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CFFSUB

The CFFSUB evaluation subroutine computes a set of friction force values for a CONTACT statement (C++ or FORTRAN). You can use a CFFSUB when the default friction force routine is not applicable to your model.

Use

Calling Sequence

SUBROUTINE CFFSUB (id, time, par, npar, loci, locj, x, xdot, nforce, area, dflag, iflag, result)

Input Arguments

area	A double-precision variable that specifies the value of the contact area.
dflag	A logical variable that Adams/Solver sets to true when it calls CFFSUB to evaluate the partial derivatives of the specified functions. Otherwise, Adams/Solver sets the dflag argument to false.
id	An integer variable that provides the identifier of the CONTACT statement requesting information from CFFSUB. From the identifier, Adams/Solver automatically recognizes other information (such as the par argument) that is available in the corresponding statement.
iflag	A logical variable that Adams/Solver sets to true when it needs the functional dependency information from CFFSUB. Functional dependencies are established with the same calls to SYSARY and SYSFNC that are later used to compute the values of the CONTACT components (see SYSARY and SYSFNC). If the iflag argument is false, the values of the user-defined expressions are computed.
loci	A double-precision array that specifies the vector from the reference marker of the CONTACT I Geometry (IGEOM) to the contact point on IGEOM. Expressed in the coordinate system of the reference marker of IGEOM.
locj	A double-precision array that specifies the vector from the reference marker of the CONTACT J Geometry (JGEOM) to the contact point on JGEOM. Expressed in the coordinate system of the reference marker of JGEOM.
nforce	A double-precision variable that specifies the value of the contact normal force.
npar	An integer variable that indicates the number of constants you specify in the USER parenthetical list. This variable provides the CFFSUB evaluation subroutine with the number of values stored in the par array.
par	A double-precision array of constants taken in order from the FRICTION_FUNCTION USER parenthetical list of the CONTACT statement.
time	A double-precision variable through which Adams/Solver conveys the current simulation time.
x	A double-precision array that specifies the values of the contact deformation. (1) - Translational deformation since the beginning of contact along the x-axis of the contact I incident marker. (2) - Translational deformation since the beginning of contact along the y-axis of the contact I incident marker. (3) - Rotational deformation since the beginning of contact about the z-axis of the contact I incident marker.
xdot	A double-precision array that specifies the slip velocities at the contact point. (1) - Translational slip velocity along the x-axis of the contact I incident marker. (2) - Translational slip velocity along the y-axis of the contact I incident marker. (3) - Relative angular velocity of contacting surfaces about the z-axis of the contact I incident marker.

Output Argument

result

A double-precision array that returns the three components of the friction force. (1) - Translational force along the x-axis of the contact I incident marker. (2) - Translational force along the y-axis of the contact I incident marker. (3) - Rotational torque about the z-axis of the contact I incident marker.

Extended Definition

The default friction model in the CONTACT statement can only model simple dynamic friction. This means that to produce a friction force, contacting bodies must be sliding at the point of contact. Effects such as static friction and frictional torque are not modeled.

If you require another force model, you can use a CFFSUB. If the algorithms use or consist of already-existing FORTRAN-77 subroutines, CFFSUB can be made to call them. You can call utility subroutines, such as AKISPL, CUBSPL, SYSARY, and SYSFNC from CFFSUB, to obtain information about system variables, user-defined variables, and splines.

The SYSARY and SYSFNC utility subroutines set functional dependencies when the CFFSUB argument iflag is true. To compute solutions efficiently, Adams/Solver must know the other variables on which each user-defined variable depends. Adams/Solver determines these functional dependencies at the beginning of the simulation by calling CFFSUB with the argument iflag set to true. Adams/Solver does this once for each CONTACT statement with a
FRICTION_FUNCTION=USER() argument.

During each call to CFFSUB, Adams/Solver records which calls you make to SYSARY and SYSFNC. Adams/Solver assumes that the CONTACT components depend on those Adams/Solver variables that are accessed through SYSARY and SYSFNC.

Using the DFLAG Variable

The use of the DFLAG variable is optional. Its purpose is to simply let you know that CFFSUB is being called to evaluate a partial derivative. One of the states on which the CFFSUB depends has been perturbed very slightly. In many situations, it is likely that major calculations in the CFFSUB are insensitive to small changes in state, and therefore, need not be recalculated. In such situations, you can structure the CFFSUB not to redo these calculations.

