Emerson Rosemount 56, Instruction Manual

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Instruction Manual Section 8: Time Proportional Control
LIQ-MAN-56 April 2017
PID Control 65
PID Control Parameters: Basic Definitions
• Setpoint – Select the desired setpoint.
• URV – The value of PV (in the above example, 14.00 pH) at which the control will be
100% On Time.
• LRV – The value of PV (in the above example 0.00 pH) at which the control will be 0% On
Time.
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.
• Time Period (range: 10 to 3,000 seconds; default 30) -- The time period for each cycle of
TPC.
• Bias (range: 0 to 100%; default 0%) – Bias is a fixed control output which allows the % on
time to be greater than zero when the measurement (PV) is at setpoint. It is used in pro-
portional only control to prevent cycling resulting from the % on time going to zero at the
setpoint.
• Relay default (Close, Open, None) – Select the relay action during a fault condition.
• 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 setpoint
divided by the output span (URV – LRV). Gain is a factor which multiplies the proportional
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 increase 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 response 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 derivative control
contributes to the control signal. Smaller values reduce the effect of derivative 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 calibration
or servicing.
• % On Time (Manual) (range: 0 100%) – When the Manual mode is chosen; this control
appears on the screen and allows you to write the constant On Time value in the Manual
mode.