Theatre Design & Technology - Summer 2003 - 50

123456789012345678901234567890121234567890123456789012345678901212345
123456789012345678901234567890121234567890123456789012345678901212345
123456789012345678901234567890121234567890123456789012345678901212345
123456789012345678901234567890121234567890123456789012345678901212345
123456789012345678901234567890121234567890123456789012345678901212345
123456789012345678901234567890121234567890123456789012345678901212345

inattentive actor, it seems rather a long time. Eventually, most of this delay was eliminated through more efficient programming, and appropriately wired Emergency Stop systems. [Note: E-Stops should never run
through the network, or a PLC for that matter. E-Stops should be
hardwired to the motor controller and brake circuit.] Taking advantage
of a distributed system allows the design to keep control of an individual
effect or operation local to that operation. While a "go" signal may be
sent over the network to initiate an operation, all input sensors for that
effect are wired and controlled by the local PLC.
There were also some issues with the 3 phase power lines going to the
motors inducing noise into the network lines. I quickly learned to keep network lines crossing power lines at right angles, minimizing interference.
One advantage to using multiple smaller PLCs in lieu of one large
system was that each processor controlled only one or two effects. This made
the programming of individual effects simple and allowed us to program
them in such a way that safety and position inputs were processed locally-
making the local effects relatively immune to a network failure. Each
modular controller was supplied with a box wired with switches that
could be used for local control of each effect if necessary. It also became
possible for the PLCs to duplicate some functions creating redundant
control in the event of a PLC, network or control interface failure.
Another significant advantage to implementing a network is that
smaller, cheaper PLCs can be purchased singly over time, adding network adapters as necessary to slowly build an inventory. As a matter of
fact, while I was at Illinois, the larger PLC system was rarely used directly
wired to an effect, with one exception: the use of analog (0 to 10 VDC)
outputs to interface with the 3 phase inverters to control motor speed.
Analog outputs are now available on smaller modular PLCs from AllenBradley and other manufacturers.
Networking PLCs also expands a programmer's ability to program
and observe or intervene in the control system in a live situation via a
personal computer. I use Rockwell Automation's RSLOGIX 500 software
to program our Allen-Bradley PLCs. Using the RSLINX software allows the
RSLOGIX application to "go online" and observe the PLC program
graphically during operation. In networked systems, I can go online to any
active PLC connected to the system, observing the operations of the PLC, updating data or troubleshooting applications. This ability to remotely
modify PLC applications avoids having to crawl under platforms to attach
programming cables to a PLC installed in an effect or moving unit.
There are many protocols and wiring topologies used to network
PLCs, but all function in the same basic way. PLC programs utilize data
files that reflect the state of inputs, outputs, timers, counters and other
functions. PLC communication instructions allow PLCs and other devices
(such as personal computers or Human-Machine Interfaces) to modify
the data in specific files on remote PLCs. An example of this might be a line of
code that causes an indicator lamp to flash on the local PLC when a unit
controlled by a remote PLC reaches its destination, or while it is underway.

4-c ad option

TD & T

S U M M E R

2 0 0 3

Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 2003