chgFreqUnit

Change frequency units of frequency-response data model

Syntax

sys_new = chgFreqUnit(sys,newfrequnits)

Description

sys_new = chgFreqUnit(sys,newfrequnits) changes units of the frequency points in sys to newfrequnits. Both Frequency and FrequencyUnit properties of sys adjust so that the frequency responses of sys and sys_new match.

Input Arguments

sys

Frequency-response data (frd (Control System Toolbox), idfrd, or genfrd (Control System Toolbox)) model.

newfrequnits

New units of frequency points, specified as one of the following values:

'rad/TimeUnit'
'cycles/TimeUnit'
'rad/s'
'Hz'
'kHz'
'MHz'
'GHz'
'rpm'

rad/TimeUnit and cycles/TimeUnit express frequency units relative to the system time units specified in the TimeUnit property.

Default: 'rad/TimeUnit'

Output Arguments

sys_new

Frequency-response data model of the same type as sys with new units of frequency points. The frequency response of sys_new is same as sys.

Examples

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Change Frequency Units of Frequency-Response Data Model

This example uses:

Control System Toolbox Control System Toolbox

Open Live Script

Create a frequency-response data model.

load('AnalyzerData');
sys = frd(resp,freq);

The data file AnalyzerData has column vectors freq and resp. These vectors contain 256 test frequencies and corresponding complex-valued frequency response points, respectively. The default frequency units of sys is rad/TimeUnit, where TimeUnit is the system time units.

Change the frequency units.

sys1 = chgFreqUnit(sys,'rpm');

The FrequencyUnit property of sys1 is rpm.

Compare the Bode responses of sys and sys1.

bodeplot(sys,'r',sys1,'y--');
legend('sys','sys1')

Figure contains 2 axes objects. Axes object 1 with ylabel Magnitude (dB) contains 2 objects of type line. These objects represent sys, sys1. Axes object 2 with ylabel Phase (deg) contains 2 objects of type line. These objects represent sys, sys1.

The magnitude and phase of sys and sys1 match because chgFreqUnit command changes the units of frequency points in sys without modifying system behavior.

Change the FrequencyUnit property of sys to compare the Bode response with the original system.

sys2 = sys;
sys2.FrequencyUnit = 'rpm';
bodeplot(sys,'r',sys2,'gx'); 
legend('sys','sys2');

Figure contains 2 axes objects. Axes object 1 with ylabel Magnitude (dB) contains 2 objects of type line. One or more of the lines displays its values using only markers These objects represent sys, sys2. Axes object 2 with ylabel Phase (deg) contains 2 objects of type line. One or more of the lines displays its values using only markers These objects represent sys, sys2.

Changing the FrequencyUnit property changes the system behavior. Therefore, the Bode responses of sys and sys2 do not match. For example, the original corner frequency at about 2 rad/s changes to approximately 2 rpm (or 0.2 rad/s).