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附录
附录一：外文原文
Temperature Control Using a
Microcontroller:
An Interdisciplinary
Undergraduate Engineering Design Project
James S. McDonald
Department of Engineering Science
Trinity University
San
Antonio, TX 78212
Abstract
This
paper
describes
an
interdisciplinary
design
project
which
was
done
under
the
author’s
supervision
by
a
group
of
four
senior
students
in
the
Department
of
Engineering Science at Trinity
University. The objective of the project was to
develop
a temperature control system
for an air-filled chamber. The system was to allow
entry
of a desired chamber temperature
in a prescribed range and
to
exhibit overshoot and
steady-state
temperature
error
of
less
than
1
degree
Kelvin
in
the
actual
chamber
temperature
step
response.
The
details
of
the
design
developed
by
this
group
of
students, based on a Motorola MC68HC05
family microcontroller, are described. The
pedagogical
value of
the problem
is also
discussed through a description of some of
the
key
steps
in
the
design
process.
It
is
shown
that
the
solution
requires
broad
knowledge
drawn
from
several
engineering
disciplines
including
electrical,
mechanical, and
control systems engineering.
1
Introduction
The design project which
is the subject of this paper originated from a
real-world
application.
A
prototype
of
a
microscope
slide
dryer
had
been
developed
around
an
OmegaTM
model
CN-390 temperature controller, and the objective
was
to develop a
custom
temperature control system to replace the Omega
system. The motivation was
that
a
custom
controller
targeted
specifically
for
the
application
should
be
able
to
achieve
the
same
functionality
at
a
much
lower
cost,
as
the
Omega
system
is
unnecessarily versatile
and equipped to handle a wide variety of
applications.
The
mechanical
layout
of
the
slide
dryer
prototype
is
shown
in
Figure
1.
The
main element of the dryer is a large,
insulated, air-filled chamber in which microscope
slides, each
with a
tissue sample encased
in
paraffin, can be set on caddies. In order
that the paraffin maintain the proper
consistency, the temperature in the slide chamber
must
be
maintained
at
a
desired
(constant)
temperature.
A
second
chamber
(the
electronics enclosure)
houses a
resistive
heater
and the temperature
controller, and a
fan mounted on the
end of the dryer blows air across the heater,
carrying heat into the
slide chamber.
This design project
was
carried out during
academic
year 1996
–
97 by
four
students
under
the
author’s
supervision
as
a
Senior
Design
project
in
the
Department of
Engineering Science at Trinity University. The
purpose of this paper is
to
describe the problem and
the students’
solution
in some detail, and
to discuss
some
of
the
pedagogical
opportunities
offered
by
an
interdisciplinary
design
project
of this type. The
students’ own report was pre
sented at
the 1997 National Conference
on
Undergraduate
Research
[1].
Section
2
gives
a
more
detailed
statement
of
the
problem,
including performance
specifications, and Section 3 describes
the students’
design.
Section 4 makes up the bulk of the paper, and
discusses in some detail several
aspects
of
the
design
process
which
offer
unique
pedagogical
opportunities.
Finally,
Section 5 offers some conclusions.
2 Problem Statement
The basic idea of the project is to
replace the relevant parts of the functionality of
an
Omega
CN-390
temperature
controller
using
a
custom-
designed
system.
The
application
dictates
that
temperature
settings
are
usually
kept
constant
for
long
periods
of
time,
but
it’s
nonetheless
important
that
step
changes
be
tracked
in
a
―reasonable‖
manner.
Thus
the
main
requirements
boil
down
to·allowing
a
chamber
temperature set-
point to be entered,·
displaying both
set-point and actual temperatures,
and·
tracking
step
changes
in
set-
point
temperature
with
acceptable
rise
time,
steady-state error,
and overshoot.
Although
not
explicitly
a
part
of
the
specifications
in
Table
1,
it
was
clear
that
the
customer
desired
digital
displays
of
set-
point
and
actual
temperatures,
and
that
set-point
temperature
entry
should
be
digital
as
well
(as
opposed
to,
say,
through
a
potentiometer setting).
3
System Design
The
requirements
for
digital
temperature
displays
and
setpoint
entry
alone
are
enough
to
dictate
that
a
microcontrollerbased
design
is
likely
the
most
appropriate.
