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ctllp05044Mistakes.qxd 4/18/2005 11:29 AM Page 4

THE FOUR BIGGEST MISTAKES IN INSTRUMENTATION ... A

mounting location closest to the opposing rollers (see Figure 1). A until some change begins to
furnace may be convenient for thickness sensor downstream from appear in the measurements
wiring purposes, but if that spot the rollers gauges the sheet and reported by the sensor. By that
happens to be in a hallway or other causes the controller to apply time, the controller’s cumulative
dead air space, the thermostat will either more or less pressure to efforts will have already overcom-
not be able to determine the aver- compensate for any out-of-spec pensated for the original error,
age temperature elsewhere in the thickness. causing an error in the opposite
house. It will only be able to main- Ideally, the thickness sensor direction. The result will be a con-
tain the desired temperature in its should be located adjacent to the stant series of up and down swings
immediate vicinity. The rest of the rollers to minimize the time in the roller pressure and a lot of
house may end up roasting or between a change in roller pres- steel ruined by lateral corrugations.
freezing. sure and the resulting change in Of course, overall process per-
the thickness measurement. formance considerations aren’t lim-
C Coonnttrroolllleerr ppeerrffoorrmmaannccee Otherwise, the controller will not be ited to how well the sensor feeds
Poor control also results when a able to detect any mistakes it may data to the controller during opera-
sensor is installed too far away have been making soon enough to tion. Other factors to consider
from the associated actuator. A dis- prevent even more of the sheet include ease of installation and
tant sensor may not be able to from turning out too thick or thin. time spent on the selection
measure the effects of the actua- Worse still, an appreciable dead process, set up routines, and any
tor’s last move in time for the con- time between the controller’s labor-intensive maintenance.
troller to make an educated deci- actions and the resulting effects on Fortunately, some instrumentation
sion about what to do next. the steel can cause the controller vendors design their sensors to
For example, consider the to become impatient. It will see no accommodate such challenges,
process of flattening hot steel into results from an initial control move, thereby improving performance
uniform sheets by means of two so it will make another and another before the system even goes on-
line. ABB, for instance, offers a
FIGURE 1. POOR SENSOR PLACEMENT swirl flowmeter that significantly
reduces the need to install special
upstream and downstream devices
to accurately measure the flow
through a pipe.
PISTON CONTROLLER
P Prrootteeccttiioonn
A steel mill is also a classic exam-
ADJUSTABLE OPTICAL
ROLLER THICKNESS ple of a harsh environment that can
GAUGE destroy inadequately protected
sensors. Fortunately, the hazards
posed by a manufacturing process
FIXED are generally obvious and can often
ROLLER
D be overcome by installing a shield
or choosing a rugged instrument.
Often overlooked, however, are
In this steel rolling example, D is the distance between the steel rollers and the thickness the effects of weather. Outdoor
gauge downstream. If D is too large, the controller will take too long to correct thickness instruments can take quite a beat-
errors and may even make matters worse by becoming impatient. ing from rain, snow, hail, and falling