Theatre Design & Technology - Summer 1998 - 66
Typical ClosedLoop System
(fig. 3) In a
USO'
Ir:er'e::€
closed-loop or
servo system, the
controller always
knows the
location of the
load.
!J;S!
PCS::-l1u:"""""arJ
COlT;J~€'
i
The disk used in an optical (photo-electric) encoder is usually made of glass with opaque lines
etched into the surface. The etched di!?k provides
high resolutions in a comp.act package but makes
an optical encoder fragile and susceptible to breakage in harsh environments.
A resolver is a rugged electromagnetic device
that acts like a transformer with a single, rotating
primary winding and, two statiohary secondary
windings. When the primary winding is energized
with a sinusoidal signal, a signal of the same frequency is produced in the secondary windings, but
.. at an amplitude dependent on the angular position
of the resolver shaft. By comparing the ampmude
of the signal in each secondary winding, the position of the resolver shaft can be determined. Since
the angle of the shaft is always known, resolvers
are absolute position devices. Unlike incremental
devices, resolvers require no initialization sequences to determine location at start up or after a
power failure. The electronics package is more
complex (expensive) for a resolver than that of an
encoder, but it may also provide agreater variety of
output signals, including BCD (binary-coded deci-,
mal), Gray code, incremental and analog.
."
..
....
-
-_ _;
pletell' before a runall'al' unit GlIl do mn' d,Ullage.)
Rotarl' limits with more th,lIl four sll'itches
are quite e.\pensil'e so electronic counters mal' be
more cost effectil'e for units requiring multiple
on-stage positions. [n a counter sl'stem. the rotation of the Il'inch drum is cOl1lerted to a series of
discrete pulses. Il'hich the counter accumulates.
Lpon reaching a preset number. the countcr
opens a sll'itch ,lIld stops thc motor. Electronic
counters are al'ailable Ilith ,L'i m'Ull' ,L'i si.\tecn
prescts. There are sel'eral Il'al'S to conl'ert thc rotation of the lI'inch drum into thc pulscs rcquircd
bl' the countcr. :\n inc.\pcnsil'c mcthod. suitable
for manl' applications. is to mount a solid-state
pro.\imitl' switch ncar thc drum sprocket that
scnses thc p,L'iSagc of (hc sprocket's lceth. Each
timc a tooth p,L'iSCS bl~ the sllitch turns on. Of
coursc this mcthod limits thc nJ;Lximum position
...
t
\1
\\
I
H
I
')
')
H
Vo:a::€
0,.,
;:r~
Encoders Versus Resolvers
r;
I/;::n
T D& T
'€'
resolution to the pilCh (distmlCe between the
teeth) of the Il'inch sprocket. or between 5/8,lIld
I inch for t\pical stage lI·inches.
Another method is to use ,lIl incremental
encoder dril'en directll' [rom the Il'inch shaft. An
incremental encoder consists of a photoelectric
sensor that reads the light passing through el'enll'
spaced slots cut into a disk rotating on the encoder shaft. The encoder outputs a square Il'ave
pulse corresponding to each slot passing in front
of the photoelectlic sensor. Incremental encoders often use 1\1'0 photo sensors to output 1\1'0
square Ilaves (A channel ,lIld Bchml11el) 90 degrees out of phase. BI' noting which sensor pulses
first, it is possible to determine the direction of
rotation as Il'ell. Electronic counters are designed
to count up if one chml11elleads ,Old dOli'll if the
other leads. FUl1hermore. bl' counting both the
rising ,lIld falling edges of each square lral'e pulsc
on both c1ml11els. the resolution of a t\pical incremental encoder GlIl be increa,ed bl' a factor
of four. This process is called quadrature ,lIld
most electronic counters prOlide quadrature inputs. Encoders are al',tilable Il'ith more than 2500
pulses per rel'Olution. or ol'er j 0.000 pulses per
rel'Olution using quadrat1ll'c. In theor\', ,lIl encoder can prOlide position resolutions of a fell'
thousandths of ml inch. [n actual practice. hOlI'cl'er. counter-based sl'stems seldom achiel'e their
theoretical resolution-there is usualll' too much
piaI' ,ullong the m'lIll' meCh,OliG~ pal1s in a I\piG~ stage Il'inch sl'stem-but thel' are good to
within a fCII' fractions of an inch.
Similar delices emplo~ing rcsolvers (see
sidebar) instead of cncodcrs are al'ailable ,lIld
mal' bc sold ,L'i progr,ullmable limit sll'itchcs.
Widclr used on assembll' lines. COI1l'Cl'ors .lIld
packaging machines. thel' arc not rcally dcsigncd
to position a load. but to initiatc othcr machine
processes at cCl1ain points in thc load's tral'e!.
For theatre. thcir application dilTers little from
counters. Oftcn prOlided II'ith onboard microproccssors to control comple.\ sequencing. progr,ullmablc limit SII'ilChes arc a bridgc bctll'ccn
simple counters ,lIld hill-bloll'll progrmllmable
logic controllers both in price .lIld capability.
Progrmlllllable logic controllers (PLCs) are
computer-like del'ices designed specifiG~h for
the control of macltinerl' or processes. Thel' include multiple intern,~ counters Il'ltich mal' be
uscd in a m
Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1998
Contents
Theatre Design & Technology - Summer 1998 - 1
Theatre Design & Technology - Summer 1998 - 2
Theatre Design & Technology - Summer 1998 - 3
Theatre Design & Technology - Summer 1998 - Contents
Theatre Design & Technology - Summer 1998 - 5
Theatre Design & Technology - Summer 1998 - 6
Theatre Design & Technology - Summer 1998 - 7
Theatre Design & Technology - Summer 1998 - 8
Theatre Design & Technology - Summer 1998 - 9
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Theatre Design & Technology - Summer 1998 - 18
Theatre Design & Technology - Summer 1998 - 19
Theatre Design & Technology - Summer 1998 - 20
Theatre Design & Technology - Summer 1998 - 21
Theatre Design & Technology - Summer 1998 - 22
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Theatre Design & Technology - Summer 1998 - 24
Theatre Design & Technology - Summer 1998 - 25
Theatre Design & Technology - Summer 1998 - 26
Theatre Design & Technology - Summer 1998 - 27
Theatre Design & Technology - Summer 1998 - 28
Theatre Design & Technology - Summer 1998 - 29
Theatre Design & Technology - Summer 1998 - 30
Theatre Design & Technology - Summer 1998 - 31
Theatre Design & Technology - Summer 1998 - 32
Theatre Design & Technology - Summer 1998 - 33
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Theatre Design & Technology - Summer 1998 - 35
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Theatre Design & Technology - Summer 1998 - 37
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https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1978fall
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1978summer
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1978spring
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1977winter
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1977fall
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1977summer
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1977spring
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1976winter
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1976fall
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1976summer
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1976spring
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1975winter
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1975fall
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1975summer
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1975spring
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1974Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1974Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1974May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1974Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1973Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1973Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1973May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1973Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1972Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1972Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1972May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1972Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1971Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1971Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1971May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1971Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1970Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1970Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1970May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1970Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965May
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