Floating Control - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CONTROL FUNDAMENTALS
THROTTLING RANGE
OFF
4
OFF
3
STAGES OFF
2
OFF
1
22
SPACE TEMPERATURE ( C)
0%
Fig. 27. Staged Reciprocating Chiller Control.
Figure 28 shows step control of sequenced DX coils and electric
heat. On a rise in temperature through the throttling range at the
thermostat, the heating stages sequence off. On a further rise after
a deadband, the cooling stages turn on in sequence.
ACTUATOR
STAGE NUMBERS
6
5
4
3
2
1
SOLENOID
VALVES
STEP
CONTROLLER
D
X
FAN DIRECT EXPANSION
COILS
Fig. 28. Step Control with Sequenced
DX Coils and Electric Heat.
A variation of step control used to control electric heat is
step-plus-proportional control, which provides a smooth
transition between stages. This control mode requires one of
the stages to be a proportional modulating output and the others,
two-position. For most efficient operation, the proportional
modulating stage should have at least the same capacity as one
two-position stage.
Starting from no load, as the load on the equipment increases,
the modulating stage proportions its load until it reaches full output.
Then, the first two-position stage comes full on and the modulating
stage drops to zero output and begins to proportion its output
again to match the increasing load. When the modulating stage
again reaches full output, the second two-position stage comes
DIFFERENTIAL
ON
ON
ON
ON
24
23
SETPOINT
LOAD 100%
C3982
SPACE OR
RETURN AIR
THERMOSTAT
D
DISCHARGE
AIR
X
MULTISTAGE
C2716
ELECTRIC HEAT
full on, the modulating stage returns to zero, and the sequence
repeats until all stages required to meet the load condition are on.
On a decrease in load, the process reverses.
With microprocessor controls, step control is usually done
with multiple, digital, on-off outputs since software allows
easily adjustable on-to-off per stage and interstage differentials
as well as no-load and time delayed startup and minimum on
and off adjustments.

FLOATING CONTROL

Floating control is a variation of two-position control and is
often called "three-position control". Floating control is not a
common control mode, but is available in most microprocessor-
based control systems.
Floating control requires a slow-moving actuator and a fast-
responding sensor selected according to the rate of response in
the controlled system. If the actuator should move too slowly,
the controlled system would not be able to keep pace with
sudden changes; if the actuator should move too quickly, two-
position control would result.
Floating control keeps the control point near the setpoint at
any load level, and can only be used on systems with minimal
lag between the controlled medium and the control sensor.
Floating control is used primarily for discharge control systems
where the sensor is immediately downstream from the coil,
damper, or device that it controls. An example of floating control
is the regulation of static pressure in a duct (Fig. 29).
FLOATING
STATIC
PRESSURE
CONTROLLER
REFERENCE
PRESSURE
ACTUATOR
DAMPER
Fig. 29. Floating Static Pressure Control.
In a typical application, the control point moves in and out
of the deadband, crossing the switch differential (Fig. 30). A
drop in static pressure below the controller setpoint causes the
actuator to drive the damper toward open. The narrow
differential of the controller stops the actuator after it has moved
a short distance. The damper remains in this position until the
static pressure further decreases, causing the actuator to drive
the damper further open. On a rise in static pressure above the
setpoint, the reverse occurs. Thus, the control point can float
between open and closed limits and the actuator does not move.
When the control point moves out of the deadband, the
controller moves the actuator toward open or closed until the
control point moves into the deadband again.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
20
STATIC
PRESSURE
PICK-UP
PICK-UP
AIRFLOW
C2717