| Using Simulink | |
Systems and Subsystems
A Simulink block diagram can consist of layers. Each layer is defined by a subsystem. A subsystem is part of the overall block diagram and ideally has no impact on the meaning of the block diagram. Subsystems are provided primarily to help in the organization aspects a block diagram. Subsystems do not define a separate block diagram.
Simulink differentiates between two different types of subsystems: virtual and nonvirtual. The main difference is that nonvirtual subsystems provide the ability to control when the contents of the subsystem are evaluated.
Flattening the Model Hierarchy
While preparing a model for execution, Simulink generates internal "systems" that are collections of block methods (equations) that are evaluated together. The semantics of time-based block diagrams doesn't require creation of these systems. Simulink creates these internal systems as a means to manage the execution of the model. Roughly speaking, there will be one system for the top-level block diagram which is referred to as the root system, and several lower-level systems derived from nonvirtual subsystems and other elements in the block diagram. You will see these systems in the Simulink Debugger. The act of creating these internal systems is often referred to as flattening the model hierarchy.
Conditionally Executed Subsystems
You can create conditionally executed subsystems that are executed only when a transition occurs on a triggering, function-call, action, or enabling input (see Creating Conditionally Executed Subsystems). Conditionally executed subsystems are atomic, i.e., the equations that they define are evaluated as a unit.
Atomic Subsystems
Unconditionally executed subsystems are virtual by default. You can, however, designate an unconditionally executed subsystem as atomic (see the Atomic Subsystem block). This is useful if you need to ensure that the equations defined by a subsystem are evaluated as a unit.
Signals
Simulink uses the term signal to refer to a time varying quantity that has values at all points in time. Simulink allows you to specify a wide range of signal attributes, including signal name, data type (e.g., 8-bit, 16-bit, or 32-bit integer), numeric type (real or complex), and dimensionality (one-dimensional or two-dimensional array). Many blocks can accept or output signals of any data or numeric type and dimensionality. Others impose restrictions on the attributes of the signals they can handle.
On the block diagram, you will find that the signals are represented with lines that have an arrow head. The source of the signal corresponds to the block that writes to the signal during evaluation of its block methods (equations). The destinations of the signal are blocks that read the signal during the evaluation of its block methods (equations). A good analogy of the meaning of a signal is to consider a classroom. The teacher is the one responsible for writing on the white board and the students read what is written on the white board when they choose to. This is also true of Simulink signals, a reader of the signal (a block method) can choose to read the signal as frequently or infrequently as so desired.
Block Methods
Blocks represent multiple equations. These equations are represented as block methods within Simulink. These block methods are evaluated (executed) during the execution of a block diagram. The evaluation of these block methods is performed within a simulation loop, where each cycle through the simulation loop represent evaluation of the block diagram at a given point in time.
Method Types
Simulink assigns names to the types of functions performed by block methods. Common method types include:
Computes the outputs of a block given its inputs at the current time step and its states at the previous time step.
Computes the value of the block's discrete states at the current time step, given its inputs at the current time step and its discrete states at the previous time step.
Computes the derivatives of the block's continuous states at the current time step, given the block's inputs and the values of the states at the previous time step.
Method Naming Convention
Block methods perform the same types of operations in different ways for different types of blocks. The Simulink user interface and documentation uses dot notation to indicate the specific function performed by a block method:
For example, Simulink refers to the method that computes the outputs of a Gain block as
The Simulink debugger takes the naming convention one step further and uses the instance name of a block to specify both the method type and the block instance on which the method is being invoked during simulation, e.g.,
| | Block Sample Times | Model Methods | |
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