COMBIN37


Control

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COMBIN37 Element Description

COMBIN37 is a unidirectional element with the capability of turning on and off during an analysis. The element has one degree of freedom at each node, either a translation in a nodal coordinate direction, rotation about a nodal coordinate axis, pressure, or temperature. Similar unidirectional elements (without remote control capability) are COMBIN14, COMBIN39, and COMBIN40. The element has many applications, such as controlling heat flow as a function of temperature (thermostat), controlling damping as a function of velocity (mechanical snubber), controlling flow resistance as a function of pressure (relief valve), controlling friction as a function of displacement (friction clutch), etc. See COMBIN37 in the Mechanical APDL Theory Reference for more details about this element.

Figure 37.1: COMBIN37 Geometry

COMBIN37 Geometry

COMBIN37 Input Data

The functioning of this element is shown in Figure 37.1: COMBIN37 Geometry. The element is defined by two pairs of nodes, these being active nodes (I, J) and optional control nodes (K, L). Generally in the cases using UX, UY, or UZ as the active degrees of freedom, the active nodes should be coincident as this eliminates the possibility of moment disequilibrium. However, for visualization purposes, it may be useful to give node J a slightly greater coordinate value than node I. The element is defined such that a positive displacement of node J relative to node I will stretch the spring. Thus, if nodes I and J are interchanged, the same nodal motions will compress the spring.

Certain parameters associated with the control nodes are used to determine whether the control element is part of the structure (on) or not (off) and, therefore, can be used to disconnect regions of the model during time dependent or iterative analyses. Other input values are stiffness (STIF), damping coefficient (DAMP), concentrated nodal masses (MASI, MASJ), on/off control values (ONVAL, OFFVAL), element load (AFORCE: positive pulls node I in the positive nodal coordinate direction, and pulls node J in the negative nodal coordinate direction), initial on/off element status (START: -1 if explicitly off, 0 if determined from starting value of control parameter, 1 if explicitly on), several nonlinear constants (C1, C2, C3, C4), and a limiting sliding force (FSLIDE).

The FSLIDE value represents the absolute value of the spring force that must be exceeded before sliding occurs. If FSLIDE is 0.0, the sliding capability of the element is removed, that is, a rigid connection is assumed. For structural analyses, units are force/length or moment/rotation for stiffness, force*time/length or moment*time/rotation for damping, force*time2/length or moment*time2/rotation for mass, and force or moment for element load. For thermal analyses with temperature degrees of freedom, stiffness represents conductance and has units of heat/time*degrees, mass represents thermal capacitance with units of heat/degrees, and element load represents heat flow with units of heat/time. Also, in analyses with pressure degrees of freedom, stiffness represents flow conductance with units of length2/time. Stiffness, damping, mass, and element load should be defined on a full 360° basis for axisymmetric analyses.

The active nodes (I, J) have only one degree of freedom each, selected with the KEYOPT(3) option. The control nodes (K, L) can have the same, or a different, degree of freedom as specified with KEYOPT(2). The KEYOPT(1) option assigns to the parameters of the control nodes either the value of the degree of freedom, the first or second derivative of the value, the integral of the value, or time, for example:

Control nodes need not be connected to any other element. If node L is not defined, the control parameter is based only upon node K. If time is the control parameter (KEYOPT(1)), control nodes K and L need not be defined.

When the element is active and used in structural analyses, the element acts like any other spring/damper/mass element (such as COMBIN14, MASS21, and COMBIN40). In addition, the element can exhibit nonlinear behavior according to the function: RVMOD = RVAL + C1|CPAR|C2 + C3|CPAR|C4, where RVMOD is the modified value of an input real constant value RVAL (identified by KEYOPT(6)), C1 through C4 are other real constants, and CPAR is the control parameter (see KEYOPT(1)). RVMOD may also be defined by user subroutine USERRC and is accessed by KEYOPT(9) = 1. Note, FSLIDE modified to a negative value is set to zero. In a field analysis, the temperature or pressure degree of freedom acts in a manner analogous to the displacement.

