Feedback Loop Performance Index Using EWMA Normalization
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Solution Overview
Problem
Traditional methods for calculating performance indices of feedback controllers in HVAC systems are difficult to evaluate and compare across systems, as they depend on the specific system and unmeasured disturbances, and are computationally expensive, limiting their application to offline batch analysis.
Innovation Solution
A method using exponentially-weighted moving averages (EWMA) to generate normalized performance indices, allowing for real-time calculation and comparison of feedback control loops by computing a first EWMA of the error signal and a second EWMA of its absolute value, with optional modification by a parameter derived from the closed loop time constant.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional performance measures are used to quantify feedback controller performance, then the measures can capture system-specific characteristics, but the measures become difficult to evaluate and cannot be directly compared across systems or controllers
Solution Approach 1:
The patent transforms the performance measurement from system-dependent parameters (mean absolute error) to a normalized parameter (performance index between 0 and 1) that is independent of system characteristics. This normalization allows direct comparison across different HVAC systems and controllers while maintaining measurement precision through the mathematical transformation of the error signal.
2Measurement precision
If traditional methods are used to calculate normalized index values, then normalized performance indices can be generated for comparison, but the calculation becomes computationally expensive and limited to offline batch analysis
Solution Approach 1:
The patent segments the calculation into two parts: an offline component that pre-calculates system characteristics (denominator of the performance index), and an online component that only calculates the numerator using real-time error signals. This segmentation enables real-time performance monitoring without the full computational burden of traditional offline-only methods.
Solution Approach 2:
The patent performs preliminary calculation of the denominator (which depends on system characteristics) offline before real-time operation. This preliminary action stores system-specific parameters that can be quickly combined with real-time error measurements to generate normalized performance indices, eliminating the need for expensive real-time batch processing.
3Measurement precision
If offline batch analysis is used for performance evaluation, then computationally expensive calculations can be performed, but real-time performance monitoring and immediate issue detection are not possible
Solution Approach 1:
The patent divides the performance index calculation into offline system characterization (denominator) and online performance monitoring (numerator). This segmentation enables real-time calculation by only requiring computation of the numerator component during operation, while the computationally intensive denominator is pre-calculated and stored for reuse.
Data Source
AI summary
A feedback controller for generating and using a normalized performance index for a feedback control loop is shown and described. The feedback controller is configured to generate an input for a control process, identify an error signal representing a difference between a setpoint and a feedback signal from the control process, compute a first exponentially-weighted moving average (EWMA) of a first function of the error signal and a second EWMA of a second function of the error signal, and to generate a normalized performance index using the first EWMA and the second EWMA.