EWMA Diagnostics - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

Controller Tool Help

Brand
Johnson Controls
Metasys
Product name
Controller Configuration Tool
Document type
User Guide
Document number
LIT-12011147
Version
13.1
Revision date
2019-12-04
Language
English

The PID controller blocks collect data about their inputs, outputs, and internally calculated variables. Inside the PID blocks, Exponentially Weighted Moving Averages (EWMA) diagnostic value calculations minimize memory requirements, reduce communication traffic, and provide an easy way to analyze collected data.

Starting at Release 5.1, all zone control based system selection trees (terminal units only; not AHUs) automatically expose the following four values for each Output Controller. The XXX label in the descriptions relates to the type of control (for example, Zone Heating, Zone Cooling, and Flow).

  • XXX-ABSERROR (Absolute Error EWMA)—The Absolute Error EWMA is the absolute value of the calculated setpoint minus the present value of the controlled variable (for example, temperature, and flow). These values are calculated in the units of the controlled variable. The moving average calculation for this value is performed at the effective period of the PID. For example, the effective period for zone temperature-based control is 60 seconds, and a VAV Box Controller box flow loop is 1 second. If this value increases in reference to the type of loop under control, the increase may indicate a control or mechanical system issue.

  • XXX-ABSEFFORT (Absolute Effort EWMA)—The Absolute Effort EWMA is the absolute value of the change in the present value from one execution of the PID to the next. These values are calculated in units of %. The moving average calculation for this value is performed at the effective period of the PID. For example, the effective period for zone temperature based control is 60 seconds, and a VAV Box Controller box flow loop is 1 second. If this value increases more than 5% to 10%, the increase may indicate excessive controlled device movement.

  • XXX-ERROR (Error EWMA)—The Error EWMA is the controlled variable setpoint minus the present value of the controlled variable. These values are calculated in the units of the controlled variable. The moving average calculation for this value is performed at the effective period of the PID. For example, the effective period for zone temperature-based control is 60 seconds, and a VAV Box Controller box flow loop is 1 second. If this value increases or decreases in reference to the type of loop under control, the value change may indicate a control or mechanical system issue. If a loop is cycling above and below its setpoint, the XXX-ERROR value tends to be close to zero. The XXX-ABSERROR is typically large with respect to the type of control loop under control.

  • XXX-EWMA (Present Value EWMA)—The Present Value EWMA is the average position of the PID present value. These values are calculated in units of %. The moving average calculation for this value is performed at the effective period of the PID. For example, the effective period for zone temperature based control is 60 seconds, and a VAV Box Controller box flow loop is 1 second. This value can be used in future diagnostic application work.

When using these values, please note that the state-based control logic stops the calculations when the PID is not active. As a best practice, keep in mind the context of the Occupied Zone Status (ZNT-STATE) value as you use this data. The present value of this state coincides with a specific Output Controller module’s PID diagnostic values. The values for the Output Controllers that are not active remain at the values from the last time the calculations ran. Additionally, you may want to view the output status of the Output Controller to show which PID is active (XXX-OUTSTATE).

EWMAs from several devices can be compared to easily identify devices with poor relative performance. Use the Diagnostic—VAV Box Error Analysis and Diagnostic—VAV Box Effort Analysis Summary Definitions in the online configuration user interface.