Normalized indices for feedback control loops

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; and

generating, 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; and

dividing 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; and

the 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; and

dividing 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; and

the 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; and

subtracting 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; and

dividing 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; and

dividing 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; and

using 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; and

using 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; and

one 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; and

a 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; and

generate 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; and

dividing 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; and

the 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; and

dividing 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; and

the 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; and

dividing 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; and

one 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; and

a 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; and

generate 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; and

calculate 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; and

use the specified closed loop time constant to calculate at least one of the first EWMA and the second EWMA.