lsiminfo

Compute linear response characteristics

Description

lsiminfo lets you compute linear response characteristics from an array of response data [y,t]. For a linear response y(t), lsiminfo computes characteristics relative to yinit and yfinal, where yinit is the initial offset, that is, the value before the input is applied, and yfinal is the steady-state value of the response.

lsiminfo uses yinit = 0 and yfinal = last sample value of y(t) unless you explicitly specify these values.

The function returns the characteristics in a structure containing the fields:

• TransientTime — The first time T such that the error |y(t) – yfinal| ≤ SettlingTimeThreshold × emax for tT, where emax is the maximum error |y(t) – yfinal| for t ≥ 0.

By default, SettlingTimeThreshold = 0.02 (2% of the peak error). Transient time measures how quickly the transient dynamics die off.

• SettlingTime — The first time T such that |y(t) – yfinal| ≤ SettlingTimeThreshold × |yfinalyinit| for tT.

By default, settling time measures the time it takes for the error to stay below 2% of |yfinalyinit|.

• Min — Minimum value of y(t).

• MinTime — Time the response takes to reach the minimum value.

• Max — Maximum value of y(t).

• MaxTime — Time the response takes to reach the maximum value.

S = lsiminfo(y,t) computes linear response characteristics from an array of response data y and corresponding time vector t. This syntax uses yinit = 0 and the last value in y (or the last value in each channel's corresponding response data) as yfinal to compute characteristics that depend on these values.

For SISO system responses, y is a vector with the same number of entries as t. For MIMO response data, y is an array containing the responses of each I/O channel.

example

S = lsiminfo(y,t,yfinal) computes linear response characteristics relative to the steady-state value yfinal. This syntax is useful when you know that the expected steady-state system response differs from the last value in y for reasons such as measurement noise. This syntax uses yinit = 0.

For SISO responses, t and y are vectors with the same length NS. For systems with NY outputs, you can specify y as an NS-by-NY array and yfinal as an NY-by-1 array. lsiminfo then returns a NY-by-1 structure array S of response characteristics corresponding to each output channel.

S = lsiminfo(y,t,yfinal,yinit) computes response characteristics relative to the response initial value yinit. This syntax is useful when your y data has an initial offset, that is, y is nonzero before the input is applied.

For SISO responses, t and y are vectors with the same length NS. For systems with NY outputs, you can specify y as an NS-by-NY array and yfinal and yinit as an NY-by-1 arrays. lsiminfo then returns a NY-by-1 structure array S of response characteristics corresponding to each output channel.

S = lsiminfo(___,'SettlingTimeThreshold',ST) lets you specify the threshold ST used in definition of settling and transient times. The default value is ST = 0.02 (2%). You can use this syntax with any of the previous input-argument combinations.

Examples

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Create the following continuous-time transfer function:

$H\left(s\right)=\frac{s-1}{{s}^{3}+2{s}^{2}+3s+4}$

sys = tf([1 -1],[1 2 3 4]);

Calculate the impulse response.

[y,t] = impulse(sys);

impulse returns the output response y and the time vector t used for simulation.

Compute the response characteristics using a final response value of 0.

s = lsiminfo(y,t,0)
s = struct with fields:
TransientTime: 22.8700
SettlingTime: NaN
Min: -0.4268
MinTime: 2.0088
Max: 0.2847
MaxTime: 4.0733

You can plot the impulse response and verify these response characteristics. For example, the time at which the minimum response value (MinTime) is reached is approximately 2 seconds.

impulse(sys) Input Arguments

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Response data, specified as one of the following:

• For SISO response data, a vector of length Ns, where Ns is the number of samples in the response data.

• For MIMO response data, an Ns-by-Ny array, where Ny is the number of system outputs.

Time vector corresponding to the response data in y, specified as a vector of length Ns.

Response steady-state value, specified as a scalar or an array.

• For SISO response data, specify a scalar value.

• For MIMO response data, specify an Ny-by-1 array, where each entry provides the steady-state response value for the corresponding system channel.

If you do not provide yfinal, then lsiminfo uses the last value in the corresponding channel of y as the steady-state response value.

Value of y before the input is applied, specified as a scalar or an array.

• For SISO response data, specify a scalar value.

• For MIMO response data, specify an Ny-by-1 array, where each entry provides the response initial value for the corresponding system channel.

If you do not provide yinit, then lsiminfo uses zero as the response initial value.

Threshold for defining settling and transient times, specified as a scalar value between 0 and 1. To change the default settling and transient time definitions (see Description), set ST to a different value. For instance, to measure when the error falls below 5%, set ST to 0.05.

Output Arguments

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Linear response characteristics, returned as a structure containing the fields:

• TransientTime

• SettlingTime

• Min

• MinTime

• Max

• MaxTime

For MIMO models or responses data, S is a structure array in which each entry contains the step-response characteristics of the corresponding I/O channel. For instance, if you provide a 3-input, 3-output model or array of response data, then S(2,3) contains the characteristics of the response from the third input to the second output.

Compatibility Considerations

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Behavior changed in R2021b