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MODEL 54eA SECTION 14.0
CALIBRATION - CONTROL
14.2 TIME PROPORTIONAL CONTROL
(TPC) MODE (Code -20)
In the TPC mode, you must establish the following
parameters which will determine how the Model
54eA controller responds to your system (see
Section 5.7):
• Setpoint
• Time period
• URV point (or 100% on)
• LRV point (or 0% on)
• Proportional
• Integral
• Derivative
The following describes how TPC works. In the exam-
ple, pH is to be controlled.
The setpoint is the pH you want to control the process
to. Time period is programmed in seconds and defines
the interval during which the controller compares the
pH input from the sensor with the setpoint. In the TPC
mode the controller divides the period up into pump on-
time (feed time) and pump off-time (blend time).
The URV setting determines how far the pH must devi-
ate from the setpoint to get the pump to be on for the
entire period. The LRV setting determines how close
the pH must be to the setpoint for the pump to be off for
the entire period. The LRV setting should always be set
at zero. When the error (the pH minus the setpoint) is
between the URV and LRV values, the relay will be
energized for some portion of the time period. As the
pH value approaches the setpoint, the pump will be
feeding for shorter and shorter intervals, and the chem-
icals will be allowed to mix for longer and longer inter-
vals of the period. This relationship is illustrated in
Figure 14-3.
The exact amount of on time and off time per period is
determined by the settings for proportional, integral,
and derivative bands. The proportional band (P) in %
is a separate adjustment that narrows (or widens) the
range of the TPC 0-100% action. Smaller values are
used for more control response. For a setpoint of 7 pH,
a URV of 2 pH, and P=100%, a pH reading of 8 would
result in a relay on (8-7)/((2-0)*(100%)) or 50% of the
time. If P was changed to 50%, the same relay would
be on (8-7)/((2-0)*50%) or 100% of the time.
The integral band is set in seconds and acts to
increase the controller output as more time is spent
away from the setpoint. A smaller value in seconds will
result in faster integration response. Too low a value
will result in excess oscillation.
The derivative band is set in % and acts to prevent
changes in the reading. This setting should generally
be set to zero for pH applications.
TPC offers precise control by forcing the pump to feed
chemical for shorter periods of time as you approach
the desired setpoint. If the process faces a large upset,
TPC mode forces the pump to feed chemical for longer
periods of time as the process deviates further from the
setpoint. This action continues until the pump is feed-
ing all the time, providing a speedy recovery from large
up-sets.
The controller can be programmed to be direct or
reverse acting, depending on the pH (or temperature)
value selected for URV. For example, if the controller is
direct acting based on pH, such as in caustic chemical
addition control, the pH will rise as chemical is added,
so the URV value will be below the LRV (i.e. below
zero). As the pH rises toward the control point value,
the pump will be on for gradually less time. Conversely,
if the controller is reverse-acting based on pH, such as
in acid addition for control, the pH will drop as acid is
added, and the URV value will be positive. The pH will
fall toward the control point value, and the pump will be
on for gradually less time.
Complete TPC configuration is explained and typical
settings for these parameters are listed in Section 5.0.
After startup, the operator needs to adjust only the 0%
On to maintain the desired chemical concentration.
FIGURE 14-3. Time Proportional Control
83