About the Structure Section

The structure section of a Simscape™ file is executed once during compilation. This section contains information on how the constituent components’ ports are connected to one another, as well as to the external inputs, outputs, and nodes of the top-level component.

The structure section begins with a connections keyword and is terminated by an end keyword. This connections block contains a set of connect constructs, which describe both the conserving connections (between nodes) and the physical signal connections (between the inputs and outputs).

In the following example, the custom component file includes the Foundation library Voltage Sensor and Electrical Reference blocks and specifies the following connections:

component VoltSG
  nodes
     p = foundation.electrical.electrical; % +
  end
  outputs
     Out = { 0.0, 'V' }; % V:right
  end
  components(ExternalAccess=observe)
     VoltSensor = foundation.electrical.sensors.voltage;
     Grnd = foundation.electrical.elements.reference;
  end
  connections
     connect(p, VoltSensor.p);
     connect(Grnd.V, VoltSensor.n);
     connect(VoltSensor.V, Out);
  end
end

In this example, the first two connect constructs specify conserving connections between electrical nodes. The third connect construct is a physical signal connection. Although these constructs look similar, their syntax rules are different.

Conserving Connections

For conserving connections, the connect construct can have two or more arguments. For example, the connections in the following example

  connections
     connect(R1.p, R2.n);
     connect(R1.p, R3.p);
  end

can be replaced with

  connections
     connect(R1.p, R2.n, R3.p);
  end

The order of arguments does not matter. The only requirement is that the nodes being connected are all of the same type (that is, are all associated with the same domain).

In the following example, the composite component consists of three identical resistors connected in parallel:

component ParResistors
  nodes
     p = foundation.electrical.electrical;
     n = foundation.electrical.electrical;
  end
  parameters
    p1 = {3 , 'Ohm'};
  end
  components(ExternalAccess=observe)
    r1 = foundation.electrical.elements.resistor(R=p1);
    r2 = foundation.electrical.elements.resistor(R=p1);
    r3 = foundation.electrical.elements.resistor(R=p1);
  end
  connections
    connect(r1.p, r2.p, r3.p, p);
    connect(r1.n, r2.n, r3.n, n);
  end
end

Connections to Implicit Reference Node

The * symbol indicates connections to a reference node in branch statements. You can also use it to indicate connections to an implicit reference node within the structure section of a component:

connections
    connect(A, *);
end

The implicit reference node acts as a virtual grounding component. A node connected to an implicit reference has all its Across variables equal to 0.

The * symbol is not domain-specific, and the same structure section can contain connections to implicit reference in different domains:

component abc
    nodes
        M = foundation.mechanical.rotational.rotational;
        N = foundation.electrical.electrical;
    end    
    connections
        connect(M,*);
        connect(N,*);
    end
end

However, multiple ports connected to an implicit reference within the same connect statement must all belong to the same domain:

connections
    connect(a, b, *);
end

The order of ports does not matter. This behavior is consistent with general connection rules for multiple conserving ports.

Physical Signal Connections

Physical signal connections are directional, therefore the connect construct has the following format:

  connect(s, d);

where s is the signal source port and d is the destination port.

There can be more than one destination port connected to the same source port:

  connect(s, d1, d2);

The source and destination ports belong to the inputs or outputs member classes. The following table lists valid source and destination combinations.

For example, consider the following block diagram.

It represents a composite component CompMeas, which, in turn, contains a composite component Valve Subsystem, as well as several Foundation library blocks. The Simscape file of the composite component would specify the equivalent signal connections with the following constructs.

Also notice that the output port of the PS Gain block is connected to the input port Conn2 of the Valve Subsystem composite block, which is another member component at the same level. Valve Subsystem is a standalone composite component, and therefore if you connect the output port of the PS Gain block to an input port of one of the member components inside the Valve Subsystem, that would violate the causality of the physical signal connections (a destination port cannot be connected to multiple sources).

Nonscalar Physical Signal Connections

Multidimensional physical signals can be useful for:

Simscape language supports nonscalar (vector-valued or matrix-valued) physical signals in inputs and outputs declarations. All signals in such vector or matrix should have the same units. For example, the following declaration

 inputs
   I = {zeros(3), 'm/s'}; % :left
 end

initializes a component input as a 3-by-3 matrix of linear velocities.

When you connect input and output ports carrying nonscalar physical signals, you can use signal indexing and concatenation at the source, but not at the destination. Scalar expansion is not allowed.

The following table shows valid syntax examples, assuming subcomponent A with output signal port A.o is being connected to subcomponent B with input signal port B.i, and all sizes and units are compatible.

You can use block parameter values for indexing inside a connect statement, for example:

connect(a.o(value(param_name, '1'), 3), b.i);

When you connect two physical signals, their units must be directly convertible. If one of the signals is declared as unitless (that is, with units of '1') , then you can connect a signal with any base units to it. However, unit conversion is not supported in this case. For example, if a.i is a 2x1 unitless input port, then this statement is valid:

connect([out1_in_meters, out2_in_seconds], a.i);

If you connect signals with different scales of the same unit with a unitless input port, the compiler alerts you to the fact that unit conversion is ignored. For example, the following statement produces a warning at compile time:

connect([out1_in_km, out2_in_mm], a.i);