IEEE
TRANSACTIONS
ON
INDUSTRIAL
ELECTRONICS,
VOL.
IE-29,
NO.
4,
NOVEMBER
1982
Design
and
Implementation
of
a
Fully
Digital
DC
Servo
System
Based
on
a
Single-Chip
Microcomputer
PEI-CHONG
TANG,
SHUI-SHONG
LU,
AND
YUNG-CHUN
WU
Abstract-A
single-chip
microcomputer
is
used
to
design
a
fully
digital
dc
servo
system
to
replace
the
conventional
analog
circuits.
This
microcomputer
performs
three
main
tasks:
the
firing
control
of
a
three-phase
full-wave
thyristor
dual
converter;
the
compensation
for
the
nonlinear
and
loading
effect
in
the
converter;
and
compensations
of
position
loop
and
rate
loop.
With
no
current
feedback
and
min-
imum
components,
this
dc
servo
system
provides
fast
transient
re-
sponse
and
high
reliability.
I.
INTRODUCTION
M
ICROPROCESSOR
applications
in
the
industrial
control
are
increasing
rapidly
in
recent
years.
For
the
thyristor-
driven
dc
servo
systems,
several
microprocessor-based
designs
have
been
implemented
to
replace
the
conventional
analog
and
discrete
digital
circuits
[1
]
-[3]
.
Ohmae
et
a!.
[1
]
used
a
bit-
slice
microprocessor
system
to
design
a
speed
regulation sys-
tem
by
a
dual-mode
current
loop-control
method.
Chan
etal.
[2]
-[3]
used
an
8-bit
microprocessor
system
to
control
the
speed
and
position
of
a
dc
motor
by
a
different
current
loop-controlled
method.
In
the
case
of
thyristor-driven
dc
servo
systems,
the
critical
problem
is
to
find
an
easy
way
to
compensate
the
nonlinear
and
loading
effect
of
thyristor
con-
verter.
From
this
point
of
view,
the
methods
proposed
by
Ohmae
and
Chan
et
al.
are
unsatisfactory.
Recently,
Tang,
Lu,
and
Wu
[4]
proposed
a
cascade
non-
linear
compensation
scheme
to
compensate
the
nonlinear
and
loading
effect
of
thyristor
converter.
In
this
scheme,
no
current
loops
were
required
and
simple
computing
algorithms
were
used.
Thus,
the
microprocessor-based
controller
design
was
simplified
in
both
hardware
and
software.
In
addition,
it
was
shown
that
this
compensated
thyristor-driven
dc
servo
system
could
be
regarded
as
a
quasi-linear
system.
Thus,
the
control-loop
design
was
simplified.
In
another
paper,
Tang,
Lu,
and
Wu
[5]
introduced
a
microprocessor-based
firing
scheme
to
provide
maximum
firing
range
with
minimum
firing
delay.
This
scheme
used
minimum
components
and
no
ad-
justment
was
required.
In
this
paper,
a
single-chip
microcom-
puter
is
used
to
implement
a
fast-response
thyristor-driven
dc
servo
system
based
on
these
two
schemes.
This
circuit
uses
fully
digital
components
with
minimum
hardware.
II.
HARDWARE
DESCRIPTION
The
schematic
diagram
of
the
dc
servo
system
based
on
a
single-chip
microcomputer
is
shown
in
Fig.
1.
The
three-phase
full-wave
thyristor
dual
converter
is
composed
of
twelve
silicon
Manuscript
received
April
21,
1981;
revised
March
9,
1982.
P.
C.
Tang
is
with
the
Institute
of
Electronics
Engineering,
National
Chiao
Tung
University,
Hsinchu,
Taiwan,
Republic
of
China.
S.
S.
Lu
is
with
the
Department
of
Mechanical
Engienering,
National
Taiwan
University,
Taipei,
Taiwan,
Republic
of
China.
Y.
C.
Wu
is
with
the
Department
of
Control
Engineering,
National
Chiao
Tung
University,
Hsinchu,
Taiwan,
Republic
of
China.
ji
I,
I;
~
>
Load
II
j
otor
L..._W
L___
Thyristor
Converter
ition
Feedback
Signal
Fig.
