Patent Number: 
Section: claims

1. A method for operating a feedback control loop and generating a normalized performance index for the feedback control loop, the method comprising:generating, by a feedback controller, an input for a control process comprising building equipment and one or more measurement devices;operating the building equipment to affect a variable state or condition within a building;operating the one or more measurement devices to measure a value of the variable state or condition within the building and generate a feedback signal representing the measured value;identifying, by the feedback controller, an error signal representing a difference between a setpoint and the feedback signal from the control process;computing, by the feedback controller, 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; andgenerating, by the feedback controller, a normalized performance index using the first EWMA and the second EWMA. 2. The method of claim 1, wherein generating the normalized performance index comprises:using the first EWMA to calculate a numerator;using the second EWMA to calculate a denominator; anddividing the numerator by the denominator to generate a normalized ratio of the first EWMA to the second EWMA. 3. The method of claim 1, wherein:the first function of the error signal is the error signal without modification; andthe second function of the error signal is an absolute value of the error signal. 4. The method of claim 3, wherein generating the normalized performance index comprises:calculating an absolute value of the first EWMA; anddividing the absolute value of the first EWMA by the second EWMA. 5. The method of claim 1, wherein:the first function of the error signal is a time-differenced error signal modified by a parameter; andthe second function of the error signal is an absolute value of the error signal. 6. The method of claim 5, further comprising:multiplying the parameter by a value of the error signal from a previous time step; andsubtracting a result of the multiplying from a value of the error signal for a current time step to generate the time-differenced error signal modified by the parameter. 7. The method of claim 5, wherein generating the normalized performance index comprises:using the first EWMA to calculate a numerator;using the second EWMA and the parameter to calculate a denominator; anddividing the numerator by the denominator to generate a normalized ratio of the first EWMA to the second EWMA modified by the parameter. 8. The method of claim 5, further comprising calculating the parameter, wherein calculating the parameter comprises:identifying a sample period and a specified closed loop time constant for the feedback control loop; anddividing the sample period by the specified closed loop time constant. 9. The method of claim 8, further comprising:identifying an integral time parameter for the feedback controller; andusing the integral time parameter as the specified closed loop time constant. 10. The method of claim 8, further comprising:using the error signal to estimate a dominant time constant for the control process; andusing the estimated time constant for the control process as the specified closed loop time constant. 11. The method of claim 1, further comprising:recursively updating the first EWMA and the second EWMA in response to receiving a new measurement of the feedback signal from the control process. 12. A system for operating a feedback control loop and generating a normalized performance index for the feedback control loop, the system comprising:a control process comprising:building equipment operable to affect a variable state or condition within a building; andone or more measurement devices operable to measure a value of the variable state or condition within the building and generate a feedback signal representing the measured value; anda feedback controller having a processing circuit comprising a processor and memory, wherein the processing circuit is configured to:generate an input for the control process;identify an error signal representing a difference between a setpoint and the 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; andgenerate a normalized performance index using the first EWMA and the second EWMA. 13. The system of claim 12, wherein generating the normalized performance index comprises:using the first EWMA to calculate a numerator;using the second EWMA to calculate a denominator; anddividing the numerator by the denominator to generate a normalized ratio of the first EWMA to the second EWMA. 14. The system of claim 12, wherein:the first function of the error signal is the error signal without modification; andthe second function of the error signal is an absolute value of the error signal. 15. The system of claim 14, wherein generating the normalized performance index comprises:calculating an absolute value of the first EWMA; anddividing the absolute value of the first EWMA by the second EWMA. 16. The system of claim 12, wherein:the first function of the error signal is a time-differenced error signal modified by a parameter; andthe second function of the error signal is an absolute value of the error signal. 17. The system of claim 16, wherein generating the normalized performance index comprises:using the first EWMA to calculate a numerator;using the second EWMA and the parameter to calculate a denominator; anddividing the numerator by the denominator to generate a normalized ratio of the first EWMA to the second EWMA modified by the parameter. 18. A system for operating a feedback control loop and generating a normalized performance index for the feedback control loop, the system comprising:a control process comprising:building equipment operable to affect a variable state or condition within a building; andone or more measurement devices operable to measure a value of the variable state or condition within the building and generate a feedback signal representing the measured value; anda feedback controller having a processing circuit comprising a processor and memory, wherein the processing circuit is configured to:identify a first exponentially-weighted moving average (EWMA) and a second EWMA, wherein the first EWMA and the second EWMA are functions of the feedback signal;calculate a ratio of the first EWMA to the second EWMA; andgenerate a normalized performance index using the calculated ratio. 19. The system of claim 18, wherein the processing circuit is configured to:identify an error signal representing a difference between a setpoint and the feedback signal from the control process;calculate the first EWMA using a first function of the error signal; andcalculate the second EWMA using a second function of the error signal. 20. The system of claim 18, wherein the processing circuit is configured to:identify a specified closed loop time constant for the feedback control loop; anduse the specified closed loop time constant to calculate at least one of the first EWMA and the second EWMA.