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tf

Create transfer function model, convert to transfer function model

Syntax

sys = tf(num,den)
sys = tf(num,den,Ts)
sys = tf(M)
sys = tf(num,den,ltisys)
tfsys = tf(sys)
tfsys = tf(sys, 'measured')
tfsys = tf(sys, 'noise')
tfsys = tf(sys, 'augmented')

Description

Use tf to create real- or complex-valued transfer function models (TF objects) or to convert state-space or zero-pole-gain models to transfer function form. You can also use tf to create generalized state-space (genss) models or uncertain state-space (uss) models.

Creation of Transfer Functions

sys = tf(num,den) creates a continuous-time transfer function with numerator(s) and denominator(s) specified by num and den. The output sys is:

  • A tf model object, when num and den are numeric arrays.

  • A generalized state-space model (genss) when num or den include tunable parameters, such as realp parameters or generalized matrices (genmat).

  • An uncertain state-space model (uss) when num or den are uncertain (requires Robust Control Toolbox™ software).

In the SISO case, num and den are the real- or complex-valued row vectors of numerator and denominator coefficients ordered in descending powers of s. These two vectors need not have equal length and the transfer function need not be proper. For example, h = tf([1 0],1) specifies the pure derivative h(s) = s.

To create MIMO transfer functions, using one of the following approaches:

  • Concatenate SISO tf models.

  • Use the tf command with cell array arguments. In this case, num and den are cell arrays of row vectors with as many rows as outputs and as many columns as inputs. The row vectors num{i,j} and den{i,j} specify the numerator and denominator of the transfer function from input j to output i.

For examples of creating MIMO transfer functions, see Examples and MIMO Transfer Function Model in the Control System Toolbox™ User Guide.

If all SISO entries of a MIMO transfer function have the same denominator, you can set den to the row vector representation of this common denominator. See "Examples" for more details.

sys = tf(num,den,Ts) creates a discrete-time transfer function with sample time Ts (in seconds). Set Ts = -1 to leave the sample time unspecified. The input arguments num and den are as in the continuous-time case and must list the numerator and denominator coefficients in descending powers of z.

sys = tf(M) creates a static gain M (scalar or matrix).

sys = tf(num,den,ltisys) creates a transfer function with properties inherited from the dynamic system model ltisys (including the sample time).

There are several ways to create arrays of transfer functions. To create arrays of SISO or MIMO TF models, either specify the numerator and denominator of each SISO entry using multidimensional cell arrays, or use a for loop to successively assign each TF model in the array. See Model Arrays in the Control System Toolbox User Guide for more information.

Any of the previous syntaxes can be followed by property name/property value pairs

'Property',Value

Each pair specifies a particular property of the model, for example, the input names or the transfer function variable. For information about the properties of tf objects, see Properties. Note that

sys = tf(num,den,'Property1',Value1,...,'PropertyN',ValueN)

is a shortcut for

sys = tf(num,den)
set(sys,'Property1',Value1,...,'PropertyN',ValueN)

Transfer Functions as Rational Expressions in s or z

You can also use real- or complex-valued rational expressions to create a TF model. To do so, first type either:

  • s = tf('s') to specify a TF model using a rational function in the Laplace variable, s.

  • z = tf('z',Ts) to specify a TF model with sample time Ts using a rational function in the discrete-time variable, z.

Once you specify either of these variables, you can specify TF models directly as rational expressions in the variable s or z by entering your transfer function as a rational expression in either s or z.

Conversion to Transfer Function

tfsys = tf(sys) converts the dynamic system model sys to transfer function form. The output tfsys is a tf model object representing sys expressed as a transfer function.

If sys is a model with tunable components, such as a genss, genmat, ltiblock.tf, or ltiblock.ss model, the resulting transfer function tfsys takes the current values of the tunable components.

Conversion of Identified Models

An identified model is represented by an input-output equation of the form y(t) = Gu(t) + He(t), where u(t) is the set of measured input channels and e(t) represents the noise channels. If Λ = LL' represents the covariance of noise e(t), this equation can also be written as: y(t) = Gu(t) + HLv(t), where cov(v(t)) = I.

tfsys = tf(sys), or tfsys = tf(sys, 'measured') converts the measured component of an identified linear model into the transfer function form. sys is a model of type idss, idproc, idtf, idpoly, or idgrey. tfsys represents the relationship between u and y.

tfsys = tf(sys, 'noise') converts the noise component of an identified linear model into the transfer function form. It represents the relationship between the noise input, v(t) and output, y_noise = HL v(t). The noise input channels belong to the InputGroup 'Noise'. The names of the noise input channels are v@yname, where yname is the name of the corresponding output channel. tfsys has as many inputs as outputs.

tfsys = tf(sys, 'augmented') converts both the measured and noise dynamics into a transfer function. tfsys has ny+nu inputs such that the first nu inputs represent the channels u(t) while the remaining by channels represent the noise channels v(t). tfsys.InputGroup contains 2 input groups- 'measured' and 'noise'. tfsys.InputGroup.Measured is set to 1:nu while tfsys.InputGroup.Noise is set to nu+1:nu+ny. tfsys represents the equation y(t) = [G HL] [u; v].

