# bj

Estimate Box-Jenkins polynomial model using time domain data

## Syntax

```sys = bj(data, [nb nc nd nf nk])sys = bj(data,[nb nc nd nf nk], Name,Value)sys = bj(data, init_sys)sys = bj(data, ___, opt)```

## Description

```sys = bj(data, [nb nc nd nf nk])``` estimates a Box-Jenkins polynomial model, `sys`, using the time-domain data, `data`. ```[nb nc nd nf nk]``` define the orders of the polynomials used for estimation.

```sys = bj(data,[nb nc nd nf nk], Name,Value)``` estimates a polynomial model with additional options specified by one or more `Name,Value` pair arguments.

`sys = bj(data, init_sys)` estimates a Box-Jenkins polynomial using the polynomial model `init_sys` to configure the initial parameterization of `sys`.

`sys = bj(data, ___, opt)` estimates a Box-Jenkins polynomial using the option set, `opt`, to specify estimation behavior.

## Input Arguments

 `data` Estimation data. `data` is an `iddata` object that contains time-domain input and output signal values. You cannot use frequency-domain data for estimating Box-Jenkins models. `[nb nc nd nf nk]` A vector of matrices containing the orders and delays of the Box-Jenkins model. Matrices must contain nonnegative integers. `nb` is the order of the B polynomial plus 1 (Ny-by-Nu matrix)`nc` is the order of the C polynomial plus 1 (Ny-by–1 matrix)`nd` is the order of the D polynomial plus 1 (Ny-by-1 matrix)`nf` is the order of the F polynomial plus 1 (Ny-by-Nu matrix)`nk` is the input delay (in number of samples, Ny-by-Nu matrix) where Nu is the number of inputs and Ny is the number of outputs. `opt` Estimation options. `opt` is an options set that configures, among others, the following: estimation objectiveinitial conditionsnumerical search method to be used in estimation Use `bjOptions` to create the options set. `init_sys` Polynomial model that configures the initial parameterization of `sys`. `init_sys` must be an `idpoly` model with the Box-Jenkins structure that has only B, C, D and F polynomials active. `bj` uses the parameters and constraints defined in `init_sys` as the initial guess for estimating `sys`. Use the `Structure` property of `init_sys` to configure initial guesses and constraints for B(q), F(q), C(q) and D(q). To specify an initial guess for, say, the C(q) term of `init_sys`, set `init_sys.Structure.c.Value` as the initial guess. To specify constraints for, say, the B(q) term of `init_sys`: set `init_sys.Structure.b.Minimum` to the minimum B(q) coefficient valuesset `init_sys.Structure.b.Maximum` to the maximum B(q) coefficient valuesset `init_sys.Structure.b.Free` to indicate which B(q) coefficients are free for estimation You can similarly specify the initial guess and constraints for the other polynomials.

### Name-Value Pair Arguments

Specify optional comma-separated pairs of `Name,Value` arguments. `Name` is the argument name and `Value` is the corresponding value. `Name` must appear inside single quotes (`' '`). You can specify several name and value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

 `'InputDelay'` Input delays. `InputDelay` is a numeric vector specifying a time delay for each input channel. Specify input delays in integer multiples of the sample time `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, where each entry is a numerical value representing 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. Default: 0 for all input channels `'ioDelay'` Transport delays. `ioDelay` is a numeric array specifying a separate transport delay for each input/output pair. Specify transport delays as integers denoting delay of a multiple of the sample time `Ts`. For a MIMO system with `Ny` outputs and `Nu` inputs, set `ioDelay` to a `Ny`-by-`Nu` array, where each entry is a numerical value representing 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. Default: 0 for all input/output pairs `'IntegrateNoise'` Logical specifying integrators in the noise channel. `IntegrateNoise` is a logical vector of length Ny, where Ny is the number of outputs. Setting `IntegrateNoise` to `true` for a particular output results in the model: $y\left(t\right)=\frac{B\left(q\right)}{F\left(q\right)}u\left(t-nk\right)+\frac{C\left(q\right)}{D\left(q\right)}\frac{e\left(t\right)}{1-{q}^{-1}}$ Where, $\frac{1}{1-{q}^{-1}}$ is the integrator in the noise channel,e(t). Default: `false(Ny,1)`(Ny is the number of outputs)

## Output Arguments

 `sys` Identified polynomial model of Box-Jenkins structure. `sys` is a discrete-time `idpoly` model which encapsulates the identified polynomial model.

## Examples

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### Identify SISO Box-Jenkins Model

Estimate the parameters of a single-input, single-output Box-Jenkins model from measured data.

