COVERSTORY
bus. The DeviceNet protocol included standard
objects with network and node status information,
but the EZ-ZONE RM’s function block
programming offered a convenient way for the
network supervisor to monitor connections with
each node. Function blocks were configured to run
a heartbeat routine in each control system. The
network supervisor queried each controller’s
heartbeat variable at regular intervals and
determined if the node connection was online.
Using Proportional Integral Derivative (PID)
with Programmable Function Blocks
Another PV thin film manufacturer needed
capabilities to customize PID control in their
deposition chamber. This chamber’s 60 control
loops were heated with tungsten lamps driven by
Watlow’s DIN-A-MITE C phase-angle fired power
controllers. Due to conditions inside the chamber,
it was impractical and costly to install a
temperature sensor for each loop. Fewer sensors
Watlow MULTICELL heater could be used if each sensor controlled a group of
adjacent zones. Without actual temperature
feedback from the adjacent zones, a method of
with flawless control performance. A full control inferring temperature or power required for
17
system consisting of three controller nodes was adjacent zones would be needed.
designed for the chamber. Each controller node
www
supported 40 to 50 loops using 11 to 14 control Experiments determined the optimal ratio of
.solar
modules and one access module. The system also sensors to zones. The 60 zones were divided into
included DeviceNet communications for linkage to 15 primary zones with each controlling 3
-pv-management.com
the supervisory system software. secondary adjacent zones.
Watlow DIN-A-MITE C
By removing temperature control from the PLC and The EZ-ZONE RM’s PID control algorithm power controller with
using a stand-alone discrete device, the calculated heater power for each primary zone phase angle fire power
manufacturer was able to improve overall while a math function controller.
performance and optimize temperature control. block calculated
secondary zone
Issue I 2010
Optimizing a System with Programmable heater power based
Function Blocks on a ratio to the
After the manufacturer’s full system was up and primary zone – a
running, some improvement opportunities became secondary zone might
apparent. An area of concern was the surge in run at 85 percent of
electrical demand when all heaters switched on at its primary zone.
the same time. Control outputs were configured for
fixed time base – a method of applying heater Using power
power that is proportioned over a segment of time. calculations for
While heater power varied from loop to loop, the adjacent zones, the
start of each on-cycle was the same for all. controller
accommodated each
Using the EZ-ZONE RM’s programmable function zone’s unique
blocks, the manufacturer programmed timers to heating requirements
progressively delay the start of each heater’s on- without the need for a
cycle. While maintaining control performance, this sensor on each zone.
technique staggered heater turn-on times and EZ-ZONE RM’s
reduced peak electrical demand. programmable function
blocks provided the tools to
An additional improvement area was focused link control of adjacent zones
around the status of nodes on the communications with the primary zone. The
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