Udwadia–Kalaba formulation

The Udwadia–Kalaba equation is a method for deriving the equations of motion of a constrained mechanical system. This equation was discovered by Firdaus E. Udwadia and Robert E. Kalaba in 1992. The fundamental equation is the simplest and most comprehensive equation so far discovered for writing down the equations of motion of a constrained mechanical system. The equation even generalizes to constraint forces that do not obey D'Alembert's principle.

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 $n$-vector is given by

$\mathbf {q} :=[q_{1},q_{2},\ldots ,q_{n}]^{\mathrm {T} }.$

Using Newtonian or Lagrangian dynamics, the unconstrained equations of motion of the system $S$ under study can be derived as

$\mathbf {M} (q,t){\ddot {\mathbf {q} }}(t)=\mathbf {Q} (q,{\dot {q}},t),$

where it is assumed that the initial conditions $\mathbf {q} (0)$ and ${\dot {\mathbf {q} }}(0)$ are known. We call the system $S$ unconstrained because ${\dot {\mathbf {q} }}(0)$ may be arbitrarily assigned. Here, the dots represent derivatives with respect to time. The $n$ by $n$ matrix $\mathbf {M}$ is symmetric, and it can be positive definite $(\mathbf {M} >0)$ or semi-positive definite $(\mathbf {M} \geq 0)$. Typically, it is assumed that $\mathbf {M}$ is positive definite; however, it is not uncommon to derive the unconstrained equations of motion of the system $S$ such that $\mathbf {M}$ is only semi-positive definite; i.e., the mass matrix may be singular. The $n$-vector $\mathbf {Q}$ denotes the total generalized force impressed on the system; it can be expressible as the summation of all the conservative forces with the non-conservative forces.

Constraints

We now assume that the unconstrained system $S$ is subjected to a set of $m$ consistent equality constraints given by

$\mathbf {A} (q,{\dot {q}},t){\ddot {\mathbf {q} }}=\mathbf {b} (q,{\dot {q}},t),$

where $\mathbf {A}$ is a known m by n matrix of rank r and $\mathbf {b}$ is a known m-vector. We note that this set of constraint equations encompass a very general variety of holonomic and non-holonomic equality constraints. For example, holonomic constraints of the form

$\varphi (q,t)=0$

can be differentiated twice with respect to time while non-holonomic 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 $\mathbf {A}$ and the $m$-vector $\mathbf {b}$. 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

$\mathbf {M} {\ddot {\mathbf {q} }}=\mathbf {Q} +\mathbf {Q} _{c}(q,{\dot {q}},t),$

where $\mathbf {Q} _{c}$—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 $\mathbf {A} {\ddot {q}}=\mathbf {b}$ 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.

Equation of motion

The solution to this central problem is given by the Udwadia–Kalaba equation. When the matrix $\mathbf {M}$ is positive definite, the equation of motion of the constrained system $S_{c}$, at each instant of time, is

$\mathbf {M} {\ddot {\mathbf {q} }}=\mathbf {Q} +\mathbf {M} ^{1/2}\left(\mathbf {A} \mathbf {M} ^{-1/2}\right)^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} ^{-1}\mathbf {Q} ),$

where the '+' symbol denotes the pseudoinverse of the matrix $\mathbf {A} \mathbf {M} ^{-1/2}$. The force of constraint is thus given explicitly as

$\mathbf {Q} _{c}=\mathbf {M} ^{1/2}\left(\mathbf {A} \mathbf {M} ^{-1/2}\right)^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} ^{-1}\mathbf {Q} ),$

and since the matrix $\mathbf {M}$ is positive definite the generalized acceleration of the constrained system $S_{c}$ is determined explicitly by

${\ddot {\mathbf {q} }}=\mathbf {M} ^{-1}\mathbf {Q} +\mathbf {M} ^{-1/2}\left(\mathbf {A} \mathbf {M} ^{-1/2}\right)^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} ^{-1}\mathbf {Q} ).$

In the case that the matrix $\mathbf {M}$ is semi-positive definite $(\mathbf {M} \geq 0)$, the above equation cannot be used directly because $\mathbf {M}$ may be singular. Furthermore, the generalized accelerations may not be unique unless the $n+m$ by $n$ matrix

${\hat {\mathbf {M} }}=\left[{\begin{array}{c}\mathbf {M} \\\mathbf {A} \end{array}}\right]$

has full rank (rank = $n$). 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 ${\hat {\mathbf {M} }}$ 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

$\mathbf {M} _{\mathbf {A} }{\ddot {\mathbf {q} }}:=(\mathbf {M} +\mathbf {A} ^{+}\mathbf {A} ){\ddot {\mathbf {q} }}=\mathbf {Q} +\mathbf {A} ^{+}\mathbf {b} :=\mathbf {Q} _{\mathbf {b} },$

