RecurDyn Contact - Becoming an Experienced User, 5th Episode: Basics of the contact algorithm #2

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This article is a continuation of the last article, RecurDyn Contact - Becoming an Experienced User, 4th Episode: Basics of contact algorithm #1

The following is the summary of the previous article.

1. RecurDyn uses Penalty Method to calculate the contact force.

2. Penalty Method assumes that two different bodies can be overlapped.

3. The amount of the overlap is called 'penetration'.

4. The contact force is (conceptually) calculated as f = kδ. (such as spring force)

5. It is important to use the appropriate K (Stiffness)

If K (Stiffness) is too small, penetration becomes big and the solver accuracy can be reduced.
If K (Stiffness) is too big, the solver becomes unstable .

Let's see some exceptions here.

Case #1)

In the figure below, let's assume that the blue (small) box is very heavy. (the mass is 100Kg, so that the downward force is 1000N)

- If K = 10N/mm, δ must be 100mm to achieve force equilibrium (the contact force = 1000N)

- If the height of the red box is 10mm, δ cannot be 100mm. Therefore, in this model, the blue box will fall down through the red box. (because force equilibrium is not achieved.)

example to show how to decide the stiffness in multibody dynamics contact

Solution #1)

When you simulate several models, sometimes you can see an excessively large penetration between 2 bodies. Please remember that this is usually caused by external force that are too big or K values that are too small.

If you want to reduce the penetration, increase K.

If you can predict the magnitude of the contact force, you can estimate the minimum K for the simulation (appropriate K). (For example, If you set K = 100,000N/mm, then force equilibrium can be achieved when δ=0.01mm)

Case #2)

Numerical analysis uses the discrete time for calculation. Therefore, when the time step is big, (as figure below) contact doesn't occur between 2 bodies at some time instance. Then on the next time step, 2 bodies may overlap excessively. (In other words, large penetration can be caused)

In the below model, at t0, δ = 0, so only gravity is applied to the upper box and it moves downward.

If δ becomes 15 at t1, the upward force (contact force) becomes 150 and the downward force (gravity) is still 30. So the net force becomes 120 (upward force: 150 - 30). As a result, the upper box can suddenly bounce upwards because abrupt upward force is applied to the upper box.

basics about multibody dynamics contact algorithm

Solution #2)

To avoid this kind of trouble, you should use time steps small enough to prevent the sudden big penetration. You can use the solver parameter, maximum time step for this solution..

Experienced users can use the Maximum Stepsize Factor in the Contact parameter dialog instead. This parameter uses smaller time step when contact between 2 bodies is expected.

For example, Maximum Stepsize Factor = 10 in the dialog below (right-side).

When 2 bodies are about to be contacted (they approach each other), RecurDyn solver reduces the time step ( 1/10 of the time step) to prevent the excessive penetration by sudden collision.

RecurDyn Contact parameter - max time step and max stepsize factor

Case #3)

This is similar to Case #2)

In the similar situation with Case #2, it is possible that the upper body just falls down instead of bouncing off.

This kind of result may be caused by the Maximum Penetration in the Contact parameter dialog. This parameter means that if the real penetration is bigger than this 'Maximum Penetration', the contact force is not calculated.

In other words, If maximum penetration = 5, since δ = 15 at t1, the contact force becomes 0. (because 15 > 5).

As a result, only the downward force (30) is applied to the upper box, and the upper box falls down.

RecurDyn Contact parameter - Multi Body Dynamics max penetration

Solution #3)

The solution is similar to the Solution #2.

The time step size needs to be reduced to prevent the excessive penetration.

**Technically, increasing maximum penetration can prevent this kind of problem, but it is recommended to reduce the time step size in most of the cases. (because smaller penetration is usually the better result.)

Please refer to this article to know more about Maximum Penetration.

If you want to know more about RecurDyn Contact, please refer to the below articles

Technical Support

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This article is a continuation of the last article, RecurDyn Contact - Becoming an Experienced User, 4th Episode: Basics of contact algorithm #1

Let's see some exceptions here.

Case #1)