The Fundamental Equation of Constrained Motion^{[1]} is a mathematical method for derivation of the equations of motion of a constrained system. This equation was originally developed by Firdaus E. Udwadia and Robert E. Kalaba in a series of papers, beginning in 1992. This equation has application in the field of analytical dynamics.
The Central Problem of Constrained Motion
In the study of the dynamics of mechanical systems, the configuration of a given system S is, in general, completely described by n generalized coordinates so that its generalized coordinate nvector is given by

q:=[q_1,q_2,\ldots,q_n]^T.
Using Newtonian or Lagrangian dynamics, the unconstrained equations of motion of the system S under study can be derived as

M(q,t)\ddot{q}(t)=Q(q,\dot{q},t),
where it is assumed that the initial conditions q(0) and \dot{q}(0) are known. We call the system S unconstrained because \dot{q}(0) may be arbitrarily assigned. Here, the dots represent derivatives with respect to time. The n by n matrix M is symmetric, and it can be positive definite (M > 0) or semipositive definite (M \geq 0). Typically, it is assumed that M is positive definite; however, it is not uncommon to derive the unconstrained equations of motion of the system S such that M is only semipositive definite; i.e., the mass matrix may be singular.^{[2]}^{[3]} The nvector Q denotes the total generalized force impressed on the system; it can be expressible as the summation of all the conservative forces with the nonconservative forces.
Constraints
We now assume that the unconstrained system S is subjected to a set of m consistent equality constraints given by

A(q,\dot{q},t)\ddot{q} = b(q,\dot{q},t),
where A is a known m by n matrix of rank r and b is a known mvector. We note that this set of constraint equations encompass a very general variety of holonomic and nonholonomic equality constraints. For example, holonomic constraints of the form

\varphi(q,t) = 0
can be differentiated twice with respect to time while nonholonomic constraints of the form

\psi(q,\dot{q},t) = 0
can be differentiated once with respect to time to obtain the m by n matrix A and the mvector b. In short, constraints may be specified that are (1) nonlinear functions of displacement and velocity, (2) explicitly dependent on time, and (3) functionally dependent.
As a consequence of subjecting these constraints to the unconstrained system S, an additional force is conceptualized to arise, namely, the force of constraint. Therefore, the constrained system S_c becomes

M\ddot{q}=Q+Q^{c}(q,\dot{q},t),
where Q^{c}—the constraint force—is the additional force needed to satisfy the imposed constraints. The central problem of constrained motion is now stated as follows:
1. given the unconstrained equations of motion of the system S,
2. given the generalized displacement q(t) and the generalized velocity \dot{q}(t) of the constrained system S_c at time t, and
3. given the constraints in the form A\ddot{q}=b as stated above,
find the equations of motion for the constrained system—the acceleration—at time t, which is in accordance with the agreed upon principles of analytical dynamics.
The Fundamental Equation of Constrained Motion
The solution to this central problem is given by the fundamental equation of constrained motion. When the matrix M is positive definite, the equation of motion of the constrained system S_c, at each instant of time, is^{[4]}^{[5]}

M\ddot{q} = Q + M^{1/2}\left(AM^{1/2}\right)^+(bAM^{1}Q),
where the '+' symbol denotes the MoorePenrose inverse of the matrix AM^{1/2}. The force of constraint is thus given explicitly as

Q^{c} = M^{1/2}\left(AM^{1/2}\right)^+(bAM^{1}Q),
and since the matrix M is positive definite the generalized acceleration of the constrained system S_c is determined explicitly by

\ddot{q} = M^{1}Q + M^{1/2}\left(AM^{1/2}\right)^+(bAM^{1}Q).
In the case that the matrix M is semipositive definite (M \geq 0), the above equation cannot be used directly because M may be singular. Furthermore, the generalized accelerations may not be unique unless the n+m by n matrix

\hat{M} = \left[\begin{array}{c} M \\ A \end{array}\right]
has full rank (rank = n).^{[2]}^{[3]} But since the observed accelerations of mechanical systems in nature are always unique, this rank condition is a necessary and sufficient condition for obtaining the uniquely defined generalized accelerations of the constrained system S_c at each instant of time. Thus, when \hat{M} has full rank, the equations of motion of the constrained system S_c at each instant of time are uniquely determined by (1) creating the auxiliary unconstrained system^{[3]}

M_A \ddot{q}:=(M+A^+A)\ddot{q} = Q + A^+b := Q_b,
and by (2) applying the fundamental equation of constrained motion to this auxiliary unconstrained system so that the auxiliary constrained equations of motion are explicitly given by^{[3]}

M_A \ddot{q} = Q_b + M_A^{1/2}(AM_A^{1/2})^+(bAM_A^{1}Q_b).
Moreover, when the matrix \hat{M} has full rank, the matrix M_A is always positive definite. This yields, explicitly, the generalized accelerations of the constrained system S_c as

\ddot{q} = M_A^{1}Q_b + M_A^{1/2}(AM_A^{1/2})^+(bAM_A^{1}Q_b).
This equation is valid when the matrix M is either positive definite or positive semidefinite! Additionally, the force of constraint that causes the constrained system S_c—a system that may have a singular mass matrix M—to satisfy the imposed constraints is explicitly given by

Q^{c} = M_A^{1/2}(AM_A^{1/2})^+(bAM_A^{1}Q_b).
References

^ Udwadia, F.E.; Kalaba, R.E. (1996). Analytical Dynamics: A New Approach. Cambridge University Press. ISBN 0521048338

^ ^{a} ^{b} Udwadia, F.E.; Phohomsiri, P. (2006). "Explicit equations of motion for constrained mechanical systems with singular mass matrices and applications to multibody dynamics". Proceedings of the Royal Society of London, Series A 462 (2071): 2097–2117.

^ ^{}a ^{b} ^{c} ^{d} Udwadia, F.E.; Schutte, A.D. (2010). "Equations of motion for general constrained systems in Lagrangian mechanics". Acta Mechanica 213 (1): 111–129.

^ Udwadia, F.E.; Kalaba, R.E. (1992). "A new perspective on constrained motion". Proceedings of the Royal Society of London, Series A 439 (1906): 407–410.

^ Udwadia, F.E.; Kalaba, R.E. (1993). "On motion". Journal of the Franklin Institute 330 (3): 571–577.
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