# Constraint: SFSF

Model ElementConstraint_SFSF defines a higher pair constraint. The constraint consists of a surface on one body rolling and sliding on a surface on a second body. The surfaces are required to have a unique contact point.

## Format

<Constraint_SFSF
id            = "integer"
label         = "Name of Constraint_SFSF element"
i_marker_id   = "integer"
i_surface_id  = "integer"
i_disp_x0     = "real"
i_disp_y0     = "real"
i_disp_z0     = "real"
j_marker_id   = "integer"
j_surface_id  = "integer"
j_disp_x0     = "real"
j_disp_y0     = "real"
j_disp_z0     = "real"
</Constraint_SFSF>

## Attributes

id
Element identification number (integer>0). This number is unique among all Constraint_SFSF elements.
label
The name of the Constraint_SFSF element.
i_marker_id
Specifies a marker that defines the coordinate system in which the i_surface points are defined. It also implicitly defines the body on which the surface is "etched". The surface moves with the body. i_marker_id may belong to any type of body: flexible, rigid, or point. The parameter is mandatory.
i_surface_id
Specifies the ID of the Reference_ParamSurface that contains the surface definition.
i_disp_x0, i_disp_y0, i_disp_z0
These three parameters specify the location of the contact point on i_surface at the input configuration as measured in the i_marker_id coordinate system. The three parameters come as a set. All three must be specified or none may be defined. These parameters are optional. 8
j_marker_id
Specifies a Reference_Marker that defines the coordinate system in which the j_surface points are defined. It also implicitly defines the body on which the surface is "etched". The surface moves with the body. j_marker_id may belong to any type of body: flexible, rigid, or point. The parameter is mandatory.
j_surface_id
Specifies the ID of the Reference_ParamSurface that contains the surface definition.
j_disp_x0, j_disp_y0, j_disp_z0
These three parameters specify the location of the contact point on j_surface at the input configuration as measured in the j_marker_id coordinate system. The three parameters come as a set. All three must be specified or none may be defined. These parameters are optional. 8

## Example

Figure 1 shows two surfaces I and J that are in continuous contact.

Surface I is defined with respect to Reference_Marker 1023; surface J is defined with respect to Reference_Marker 2046. Reference_ParamSurface 123 defines surface I and Reference_ParamSurface 246 defines surface J. The patch containing the contact point is also shown in figure 1.

An initial guess for the contact point on both surfaces is defined. Assume the initial contact point location on surface J, as measured in the coordinate system of 2046, is [1.466, 5.66, 0.1]. Assume the contact point location on surface I, as measured in the coordinate system of 1023, is [-0.522, -0.852, -0.453].

The Constraint_SFSF object may be defined as follows:

<Constraint_SFSF
id            = "1"
i_marker_id   = "1023"
i_surface_id  = "123"
i_disp_x0     = "1.466"
i_disp_y0     = "5.66"
i_disp_z0     = "0.1"
j_marker_id   = "2046"
j_surface_id  = "246"
j_disp_x0     = "-0.522"
j_disp_y0     = "-0.852"
j_disp_z0     = "-0.452" >
</Constraint_SFSF>