Trend Flatline


I have some simple two pen trends that track a setpoint and actual value. The setpoint follows a constant rate/slope linear ramp for a period of time, then decreases to a constant value soaking period. It seems the during the ramping period the actual value is plotted correctly, but he setpoint value remains a straight line for the duration of the ramp. Once the ramp completes and the setpoint drops to soak value, the setpoint pens is correctly drawn, making both pens correct. The attached images show during and after ramp, with the light blue line being the setpoint.


This is due to the compression mechanism for history. During the period of time that the setpoint value is being show incorrectly, the real value is trending along a single “corridor”. Since values are only stored when they go outside of their corridor, the second value (the top of the ramp, which causes the graph to draw correctly) is only stored after the setpoint value goes back down.

Basically, this is a known issue. In order to fix this, we intend to make the history system use the last value of the tag as the “temporary value”, instead of projecting out the last stored value, as it currently does. The only other solutions would be to not show any projection (in which case you would be posting that the value wasn’t showing for that period of time), or the only current work around: turn on the “max age” setting on the history settings for the tag, and set it to something reasonable. This will cause values to be stored, effectively bypassing the compression.

I’ve escalated the ticket for this, because while this doesn’t come up much, it obviously can be rather misleading. Hopefully we can get it taken care of for 7.4.