Tips for Use Cases Involving $condition.inverse()

[John Cox]

Seeq's .inverse() Formula function creates a new condition that is the inverse of the capsules of another condition. It is often used for identifying time periods between the capsules in an event related condition. For example, a user may create an "event" condition which represents equipment changes or maintenance events, and they then want to quantify the time duration in between the events, as well as the time elapsed from the last event to the current time. It may be important to statistically analyze the historical time between events, or they may want to be notified if the time since the most recent event exceeds some guideline value. 

A common and possibly confusing issue encountered by users is that the most recent "Time Since...." capsule (created with $EventCondition.inverse()) may extend indefinitely into the future, making it impossible to quantify the time elapsed from the last event to the current time. This issue is easily resolved with the approach shown in step 3 in the example below.

1. The user already has an event condition created named "Filter Changes", which represents maintenance events on a process filter which removes particulates. The user wants to monitor the time elapsed between filter changes and therefore creates a "Time Since Filter Change" condition using $FilterChanges.inverse():

2. Signal from Condition is used to create the "Calculated Time Since Filter Change" signal, and the user chooses to place the result at the end of each capsule. Because the most recent capsule in the "Time Since Filter Change" condition extends past the current time and indefinitely into the future, the duration of that capsule can't be quantified (and it of course exceeds any specified maximum capsule duration). The user may be confused by the missing data point for the signal (see the trend screenshot below), and the missing value is an important result needed for monitoring.

 

3. The issue is easily resolved by clipping the most recent "Time Since Filter Change" capsule at the current time by adding .intersect(past()) to the Formula. This ensures the most recent "time since" capsule will not extend past the current time, by intersecting it with the "past" capsule. The "Calculated Time Since Filter Change" signal (lane 3 in screenshot, based on the clipped condition) updates dynamically as time moves forward, giving the user near real time information on the time elapsed.