1.
Block
diagram
of
the
position
servo
system.
controlled
rectifiers
(SCR's).
This
converter
is
used
to
drive
a
separately
excited
dc
motor
in
a
four-quadrant
operation.
The
motor
field
is
maintained
constant.
One
incremental
photoencoder
is
coupled
to
the
motor
shaft
as
the
only
feed-
back
transducer.
Two-phase
outputs
of
the
photoencoder
are
used
to
generate
the
position
and
the
derived-speed
feedback.
Upon
receiving
the
pulse-train-like
command,
this
control
cir-
cuit
will
control
both
speed
and
position
of
the
thyristor-
driven
dc
motor.
The
block
diagram
of
this
fully
digital
circuit
is
shown
in
Fig.
2
and
described
below.
A.
Input
Buffer
Input
buffer
functions
as
the
interface
among
command,
photoencoder,
and
the
single-chip
microcomputer.
For
the
purpose
of
tracking-system
applications,
command
inputs
are
two
pulse-train
signals.
One
is
the
clockwise
command
and
the
other
is
the
counterclockwise
command.
By
using
three
divide-
by-sixteen
up-down
counters,
the
position
command
is
obtained
by
integrating
these
pulse-train
signals.
Outputs
of
the
incremental
photoencoder
are
two-phase
pulse-train
signals.
The
phase
difference
between
them
is
about
900.
After
the
multiply-by-four
decoder,
these
two-
phase
signals
are
decoded
to
clockwise
and
counterclockwise
signals.
Then,
the
position
feedback
is
obtained
by
using
the
12-bit
up-down
counter
similar
to
the
one
used
for
position
command.
A
microcomputer-controlled
12-bit
two-to-one
multiplexer
is
used
to
read
in
the
digitized
position
command
and
feedback.
This
multiplexer
is
desired
in
order
to
decrease
the
number
of
microcomputer
I/O
required.
B.
Microcomputer
Controller
The
microcomputer
used
is
Zilog
Z8
single-chip
micro-
computer.
The
Z8
microcomputer
introduces
a
new
level
of
0278-0046/82/1
100-0295$00.75
©
1982
IEEE
295
IEEE
TRANSACTIONS
ON
INDUSTRIAL
ELECTRONICS,
VOL.
IE-29,
NO.
4,
NOVEMBER
1982
Fig.
2.
Block
diagram
of
the
control
circuit.
Position
and
Rate
Loop
c
ascade
Nonlinear
IThyristor
Firing
Compwetor
I
Compensator
I
Controller
Fig.
3.
Block
diagram
of
the
microcomputer
controller.
sophistication
of
single-chip
architecture
with
2K
bytes
of
internal
ROM,
124
bytes
of
on-chip
RAM,
two
programmable
8-bit
timers,
and
32
bits
of
programmable
I/O.
The
block
diagram
of
the
microcomputer
controller
is
illustrated
in
Fig.
3.
The
functions
performed
by
the
micro-
computer
will
be
detailed
in
the
next
section.
Because
most
complex
works
are
executed
by
microcomputer
software,
the
hardwares
are
just
buffers
and
drivers.
C
Output
Driver
The
output
driver
functions
as
the
interface
between
the
thyristor
dual
converter
and
the
single-chip
microcom-
puter.
The
design
of
the
output
driver
is
based
on
the
work
by
Tang,
Lu,
and
Wu
[5].
The
three-phase
ac
source
signals,
digitized
by
using
the
power
signal
operator,
are
sent
to
the
microcomputer
for
firing
control.
In
each
600,
of
ac
source,
the
base
interrupt
signal
is
generated
at
each
zero
crossing
of
the
three-phase
ac
source.
This
signal
is
used
to
synchronize
the
microcomputer
and
ac
source.
The
current
direction
detector
is
used
to
detect
the
bi-
direction
current
ON-OFF
signals.
These
ON-OFF
signals
are
sent
to
the
microcomputer
for
crossover
damage
protection
of
thyristor
dual
converter.
The
demultiplexer
and
the
gate
driver
are
used
to
distribute
the
SCR
firing
signals
to
each
gate
of
chosen
SCR's.
