Emerson Rosemount 56, Instruction Manual

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Instruction Manual Section 7: PID Control
LIQ-MAN-56 April 2017
PID Control 61
Note: If you want the control output to increase as PV (in this case pH) increases, URV should
be greater than LRV. This is direct acting control action. Examples of direct acting control are
the addition of acid to decrease pH and adding water to a solution to decrease the
concentration.
If you want the control output to decrease as PV increases, i.e. reverse acting control action, the
URV should be less than LRV. Examples of reverse acting control are adding caustic to increase
pH and adding a concentrated solution to water to make a solution of lower concentration.
• Bias (range: 0 to 100%; default 0%) – Bias is a fixed control output which allows the con-
trol output to be greater than zero when the measurement (PV) is at setpoint. It is used
in proportional only control to prevent cycling resulting from the control output going
to zero at the setpoint.
• Gain (range: 0.0 to 1000.0; default 0.0) – In proportional (P) only control, the output is
directly proportional to the difference between the process variable (PV) and the set-
point divided by the output span (URV – LRV). Gain is a factor which multiplies the pro-
portional output to meet the requirements of the process being controlled. Using Gain
values less than 1 reduce the proportional output while Gain values greater than 1 in-
crease the proportional output.
• Integral (Reset, I) (range: 0 to 3,000 seconds; default 0 seconds) – Integral repeats the
proportional action in a time period given by the reset time (I). The reset time is given
in seconds per repeat and is adjustable from 0 to 3,000 seconds. Integral control acts
as an automatic bias which increases or decreases the overall control output in re-
sponse to the error (PV – SP) to keep the PV at the setpoint.
• Derivative (Rate, D) (range: 0 to 3,000 seconds; default 0 seconds) –Derivative action,
gives an immediate control output in response to changes in the proportional output
with time (derivative). The amount of increase or decrease depends on the rate of
change of the error. The rate constant (D) allows the user to adjust the amount deriva-
tive control contributes to the control signal. Smaller values reduce the effect of deriva-
tive control.
• Mode: Mode has two settings, Auto (Automatic) and Man (Manual). In the Auto mode
the control output is controlled automatically by the PID algorithm. In manual mode,
the control output can be set to a constant value; this is useful during transmitter cali-
bration or servicing.
• Value (Manual) (range: 0 100%) – When the Manual mode is chosen; this control ap-
pears on the screen and allows you to write the constant control output value in the Man-
ual mode.
Using the Next button lead to the final PID control window:
7.2.4 Transport Time
Transport Time makes it possible to apply PID control action to a process flowing in a pipe for
a short period of time (run time), and then hold the control output fixed to allow the treated
sample time to mix and travel to the pH or other analytical sensor (transport time). If
properly tuned the PV should reach the setpoint after successive time periods.
It is best used when raw sample pH (or concentration) remains relatively steady for long
periods of time, as is the case for samples flowing from a large body of water. It should not
be used where process upsets are possible because the delay in applying control will make
recovery from the upset slow, and can result in overshoot after the incoming sample has
returned to a normal range.