    Tip   An identified nonlinear model cannot be converted into a transfer function. Use linear approximation functions such as linearize and linapp.

Creation of Generalized State-Space Models

You can use the syntax:

gensys = tf(num,den)

to create a Generalized state-space (genss) model when one or more of the entries num and den depends on a tunable realp or genmat model. For more information about Generalized state-space models, see Models with Tunable Coefficients.

Examples

Example 1

Transfer Function Model with One-Input Two-Outputs

Create the one-input, two-output transfer function

with input current and outputs torque and ang velocity.

To do this, enter

num = {[1 1] ; 1};
den = {[1 2 2] ; [1 0]};
H = tf(num,den,'inputn','current',...
                      'outputn',{'torque' 'ang. velocity'},...
                          'variable','p')

These commands produce the result:

Transfer function from input "current" to output...
              p + 1
 torque:  -------------
          p^2 + 2 p + 2
 
                 1
 ang. velocity:  -
                 p

Setting the 'variable' property to 'p' causes the result to be displayed as a transfer function of the variable p.

Example 2

Transfer Function Model Using Rational Expression

To use a rational expression to create a SISO TF model, type

s = tf('s');
H = s/(s^2 + 2*s +10);

This produces the same transfer function as

h = tf([1 0],[1 2 10]);

Example 3

Multiple-Input Multiple-Output Transfer Function Model

Specify the discrete MIMO transfer function

with common denominator d (z) = z + 0.3 and sample time of 0.2 seconds.

nums = {1 [1 0];[-1 2] 3};
Ts = 0.2;
H = tf(nums,[1 0.3],Ts)     % Note: row vector for common den. d(z)

Example 4

Convert State-Space Model to Transfer Function

Compute the transfer function of the state-space model with the following data.

To do this, type

sys = ss([-2 -1;1 -2],[1 1;2 -1],[1 0],[0 1]);
tf(sys) 

These commands produce the result:

Transfer function from input 1 to output:
s - 4.441e-016
--------------
s^2 + 4 s + 5
 
Transfer function from input 2 to output:
s^2 + 5 s + 8
-------------
s^2 + 4 s + 5

Example 5

Array of Transfer Function Models

You can use a for loop to specify a 10-by-1 array of SISO TF models.

H = tf(zeros(1,1,10)); 
s = tf('s')                                                  
for k=1:10,                                                              
		H(:,:,k) = k/(s^2+s+k);                                               
end                                                                      

The first statement pre-allocates the TF array and fills it with zero transfer functions.

Example 6

Tunable Low-Pass Filter

This example shows how to create the low-pass filter F = a/(s + a) with one tunable parameter a.

You cannot use ltiblock.tf to represent F, because the numerator and denominator coefficients of an ltiblock.tf block are independent. Instead, construct F using the tunable real parameter object realp.

  1. Create a tunable real parameter.

    a = realp('a',10);   
    

    The realp object a is a tunable parameter with initial value 10.

  2. Use tf to create the tunable filter F:

    F = tf(a,[1 a]);

F is a genss object which has the tunable parameter a in its Blocks property. You can connect F with other tunable or numeric models to create more complex models of control systems. For an example, see Control System with Tunable Components.

Example 7

Extract the measured and noise components of an identified polynomial model into two separate transfer functions. The former (measured component) can serve as a plant model while the latter can serve as a disturbance model for control system design.

load icEngine;
z = iddata(y,u,0.04);
nb = 2; nf = 2; nc = 1; nd = 3; nk = 3;
sys = bj(z, [nb nc nd nf nk]);

sys is a model of the form: y(t) = B/F u(t) + C/D e(t), where B/F represents the measured component and C/D the noise component.

sysMeas = tf(sys, 'measured') 
sysNoise = tf(sys, 'noise')

Alternatively, use can simply use tf(sys) to extract the measured component.

Discrete-Time Conventions

The control and digital signal processing (DSP) communities tend to use different conventions to specify discrete transfer functions. Most control engineers use the z variable and order the numerator and denominator terms in descending powers of z, for example,

The polynomials z2 and z2 + 2z + 3 are then specified by the row vectors [1 0 0] and [1 2 3], respectively. By contrast, DSP engineers prefer to write this transfer function as

and specify its numerator as 1 (instead of [1 0 0]) and its denominator as [1 2 3].

tf switches convention based on your choice of variable (value of the 'Variable' property).

