Air flow control system and method for a dual duct system

A dual-duct HVAC system for providing a desired comfort level in a room, includes a controller that carries out a simple operation that determines the total open damper positions for dual dampers in a dual duct system to effect a desired air flow. Using the total open damper position information as damper control information, the controller also determines a relative damper position difference required between the two dampers to effect a desired temperature level while also meeting the air flow requirement as determined from the total open damper position information. The combination of the total open damper position information and the relative damper position difference is used as control information to control both dampers. Preferably, the controller gives priority of air flow control over temperature control by determining a valid damper position difference range for use in conjunction with the total open damper position information.

BACKGROUND OF THE INVENTION 
The invention relates generally to HVAC (heating, ventilating and air 
conditioning) systems and more particularly to systems and methods for 
controlling temperature and/or air flow in a dual duct system. 
The temperature and ventilation of an area within a building may be 
controlled through the use of a dual duct terminal box. The terminal box 
typically includes a hot air inlet duct, a cold air inlet duct, a mixing 
area where mixing of the hot and cold air occurs, and an outlet duct for 
passing the mixed air to the area. The temperature and ventilation for the 
room may be controlled by modulating the air flow rate of warm or cool air 
supplied to the mixing area. This is typically accomplished by the use of 
a damper or valves in each of the hot air inlet duct and the cold air 
inlet duct which are typically controlled by a control system. The dampers 
are used to regulate the rate of air flow exiting the mixing box and the 
air temperature exiting the mixing box. Each damper may be positioned in a 
separate air duct. 
Several systems are known for controlling the dampers to obtain a desired 
comfort level within the room. One known system involves treating the HVAC 
system as two separate single-input single-output (SISO) systems wherein 
one control loop operates one damper, usually the cold air damper, to 
regulate the total air flow while another control loop operates the other 
damper, such as the hot air damper, to control the temperature in the 
room. However, a problem arises with such a system since increasing the 
air flow using the cold air damper will also reduce the temperature of the 
air. The control system for the temperature then determines that the air 
temperature is too low and, consequently, opens the hot air damper which 
increases the total flow and leads to the cold air damper closing again. 
As a result, the control performance of the system tends to be poor since 
the system does not hold temperature and flow set points very well. 
Another problem arises when one damper reaches an end of its stroke (i.e., 
in a fully open or fully closed position). At such a point, the HVAC 
system loses control of the variable associated with the damper. For 
example, if the damper is the air flow control damper, the control loop 
for operating that damper reaches a maximum condition so that the damper 
position can not be changed to properly effectuate the necessary air flow 
requirement. 
Another known approach for controlling dual duct systems is to mechanically 
link the hot and cold dampers to control air temperature and to add a 
separate flow control damper in the outlet duct to control air flow to the 
area. However, the added complexity of the mechanical linkage between the 
hot and cold dampers typically reduces system reliability by increasing 
the number of moving parts. Also, the additional flow control damper 
increases the cost and control complexity of the control system. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide an 
improved dual duct control system for overcoming the above problems. 
It is also an object of the present invention to provide an improved method 
and system for controlling temperature and air flow when the dampers are 
not at their limits while providing absolute priority to flow control or 
temperature control when either of the dampers reaches its physical 
limits. 
It is yet a further object of the present invention to provide an improved 
control method and system for a dual duct system which may provide 
suitable comfort levels through the use of a lower cost system. 
Another object of the present invention is to provide such a control method 
and system for a dual duct system which electrically controls direct 
movement of a plurality of dampers in the same direction to control air 
flow and electrically controls direct movement of a plurality of dampers 
in the opposite direction to control temperature. 
It is yet a further object of the invention to provide such a control 
method and system for a dual duct system which generates an electric 
control signal for both dampers in response to each of the air flow 
adjustment signal and the air temperature adjustment signal so that both 
dampers are moved to control temperature and both dampers are moved to 
control air flow. 
An improved control system for a dual duct system includes a set point 
selector for selecting a desired temperature set point for the area and 
for selecting a desired air flow set point for the area. An outlet duct 
temperature sensor generates a feedback temperature signal indicative of a 
air temperature in the area or air temperature to the area. An outlet duct 
air flow sensor generates a feedback air flow signal indicative of an 
amount of air flow to the area or in the area. 
