Theatre Design & Technology - Summer 1998 - 67

or fliction Iliulin Ule mechmliG~ components of
the s~"Stem. In this case the controller prO\ides :Ul
integration function Irhich Irill boost the drire's
OUlpUl until the load mores to the specified position. Finallr. if ~le position error is large mld the
controller responds proportion:~jr. it mar more
toO qUicklr mld orershoOl ilS intended mark
Therefore. it is necess:u~' for the controller to
knoll' not onlr the size of the error. but ruso hO\r
quicklr the error is chmlging. BI compaIing the
torque required to accelerate the load. ilS speed
:md pOSition. the controller cml deIire the proper
response mld adjust its output on the Or to hit the
intended mm'k exac~l: These functions. Proportiol/a!. II/tegral and Derimtire f0I111 the ChlSSic
PID filter. Irllich makes exact position control
possible. In engineeling terms. the propOl1ion:~
component affeclS 51'stem response. the integration component adjusts the deadb:md mld the deriratire component m:tintains S\"Stem tability.
Each of these functions is adjustable either
through h:u'd-Ilired controls or softlrare.
OUlput ~'om the controller is l\Vicruh' :m
mlruog mltage between + 10 roilS and -10 mlts
DC. The dlire. if a four quadrant model. Ilill intet'
pret all positire mltages as a forw:u'd commaJld
:mdnegatire roltages as a rererse COllllllaJld. Because the control sign:~ is relatirelr small com-

pared to the OUlpUl of the drire. chires that accept
e:i:ternal control signals are oflen called amplifiers. The drire is usu:~h' designed to prO\ide the
motor Irith ma.ximumtorque or nuximum I'elocity. depending on the dlire. at plus or minus 10
lolts. :\t zero milS. ~le motor is still. .\Iost modem
dIires. Irhether four quadrant DC conreners.
four quadrant AC inreners or serI'o amplifiers accept this industri:~ sign:~ standard. Hence it is
possible to mi\ br:mds of controllers. motors :md
dlires when assembling a motion control 51"Stem.
~hmy manufacnlrel"S prO\ide complete motion
control packages. inducting ~le motor. controller. dlire. cables :md soli\\:u·e.
The one element missing [rom gener:~-pur-
pose motion control packages is the end-user intelface. The controller mal' onlr be prorided Irith
a terminru progrmll that rulO\rs a host computer
to issue instl1lctions or to dO\rnload progrmllS to
the controller. The consumer is e:i:pected to derelop the user inteIface for his specific application by progrmnming in one of sererru standard
I:mguages like C+ +. \"isu:~ Basic. or, perhaps. a
controllel'specific propIietary hmguage. \"endors
olien prO\ide a set of sofnrm'e tools mld libr:uies
to simplify the task of programming. but ~le e m'e
geared towm'd generru industry and not theatre.
The resuh is that. Irhile it is a fairly simple maner

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to create a specific motion profile in modem
controllers. linking seren~ motion profiles together in something resembling theatriG~ cues is
more ctifficult.
Recognjzing the potentiru of this rapicllr
erohing technology. numerous theatre ligging
comp:uues haYe begun dereloping I1lmkey motion control SI"Stems that address the unique
needs of theatre. Each rear the list of motion control rendors at the lSITI' Stage E\position groll's
longer. lnlike lighting consoles. hO\rerer. ulere
are fell' simil:uities :uld linle consensus mnong
the u er interfaces :md control p:Ulels offered by
competing rendOl"S. II is still a Oedgling business
mld the issues of motion control for the stage are
comple:i:. Simplr restoring to the top of a cue is
far more difficult :md dangerous Irith machinery
than it is for lighting or sound. especiallr if the
cue was interrupted. The position of units on the
stage or the tinung of sequences mar nOl pernut
simply rerersing the cue. Vendors employ cliffel'
ent strategies to meet the complex demmlds of
theatriG~ motion control and the result is a
hodgepodge of proprietal~' user intelfaces. This
Ililllikely change. hO\rerer. as rendors adopt the
bener ideas of ~leir competitors-something
that is getting easier to do as control S\ tems
more to the Pc.

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