Theatre Design & Technology - Winter 2005 - 27

timing between loads-the calculated velocity was little changed.
Trapping the nether end of the hose merely made the hose snake
into a serpentine shape, which became more pronounced with a
greater load. This was an effect I had not anticipated and I
could see that it did not bode well for computer control.
Our next test on the linear test bed was to put the system
under computer control. The San Diego State University Theatre, along with several other theatres on the west coast, uses a
software program called MCued, developed in-house by
Joanne Rock, to control stage machinery. It is based on Galil
servo control hardware and communicates with our stage machinery via an analog +/-10VDC control signal. A positive 10
VDC applied to a machine represents full-speed forward,
whereas a negative 10 VDC represents full-speed in reverse.
Speed and direction is completely variable between these two
extremes. This is exactly the kind of signal the proportional
valve was designed to receive, so it was a simple matter of connecting cables to the right places.
The flow meter (fig. 12) was inserted into the hydraulic
circuit to provide the quadrature signal required by the servo
controller to close the velocity and position loops. Like most
closed-loop systems, our system required tuning. This is a
matter of sending a signal to the valve and observing how
closely the valve's motion corresponds to the applied signal in
special monitoring software provided by Galil. By adjusting
three parameters: proportional, integral and derivative, wellconstructed servo systems can be made to follow an applied
signal with great precision. After years of observing lagging
motor responses, the valve's response to the applied signal
looked nearly perfect, even though the flow meter's quadrature output was rather coarse compared to motor-mounted
encoders. The flow meter generated approximately 1000
pulses per foot, compared to over two hundred thousand
counts per foot with some motorized winches. The valve's
quick response was likely due to its low mass compared to
that of a large motor's rotor.
I wrote a few cues into MCued that would run the cart
back and forth and then stop the cart at the center of the hose.
The motion of the pointer on the MCued screen and the movement of the cart agreed exactly-no matter what the load. This
was more in line with my expectations, given the ratio of the
force to the load. But when we measured the cart's location
upon its return to center it was off by nine inches! The effect
was more pronounced the further the cart was from the pressure end of the hose when the run started. Furthermore, the
greater the load the more the hose stretched. Obviously, while
the computer could accurately measure the volume of fluid in
the hose, it could not account for how much the hose
stretched to accommodate that volume. Accounting for hose
stretch might be possible with strain gauges and the like, but it
certainly added complexity to computerize control. Although
this was a great disappointment, I was please with how MCued
and the proportional valve worked together.
While we had the stage we performed a few more tests
and filmed them for the 2004 USITT convention in Long Beach.
We ran Rover on the unconstrained (no track) serpentine layout
and also the spiral layout. Each time Rover navigated the hose
26

W I N T E R

2 0 0 5

TD & T

gracefully. And while we had no intention of duplicating Elbin
Cleveland's efforts, we did try a rudimentary lift. By pulling the
center of the spiral layout up about ten vertical feet and attaching it to a locked-off stage batten, we were able to run Rover
around the spiral on the floor and then directly up the vertical
portion of the hose (fig. 13). Furthermore, Rover climbed the
hose without hesitation even when we loaded it with 260
pounds (Matt). It would be a simple matter to tie one end of
the hose to the grid and link Rover to an arbor to run a line set
in and out. However, at one foot per second it would be a
rather long move. Although there is sufficient force to overcome the additional 50 feet of head pressure in the vertically
oriented hose, stretch would once again make accurate positioning difficult. The result might be goods piled on the floor
or drops bumping and sagging. We elected not to try it.

The Application of Reverse Peristalsis to New Theatrical
Motion Profiles
The original impetus for this study was to attempt new or otherwise difficult motion profiles with the reverse peristaltic system.
We had already demonstrated unconfined serpentine and spiral
profiles, which would be difficult or impossible to achieve with
typical winches and cables or chains. Now it was time to put the
system to the test in a tracked system. Matt designed and built a
diabolical device we called the Gauntlet, which featured several 90° and 180° turns with a radius of 24˝, and a hill that
took the Rover up and over the first part of the track (fig. 14).
The rollerblade wheels proved to be a serendipitous choice,
since they held Rover in the center of the track against the
hose's attempt to straighten out against the sidewalls of the
track. The hose might writhe around as it pushed the rover
down the track, but Rover and its attachment to a scenic element would stay centered under the a˝ groove in the deck.
This was the big moment: after nearly two years of development, this was the first test of the reverse peristaltic system
within a genuine theatrical track, hauling a wheel-less skid
with a load (Matt) aboard. The track creaked and groaned as
it dealt with the force of the hose against the sidewalls, but
Matt moved along as though riding a magic carpet. He made it
through the first turn when I noticed hydraulic fluid spilling
from the end of the track. A rogue staple and a loose screw
had punctured the hose in two places. We were dead in the
water-or hydraulic fluid in this case. In a flurry of activity
over the next two days, Matt ripped the track apart searching
for more screws and staples while I tried to locate a local
source for a new hose. A show was to open on stage within a
week and, while the director and designers had shown remarkable forbearance while we performed our experiments
(and cleaned up hydraulic fluid), they needed the stage.
Matt had the Gauntlet reassembled and I found a hose by
an offshore manufacturer that claimed to be rated to 400 psi,
but whether that was burst pressure or working pressure the
vendor could not say. No matter, a good hose was weeks away,
so this one would have to do. I also discovered that the new
hose used a different thread than the original and that put us
back a day waiting for new adapters to arrive from McMasterCarr. (See "Pipe and Hose Thread Standards" for explanation



Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 2005

Contents
Theatre Design & Technology - Winter 2005 - 1
Theatre Design & Technology - Winter 2005 - 2
Theatre Design & Technology - Winter 2005 - 3
Theatre Design & Technology - Winter 2005 - Contents
Theatre Design & Technology - Winter 2005 - 5
Theatre Design & Technology - Winter 2005 - 6
Theatre Design & Technology - Winter 2005 - 7
Theatre Design & Technology - Winter 2005 - 8
Theatre Design & Technology - Winter 2005 - 9
Theatre Design & Technology - Winter 2005 - 10
Theatre Design & Technology - Winter 2005 - 11
Theatre Design & Technology - Winter 2005 - 12
Theatre Design & Technology - Winter 2005 - 13
Theatre Design & Technology - Winter 2005 - 14
Theatre Design & Technology - Winter 2005 - 15
Theatre Design & Technology - Winter 2005 - 16
Theatre Design & Technology - Winter 2005 - 17
Theatre Design & Technology - Winter 2005 - 18
Theatre Design & Technology - Winter 2005 - 19
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