WORKSHOP 12 BOUNCING BALL

WORKSHOP 12 – BOUNCING BALL Problem statement Make the ball bounce on the plane in the given model.

WORKSHOP 12 – BOUNCING BALL Model description The given model has all of the geometry and parts defined (including mass properties). In addition, there is already a revolute joint that connects the diving board to the ground. You are going to add a torsion spring and a contact force to the model.

WORKSHOP 12 – BOUNCING BALL Getting started Create a new database. Load the file ball_start.cmd. Simulate the given model for 1 second, 100 steps. Animate the simulation results. The diving board should fall like a pendulum. The ball should just free fall.

WORKSHOP 12 – BOUNCING BALL Getting to know the model

WORKSHOP 12 – BOUNCING BALL Getting to know the model Notice the following: The ball and diving_board parts have user-specified mass & inertia properties. The revolute joint is at the global origin (0,0,0). Its I and J markers are marker_201 and marker_101, respectively. The board dimensions are: 1000 mm long, 100 mm deep, and 20 mm thick mass = 2 kg The ball dimensions are: 50 mm radius

WORKSHOP 12 – BOUNCING BALL Changing the pendulum: To change the pendulum into a diving board: Create a Torsion Spring element using the Torque element found in the Applied Forces panel at the diving_board to ground joint. Specify Two Bodies for the Run-time Direction Use the diving_board and ground as the two bodies. Use the joint to locate the spring Specify K and C as the Characteristic where: KT = 30,000 After the force has been created, modify the force element and change the damping portion of the expression so that damping has a value of 100. Save the model. Simulate the model for 1 second, 100 steps and then view the animation.

WORKSHOP 12 – BOUNCING BALL Check that the spring element is proper: Write the Adams/Solver dataset (.adm) for this model to the Info Window: Do a File -> Export, Adams/Solver Dataset and select ‘Write to Window’. Look through the model definition and find the SFORCE statement. Ensure that: It is of type ROTATIONAL The I and J marker definitions are sensible The function expression is sensible.

WORKSHOP 12 – BOUNCING BALL Review the last animation: Graphic of force in torsion spring The board will likely not be stationary so you can modify the force and: Increase the spring stiffness to 1e5 Increase the spring damping to 1e4 You must have this portion of the model working properly before you move on to the next section.

WORKSHOP 12 – BOUNCING BALL Creating contact To add contact between the ball and the diving board: Create a vector force between the ball and diving board: Select the ball as the first body and the diving board as the second body. Select the ball cm marker (MARKER_300) for the location Modify the VFORCE Reference Marker: Modify the newly-created VFORCE element and specify that the Reference Marker be marker_200 on the board. This ensures that the force orientation is always derived from the marker moving with the diving board. Determine markers for the IMPACT expression: The IMPACT function requires marker displacement and velocity as inputs, so you must first identify suitable markers. A good choice might be the I-marker in the VFORCE (on the ball part) and marker_200 on the board. Do an Info on the VFORCE to determine the I-marker name. This is used in the next step.

WORKSHOP 12 – BOUNCING BALL Creating contact (cont): Create the IMPACT expression in steps: Right-click the Y Force field in the VFORCE and go to Function Builder. Using the Displacement functions, insert an expression into the text field for the ball height with respect to the diving board. An example might be: DY(MARKER_300, MARKER_200, MARKER_200) Insert a similar expression for the ball velocity, something like: VY(MARKER_300, MARKER_200, MARKER_200) Use these expressions to create an IMPACT function between the ball and the board. Refer to the IMPACT documentation for the various parameters and create an expression that looks similar to this (your marker IDs may vary): IMPACT(DY(MARKER_300, MARKER_200, MARKER_200), VY(MARKER_300, MARKER_200, MARKER_200), 50.0, 100000, 1.5, 10, 0.1) Enter zero for X and Z Force field.

WORKSHOP 12 – BOUNCING BALL Simulate and animate: Simulate the model and animate. The ball should hit the diving board and bounce up. If you let Adams/Solver simulate long enough, you will notice that the ball continues to make contact with the board even after it has gone beyond the graphic. It does not fall off. Mathematically, the ball is hitting an infinite plane. To change this behavior, move on to the next section, Optional tasks.

WORKSHOP 12 – BOUNCING BALL Optional task 1: To make the ball fall when it reaches the right end of the board: Right-click and modify the function expression for the IMPACT. Multiply the IMPACT function by a STEP function that turns the force off when the ball reaches the end of the graphic. The STEP function would look something like this:

WORKSHOP 12 – BOUNCING BALL Optional task 1 (Cont.) In the example given on the previous page, the RM’s y-axis defined the normal force direction; therefore, in the VFORCE, you would edit the FY function such that it looks something like this: FY=IMPACT(...)*STEP(DX(300,201,201), 500,1,1050.01,0)

WORKSHOP 12 – BOUNCING BALL Optional task 2: Create a CONTACT element between the ball and diving board. Use the parameters shown to mimic the IMPACT model. Right-click and Deactivate the VFORCE element. Re-run the model and note the behavior: the system should behave in a similar way.