The system includes a controller having a temperature control stage, an air 
flow control stage and a damper position control stage. The controller 
generates an air flow adjustment signal, such as a damper position sum 
signal, based upon the air flow set point and the air flow signal. The air 
flow adjustment signal represents a total amount of damper opening 
position required for the combination of both dampers to effectuate the 
desired air flow. 
The controller also generates a temperature adjustment signal, such as a 
damper position difference signal, that corresponds to the total relative 
position difference required between the two dampers to effectuate the set 
point temperature. The temperature adjustment signal is based upon the 
temperature set point and the feedback temperature signal. 
The controller generates electric damper position control signals to 
electrically control both dampers in response to each of the air flow 
adjustment signal and the air temperature adjustment signal by generating 
damper position control signals for both dampers. Hence the flow 
adjustment signal influences the movement of both dampers, and the 
temperature adjustment signal influences movement of both dampers. The 
controller detects when one of the dampers is at an end of its stroke and 
gives priority to one of the control parameters (air flow and 
temperature). 
In a further embodiment, the controller prioritizes air flow control over 
temperature control by dynamically determining an acceptable damper 
position difference range, based on the air flow adjustment signal and a 
known position of each damper. The acceptable damper position difference 
range represents a range of relative damper position settings wherein the 
damper opening for both dampers achieves the air flow requirement and the 
position difference between the dampers does not cause either of the 
dampers to exceed their stroke. Generating this predetermined acceptable 
operating range improves the response characteristics of the control 
system by substantially preventing a reset wind-up condition. Accordingly, 
the controller generates damper position control signals for each damper 
that fall within the damper position difference range. Priority may 
alternatively be given to temperature control when one of the dampers has 
reached an end of its stroke. 
A method for controlling air flow in a multiduct HVAC system includes 
generating an air flow adjustment signal based upon the air flow set point 
and the air flow signal. The air flow adjustment signal represents a total 
amount of damper opening position required for the combination of both 
dampers to effectuate the desired air flow. The method further includes 
generating a temperature adjustment signal based upon the temperature set 
point and the temperature signal. The temperature adjustment signal 
represents a difference in damper position between the dampers necessary 
to effect the temperature set point. 
The method further includes the step of electrically controlling both 
dampers in response to each of the air flow adjustment signal and the air 
temperature adjustment signal in an effort to effect both the selected air 
flow set point and the selected temperature set point. The step of 
electrically controlling both dampers may include generating an electric 
damper position control signal for concurrently controlling both dampers. 
To effect priority of one control parameter over the other, the method may 
further include determining whether either of the dampers is at an end of 
its stroke and then prioritizing air flow control over temperature control 
when at least one of the dampers has reached an end of its stroke. 
Consequently, the dampers are electrically controlled to effect the air 
flow set point at the expense of attaining the temperature set point. 
Where temperature control is selected as the priority parameter, the 
method may include the step of prioritizing temperature control over air 
flow control when at least one of the dampers has reached an end of its 
stroke.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention utilizes a controller that carries out a simple operation 
that determines the total open damper positions for two dampers in a dual 
duct system to effect a desired air flow. Using the total open damper 
positions as a fixed control value, the controller then determines a 
relative damper position difference between the two dampers that will 
effect both a desired temperature level while meeting the desired air flow 
requirement. In the preferred embodiment (FIG. 2b), the controller gives 
priority of air flow control over temperature control by determining a 
valid damper position difference range. 
Referring to FIG. 1, a novel dual duct HVAC system 10 conditions the 
temperature and air flow to provide a desired comfort level in an area, 
such as a room within a building, through the use of a terminal box 11. 
Blowers (not shown) circulate temperature conditioned air to mixing area 
12 through two separate supply air ducts 16 and 18. Supply air in air duct 
16 is heated by a heater 20 prior to entering the mixing area 12. The 
heater 20 may be any suitable heating mechanism. The supply air in air 
duct 18 is cooled by heat exchanger 22 before entering the area 12. The 
heat exchanger 22 may be any suitable cooling mechanism. An outlet duct 24 
from the mixing area 12 serves as an inlet duct to the room. 