Figure 2 shows
a block diagram of the
students’
design.
The
microcontroller, a MotorolaMC68HC705B16 (6805 for
short), is the heart of
the system. It
accepts inputs from a simple four-key keypad which
allow specification
of
the
set-point
temperature,
and
it
displays
both
set-point
and
measured
chamber
temperatures
using
two-digit
seven-segment
LED
displays
controlled
by
a
display
driver.
All these
inputs and outputs are accommodated by
parallel ports on
the 6805.
Chamber temperature
is
sensed using a pre-calibrated thermistor and
input via one of
the 6805’s
analog
-to-digital
inputs. Finally, a pulse-width
modulation (PWM) output
on
the 6805
is
used
to drive a relay which switches
line power to
the resistive
heater
off and on.
Figure 3 shows a more detailed
schematic of the electronics and their interfacing
to
the
6805.
The
keypad,
a
Storm
3K041103,
has
four
keys
which
are
interfaced
to
pins PA0{ PA3 of Port A, configured as
inputs. One key
functions as
a mode switch.
Two
modes are
supported:
set
mode and run
mode. In set
mode two of the
other keys
are
used to
specify
the set-point
temperature: one
increments
it and one decrements.
The
fourth
key
is
unused
at
present.
The
LED
displays
are
driven
by
a
Harris
Semiconductor
ICM7212
display
driver
interfaced
to
pins
PB0{PB6
of
Port
B,
configured
as
outputs.
The
temperature-sensing
thermistor
drives,
through
a
voltage
divider, pin AN0 (one of eight analog
inputs). Finally, pin PLMA (one of two
PWM
outputs) drives the heater relay.
Software
on
the
6805
implements
the
temperature
control
algorithm,
maintains
the
temperature
displays,
and
alters
the
set-
point
in
response
to
keypad
inputs.
Because
it
is
not complete at
this
writing,
software will
not
be discussed
in detail
in
this paper. The control algorithm in
particular has not been determined, but
it is likely
to
be
a
simple
proportional
controller
and
certainly
not
more
complex
than
a
PID.
Some
control design issues will be discussed in Section
4, however.
4 The Design Process
Although
essentially
the
project
is
just
to
build
a
thermostat,
it
presents
many
nice
pedagogical
opportunities.
The
knowledge
and
experience
base
of
a
senior
engineering
undergraduate
are
just
enough
to
bring
him
or
her
to
the
brink
of
a
solution to various
aspects of the problem. Yet, in each case,
realworld considerations
complicate the
situation significantly.
Fortunately
these complications are
not
insurmountable,
and the
result
is a
very
beneficial design experience.
The remainder of
this
section
looks at a
few
aspects of
the problem which present
the type of learning opportunity just described.
Section 4.1
discusses
some
of
the
features
of
a
simplified
mathematical
model
of
the
thermal
properties
of
the
system
and
how
it
can
be
easily
validated
experimentally.
Section
4.2
describes
how
realistic
control
algorithm
designs
can
be
arrived
at
using
introductory
concepts
in
control
design.
Section
4.3
points
out
some
important
deficiencies
of
such
a
simplified
modeling/control
design
process
and
how
they
can
be
overcome
through
simulation.
Finally,
Section
4.4
gives
an
overview
of
some
of
the
microcontroller-related
design
issues
which
arise
and
learning
opportunities
offered.
4.1
MathematicalModel
Lumped-element
thermal systems are described in almost any
introductory linear
control
systems
text,
and
just
this
sort
of
model
is
applicable
to
the
slide
dryer
problem. Figure 4 shows a second-order
lumped-element
thermal
model of the slide
dryer.
The state
variables are the
temperatures
Ta of
the air
in
the
box and
Tb of
the
box
itself.
The
inputs
to
the
system
are
the
power
output
q(t)
of
the
heater
and
the
ambient
temperature
T?
.
ma
and
mb
are
the
masses
of
the
air
and
the
box,
respectively,
and
Ca
and
Cb
their
specific
heats.
μ
1
and
μ
2
are
heat
transfer
coefficients
from
the
air
to
the
box
and
from
the
box
to
the
external
world,
respectively.
