Note: Descriptions are shown in the official language in which they were submitted.
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FLOW CONTROLLER
BACKGROUND OF THE I~VE~TION
The present invention relates to a flowcontroller
for controlling the flow of fluid between an inlet and an
outlet dependent upon the pressure drop across the gate
which is controlling the flow. Such a flow ~ontrol
arrangement can be used for controlling the amount of air
moving through an air conditioning duct, the amount of
water or other type fluid moving through a pipe under
control of a valve, the amount of humidified air moving
through an air conditioning duct under control of a damper,
or the like. For purposes herein, flow is defined as the
volume of fluid moving through a pipe or duct or the like
- per unit time.
In the control of variable air volume boxes,
for example, several control approaches havebeenheretofore
adopted. Hot wire anemometers, heated thermistors,
Pitot tubes and deflecting jet streams have been used by
contro1 systems for sensing a characteristic of the fluid
moving through the duct which characteristic can be related
to flow. All of these systems, while they have heretofore
performed satisfactorily, have drawbacks. Typically, the
signal processing systems which are required by these types
of sensors are complex. For example, in t~e Pitot tube
approach, one of the two Pitot tubes senses both velocity
pressure and static pressure and the other tube senses
static pressure. The static pressure sensed by the second
tube must be subtracted from the velocity pressure and
static pressure seilsed bythe first tube in order to eliminate
static pressure from the sensed signal. Then t~e square
root of the velocity pressure must be taken in order to
derive a signal related to t~e velocity of the fluid moving
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through the pipe or duct. Thus, the signal yrocessing
which must be performed even before the control system
can use it -to control the final end element makes the
system overly complicated. Furthermore, the system then
controls velocity, that is the rate at which the fluid
moves through the duct or pipe, rather than flow.
Moreover, pressure drop is easier to sense and
to read than velocity pressure because the typical pressure
drop within a box ll varies from a ~ inch water column t~
4 inches of water column whereas velocity pressure can
vary from .015 inches wat~r column to 1 inch water column.
Pressure drop also gives a better reading of average flow.
If velocity pressure is used, then several readings across
box 11 should be used for developing a signal relating to
flow through the box. However, by sensing pressure drop,
multiple readings can be eliminated.
Multiple readings may also be necessary with
the other types of sensors listed above. For example,
fluid moving through a duct may not have a constant velocity
profile across the duct. Thus, if velocity is to be sensed~
the sensing must be done at several points so that an
average velocity can be determined. Such sensing requires
the use of multiple sensors thus increasing ~he c~mple~ity
of the system.
Similarly, other types of systems which have
been used in the past to sense velocity rather than flow
have required the use of a complex signal processing
arrangement. It is more desirabie, instead, to maintain
a desired flow independent of static pressure changes.
The system which is used to control the flow
must be stable and not subject to droop. Fixed speed,
floating control systems can have stability problems. Such
systems do not provide an output signal which relate to
how much the controlling element such as a damper must
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move in order to provide the correct control but they
rather provide an output having a predetermined speed
regaLdless of the size of the error signal.
Proportional control systems, on ~he other hand,
will provide an output signal based upon the magnitude of
the error signal and thus in effect tells the controlling
element how much to move in response to the error signal.
Proportional control systems are stable ~ut suffer rom
droop because there mus~ be an error signal, i.e. a
difference between the actual condition being s~nsed and
the desired condition, in order to hold the load at a
position to maintain the conditions at the desired level.
Proportional control systems are also generally thought
to be linear and, therefore, the am~unt of correction is
linearly related to offset. The problem of using this
type of approach in flow control is ~h~t, in flow control,
the amount of correction needed is not constant and varies
wit'n the pressure drop and the desired flow.
A third control approach which has been adopted
in prior art systems is to provide a variable speed floating
control system in which the speed of the actuator movement
is dependent upon the amount of deviation between the
sensed condition and the desired condition. miS t~pe of
system provides integral control WlliCh results in a less
~5 complex system and elLminates droop.
The present invention eliminates many of these
disadvantages and adopts many ofthe advantages byproviding
essentially a proportional control system having variable
gain. Thus, the system is stabl~, linear and does not
suffer from droop.
