Autopilot Model-Based Design for ARP-4754A, DO-178C and DO-331

by admin in , on January 8, 2020

This demo provides an example of how to apply Model-Based Design to an ARP-4754A/DO-178C/DO-331 project using The MathWorks tools. The demo includes many of the development and verification lifecycle stages called out by ARP-4754A, DO-178C and DO-331.

The demo, which is based on a very simple autopilot implementation, has been fully tested in Release R2013b, but may work with newer releases. The autopilot in this demo is controlling a DeHavilland Beaver airframe.

This document also contains macros, so if macros were disabled when opening this document, links within the document will not work properly. The links to help for the various products assume that the help documentation has been installed along with the product. Those links will not work properly if only web based help is used for your MATLAB installation.

In most cases, where the proper products are not installed, the demo provides links to pre-generated reports, such that the product licenses are not necessary to view many of the artifacts.

Lifecycle Overview

The following lifecycle stages defined in ARP-4754A, DO-178C and DO-331  are covered in this demonstration:

* ARP-4754A Section 5.3 Requirements Capture
* ARP-4754A Section 5.4 Requirements Validation
* ARP-4754A Section 5.5 Implementation Verification
* DO-178C Section 2.1 System Requirements Allocated to Software
* DO-331 Section MB.5.1 Software Requirements Process
* DO-331 Section MB.5.2 Software Design Process
* DO-178C Section 5.3 Software Coding Process
* DO-331 Section MB.5.5 Software Development Process Traceability
* DO-331 Section MB.6.3.1 Reviews and Analyses of High-Level Requirements
* DO-331 Section MB.6.3.2 Reviews and Analyses of Low-Level Requirements
* DO-331 Section MB.6.3.3 Reviews and Analyses of Software Architecture
* DO-331 Section MB.6.3.4 Reviews and Analyses of Source Code
* DO-331 Section MB.6.5 Software Verification Process Traceability
* DO-331 Section MB.6.7 Model Coverage Analysis for Design Models
* DO-331 Section MB.6.8.1 Model Simulation for Verificaiton of the Model
* DO-331 Section MB.6.8.3 Simulation Cases, Proceddures and Results

System Requirements

The system requirements for the roll axis of the autopilot are provided  in a Microsoft Word document. Each requirement is implemented in a section of the document which also contains rationale for the requirement. The requirements are traced to the design model via electronic hyperlinks using Simulink Verification and Validation.

<matlab:enableservice('AutomationServer',true);winopen('do178b_autopilot_requirements.docx');open_system('roll_ap');
Open the requirements document (requires MSWord license) and the roll_ap model>

To view a block in the model that traces to a requirement, double click on one of the hyperlinks to the right of the section heading. The *roll_ap* model will open and the block linked to the requirement will be highlighted in the model. To view a requirement in the document from the model, right click on a block, select Requirements and then one of the requriement descriptions. This will go to the document and highlight the section in the docuemnt containing that requirement.

For more information about the *Requirements Management Interface* see the *Help for Simulink Verification and Validation*. This describes how to interface to various document formats, including Word, as well as how
to interface to DOORS.

<matlab:doc('slvnv'); Help for Simulink Verification and Validation>

System Requirements Validation

Requirements validation is defined as showing that the requirements are complete and correct. Requirements validation is typically performed by reviewing the requirements using a checklist, or similar means, based on
requirements standards for the project.

ARP-4754A also calls out modeling, simulation and prototyping as methods  of validating requirements. As the system design is developed, as described in the following section on system design, simulation of the  model may be used to ensure completeness and correctness of the requirements. This simulation may uncover undesirable behavior that was not considered in the requirements or it may be determined that the requirements are not verifiable. Hardware prototyping may also be used to validate requirements. Simulink, Real-Time Workshop and xPC Target  allow hardware in the loop prototyping that can be used as a part of requirements validation. Hardware prototyping is not covered in this demo.

System Design

The system design is implemented in the Simulink model *do178b_dhc2*.

