State Definition

In view of the above discussion, it is perhaps plausible that the
*state*
of a physical
system at time
can be defined as a collection of *state
variables*
, wherein each state variable
is a
*physical amplitude* (pressure, velocity, position,
)
corresponding to a *degree of freedom* of the system. We define a
*degree of freedom* as a single dimension of *energy
storage*. The net result is that it is possible to compute the
stored energy in any degree of freedom (the system's ``memory'') from
its corresponding state-variable amplitude.

For example, an *ideal mass*
can store only *kinetic
energy*
, where
denotes the
mass's velocity along the
axis. Therefore, *velocity* is the
natural choice of state variable for an ideal point-mass.
Coincidentally, we reached this conclusion independently above by
writing
in state-space form
. Note that a
point mass that can move freely in 3D space has three degrees of
freedom and therefore needs three state variables
in
its physical model. In typical models from musical acoustics (*e.g.*,
for the piano hammer), masses are allowed only one degree of freedom,
corresponding to being constrained to move along a 1D line, like an
ideal spring. We'll study the ideal mass further in §7.1.2.

Another state-variable example is provided by an *ideal spring*
described by Hooke's law
(§B.1.3), where
denotes the spring
constant, and
denotes the spring displacement from rest. Springs
thus contribute a force proportional to displacement in Newtonian
ODEs. Such a spring can only store the physical *work* (force
times distance), expended to displace, it in the form of
*potential energy*
. More about
ideal springs will be discussed in §7.1.3. Thus,
*spring displacement* is the most natural choice of state
variable for a spring.

In so-called RLC electrical circuits (consisting of resistors , inductors , and capacitors ), the state variables are typically defined as all of the capacitor voltages (or charges) and inductor currents. We will discuss RLC electrical circuits further below.

There is no state variable for each resistor current in an RLC circuit
because a resistor dissipates energy but does not store it--it has no
``memory'' like capacitors and inductors. The state (current
,
say) of a resistor
is determined by the voltage
across it,
according to Ohm's law
, and that voltage is supplied by the
capacitors, inductors, and voltage-sources, etc., to which it is
connected. Analogous remarks apply to the *dashpot*, which is
the mechanical analog of the resistor--we do not assign state
variables to dashpots. (If we do, such as by mistake, then we will
obtain state variables that are linearly dependent on other state
variables, and the order of the system appears to be larger than it
really is. This does not normally cause problems, and there are many
numerical ways to later ``prune'' the state down to its proper order.)

Masses, springs, dashpots, inductors, capacitors, and resistors are
examples of so-called *lumped* elements. Perhaps the simplest
*distributed* element is the continuous ideal delay line.
Because it carries a *continuum* of independent amplitudes, the
order (number of state variables) is *infinity* for a continuous
delay line of any length! However, in practice, we often work with
*sampled, bandlimited* systems, and in this domain, delay lines
have a finite number of state variables (one for each delay element).
Networks of lumped elements yield finite-order state-space models,
while even one distributed element jumps the order to infinity until
it is bandlimited and sampled.

In summary, a state variable may be defined as a physical amplitude
for some energy-storing degree of freedom. In models of mechanical
systems, a state variable is needed for each ideal spring and point
mass (times the number of dimensions in which it can move). For RLC
electric circuits, a state variable is needed for each capacitor and
inductor. If there are any switches, their state is also needed in
the state vector (*e.g.*, as boolean variables). In discrete-time
systems such as digital filters, each unit-sample delay element
contributes one (continuous) state variable to the model.

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