Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 17

Proportional-Integral (PI) control: A control algorithm that
combines the proportional (proportional response) and
integral (reset response) control algorithms. Reset
response tends to correct the offset resulting from
proportional control. Also called "proportional-plus-
reset" or "two-mode" control.
Proportional-Integral-Derivative (PID) control: A control
algorithm that enhances the PI control algorithm by
adding a component that is proportional to the rate of
change (derivative) of the deviation of the controlled
variable. Compensates for system dynamics and
allows faster control response. Also called "three-
mode" or "rate-reset" control.
Reset Control: See Compensation Control.
Sensing element: A device or component that measures the
value of a variable.
Setpoint: The value at which the controller is set (e.g., the
desired room temperature set on a thermostat). The
desired control point.
Short cycling: See Cycling.
Step control: Control method in which a multiple-switch
assembly sequentially switches equipment (e.g.,
electric heat, multiple chillers) as the controller input
varies through the proportional band. Step controllers
MEASURED
VARIABLE OUTDOOR
AIR
-2
OUTDOOR
AIR
CONTROLLED
MEDIUM
MEASURED
VARIABLE
HOT WATER
RETURN
ENGINEERING MANUAL OF AUTOMATIC CONTROL
RESET SCHEDULE
15 55
-15 90
HW
OA
SETPOINT
TEMPERATURE
CONTROL
CONTROLLED
POINT
VARIABLE
HOT WATER
SUPPLY
TEMPERATURE
HOT WATER
SUPPLY
64
AUTO
Fig. 1. Typical Control Loop.
may be actuator driven, electronic, or directly activated
by the sensed medium (e.g., pressure, temperature).
Throttling range: In a proportional controller, the control point
range through which the controlled variable must pass
to move the final control element through its full
operating range. Expressed in values of the controlled
variable (e.g., Kelvins or degrees Celsius, percent
relative humidity, kilopascals). Also called
"proportional band". In a proportional room
thermostat, the temperature change required to drive
the manipulated variable from full off to full on.
Time constant: The time required for a dynamic component,
such as a sensor, or a control system to reach 63.2
percent of the total response to an instantaneous (or
"step") change to its input. Typically used to judge
the responsiveness of the component or system.
Two-position control: See on/off control.
Zero energy band: An energy conservation technique that
allows temperatures to float between selected settings,
thereby preventing the consumption of heating or
cooling energy while the temperature is in this range.
Zoning: The practice of dividing a building into sections for
heating and cooling control so that one controller is
sufficient to determine the heating and cooling
requirements for the section.
70
PERCENT
71
7
CONTROL FUNDAMENTALS
ALGORITHM IN
CONTROLLER
SETPOINT
SETPOINT
INPUT
OUTPUT
41
OPEN
FINAL CONTROL
ELEMENT
VALVE
STEAM
CONTROL
FLOW
AGENT
MANIPULATED
VARIABLE
M15127