Theatre Design & Technology - Spring 1979 - 12

environment, regardless of what motive force is employed.
Firstly, an extremely low operational noise level is essential,
as many of the movements take place during the performance-often during speech from the actors. With theatres
being specifically designed to transmit acoustically from
stage to auditorium and modern theatres especially being
most efficient in this respect, even noises of a very low order
are unacceptable. Generally speaking, the standard criteria is
taken as NC20, measured from any seat in the auditorium.
Noise criteria or NC curves define allowable noise level in dB
in octave bands at frequencies between 37.5 and 9600 Hz.
NC20 relates to an extremely low level of noise by normal
industrial standards.
Secondly, position control must be precise, usually to
within ±3mm for stage elevators and ±5mm for scenery
flying systems.
In the quest for remote, smooth, and accurate control,
conventional proprietry hydraulic valves were generally found
unacceptable and the need for electro-hydraulic proportional
flow control valves demonstrated. This type of valve had been
developed for similar applications on mobile and industrial
equipment and is primarily suited for open loop applications
where the controlled variable can be seen by the operator.
The principle of operation is identical to high performance
closed loop electro-hydraulic servovalves but construction
characteristics are suited to a less sophisticated environment.

Characteristics of the Valves

1. A physical configuration similar to existing mobile valves,
i.e., stackable with through porting.
2. Rugged construction and the capability of operation in
extreme environments.
3. Reasonable insensitivity to contamination, giving reliability
in systems protected by 25 micron filters.
4. Precise metering characteristics over the rated flow range.
5. Minimum dead band consistent with good load holding.
6. Maximum delay between operator input and spool maximum flow rate of 0.2 seconds to assure responsive control.
7. Manual override for use in the event of control system
failure.
T he principle of operation of this type of valve begins when
a signal from the control station to the valve is made electrically via a low voltage DC current. This is an important feature
in its own right because the control point can be remote from
the valve and hydraulic circuits and in theatrical applications
it is not unusual for the control point itself to be mobile at the
end of a wandering lead.
The valve is comprised of torque motor, pilot stage, and
valve spool. The torque motor includes polepieces, magnets,
and an armature, the armature being supported for limited
movement by a flexure tube. The flexure tube also provides a
fl uid seal between the hydraulic and electro-magnetic portions of the valve. The flapper is attached to the center of the
armature extending down through the flexure tube into the
pilot stage.
A nozzle is located on either side of the flapper so that the
flapper motion induced by a differential current in the coils
10

Theatre Design & Technology

ElECTRICAL
TERMINAL BLOCK

SPOOL

INLET
ORIFICE
FILTER

C2
RETURN

Design details of the Series 60 proportional control valve which comprises a torque motor, pilot stage, and valve spool.

varies the nozzle openings. Pressurized fluid is supplied to
each nozzle through an inlet orifice protected by a 40 micron
nominal rated filter. Differential pressure caused by the flapper movement between the nozzles is applied to the end of
the spool. A feedback wire is deflected by the resulting spool
movement so that a feedback torque is applied to the armature, recentering the flapper between the nozzles and eliminating the differential pressure across the spool.
Thus, for each current level in the coils, a stage of equilibrium is reached in the pilot stage which corresponds to a
specific spool position. This feature, because of the inherent
balanced characteristic, gives good control in a wide variety
of environments. The principle has several advantages, not
the least being that the speed of the movement of the actuator, and therefore, that of the machinery the actuator is controlling, is infinitely variable from zero to the predetermined
full speed.
An SG cast iron body, with cored porting, houses a conventional four-way configuration spool while a selected fit
between spool and body has been avoided so that spool
interchangeability is possible. A sealed pressure die-cast motor cap provides physical and environmental protection for
the torque motor and, in addition, houses the terminal block
for all electrical connections.
We can now examine three theatrical installations where
this type of valve has been in use in the control of the hydraulic circuits.

Stage at the Royal Opera House

One of the most complicated stage sets ever built in the
UK, from the control point of view, is that currently used at the
USITT /Spring, 1979

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