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Discrete Derivative - Compute discrete time derivative

Library

Discrete

Description

The Discrete Derivative block computes an optionally scaled discrete time derivative as follows

where and are the block's input and output at the current time step, respectively, is the block's input at the previous time step, is a scaling factor, and is the simulation's discrete step size, which must be fixed.

Guidelines for Usage in Triggered Subsystems

When you use the Discrete Derivative block in triggered subsystems, follow these guidelines:

When the Sample time type parameter of the trigger port is triggered, verify that your model does not trigger the subsystem at the first time step.
If your model triggers the subsystem at the first time step, the Discrete Derivative block generates a divide-by-zero warning message. This behavior occurs because the time step at t = 0 is zero.
When the Sample time type parameter of the trigger port is periodic, your model can trigger the subsystem at any time step.

Data Type Support

The Discrete Derivative block supports all numeric Simulink data types, including fixed-point data types.

For a discussion on the data types supported by Simulink software, see Data Types Supported by Simulink.

Parameters and Dialog Box

The Main pane of the Discrete Derivative block dialog box appears as follows:

Gain value: Scaling factor used to weight the block's input at the current time step.
Initial condition for previous weighted input K*u/Ts: Set the initial condition for the previous scaled input.

The Signal Attributes pane of the Discrete Derivative block dialog box appears as follows:

Output minimum

Specify the minimum value that the block should output. The default value, [], is equivalent to -Inf. Simulink software uses this value to perform:

Simulation range checking (see Checking Signal Ranges)
Automatic scaling of fixed-point data types

Output maximum

Specify the maximum value that the block should output. The default value, [], is equivalent to Inf. Simulink software uses this value to perform:

Simulation range checking (see Checking Signal Ranges)
Automatic scaling of fixed-point data types

Output data type

Specify the output data type. You can set it to:

A rule that inherits a data type, for example, Inherit: Inherit via back propagation
The name of a built-in data type, for example, single
The name of a data type object, for example, a Simulink.NumericType object
An expression that evaluates to a data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output data type parameter.

See Specifying Block Output Data Types for more information.

Lock output data type setting against changes by the fixed-point tools

Select to lock the output data type setting of this block against changes by the Fixed-Point Tool and the Fixed-Point Advisor. For more information, see Fixed-Point Tool and Fixed-Point Advisor in the Simulink Fixed Point documentation.

Integer rounding mode

Select the rounding mode for fixed-point operations. For more information, see Rounding in the Simulink Fixed Point User's Guide.

Saturate to max or min when overflows occur

If selected, fixed-point overflows saturate. Otherwise, they wrap.

Characteristics

Direct Feedthrough	Yes
Scalar Expansion	Yes, of inputs and gain