It’s
not
hard to show that the
(linearized) state equationscorresponding to
Figure
4 are
Taking
Laplace
transforms
of
(1)
and
(2)
and
solving
for
Ta(s),
which
is
the
output of interest,
gives the following open-loop model of the thermal
system:
where
K
is
a
constant
and
D(s)
is
a
second-order
polynomial.K,
tz,
and
the
coefficients of D(s) are functions of
the variousparameters appearing in (1) and (2).Of
course the various parameters in (1)
and (2) are completely unknown, but it’s not
ha
rd
to
show
that,
regardless
of
their
values,
D(s)
has
two
real
zeros.
Therefore
the
main
transfer
function of
interest
(which
is
the one
from Q(s
), since we’ll
assume constant
ambient temperature)
can be written
Moreover,
it’s
not
too
hard to
show
that
1=tp1 <1=tz <1=tp2,
i.e.,
that the
zero
lies between the two poles. Both of
these are excellent exercises
for the
student, and
the result is the openloop pole-zero
diagram of Figure 5.
Obtaining a complete thermal
model, then,
is reduced to
identifying
the constant
K and
the
three
unknown time
constants
in
(3). Four
unknown parameters
is quite
a
few, but simple experiments show that
1=tp1 _ 1=tz;1=tp2 so that tz;tp2 _ 0 are good
approximations. Thus the open-loop
system is essentially first-order and can
therefore
be written
(where the subscript p1 has
been dropped).
Simple open-loop step
response experiments show that,for a wide range of
initial
temperatures and heat inputs, K
_0:14 _=W and t _ 295 s.1
4.2 Control
System Design
Using the
first-order
model of (4)
for the open-loop transfer
function Gaq(s) and
assuming
for the moment that linear control of the heater
power output q(t) is possible,
the
block diagram of Figure 6 represents the closed-
loop system.
Td(s)
is the
desired,
or
set-point,
temperature,C(s)
is
the
compensator
transfer
function,
and
Q(s)
is
the
heater output in watts.
Given
this
simple
situation,
introductory
linear
control
design
tools
such
as
the
root
locus
method
can
be
used
to
arrive
at
a
C(s)
which
meets
the
step
response
requirements on
rise
time, steady-state error, and overshoot
specified
in
Table 1.
The
upshot,
of
course,
is
that
a
proportional
controller
with
sufficient
gain
can
meet
all
specifications.
Overshoot
is
impossible,
and
increasing
gains
decreases
both
steady-state error and
rise time.
Unfortunately, sufficient
gain
to
meet
the specifications
may
require
larger
heat
outputs
than
the
heater
is
capable
of
producing.
This
was
indeed
the
case
for
this
system,
and
the
result
is
that
the
rise
time
specification
cannot
be
met.
It
is
quite
revealing to the student how useful
such an oversimplified model, carefully arrived
at,
can be in determining overall
performance limitations.
4.3 Simulation
Model
Gross
performance
and
its
limitations
can
be
determined
using
the
simplified
model of Figure
6, but
there are a
number of
other aspects of
the closed-loop system
whose
effects
on
performance
are
not
so
simply
modeled.
Chief
among
these
are
·
quantization
error
in
analog-
to-digital
conversion
of
the
measured
temperature
and ·
the use of PWM to control the heater.
Both of
these are
nonlinear and
time-varying
effects, and
the only practical
way
to study them is through
simulation (or experiment, of course).
Figure
7
shows
a
SimulinkTM
block
diagram
of
the
closed-loop
system
which
incorporates
these
effects.
A/D
converter
quantization
and
saturation
are
modeled
using
standard
Simulink
quantizer
and
saturation
blocks.
Modeling
PWM
is
more
complicated and
requires a custom S-function to represent it.
This
simulation
model
has
proven
particularly
useful
in
gauging
the
effects
of
varying the basic PWM parameters and
hence selecting them appropriately.
(I.e.,
the
longer the
period, the larger the temperature error PWM
introduces. On the other hand,
a
long
period
is
desirable
to
avoid
excessive
relay
―chatter,‖
among
other
things.)
PWM
is
often
difficult
for
students
to
grasp,
and
the
simulation
model
allows
an
exploration of its operation and
effects which is quite revealing.