```load iddata1 z1; nb = 2; nc = 2; nd = 2; nf = 2; nk = 1; sys = bj(z1,[nb nc nd nf nk]); ```

`sys` is a discrete-time `idpoly` model with estimated coefficients. The order of sys is as described by `nb`, `nc`, `nd`, `nf`, and `nk`.

Use `getpvec` to obtain the estimated parameters and `getcov` to obtain the covariance associated with the estimated parameters.

### Estimate a Multi-Input, Single-Output Box-Jenkins Model

Estimate the parameters of a multi-input, single-output Box-Jenkins model from measured data.

```load iddata8 nb = [2 1 1]; nc = 1; nd = 1; nf = [2 1 2]; nk = [5 10 15]; sys = bj(z8,[nb nc nd nf nk]); ```

`sys` estimates the parameters of a model with three inputs and one output. Each of the inputs has a delay associated with it.

### Estimate Box-Jenkins Model Using Regularization

Estimate a regularized BJ model by converting a regularized ARX model.

Load data.

```load regularizationExampleData.mat m0simdata ```

Estimate an unregularized BJ model of order 30.

```m1 = bj(m0simdata(1:150), [15 15 15 15 1]); ```

Estimate a regularized BJ model by determining Lambda value by trial and error.

```opt = bjOptions; opt.Regularization.Lambda = 1; m2 = bj(m0simdata(1:150), [15 15 15 15 1], opt); ```

Obtain a lower-order BJ model by converting a regularized ARX model followed by order reduction.

```opt1 = arxOptions; [L,R] = arxRegul(m0simdata(1:150), [30 30 1]); opt1.Regularization.Lambda = L; opt1.Regularization.R = R; m0 = arx(m0simdata(1:150),[30 30 1],opt1); mr = idpoly(balred(idss(m0),7)); ```

Compare the model outputs against data.

```compare(m0simdata(150:end), m1, m2, mr, ... compareOptions('InitialCondition','z')); ```

### Configure Estimation Options

Estimate the parameters of a single-input, single-output Box-Jenkins model while configuring some estimation options.

Generate estimation data.

```B = [0 1 0.5]; C = [1 -1 0.2]; D = [1 1.5 0.7]; F = [1 -1.5 0.7]; sys0 = idpoly(1,B,C,D,F,0.1); e = iddata([],randn(200,1)); u = iddata([],idinput(200)); y = sim(sys0,[u e]); data = [y u]; ```

`data` is a single-input, single-output data set created by simulating a known model.

Estimate initial Box-Jenkins model.

```nb = 2; nc = 2; nd = 2; nf = 2; nk = 1; init_sys = bj(data,[2 2 2 2 1]); ```

Create an estimation option set to refine the parameters of the estimated model.

```opt = bjOptions; opt.Display = 'on'; opt.SearchOption.MaxIter = 50; ```

`opt` is an estimation option set that configures the estimation to iterate 50 times at most and display the estimation progress.

Reestimate the model parameters using the estimation option set.

```sys = bj(data,init_sys,opt); ```

`sys` is estimated using `init_sys` for the initial parameterization for the polynomial coefficients.

To view the estimation result, enter `sys.Report`.

### Estimate MIMO Box-Jenkins Model

Estimate a multi-input, multi-output Box-Jenkins model from estimated data.

Load measured data.

```load iddata1 z1 load iddata2 z2 data = [z1, z2(1:300)]; ```

`data` contains the measured data for two inputs and two outputs.

Estimate the model.

```nb = [2 2; 3 4]; nc = [2;2]; nd = [2;2]; nf = [1 0; 2 2]; nk = [1 1; 0 0]; sys = bj(data, [nb nc nd nf nk]);```

The polynomial order coefficients contain one row for each output.

`sys` is a discrete-time `idpoly` model with two inputs and two outputs.

## Alternatives

To estimate a continuous-time model, use:

## More About

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### Box-Jenkins Model Structure

The general Box-Jenkins model structure is:

$y\left(t\right)=\sum _{i=1}^{nu}\frac{{B}_{i}\left(q\right)}{{F}_{i}\left(q\right)}{u}_{i}\left(t-n{k}_{i}\right)+\frac{C\left(q\right)}{D\left(q\right)}e\left(t\right)$

where nu is the number of input channels.

The orders of Box-Jenkins model are defined as follows:

## References

[1] Ljung, L. System Identification: Theory for the User, Upper Saddle River, NJ, Prentice-Hal PTR, 1999.

## See Also

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