As illustrated in Figure 37.2: COMBIN37 Behavior as a Function of Control Parameter, the KEYOPT(4) and KEYOPT(5) options, when used in combination with ONVAL and OFFVAL, set the control behavior of the element. The element is either on or off depending on the position of the control parameter with respect to the values of ONVAL and OFFVAL. Also, note that when KEYOPT(4) = 0 and the control parameter (CPAR) is within the ONVAL/OFFVAL interval, the element's status depends on the direction of the CPAR (that is, on going from on to off, and vice-versa). If ONVAL = OFFVAL = 0.0 (or blank), the on/off capability is ignored and the element is always active.

A summary of the element input is given in "COMBIN37 Input Summary". A general description of element input is given in Element Input.

COMBIN37 Input Summary

Nodes

I, J, K, L (or I, J, K or I, J)

Degrees of Freedom

UX, UY, UZ, ROTX, ROTY, ROTZ, PRESS, or TEMP (depending on KEYOPT(2) and KEYOPT (3) below)

Real Constants
STIF, DAMP, MASJ, ONVAL, OFFVAL, AFORCE,
MASI, START, C1, C2, C3, C4,
FSLIDE
See Table 37.1: COMBIN37 Real Constants for a description of the real constants
Material Properties

MP command: ALPD, BETD, DMPR, DMPS

Surface Loads

None

Body Loads

None

Special Features
KEYOPT(1)

Control parameter:

0, 1 -- 

Control on value (UK-UL) (or UK if L not defined)

2 -- 

Control on first derivative of value with respect to time

3 -- 

Control on second derivative of value with respect to time

4 -- 

Control on integral of value with respect to time (zero initial condition assumed)

5 -- 

Control on time value (KEYOPT(2) and nodes K and L ignored)

KEYOPT(2)

Degree of freedom for control nodes (K and L):

N -- 

Use degree of freedom N as listed for KEYOPT(3) (defaults to KEYOPT(3))

KEYOPT(3)

Degree of freedom for active nodes (I and J):

0, 1 -- 

UX (Displacement along nodal X axes)

2 -- 

UY (along nodal Y)

3 -- 

UZ (along nodal Z)

4 -- 

ROTX (rotation about nodal X axes)

5 -- 

ROTY (about nodal Y)

6 -- 

ROTZ (about nodal Z)

7 -- 

PRESS

8 -- 

TEMP

KEYOPT(4)

ON-OFF range behavior (see Figure 37.2: COMBIN37 Behavior as a Function of Control Parameter):

0 -- 

Overlapping ranges

1 -- 

Unique ranges

KEYOPT(5)

ON-OFF position behavior (see Figure 37.2: COMBIN37 Behavior as a Function of Control Parameter):

0 -- 

OFF-either-ON (or OFF-ON-OFF if unique)

1 -- 

ON-either-OFF (or ON-OFF-ON if unique)

KEYOPT(6)

Real constants used for RVMOD function (used if C1 or C3 is not equal to zero; see "COMBIN37 Input Data"):

0, 1 -- 

Use STIF for nonlinear function. (Both STIF and FSLIDE cannot be zero).

2 -- 

Use DAMP

3 -- 

Use MASJ

4 -- 

Use ONVAL

5 -- 

Use OFFVAL

6 -- 

Use AFORCE

7 -- 

Use MASI

8 -- 

Use FSLIDE

KEYOPT(7)

Element level time increment control:

0 -- 

No control

1 -- 

Predictions are made to achieve a reasonable time (or load) increment

KEYOPT(9)

Method to define nonlinear behavior:

0 -- 

Use RVMOD expression for real constant modifications

1 -- 

Real constants modified by user subroutine USERRC


Note:  See the Guide to User-Programmable Features in the Programmer's Reference for information about user-written subroutines.