<matlab:open_system('do178b_dhc2'); Open the system model (requires Aerospace Blockset license)>

The top level of the model consists of the following subsystems:

* DeHavilland Beaver Airframe (plant model)
* Environment (atmosphere, gravity and wind models)
* Signal Conditioning (electrical to mechanical interfaces)
* Air Data Sensor (sensor feedback model)
* Inertial Data Sensor (sensor feedback model)
* Cockpit Controls (pilot interface)
* Engage and Mode Panel (autopilot interface/test stimulus)
* Reference Signals (autopilot interface/test stimulus)
* Verification_Blocks(requirements based test verification)

The top level of the model also contians the following model reference block:

* Autopilot (controller model)

<matlab:open_system('Autopilot'); Open the Autopilot model>

The layout of the top level illustrates a basic system architecture that not only represents the system design, but also provides the capability to verify that the system design satisfies the system requirements. The top level of the model includes a model information block showing the model version and date that the model was last saved. The use of this model will be discussed in the following sections in more detail.

System Design Verification

With the system design implemented in the Simulink model *do178b_dhc2*, it is possible to verify the design against the system requirements without having to first implement that design in actual hardware and software. This is the advantage of Model-Based Design.

At the top level of the model *do178B_dhc2*, there are three signal builder blocks provided to conduct specific tests during simulation. These blocks are:

* Engage and Mode Panel (autopilot interface/test stimulus)
* Reference Signals (autopilot interface/test stimulus)
* Cockpit Controls (pilot interface)

Each of these blocks can be opened in the model by double clicking them to see the definition of the test signals and test groups within the signal builder blocks. These blocks also contain traceability back to the system requirements document for the requirements that will be tested using each group of signals. For more detailed information about how to use the signal builder blocks, see the help section for that block in Simulink Help.

<matlab:doc('simulink/signalbuilder'); Help for Signal Builder block>

Signal stimulation can also be accomplished via other means, such as using other Simulink source blocks or by using From Workspace blocks in conjunction with MATLAB scripts or MAT files that load data to the  workspace. It is also possible to use the Data Import/Export Configuration Parameters dialogue to get data from the MATLAB workspace.

Of course test stimulus alone is not enough to verify the system design is satisfying the requirements. Assertion blocks, which are contained in the subsystem *Verification_Blocks*, can be used to determine whether or not the system requirements are satisfied by the design during simulation. To see the verification blocks open this subsystem.

<matlab:open_system('do178b_dhc2');open_system('do178b_dhc2/Verification_Blocks');
Open Verification_Blocks subsystem>

The following four Assertion Blocks can be found in this subsystem

* Check Dynamic Roll
* Check Roll Range
* Check Roll Rate Range
* Check Aileron Range

The assertions *Check Roll Range*, *Check Roll Rate Range* and *Check Aileron Range* provide minimum and maximum values for roll angle, roll rate and aileron angle respectively. These assertions are enabled during all test cases and the values are based on the *Autopilot Requirements Document*, including tolerances. For example, the requirement for the maximum roll angle is 30 degrees +/-10%, which means the allowable roll angle is from -33 degrees to +33 degrees.

The assertion *Check Dynamic Roll* provides a window between minimum and maximum values that is dependent upon the test case being executed. The signal builder block *Roll References* is used to provide the minimum and maximum values for each test case using signal groups. This block can be  opened in the model by double clicking it to see the definition of the test signals and test groups within the signal builder block. This block also contains traceability back to the system requirements document for the requirements that will be tested using each group of signals.

The use of three signal builder blocks for test stimulation and one signal builder block for assertion control requires that the selected groups of each of these blocks be coordinated. *MATLAB Report Generator* is used for this coordination of the test cases and also to provide automated test case execution.

<matlab:setedit('Roll_AP_Tests_loop_sim'); Open Report Generator for this demo (requires MATLAB and Simulink Report Generator licenses)>

MATLAB Report Generator provides a framework for setting up simulations,  executing them, and finally generating a test report. For more  information about MATLAB Report Generator, see the help.

