# BISTOP

The BISTOP function models a gap element.

## Format

$\text{Bistop}\left(x,\stackrel{˙}{x},{x}_{1},{x}_{2},k,e,{c}_{\mathrm{max}},d\right)$

## Description

It can be used to model forces acting on a body while moving in the gap between two boundary surfaces, which act as elastic bumpers. The properties of the two boundary surfaces can be tuned as desired.

## Arguments

$x$
The expression used for the independent variable. For example, to use the z-displacement of I marker with respect to J marker as resolved in the reference frame of the RM marker as the independent variable, specify $x$ as DZ({marker_i.idstring}, {marker_j.idstring}, {marker_rm.idstring}).
$\stackrel{˙}{x}$
The time derivative of the independent variable. For example, if $x$ is specified as above, then $\stackrel{˙}{x}$ will be VZ({marker_i.idstring}, {marker_j. idstring}, {marker_rm.idstring}).
${x}_{1}$
The lower bound of $x$ . If $x$ is less than ${x}_{1}$ , the bistop function returns a positive value. The value of ${x}_{1}$ must be less than the value of ${x}_{2}$ .
${x}_{2}$
The upper bound of $x$ . If $x$ is greater than ${x}_{2}$ , the bistop function returns a negative value. The value of ${x}_{2}$ must be greater than the value of ${x}_{1}$ .
$k$
The stiffness of the boundary surface interaction. It must be non-negative.
$e$
The exponent of the force deformation characteristic. For a stiffening spring characteristic, $e$ must be greater than 1.0 and for a softening spring characteristic, $e$ must be less than 1.0. It must always be positive.
${c}_{\text{max}}$
The maximum damping coefficient. It must be non-negative.
$d$
The penetration at which the full damping coefficient is applied. It must be positive.

(1)

## Example

<Force_Vector_TwoBody
id                    = "30101"
type                  = "ForceOnly"
i_marker_id           = "30102031"
j_floating_marker_id  = "30101031"
ref_marker_id         = "30101010"
fx_expression         = "BISTOP(DX(30102030,30101010,30101010),VX(30102030,30101010,30101010),0.5,9.5,10000000,2.1,1,0.001)"
fy_expression         = "0"
fz_expression         = "0"
/>