Limiting the Integral 的使用

Defining Initial Conditions

You can define the initial conditions as a parameter on the block dialog box or input them from an external signal:

• To define the initial conditions as a block parameter, specify the Initial condition source parameter as internal and enter the value in the Initial condition field.

• To provide the initial conditions from an external source, specify the Initial condition source parameter as external. An additional input port appears under the block input.

  Note   If the integrator limits its output (see Limiting the Integral), the initial condition must fall inside the integrator's saturation limits. If the initial condition is outside the block saturation limits, the block displays an error message.

Limiting the Integral

To prevent the output from exceeding specifiable levels, select the Limit output check box and enter the limits in the appropriate parameter fields. This action causes the block to function as a limited integrator. When the output reaches the limits, the integral action is turned off to prevent integral wind up. During a simulation, you can change the limits but you cannot change whether the output is limited. The block determines output as follows:

• When the integral is less than or equal to the Lower saturation limit, the output is held at the Lower saturation limit.

• When the integral is between the Lower saturation limit and the Upper saturation limit, the output is the integral.

• When the integral is greater than or equal to the Upper saturation limit, the output is held at the Upper saturation limit.

To generate a signal that indicates when the state is being limited, select the Show saturation port check box. A saturation port appears below the block output port.

The signal has one of three values:

• 1 indicates that the upper limit is being applied.

• 0 indicates that the integral is not limited.

• –1 indicates that the lower limit is being applied.

When you select this check box, the block has three zero crossings: one to detect when it enters the upper saturation limit, one to detect when it enters the lower saturation limit, and one to detect when it leaves saturation.