tunablePID2

Tunable two-degree-of-freedom PID controller

Syntax

```blk = tunablePID2(name,type) blk = tunablePID2(name,type,Ts) blk = tunablePID2(name,sys) ```

Description

Model object for creating tunable two-degree-of-freedom PID controllers. `tunablePID2` lets you parametrize a tunable SISO two-degree-of-freedom PID controller. You can use this parametrized controller for parameter studies or for automatic tuning with tuning commands such as `systune`, `looptune`, or the Robust Control Toolbox™ command `hinfstruct`.

`tunablePID2` is part of the family of parametric Control Design Blocks. Other parametric Control Design Blocks include `tunableGain`, `tunableSS`, and `tunableTF`.

Construction

`blk = tunablePID2(name,type)` creates the two-degree-of-freedom continuous-time PID controller described by the equation:

`$u={K}_{p}\left(br-y\right)+\frac{{K}_{i}}{s}\left(r-y\right)+\frac{{K}_{d}s}{1+{T}_{f}s}\left(cr-y\right).$`

r is the setpoint command, y is the measured response to that setpoint, and u is the control signal, as shown in the following illustration.

The tunable parameters of the block are:

• Scalar gains `Kp`, `Ki`, and `Kd`

• Filter time constant `Tf`

• Scalar weights `b` and `c`

The `type` argument sets the controller type by fixing some of these values to zero (see Input Arguments).

`blk = tunablePID2(name,type,Ts)` creates a discrete-time PID controller with sample time `Ts`. The equation describing this controller is:

`$u={K}_{p}\left(br-y\right)+{K}_{i}IF\left(z\right)\left(r-y\right)+\frac{{K}_{d}}{{T}_{f}+DF\left(z\right)}\left(cr-y\right).$`

IF(z) and DF(z) are the discrete integrator formulas for the integral and derivative terms, respectively. The values of the `IFormula` and `DFormula` properties set the discrete integrator formulas (see Properties).

`blk = tunablePID2(name,sys)` uses the dynamic system model, `sys`, to set the sample time, `Ts`, and the initial values of all the tunable parameters. The model `sys` must be compatible with the equation of a two-degree-of-freedom PID controller.

Input Arguments

`name`

PID controller `Name`, specified as a character vector such as `'C'` or `'2DOFPID1'`. (See Properties.)

`type`

Controller type, specified as one of the values in the following table. Specifying a controller type fixes up to three of the PID controller parameters.

Value for `type`Controller TypeEffect on PID Parameters
`'P'`Proportional only`Ki` and `Kd` are fixed to zero; `Tf` is fixed to 1; `Kp` is free
`'PI'`Proportional-integral`Kd` is fixed to zero; `Tf` is fixed to 1; `Kp` and `Ki` are free
`'PD'`Proportional-derivative with first-order filter on derivative action`Ki` is fixed to zero; `Kp`, `Kd`, and `Tf` are free
`'PID'`Proportional-integral-derivative with first-order filter on derivative action`Kp`, `Ki`, `Kd`, and `Tf` are free

`Ts`

Sample time, specified as a scalar.

`sys`

Dynamic system model representing a two-degree-of-freedom PID controller.

Properties

`Kp,Ki,Kd,Tf,b,c`

Parametrization of the PID gains `Kp`, `Ki`, `Kd`, the filter time constant, `Tf`, and the scalar gains, `b` and `c`.

The following fields of `blk.Kp`, `blk.Ki`, `blk.Kd`, `blk.Tf`, `blk.b`, and `blk.c` are used when you tune `blk` using a tuning command such as `systune`:

FieldDescription
`Value`Current value of the parameter. `blk.b.Value`, and `blk.c.Value` are always nonnegative.
`Free`

Logical value determining whether the parameter is fixed or tunable. For example:

• If `blk.Kp.Free = 1`, then `blk.Kp.Value` is tunable.

• If `blk.Kp.Free = 0`, then `blk.Kp.Value` is fixed.

`Minimum`

Minimum value of the parameter. This property places a lower bound on the tuned value of the parameter. For example, setting ```blk.Kp.Minimum = 0``` ensures that `Kp` remains positive.

`blk.Tf.Minimum` must always be positive.

`Maximum`Maximum value of the parameter. This property places an upper bound on the tuned value of the parameter. For example, setting ```blk.c.Maximum = 1``` ensures that `c` does not exceed unity.

`blk.Kp`, `blk.Ki`, `blk.Kd`, `blk.Tf`, `blk.b`, and `blk.c` are `param.Continuous` objects. For more information about the properties of these `param.Continuous` objects, see the `param.Continuous` (Simulink Design Optimization) object reference page.

`IFormula, DFormula`

Discrete integrator formulas IF(z) and DF(z) for the integral and derivative terms, respectively, specified as one of the values in the following table.

ValueIF(z) or DF(z) Formula
`'ForwardEuler'`

`$\frac{{T}_{s}}{z-1}$`

`'BackwardEuler'`

`$\frac{{T}_{s}z}{z-1}$`

`'Trapezoidal'`

`$\frac{{T}_{s}}{2}\frac{z+1}{z-1}$`

Default: `'ForwardEuler'`

`Ts`

Sample 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. Unspecified sample time (`Ts = -1`) is not supported for PID blocks.

Changing this property does not discretize or resample the model.

Default: `0` (continuous time)

`TimeUnit`

Units for the time variable, the sample time `Ts`, and any time delays in the model, specified as one of the following values:

• `'nanoseconds'`

• `'microseconds'`

• `'milliseconds'`

• `'seconds'`

• `'minutes'`

• `'hours'`

• `'days'`

• `'weeks'`

• `'months'`

• `'years'`

Changing this property has no effect on other properties, and therefore changes the overall system behavior. Use `chgTimeUnit` to convert between time units without modifying system behavior.

Default: `'seconds'`

`InputName`

Input channel name, specified as a character vector or a 2-by-1 cell array of character vectors. Use this property to name the input channels of the controller model. For example, assign the names `setpoint` and `measurement` to the inputs of a 2-DOF PID controller model `C` as follows.

`C.InputName = {'setpoint';'measurement'};`

Alternatively, use automatic vector expansion to assign both input names. For example:

`C.InputName = 'C-input';`

The input names automatically expand to `{'C-input(1)';'C-input(2)'}`.

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

Input channel names have several uses, including:

• Identifying channels on model display and plots

• Specifying connection points when interconnecting models

Default: `{'';''}`

`InputUnit`

Input channel units, specified as a 2-by-1 cell array of character vectors. Use this property to track input signal units. For example, assign the units `Volts` to the reference input and the concentration units `mol/m^3` to the measurement input of a 2-DOF PID controller model `C` as follows.

`C.InputUnit = {'Volts';'mol/m^3'};`

`InputUnit` has no effect on system behavior.

Default: `{'';''}`

`InputGroup`

Input channel groups. This property is not needed for PID controller models.

Default: `struct` with no fields

`OutputName`

Output channel name, specified as a character vector. Use this property to name the output channel of the controller model. For example, assign the name `control` to the output of a controller model `C` as follows.

`C.OutputName = 'control';`

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

Input channel names have several uses, including:

• Identifying channels on model display and plots

• Specifying connection points when interconnecting models

Default: Empty character vector, `''`

`OutputUnit`

Output channel units, specified as a character vector. Use this property to track output signal units. For example, assign the unit `Volts` to the output of a controller model `C` as follows.

`C.OutputUnit = 'Volts';`

`OutputUnit` has no effect on system behavior.

Default: Empty character vector, `''`

`OutputGroup`

Output channel groups. This property is not needed for PID controller models.

Default: `struct` with no fields

`Name`

System name, specified as a character vector. For example, `'system_1'`.

Default: `''`

`Notes`

Any text that you want to associate with the system, stored as a string or a cell array of character vectors. The property stores whichever data type you provide. For instance, if `sys1` and `sys2` are dynamic system models, you can set their `Notes` properties as follows:

