I have a trend where I am displaying the speed of a motor. This motor runs at a fixed speed (occasionally the operators will adjust the set point, but this is a maybe-once-a-day type occurrence). The motor starts and stops as necessary according to process conditions irrelevant to the questions at hand.
Here we can see a trend of the motor speed (green/blue), as well as the motor running feedback from the drive (orange). I have configured the motor speed trend pen with “step after” interpolation. I would expect the motor speed trend to essentially mimic the motor running signal with only a very slight trapezoidal shape at the start and the end for the ramp up/down time (a few seconds). However, it doesn’t do this:
Instead the trend line appears to ramp up/down over the entire length of the motor run time, which in most cases is several hours.
When I change the motor speed pen to be a scatter plot, we can see that there appear to actually be data points at these locations:
…which (a) should not be the case because the motor is not ramping up over the course of two hours, it’s running at a fixed speed, and (b) the historical data setup should not be storing data that way.
Out of curiosity, I set up a display with a table, and bound the table data to a tag history query for that motor. What I’m seeing is that as the motor ramps up to speed, data points are stored several times per second, at increments of less than 0.01Hz, while the history configuration suggests that it should not log a value until the difference is 1.0Hz.
Following that, I just have page after page of the exact same speed logged, again, several times per second, despite the fact that it should not be logged while at the same speed for at least an hour, according to the tag history settings. Given that the minimum time between samples is 1s, I can’t see how it could ever be sampling multiple times on the same second in any case.
What’s going on here?