SUMMARY OF THE INVENTION
Thus, the present invention provides a flow
control system having a gate element located between an
inlet and ~n outlet wherein the position of the gate element
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controls the flow of fluid between the inlet and the outlet,
the gate element having a pressure drop thereacross, a
pressure drop sensor for sensing the pressure drop across
the gate element, a processor control responsive to the
pressure drop sensor for providing an output signal having
a value dependent upon the gate element position which
will allow a desired amount of flow between the inlet and
the outlet based ùpon the pressure drop across the gate
element, and a motor responsive to the output signal for
operating the gate element to a position to provide the
desired flow.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages will
become more apparent from a detailed consideration of the
invention when taken in conjunction with the drawings in
- w:~ich the single figure of the drawing shows a schematic
diagram of the present invention.
DETAILED DESCRIPTION
The system according to the present invention
is shown in the drawing controlling a variable air volume
box il in an air conditioning system which receives inlet
air from inlet duct 12 and discharges outlet air from
- outlet duct 13. It should be recognized, however, that
the system according to the present invention can be used
in humidification systems and valvin~ systems such that
either the gate element 14 is a d~mper such as that shown
in the drawing or a valve plug or similar valve mechanism
in the case of a valve system.
As shown in the drawing, s~atic pressure sensing
t~be 15 is located upstream of damper 14 and static pressure
sensing tube 16 is located downstream of damper 14. S~atic
pressure sensing tubes lS and 16 are connected to transducer
17 which transduces the differential pressure between static
pressure sensors 15 and 16 into an electrical signal and
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converts the electrical signal into a digi~al signal for
supply to microprocessor controller 18.
Based upon the pressure drop signal which it
receives from transducer 17, microprocessor controller 18
will determine the position of damper 14 which will provide
for the desired flow through box 11. Thus, microprocessor
controller 18 issues a control signal to stepper motor 19
for driving damper 14 to the position for providing the
desired flow through box 11.
An equation can be written to describe the flow
through box 11 dependent upon the particular box chosen.
For example, if box 11 is chosen to be a Metalaire box,
then the following equation is a good approximation of
the flow of air moving through the box as a function of
t'ne pressure drop across damper 14:
FLOW(CFM) - ~sin (3+45) - sin 45D~Kl ~ (1)
where 9 is the angle of the damper between its position
as snown in thè drawing and its normal closed position
shown. by dached l.ine 21, ~1 is a constant term which
depends upon the design parameters of box 11, ~P is the
pressure drop across damper 14 as sensed by the circuit
15-17, and FLOW is the flow of the air or fluid moving
through box 11 from inlet 12 to outlet 13 as a function
of cubic feet per minute. Equation 1 can be rewritten
as:
. LOW
sin (~+45~) = X~ + .707. (2)
Simplifying Equation 2, the anglP ~ at which damper 14
must be moved in order to achieve the desired flow can be
given by the expression:
FLOW
30 ~ = sin 1 (K~ .707) _ 45~. (3)
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This expression then determines the damper angle for ~he
desired flow.
In a thermostatically controlled system, the
desired flow canbe provided bya thermostat sensing circuit.
~nus, in the drawing, thermostat 22, which may be electric
or electronic but is s~own as a pnèumatic thermostat,
provides a pneumatic output signal to transducer 23 which
converts the pneumàtic signal into a digital signal fox
supply to microprocessor controller 18. Thus, the desired
flow can be given by the expres~ion:
FLOW = K2~T (4)
where K2 is a constant term dependent upon box parameters.
Substitutin~ Equation 4 into Equation 3, the damper angle
as a function of both temperature and pressure can be
given by the expression:
~T
~ ~ = sin~l ( 2 ~ .707) _ 45~. (5)
"~, Kl~
Microprocessor controller 18 can then be set up
for determining the damper angle to provide the proper
flow as a function of thedeviation ofthe actual temperature
from the desired temperature, QT, and the pressure drop
~P across damper 14. In effect then, the thermostat
determines the desired flow. Microprocessor 18 issues a
signal, ~, to stepper motor 19 to maintain this desired
flow even though static pressure fluctuates. The signal
9then is the damper position which will provide the desired
flow.
An alternative to using ~he Equation 5 is to
provide a curve fit for box 11 which involves a looXup
~able for each ~T which will provide damper angle ~ as a
function of the pressure drop across damper 14.
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Thus, microprocessor con~roller 18 provides an
output signal to stepper motor 19. This output signal
has a value representing the damper position which will
result in the desired flow through box 11 as a function
of the pressure drop across damper 14.