```sys1.Notes = "sys1 has a string."; sys2.Notes = 'sys2 has a character vector.'; sys1.Notes sys2.Notes```
```ans = "sys1 has a string." ans = 'sys2 has a character vector.' ```

Default: `[0×1 string]`

`UserData`

Any type of data you want to associate with system, specified as any MATLAB® data type.

Default: `[]`

Examples

Tunable Two-Degree-of-Freedom Controller with a Fixed Parameter

Create a tunable two-degree-of-freedom PD controller. Then, initialize the parameter values, and fix the filter time constant.

```blk = tunablePID2('pdblock','PD'); blk.b.Value = 1; blk.c.Value = 0.5; blk.Tf.Value = 0.01; blk.Tf.Free = false; blk```
```blk = Parametric continuous-time 2-DOF PID controller "pdblock" with equation: s u = Kp (b*r-y) + Kd -------- (c*r-y) Tf*s+1 where r,y are the controller inputs and Kp, Kd, b, c are tunable gains. Type "showBlockValue(blk)" to see the current value and "get(blk)" to see all properties. ```

Controller Initialized by Dynamic System Model

Create a tunable two-degree-of-freedom PI controller. Use a two-input, one-output `tf` model to initialize the parameters and other properties.

```s = tf('s'); Kp = 10; Ki = 0.1; b = 0.7; sys = [(b*Kp + Ki/s), (-Kp - Ki/s)]; blk = tunablePID2('PI2dof',sys)```
```blk = Parametric continuous-time 2-DOF PID controller "PI2dof" with equation: 1 u = Kp (b*r-y) + Ki --- (r-y) s where r,y are the controller inputs and Kp, Ki, b are tunable gains. Type "showBlockValue(blk)" to see the current value and "get(blk)" to see all properties. ```

`blk` takes initial parameter values from `sys`.

If `sys` is a discrete-time system, `blk` takes the value of properties, such as `Ts` and `IFormula`, from `sys`.

Controller with Named Inputs and Output

Create a tunable PID controller, and assign names to the inputs and output.

```blk = tunablePID2('pidblock','pid'); blk.InputName = {'reference','measurement'}; blk.OutputName = {'control'};```

`blk.InputName` is a cell array containing two names, because a two-degree-of-freedom PID controller has two inputs.

Tips

• You can modify the PID structure by fixing or freeing any of the parameters. For example, `blk.Tf.Free = false` fixes `Tf` to its current value.

• To convert a `tunablePID2` parametric model to a numeric (nontunable) model object, use model commands such as `tf` or `ss`. You can also use `getValue` to obtain the current value of a tunable model.

Compatibility Considerations

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Behavior changed in R2016a