4.4
The Microcontroller
Simple closed-loop
control, keypad reading, and display control are
some of
the
classic
applications
of
microcontrollers,
and
this
project
incorporates
all
three.
It
is
therefore an excellent all-around
exercise
in
microcontroller
applications. In addition,
because
the project
is
to
produce an actual packaged prototype,
it won’t do
to
use a
simple evaluation
board with the I/O pins
jumpered to
the target system.
Instead,
it’s
necessary
to
develop a complete embedded application.
This entails the
choice of
an
appropriate part
from the
broad range offered
in a typical
microcontroller
family and
learning
to
use
a
fairly
sophisticated
development
environment.
Finally,
a
custom
printed-
circuit
board
for
the
microcontroller
and
peripherals
must
be
designed
and
fabricated.
Microcontroller Selection.
In view of existing local expertise,
the Motorola line
of
microcontrollers
was chosen
for
this project. Still,
this does
not
narrow the choice
down
much. A
fairly disciplined
study of system requirements
is
necessary to specify
which
microcontroller,
out
of
scores
of
variants,
is
required
for
the
job.
This
is
difficult
for
students,
as
they
generally
lack
the
experience
and
intuition
needed
as
well as
the perseverance to wade through
manuf
acturers’ selection
guides.
Part of the problem
is in choosing methods for interfacing the various
peripherals
(e.g.,
what
kind
of
display
driver
should
be
used?).
A
study
of
relevant
Motorola
application notes
[2, 3, 4] proved very helpful in understandingwhat
basic approaches
are available, and
what microcontroller/peripheral combinations
should be considered.
The
MC68HC705B16
was
finally
chosen on the basis of
its availableA/D
inputs
and PWMoutputs as
well as 24 digital I/O lines. In retrospect this
is probably overkill,
as
only
one
A/D
channel,
one
PWM
channel,
and
11
I/O
pins
are
actually
required
(see
Figure
3).
The
decision
was
made
to
err
on
the
safe
side
because
a
complete
development system
specific to the chosen part was necessary, and the
project budget
did
not
permit
a
second
such
system
to
be
purchased
should
the
first
prove
inadequate.
Microcontroller
Application
Development.
Breadboarding
of
the
peripheral
hardware,
development
of
microcontroller
software,
and
final
debugging
and
testing
of a
custom printed-circuit board
for the
microcontroller and peripherals all
require a
development
environment
of
some
kind.
The
choice
of
a
development
environment,
like
that
of
the
microcontroller
itself,
can
be
bewildering
and
requires
some
faculty
expertise.
Motorola
makes
three
grades
of
development
environment
ranging
from
simple evaluation boards (at around
$$100) to full-blown real-time in-circuit emulators
(at
more
like
$$7500).
The
middle
option
was
chosen
for
this
project:
the
MMEVS,
which
consists
of
_
a
platform
board
(which
supports
all
6805-family
parts),
_
an
emulator
module
(specific
to
B-series
parts),
and
_
a
cable
and
target
head
adapter
(package-specific).
Overall,
the
system
costs
about
$$900
and
provides,
with
some
limitations,
in-circuit
emulation
capability.
It
also
comes
with
the
simple
but
sufficient software development
environment RAPID [5].
Students
find
learning to
use this type of system
challenging, but
the
experience
they
gain
in
real-world
microcontroller
application
development
greatly
exceeds
the
typical first-course experience using
simple evaluation boards.
Printed-
Circuit
Board.
The
layout
of
a
simple
(though
definitely
not
trivial)
printed-circuit
board is another practical learning opportunity
presented by this project.
The
final
board
layout,
with
package
outlines,
is
shown
(at
50%
of
actual
size)
in
Figure
8.
The
relative
simplicity
of
the
circuit
makes
manual
placement
and
routing
practical
—
in
fact,
it
likely
gives
better
results
than
automatic
in
an
application
like
this
—
and
the
student
is
therefore
exposed
to
fundamental
issues
of
printed-circuit
layout
and
basic
design
rules.
The
layout
software
used
was
the
very
nice
package
pcb,2
and
the
board
was
fabricated
in-house
with
the
aid
of
our
staff
electronics
technician.
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