Comparing start and end values during an event

[Allison Buenemann]

I want to identify the change in value of my signal after a change has occurred. My signal is generally constant (with typical noise) aside from when an event occurs during which there is a step change in the value of the signal. 

Edited December 1, 2023 by Rupesh Parbhoo
Split topic into problem and solution for the general discussion,

[Rupesh Parbhoo]

In order to compare the actual value of the signal before and after some event, you must first identify the event. 

Method 1: Running Delta. Note: an approach using the running delta function can be preferred to the derivative function (Method 2) when step changes are present as the derivative is infinite during these steps. Using the derivative function works best if the signal is first cleansed/filtered and the interpolation method changed to linear. 

1. Use Seeq Formula to create a new signal that is the running delta of the original signal. Use the syntax: $OrigianlSignal.runningdelta()
   1. Use the “Search Documentation” bar on the right side of the Formula window to learn more about the running delta function and syntax
2. Once the new running delta signal has been created, do a value search on when it exceeds some threshold to identify the time periods over which you would like to know the start and end values.
   1. Note: if the step changes are occurring in both the increasing and decreasing directions, you can still identify all events by using Seeq Formula to take the absolute value of your running delta function ($runningDeltaSignal.abs()) and applying the value search tool to the absolute value signal. 
   2. The time periods identified using value search can be further manipulated to yield exactly the time periods that you are interested in using Seeq formula and the grow(), shrink(), and/or move() functions.
3. Once the events during which the step change occurs have been identified, you can calculate statistics on them using the Signal from Condition tool. The inputs to the tool will be your original signal and the new condition that you’ve created to identify the events. A few statistics that you may be interested in:
   1. Delta: the end value minus the start value of the signal. This would give you the value drop (or gain) during one of these events.
   2. Value at start: this will grab the value of the original signal at the start of the event
   3. Value at end: this will grab the value of the original signal at the end of the event

Method 2: Derivative. Best approach for most rate of change problems, excluding step changes. 

1. Use Low Pass Filter tool or Seeq Formula agileFilter() function to cleanse the original signal to remove typical noise from signal. 
2. Use Seeq Formula to create a new signal that is the derivative of the cleansed signal. Use the syntax: $CleansedSignal.derivative()
   1. Use the “Search Documentation” bar on the right side of the Formula window to learn more about the derivative() function and syntax
3. Once the new derivative signal has been created, do a value search on when it exceeds some threshold to identify the time periods over which you would like to know the start and end values.
   1. Note: if the value changes are occurring in both the increasing and decreasing directions, you can still identify all events by using Seeq Formula to take the absolute value of your derivative function ($derivativeSignal.abs()) and applying the value search tool to the absolute value signal. 
   2. The time periods identified using value search can be further manipulated to yield exactly the time periods that you are interested in using Seeq formula and the grow(), shrink(), and/or move() functions.
4. Once the events during which the value change occurs have been identified, you can calculate statistics on them using the Signal from Condition tool. The inputs to the tool will be your original signal and the new condition that you’ve created to identify the events. A few statistics that you may be interested in:
   1. Delta: the end value minus the start value of the signal. This would give you the value drop (or gain) during one of these events.
   2. Value at start: this will grab the value of the original signal at the start of the event
   3. Value at end: this will grab the value of the original signal at the end of the event