These
SCR
firing
signals
are
generated
by
the
microcomputer
under
the
control
of
a
firing
control
software.
III.
SOFTWARE
DESCRIPTION
Most
functions
of
the
servo
controller
are
performed
by
the
microcomputer
software.
As
shown
in
Fig.
3,
there
are
three
main
tasks
in
the
microccomputer
software
which
in-
cludes
position
and
rate-loop
compensation,
cascade
nonlinear
compensation,
and
thyristor
firing
control.
A.
Position
and
Rate-Loop
Compensation
The
position
and
rate-loop
compensator
compares
the
position
command
and
feedback,
then
generates
a
voltage
command
VC
as
its
output.
The
first
important
job
is
to
obtain
the
correct
position,
command,
and
feedback.
Be-
cause
the
length
of
hardware
counter
is
only
12
bits,
the
over-
flow
of
counter
happens
frequently.
To
correct
the
overflow
error
of
the
counter,
the
foLlowing
code
correction
technique
is
used.
Due
to
the
frequency
response
of
the
photoencoder,
it
is
assumed
that
the
input
signal
frequency
is
less
than
700
kHz.
In
one
period
of
360-Hz
sampling
time,
the
maximum
number
integrated
by
the
counter
is
less
than
2047
pulses,
which
can
be
handled
by
an
11
-bit
counter.
Thus,
the
remaining
bit
in
the
12-bit
counter
is
used
to
check
the
overflow
of
11
-bit
data,
and
the
software
counter
can
then
be
constructed
by
inte-
grating
these
overflows.
In
this
way,
the
16-bit
position
com-
mand
and
feedback
are
constructed
by
combining
the
4-bit
software
counter
and
the
12-bit
hardware
counter.
By
differentiating
the
position
feedback,
the
speed
feed-
back
is
obtained
for
rate-loop
compensation.
Due
to
the
linear
relationship
between
the
motor
speed
and
the
induced
EMF,
the
speed
signal
can
be
used
as
the
voltage
measurement
by
multiplying
a
gain
constant
AK3.
The
other
two
param-
eters,
AK1
and
AK2,
are
used
to
construct
a
proportional
controller
with
inner
rate
loop.
B.
Cascade
Nonlinear
Compensation
The
cascade
nonlinear
compensation
is
based
on
the
design
by
Tang,
Lu,
and
Wu
[4].
Using
the
difference
between
the
voltage
command
and
voltage
measurement
as
a
new
param-
eter,
the
firing-angle
command
is
obtained
by
searching
two
tables.
One
is
used
to
get
the
firing
angle
under
the
continuous
current
mode
and
the
other
is
used
to
modify
this
firing
angle
296
TANG
et
al.:
DESIGN
AND
IMPLEMENTATION
OF
SERVO-SYSTEM
BASED
ON
A
MICROCOMPUTER
External
Torque
Cowmrand
Input
Fig.
5.
Linear
model
of
the
position
servo.
Fig.
4.
Flow
chart
of
the
microcomputer
controller.
under
the
discontinuous
current
mode.
In
addition
to
the
com-
pensation
of
nonlinear
and
loading
effect
in
the
thyristor
dual
converter,
the
difference
between
the
voltage
command
and
voltage
measurement
can
be
limited
to
a
safe
region
so
that
it
functions
as
a
current
limiter.
The
sign
of
this
voltage
dif-
ference
is
also
used
as
the
current
direction
command
for
the
thyristor
dual
converter.
Thus,
the
cascade
nonliner
compen-
sation
software
computes
the
thyristor
firing
angle
and
direc-
tion
commands
used
for
the
thyristor
firing
control.
C
Thyristor
Firing
Control
This
controller
is
based
on
the
design
by
Tang,
Lu,
and
Wu
[5].
The
flowchart
is
shown
in
Fig.
4.
The
control
cycle
has
a
period
of
about
2.7
ms
and
is
synchronized
with
the
ac
source.
At
the
start
of
each
control
cycle,
the
firing-angle
command
calculated
in
last
control
cycle
is
used
to
set
the
delay
time
of
the
timer,
then
the
microcomputer
proceeds
to
calculate
the
next
firing
angle
command.