<matlab:doc('rptgen'); Help for Report Generator>

The report file *Roll_AP_Tests_loop_sim.rpt* performs the following sequence to run the simulation cases and procedures, as well as verifying the results:

* Opens the test harness model and inserts a snapshot into the report
* Opens the autopilot model and inserts a snapshot into the report
* Opens the verification subsystem in the test harness model and inserts a snapshot into the report
* Sets up arrays for Signal Builder block cases, initial Roll Angles, Test Descriptions and Test Criterias for 20 different tests
* Executes a loop of 20 simulation cases that include the following:

# Lists the requirements being verified for each individual iteration
# Lists a description of the test for each individual iteration
# Lists the test results criteria for each individual iteration
# Sets the initial roll angle for each individual iteration
# Sets the Signal Builder blocks cases for each individual iteration
# Runs the simulation of the test harness for each individual iteration
# Generates a plot of the simualtion results for each individual iteration
# Generates a pass or fail indication of the results for each individual iteration

* Inserts a link to the model coverage cummulative summary report
* Provides a summary pass or fail indication for the cumulative results

To run the simulations, click on the link below, this will take a few minutes to execute.

<matlab:report('Roll_AP_Tests_loop_sim.rpt'); Generate Simulation Report (requires MATLAB and Simulink Report Generator licenses)>

A pre-generated report is also available. To view this report click on the link below.

<matlab:winopen('Roll_AP_Tests_loop_sim.pdf'); Open the simulation report for roll_ap>

A Model Coverage report is also generated during testing. To view this report click on the link below.

<matlab:winopen('roll_ap_cov.html'); Open the Model Coverage report for roll_ap>

Software Requirements

From the system design, specific functions are allocated to software in order to form the software requirements. For this system design, the *Autopilot* subsystem in *do178b_dhc2* is being allocated to software, and therefore becomes the high level software requirements.

<matlab:open_system('Autopilot'); Open Autopilot model>

It can be seen within the *Autopilot* model that the architecture is further broken down into *Roll_Autopilot*, *Pitch_Autopilot* and *Yaw_Damper*. This provides several important features in the design:

* The roll, pitch and yaw axes of the autopilot are all configured as separate files which can be reviewed and tested independently
* The source code for the roll, pitch and yaw axes are all configured as separate files which can be reviewed separately
* The test harness is configured in a completely separate file and changes to the test harness do not affect the models used for embedded code generation

We can now open the separate *roll_ap* model to see the actual functionality that is allocated to software.

<matlab:open_system('roll_ap'); Open roll_ap model>

It can be seen within the *roll_ap* model that the architecture is further broken down into *Heading Mode*, *Roll Reference* and *Basic Roll Mode* systems. *Heading Mode* and *Basic Roll Mode* are  separate reference models, while *Roll Reference* is a subsystem. Either of these reference models or the subsystem can be opened, and the basic functionality comprised of Simulink blocks will be found.

The top level of the model includes a model information block showing the model version and date that the model was last saved.

To view a requirement associated with a block in the model, right click on the Mode Switch block, go to Requirements on the tab and then select  “1.2. Roll Hold Mode” and the Autopilot Requirements document will open With section 1.2 Roll hold Mode highlighted.

Software Requirements Verification

For DO-331 it is necessary to have requirements verification artifacts with respect to compliance to the system requirements, accuracy and consistency, verifiability and algorithm accuracy. The simulation activities described in the previous section can be used to cover these  objectives in DO-331.

For ARP-4754A, DO-178C and DO-331, it is necessary to have an artifact that shows traceability between System Requirements and Software Requirements. A *System Design Description* report can be generated using the *Simulink Report Generator* and this document contains a section that includes the traceability information from the model to the requirements document. This report may take a few minutes to complete.

<matlab:open_system('roll_ap');rptcfg=StdRpt.SDD(bdroot);rptcfg.run();
Generate the System Design Description report (requires MATLAB and Simulink Report Generator licenses)>

A pre-generated report can be opened.