Table 37.1: COMBIN37 Real Constants

No.NameDescription
1STIFSpring stiffness
2DAMPDamping coefficient
3MASJNodal mass at node J
4ONVAL "ON" control value
5OFFVAL "OFF" control value
6AFORCEElement load
7MASINodal mass at node I
8STARTInitial on/off element status
9C1First scalar in RVMOD equation
10C2First exponent in RVMOD equation
11C3Second scalar in RVMOD equation
12C4Second exponent in RVMOD equation
13FSLIDELimiting sliding force

COMBIN37 Output Data

The solution output associated with the element is in two forms:

The active nodal displacements and forces correspond to the degree of freedom selected with the KEYOPT(3) option. For axisymmetric analysis, the element forces are expressed on a full 360° basis.

The element value STRETCH is the relative deflection at the end of the substep less the amount of sliding (for example, UJ-UI-SLIDE). STATUS and OLDST indicate if the element is on or off at the end of the current and previous substeps, respectively. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

Figure 37.2: COMBIN37 Behavior as a Function of Control Parameter

COMBIN37 Behavior as a Function of Control Parameter

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file jobname.out. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 37.2: COMBIN37 Element Output Definitions

NameDefinitionOR
ELElement NumberYY
ACTIVE NODESNodes - I, JYY
CONTROL NODESNodes - K, LYY
XC, YC, ZCLocation where results are reportedY 5
CONTROL PARAMCPAR value (see KEYOPT(1)) of the control nodesYY
STATElement status 1 1
OLDSTSTAT value of the previous time step 1 1
UIDisplacement of node I 2 2
UJDisplacement of node J 2 2
UKDisplacement of node K 2 2
ULDisplacement of node L 2 2
STRETCHRelative displacement 2 2
SFORCESpring force in element 2 2
AFORCEApplied force in the element 2 2
SLSTATSliding status 3 3
OLDSLSSliding status value of the previous time step 3 3
SLIDEAmount of sliding 4 4

  1. If the value of the element status is:

    0 - OFF

    1 - ON

  2. For the thermal and fluid options, analogous items are output. Thermal option output items TEMPI, TEMPJ, TEMPK, TEMPL, DELTEMP, SHEAT, and AHEAT and fluid option output items PRESI, PRESJ, PRESK, PRESL, DELPRES, SFLOW, and AFLOW are respectively analogous to output items UI, UJ, UK, UL, STRETCH, SFORCE, and AFORCE.

  3. Output only if FSLIDE is greater than zero. If the value of the sliding status is:

    0 - No sliding

    1 - Sliding right (node J moving to right of node I)

    -1- Sliding left (node J moving to left of node I)

  4. If FSLIDE is greater than zero

  5. Available only at centroid as a *GET item.

Table 37.3: COMBIN37 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 37.3: COMBIN37 Item and Sequence Numbers:

Name

output quantity as defined in the Table 37.2: COMBIN37 Element Output Definitions

Item

predetermined Item label for ETABLE command

E

sequence number for single-valued or constant element data

Table 37.3: COMBIN37 Item and Sequence Numbers

Output Quantity Name ETABLE and ESOL Command Input
ItemE
SFORCESMISC1
AFORCESMISC2
STATNMISC1
OLDSTNMISC2
SLSTATNMISC3
OLDSLSNMISC4
STRETCHNMISC5
UINMISC6
UJNMISC7
UKNMISC8
ULNMISC9
CPARNMISC10
SLIDENMISC11

Analogous thermal and fluid option output items use the same item and sequence numbers. See footnote 2 of Table 37.2: COMBIN37 Element Output Definitions.

COMBIN37 Assumptions and Restrictions

  • The element may have only one degree of freedom per node which is specified in the nodal coordinate system (see Elements That Operate in the Nodal Coordinate System).

  • The element assumes only a one-dimensional action.

  • Nodes I and J may be anywhere in space (preferably coincident).

  • No moment effects are included due to noncoincident nodes. That is, if the nodes are offset from the line of action, moment equilibrium may not be satisfied.

  • The nonlinear capabilities of the element operate only in static and nonlinear transient dynamic analyses.

  • If used in other analysis types, the element maintains its initial status (on or off), throughout the analysis.

  • The real constants for this element are not allowed to be changed from their initial values.

  • The element can not be deactivated with the EKILL command.

  • Only the lumped mass matrix is available.

COMBIN37 Product Restrictions

There are no product-specific restrictions for this element.