Refrigeration – With indicator or tester – Diverse function indicators or testers
Reexamination Certificate
1999-08-05
2001-05-01
Tanner, Harry B. (Department: 3744)
Refrigeration
With indicator or tester
Diverse function indicators or testers
C236S094000
Reexamination Certificate
active
06223544
ABSTRACT:
FIELD OF THE TECHNOLOGY
The present invention relates to control systems for eating, ventilating and air conditioning (HVAC) systems, and in particular to mechanism that detect fault conditions in such systems.
BACKGROUND OF THE INVENTION
Central air handling systems provide conditioned air to rooms within a building. A wide variety of such systems exist such as constant volume and variable-air-volume air-handling units (A.U.). In a typical A.U.
10
, as shown in
FIG. 1
, air returns from the conditioned rooms through the return air duct
11
being drawn by a return fan
12
. Depending on the positions of an exhaust damper
13
and a recirculation damper
14
, the return air may be exhausted outside the building or go from the return air duct
11
to a mixed air plenum
15
, becoming recirculated air. In the mixed air plenum
15
, fresh outside air, drawn through inlet damper
16
,is mixed with recirculated air, and the mixture then passes through a filter
17
, a cooling coil
18
, a heating coil
19
, and a supply fan
20
. The temperatures and flow rates of the outdoor and recirculated air streams determine the conditions at the exit of the mixed air plenum. At most only one of the cooling and heating coils
18
or
19
will be active at any given time assuming the sequencing control strategy is implemented properly and there are no valve leaks or other faults in the system. After being conditioned by the coils, the air is distributed to the zones through the supply air duct
21
.
The cooling coil
18
, heating coil
19
, and dampers
13
,
14
and
16
of air-handling unit
10
are operated by a feedback controller
22
having control logic which determines the proper combination of system components to activate for maintaining the supply air temperature at the desired value at any given time. The controller
22
implements a control strategy which regulates the mixture of outside air with mechanical cooling or heating provided by the coils
18
and
19
to efficiently condition the air being supplied to the rooms. Such control is predicated on receiving accurate sensor data regarding conditions in the rooms and outside the building, as well as within the air handling unit
10
. The controller
22
receives an input signal on line
26
which indicates the desired temperature (a control setpoint) for the supply air temperature. An outdoor air temperature sensor
23
provides a signal indicative of the temperature of the air entering the system and a supply air temperature sensor
24
produces a signal which indicates the temperature of the air being fed to the supply air duct
21
. An optional sensor
25
may be installed to sense the temperature of the air in the return air duct
11
.
A number of faults may occur which adversely affect the operation of the air handling unit
10
. For example, a sensor error, such as a complete failure, an incorrect signal or excessive signal noise, can produce faulty operation. In addition errors may be due to stuck or leaky dampers and valves for the heating and cooling coils
18
and
19
, as well as fan problems.
Previous approaches to providing a robust control system that was more immune from fault related problems utilized multiple sensors to measure the same physical quantity and special sensors for directly detecting and diagnosing faults. Other approaches involved limit checking in which process variables are compared to thresholds, spectrum analysis for diagnosing problems, and logic reasoning approaches.
Many of the previous fault detection and diagnostic techniques for HVAC systems were based on analyzing the system after it has reached a steady-state condition. Observations of process inputs and outputs enter the steady-state fault detection system which then determines if the system has been operating in steady-state. If the system reaches a steady-state condition, then the fault detection system can determine whether faults are present. If the system does not reach a steady-state condition, then the fault detection system issues a command that the system is not in steady-state. Non-steady state operation can be caused by poorly tuned control systems, oversized control valves, or control valves with poor authority.
The HVAC industry is very cost sensitive. Consequently, there often are very few sensors installed on HVAC systems, which makes it difficult to detect faults when only a few parameters are being monitored. In addition, the behavior of HVAC equipment is non-linear and loads are time varying; factors which further complicate accurate fault detection.
SUMMARY OF THE INVENTION
The present invention is a new method for integrated control and fault detection of air-handling systems which are operated by a finite state machine controller. The method can be used to detect faults in existing air handling units without having to incorporate additional sensors. The control system does not have to be in steady-state operation to perform fault detection, i.e., the control loops may be oscillating due to poor tuning or a limit cycle due to oversized valves or too small a valve authority. The present control method is fault tolerant, in that if a fault is detected, the system still is able to maintain control of the air handling unit. The method described is able to detect a number of faults in air-handling systems, such as stuck dampers and actuators, a too high or too low ventilation flow, leaking air dampers, and leakage through closed heating and cooling valves.
The fault detection method includes gathering operational data regarding performance of the HVAC system. That operational data occasionally is evaluated against predefined criteria either for a current state in which the finite state machine controller is operating or for a given transition which has occurred. Based on results of the evaluation, a determination is made whether an fault condition exists.
In the preferred embodiment, the operational data is checked when the controller is in a given state to determined whether the HVAC system control is saturated in a manner that can not be overcome by a transition to another state. Saturation occurs when controller remains in a given operational mode for a predetermined period of time without being able to adequately control the environment of the building. For example, the controller is in the mechanical heating mode, but can not heat the environment to the desired temperature.
Preferably the fault detection method may compare the actual performance to a model of the HVAC system upon the occurrence of a transition between control states. Such a comparison can produce a residual value indicative of the degree that the actual performance matches the model. The magnitude of the residual then is employed to determine whether a fault condition exists and the possible causes.
REFERENCES:
patent: 4325223 (1982-04-01), Cantley
patent: 4611470 (1986-09-01), Enström
patent: 5582021 (1996-12-01), Masauji
patent: 5963458 (1999-10-01), Cascia
Haas George E.
Johnson Controls Technology Co.
Quarles & Brady
Tanner Harry B.
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