3501 #7: Rigid Body Motion

Assignment 7: Rigid Body Motion

Due date: Wednesday, Dec 1

This assignment will let you build a simple rigid body simulator (without collisions). You will implement torques and orientation changes in a simple setting. This is a bonus assignment. If you do this assignment, your mark on the assignment will replace one of your the other assignments (either one that you missed, or the assignment on which you got the lowest mark.)

Part 1: setup

Download the submarine demo from the 3501 site.
Run it to make sure it works before starting to modify it.
Look carefully at the Boat.cs file, which has the setup and simulation code for the submarine. Notice that there is an orientation quaternion, but no change in orientation, and forces, but no torques.

Part 2: Torques and Angular Momentum

The submarine has four "hitpoints", locations where off-centre forces can be applied. The system is presently calculating buoyant forces for these locations, but not torques.

Calculate a torque for each hitpoint (r x F). Be VERY careful what you use for r! The simplest thing is to use the centre of mass of the submarine as the origin and to do calculations in the submarine's body space, but in order to do so you need to transform the world-coordinate into body space. Alternatively, you can transform the body coordinates into world space.

Once you have suitable torques, update L (angular momentum) and use omega = Iinv * L to find angular velocity. Then, you have all the information you need to update the complete state vector, including the orientation q. Remember to renormalize your orientation quaternion at every timestep.

Part 3: Damping

You will probably notice that the system is rather unstable: the submarine can start spinning with ever-increasing speeds. This is because the rotational motion is not damped. Introduce damping torques.

The damping force at a point will be -k*v, for a damping constant k and linear velocity v. The movement of a point is not the movement of the whole body! Recall that the rotational velocity of a point (i.e., relative to the centre of mass) is omega x r. Again, remember to keep your coordinate systems straight when you calculate r: as described, r is the vector to the centre of mass. Once the damping forces are known, apply the damping forces and torques to the body -- one for every underwater hitpoint.

Part 4: Bonus

For a bonus of up to +10%, turn the simulation into a game by trying to keep the submarine stable despite stormy seas. At random intervals, assign a random impulse to one of the four hitpoints of the submarine. Also, give the user the ability to apply small known impulses to the hitpoints using keypresses. Track how much the submarine is tipped over (vertical vector in body coordinate frame is far from world vertical -- hint, this is the same problem as the lander question on the midterm) and if it exceeds some value, the game is lost. Keep score by marking how much time the player can keep the submarine up.

Handing it in

Hand in your project using WebCT. The easiest thing to do is to create a single zipped folder and submit that, rather than all the individual files. It might take a while to upload, so be prepared to take a little break while the files are in transit.