Theatre Design & Technology - Summer 1998 - 65

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Pulse-Width
Modulation
(fig, 2) When a
single voltage is
rapidly switched
ON and OFF, the
wider the ON
pulses the
greater the
average output
voltage.

merell' replaced Ule muscle but not ule brain of
ule 51 [em.
The ne"1 step tOll'arcis tme automation emplo~ limit S\litches to stop a urut on its m;H'k. In
ulis ~ tem the operator presses a GO bunon
\\hich latches the motor on ulrough a relal'. lpon
reaching the limit S\litch. Ule relay circuit is
opened ;HId Ule motor comes to a stop. ~luJtiple
positions require multiple Iinlit S\litche ;HId
ome me;uls for the operator to select \\'hich
S\litch is actil'e. Linlit S\litches mal' be placed in
the tracks for \\'agons or along ule lim of tUllJtables, but this method makes adjusting ule t;H'get
positions more difficult. Directors do not look
kincllv on interrupting rehearsals willie a caqJentel' lips up the deck to adjust ule position of a
limit S\litch. A bener meulOd is to use a rot;Hy
Unlit s\\itch chi\'en by a small chain or cogged
belt off ule winch itself.
TIl'o or more S\litches ;H'e usually contained
WiU1.i.t1 a single rot;H'y linlit enclosure. Four
S\l'itches is the nlirumum reconUJ1endednumber
for uleallicaluse. This allOlI'S for an on-stage and
;HI off-stage position and 111'0 ol'er-trmellinlits for
each end of the track. (IIard-\\ired ol'er-tral'el
Iinlits ;H'e recommended for ;HII' mech;ulized
urut, reg;H'dless of ule control ~'Stem. TIley
should be designed to shut ule ~'Stem dOl\l1 com-

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called regenerative braking. Therefore. DC motors mal' function in one of four distinct modes:
as a motor driling fomard; as a generator producing torque against ule motor's fonmrd rotation (braking): as a motor dIiling in reverse: as a
generator producing torque against Ule motor's
rel'erse rotation (braking). Each of these modes
is called a quadrant and dtil'es are mailable Ulat
operate in one. 111'0 or four quadrants. The four
quadram regeneratil'e dtil'e is Ule most useful for
Uleauical applications because speed comrol, acceleration. deceleration and rel'ersal are handled
bl' electroruc means \\iUlOut ule need for mechanical comactors. Tllis in nIl11 makes ule de-

sign ;U1d intetfacing of control systems much simpler.
Once the t~ pe of motor and dri\'e has been
deternlined, ule next question is how to control it.
The easiest Ira\' to control a mototized system is
to hand ule control box to a human operator. The
operator selects speed and direction and ulen
Il'atches for spike marks on the \\inch cable or
Ule 0001' to stop a lIIlit on its mark. Tllis ~'Stem is
Ule most acL1ptable, easiest to progranl but often
the most unreliable and, for long production
nll1S, ule most expensil'e. Tllis ~ tem is not tndy
automated. since starting and stopping still requires human interl'ention. The maclline has

Canadian Institute for Theatre Technology
in cooperation with the

University of Alberta Department of Drama
presents the

1998 CITT Annual Conference
"Directions
II

August 20-23, 1998
Timms Centre for the Arts
Edmonton, Alberta
Toll Free Information: 1-888-271-3383
1D&1

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Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1998