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Advanced
process control and fault monitoring: analog vs. digital
Typically, analog designs provide minimal process and fault information.
Only critical information is monitored. In an electric forehearth
application operating under current control, electrode current
is the only parameter being controlled and monitored The process
control system will send a signal (typically 4-20 mADC) to set
desired zone outputcurrent.
In addition,
a transducer (also with 4- 20 mADC output) will be used to monitor
output current delivered to the forehearth. Both the output set
point control signal and output transducer signal are sent, via
wire connections, between the power control panel containing SCR
power controllers and the process control system.
Through these
individual wires, the operator in the control room sets the desired
output and monitors the results. Besides monitoring and controlling
specific parameters such as output current, analog equipment uses
generic fault indications such as I zone failure' or 'power controller
failure.' These single fault indications used to keep equipment
and wiring costs to a minimum - have multiple meanings. They could
be indicating SCR over temperature, a blown fuse, SCR over current
shutdown, load failure, or some other malfunctions.
To further
understand the problem, the control room operator must send service
or maintenance personnel out to the control panel location to
determine exactly what is causing the generic fault indication.
With analog
technology, each additional fault requires control relays, wiring
and digital inputs on the process controller. If each piece of
information is to be displayed in the control room, the cost of
showing each fault can be substantial, especially for multi-zone
systems.
 Digital
power controllers have numerous advantages over their analog counterparts.
All parameters and faults directly related to each power controller,
or zone of control, are monitored, and many are controllable.
These parameters include: input and output phase-to-phase voltage,
input frequency, input and output per phase current, input kVA
and output kW, power factor and kW-hour.
Furthermore,
the single-phase and three-phase digital power controllers monitor
a variety of faults, including: input high voltage, frequency
out of tolerance, over current shutdown, SCR over temperature
and phase loss.
Parameters
that are digitally controllable include output voltage, current
and power. This level of functionality is driven by the nature
of the digital design.
In an analog
design, each additional parameter to control or monitor current
flow meant more circuitry and higher equipment costs. With digital
power controllers, that paradigm no longer exists because software
in the microprocessor dictates the unit's functionality.
A
better way to achieve local process control
Glass manufacturers are accustomed to using relatively simple
methods for achieving local control of SCR power controllers.
Typically, on/off pushbuttons, control potentiometers, and analog
meters are provided locally and are often located on the enclosure
door of the panel contaming the specific SCR power controller.
In power control zones for the BATH portion of a flat glass manufacturing
line, the local controls for each of the 28-33 zones consist of
a power set point potentiometer, power meter (analog or digital),
on/off pushbuttons or a selector switch and a 'power on' pilot
light. These devices allow for individual zone control locally
at the power panel during startup, and when there are problems
with the central process controller.
As with other
aspects of an analog design, functionality is kept to a minimum
due to the incremental costs to add features. Local Digital Controllers
(LDCs) have forever changed what is expected of local control
capabilities. With a mere push of a button, the LDC provides on/off
control and set point adjustment, and also displays voltage, current,
and power levels
The
ideal connectivity package
Analog designs transfer process and control information through
control wire (typically 'twisted-pair') connections carrying control
signals and other relay type contacts representing faults and
alarms.
In the BATH application in flat glass manufacturing, each SCR
power controller zone accepts a 4-20 mADC control signal for its
power set point and sends a 4-20 mADC signal proportional to output
power back to the process controller.
In addition,
an analog SCR power controller accepts relay contacts for On/off
control and provides a relay contact for zone fault. This configuration
means that, per zone, there are up to 2 sets of twisted-pair control
wires and 3 sets of wires for relay contacts. These wires must
be physically 'run' from the power panel to the process controller
in the control room. Multiply those numbers by the 28-33 zones
the application requires, and one can easily see that a lot of
wire, conduit, and labor are expended just to achieve a minimal
level of control.
In contrast,
digital SCR power controllers are capable of continuously monitoring
and controlling a multitude of parameters and faults that are
unfeasible for analog equipment to detect or regulate
Another benefit
of digital power control is that single network connections do
not 'run' back to the control room. Instead, they are terinitiated
at the power panels 'in network 'tap' boxes. A single network
cable is then 'run' to the control room and to the network interface
of the process controller. In BATH applications, a single digital
network connection replaces 140-165 analog control wire connections
between the process controller and the power panels. The savings
in labor and materials are significant.
Conclusion
Single- and three-phase digital power controllers designed to
operate on 24 to 600 volt RMS at 50/60 Hz are ideal for glass
and fiberglass manufacturing applications. Digital technology
allows for independent, remote operation of SCR power controllers
and eliminates calibration and hardware considerations that formerly
constrained the ability to precisely monitor and control electric
current in glassmaking operations.
About
the author
Christopher M. McCormick is business manager of power control
systems for Spang Power Electronics - Mentor, Ohio
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