB?