SUBROUTINE AB09MD( DICO, JOB, EQUIL, ORDSEL, N, M, P, NR, ALPHA,
$ A, LDA, B, LDB, C, LDC, NS, HSV, TOL, IWORK,
$ DWORK, LDWORK, IWARN, INFO )
C
C PURPOSE
C
C To compute a reduced order model (Ar,Br,Cr) for an original
C state-space representation (A,B,C) by using either the square-root
C or the balancing-free square-root Balance & Truncate (B & T)
C model reduction method for the ALPHA-stable part of the system.
C
C ARGUMENTS
C
C Mode Parameters
C
C DICO CHARACTER*1
C Specifies the type of the original system as follows:
C = 'C': continuous-time system;
C = 'D': discrete-time system.
C
C JOB CHARACTER*1
C Specifies the model reduction approach to be used
C as follows:
C = 'B': use the square-root Balance & Truncate method;
C = 'N': use the balancing-free square-root
C Balance & Truncate method.
C
C EQUIL CHARACTER*1
C Specifies whether the user wishes to preliminarily
C equilibrate the triplet (A,B,C) as follows:
C = 'S': perform equilibration (scaling);
C = 'N': do not perform equilibration.
C
C ORDSEL CHARACTER*1
C Specifies the order selection method as follows:
C = 'F': the resulting order NR is fixed;
C = 'A': the resulting order NR is automatically determined
C on basis of the given tolerance TOL.
C
C Input/Output Parameters
C
C N (input) INTEGER
C The order of the original state-space representation, i.e.
C the order of the matrix A. N >= 0.
C
C M (input) INTEGER
C The number of system inputs. M >= 0.
C
C P (input) INTEGER
C The number of system outputs. P >= 0.
C
C NR (input/output) INTEGER
C On entry with ORDSEL = 'F', NR is the desired order of the
C resulting reduced order system. 0 <= NR <= N.
C On exit, if INFO = 0, NR is the order of the resulting
C reduced order model. For a system with NU ALPHA-unstable
C eigenvalues and NS ALPHA-stable eigenvalues (NU+NS = N),
C NR is set as follows: if ORDSEL = 'F', NR is equal to
C NU+MIN(MAX(0,NR-NU),NMIN), where NR is the desired order
C on entry, and NMIN is the order of a minimal realization
C of the ALPHA-stable part of the given system; NMIN is
C determined as the number of Hankel singular values greater
C than NS*EPS*HNORM(As,Bs,Cs), where EPS is the machine
C precision (see LAPACK Library Routine DLAMCH) and
C HNORM(As,Bs,Cs) is the Hankel norm of the ALPHA-stable
C part of the given system (computed in HSV(1));
C if ORDSEL = 'A', NR is the sum of NU and the number of
C Hankel singular values greater than
C MAX(TOL,NS*EPS*HNORM(As,Bs,Cs)).
C
C ALPHA (input) DOUBLE PRECISION
C Specifies the ALPHA-stability boundary for the eigenvalues
C of the state dynamics matrix A. For a continuous-time
C system (DICO = 'C'), ALPHA <= 0 is the boundary value for
C the real parts of eigenvalues, while for a discrete-time
C system (DICO = 'D'), 0 <= ALPHA <= 1 represents the
C boundary value for the moduli of eigenvalues.
C The ALPHA-stability domain does not include the boundary.
C
C A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
C On entry, the leading N-by-N part of this array must
C contain the state dynamics matrix A.
C On exit, if INFO = 0, the leading NR-by-NR part of this
C array contains the state dynamics matrix Ar of the reduced
C order system.
C The resulting A has a block-diagonal form with two blocks.
C For a system with NU ALPHA-unstable eigenvalues and
C NS ALPHA-stable eigenvalues (NU+NS = N), the leading
C NU-by-NU block contains the unreduced part of A
C corresponding to ALPHA-unstable eigenvalues in an
C upper real Schur form.
C The trailing (NR+NS-N)-by-(NR+NS-N) block contains
C the reduced part of A corresponding to ALPHA-stable
C eigenvalues.
C
C LDA INTEGER
C The leading dimension of array A. LDA >= MAX(1,N).
C
C B (input/output) DOUBLE PRECISION array, dimension (LDB,M)
C On entry, the leading N-by-M part of this array must
C contain the original input/state matrix B.
C On exit, if INFO = 0, the leading NR-by-M part of this
C array contains the input/state matrix Br of the reduced
C order system.
C
C LDB INTEGER
C The leading dimension of array B. LDB >= MAX(1,N).
C
C C (input/output) DOUBLE PRECISION array, dimension (LDC,N)
C On entry, the leading P-by-N part of this array must
C contain the original state/output matrix C.
C On exit, if INFO = 0, the leading P-by-NR part of this
C array contains the state/output matrix Cr of the reduced
C order system.
C
C LDC INTEGER
C The leading dimension of array C. LDC >= MAX(1,P).
C
C NS (output) INTEGER
C The dimension of the ALPHA-stable subsystem.
C
C HSV (output) DOUBLE PRECISION array, dimension (N)
C If INFO = 0, the leading NS elements of HSV contain the
C Hankel singular values of the ALPHA-stable part of the
C original system ordered decreasingly.
C HSV(1) is the Hankel norm of the ALPHA-stable subsystem.
C
C Tolerances
C
C TOL DOUBLE PRECISION
C If ORDSEL = 'A', TOL contains the tolerance for
C determining the order of reduced system.
C For model reduction, the recommended value is
C TOL = c*HNORM(As,Bs,Cs), where c is a constant in the
C interval [0.00001,0.001], and HNORM(As,Bs,Cs) is the
C Hankel-norm of the ALPHA-stable part of the given system
C (computed in HSV(1)).
C If TOL <= 0 on entry, the used default value is
C TOL = NS*EPS*HNORM(As,Bs,Cs), where NS is the number of
C ALPHA-stable eigenvalues of A and EPS is the machine
C precision (see LAPACK Library Routine DLAMCH).
C This value is appropriate to compute a minimal realization
C of the ALPHA-stable part.
C If ORDSEL = 'F', the value of TOL is ignored.
C
C Workspace
C
C IWORK INTEGER array, dimension (LIWORK)
C LIWORK = 0, if JOB = 'B';
C LIWORK = N, if JOB = 'N'.
C
C DWORK DOUBLE PRECISION array, dimension (LDWORK)
C On exit, if INFO = 0, DWORK(1) returns the optimal value
C of LDWORK.
C
C LDWORK INTEGER
C The length of the array DWORK.
C LDWORK >= MAX(1,N*(2*N+MAX(N,M,P)+5) + N*(N+1)/2).
C For optimum performance LDWORK should be larger.
C
C Warning Indicator
C
C IWARN INTEGER
C = 0: no warning;
C = 1: with ORDSEL = 'F', the selected order NR is greater
C than NSMIN, the sum of the order of the
C ALPHA-unstable part and the order of a minimal
C realization of the ALPHA-stable part of the given
C system. In this case, the resulting NR is set equal
C to NSMIN.
C = 2: with ORDSEL = 'F', the selected order NR is less
C than the order of the ALPHA-unstable part of the
C given system. In this case NR is set equal to the
C order of the ALPHA-unstable part.
C
C Error Indicator
C
C INFO INTEGER
C = 0: successful exit;
C < 0: if INFO = -i, the i-th argument had an illegal
C value;
C = 1: the computation of the ordered real Schur form of A
C failed;
C = 2: the separation of the ALPHA-stable/unstable diagonal
C blocks failed because of very close eigenvalues;
C = 3: the computation of Hankel singular values failed.
C
C METHOD
C
C Let be the following linear system
C
C d[x(t)] = Ax(t) + Bu(t)
C y(t) = Cx(t) (1)
C
C where d[x(t)] is dx(t)/dt for a continuous-time system and x(t+1)
C for a discrete-time system. The subroutine AB09MD determines for
C the given system (1), the matrices of a reduced order system
C
C d[z(t)] = Ar*z(t) + Br*u(t)
C yr(t) = Cr*z(t) (2)
C
C such that
C
C HSV(NR+NS-N) <= INFNORM(G-Gr) <= 2*[HSV(NR+NS-N+1)+...+HSV(NS)],
C
C where G and Gr are transfer-function matrices of the systems
C (A,B,C) and (Ar,Br,Cr), respectively, and INFNORM(G) is the
C infinity-norm of G.
C
C The following procedure is used to reduce a given G:
C
C 1) Decompose additively G as
C
C G = G1 + G2
C
C such that G1 = (As,Bs,Cs) has only ALPHA-stable poles and
C G2 = (Au,Bu,Cu) has only ALPHA-unstable poles.
C
C 2) Determine G1r, a reduced order approximation of the
C ALPHA-stable part G1.
C
C 3) Assemble the reduced model Gr as
C
C Gr = G1r + G2.
C
C To reduce the ALPHA-stable part G1, if JOB = 'B', the square-root
C Balance & Truncate method of [1] is used, and for an ALPHA-stable
C continuous-time system (DICO = 'C'), the resulting reduced model
C is balanced. For ALPHA-stable systems, setting TOL < 0, the
C routine can be used to compute balanced minimal state-space
C realizations.
C
C If JOB = 'N', the balancing-free square-root version of the
C Balance & Truncate method [2] is used to reduce the ALPHA-stable
C part G1.
C
C REFERENCES
C
C [1] Tombs M.S. and Postlethwaite I.
C Truncated balanced realization of stable, non-minimal
C state-space systems.
C Int. J. Control, Vol. 46, pp. 1319-1330, 1987.
C
C [2] Varga A.
C Efficient minimal realization procedure based on balancing.
C Proc. of IMACS/IFAC Symp. MCTS, Lille, France, May 1991,
C A. El Moudui, P. Borne, S. G. Tzafestas (Eds.),
C Vol. 2, pp. 42-46.
C
C NUMERICAL ASPECTS
C
C The implemented methods rely on accuracy enhancing square-root or
C balancing-free square-root techniques.
C 3
C The algorithms require less than 30N floating point operations.
C
C CONTRIBUTOR
C
C A. Varga, German Aerospace Center, DLR Oberpfaffenhofen.
C February 1999. Based on the RASP routines SADSDC, SRBT and SRBFT.
C
C REVISIONS
C
C Mar. 1999, V. Sima, Research Institute for Informatics, Bucharest.
C Nov. 2000, A. Varga, DLR Oberpfaffenhofen.
C
C KEYWORDS
C
C Balancing, minimal realization, model reduction, multivariable
C system, state-space model, state-space representation.
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ZERO, ONE, C100
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0, C100 = 100.0D0 )
C .. Scalar Arguments ..
CHARACTER DICO, EQUIL, JOB, ORDSEL
INTEGER INFO, IWARN, LDA, LDB, LDC, LDWORK, M, N, NR,
$ NS, P
DOUBLE PRECISION ALPHA, TOL
C .. Array Arguments ..
INTEGER IWORK(*)
DOUBLE PRECISION A(LDA,*), B(LDB,*), C(LDC,*), DWORK(*), HSV(*)
C .. Local Scalars ..
LOGICAL DISCR, FIXORD
INTEGER IERR, IWARNL, KT, KTI, KU, KW, KWI, KWR, LWR,
$ NN, NRA, NU, NU1, WRKOPT
DOUBLE PRECISION ALPWRK, MAXRED
C .. External Functions ..
LOGICAL LSAME
DOUBLE PRECISION DLAMCH
EXTERNAL DLAMCH, LSAME
C .. External Subroutines ..
EXTERNAL AB09AX, TB01ID, TB01KD, XERBLA
C .. Intrinsic Functions ..
INTRINSIC DBLE, INT, MAX, MIN, SQRT
C .. Executable Statements ..
C
INFO = 0
IWARN = 0
DISCR = LSAME( DICO, 'D' )
FIXORD = LSAME( ORDSEL, 'F' )
C
C Test the input scalar arguments.
C
IF( .NOT. ( LSAME( DICO, 'C' ) .OR. DISCR ) ) THEN
INFO = -1
ELSE IF( .NOT. ( LSAME( JOB, 'B' ) .OR. LSAME( JOB, 'N' ) ) ) THEN
INFO = -2
ELSE IF( .NOT. ( LSAME( EQUIL, 'S' ) .OR.
$ LSAME( EQUIL, 'N' ) ) ) THEN
INFO = -3
ELSE IF( .NOT. ( FIXORD .OR. LSAME( ORDSEL, 'A' ) ) ) THEN
INFO = -4
ELSE IF( N.LT.0 ) THEN
INFO = -5
ELSE IF( M.LT.0 ) THEN
INFO = -6
ELSE IF( P.LT.0 ) THEN
INFO = -7
ELSE IF( FIXORD .AND. ( NR.LT.0 .OR. NR.GT.N ) ) THEN
INFO = -8
ELSE IF( ( DISCR .AND. ( ALPHA.LT.ZERO .OR. ALPHA.GT.ONE ) ) .OR.
