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| R2012a Documentation → Control System Toolbox | |
Learn more about Control System Toolbox |
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blk = ltiblock.tf(name,Nz,Np)
blk = ltiblock.tf(name,Nz,Np,Ts)
blk = ltiblock.tf(name,sys)
Model object for creating tunable SISO transfer function models of fixed order. ltiblock.tf lets you parametrize a transfer function of a given orderfor parameter studies or for automatic tuning with hinfstruct (requires Robust Control Toolbox).
ltiblock.tf is part of the Control Design Block family of parametric models. Other Control Design Blocks includeltiblock.pid, ltiblock.ss, and ltiblock.gain.
blk = ltiblock.tf(name,Nz,Np) creates the parametric SISO transfer function:
n = Np is the maximum number of poles of blk, and m = Nz is the maximum number of zeros. The tunable parameters are the numerator and denominator coefficients a0, ..., am and b0, ..., bn–1. The leading coefficient of the denominator is fixed to 1.
blk = ltiblock.tf(name,Nz,Np,Ts) creates a discrete-time parametric transfer function with sampling time Ts.
blk = ltiblock.tf(name,sys) uses the tf model sys to set the number of poles, number of zeros, sampling time, and initial parameter values.
name |
String specifying the Name of the parametric transfer function blk. (See Properties.) |
Nz |
Nonnegative integer specifying the number of zeros of the parametric transfer function blk. |
Np |
Nonnegative integer specifying the number of poles of the parametric transfer function blk. |
Ts |
Scalar sampling time. |
sys |
tf model providing number of poles, number of zeros, sampling time, and initial values of the parameters of blk. |
To convert an ltiblock.tf parametric model to a numeric (non-tunable) model object, use model commands such as tf, zpk, or ss.
num, den |
Parametrization of the numerator coefficients am, ..., a0 and the denominator coefficients 1,bn–1, ..., b0 of the tunable transfer function blk. blk.num and blk.den are param.Continuous objects. For general information about the properties of these param.Continuous objects, see the param.Continuous object reference page. The following fields of blk.num and blk.den are used when you tune blk using hinfstruct:
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Ts |
Sampling time. For continuous-time models, Ts = 0. For discrete-time models, Ts is a positive scalar representing the sampling period 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. Default: 0 (continuous time) | ||||||
TimeUnit |
String representing the unit of the time variable, any time delays in the model (for continuous-time models), and the sampling time Ts (for discrete-time models). TimeUnit can take the following values:
Changing this property changes the overall system behavior. Use chgTimeUnit to convert between time units without modifying system behavior. Default: 'seconds' | ||||||
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 software automatically expands the input names 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:
Default: Empty string '' for all input channels | ||||||
InputUnit |
Input channel units. Use InputUnit to keep track of input signal units. Set InputUnit to a string for single-input model, or to a cell array of strings for a multi-input model. InputUnit has no effect on system behavior. Default: Empty string '' for all input channels | ||||||
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 whose field names are the group names and whose field values are the input channels belong 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') Default: Struct with no fields | ||||||
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 software automatically expands the output names 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:
Default: Empty string '' for all input channels | ||||||
OutputUnit |
Output channel units. Use OutputUnit to keep track of output signal units. Set OutputUnit to a string for single-input model, or to a cell array of strings for a multi-input model. OutputUnit has no effect on system behavior. Default: Empty string '' for all input channels | ||||||
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 whose field names are the group names and whose field values are the output channels belong 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',:)
Default: Struct with no fields | ||||||
Name | System name. Set Name to a string to label the system. Default: '' | ||||||
Notes | Any text that you wish to associate with the system. Set Notes to a string or a cell array of strings. Default: {} | ||||||
UserData | Any type of data you wish to associate with system. Set UserData to any MATLAB data type. Default: [] |
Create a parametric SISO transfer function with two zeros, four poles, and at least one integrator.
A transfer function with an integrator includes a factor of 1/s. Therefore, to ensure that a parametrized transfer function has at least one integrator regardless of the parameter values, fix the lowest-order coeffiecient of the denominator to zero.
blk = ltiblock.tf('tfblock',2,4); % two zeros, four poles
blk.den.Value(end) = 0; % set last denominator entry to zero
blk.den.Free(end) = 0; % fix it to zero |
Create a parametric transfer function, and assign names to the input and output.
blk = ltiblock.tf('tfblock',2,3);
blk.InputName = {'error'}; % assign input name
blk.OutputName = {'control'}; % assign output namegenss | ltiblock.gain | ltiblock.pid | ltiblock.tf
Learn more about resources for designing, testing, and implementing control systems.
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