Tip:

If the SYSARY or SYSFNC subroutines are called to access angular displacements, the values returned by CFFSUB may contain discontinuities. Discontinuities occur if there is an Euler singularity. To avoid the Euler singularity (and thus the discontinuities), use the RCNVRT utility subroutine to convert the rotational angles from Euler angles to some other coordinate system that does not encounter a singularity. • If the calculations always use the same SYSARY and SYSFNC calls through the whole simulation, and you have no initialization to do, you do not need to check the flag argument at all. You can just call SYSARY and/or SYSFNC, compute the user-defined variable value, and return to Adams/Solver (FORTRAN)

Caution:

When the iflag argument is true: • You must make all the calls to SYSARY and SYSFNC as they are made to compute the component values of the CONTACT statement. This ensures that Adams/Solver has the proper functional dependencies. In general, failure to account for dependencies in the CONTACT statement components can make it difficult for Adams/Solver to converge to a solution and/or can force Adams/Solver to take small integration steps, potentially causing large increases in execution time. • SYSARY and SYSFNC return zero values for system and user-defined variables. When you use Adams/Solver, computations that divide by these values result in fatal errors. You should check for nonzero values, or ensure the iflag argument is set to false, before dividing by these values.

Fortran - Prototype

A sample structure for CFFSUB is shown next. The comments explain how the subroutine works.

SUBROUTINE CFFSUB(ID, TIME,PAR, NPAR, LOCI, LOCJ, X, XDOT,      
& NFORCE, DFLAG, IFLAG, FORCE)
C
C === Type and dimension statements ===================       

IMPLICIT NONE       
INTEGER ID       
DOUBLE PRECISION TIME       
DOUBLE PRECISION PAR( * )       
INTEGER NPAR       
DOUBLE PRECISION LOCI(3)       
DOUBLE PRECISION LOCJ(3)      
 DOUBLE PRECISION X(3)       
DOUBLE PRECISION XDOT(3)       
DOUBLE PRECISION NFORCE       
LOGICAL DFLAG       
LOGICAL IFLAG       
DOUBLE PRECISION FORCE(3)

C
C  Input parameters
C
C  ID    Identifier of calling CONTACT statement
C  TIME  Current timeC  PAR   Array containing passed parameters
C    PAR(1) - stiction coefficient
C    PAR(2) - friction coefficient
C    PAR(3) - stiction velocity
C    PAR(4) - friction velocity
C  NPAR   Number of passed parameters
C  LOCI   contact point location on I in I coordinates
C  LOCI   contact point location on J in J coordinates
C  X      sliding displacement since the beginning of contact
C   X(1) - translational deformation in x
C   X(2) - translational deformation in y
C   X(3) - rotational deformation about z
C  XDOT slip velocity of contact point
C   XDOT(1) - slip velocity in x
C   XDOT(2) - slip velocity in y
C   XDOT(3) - relative angular velocity about z
C  NFORCE contact normal force
C
C  components returned to ADAMS
C
C  FORCE Array (dimension 3) of computed CNFORC
C    FORCE(1) - force in x direction
C    FORCE(2) - force in y direction
C    FORCE(3) - torque about z axis
C  Local variable and parameter definitions
C       

DOUBLE PRECISION US       
DOUBLE PRECISION UD       
DOUBLE PRECISION VS       
DOUBLE PRECISION VD       
DOUBLE PRECISION H0, H1, X0, X1, TEMP1, TEMP2       
LOGICAL ERRFLG

C
C ===Executable code ==================================
C       

US = PAR(1)       
UD = PAR(2)       
VS = PAR(3)       
VD = PAR(4)

X0 = -VS       
H0 = -1       
X1 = VS       
H1 = 1            

CALL STEP(XDOT(1), X0, H0, X1,H1, 0, TEMP1, ERRFLG)       
CALL ERRMES(ERRFLG,'ERROR CALLING STEP',ID,'STOP')

X0 = VS       
H0 = US       
X1 = VD       
H1 = UD

CALL STEP(XDOT(1), X0, H0, X1, H1, 0, TEMP2, ERRFLG)       
CALL ERRMES(ERRFLG,'ERROR CALLING STEP',ID,'STOP')

FORCE(1) = -NFORCE*TEMP1*TEMP2       
FORCE(2) = 0.0       
FORCE(3) = 0.0       

RETURN       
END

C Style - Prototype

typedef void adams_c_CFFSUB(const struct sAdamsContactFriction* fric, double TIME, const double* LOCI, const double* LOCJ, const double* X, const double* XDOT, double NFORCE, double AREA, int DFLAG, int IFLAG, double* VALUES );

typedef void STDCALL adams_f77_CFFSUB(const int* ID, const double* TIME, const double* PAR, const int* NPAR, const double* LOCI, const double* LOCJ, const double* X, const double* XDOT, const double* NFORCE, const double* AREA, const int* DFLAG, const int* IFLAG, double* VALUES );

 

struct sAdamsContact

{

int ID;

int nIGEOM;

int nJGEOM;

int* IGEOM;

int* JGEOM;

int* IFLIP_GEOM;

int* JFLIP_GEOM;

};

struct sAdamsContactFriction

{

struct sAdamsContact contact;

int NPAR;

const double* PAR;

};