Supply air duct 16 includes an air flow regulating mechanism such as a 
valve mechanism or damper 26 controlled through damper actuator 27a under 
the control of a controller 28. Similarly, air supply duct 18 includes a 
damper 30 controlled through damper actuator 27b also under the control of 
the controller 28. The dampers 26 and 30 are used to vary the degree of 
opening in the ducts, which in turn varies air flow (the amount of hot or 
cold air) entering the mixing area 12. 
A duct temperature sensor 32 generates a feedback air temperature signal 33 
for the controller 28 indicative of the air temperature in outlet duct 24. 
Air flow sensor 34 generates a sensed feedback air flow signal 35 for the 
controller 28 indicative of the outgoing air flow from outlet air duct 24. 
The air temperature sensor 32 may be any suitable air temperature sensing 
device, such as a thermistor. The air flow sensor 34 may be any suitable 
air flow sensing device. The controller may be any suitable microprocessor 
based computer such as a Unitary Controller, manufactured by Landis & Gyr 
Powers, Inc., Buffalo Grove, Ill. 
A set point selector 36, such as a temperature control knob and flow 
control knob, facilitates the selection of a desired air flow set point 
and temperature set point for the room. The set point selector 36 
generates a temperature set point input signal 38 and an air flow set 
point input signal 40 for the controller 28. The controller 28 supplies 
damper control signals 42 and 44 to both the hot damper actuator 27a and 
the cold damper actuator 27b, respectively, to simultaneously effect the 
desired air flow and temperature for the room. It will be recognized that 
although description of the invention is being made with respect to a 
terminal box, the invention may be applied to any suitable dual duct 
configuration. For example, the dual ducts may directly enter the room and 
the outlet duct may draw air from the room. Hence the temperature and flow 
sensors 32 and 34 may be located in the room or any other suitable 
location. 
FIG. 2a broadly depicts one embodiment of the invention and shows the 
controller 28 having a temperature control stage 52, a flow control stage 
54 and a damper control stage 56. Each of these stages may be formed by 
software routines and associated data storage registers or buffers. It 
will also be recognized that discrete electric components may also be 
used. 
The flow control stage 54 determines the amount of required air flow by 
comparing the air flow set point signal 40 to the sensed feedback air flow 
signal 35. The result is a damper position sum signal 60. This required 
air flow amount indicates the required damper position settings for both 
dampers in the two air ducts necessary to achieve the air flow set point. 
The damper position sum signal 60 corresponds to the required sum of both 
damper control signals 42 and 44 necessary to achieve the air flow set 
point. The damper position sum signal 60 therefore corresponds to desired 
air flow control. An increase in the damper position sum signal 60 
requires an increase in air flow from the two ducts. 
The temperature control stage 52 compares the temperature set point signal 
38 with the sensed temperature signal 33 to determine the amount of 
combined damper position opening necessary to reach the set point 
temperature. The result is a damper position difference signal 62 that 
corresponds to the total relative position difference required between the 
two dampers to effectuate the set point temperature. Consequently the 
damper position difference signal 62 corresponds to desired temperature 
control. The output from the flow control stage 54 is the sum of the two 
damper control signals 42 and 44 and the output from the temperature 
control stage 52 is the difference between the two control signals 42 and 
44 when both control parameters (air flow and temperature) can be 
simultaneously achieved. 
The damper control stage 56 determines the damper control signals 42 and 44 
based on the damper position sum signal 60, and the damper position 
difference signal 62. These signals may be represented as data stored in a 
register. The damper control signals 42 and 44 are represented in terms of 
a signal necessary to position a damper to a given open position. Hence a 
damper control signal equal to "85" corresponds to a damper position 
signal required to move the damper so that the damper is 85% open. A fully 
open damper is considered to be 100% open whereas a fully closed damper is 
considered to be 0% open. 
The damper position sum signal 60 and damper position difference signal 62 
are used by the position control stage 56, to perform the following linear 
transformations: 
EQU hot=(sum+difference)/2 
EQU cold=(sum-difference)/2 
where "sum" is defined as: sum=hot+cold; and "difference" is defined as: 
difference=hot-cold. .cent.Hot" refers to the percent open of the hot duct 
damper 26 and "cold" refers to the percent open of the cold duct damper 
30. 
