Systems and methods for steady state detection

1. A building management system comprising:

connected equipment configured to measure a plurality of monitored variables; and

a predictive diagnostics system comprising:

a communications interface configured to receive samples of the monitored variables from the connected equipment;

a steady state detector configured to:

recursively update a mean and a variance of the samples each time a new sample is received;

identify whether each of the samples reflects a steady state or a transient state of operation of the connected equipment using the mean and the variance; and

associate each of the samples to the steady state or the transient state as identified; and

a controller configured to adjust an operation of the connected equipment based on the steady state or the transient state as identified.

2. The building management system of claim 1 , wherein the predictive diagnostics system further comprises a modeler configured to construct a model for predicting performance of the connected equipment using only the samples associated with the steady state, wherein the controller is configured to adjust the operation of the connected equipment using the model.

3. The building management system of claim 1 , wherein the steady state detector is further configured to:

recursively update a slope of the mean, a slope of the variance, and a second derivative of the variance of the samples each time a new sample is received; and

combine the slope of the mean and the slope of variance to generate a combined slope;

wherein the steady state detector is configured to use the combined slope and the second derivative of the variance of the samples to identify the steady state or the transient state.

4. The building management system of claim 3 , wherein the steady state detector is configured to:

determine whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the transient state, determine whether the combined slope is no greater than a first threshold and whether the second derivative of the variance is positive; and

in response to determining that the combined slope is no greater than a first threshold and the second derivative of the variance is positive, determine that the operation of the connected equipment has switched to the steady state and associate the new sample with the steady state.

5. The building management system of claim 4 , wherein determining that the combined slope is no greater than a first threshold and that the second derivative of the variance is positive comprises determining that the combined slope is no greater than a first threshold and that the second derivative of the variance is positive for several consecutive samples.

6. The building management system of claim 3 , wherein the steady state detector is configured to:

determine whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the transient state, determine whether the combined slope is no greater than a first threshold and whether the second derivative of the variance is positive; and

in response to determining that the combined slope is greater than a first threshold, or that the second derivative of the variance is not positive, or both, determine that the operation of the connected equipment remains in the transient state and associate the new sample with the transient state.

7. The building management system of claim 3 , wherein the steady state detector is configured to:

determine whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the steady state, determine whether the combined slope is greater than a first threshold; and

in response to determining that the combined slope is greater than a first threshold, determine that the operation of the connected equipment has switched to the transient state and associate the new sample with the transient state.

8. The building management system of claim 7 , wherein determining that the combined slope is greater than the first threshold comprises determining that the combined slope is greater than the first threshold for several consecutive samples.

9. The building management system of claim 3 , wherein the steady state detector is configured to:

determine whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the steady state, determine whether the combined slope is greater than a first threshold; and

in response to determining that the combined slope is greater than a first threshold, determine whether a change of the variance of the samples is greater than a second threshold; and

in response to determining that the change of the variance is greater than the second threshold, determine that the operation of the connected equipment has switched to the transient state and associate the new sample with the transient state.

10. The building management system of claim 3 , wherein the steady state detector is configured to:

determine whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the steady state, determine whether the combined slope is greater than a first threshold; and

in response to determining that the combined slope is no greater than a first threshold, determine that the operation of the connected equipment remains in the steady state and associate the new sample with the steady state.

11. A method for monitoring and controlling connected equipment in a building management system, the method comprising:

measuring a plurality of monitored variables at the connected equipment;

receiving samples of the monitored variables at a predictive diagnostics system;

recursively updating a mean and a variance of the samples each time a new sample is received;

identifying whether each of the samples reflects a steady state or a transient state of operation of the connected equipment using the mean and the variance;

associating each of the samples to the steady state or the transient state as identified; and

adjusting an operation of the connected equipment based on the steady state or the transient state as identified.

12. The method of claim 11 , further comprising:

constructing a model for predicting performance of the connected equipment using only the samples associated with the steady state; and

adjusting the operation of the connected equipment using the model.

13. The method of claim 11 , further comprising:

recursively updating a slope of the mean, a slope of the variance, and a second derivative of the variance of the samples each time a new sample is received; and

combining the slope of the mean and the slope of variance to generate a combined slope; and

using the combined slope and the second derivative of the variance of the samples to identify the steady state or the transient state.

14. The method of claim 13 , further comprising:

determining whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the transient state, determining whether the combined slope is no greater than a first threshold and whether the second derivative of the variance is positive; and

in response to determining that the combined slope is no greater than a first threshold and the second derivative of the variance is positive, determining that the operation of the connected equipment has switched to the steady state and associating the new sample with the steady state.

15. The method of claim 13 , further comprising:

determining whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the transient state, determining whether the combined slope is no greater than a first threshold and whether the second derivative of the variance is positive; and

in response to determining that the combined slope is greater than a first threshold, or that the second derivative of the variance is not positive, or both, determining that the operation of the connected equipment remains in the transient state and associating the new sample with the transient state.

16. The method of claim 13 , further comprising:

determining whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the steady state, determining whether the combined slope is greater than a first threshold; and

in response to determining that the combined slope is greater than a first threshold, determining that the operation of the connected equipment has switched to the transient state and associating the new sample with the transient state.

17. The method of claim 13 , further comprising:

determining whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the steady state, determining whether the combined slope is greater than a first threshold;

in response to determining that the combined slope is greater than a first threshold, determining whether a change of the variance of the samples is greater than a second threshold; and

in response to determining that the change of the variance is greater than the second threshold, determining that the operation of the connected equipment has switched to the transient state and associating the new sample with the transient state.

18. The method of claim 13 , further comprising:

determining whether a current state of the operation of the connected equipment is the steady state or the transient state;

in response to determining that the current state is the steady state, determining whether the combined slope is greater than a first threshold; and

in response to determining that the combined slope is no greater than a first threshold, determining that the operation of the connected equipment remains in the steady state and associate the new sample with the steady state.

19. A heating, ventilation, or air conditioning (HVAC) system comprising:

connected equipment configured to measure a plurality of monitored variables;

a predictive diagnostic system configured to:

receive samples of the monitored variables from the connected equipment;

recursively update a variance of the samples, a slope of the variance, and a second derivative of the variance each time a new sample is received;

determine whether the slope is no greater than a first threshold and whether the second derivative of the variance is positive; and

in response to determining that the slope is no greater than the first threshold and the second derivative of the variance is positive, associate the new sample with a steady state operation of the connected equipment; and

a controller configured to adjust an operation of the connected equipment based on whether the new sample is associated with the steady state operation or a transient state.

20. The HVAC system of claim 19 , wherein the predictive diagnostic system further comprises a modeler configured to construct a model for predicting performance of the connected equipment using only the samples associated with the steady state, wherein the controller is configured to adjust the operation of the connected equipment using the model.