<matlab:open('roll_ap.pdf');
Open System Design Description for roll_ap model>

This report provides a complete description of the model, including:

* Snapshots of the top level model and all subsystems
* Interfaces for the top model and all subsystems
* Block execution order
* Details of all block dialogue settings
* Details of all parameters and signals used in the model
* Traceability from the model objects to the requirements document
* Details of all the model configuration settings
* Glossary and report explanation

For more information about the *Requirements Management Interface* see the *Help for Simulink Verification and Validation*. This describes how to interface to various document formats, including Word, as well as how
to interface to DOORS.

<matlab:doc('slvnv'); Help for Simulink Verification and Validation>

As a part of the DO-178C lifecycle, the high and low level requirements must be verified to comply with standards and it must also be shown that the requirements are compatible with the target computer. The Model Advisor provides the capability to perform static checks on the model in order to verify many standards automatically and to verify certain code generator option settings related to hardware compatibility. To generate the Model Advisor report for the roll_ap model, select the link below. This may take a few minutes to complete.

<matlab:open_system('roll_ap');ma=Simulink.ModelAdvisor.getModelAdvisor('roll_ap');IDs=ma.getCheckForGroup('Simulink%20Verification%20and%20Validation|Modeling%20Standards|DO-178C/DO-331%20Checks');marpt=ModelAdvisor.run('roll_ap',IDs);marpt{1}.viewReport;
Generate roll_ap model advisor report (requires Simulink Verification and Validation license)>

A pre-generated report can be opened.

<matlab:winopen([pwd%20'\slprj\modeladvisor\roll__ap\report.html']);
Open Model Advisor Report for roll_ap model>

Software Design and Architecture

The software design and architecture is defined within the roll_ap model and the Heading_Mode and attitude_controller reference models for the control portion of the roll autopilot function. For this function the
code will be generated directly from the high level software requirements, therefore the model satisfies both the high and low level software requirements.

To view the architecture of the roll_ap model and it’s referenced models open the following:

<matlab:view_mdlrefs('roll_ap'); Open Model Dependency Viewer for roll_ap>

To view the architecture of the entire autopilot model, it’s referenced modelsopen the following:

<matlab:view_mdlrefs('Autopilot'); Open Model Dependency Viewer for Autopilot>

There will be additional design and architecture activities that will be required beyond just the model in order to fully define the high and low level software requirements. For example, there will be requirements and design associated with processing the inertial reference and air data sensor inputs, passing those inputs to the model inputs, and scheduling these two task in the proper order. These activities are not included as
a part of this demo.

Software Design Verification

Because the high and low level software requirements have been combined  for this demo, the software requirements verification activities previously discussed are being used to also cover the software design
verification activities.

Source Code

The source code is generated from the roll_ap model using Embedded Coder. The *ert.tlc* target is used for this demo. The use of this target provides a good baseline for generating traceable and MISRA compliant code. The Model Advisor for this demo is using configuration settings necessary for MISRA compliance and high integrity code.

The code for the autopilot and roll_ap models and therir referenced  models can be generated and an HTML report can be viewed by clicking the following link.

<matlab:slbuild('Autopilot');
Generate code and open code generation report for Autopilot (requires Embedded Coder license)>

A pre-generated code generation report can be opened.

<matlab:winopen([pwd%20'\Autopilot_ert_rtw\html\Autopilot_codegen_rpt.html']);
Open Code Generation Report>

The C code file for roll_ap can be opened from the HTML report by first clicking on the roll_ap Referenced Model link, then clicking on roll_ap.c. To see how the code traces to the model go to line 79 and click on the link ‘<S3>/FixPt Unit Delay1’. This will open the roll_ap model and highlight the Unit Delay block in the subsystem *Roll Reference* of roll_ap model. Other trace links can be found in the code and examined if desired. It can also be seen that code within the atomic subsystem, *Roll Reference* is clearly commented in the code file and contained on contiguous lines.

Source Code Verification

Source code verification is performed using *Simulink Code Inspector* and *Polyspace*. Simulink Code Inspector verifies that the source code structurally and functionally matches the model. The Simulink Code Inspector also verifies that the model and code trace to each other. To run the code inspection on Autopilot and all of it’s referenced models, click on the link below.

