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78 CHAPTER THREE
at the special-purpose computers first. It’s likely we’ll be choosing a general-purpose
computer for the robot, but special-purpose computers can bring many advantages to the
design. Before we take a close look at the architecture of the general-purpose computer,
here is a quick tour of the basic architectures of some special-purpose computers.
Analog Computers

Webster’s dictionary defines analog as “something similar to something else; a mecha-
nism in which data is represented by continuously variable physical quantities.” Analog
computers are commonly perceived as a throwback to the early days of computing
machinery. Even now, all electronic computers use analog electronic signals to support
their calculations. General-purpose digital computers, however, restrict the analog elec-
tronic signals to just two voltage levels representing binary 1 and binary 0 in an effort
to gain speed. Analog computers have no such voltage restrictions for signals. Instead,
signals vary throughout the range of voltages that the analog computer electronics can
support. A single analog signal can directly represent, for example, the speed of the
8
wind from 0 to 255 mph. A general-purpose computer needs eight signals (2 256)
to represent the same range of values for the wind.
Analog computers use analog electronics, such as operational amplifiers, to build cir-
cuits to simulate the behavior of complex systems. They are especially good at simu-
lating systems that are governed by differential equations. The second-order control
system described elsewhere in the book is a prime example. With just one operational
amplifier, an analog computer can fully simulate the same curves and parametric con-
trols we have already looked at. The front of an analog computer looks like a giant
switchboard with lots of places to plug in wires.
To program an analog computer, an engineer uses patch wires to plug together the
required building blocks. Knobs on the analog computer can be rotated to enter the val-
ues for the desired frequency and damping. The engineer starts the computer and a
meter needle shows the resulting curve over the span of a couple of seconds of simu-
lated time. In the example of our robot’s second-order system, overshoot is evident if
the meter needle goes too high before settling down. Ringing can be seen as the oscil-
lation of the needle back and forth while it settles down.
Analog computers have dropped by the wayside for two basic reasons:
A general-purpose computer can be programmed to simulate an analog computer,
obviating the need for the analog hardware.
General-purpose computers can be programmed in different ways to solve the
same problems. Instead of simulating the analog computer (which simulates the
real-world problem), a general-purpose computer can be programmed to simulate
the real-world problem directly.