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12-19-2016 08:36 AM

Hello,

This is my first post, so apologies if I have not fully grasped any of the forum ettiquite yet.

I am having some issues with 3d contact in NX motion, specifically with the reaction forces 'chattering'. My current method is to start with the recommended material settings, set the local penetration depth to 0 and then generally ramp up the stiffness as required to prevent any parts from penetrating each other and acheive a smooth reaction force graph.

When it works, it works beautifully. But simple changes to the geometry can cause the contacts to stop working properly, creating chatter and making the results a bit useless.

Can someone provide explanations for what any of these parameters are please?

- Global Maximum Penetration
- Local Maximum Penetration
- Plane Tolerance Factor
- Bounding Buffer Length
- Maximum Facet Size Factor
- Maximum Step Size Factor

Many thanks for your help in advance.

Solved! Go to Solution.

2 REPLIES

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12-20-2016 11:52 AM - edited 12-20-2016 11:53 AM

- Global Maximum Penetration

Sets the threshold value for determining how the solver calculates the penetration used to calculate the contact normal force. When the calculated penetration depth is greater than this threshold, no contact force is generated.

This threshold prevents the action body from completely penetrating the base body Such penetration would cause the non-contacting surfaces to face each other, and create a false contact.

For a description of how the solver uses this threshold, see Penetration depth calculation for the RecurDyn solver. You can let the solver calculate this value dynamically or you can set it manually:

- Local Maximum Penetration

Available only for 3D contacts. Sets the threshold value for determining how the solver calculates the penetration used to calculate the contact normal force. You can let the solver calculate this value dynamically or you can set it manually. For a description of how the solver uses this threshold, see Penetration depth calculation for the RecurDyn solver.

Penetration depth calculation for the RecurDyn solver:

After the solver determines that contact has been made between two bodies, it calculates the depth of penetration for the action body into the base body (this penetration value is used to calculate the contact normal force). The solver calculates this penetration depth using one of two algorithms: the *local* or *global* algorithm.

The solver chooses the algorithm to use based on threshold values in the **Local Maximum Penetration** and **Global Maximum Penetration** parameters. You can specify threshold values for these parameters or you can let the software calculate them automatically.

The solver uses the following process to calculate the penetration depth:

- Estimates the penetration depth.
- Compares the estimated penetration depth to the threshold values:
- If the estimated penetration depth is between 0 and the
**Local Maximum Penetration**threshold, the solver calculates the penetration depth using the local algorithm.

- If the estimated penetration depth is greater than the
**Local Maximum Penetration**threshold, the solver calculates the penetration depth using the global algorithm.

- Compares the calculated penetration depth to the
**Global Maximum Penetration**threshold:

- If the penetration depth is less than the
**Global Maximum Penetration**threshold, the solver uses that depth to generate the contact force.

- If the penetration depth is greater than the
**Global Maximum Penetration**threshold, no contact force is generated.

The **Global Maximum Penetration** threshold prevents the action body from completely penetrating the base body. Such penetration would cause the non-contacting surfaces to face each other, and create a false contact.

In most cases, contact problems solve faster when only the global algorithm is used. However, the local algorithm is sometimes needed to achieve accurate results, depending on the geometry involved in the contact.

- Plane Tolerance Factor

The precision factor (of 1-10) for partitioning the action body surface (or base body surface) into facets for representation in the contact calculation.

The default value of **3** provides reasonable representations for almost all contact problems. If you want to adjust the facet precision, it is recommended that you adjust the Maximum Facet Size Factor rather than this parameter.

- Bounding Buffer Length

Before calculating the collision between the two bodies, the solver performs a pre-contact search using bounding boxes around the base and action bodies. The Bounding Buffer Length defines the offset of the pre-search bounding box from the real bounding box for each body.

(1) is the real bounding box and (2) is the Bounding Buffer Length

The default value is calculated as: 0.5 * RLength

where RLength= (Geometry Volume)^(1/3) or (Surface Area)^(1/2)

Increase this value by a factor of 10 if contact is not detected well or if there is irregular rebound between the bodies in the solved solution.

You can define this value for the action body and base body.

- Maximum Facet Size Factor

Sets the maximum length of facets on the surfaces of the contact bodies. For efficiency, the solver partitions the surfaces of the contact bodies into small, triangular surface facets. You can enter a value of 0-10. The default value is 2. A smaller value creates more facets, which increases precision in the contact solution. If the bodies in contact have a mix of curved and flat surfaces, use a value of 0.5 or 1.0. You can define this value for the action body and base body by selecting the check box.

- Maximum Step Size Factor

Reduces the integrator maximum step size temporarily, as the action body approaches the base body during the contact portion of the simulation. A larger **Maximum Step Size Factor** results in smaller step sizes, which provide greater solver precision. This parameter works with the **Maximum Step Size** solver parameter to allow for analysis precision during contact without unnecessarily increasing the solve time. For collision contact, try using a **Maximum Step Size Factor** equal to **10** or **20.**

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12-21-2016 03:12 AM

Thanks so much, this is exactly what I have been looking for. Its a bit difficult adjusting settings that you have no idea about.

Very informative and clear answer, cheers

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