When
the
timer
counts
down
to
zero,
a
signal
is
generated
to
interrupt
the
main
process,
and
the
SCR
firing
signals
are
sent
to
fire
the
chosen
SCR's.
IV.
SERVO
SYSTEM
DESIGN
The
overall
system
is
a
nonlinear,
sampled-data
control
system.
The
sampling
rate
is
360
Hz
and
the
maximum
de-
lay
time
including
computing
time
and
firing
delay
is
two
sampling
periods,
about
5.5
ms.
The
exact
analysis
of
the
overall
system
is
difficult,
but
the
approximate
solution
is
simple
[4].
Because
the
system
time
constant
is
much
greater
than
the
sampling
period
and
due
to
the
linearization
effect
of
the
cascade
nonlinear
compensation,
the
overall
system
can
be
approximated
to
a
linear
and
continuous
control
system.
Thus,
most
of
the
design
methods
based
on
the
linear
and
continuous
control
theory
can
be
applied
to
this
micro-
computer-based
dc
servo
system.
One
of
the
simplest
methods
used
is
a
proportional
control
with
inner
rate
loop,
as
shown
in
Fig.
5.
The
step-by-step
design
from
inner
loop
to
outer
loop
is
chosen
to
solve
this
problem.
In
the
experiment,
the
motor
used
is
a
0.5-kW
permanent
magnet
dc
motor
(90
V,
15
A,
3000
rpm).
The
only
feedback
transducer
is
a
photoencoder
with
2000
ppr.
With
a
200-V
peak-to-peak
ac
source
and
the
parameters
AKl,
AK2,
and
AK3
set
at
0.039,
5.45,
and
0.98,
respectively,
the
open-loop
gain
is
43.3/s.
V.
EXPERIMENTAL
RESULTS
AND
DISCUSSIONS
The
open-loop
step
response
of
the
uncompensated
thyristor
dual
converter
is
shown
in
Fig.
6(a).
Because
of
the
semi-
conductor
characteristics
of
SCR's,
no
decelerating
torque
is
applied
to
the
motor
during
the
transcience
from
high
speed
to
low
speed.
The
settling
times
in
each
transcience
of
ac-
celeration
and
deceleration
are
approximately
220
ms
and
660
ms,
respectively.
With
the
use
of
the
cascade
nonlinear
com-
pensator,
the
transient
response
is
improved
in
both
tran-
sience
of
acceleration
and
deceleration.
The
settling
time
of
compensated
thyristor
dual
converter
is
about
160
ms
for
both
acceleration
and
deceleration,
as
shown
in
Fig.
6(b).
In
the
experiments,
the
current
limiter
in
the
cascade
nonlinear
compensator
is
set
at
about
5
A
to
show
its
effect.
As
is
evident
from
Fig.
6(b),
both
the
acceleration
and
deceleration
are
limited
by
this
current
limiter.
If
the
open-loop
command
change
is
so
small
that
the
current
limit
does
not
occur,
as
in
the
case
of
Fig.
6(c),
the
transient
response
are
very
fast
and
the
same
for
both
acceleration
and
deceleration.
The
settling
time
in
this
case
is
about
40
ms.
As
can
be
seen
from
Fig.
6,
the
benefit
of
the
cascade
non-
linear
compensation
is
obvious.
With
the
use
of
the
cascade
nonlinear
compensation,
the
step
responses
of
the
rate
loop
and
the
position
loop
are
shown
in
Figs.
7
and
8,
respectively.
The
settling
time
of
the
rate-loop
response
and
the
position-
loop
response
are
about
80
ms
under
the
condition
of
current
limiting.
The
transient
responses
are
the
same
in
both
direc-
tions.
Because
only
a
simple
proportional
controller
is
used
for
the
rate
loop,
there
is
some
steady-state
error
in
the
rate-
loop
response,
as
shown
in
Fig.
7.
For
a
type-1
control
system,
no
steady-state
error
exists
in
the
response
of
position
loop
if
there
is
no
external
torque.
The
memory
used
in
this
system
is
about
1.4K
bytes
and
the
maximum
computing
time
in
one
control
cycle
is
about
1.5
ms,
as
shown
in
Table
I.