For example, a damper position sum value of 75 represents that the combined 
damper positions for both ducts are 75% of the full open positions. Hence, 
damper 26 could be positioned to be open 50% and damper 30 could be 
positioned to be open 25%, so that the damper position sum value equals 
75% open. Consequently, the damper control stage 56 sends an appropriate 
damper control signal 44 to the hot damper actuator 27a l indicative of 
moving damper 26 to be 50% open. Likewise, the damper control stage 56 
generates a damper control signal 42 for cold damper actuator 27b which 
allows the damper 30 to be 25% open. 
However, since the temperature must also be controlled, the damper position 
difference signal 62 and the damper position sum signal 60, both serve as 
inputs to the position control stage 56 to determine the control signals 
40 and 42. Therefore, each of the two signals 60 and 62 are used to 
generate two suitable control signals 40 and 42 so that each of the 
signals 60 and 62 influence both of the dampers. To illustrate, TABLE 1 
shows various damper control signal valves for signals 40 and 42 generated 
by the controller as derived from the damper position sum signal 60 and 
difference signal 62 using the above mentioned linear transformations. 
TABLE 1 
______________________________________ 
SUM DIFFERENCE 
CASE VALUE VALUE HOT COLD 
______________________________________ 
1 0 0 0 0 
2 100 0 50 50 
3 200 0 100 100 
4 100 100 100 0 
5 100 -100 0 100 
6 100 50 75 25 
7 30 -10 10 20 
8 150 -10 70 80 
9 50 50 50 0 
10 50 60 X X 
11 150 50 100 50 
12 150 60 X X 
______________________________________ 
As shown in Table 1, the control signals are determined based on a damper 
position sum value and a damper difference value. These values are 
numerical representations of the damper position sum signal 60 and the 
damper position difference signal 62, respectively. When the difference 
value is zero, indicating that the damper positions are the same, the hot 
and cold damper position signal values (corresponding to the position 
signal as 42 and 44) are both the same and the sum can be between 0 and 
200 (Cases 1, 2, 3). When the difference value between damper positions is 
positive, the hot damper is open more than the cold damper (Case 4). When 
the difference in damper positions is negative, the cold damper should be 
open more than the hot damper (Case 5). When the sum is 100, the 
difference between damper positions can be between -100 and +100 (Cases 2, 
4, 5). 
As indicated, there are other combinations of the sum and difference damper 
positions that are impossible because of limits on the hot and cold 
dampers (Cases 10 and 12). To adjust to such conditions, the damper 
position sum signal 60 may serve as a priority air flow control value. The 
controller 28, through the damper control stage 56, enforces a priority of 
the damper position sum signal 60 over the damper position difference 
signal 62 when the combination would produce invalid damper control 
signals 42 and 44. The controller 28 applies absolute priority to the sum 
signal 60 over the difference signal 62 so that air flow is given priority 
over temperature control. 
For example, in Case 10 (TABLE 1) where the sum value of the dampers is 50, 
but the temperature control stage determines that the desired air 
temperature (set point temperature) requires a damper difference value of 
60, indicating that additional hot air flow is required, the hot damper 
value (damper control signal 44) may be 50 and the cold damper value 
(damper control signal 42) may be 0 so that the allowable maximum 
difference value is 50 (sum). Therefore the air flow will be controlled 
properly at the expense of the temperature. Accordingly, the control 
system utilizes simple transformations to electrically control both 
dampers to provide the required air flow to the area. 
As described, the damper sum signal 60 and the damper difference signal 62 
serve as control information for generating both interdependent damper 
control signals 42 and 44 so that the system 10 controls (moves) both 
dampers each time flow control or temperature control is necessary. Each 
signal 60 and 62 influence the control of both dampers. Accordingly, the 
aforedescribed simple control system provides a unique de-coupling of flow 
control and temperature control because the sum signal 60 has a strong 
effect on flow control and a more negligible effect on temperature 
control. For example, when no temperature change is necessary, both 
dampers will open the same amount to effectuate the proper air flow 
because the controller 28 electrically controls the damper actuators 27a 
and 27b to move both dampers (two control signals 42 and 44 are 
generated). Unlike conventional dual-duct control systems, both dampers 
are electrically controlled to move to facilitate a change for either air 
temperature or air flow. Each signal 60 and 62 has some control over both 
dampers. However, when a conflict between control parameters arises, one 
parameter is given priority over the other. The controller moves the 
dampers in the same direction to control air flow and moves the dampers in 
an opposite direction to control temperature. 
