4.37 COMBIN37 Control

4.37 COMBIN37 Control (UP19980821 ) 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. A control element with more capabilities (six degrees of freedom and large deflection) is described in COMBIN7. 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 Section 14.37 of the ANSYS Theory Reference for more details about this element.

Figure 4.37-1 COMBIN37 Control Element



4.37.1 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 4.37-1. The element is defined by two pairs of nodes, these being active nodes (I, J) and optional control nodes (K, L). The active nodes define the position of the element. 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, thus, 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 the nodes together (or pulls into the element)), 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:

CPAR = UXK -UXL
CPAR = d(TK -TL )/dt
CPAR = d2(ROTZK -ROTZL )/dt2
CPAR =
CPAR = t

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 4.37-2, 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 (i.e., 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 Table 4.37-1. A general description of element input is given in Section 2.1.

Table 4.37-1 COMBIN37 Input Summary

Element Name

COMBIN37

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

Material Properties

None

Surface Loads

None

Body Loads

None

Special Features

Nonlinear, Adaptive descent

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 - Pressure degree of freedom element
8 - Temperature degree of freedom element

KEYOPT(4)

ON-OFF range control behavior (see Figure 4.37-2)
0 - Overlapping ranges
1 - Unique ranges

KEYOPT(5)

ON-OFF position control behavior (see Figure 4.37-2)
0 - OFF-either-ON (or OFF-ON-OFF if unique)
1 - ON-either-OFF (or ON-OFF-ON if unique)

KEYOPT(6)

Used if C1 or C3 is not equal to zero (see Section 4.37.3)
0, 1 - Use STIF for nonlinear function
2 - Use DAMP
3 - Use MASJ
4 - Use ONVAL
5 - Use OFFVAL
6 - Use AFORCE
7 - Use MASI
8 - Use FLSIDE

KEYOPT(9)

0 - Use RVMOD expression for real constant modifications
1 - Real constants modified by user subroutine USERRC
(see the Guide to ANSYS User Programmable Features information about user written subroutines)

4.37.2 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 (e.g., 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 Section 2.2. See the ANSYS Basic Analysis Procedures Guide for ways to view results.

Figure 4.37-2 COMBIN37 Behavior As a Function of Control Parameter



The following notation is used in Table 4.37-2:

A colon (:) in the Name column indicates the item can be accessed by the Component Name method [ETABLE, ESOL] (see Section 2.2.2). The O and R columns indicate the availability of the items in the file Jobname.OUT (O) or in the results file (R), a Y indicates that the item is always available, a number refers to a table footnote which describes when the item is conditionally available, and a - indicates that the item is not available.

Table 4.37-2 COMBIN37 Element Output Definitions

Name

Definition

O

R

EL

Element number

Y Y
ACTIVE NODES

Nodes - I, J

Y Y
CONTROL NODES

Nodes - K, L

Y Y
CENT: X, Y, Z

Center location XC, YC, ZC

- Y
CONTROL PARAM

CPAR value (see KEYOPT(1)) of the control nodes

Y Y
STAT

Element status

1 1
OLDST

STAT value of the previous time step

1 1
UI

Displacement of node I

2 2
UJ

Displacement of node J

2 2
UK

Displacement of node K

2 2
UL

Displacement of node L

2 2
STRETCH

Relative displacement

2 2
SFORCE

Spring force in element

2 2
AFORCE

Applied force in the element

2 2
SLSTAT

Sliding status

3 3
OLDSLS

Sliding status value of the previous time step

3 3
SLIDE

Amount of sliding

4 4
1. If the value of the element status is:
0 - OFF
1 - ON

2. For the thermal option analogous items TEMPI, TEMPJ, DELTEMP, SHEAT, AHEAT are output. For the fluid option analogous items PRESI, PRESJ, DELPRES, SFLOW, AFLOW are output.

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

Table 4.37-3 lists output available through the ETABLE command using the Sequence Number method. See Chapter 5 of the ANSYS Basic Analysis Procedures Guide and Section 2.2.2.2 of this manual for more information. The following notation is used in Table 4.37-3:

Table 4.37-3 COMBIN37 Item and Sequence Numbers for the ETABLE and ESOL Commands

Name

Item

E

SFORCE

SMISC

1
STAT

NMISC

1
OLDST

NMISC

2
SLSTAT

NMISC

3
OLDSLS

NMISC

4
STRETCH

NMISC

5
UI

NMISC

6
UJ

NMISC

7
UK

NMISC

8
UL

NMISC

9
CPAR

NMISC

10
SLIDE

NMISC

11

4.37.3 Assumptions and Restrictions

The element may have only one degree of freedom per node which is specified in the nodal coordinate system (see Section 2.3.2). 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 the 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. Also, the element may not be deactivated with the EKILL command. Only the lumped mass matrix is available.

4.37.4 Product Restrictions