Two Reasons Why a Controls Laboratory Needs an Analog Computer
Two Reasons Why a Controls Laboratory Needs
R. H. Spiess
Cc
an Analog Computer*
Inc., 305 D
Road, IL 60010, USA
Science and technology advance neither steadily nor continuously. Concepts deemed obsolete and
confined to oblivion find their niche in new developments. Discarded old methods revive as the key
ingredients to new designs
f Computer simulation, as applied
to system design, was as founded. The operational amplifier, to become the basis for modern linear
circuitry, was perfected..
Valuable as it was, analog simulation’s quasi-hardware approach to analysis aggravated users as much
as it aided them. As digital simulation languages arrived, simulation engineers readily adopted them.
When the microprocessor transformed digital computers into inexpensive circuit components (more
like gates, flip-flops, amplifiers, et
But, the discrete digital and conti 9g worlds are not
variable resolution limitations create problems
Control system designs, especially microprocessor based ones, need laboratory development, For
hands-on testing, the analog computer is as handy an instrument as a controls engineer could have. Its
two unique ahd valuable functions are:
Simulator of Systems to be Controlled ... The electrical analogs of physical models, analog computer
‘Timing discontinuities and
Programmable Linear Circuits Manifold... The terminal points for high quality, linear circuit devices,
analog computer patch panels offer the only formal means of programing linear signal processing,
interface and control circuits.
‘This paper offers a discussion of these two analog computing uses.
*Paper submitted to the American Control Conference, Seattle, WA, USA, 18-20 June 1986.
ANALOG COMPUTER REVIEW
Analog computing owes its existance to the develop-
ment of the operaational amplifier. In the late 1930’s the
operational amplifier was refined to a point of becoming
a functional circuit component. Its usefulness was
demonstrated during World War II where active circuits
computed anti-aircraft fire control projectile trajectories.
Soon after the war ended the tech- nology was applied to
general applications. By the early 1950’s, the patch panel
was adopted and analog computer simulation was being
used enthusiastically by aircraft and other dynamic sys-
tem design engineers.
As the joperaicnal amplifier was vital to the analog
the anal was to the
operational amplifier. The builders and users of analog
computers were the driving forces that led to the
amplifier’s role in today’s linear circuit technology.
Analog simulation brought to focus its versatility. Analog
computer designs struggled with its difficult stabilty
Analog p dits useful-
ness.
The operational amplifier’s unique function was then,
as it is now, to force virtual ground points throughout a
circuit so that its components can be isolated and treated
simply as an input/ output transfer function. By selecting
amplifier networks (resistors, capacitors, transconduc-
tors) a variety of transfer function devices
ANALOG COMPUTERS IN THE
LABORATORY
A controls laboratory will likely support one of the
following:
Project Development.. where laboratory ap-
paratus is dedicated to a specific project, remain-
ing intact until the its completion.
General Development... .where the apparatus is
selected and d for general
use.
Education...where workstations are structured for
expreriments to support lecture presentations.
Asa general purpose research, development and
d the basic of figure 1
is suggested.
TYPICAL CONTROL DESIGN WORKSTATION
MICROCOMPUTER
POWER AMPLIFIER
integrtors, multipliers, etc.) become available for circuit
use
‘analog instrument
Figure 1.
Insuch a configuration, the user may work easily and
interchangeably in the following media:
Analog Simulation...Both the controller and
Ranienito ied are eeaulated Gu the
analog computer.
Analog Control of the Hardware...The control-
ler is patched and run from the analog computer.
Digital Control of the Simulator...The digital
computer controls the analog simulator.
Digital Control of Hardware...The digital com-
puter controls the hardware.
Analog/Digital Control of A Combined Ap-
paratus/Analog Simulator...By adding simu-
lated poles and zeros, simple hardware is made
to function as a more complex mechanism.
SIMULATOR OF THE MECHANISM TO BE
CONTROLLED
Natural systems are fundamentally continuous. It may
or may nor be realistic to simulate continuous systems
with discrete data. It may or may not be workable to
sample a natural system as discrete data and control it
with discrete ds. Digital
and control designs ultimately demand near zero sam-
pling periods and near infinite resolution of system vari-
ables.
At some point a digital method will fail becaue of
ig time or data
resolution.
As the electrical analogs of real systems, analog com-
puter simulations produce the same continuous, infinite
resolution variables as those found in natural eviron-
ments,
Analog simulations synthesize continuous vari-
ables with a realism that is unattainable by digital
methods.
Simulations in general enable a design to be tested for
theoretical validity before being exposed to real world
difficulties.
In testing a design, analog simulators are direct
replacements for actual hardware.
While analog simulations are useful for both analog
and digital control design, they are especially valuable for
testing digital controllers:
First, a simulation of the total system will likely consider
only the theoretical validity of the equivalent analog
controller. Numerical simulation techniques not be well
suited for simulating hybrid analog/digital systems.
Mixing discrete and continuous operations adds
programming difficulties that are avoided when using an
analog simulator.
Second, testing the controller hardware is more com-
plex and, thereby, more critical to the digital design.
Where analog controllers are circuits of operational
amplifiers, directly compatible with system analog i in-
tr digital Hers are not. Theyi
discontinuities that, at times, exert unpredictable non-
linear effects.
Analog simulators offer ideal testing grounds for digi-
tal controllers, superior to real systems in the following
two ways:
They behave like real mechanisms, respond to and
produce the same continuous voltages
but, their behavior is predictable, the exact
response of analytical models or transfer func-
tions, and they can be altered to suit test condi-
tions--where parameters may be changed,
non-linearities, noise, etc. added, and models
reprogrammed.
Second, the designer can evaluate easily all key
variables,
where displacements, velocities, errors, etc. are
programmed as operational amplifier outputs.
THE PROGRAMMABLE LINEAR CIRCUITS
MANIFOLD
bling blocks to digital/analog de-signs are, more
often than not, analog circuits. No matter how important
the digital processor is, no matter how much of the ap-ap-
plication is handled by digital software, if the program is
interfaced to an analog system there will be analog cir-
cuits.
To connect the discrete digital and continuous
analog worlds, bridges need be built: Instrumen-
tation signals need to be amplified. Variables
need be to scaled. Noise needs to be filtered.
Analog computers offer the only means to build the
bridges as patch panel programs rather than breadboard
circuits.
Some advantages of the general purpose patch panel
over dedicated breadboard circuits are:
Speed...A patched program can be up and running
in a fraction of the time needed to design and test
a dedicated circuit.
Reliablity...Developed for general purpose use,
analog computing devices operate stabily under
both resistive and capacitive loading.
Accuracy...Precision amplifier networks and high
resolution parameter settings are inherent analog
computer features.
Versatility...Unlike a dedicated circuit, a program
is easily changed to meet unanticipated demands.
Cost...Savings are realized both from eliminating
the custom design cost and by spreading the pur-
chase costs over multiple uses.
CONCLUSION
To best meet hard: ¢ testing and devel Te-
a controls | y needs to be « quipped
with a small general purpose analog computer.
305 Devonshire Road, Barrington, Illinois 60010, tel & fax (708) 381-7560