Now that we’ve created Python components representing the three vehicle subsystems, we need to connect them so that they can be executed in sequence. Recall that an OpenMDAO component that contains a collection of other components is called an Assembly. The Assembly allows a set of components to be linked together by connecting their inputs and outputs. The data connections define an execution order based on their dependencies, i.e., components that are upstream in the data flow will be executed prior to those downstream so that input data to a component will always be valid with respect to the other parts of the workflow. A component will not execute if its inputs have not changed since its last execution. An Assembly contains a Driver called driver that controls iterations over the workflow.

For the vehicle simulation, a Vehicle assembly is needed that can sequentially execute the Transmission, Engine, and Chassis components.

from openmdao.main.api import Assembly
from openmdao.lib.datatypes.api import Float

from openmdao.examples.enginedesign.transmission import Transmission
from openmdao.examples.enginedesign.chassis import Chassis
from openmdao.examples.enginedesign.engine_wrap_c import Engine

class Vehicle(Assembly):
    """ Vehicle assembly. """

    def configure(self):

        # Create component instances

        self.add('transmission', Transmission())
        self.add('engine', Engine())
        self.add('chassis', Chassis())

        # Set up the workflow
        self.driver.workflow.add(['transmission', 'engine', 'chassis'])

The Engine, Transmission, and Chassis components all need to be imported so their instances can be created; they can be added to the assembly with add. Please notice that an assembly inherits from Assembly instead of from Component. We also need to add the instances to the driver’s workflow using the add method. Note that the order we add these doesn’t matter here, because the execution order is inferred from the data connections, which we now need to make.

Now that the components are instantiated in the assembly, they need to be hooked up:


The first argument in the call to self.connect is the output variable of the source component instance, and the second argument is the input variable of the target component instance. For a connection to be valid, the units of the output and input must be compatible (e.g., length, speed, etc.) If they differ within the same class (e.g., meters vs. inches), then the value is converted from the source unit to the target unit before setting the value at the input. If the classes are incompatible (e.g., meters vs. seconds), then an exception is raised during execution.

The Vehicle assembly also has inputs and outputs, and it can be hooked up to other components and included in other assemblies once its variables are defined. We would like to promote all of the design and simulation variables from the Engine, Transmission, and Chassis components to the input and output of the Vehicle assembly. OpenMDAO includes a shortcut for doing this quickly by creating passthroughs:

# ...
# ...
# ...
# ...

The create_passthrough function creates an identical variable in the assembly and connects it to the corresponding component variable. So now, all of the design variables are available as variables in any simulation that includes an instance of the vehicle model.

However, the engine tutorial throws you a curve ball here. The Engine and Chassis components are defined with SI units, but the Transmission component is defined with English units. We have two inputs – the tire circumference and the vehicle velocity – that are each used by two components with different units. The create_passthrough function creates an exact copy of the variable, so we cannot use it here. Instead, we must connect them manually by declaring variables in our assembly.

class Vehicle(Assembly):
    """ Vehicle assembly. """

    tire_circumference = Float(75.0, iotype='in', units='inch',
                        desc='Circumference of tire (inches)')

    velocity = Float(75.0, iotype='in', units='mi/h',
               desc='Vehicle velocity needed to determine engine RPM (mi/h)')

Now these inputs are available to connect to the components, so we connect them manually.

             ['chassis.velocity', 'transmission.velocity'])
             ['chassis.tire_circ', 'transmission.tire_circ'])

This ensures that the units for these inputs to the Vehicle are converted properly for use in the Chassis and Transmission components. While this might seem redundant, it demonstrates a way that Assemblies can be used to define a more consistent external interface.

Executing the Vehicle Assembly

We can manipulate the Vehicle Assembly in the Python shell just like we did with the engine component above. As inputs, the Vehicle takes a commanded velocity, throttle position, a gear position, and a set of vehicle design parameters, and outputs the vehicle’s instantaneous acceleration and rate of fuel burn.

>>> from openmdao.main.api import set_as_top
>>> from openmdao.examples.enginedesign.vehicle import Vehicle
>>> my_car = set_as_top(Vehicle())
>>> my_car.velocity = 25.0
>>> my_car.current_gear = 3
>>> my_car.throttle = .5
>>> my_car.acceleration
>>> my_car.fuel_burn

When we run the Vehicle, we are performing a simple multidisciplinary analysis via the OpenMDAO framework. Try setting the simulation variables to other values, including ones that should trigger an exception. (One way to do this is to command a high velocity in first gear, which should violate the maximum RPM that the engine allows.)

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