Ask the right question: Architectural size

Recently a client posed a question I have heard a number of times, “How many States can I have in my model before there are problems”?  On the surface, this seems like an O.K. question, however, when we dig in a little we see the inherent assumptions with the question.

Size matters, after a fashion

AsImage result for size matters a basic metric, the number of states in a model is meaningless, it is akin to the question “how many lines of code before there are problems”?  If someone said they had a program with one function and 100,000 lines of code you would assume that it was problematic in its complexity.  On the other hand, if they said the program had 100 functions you would think that the model was well architected.  Going to the other extream if the function had 1,000 functions you may think that they have created architectural problems of increased complexity.

No one builds a house with a Swiss army knife

Models are tools, Victorinox Swiss Champ XAVT in red transparent - 1.6795.XAVTthey perform functions in response to inputs.  It is possible to build a single function that performs a 1,000 different functions but that is rarely the correct way to go.

Rather each model should be viewed as a Image result for toolboxspecialized tool performs a function or set of related functions.  Again this relates to the “100 or 1,000” functions for a 100,000 lines of code.  I generally consider something a “related function” if

  • Uses the same inputs:  E.g. the function does not need to import additional data
  • Is used at the same time: E.g. the information is used in the same larger problem you are trying to solve.

For example, calculating the wheel speed and wheel torque in the same ABS braking function makes sense as they use the same input data (generally a PWM encoder) and are used at the same time (to determine the brake pulse width).  However, calculating mileage in that function, which can be derived from the wheel speed, does not make sense as it is not part of the same problem you are trying to solve.

Keeping it in memory…

In Image result for you can remember 7 peoplethis instance, I am talking about the developer’s memory. Above a given size and complexity it becomes difficult for a developer to remember all the parts of a function operate.

As a general rule of thumb, I try to stick to  a “depth of 3” limit.  No subsystems or nested states more than three levels deep.  If there is a need for greater depth I look to see if there is a way to decompose the model or chart into referenced models and charts.  One note, when measuring “depth” the count stops when a referenced model or chart is encountered as these are assumed to be atomic systems developed independently from the parent.

Benefits of decomposition

The Image result for decomposingfollowing benefits are realized through the decomposition of models

  1. Simplified testing: large functions have a large number of inputs, outputs, and possible responses.  Smaller models have reduced testing criteria.
  2. Simplified requirements linking: Generally, well decomposed aligns with the requirements by not clumping disparent functionality together.
  3. Improved reusability: Smaller functions are more likely to be generic or easily customizable.
  4. Improved readability: A smaller model can more quickly be reviewed and analyzed then a larger model.

What is the correct question?

There are two questions I would ask:

  1. How do I make the model functionally correct?
  2. How do I make the model readable?

For guidelines on that topic, you can read my Stateflow Best Practices document.

 

 

The Model-Based Design Workflow…

The following is an idealized Model-Based Design workflow, from initial requirements to product release.  The workflow assumes a multi-person team with resources to support multiple roles.

It all starts with requirements…

Ideally, the process starts with foundational tools and processes in place.  These consist of

  • Modeling guidelines:  Covers model architecture, data usage, and model clarity
  • Testing guidelines: How the system will be validated
  • Requirements tracking: A system for tracking the compliance and changes to requirements
  • Bug reporting: A system for tracking issues as they are discovered; this is tightly coupled to the requirements tracking.
  • Support infrastructure: The supporting infrastructure includes tools such as version control and CI systems.
  • Project timeline: The project timeline provides the objects for the completion of the project and the resource allocation (e.g people)

wf_p1

From the initial high-level requirements derived requirements and derived tests are defined.  These documents, along with modeling guidelines and testing guidelines are used to create the initial system and component level models and their associated test cases.  These models and tests are entered into the requirements tracking system and assigned to the appropriate teams.

Project development

During the development phase. both the models and tests points are elaborated by adding in details from the requirement and testing documents.  As these features are added to the system the models are validated against the test cases.

wf_p2

As issues are uncovered, either way in which the design is lacking or bugs are discovered information is added to the bug and requirements tracking systems.  This workflow goes on in parallel for each developer/model.

 

This development goes on in parallel for both the system under development and any supporting models such as plant and environmental models.