Variable

Convention

'z' (default), 'q'

Use the row vector [ak ... a1 a0] to specify the polynomial (coefficients ordered in descending powers of z or q).

'z^-1'

Use the row vector [b0 b1 ... bk] to specify the polynomial (coefficients in ascending powers of z-1).

For example,

g = tf([1 1],[1 2 3],0.1);

specifies the discrete transfer function

because z is the default variable. In contrast,

h = tf([1 1],[1 2 3],0.1,'variable','z^-1');

uses the DSP convention and creates

See also filt for direct specification of discrete transfer functions using the DSP convention.

Note that tf stores data so that the numerator and denominator lengths are made equal. Specifically, tf stores the values

num = [0 1 1]; den = [1 2 3];

for g (the numerator is padded with zeros on the left) and the values

num = [1 1 0]; den = [1 2 3];

for h (the numerator is padded with zeros on the right).

Properties

tf objects have the following properties:

<argumentlist>

num

Transfer function numerator coefficients.

For SISO transfer functions, num is a row vector of polynomial coefficients in order of descending power (for Variable values s, z, p, or q) or in order of ascending power (for Variable values z^-1 or q^-1).

For MIMO transfer functions with Ny outputs and Nu inputs, num is a Ny-by-Nu cell array of the numerator coefficients for each input/output pair.

den

Transfer function denominator coefficients.

For SISO transfer functions, den is a row vector of polynomial coefficients in order of descending power (for Variable values s, z, p, or q) or in order of ascending power (for Variable values z^-1 or q^-1).

For MIMO transfer functions with Ny outputs and Nu inputs, den is a Ny-by-Nu cell array of the denominator coefficients for each input/output pair.

Variable

String specifying the transfer function display variable. Variable can take the following values:

  • 's' — Default for continuous-time models

  • 'z' — Default for discrete-time models

  • 'p' — Equivalent to 's'

  • 'q' — Equivalent to 'z'

  • 'z^-1' — Inverse of 'z'

  • 'q^-1' — Equivalent to 'z^-1'

The value of Variable is reflected in the display, and also affects the interpretation of the num and den coefficient vectors for discrete-time models. For Variable = 'z' or 'q', the coefficient vectors are ordered in descending powers of the variable. For Variable = 'z^-1' or 'q^-1', the coefficient vectors are ordered as ascending powers of the variable.

ioDelay

Transport delays. ioDelay is a numeric array specifying a separate transport delay for each input/output pair.

For continuous-time systems, specify transport delays in the time unit stored in the TimeUnit property. For discrete-time systems, specify transport delays in integer multiples of the sampling period, Ts.

For a MIMO system with Ny outputs and Nu inputs, set ioDelay to a Ny-by-Nu array. Each entry of this array is a numerical value that represents the transport delay for the corresponding input/output pair. You can also set ioDelay to a scalar value to apply the same delay to all input/output pairs.

InputDelay — Input delays0 (default) | scalar | vector

Input delay for each input channel, specified as a numeric vector. For continuous-time systems, specify input delays in the time unit stored in the TimeUnit property. For discrete-time systems, specify input delays in integer multiples of the sampling period Ts. For example, InputDelay = 3 means a delay of three sampling periods.

For a system with Nu inputs, set InputDelay to an Nu-by-1 vector. Each entry of this vector is a numerical value that represents the input delay for the corresponding input channel.

You can also set InputDelay to a scalar value to apply the same delay to all channels.

OutputDelay

Output delays. OutputDelay is a numeric vector specifying a time delay for each output channel. For continuous-time systems, specify output delays in the time unit stored in the TimeUnit property. For discrete-time systems, specify output delays in integer multiples of the sampling period Ts. For example, OutputDelay = 3 means a delay of three sampling periods.

For a system with Ny outputs, set OutputDelay to an Ny-by-1 vector, where each entry is a numerical value representing the output delay for the corresponding output channel. You can also set OutputDelay to a scalar value to apply the same delay to all channels.

Ts

Sampling time. For continuous-time models, Ts = 0. For discrete-time models, Ts is a positive scalar representing the sampling period. This value is expressed in the unit specified by the TimeUnit property of the model. To denote a discrete-time model with unspecified sampling time, set Ts = -1.

Changing this property does not discretize or resample the model. Use c2d and d2c to convert between continuous- and discrete-time representations. Use d2d to change the sampling time of a discrete-time system.

