ETL 1110-2-553
30 May 97
Appendix B
dangerous condition of the gate being suspended
while the lifting cables were lying slack on the
Operation of PLC Systems
machinery room floor. While sensors had been
installed on the lifting cable sheaves to detect move-
B-1. Advantages of PLC System
ment, it was never considered that the sheaves could
be moving when the gate was not. Special
a. General. The following operational descrip-
instrumentation was placed upon the gate to detect
tion is typical of a lock with miter gates. Although
slacking of the cables. Because of the serious nature
the operational requirements may differ, locks with
of this problem, several redundant systems were put
sector gates or lift gates can also use a PLC system.
in place. First, slack cable limit switches were
installed at a horizontal run of the lifting cables, then
b. Overfilling. At some locks problems exist
photocells were installed in a vertical run, and,
with overfilling and overemptying of the lock cham-
finally, angle encoders were installed to determine
bers causing time delays and undue stress on the
movement of the counterweight sheaves. In addition,
gates and mechanical equipment. The valves can be
it was decided to begin lowering the gate with a
programmed with a PLC to start closing before the
small amount of head against the gate. This required
chamber is completely empty or full. The exact time
constant recalculation of the head differential because
when the valves begin to close could be programmed
of the continuously fluctuating water levels. The
to vary with the difference between the pool and the
PLC was simply reprogrammed to accommodate the
tailwater elevations since this is what most affects the
changes. The lift gate at Melvin Price has a third
rate of filling and emptying. The speed of the valves
leaf which is essentially a movable sill. This gate is
could also be adjusted to compensate for the rate of
raised and lowered only infrequently under certain
filling and emptying at different heads. A special
extreme head conditions. The PLC is programmed to
case could be programmed for open river conditions.
automatically adjust this gate during the lockage
operation so as not to cause delays to traffic.
c. Improper miter/auto-emptying. Serious dam-
age can occur to miter gates when they do not miter
e. Voltage drop reduction. A PLC can be
properly due to some obstruction or gate malfunction.
programmed to stagger start motors. This reduces the
If this condition occurs, it is important not to put full
simultaneous inrush current, thus reducing system
head on the gates. A PLC can be programmed to
voltage drop during motor starting. As a result, a
$crack# open the filling valves and put a small
savings can be realized in the sizing of system power
amount of pressure on the gates. If the gates do not
distribution equipment.
come to a proper miter after reaching a prepro-
grammed head differential, the filling valves can be
f. Troubleshooting. Troubleshooting an RBS
closed and the emptying valves opened. The operator
requires that an electrician physically move from
could be given an alarm of improper miter. Such a
point to point and take measurements of system para-
feature is a quick programming implementation in a
meters. This can be a hazardous and time consuming
PLC system.
procedure. A PLC system offers the convenience and
efficiency of troubleshooting the system from a per-
d. Lift gate control. Lift gates present special
sonal computer. Once the nature and location of the
problems to navigation locks because a lift gate is
problem is identified, an electrician usually needs
hoisted from each side of the lock. The hoisting
only to replace a modular component and the system
machinery has to operate synchronously to avoid
in back online.
skewing or $cocking# of the gate. Such fine position
g. Remote troubleshooting. With a PLC sys-
control and automatic skew correction can be done
tem, a systems engineer or technician with a personal
easily with a PLC, but would involve numerous
computer, modem, and standard phone line can net-
relays, timers, and position counters in a relay sys-
work with the PLC for troubleshooting assistance and
tem. A problem developed with the lowering of the
reprogramming at any time, from any place.
lift gate at Melvin Price Locks and Dam. For reasons
not thoroughly understood, the lift gate would occa-
sionally get stuck in the slot. The machinery had no
capable of generating the control sequences necessary
way of knowing that the gate had stopped. The
to automate lock and dam operations. Currently, only
cables continued to unreel, creating an extremely
the first steps have been taken in that direction.
B-1
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