# Force: Spring Damper

Model ElementForce_SpringDamper defines a spring damper force acting between two Reference_Markers.

## Description

The element can be translational (applies a force) or rotational (applies a torque). In both cases, the force is characterized by a stiffness coefficient, a damping coefficient, a free-length and a preload.

## Format

<Force_SpringDamper
id               = "integer"
[ label            = "string" ]
i_marker_id      = "integer"
j_marker_id      = "integer"
type             = { "TRANSLATIONAL" | "ROTATIONAL" }
stiffness        = "real"
damping          = "real"
length           = "real"
</Force_SpringDamper>

## Attributes

id
Element identification number (integer > 0). This number is unique among all Force_SpringDamper elements.
label
The name of the Force_SpringDamper element.
i_marker_id
Specifies the Reference_Marker at which the force or torque is applied. This is designated as the point of application of the force.
j_marker_id
Specifies the Reference_Marker at which the reaction force or torque is applied. This is designated as the point of application of the reaction force.
type
Specifies the type of Force_SpringDamper. Select from "TRANSLATIONAL" and "ROTATIONAL".
"TRANSLATIONAL"
Implies that the element applies a force between the two Reference_Markers. No torque is applied.
"ROTATIONAL"
Implies that the element applies a torque between the two Reference_Markers. No force is applied.
stiffness
Specifies the spring stiffness. For translational spring-dampers, this has units of force/unit translational deflection. For rotational spring-dampers, this has units of torque/radian. Note that stiffness0. A negative value is not allowed.
damping
Specifies the damping coefficient. For translational spring-dampers, this has units of force/unit translational deflection velocity. For rotational spring-dampers, this has units of torque/unit angular deflection velocity.
Note: damping0. A negative value is not allowed.
length
Specifies the free-length or the installed (reference) length of the spring.
Note: length > 0. A zero or negative value is not legal.
Specifies any assembly or pre-load in the spring-damper. This is the force or torque when the distance or angle between the I and J markers is equal to the value specified by length.
A positive value implies a force that tends to increase the distance or a torque that tends to increase the angle (repulsive), and vice versa.

## Example

The first example shows how to model a translational spring damper.

Assume a spherical steel ball of radius 0.0313m (approximately 3 cm) is suspended from the ceiling by a spring damper. The ball has a mass of 1 Kg, and inertia Ixx=Iyy=Izz=4.89E-4 Kg.m2. The spring properties are as follows:
• Stiffness = 1200 N/m.
• Damping = 40 Ns/m.
• Free Length = 0.5m.

The ball is pushed vertically up until its attachment to the spring is at a distance of 0.4m from the ceiling. The ball is then let loose, and it starts oscillating. The model is shown schematically in Figure 1. Also shown are sample results for the oscillatory motion of the ball.

The Force_SpringDamper modeling element for this scenario is:

< Force_SpringDamper
id                   = "18"
i_marker_id          = "22"
j_marker_id          = "11"
type                 = "TRANSLATIONAL"
stiffness            = "1200"
damping              = "10"
length               = "0.5"
</ Force_ SpringDamper >

The image below illustrates the use of a rotational spring damper that acts on the second revolute joint of a two link (double) pendulum. An initial angular velocity of 40 rads/sec sets the system in motion. The spring-damper parameters are shown in the image. The plot shows the torque generated by the spring-damper.

The rotational spring-damper definition for this model is:

<Force_SpringDamper
id                   = "18"
i_marker_id          = "1029"
j_marker_id          = "1039"
type                 = "ROTATIONAL"
stiffness            = "500"
damping              = "50"
length               = "0.3"
</ Force_SpringDamper >