Energy Efficiency

Evaluation of an AHU Fault Detection Scheme Based on Finite State Machine Sequencing Control

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Principal Investigator: John E. Seem, Johnson Controls, Inc., and John M. House, Iowa Energy Center

Technical Area: Energy Efficiency

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Abstract:
Numerous studies in the literature cite the successful application of fault detection and diagnostic methods to simulation and laboratory data from central air-handling units; however, unstable control, a lack of standard control sequences, and data handling challenges have limited their application in the field. This paper describes a new method for integrated
control and fault detection of central air-handling units that addresses these challenges found in the field. The method is based on finite state machine sequencing logic and utilizes thirteen residuals that compare measured conditions to model-based expected conditions. The residuals are derived from mass and energy balances applied at specific operating conditions where steady-state conditions are imposed on the air-handling unit by the sequencing logic. For faulty operation, one or more of the residuals is expected to have a value that is significantly different from zero, the expected value for normal operation.

The simulations were of a one-year period and utilized a 2.5 second time step to enable local loop control of the various AHU processes. The faults simulated consisted of the following: 1) Positive and negative offset faults of the supply air temperature, return air temperature, mixed air temperature, and outdoor air temperature; 2) Recirculation air damper stuck open, stuck closed, stuck half-way open; 3) Leakage of the recirculation air damper; 4) Cooling and heating coil valves stuck partially open (faults were simulated individually, not simultaneously); and 6) Leakage of the cooling and heating coil valves (faults were simulated individually, not simultaneously). In all, 16 fault cases were simulated.

With the exception of the return temperature sensor offset faults, at least two of the 13 residuals are noticeably affected by each fault and could be used as the basis for fault detection. In addition, the results indicate that residuals r1 and r2, which require only measurements of the supply air and outdoor air temperatures, may be adequate for detecting all of the damper and valve faults considered, with the exception of the stuck closed recirculation air damper fault. With the addition of return air temperature measurement, residuals r3 and r4 can be calculated and used to detect the stuck closed recirculation air damper fault. The results indicate the residuals enable these common air-handling unit faults to be clearly differentiated from normal operation, although the degree of differentiation will clearly depend on the severity of the fault.

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