Machine Problem 4/5: Lunar Lander

Introduction

A lunar lander is controlled by rocket engines. Rocket engines produce a constant force in the opposite direction that they are fired. The main truster points in the downward direction and is used to counteract the force of gravity. Side thusters control movement in the horizontal direction.

The goal of this machine problem is gently land your craft on the surface of the moon. Your lander will be damaged or destroyed if you descend too fast. The craft will also be destroyed if you attemped to land on a surface other than the landing pad. You will find yourself lost in space if you stray too far away from the destination. Finally, you will lose control of the thrusters if you run out of fuel.

For more information about the real lunar landings, see the references below:

• A brief history of NASA's lunar landing missions is summarized in Apollo Expeditions to the Moon - CHAPTER 11.1.
• Documentation of the Apollo 11 mission: Video clip of landing

Implementation

Interrupts are used to read the keyboard and perform real-time calculations at periodic intervals. The left, right, and downward arrow keys are used to control the operations of the thrusters. By reading scan codes directly from the keyboard, the program is able to decode simultaneous keypresses. A Thurster operates at full capacity while the button is pressed.

The timer is used to periodically compute new values of acceleration, velocity, and position. The function which performs these calculations is chained onto the default system timer interrupt vector and called 18 times a second.

Physical Equations

Recall that acceleration (A) is the time derivative of velocity (V). Velocity (V) is the time derivative of position (X). For this problem, it is convienient to calculate the piece-wise integral of acceleration to obtain velocity and the piece-wise integral of velocity to calculate the position. Using V_i-1 as the previous velocity and X_i-1 as the previous postion, new values of A, V, and X can be calculated at each instant of time as shown below:

A_i = Sum of forces / Mass
V_i = V_i-1 + A_i * dt
X _i= X_i-1 + V_i * dt

Numeric Representation

Data Display

Procedures

• InstTime
  • Purpose: Install the timer interrupt
  • Description:
    This routines should chain the procedure MyTimeInt into list of functions called by IRQ 0. It should preserve a pointer to the original timer vector for use by MyTimeInt and DeInstallTime.
  • Notes:
    The library function always installs the library-based MyTimeInt routine.

• MyTimeInt
  • Purpose: Calculate new acceleration, velocity, position, time, and fuel values every 1/18th of a second.
  • Description:
    
    • As described above, velocity and acceleration are computed as piece-wise integrals. This routine performs these calculations on the periodic interval of the default timer (1/18 of a second). The program reads modifieds the variables:
      x, v_x, a_x, y, v_y, a_y, G, T, TL, and TR
      (see variable section of given mp4.asm).
    • Time should be incremented each time the procedure is called.
    • Fuel should be decreased by the amount of thrust that each thruster is exerting. If fuel runs out, T, TL, TR should be reset to zero.
    • ExitFlag should be set to 1 if the ship touches the ground (y < 0) or gets lost in space (i.e., moves outside the edges of the window on the screen).
  • Notes:
    
    • Remember that this is an interrupt service rotuine. Be sure to save all registers and exit with IRET.
    • Review the on-line lecture notes, especially Lecture 14 and Lecture 15.

• DeInstallTime
  • Purpose: Restore original timer vector
  • Description:
    This routine should be called just before you exit your program. It should restore the default vector for IRQ0.

• InstKey
  • Purpose: Install a new keyboard handler.
  • Description:
    This function should replace (not chain) the the default keyboard interrupt vector with the function MyKeyInt. It should preserve a pointer to the original keyboard vector for use by DeInstallKey.

• MyKeyInt
  • Purpose: Read scan codes from the keyboard
  • Description:
    This function replaces the default keyboard vector and is used for the duration of the program. The routine should respond to the escape key and to the left, right, and down arrow buttons. When the following keys are pressed, the following actions should occur:
    • ESC: Set ExitFlag=1.
    • Left Arrow: Set TR=2 (Need right thrust to go left)
    • Right Arrow: Set TL=2 (Need left thrust to go right)
    • Down Arrow: Set T=5
  • Notes:
    Remember that this is an interrupt service rotuine. Be sure to save all registers and exit with IRET. The keyboard interface is documented in the lab manual (pagees 80-81), in the textbook, and on-line in the PC-GPE as keyboard.txt.html

• DeInstallKey
  • Purpose: Restore original keyboard vector
  • Description:
    This routine should be called just before you exit your program. It should restore the default vector for IRQ1.

• DrawScreen
  • Purpose: Draw the static portion of the graphic screen
  • Description:
    This routine should draw all the static text, boxes, and graphics on the screen that do not change as the lunar lander program runs.
  • Notes:
    
    • You may find many parts of your MP3 code useful for this routine.
    • You will want to create modular components to avoid coding repetitive operations.
    • The landing pad appears in the middle of the screen at X=0
    • You are encouraged and welcome to exercise artistic freedom in the implementation of your screen. At minimum, however, you must support the features of the library function.

• ReDrawScreen
  • Purpose:Redraw dynamic portions of graphics screen
  • Description:
    This routine redraws the portions of the screen that change as the lunar lander moves.
  • Notes:
    
    • The values displayed are contained within the variables: x, v_x, a_x, y, v_y, a_y, init_fuel, and fuel.
    • Recall that numbers should be displayed in units of m and m/s (not m/324 and m/s/18).
    • Display thrust if T, TR, and/or TL are non-zero.
    • For the windowed display, horizontal columns represent 2 m and vertical rows represent 4 m.

• FinalScreen
  • Purpose:Prints the messages shown at the end of the program.
  • Description:
    To land successfully, the lander must:
    • Hit the pad within 4 m
    • Avoid straying off the screen
    • Have a slow vertical velocity. Specifically:
      • 0 - 3 m/s: Perfect Landing
      • 3 - 6 m/s: Slight Damage
      • 6 - 9 m/s: Severe Damage
      • 9+ m/s: Crash!
    After accessing the status of the landing, the program should display the flight time and amount of fuel used.
  • Notes:
    
    • Print flight time in seconds, not s/18.
    • Again, artistic freedom is encouraged as long as your program has at least the same features. Sounds effects and/or an animation could easily earn that ever-so-sought-after Gold star of programming excellence!

• LandingPad
  • Purpose:Add another landing pad on the moon.
  • Description:
    Add an another flat landing pad on the right hand-side of the screen.
    Change the program to allow landing at that site as well as the original.
  • Notes:
    
    • This is not a procedure. Just a few modifications to existing data and and procedures.
    • Use your imagination to make the landing pad colorful.

Points

You are urged to test each routine as you write it. It is nearly impossible to debug a program if there are errors in the routines that it calls. Note that library routines always call other library routines. You need to call your routines directly to verify their functionality.

• DrawScreen: 15 pts.
• ReDrawScreen: 10 pts.
• FinalScreen: 15 pts.

• LandingPad: 10 pts.

• InstTime: 5 pts
• MyTimeInt: 15 pts
• DeInstallTime: 5 pts

• InstKey: 5 pts
• MyKeyInt: 15 pts
• DeInstallKey: 5 pts

  (You may complete procedures in any order)

Preliminary Procedure

• You will begin MP4 with the following files:
  • MP4.EXE: The fully functional program using library functions
  • MP4.ASM: Program Framework
  • LIBMP4.LIB: Working versions of all routines
  • LIB306.LIB: Useful (and free) routines
  • Makefile
• You can obtain these files via one of the methods listed below:
  • Download all files as: MP4.ZIP (Ver 2.1)

Clearifications and Erratica

• Version 2.0: Original Release (3/22/03, 1pm)
• Version 2.1: Correctly displays numbers for altitude, velocity, distance

MP4.ASM