FIG. 2b shows the controller 28 adapted for giving absolute priority of 
flow control (the sum signal 60) over temperature control (the difference 
signal 62) through the use of a damper position range generating stage 64. 
The damper position sum signal 60 serves as an input signal for the damper 
position range stage 64 and the damper position control stage 56. 
The position range stage 64 and damper position control stage 56 
dynamically determine an acceptable damper position difference range 66. 
An acceptable range includes the range of damper positions wherein the sum 
value is actually met and difference value will not cause either of the 
dampers to exceed their stroke. The temperature control stage 52 use the 
difference range 66 to select appropriate damper difference signals 62 
which will facilitate reaching or approaching the set point temperature 
value 38. 
The damper position range generating stage 64 determines the damper 
position control signal difference range 66 based on the damper sum value 
and determines the minimum and maximum difference signal values. The 
damper control stage 56 gives priority to the damper sum value so that air 
flow takes priority over temperature control when one of the dampers is at 
the end of its stroke or otherwise prevented from moving to a suitable 
position, e.g., when movement of one of the dampers causes the control 
signal to fall outside the difference range. Priority for flow control 
when the dampers are at such physical limits is accomplished by 
dynamically and continuously determining the limits of the damper 
difference signal 66 so that the temperature control stage 52 continuously 
generates the acceptable damper position difference signal 62. It will be 
recognized that other mechanisms may be used to determine whether a damper 
is at an end of its stroke. For example, a position sensor may be affixed 
to the damper and send a signal when the damper is completely open or 
completely closed. 
The controller 28 is calibrated so that a 0 position value corresponds to 
one end of the damper's stroke (i.e., fully closed damper) and 100 
position value corresponds to the other end of the damper's stroke (i.e., 
fully open). The damper position difference range 66 is determined by the 
position range stage based on the following linear transformations: 
(a.) allowable maximum difference value=smaller of (sum or (200-sum)); and 
(b.) allowable minimum difference value=-(allowable maximum difference 
value). 
The position difference range 66 is based on the sum signal so that air 
flow control is given priority over temperature control. The position 
range stage 64 determines whether either of the dampers is at an end of 
it's stroke when the damper difference signal reaches the allowable 
maximum difference or allowable minimal difference. The acceptable 
position difference range 66 provides the temperature control stage 52 
with a proper range of damper difference position settings so that air 
flow control is given priority. 
Alternatively, FIG. 2c shows the controller 28 adapted to give priority of 
temperature control over air flow control. Analogous to the sum signal 60 
of FIG. 2b, the damper position difference signal 62 serves as an input 
variable to the damper position range stage 64 and the damper position 
control stage 56 so that the controller can dynamically determine an 
acceptable damper position sum value range 68. An acceptable range 
includes the range of damper position values wherein the difference value 
is actually met and the sum value will not cause either of the dampers to 
exceed their stroke. 
FIG. 3 shows the method for controlling the comfort level in the area using 
the system shown in FIGS. 1 and 2a-2c. The method starts at block 70. A 
temperature set point is selected as shown in block 72 representing the 
desired temperature in the room or mixing area 12. This may be 
accomplished by programming the set point into the memory of the 
controller or adjusting a temperature set dial such as that found on a 
thermostat control panel, or any other mechanism for adjusting the set 
point. 
As shown in block 74, the desired air flow set point is selected in a 
similar manner as the temperature select point. Based on the feedback air 
flow signal 35 and the selected air flow set point, the controller 28 
generates the damper position sum signal 60 indicating the required damper 
openings from both dampers to achieve the air flow set point as shown in 
block 76. 
