to the processor operation. No messag-
ing is needed because all of the tags are
automatically updated by the processors
to the other processors. Our set of tags
included reals, integers and bits, so we
made a custom data type for each PLC’s
produced tags and then designated them
appropriately as “produced” or “con-
sumed” in each processor.
We also included a watchdog bit that
would toggle every second and would
produce an error message if it failed to
change state in two seconds. We each
also used that failed status to disable
equipment that was being commanded
into operation by the other PLC’s pro-
duced tags.
Finally, I showed all of these bits,
integers and reals in a Panelview screen
showing the Chinese what I was sending
them and what I was or wasn’t getting
from them. That screen sure cured a
lot of malfunctions and “arguments”
quickly with the Chinese when things
didn’t work right with the operations.
For control devices on the press, we
usually use Rockwell Automation’s
Allen-Bradley 8 02T limit switches, Turck
30 mm proximity sensors and Sick photo
eyes, specifically the WT260 retroreflec-
tive type.
We like the Allen-Bradley limit
switches because they have been tested
to survive millions of operations, and we
have never had a callback on them. We
use the Turck proximity sensors for no
particular reason except that we have
also never had a callback on them.
We have also used Omron’s E3G series
photo eyes and Omron’s E2E series un-
shielded proximity switches with good
results. Both the Sick and Omron photo
eyes handle universal voltages—24 V to
240 V ac/dc, which makes them possible
for use with all of the control voltages
we encounter.In the motor control center (MCC), we
use either Siemens or Rockwell Automa-
tion motor contactors with motor circuit
protectors, as well as Siemens circuit
breakers, AutomationDirect terminals
and distribution blocks and Panduit
wireways. All VFDs are the same brand
as the PLC, which in this case is Allen-
Bradley.
In the operator controls cabinets we
use remote racks by the same brand as
the PLC manufacturer—once again, here
it is Rockwell Automation’s Allen-Brad-
ley—Rockwell Automation push buttons
and pilot lights, AutomationDirect ter-
minals and Panduit wireways.
We usually use Rittal for all of the cab-
inets, and we include fans with filters, or
we include air-conditioning systems if
the factory environment will be hot.We use AutomationDirect five-port
and eight-port unmanaged Ethernet
switches in every cabinet and have
had great success with them. We also
usually include Rockwell Automation’s
Allen-Bradley 9 300-ENA network ad-
dress translator module when using that
same brand of PLCs. Finally, we use an
AutomationDirect door-mounted Eth-
ernet port with power outlet on the PLC
cabinet door.
The press is driven by two 4 0-hp
motors at 380 V, 50 Hz to power the
hydraulics. Whenever I order VFDs for
these pumps, I always get the next-
highest-rated drive, in this case 5 0-hp
drives, to be sure there are no current
overload issues, since we tend to push
these motors over the limit for short
periods of time.28 / May 2019 / ControlDesign.comcover story
The gang’s all here
Figure 3: The field technician (from left); the Chinese interpreter, Zhou; and Stephen Jones are dwarfed by
the 80-ft-tall press.CD1905_26_35_CoverStory.indd 28 4/29/19 12:22 PM