Simulink®

Transport Delay - Delay input by given amount of time

Library

Continuous

Description

The Transport Delay block delays the input by a specified amount of time. It can be used to simulate a time delay.

At the start of the simulation, the block outputs the Initial output parameter until the simulation time exceeds the Time delay parameter, when the block begins generating the delayed input. The Time delay parameter must be nonnegative.

The block stores input points and simulation times during a simulation in a buffer whose initial size is defined by the Initial buffer size parameter. If the number of points exceeds the buffer size, the block allocates additional memory and Simulink^® software displays a message after the simulation that indicates the total buffer size needed. Because allocating memory slows down the simulation, define this parameter value carefully if simulation speed is an issue. For long time delays, this block might use a large amount of memory, particularly for dimensionalized input.

When output is required at a time that does not correspond to the times of the stored input values, the block interpolates linearly between points. When the delay is smaller than the step size, the block extrapolates from the last output point, which can produce inaccurate results. Because the block does not have direct feedthrough, it cannot use the current input to calculate its output value. To illustrate this point, consider a fixed-step simulation with a step size of 1 and the current time at t = 5. If the delay is 0.5, the block needs to generate a point at t = 4.5. Because the most recent stored time value is at t = 4, the block performs forward extrapolation.

The Transport Delay block does not interpolate discrete signals. Instead, it returns the discrete value at the required time.

This block differs from the Unit Delay block, which delays and holds the output on sample hits only.

Using linmod to linearize a model that contains a Transport Delay block can be troublesome. For more information about ways to avoid the problem, see Linearizing Models .

Data Type Support

The Transport Delay block accepts and outputs real signals of type double.

Parameters and Dialog Box

Time delay

The amount of simulation time that the input signal is delayed before being propagated to the output. The value must be nonnegative.

Initial output

Specifies the output of the block at simulation time 0.

Initial buffer size

The initial memory allocation for the number of points to store.

Use fixed buffer size

Specifies use of a fixed-size buffer to save input data from previous time steps. The Initial buffer size parameter specifies the buffer's size. If the buffer is full, new data replaces data already in the buffer. Simulink software uses linear extrapolation to estimate the output value if it is not in the buffer. This option can save memory if the input data is linear. If the input is not linear, this option may yield inaccurate results.

Note   ERT or GRT code generation uses a fixed-size buffer even if you do not select this check box.

Direct feedthrough of input during linearization

Causes the block to output its input during linearization and trim. This sets the block's mode to direct feedthrough.

Enabling this check box can cause a change in the ordering of states in the model when using the functions linmod, dlinmod, or trim. To extract this new state ordering, use the following commands.

First compile the model using the following command, where model is the name of the Simulink model.

    [sizes, x0, x_str] = model([],[],[],'lincompile');

Next, terminate the compilation with the following command.

  model([],[],[],'term');

The output argument, x_str, which is a cell array of the states in the Simulink model, contains the new state ordering. When passing a vector of states as input to the linmod, dlinmod, or trim functions, the state vector must use this new state ordering.

Pade order (for linearization)

The order of the Pade approximation for linearization routines. The default value is 0, which results in a unity gain with no dynamic states. Setting the order to a positive integer n adds n states to your model, but results in a more accurate linear model of the transport delay.

Characteristics

Transfer Fcn Real Zero

Trigger