The controller then determines the relative hot and cold damper difference 
range 66, based on the damper position sum signal 60 and the known damper 
position as previously described, as shown in block 78. In block 80, the 
controller determines the damper position difference signal based on the 
damper position range 66, the feedback temperature 33 and the set point 
temperature 38 as previously described. 
Suitable damper position control signals 42 and 44 are generated based on 
the damper difference signal 62 and sum signal 60, as indicated in block 
82. The controller 28 electrically controls both dampers in response to 
each of the air flow adjustment signal and the air temperature adjustment 
signal in an effort to effect both the selected air flow set point and 
said selected temperature set point. Hence, the controller outputs 
suitable damper position control signals 42 and 44 to the damper actuators 
27a and 27b as shown in block 84. 
Where the controller is unable to effect both the set amount of air flow 
and the set temperature due to one or both of the dampers being at an end 
of its stroke, the controller will give priority to one of the parameters, 
such as air flow control. The manufacturer may set the priority control 
parameter. The controller determines whether either of the dampers is at 
an end of its stroke and moves the dampers to achieve the set air flow 
such that the dampers are electrically controlled to effect the air flow 
set point at the expense of attaining the temperature set point. The 
process ends as shown in block 86 and the controller continues to repeat 
the method on a continuous basis to ensure a continuous proper level of 
air flow and temperature control of the area. 
FIG. 4a represents the controller output to the damper actuators in terms 
of hot damper and cold damper position values. The gray area 88 is the 
valid range of damper control signals 42 and 44. The range is limited by 
the stroke of each damper. As shown by line 90, when the position 
difference range is zero, an equal damper position is arranged for each 
duct. The X-axis shows the percentage of the cold damper position from 0% 
open to a 100% open, whereas the Y-axis indicates the hot damper open 
position from 0% open to 100% open. 
FIG. 4b illustrates the controller output control signals to the damper 
actuators in terms of the sum and difference values between damper 
positions. The X-axis represents the sum range of both dampers being from 
0% to 200% wherein each damper may be open 100%. The Y-axis represents the 
difference value between damper positions having the range of -100 to 
+100. The shaded area 88 indicates the acceptable operating range for 
suitable controller outputs for the system. 
An alternative method may use the sum value as previously described and a 
ratio of the hot damper position to the sum value so that flow control is 
still prioritized over temperature control. Referring back to TABLE 1 
(cases 7 and 8), when the sum value changes from 30 to 150 at constant 
difference, a mix of hot and cold air can be expected to get much more 
neutral. The ratio of the hot damper to the sum facilitates a similar 
function as the difference in the previously described embodiment. Hence, 
the system may keep a constant ratio between hot and sum so that the 150 
sum would be reached by combining 50 hot with 100 cold. This also isolates 
the temperature control from the flow control. FIG. 5 illustrates the 
controller output in terms of the sum and ratio embodiment just described. 
Another modification to the aforedescribed sum/difference methodology may 
be used where different sized ducts are used or different types of dampers 
are used. It may be beneficial to weight one of the damper position 
settings with a weighing factor to compensate for a difference in duct 
size or air flow volume rate. For example, where a hot duct has a larger 
cross section than the cold air duct, the flow rates may be different. 
Consequently, a hot damper position or the cold damper position may be 
weighted accordingly, to compensate for the change in duct air flow rate. 
The following equations may be used to determine the sum and difference 
with a weighing factor which may then be incorporated in the system 
described with reference to FIGS. 1-3: 
EQU sum=hot+weighing factor*cold 
EQU difference=hot-weighing factor,cold. 
The inventive system eliminates the need for complex mechanical linkages 
between dampers and offers the ability to give absolute priority of one 
control parameter over another. The system generates an electric control 
signal for both dampers in response to each of the air flow adjustment 
signal and the air temperature adjustment signal. Both dampers are moved 
to control temperature and both dampers are moved to control air flow. 
Specific embodiments of a novel system and method for a dual duct system 
have been described for the purposes of illustrating the manner in which 
the invention may be used and made. It should be understood that the 
implementation of other variations and modifications of the invention, in 
its various aspects, will be apparent to those of ordinary skill in the 
art, and that the invention is not limited by the specific embodiments 
described herein. Various features of the present invention are set forth 
in the following claims.