Andi’s Autopilot

1. Andi’s Autopilot A Hard Real-Time Control Application Broderic Gonzales CS 722 May 2, 2005 v.05

2. Purpose • “Cruise control” for an airplane. • During flight the autopilot must maintain a plane’s: • Altitude: keep the plane level at its current height • Direction: keep the plane traveling in its current heading • Attitude: keep the plane’s orientation stable • The autopilot is a hard real-time system.

3. What makes a system real time?1 • Timeliness • Reactiveness • Concurrency • Device abstractions • Distributive • State-dependency • Dynamic internal structure 1 Coad[230]

4. Timeliness • Timing constraints if not met cause unwanted loss of data. • The autopilot uses separate, prioritized threads of control.

5. Reactiveness • Immediately identify, respond and process changes in the system. • The autopilot continuously polls instruments and reports deviations to the autopilot to correct the plane’s flight.

6. Concurrency • Multiple activities may be taking place at the same time. • The autopilot uses multiple threads of control. Each thread does a single task and is assigned to different subsystems to either monitor an instrument or manipulate a control.

7. Device Abstractions • Abstract representations of both the physical (problem domain) and the logical (program design.) • The autopilot abstracts the instruments and controls as objects of the airplane. • The autopilot is built on a framework layer that abstracts the implementation details of the operating system.

8. Distributive • Responsibility of controls is spread amongst objects. • The autopilot allows the instrumentation to maintain only the information that is relevant to them.

9. Design Overview

10. Main Classes

11. Airplane::Airplane() :_elevators(0), _ailerons(0), _rudders(0) { Controls* controls = Controls::On(); _elevators = controls->GetElevators(); _ailerons = controls->GetAilerons(); _rudders = controls->GetRudders(); } bool Airplane::ActivateAutopilot() { bool elevatorsActivated = _elevators->ActivateAltitudeMaint(); bool aileronsActivated = _ailerons->ActivateHeadingMaint(); bool ruddersActivated = _rudders->ActivateHeadingMaint(); // Was activation of sensor monitors successful? if( elevatorsActivated && aileronsActivated && ruddersActivated ) return true; else return false; } Airplane Class Implementation Details

12. Elevators Class Details

13. Elevators Class Details Continued bool Elevators::ActivateAltitudeMaint( ) { Initialize(); bool status = false; // Create thread to monitor for updates. taskId = _task->Start( &CallBack, (void*)this); if( _taskId == Sthread::GOOD_TID ) status = true; // Spawn subthreads. if( status ) status = Activate(); return status; }

14. Elevators Class Details Continued float Elevators::CalcAdjustments() { float length = 0.0; if( _altChgRate != 0.0 ) length = RATE *(_altDeviation/_altChgRate); float angle = 0.0; if( length != 0.0 ) angle = atan( length/_altDeviation ); return (_pitch + angle); }

15. Altimeter Class Detail void Altimeter::MonitorAltitudeDeviation( void* pid ) { Altimeter* a = (Altimeter*)pid; for( ;; ){ // Lock. a->_mutex->Acquire(); float current = a->ReadAltitude(); a->_altDeviation = a->CalcDeviation( current ); if( a->IsReportable() ){ a->ReportDeviation(); } // Unlock a->_mutex->Release(); } } Thread function example

16. Vertical Speed Gyro Detail void VerticalSpeedGyro::Register( Elevators* e ) { // May eventually want to store // this into a subscribers list. _elevators = e; } void VerticalSpeedGyro::ReportChangeRate( float changeRate ) { _elevators->Update( VSI, changeRate ); } Register-Update example

17. Attitude Gyro Detail bool AttitudeGyro::DeactivatePitchMonitoring() { _pitchMutex->Acquire(); _pitchTask->Cancel(); _pitchMutex->Release(); return true; } Perhaps, the composite pattern would’ve helped solve the complexity of this class.

18. System Interaction Classes Test drivers. They generated data for the instruments.

19. SI Classes - Details Composite Pattern Not really a gyro

20. OS Abstraction Layer Although, these classes are concrete, their design is intended to act as a framework for this autopilot to work on different OS.

21. OS Abstraction Layer Details Relying on POSIX threads

22. References • Coad, Peter. Object Models: Strategies, Patterns, & Applications. 2nd ed. New Jersey:Prentice Hall,1997