System integration and release

As the maturity of individual components matures the system level integration and validation rigor is increased.  At this stage of development testing of the derived objects, e.g. generated code, increases in frequency and rigor.

wf_p3

Wrap up

After the release of the project, the components from the development should be examined for reusability.  Beyond components processes and guidelines should be reviewed for updates to the best practices.

wf_p4

Statistical variations in plant models

Plant models, whether based on first principals physics or regression models based on real-world data are a cornerstone of the Model-Based Design controls development process.  During the initial development of the control algorithms a “static” physical model is sufficient; however when the development moves into the diagnostic and release phase physical models that demonstrate real-world variation are required.

Variations, not noise…

In Image result for variationa previous blog, I wrote about the importance of noise in testing.   Variations are different from noise in that they are a constant offset.  For instance, my height will always be 6’3″ while my wife Deborah’s will be 5’10”.  If we design an airbag system assuming everyone was 5’10” then there could be issues when the first 6’3″ person is in the car with an accident.

Working with variations

If we continue the “body variations” example and think of all the variables associated with the body, height, weight, leg length, arm length… we will observe two things

  1. There is a correlation between some variables:  In general leg length increases as height increases, as does weight.
  2. There are outliers:  While there are general correlations between properties there are still outliers which cannot be ignored.

So given these two considerations how do we proceed?

Data at the boundaries, data in the center

Test Image result for at the boundarydata should be defined that includes both data at the boundaries and in the ‘center’ of the test space.  Data at the boundaries exercises the edge cases while the data in the center is used to validate the mainline behavior.  When considering which boundary conditions to include consider the following issues.

  1. For discreet variations:  In instances where the variations are discreet, e.g. on/off, flow/no-flow all discreet instances should be included
  2. For continuous variations: In the example of height, values at the endpoints should be selected along with a set of points within the range.  (The total number should be a function of what a nominal unit is in the range.  For instance, if we took a height range from 4’10” to 6’6″ and assumed the nominal unit of 1″ then perhaps a spacing of 6″ would be reasonable)

Variations and variations… working with multiple variations

In any real-world system, there are multiple parameters that will vary.  Selecting which combination of variations (outliers and central points) needs to be determined in a rigorous fashion.  In an upcoming post, I will cover how six sigma style approaches can be used to determine which points should be selected.

 

ICDs and collaboratve verification

Interface Control Documents (ICD) are a method for specifying the functional interface of a component or system.  Used correctly they prevent integration errors and promote formal development practices.

What is in the document?

At a minimum, the ICD consists of the following information

  • Input specification
    • Name
    • Data type
    • Dimension
    • Reference/value
  • Output specification
    • Name
    • Data type
    • Dimension
    • Reference/value
  • Global data used
  • Calling method
    • Periodic or event-driven
    • Reusable / non-reusable

The I/O and global data are generally well understood.  Specification of the calling method is required to understand how time-dependent functions such as integrators or transfer functions will behave.

Additional data may include information such as signal range, update rate, units…  All of this information should be derived from the requirement specification.  (Note: the ICD is sometimes viewed as a derived requirement document)

How is the ICD used?

The ICD provides a baseline baseLinefor the interface to the software component.  Once the ICD has been defined the initial model can be created.  This is sometimes called a “shell model.”  The shell model has all of the inputs and outputs as defined by the ICD document.  The shell model can then be integrated into the system level model (or another integration model) for a system lockdown.  This integration model provides the first level of testing of the interface.  If the interface of the shell model changes the integration model will break.

ICD_WF

Test utility pattern: Simulink Test

As I have written about in previous posts I recommend the use of reusable test utilities.  When working in the text-based MATLAB environment how to create reusable utilities is easily understood; they are simply MATLAB functions.  However,  within the Simulink Test graphical environment, it may not be as clear.

Libraries and Functions

Fortunately, there is a solution; if there wasn’t there would be no post today.  Within the Simulink Test environment, calls can be made to functions.  The functions can be either return a value (or values) or directly set an assert or verify flag.

testCalls

The functions are imported from a Simulink Library and can be constructed from MATLAB or Simulink Function blocks.

testLibrarySetup

In the case of MATLAB functions that are placed in a Stateflow block with the functions export option selected.

exportFunction

So there you have it, a simple solution to reusable test utilities within the Simulink Test environment.