What the axis in friction coeff mean?

[horsefacelee]

I am currently using the vortex dynamic engine to do simulation. I am confused of the "axis orientation" in the setting for material property. I wonder why it is defaultly define as [0 0 1] (i.e., z axis), does that represent the direction of normal force? Or the direction of friction force? In addition,if I want to define the friction of 2 different object, where and how can I define it?

[coppelia]

Hello,

With a surface, you have 2 friction directions that you could handle separately. E.g. you could say that in one direction, the friction is 1, and in the direction perpendicular to the first direction, the friction is 0. This would allow you to mimic a surface structure that lets objects slide along the second direction, but grip along the first direction.

With the Vortex engine, both directions have the same coefficient of friction, by default. So in that case, the Axis orientation item has no meaning. But if you uncheck the item Linear secondary axis --> Use same values as linear primary axis, then both directions can be handled individually. In that case, what is the first direction or primary axis? The first direction (or primary axis) is the projection on the contact surface of the Axis orientation vector, expressed in the shape's reference frame.

Cheers

[horsefacelee]

Hello,

With a surface, you have 2 friction directions that you could handle separately. E.g. you could say that in one direction, the friction is 1, and in the direction perpendicular to the first direction, the friction is 0. This would allow you to mimic a surface structure that lets objects slide along the second direction, but grip along the first direction.

With the Vortex engine, both directions have the same coefficient of friction, by default. So in that case, the Axis orientation item has no meaning. But if you uncheck the item Linear secondary axis --> Use same values as linear primary axis, then both directions can be handled individually. In that case, what is the first direction or primary axis? The first direction (or primary axis) is the projection on the contact surface of the Axis orientation vector, expressed in the shape's reference frame.

Cheers

I am still a little confused.
Suppose I only want friction coeff. on x direction is 0.8, y is 0.5. If I set the axis as [0 0 1] will leave no projection on the contact plane right? If so what direction will the force apply? Does that mean friction coeff. is the same in both direction? So if I want different friction coeff. for different axis, what should I do? Add sth to the x and y direction, like [0.577 0.577 0.577]?

Also, suppose I want to make two different object contact a slope, which I create a plane to represent it. Whether the two objects or the plane have the friction coeff. i.e., three parameters now. How to define friction coeff. in such condition?

In addition, would you mind give me a brief explaination about the "friction model" ?

[coppelia]

Imagine you have a sphere. Now, what is the X direction on that sphere? What is the Y direction?

Imagine now you have a reference frame attached to that sphere. And you visualize the Axis orientation vector in that reference frame. Then, that vector represents the linear primary axis.On that sphere also, you have exactly 2 points where the linear primary axis is not clearly defined. But on all other points on the sphere, the linear primary axis is well-defined.

If you have two objects with 2 different friction coefficients that touch, then the final coefficient of friction is usually given by the multiplication of both. i.e. if one surface has no friction, then final friction is also zero. If both surfaces have a friction of 1, then the final friction is also 1.

Cheers

[horsefacelee]

now I am totally lost.
First, I always thought axis of orientation is "3 axis", i.e., the x,y,z axis of the new frame in reference frame, which is the column vectors of orientation matrix. So here we only have one vector for that really confused me.

Also, as far as I know, friction force is F(friction)=coeff.*normal force.
I am not sure whether I interpret this correctly, I suppose in this simulation, we can define object slide in one direction while friction force acting on another direction. However, I still can't understand the relationship between the "friction force definition" and "parameters in v-rep".

[martin_simulation]

Hi,

I will try to clarify the situation.

For the friction model: The friction model allows setting the friction boundaries (FB). You can either specify FB yourself (box model) or you can use scaled box so that the FB=Fn*friction coefficient where Fn is the normal force. To make long story short, this requires soilving the equations more than once since Fn is initialy unknown. The scaled box fast is an optimization where Fn is used from the same contact's Fn at previous frame so only one solve is required. If the viscous friction force as compute using the -gamma*v equation is within bounds then you have a viscous friction otherwise it is bound according to the friction model you use.

Since each shape has a material, during a collision we need to come up with a combo of those two materials. In V-Rep you have no control on the combo. The result is we use the lowest contact stiffness, the simplest friction modela (order is from high to low: box, scaledboxfast, scaledbox) and the lowest friction coefficient.

If one of the material is set to neutral, the other one is used. If one is set to None, there will be no friction for this axis.

For the friction direction, it is specified on shape's frame. If you set the direction to 0,0,0 (automatic mode), it is ignored and the direction is computed automatically.
If you set it to a valid vector V , the primary friction direction will be N x V x N where N is the contact normal direction. If result is 0,0,0, the primary direction fals back to auto mode. Normally, most shape are in auto mode so if a shape is not in automatic mode, it will be the one providing the direction. If the two colliding shapes want to specify a direction, one of the shape will have priority according to the following priority list (from high to low priotity):

Plane,Sphere,Box,Cylinder,TriangleList,Capsule,HeightField,Convex,TriangleMesh,

I hope this answers your questions.

Best regards,

Martin