Re: [Pyode-user] Not-really-fixed joints

[Hart's A. <bha...@ya...>]

Hi Ethan,

not sure if this will do what you want, but when i want to make an object follow a point i use a
magnet function, it won't be perfect, it will slip a little, try adjusting the value of magnet

target = vec3( x,y,z )
magnet = 100
v1 = vec3( body.getPosition() )
v2 = target-v1
body.addForce( v2.normalize()*magnet )

-brett


--- Ethan Glasser-Camp <gl...@cs...> wrote:

> Hi,
> 
> In a project I'm working on, there are objects which are "static",
> which means they affect objects without objects affecting them back.
> So an object can be joined using a joints to a "static" object, and
> the static object will pull the other object along without changing
> velocity.
> 
> I've been having a hard time figuring out how to simulate those
> objects. Enclosed is my current approach. Body2 is joined, via body1,
> to a static object which isn't actually in the simulation -- let's
> call it "body0". Body0 moves in the negative y direction at 50 units
> per second, so body1's position is forced to match body0 every tick.
> After about 40 ticks, this causes my simulation to freak out and move
> objects to (nan, nan, nan).
> 
> Is there a better approach to simulate this kind of thing? If not, how
> can I work around this behavior? I've tried playing with CFM and ERP
> parameters, both global and of the joints, but haven't found anything
> useful.
> 
> Ethan
> > import ode
> import math
> 
> world = ode.World()
> world.setCFM(1e-8)
> 
> # Create two bodies
> body1 = ode.Body(world)
> body1.setPosition((0,0,0))
> j0 = ode.HingeJoint(world)
> j0.attach(ode.environment, body1)
> j0.setAxis((0, 0, 1))
> j0.setAnchor((0, 0, 0))
> 
> body2 = ode.Body(world)
> body2.setPosition((200, 0, 0))
> M = ode.Mass()
> M.setSphere(2500, 0.05)
> body2.setMass(M)
> 
> j1 = ode.SliderJoint(world)
> j1.attach(body1, body2)
> j1.setAxis((200, 0, 0))
> j1.setParam(ode.ParamLoStop, 0)
> j1.setParam(ode.ParamHiStop, 4)
> 
> # Simulation loop...
> 
> fps = 50
> dt = 1.0/fps
> loops = 0
> 
> while loops < 50:
>     loops += 1
>     print body1.getPosition(), body2.getPosition(), j1.getPosition(), j1.getPositionRate()
> 
>     def down(pos):
>         return pos[0], pos[1]-1, pos[2]
>     move = down
> 
>     body1.setPosition(move(body1.getPosition()))
>     j0.setAnchor(move(j0.getAnchor()))
>     #body2.setLinearVel((1, 0, 0))
> 
>     # Next simulation step
>     world.step(dt)
> 
> print j0.getAnchor(), body1.getPosition(), body2.getPosition(), j1.getAxis()
> > -------------------------------------------------------------------------
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