In
most
servo
control
systems,
some
complex
controllers
are
usually
used
to
achieve
the
higher
performance,
such
as
a
lead-lag
compensator,
a
PID
controller,
or
a
second-order
compensator.
The
on-line
cap-
ability
in
the
microcomputer
used
in
this
system
is
so
small
that
a
more
complex
compensator
can
also
be
implemented.
In
addition,
the
fully
digital
organization
and
microcomputer
software
control
make
it
easy
to
link
this
system
with
other
microcomputer
systems.
Thus,
this
system
can
be
part
of
a
computer
network
for
adaptive
control,
automatic
testing,
automatic
design,
or
other
processes
for
industrial
application
needs.
297
IEEE
TRANSACTIONS
ON
INDUSTRIAL
ELECTRONICS,
VOL.
IE-29,
NO.
4,
NOVEMBER
1982
ANGLE
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Step
response
of
position
loop;
position
command
from
0
rad
to
1.57
rad.
TABLE
I
LIST
OF
MEMORY
AND
TIME
USED
(b)
1
200
800
400
0k
I.
.{t'
'It,
Ut!
["t
K
r
lfl
1t-jt'!
:
(c)
Fig.
6.
Step
response
of
the
compensated
thyristor
dual
converter
with
dc
motor.
(a)
Without
cascade
nonlinear
compensator;
firing-angle
command
from
1000
to
60°.
(b)
With
cascade
nonlinear
compen-
sator;
voltage
command
from
6
V
to
70
V.
(c)
With
cascade
non-
linear
compensator;
voltage
command
from
12
V
to
24
V.
Fig.
7.
Step
response
of
rate
loop;
rate
command
from
180
rpm
to
1400
rpm.
memory
used(kbytes)
time
used(ms)
position
and
rate
compensation
0.41
1.05
cascode
nonlinear
compensot
ion
0.52
0.1
(including
tables)
thyristor
firing
control
0.2
0.4
(including
tables)
initialization
program
0
27
total
1.4
1.55
z
8
provided
2.0
sampling
period
2.77
reserved
for
future
use
0.6
1.22
VI.
CONCLUSION
The
quasi-linearization
effect
of
a
compensated
thyristor
dual
converter
based
on
the
principle
of
cascade
nonlinear
compensation
proposed
by
Tang,
Lu,
and
Wu
is
confirmed
by
the
experiment.
Also,
the
equivalent
effect
of
current
limiting
is
possible
through
software
control
without
using
any
current
feedback.
With
minimum
hardware
and
good
performance,
this
single-chip
microcomputer
design
is
suitable
for
a
fast-response
high-power
dc
servo
system
for
industrial
control.
REFERENCES
[
]
T.
Ohmae
et
al.,
"A
microprocessor-controlled
fast-response
speed
regulator
with
dual-mode
current
loop
for
DCM
drives,"
IEEE
Trans.
Ind.
Appi.,
vol.
IA-16,
pp.
388-394,
May
1980.
[2]
Y.
T.
Chan
et
al.,
"A
Microprocessor-based
current
controller
for
SCR-DC
motor
drivers,"
IEEE
Trans.
Ind.
Electron.
Contr.
In-
strum.,
vol.
IECI-27,
pp.
169-176,
Aug.
1980.
[3]
J.
B.
Plant
et
al.,
"Microprocessor
control
of
position
or
speed
of
an
SCR-DC
motor
drive:'
IEEE
Trans.
Ind.
Electron.
Contr.
Instrum.,
vol.
IECI-27,
pp.
228-234,
Aug.
1980.
[4]
P.
C.
Tang
et
al.,
"A
cascade
nonlinear
compensation
of
the
thyristor
dual
converter
for
DC
motor
drives,"
to
be
published.
[5]
P.
C.
Tang
et
al.,
"Microprocessor-based
design
of
a
firing
circuit
for
a
three-phase
full-wave
thyristor
dual
converter,"
IEEE
Trans.
Ind.
Electron.,
vol.
IE-29,
pp.
67-73,
Feb.
1982.
SPEED(RPM)
SPEED
(a)
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n
SPEED(RPM)
CURRENT
(AMP)I5
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itat:
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