Theatre Design & Technology - Spring 1996 - 21

Scenic Technologies and Show Tech depend on PLCs to
control the specialized machines in shows like Dam//
Yankees, Sunset Bou/evard and HolV To Succeed In
Business. The increasingly high-tech spectacle of Las Vegas shows like Buccaneer Bay and EFX would be impossible without these devices.
PLCs come in a \\~de vaIiety of brands and models.
On the high-end are modulaI' systems featuring a central
processing core with an open back-plane onto which
specialized control or sensor modules are mounted. Depending on the modules selected, these systems may cost
several thousand dollars. On the other end of the scale is
the "micro" PLC. Several manufacturers, GEIFanuc,
Allen-Bradley and Aromat among otilers, offer a micro
PLC which lacks tile modular design of its more-expensive cousins, but still provides the solid-state equivalent
of more than a thousand relays, and hundreds of
counters and timers. These units run about $350.00 for a
"starter kit" which usually includes the PLC, programming software and a serial cable for downloading programs from a personal computer.
Physical differences among brands are minor. In
general, tile micro PLC consists of a plastic housing that
is a bit smaller tilan a brick or a paperback copy of At/as
Shrugged and considerably lighter than eitiler one. Screw
terminals are provided for all inputs, outputs and power
connections. Generally, the input terminals are arrayed
along one side of tile unit and tile output terminals along
tile opposite side. These terminals are called VO (input!
output) points and tile size of the unit is specified by ilie
combined number of points. The smallest of tile micro
PLCs are commonly 14 point units with 8 inputs and 6
outputs, but they are often provided witil a port to add an
expander unit which usually doubles tile available VO.
Micro PLCs are furtiler specified by tile type of inputs available: digital or analog. Analog PLCs are used to
control de\~ces tilat supply or require continuously variable data such as temperature or pressure and are not as
useful for controlling stage machinery. Digital units, on
tile otiler hand, use simple ON/OFF devices like pushbuttons or toggle switches for input. Connecting an input
device to a digital micro PLC is simply a matter of connecting one wire from a switch to an input terminal and
the otiler wire to tile common temlinal. For tiuee-\vire,
solid state sensors like proxinlity or photo-electric
switches, most micro PLCs prO\~de a 24 VDC ternlinal adjacent to the input ternlinals (VPP in Figure I) to provide
power for the sensor. In tllis case, one wire connects to
the common, one to tile power supply and the load \\~re
to one of the inputs. In either case, closing tile switch
turns the input ON. Most micro PLCs are capable of supplying 75 to 100 ma for external sensors and the individual inputs typically draw less than 10 mao
Outputs are either single-pole, single-throw relays
(simple 0 /OFF switches) or the transistorized equivalent. Of the two, the relay output is the easiest to implement and requires little or no additional circuitry; one
merely hooks a source of power to one side (terminal)

of the output relay and the load to the orller and that is it.
Load limitations vary among brands, but output relays
aI'e typically capable of 2 to 5 amperes at 250VAC or
30VDC for resistive loads.
If the input devices are simple 0 /OFF switches and
the outputs are simple ON/OFF s\\~tches, what good is a
PLC? Why bother witil the middleman? The answer, of
course, is progranlmability. Using only tilese simple 01 /
OFF s\vitches, PLCs can perform quite complex control
sequences based on an internally stored progranl.
For most PLCs, full-size or nlicro, the programming
language is called Relay Ladder Logic, which is a reflection of the PLCs origins. Working with Relay Ladder Logic
(RLL) differs from traditional computer programming.
PLC progranls are not written, rather, tiley are drawn and
tile resulting diagranl resembles a ladder, hence the
nan1e. Software bundled \\~th the starter kits allows the
programmer to build the diagrams on a personal computer by selecting from a menu of elements. Once the
ladder is complete, tile progranl is up-loaded to tile PLC
via a serial cable.
It may be daunting to consider learning yet another
progranlIlling language, but RLL is really quite simple,
consisting of only two primary commands: ON and OFF.
To demonstrate how simple RLL really is, we \\ill build a
progranl to prevent the Forest of Arden (rom wandering
off in the wrong direction using only ON/OFF switches
and ON/OFF commands.
For ilie purposes of tilis demonstration let's assume
tilat ilie Forest of Arden is moved by a standard stage
cable \\~nch powered by a permanent magnet DC motor,
which, in turn, is driven by a regenerative motor drive.
Selecting a direction for the regenerative drive requires
only a light duty, single-pole/double-ilirow toggle switch,
or, for our purposes, a PLC. To complete our system, we
\\ill place a limit s\\~tch at each end o( ilie forest's track
and give the operator a single, large, green push-button
to put tile forest in motion. The PLC will decide wllich direction is appropriate and when to stop.
Figure I shows how each of ilie components are
connected to a typical PLC. One wire from the operator's
push-button is connected to Input I and the oilier wire to
the PLe's common. Likewise, each Unlit switch is connected to an input and to rlle common. Activating a
S\vitch completes ilie circuit and turns its corresponding
input 01 . On rlle output side, the fonvard and reverse
wires from the regenerative drive are connected to Outputs I and 2 respectively. The drive's common \\~re is
connected to ilie common terminal shared by ilie two
outputs. In tllis case, activating an output will connect either the forward or reverse wire to the drive's common
and tile motor will start. Each input and output has a corresponding indicator light on the face of the PLC that
lights when tile input or output is ON.
Figure 2 shows a simple RLL progranl. On tile left
and right are two vertical lines called rails, which are
schematic representations of the actual power rails, were
these electro-mechanical relays. "Power" flows down ilie
TD&T

S P R I. G

Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 1996