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There are several tabs in the Property dialog of the FFlex body. In this article, the parameters in the Connecting Parameter tab will be explained.

Let me introduce the summary of this article.  

1. Generally, to use (check) 'Use Force Connector' provides faster simulation and better convergency.
2. But for more strict calculation, it is recommended to uncheck 'Use Force Connector'.
3. In RecurDyn, when you create a force or a joint on the FFlex body, a virtual body called 'Connecting Body' is used.

Let me explain in detail.

When you create a joint between a rigid body and a FFlex body, the joint doesn't connect those two bodies directly. To improve the performance of solver, RecurDyn creates a virtual body called, 'Connecting Body' between them.

Consequently, the connection is as below.

The parameters related to this connecting body can be set in Connecting Parameters tab.

(Connecting Body is a virtual rigid body.)

Descriptions of each parameter

1. Use Force Connector: decides the way how to handle the connection between the Connecting Body and the FFlexBody
   1. When it is checked (ON): Force Type
      1. Connector is treated as force (like a bushing force)
         1. Better simulation speed and convergency
      2. But the connecting body is not completely fixed to the FFlex Body
         1. For example, even if a fixed joint is connected between a rigid body and a FFlex body, they can have relative displacement
         2. However, since the default stiffness is very high, so that they can be 'almost fixed'
   2. When it is unchecked (OFF): Constraint Type
      1. Connector is treated as a fixed joint
      2. Because the constrained DOFs increase, the simulation speed can get slower
      3. But the connecting body and the body can be fixed completely
2. Material Property of Connecting Body 
   1. The default values (mass and moment of inertia) are very small to minimize the impact of the connecting body
      1. mass = 1e-6 kg, moment of inertia = 1 kgmm^2
   2. If the bodies are very light, the material property of Connecting body must be changed
   3. For example, 1/1,000,000 of the mass and the moment of inertia of the bodies or smaller value is recommended
3. Property of Connecting Force
   1. Property of the virtual connector
   2. Only meaningful when Use Force Connector is checked
4. Sync.FDR
   1. You can generally use the default setting as it is.
   2. If this option is checked, all FDRs (RBEs) in the FFlexBody will share the same "Use Force Connector" setting of the FFlexBody.
   3. If you want to apply different "Use Force Connector" settings for each FDR, uncheck this option and adjust the settings for each FDR in the dialog box below.

Below clip shows the difference between Constraint Type (Use Force Connector = OFF) and Force Type (Use Force Connector = ON)

• Left one (Constraint Type): Two bodies are fixed completely
• Right one (Force Type): The right body moves a little

(In this example, very small stiffness was used to show the difference.)

In general, Force type can also fix those bodies very tightly because the stiffness used is big enough

Summary again...)

1. Use Force Connector
   1. Generally, 'Constraint Type (Use Force Connector = OFF)' is more realistic setting because a joint or force is applied to a fixed position. (From V9R3, Constraint Type is the default setting.)
   2. However, if 'fixed location' is not so critical condition, 'Force Type (Use Force Connector = ON)' is recommended to improve the simulation speed or convergency
   3. In other words, 'Force Type' is recommended in  general
   4. But, if a very strict simulation is desired, firstly, use 'Constraint Type' then simulate again with 'Force Type', and compare the accuracy and simulation speed to decide which option you need to use.
2. Material Property of Connecting Body
   1. This is a property of the virtual body, which is rigid.
   2. The mass, moment of inertia of the connecting body need to be decided according to the mass and moment of inertia of the action body and the base body to minimize the impact.

* Multi-Body Dynamics Software RecurDyn

* MFBD (Multi Flexible Body Dynamics)