Modular Trade Studies with SysML Simulation.pptx

Modular Trade Studies with SysML Simulation
David Hetherington
System Strategy, Inc
2021 INCOSE Western States Regional Conference – San Diego, CA
Copyright © 2021 by David Hetherington Permission granted to INCOSE to publish and use

Agenda - Modular Trade Studies with SysML Simulation
 The Trade Study Problem
 Implementation in Cameo Systems Modeler
 Implementation in IBM Rhapsody
2021 WSRC - Modular Trade Studies with SysML Simulation | David Hetherington 2

The Trade Study Problem
Neither the SysML Standard nor the main textbooks make it obvious how
to actually execute a trade study

Drivetrain – A Typical Trade Study Problem
Vehicle
Gasoline
Drivetrain
Diesel
Drivetrain
Battery Electric
Drivetrain
Fuel Cell
Drivetrain
System of Interest Candidates for Drivetrain Subsystem

Assessing the Utility of a Drive Train
Drivetrain
Mass?
Volume?
Torque?
Fuel
Range?
Attribute Points Polarity Measure
Mass 20 Less is
better
% of 1000kg
Volume 20 Less is
better
% of 2 m3
Torque 30 More is
better
% of 1000 Nm
Fuel
Range
30 More is
better
% of 1000 km

First Try – Taking the Textbooks at Face Value
This would be a mess. While this ultra-simple example might not look too bad,
cross-connecting everything all in one model would get very tangled.

Instances and Generalization are the Key
1. Simulation (both Cameo and Rhapsody) runs off of instances, not off of block
definitions. When you tell either tool to simulate a block, the tool actually creates
an instance underneath the covers and runs that instance to execute the
simulation.
2. Composed instances can be assembled manually in both tools.
3. Generalization 101 – A specialized class can be used anywhere its general
parent can be used.

Strategy – the System of Interest
1. The vehicle has only one (generic) drivetrain
part.
2. The generic drivetrain contains value properties
for the key attributes.
3. A parametric simulation at the vehicle level
evaluates the utility of the drivetrain from its value
properties.

Strategy – Subsystems Being Studied
1. The four different drivetrain blocks specialize
the generic drivetrain block.
2. Each specialized drivetrain block includes the
four inherited value properties.
3. Each specialized drivetrain block can add as
much unique modeling complexity as needed to
evaluate and deliver values in the four value
properties.
4. Instances for each specialized drivetrain can be
manually inserted into the part slot for the generic
drivetrain in an instance of the vehicle.

Implementation in Cameo Systems Modeler
Cameo Step-by-Step

Set Up the Trade Study
In order to set up a parametric
diagram for simulation, it is
helpful to have a single block
that owns all of the elements.

Create the Parametric Diagram
In order to use simulation, the
names of the constraint
parameters need to be “Script
Friendly”
Connect everything with binding
connectors.

Set the Constraint Equation
Setting the language to “Built-In
Math” is critical
Implicitly declare local variables
(“mass_score”) to make the
equation more modular.

Create Vehicle Instances
Create vehicle instances, one for
each sort of drive train. Notice
that at this point, the vehicle
instances do not (yet) included
drivetrain instances.

Create Drivetrain Instances
Create one instance for each
type of drivetrain.
In order to keep the length of this presentation manageable
and focus on the core technique, we will just directly set the
“answer” for each drivetrain instance. HOWEVER, it is
definitely possible to create a unique simulation approach for
each drivetrain type to more accurately calculate the value
properties for that specific type of drivetrain.

Install the Drivetrain Instances in the Vehicle Instances
Create a BDD with the four
vehicle instances.
The drivetrain instances can be “installed” by simply
dragging them to their respective vehicle instances.

Create the Trade Study Instances
Create four trade study instances, each one with an analysis
constraint block and with a results block, but without a vehicle (yet).

Install the Vehicles in the Trade Study Instances
Once again, install each vehicle
instance into its corresponding
trade study instance.

Create an Instance Table for the Trade Study Instances
Now you are ready to create an instance table for the trade study instances.
Select the columns you want to display. Since everything is owned ultimately
by the trade study block, you can display any of the value properties in any of
the nested part blocks. Drag the columns around as needed.
Notice that the score is still “0” for each instance….

Run the Trade Study
Select all of the rows. Pull down the evaluation menu. Select “Evaluate
Selected Rows”.

Done
The scores for the different trade study instances have been calculated.
At this point there are several options:
1. The numerical results can be pushed back into the model instances. (This is an explicit step
if you want to do this. The instances are not updated by default.)
2. The results can be exported to a spreadsheet using the “Export” tool.
3. The results can be copied and pasted into any other software application (including
Microsoft Excel) that can accept the data.

Implementation in IBM Rhaposdy
Rhaposdy Step-by-Step

Set Up the Trade Study
We can set up in Rhapsody the
same way we set up in Cameo.
Rhapsody seems to be a bit
more flexible than Cameo about
how parametric diagrams are
constructed, but this structure
makes sense and helps us keep
our efforts straight.

Create the Parametric Diagram
Again we will need
some intermediate
variables for the
complicated calculation.
In Rhapsody these must
be defined as extra
constraint parameters
and must be shown in
the diagram.
Connect as before.

Set the Constraint Equation
Rhapsody constraints apparently
can’t span lines.
Instead, five separate constraints
are needed, with one line of
equation for each constraint.
The equation language is for the
open source MAXIMA solver, but
happens to be identical to the
built-in math used in Cameo.

Install Maxima
Get Maxima from:
https://maxima.sourceforge.io/
It is free and very easy to install.

Play with Maxima to Make Sure it is Working
Once installed, Maxima will run
from your Windows start menu.
It presents a command line
interpreter similar to MATRLAB.
Doing simple things can be a bit
obtuse, but it can do incredibly
powerful calculations.
Check online for tutorials.

Add PCE Profile to Rhapsody

Make Sure PCE is Configured

Create Vehicle Instances
We can create vehicle instances
in Rhapsody in the same
manager as in Cameo and
manually configure the drivetrain
in each one….
However, they end up being sort
of irrelevant because the
Rhapsody Parametric Constraint
Evaluator does not work fully
with nested instances.

Create Drivetrain Instances
Create one instance for each
type of drivetrain.
Again, we set the values directly for each instance.

Add Constraint Views
Next we create one
ConstraintView element for each
trade study instance.
These can be created by right-
clicking on the package and
selecting “Add New”
Right-click on each
ConstraintView and use “Add
New” to add a reference to the
parametric diagram and another
reference to the drivetrain
instance.

Open the Constraint View
Values are picked up from the
instance.
Clear the result, click “Evaluate”
PCE will calculate the result.
Results can be written back into
the model or exported to a CSV
file.

Questions?
 If you would like a copy of the presentation or of the models, send me an e-
mail:
– David_Hetherington@ieee.org
– dhetherington@systemxi.com

2021 WSRC - Modular Trade Studies with SysML Simulation | David Hetherington
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