$ ( .NOT.DISCR .AND. ALPHA.GT.ZERO ) ) THEN
INFO = -9
ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
INFO = -11
ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
INFO = -13
ELSE IF( LDC.LT.MAX( 1, P ) ) THEN
INFO = -15
ELSE IF( LDWORK.LT.MAX( 1, N*( 2*N + MAX( N, M, P ) + 5 ) +
$ ( N*( N + 1 ) )/2 ) ) THEN
INFO = -21
END IF
C
IF( INFO.NE.0 ) THEN
C
C Error return.
C
CALL XERBLA( 'AB09MD', -INFO )
RETURN
END IF
C
C Quick return if possible.
C
IF( MIN( N, M, P ).EQ.0 ) THEN
NR = 0
DWORK(1) = ONE
RETURN
END IF
C
IF( LSAME( EQUIL, 'S' ) ) THEN
C
C Scale simultaneously the matrices A, B and C:
C A <- inv(D)*A*D, B <- inv(D)*B and C <- C*D, where D is a
C diagonal matrix.
C Workspace: N.
C
MAXRED = C100
CALL TB01ID( 'All', N, M, P, MAXRED, A, LDA, B, LDB, C, LDC,
$ DWORK, INFO )
END IF
C
C Correct the value of ALPHA to ensure stability.
C
ALPWRK = ALPHA
IF( DISCR ) THEN
IF( ALPHA.EQ.ONE ) ALPWRK = ONE - SQRT( DLAMCH( 'E' ) )
ELSE
IF( ALPHA.EQ.ZERO ) ALPWRK = -SQRT( DLAMCH( 'E' ) )
END IF
C
C Allocate working storage.
C
NN = N*N
KU = 1
KWR = KU + NN
KWI = KWR + N
KW = KWI + N
LWR = LDWORK - KW + 1
C
C Reduce A to a block-diagonal real Schur form, with the
C ALPHA-unstable part in the leading diagonal position, using a
C non-orthogonal similarity transformation A <- inv(T)*A*T and
C apply the transformation to B and C: B <- inv(T)*B and C <- C*T.
C
C Workspace needed: N*(N+2);
C Additional workspace: need 3*N;
C prefer larger.
C
CALL TB01KD( DICO, 'Unstable', 'General', N, M, P, ALPWRK, A, LDA,
$ B, LDB, C, LDC, NU, DWORK(KU), N, DWORK(KWR),
$ DWORK(KWI), DWORK(KW), LWR, IERR )
C
IF( IERR.NE.0 ) THEN
IF( IERR.NE.3 ) THEN
INFO = 1
ELSE
INFO = 2
END IF
RETURN
END IF
C
WRKOPT = DWORK(KW) + DBLE( KW-1 )
C
IWARNL = 0
NS = N - NU
IF( FIXORD ) THEN
NRA = MAX( 0, NR-NU )
IF( NR.LT.NU )
$ IWARNL = 2
ELSE
NRA = 0
END IF
C
C Finish if only unstable part is present.
C
IF( NS.EQ.0 ) THEN
NR = NU
DWORK(1) = WRKOPT
RETURN
END IF
C
NU1 = NU + 1
C
C Allocate working storage.
C
KT = 1
KTI = KT + NN
KW = KTI + NN
C
C Compute a B & T approximation of the stable part.
C Workspace: need N*(2*N+MAX(N,M,P)+5) + N*(N+1)/2;
C prefer larger.
C
CALL AB09AX( DICO, JOB, ORDSEL, NS, M, P, NRA, A(NU1,NU1), LDA,
$ B(NU1,1), LDB, C(1,NU1), LDC, HSV, DWORK(KT), N,
$ DWORK(KTI), N, TOL, IWORK, DWORK(KW), LDWORK-KW+1,
$ IWARN, IERR )
IWARN = MAX( IWARN, IWARNL )
C
IF( IERR.NE.0 ) THEN
INFO = IERR + 1
RETURN
END IF
C
NR = NRA + NU
C
DWORK(1) = MAX( WRKOPT, INT( DWORK(KW) ) + KW - 1 )
C
RETURN
C *** Last line of AB09MD ***
END