Mechanism: Cam connections - Bug?

[Jon_Bradley]

Using Wildfire 2.0, I am trying to run a dynamic analysis on a simple bicycle model rolling down a sloped surface.

The bicycle assembly simple has a chassis plus two wheels each pin jointed to allow rotation. This is placed into an assembly by a planar connection. Also a sloped 'track' part is placed into the assembly below the bicycle, and cam connections created between the bicycle wheel surfaces and the track surface, to give the effect of friction acting between the wheel/ground.
Gravity and mass are defined.
When I run a dynamic analysis, the model falls under gravity until it hits the track (as it should), but then it stops dead there, held due to the friction coeff set in the cam connection - the wheels won't rotate (so it doesn't roll down the hill) and I can't understand why. It seems to be ignoring the rotation of the pin joint in the wheels.
(when the coeff of friction is reduced, the bicycle wheels slide down the hill - however the wheels are NOT rotating)

Also the bicycle itself remains in whatever angle it is originally placed at, when one wheel hits the ground, by laws of physics the model should then rotate under gravity so that the other wheel then subsequently hits the ground. However it does not do this. This seems to be ignoring the rotational degree of freedom within the planar connection definition.

I have spent hours trying to correct this. Got nowhere. I am suspicious that it may be a bug in the software... Has anyone else had this problem? Does anyone know if I am doing something wrong?


Here I have uploaded a zip of the assembly: 2006-11-22_173142_Connection_problem.zip



I would really appreciate some help with this! My dissertation relies on it!

Thanks, Jon.
Edited by: Jon_Bradley

 

[jbuckl]

Jon I didnt look at the file yet, but i think i had this problem on a different cam situation.


just a suggestion at the moment..... try to redefine the track so that the slope to flat surface has no break ie the contact surface is defined by a spline instead of a line-arc-lineor line to line(line to line will require the cam joint to include 'liftoff')


Maybe if you didnt select lift-off this would be the first thing to try.


Jbuckl

 

[Jon_Bradley]

Thanks for the reply Jbuckl.


I'm afraid I don't really follow what you mean... The cam surfaces are defined as follows:


CAM1 - cylindrical surace of one of the wheels


CAM2 - the 3 surfaces (sloped plane,curve,flat plane) of the 'track'


Lift-off is enabled, and cam friction has been defined. (another cam is defined in the same way for the other wheel)


Here is an image of the initial condition - note the yellow lines representing the connections present. I have clicked on gravity to show the purple arrow, representing gravity direction.


View attachment 2909


and here is where it lies after running a dynamic analysis. The bicycle has fallen onto the track under gravity, however the wheels haven't rotated, and the bicycle itself hasn't rotated, despite its C of G clearly being inbetween the 2 wheels.


View attachment 2910


I would really appreciateif you do get time to take a look at the ProE files in my first post. I've spent hours and hours trying to solve this and got nowhere!


Many thanks


Jon Bradley.
Edited by: Jon_Bradley

 

[jbuckl]

Ok jon, I looked ......line=plane...try to define the plane without the tangency lines by using a surface sweep that selects a composite datum curve to sweeep across the width. redefine the cam connection to use this new spline driven surface.


you already got the lift off ok.