TimeUnit

String representing the unit of the time variable. For continuous-time models, this property represents any time delays in the model. For discrete-time models, it represents the sampling time Ts. Use any of the following values:

  • 'nanoseconds'

  • 'microseconds'

  • 'milliseconds'

  • 'seconds'

  • 'minutes'

  • 'hours'

  • 'days'

  • 'weeks'

  • 'months'

  • 'years'

Changing this property changes the overall system behavior. Use chgTimeUnit to convert between time units without modifying system behavior.

InputName

Input channel names. Set InputName to a string for single-input model. For a multi-input model, set InputName to a cell array of strings.

Alternatively, use automatic vector expansion to assign input names for multi-input models. For example, if sys is a two-input model, enter:

sys.InputName = 'controls';

The input names automatically expand to {'controls(1)';'controls(2)'}.

You can use the shorthand notation u to refer to the InputName property. For example, sys.u is equivalent to sys.InputName.

Input channel names have several uses, including:

  • Identifying channels on model display and plots

  • Extracting subsystems of MIMO systems

  • Specifying connection points when interconnecting models

InputUnit

Input channel units. Use InputUnit to keep track of input signal units. For a single-input model, set InputUnit to a string. For a multi-input model, set InputUnit to a cell array of strings. InputUnit has no effect on system behavior.

InputGroup

Input channel groups. The InputGroup property lets you assign the input channels of MIMO systems into groups and refer to each group by name. Specify input groups as a structure. In this structure, field names are the group names, and field values are the input channels belonging to each group. For example:

sys.InputGroup.controls = [1 2];
sys.InputGroup.noise = [3 5];

creates input groups named controls and noise that include input channels 1, 2 and 3, 5, respectively. You can then extract the subsystem from the controls inputs to all outputs using:

sys(:,'controls')

OutputName

Output channel names. Set OutputName to a string for single-output model. For a multi-output model, set OutputName to a cell array of strings.

Alternatively, use automatic vector expansion to assign output names for multi-output models. For example, if sys is a two-output model, enter:

sys.OutputName = 'measurements';

The output names to automatically expand to {'measurements(1)';'measurements(2)'}.

You can use the shorthand notation y to refer to the OutputName property. For example, sys.y is equivalent to sys.OutputName.

Output channel names have several uses, including:

  • Identifying channels on model display and plots

  • Extracting subsystems of MIMO systems

  • Specifying connection points when interconnecting models

OutputUnit

Output channel units. Use OutputUnit to keep track of output signal units. For a single-output model, set OutputUnit to a string. For a multi-output model, set OutputUnit to a cell array of strings. OutputUnit has no effect on system behavior.

OutputGroup

Output channel groups. The OutputGroup property lets you assign the output channels of MIMO systems into groups and refer to each group by name. Specify output groups as a structure. In this structure, field names are the group names, and field values are the output channels belonging to each group. For example:

sys.OutputGroup.temperature = [1];
sys.InputGroup.measurement = [3 5];

creates output groups named temperature and measurement that include output channels 1, and 3, 5, respectively. You can then extract the subsystem from all inputs to the measurement outputs using:

sys('measurement',:)

Name

System name. Set Name to a string to label the system.

Notes

Any text that you want to associate with the system. Set Notes to a string or a cell array of strings.

UserData

Any type of data you wish to associate with system. Set UserData to any MATLAB® data type.

SamplingGrid

Sampling grid for model arrays, specified as a data structure.

For model arrays that are derived by sampling one or more independent variables, this property tracks the variable values associated with each model in the array. This information appears when you display or plot the model array. Use this information to trace results back to the independent variables.

Set the field names of the data structure to the names of the sampling variables. Set the field values to the sampled variable values associated with each model in the array. All sampling variables should be numeric and scalar valued, and all arrays of sampled values should match the dimensions of the model array.

For example, suppose you create a 11-by-1 array of linear models, sysarr, by taking snapshots of a linear time-varying system at times t = 0:10. The following code stores the time samples with the linear models.

 sysarr.SamplingGrid = struct('time',0:10)

Similarly, suppose you create a 6-by-9 model array, M, by independently sampling two variables, zeta and w. The following code attaches the (zeta,w) values to M.

[zeta,w] = ndgrid(<6 values of zeta>,<9 values of w>)
M.SamplingGrid = struct('zeta',zeta,'w',w)

When you display M, each entry in the array includes the corresponding zeta and w values.

M
M(:,:,1,1) [zeta=0.3, w=5] =
 
        25
  --------------
  s^2 + 3 s + 25
 

M(:,:,2,1) [zeta=0.35, w=5] =
 
         25
  ----------------
  s^2 + 3.5 s + 25
 
...
</argumentlist>

More About

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Algorithms

tf uses the MATLAB function poly to convert zero-pole-gain models, and the functions zero and pole to convert state-space models.

See Also

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Tutorials

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