## Specify PI-D and I-PD Controllers

PI-D and I-PD controllers are used to mitigate the influence of changes in the reference signal on the control signal. These controllers are variants of the 2DOF PID controller.

The general formula of a parallel-form 2DOF PID controller is:

$$u=P(br-y)+I\frac{1}{s}(r-y)+D\frac{N}{1+N\frac{1}{s}}(cr-y).$$

Here, *r* and *y* are the reference input and measured
output, respectively. *u* is the controller output, also called the
*control signal*. *P*, *I*, and
*D* specify the proportional, integral, and derivative gains, respectively.
*N* specifies the derivative filter coefficient. *b* and
*c* specify setpoint weights for the proportional and derivative
components, respectively. For a 1DOF PID controller, *b* and
*c* are equal to 1.

If *r* is nonsmooth or discontinuous, the derivative and proportional
components can contribute large spikes or offsets in *u*, which can be
infeasible. For example, a step input can lead to a large spike in *u*
because of the derivative component. For a motor actuator, such an aggressive control signal
could damage the motor.

To mitigate the influence of *r* on *u*, set
*b* or *c*, or both, to 0. Use one of the following
setpoint-weight-based forms:

PI-D (

*b*= 1 and*c*= 0) — Derivative component does not directly propagate changes in*r*to*u*, whereas the proportional component does. However, the derivative component, which has a greater impact, is suppressed. Also referred to as the*derivative of output controller*.The general formula for this controller form is:

$$u=P(r-y)+I\frac{1}{s}(r-y)-D\frac{N}{1+N\frac{1}{s}}y.$$

I-PD (

*b*= 0 and*c*= 0) — Proportional and derivative components do not directly propagate changes in*r*to*u*.The general formula for this controller form is:

$$u=-Py+I\frac{1}{s}(r-y)-D\frac{N}{1+N\frac{1}{s}}y.$$

The following plot shows *u* for different PID forms for a step
reference. The 1DOF PID controller results in a large spike when the reference changes from 0
to 1. The PI-D form results in a smaller jump. In contrast, the I-PD form does not react as
much to the change in *r*.

You can tune the *P*, *I*, *D*, and
*N* coefficients of a PI-D or I-PD controller to achieve the desired
disturbance rejection and reference tracking.

### Specify PI-D and I-PD Controllers Using PID Controller (2DOF) Block

To specify a PI-D or I-PD Controller using the PID Controller (2DOF) or
Discrete PID Controller (2DOF) blocks, open the block dialog. In the
**Controller** menu, select `PID`

.

For a PI-D controller, enter

`1`

in the**Setpoint weight (b)**box, and`0`

in the**Setpoint weight (c)**box.For an I-PD controller, enter

`0`

in the**Setpoint weight (b)**box, and`0`

in the**Setpoint weight (c)**box.

For an example that demonstrates the PI-D and I-PD controller forms, type
`ex_scd_pid2dof_setpoint_based_controllers`

. This opens a model that
compares the performance of a 1DOF PID, a PI-D, and an I-PD controller.

### Automatic Tuning of PI-D and I-PD Controllers

You can use **PID Tuner** to automatically tune PI-D and I-PD controllers while
preserving the fixed *b* and *c* values. To do so:

In the model, open the block. In the block dialog box, in the

**Controller**menu, select`PID`

.Click

**Tune**.**PID Tuner**opens.In

**PID Tuner**, in the**Type**menu, select`PI-DF`

or`I-PDF`

.**PID Tuner**retunes the controller gains, fixing*b*= 1 and*c*= 0 for PI-D, and*b*= 0 and*c*= 0 for I-PD.

You can now analyze system responses as described in Analyze Design in PID Tuner.

## See Also

PID Controller (2DOF) | Discrete PID Controller (2DOF)