<matlab:config=slci.Configuration('Autopilot');config.setTopModel(true);config.setGenerateCode(false);config.setFollowModelLinks(true);config.inspect;open([pwd%20'\slprj\slci\Autopilot_summaryReport.html']);
Inspect Autopilot and reference model code and generate summary report (requires Simulink Code Inspector license)>

A pre-generated code inspection report can be opened.

<matlab:winopen([pwd%20'\slprj\slci\Autopilot_summaryReport.html']);
Open Code Inspection Report>

PolySpace Products perform verification activities on the source code. This includes a MISRA-AC-ACG checker that can verify the code conforms to  standards. A static analysis tool performs checks on the code to look for potential run time errors, unreachable code, un-initialized variables or data coupling issues. To run the code verification on Autopilot, click onthe link below.

<matlab:open_system('Autopilot');psopts=pslinkoptions('Autopilot');resultsFolder=pslinkrun('Autopilot',psopts,true);
Analyze Autopilot and reference model code and generate Developer Report (requires Polyspace Bug Finder and Code Prover licenses>

A pre-generated report can be opened.

<matlab:open([pwd%20'\results_Autopilot\Autopilot\Polyspace-Doc\Autopilot_Developer.pdf']);
Open Developer Report>

Executable Object Code

The executable object code will be generated from the source code using third party compilers/linkers. Embedded Coder can assist in the generation of make files for the software build process or this can be done manually. See the Embedded Coder Help for more information about this capability.

<matlab:doc('ecoder'); Help for Embedded Coder>

Executable Object Code Verification

The executable object code may be verified using the Processor-In-The_Loop mode (PIL) availble with Embedded Coder. This allows the simulation cases run using the report generator to be re-run easily on a traget board connected to Simulink via a serial bus. For more information about PIL mode, see the Embedded Coder help:

<matlab:doc('ecoder'); Help for Embedded Coder>

PIL mode is not currently part of this demo.

During requirements based testing it is also necessary to perform structural coverage analysis on the code to measure statement (Levels A, B and C), decision (Levels A and B) and modified condition/decision coverage (Level A). This is normally considered to be one of the most expensive and time consuming aspects of DO-178C objectives.

During execution of the tests on the object code using the PIL mode, it is recommended to use a code coverage tool to evaluate the structural coverage of the code. There are many third party coverage tools available for analyzing code coverage. Some of these tools instrument the code, in which case the tests need to be run on instrumented and un-instrumented versions of the object code. Embedded Coder provides integration with Bullseye and LDRA code coverage tools.

Although PIL mode is not supported in the demo, SIL mode software in-the- loop) is supported. SIL mode uses teh source code genrated for the final target, but is compiled ot run on the host computer, thus the executable object code doe snot match that of the real target hardware. However, for demonstration purposes this effectively shows how the PIL mode could be used to verify the software.

For this part of the demo, the Model Reference block for the Autopilot in the top level of the do178b_dhc2 model is set to run in  Software-in-the-loop (SIL) mode, and model coverage recording is set to off.

To run the tests, click on the link below, this will take a few minutes to execute.

<matlab:report('Roll_AP_Tests_loop_SIL.rpt'); Generate SIL Test Report>

A pre-generated report is also available. To view this report click on the link below.

<matlab:winopen('Roll_AP_Tests_loop_SIL.pdf'); Open the SIL test report for roll_ap>

Conclusion

The purpose of this demo was to provide a possible approach to satisfying many of the objectives of DO-178C when using a Model-Based Design process. Other approaches and variations to the approach in this demo are also possible.

Here is a list of potential advantages when using Model-Based Design with The MathWorks tools:

* Early requirements validation as the design process starts
* Verification of the design and elimination of errors early in development
* Test case and procedure development early in the project, not as an afterthought
* Coverage metrics early in the project, no surprises or extra work at the end
* Improved code quality via automatic code generation
* Automated source code analysis and standards compliance checking
* Automated test execution provides simplified but thorough regression testing
* Complete traceability between requirements, design, code and test cases

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    January 8, 2020

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    January 8, 2020

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