Systems
How do you…
map
…feel?
cool
hot
can be Static or Dynamic
which can be Linear or Closed-loop
…know?
which can be Recirculating or Self-regulatory
path
…do?
handle
button
which can be First- or Second-order
which can be Self-adjusting or Learning
• Bill Verplank’s interaction model
• Types of systems
the aperture of the steam-valve; with less steam
the piston fills less quickly, turning the wheel less
quickly. As the wheel slows, the governor expands
the valve aperture, increasing steam and thus
increasing the speed of the wheel. The piston provides input to the wheel, but the governor translates the output of the wheel into input for the piston. This is a self-regulating system, maintaining
the speed of the wheel—a classic feedback loop.
Of course, the steam engine does not operate entirely on its own. It receives its “goal” from
outside; a person sets the speed of the wheel by
adjusting the length of the linkage connecting the
fly-ball governor to the steam valve. In Haque’s
terminology, the transfer function is changed.
Our model of the steam engine has the same
underlying structure as the classic model of interaction described earlier! Both are closed information loops, self-regulating systems, first-order
cybernetic systems. While the feedback loop is a
useful first approximation of human computer
interaction, its similarity to a steam engine may
give us pause.
The computer-human interaction loop differs
from the steam-engine-governor interaction loop
in two major ways. First, the role of the person: The
person is inside the computer-human interaction
loop, while the person is outside the steam-engine-governor interaction loop. Second, the nature of the
system: The computer is not characterized in our
model of computer-human interaction. All we know
is that the computer acts on input and provides output. But we have characterized the steam engine in
some detail as a self-regulating system. Suppose we
characterize the computer with the same level of
detail as the steam engine? Suppose we also characterize the person?
Types of Systems
So far, we have distinguished between static and
dynamic systems—those that cannot act and thus
have little or no meaningful effect on their environment (a chair, for example) and those that can
and do act, thus changing their relationship to the
environment.
Within dynamic systems, we have distinguished
between those that only react and those that interact—linear (open-loop) and closed-loop systems.
Some closed-loop systems have a novel property—they can be self-regulating. But not all closed-loop systems are self-regulating. The natural cycle
of water is a loop. Rain falls from the atmosphere
and is absorbed into the ground or runs into the
sea. Water on the ground or in the sea evaporates
into the atmosphere. But nowhere within the cycle
is there a goal.
A self-regulating system has a goal. The goal
defines a relationship between the system and its
environment, which the system seeks to attain and
maintain. This relationship is what the system regulates, what it seeks to keep constant in the face
of external forces. A simple self-regulating system
(one with only a single loop) cannot adjust its own
goal; its goal can be adjusted only by something
outside the system. Such single-loop systems are
called “first order.”
Learning systems nest a first self-regulating
system inside a second self-regulating system. The