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

$\mathbf {M} _{\mathbf {A} }{\ddot {\mathbf {q} }}=\mathbf {Q} _{\mathbf {b} }+\mathbf {M} _{\mathbf {A} }^{1/2}(\mathbf {A} \mathbf {M} _{\mathbf {A} }^{-1/2})^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} _{\mathbf {A} }^{-1}\mathbf {Q} _{\mathbf {b} }).$

Moreover, when the matrix ${\hat {\mathbf {M} }}$ has full rank, the matrix $\mathbf {M} _{\mathbf {A} }$ is always positive definite. This yields, explicitly, the generalized accelerations of the constrained system $S_{c}$ as

${\ddot {\mathbf {q} }}=\mathbf {M} _{\mathbf {A} }^{-1}\mathbf {Q} _{\mathbf {b} }+\mathbf {M} _{\mathbf {A} }^{-1/2}(\mathbf {A} \mathbf {M} _{\mathbf {A} }^{-1/2})^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} _{\mathbf {A} }^{-1}\mathbf {Q} _{\mathbf {b} }).$

This equation is valid when the matrix $\mathbf {M}$ is either positive definite or positive semi-definite! Additionally, the force of constraint that causes the constrained system $S_{c}$—a system that may have a singular mass matrix $\mathbf {M}$—to satisfy the imposed constraints is explicitly given by

$\mathbf {Q} _{c}=\mathbf {M} _{\mathbf {A} }^{1/2}(\mathbf {A} \mathbf {M} _{\mathbf {A} }^{-1/2})^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} _{\mathbf {A} }^{-1}\mathbf {Q} _{\mathbf {b} }).$

Non-ideal constraints

Suppose that the virtual work of a displacement is

$W^{c}(t)=\mathbf {v} ^{\mathrm {T} }(t)\mathbf {C} (q,{\dot {q}},t)$

The vector $\mathbf {C} (q,{\dot {q}},t)$ describes the non-ideality of the virtual work and may be related, for example, to a force of friction. This is a generalized D'Alembert's principle, where the usual form of the principle has vanishing virtual work with $\mathbf {C} (q,{\dot {q}},t)=0$.

The Udwadia–Kalaba equation is modified by an additional non-ideal constraint term to

$\mathbf {M} {\ddot {\mathbf {q} }}=\mathbf {Q} +\mathbf {M} ^{1/2}\left(\mathbf {A} \mathbf {M} ^{-1/2}\right)^{+}(\mathbf {b} -\mathbf {A} \mathbf {M} ^{-1}\mathbf {Q} )+\mathbf {M} ^{1/2}\left[\mathbf {I} -\left(\mathbf {A} \mathbf {M} ^{-1/2}\right)^{+}\mathbf {A} \mathbf {M} ^{-1/2}\right]\mathbf {M} ^{1/2}\mathbf {C}$

Example

The method can solve the inverse Kepler problem of determining the force law that corresponds to the orbits that are conic sections. We take there to be no external forces (not even gravity) and instead constrain the particle motion to follow orbits of the form

$r=\epsilon x+l$

where $r={\sqrt {x^{2}+y^{2}}}$, $\epsilon$ is the eccentricity, and $l$ is the semi-latus rectum. Differentiating twice with respect to time and rearranging slightly gives a constraint

$(x-r\epsilon ){\ddot {x}}+y{\ddot {y}}=-{\frac {(x{\dot {y}}-y{\dot {x}})^{2}}{r^{2}}}$

We also assume that angular momentum about the focus is conserved as

$m(x{\dot {y}}-y{\dot {x}})=L$

with time derivative

$x{\ddot {y}}-y{\ddot {x}}=0$

We can combine these two constraints into the matrix equation

${\begin{pmatrix}x-r\epsilon &y\\y&-x\end{pmatrix}}{\begin{pmatrix}{\ddot {x}}\\{\ddot {y}}\end{pmatrix}}={\begin{pmatrix}-{\frac {L^{2}}{m^{2}r^{2}}}\\0\end{pmatrix}}$

The constraint matrix has inverse

${\begin{pmatrix}x-r\epsilon &y\\y&-x\end{pmatrix}}^{-1}={\frac {1}{lr}}{\begin{pmatrix}x&y\\y&-(x-r\epsilon )\end{pmatrix}}$

We assume the body has a simple, constant mass. The force of constraint is therefore the expected, central inverse square law

$\mathbf {F} _{c}=m\mathbf {A} ^{-1}\mathbf {b} ={\frac {m}{lr}}{\begin{pmatrix}x&y\\y&-(x-r\epsilon )\end{pmatrix}}{\begin{pmatrix}-{\frac {L^{2}}{m^{2}r^{2}}}\\0\end{pmatrix}}=-{\frac {L^{2}}{mlr^{2}}}{\begin{pmatrix}\cos {\theta }\\\sin {\theta }\end{pmatrix}}$