Tuesday, October 1, 2013

MSAT to Automation, MSAT to Automation. Come in, Automation

When I was running cell culture campaign monitoring and we were using PI to review trends to understand physical phenomenon, there were times the trends didn't make any sense.

After digging a little, we found out that the data was simply recording with too little resolution either in the Y-direction or the X-direction.

Here's a short blog post describing the words to say to Automation (as well as some perspective) to get some more resolution in your data.

Compression Settings

If the data seems sparse in the Y-direction (e.g., you expect to see oscillation but only see a straight line), it could because the compression settings are such that too much data gets filtered out. For OSI PI, there are two types of filter settings: (1) exception and (2) compression.

Exception is responsible for filtering out repeat data between the data interface and PI.

Compression is responsible for filtering out repeat linear data within PI (between the snapshot and the archive).

Every point attribute can be viewed from within PI ProcessBook. view pi point attribute And if you find that your exception or compression settings are too wide, view them within PI and make a note of what they ought to be, then go on and tell your Automation team.

In my experience, you'll find a reluctance within Automation for changing the individual settings on points. Generally, there is a standard or a rule that is applied uniformly to the set of points. For example, you're using Broadley-James pH probes in both cell culture and purification and we (cell culture) ask for a 0.005 compdev on bioreactor pH probes, shouldn't the buffer prep pH probes also be set to 0.005 compdev?

Automation has to balance the tension between customer (your) needs as well as defensible system configuration.

Generally speaking, you're going to be asking for changes to compdev of excdev point attributes, and if you're asking for more data to be collected, you want these numbers to be smaller.

Scan Rate Settings

What if after improving compression to filter out less data you still find that there is not sufficient resolution in the data to observe the physical phenomena that you know is happening? Well, the only place left to check is in the scan rate of the data... sparseness of data along the X-axis.

A point's scan rate is set based on a list of pre-defined intervals in the data interface. The data interface is a piece of software that transfers data from the source (control system) to the destination (data historian). If the interface is configured well, it will have sensible scan rates:
  1. Every second
  2. Every 2 seconds
  3. Every 5 seconds
  4. Every 10 seconds
  5. Every 20 seconds
  6. Every 30 seconds
  7. Every minute
  8. Every 5 minutes
It isn't always like this, but very often you'll see these intervals. The scan rates are defined in the interface configuration file and once set, they rarely change. The way it works is this, the first entry in the interval configuration is gets assigned: 1... the second entry: 2... the third entry: 3.

And whatever you set the point's location4 attribute is what it's scan rate is.

So suppose 00:00:05 is the third entry. Then a point whose location4=3 has a scan rate of every-5-seconds.

In a lot of cases, you simply tell your PI administrator you want the scan rate to be "every second," after which he's on the hook for looking up that scan rate in the interface. But FYI, if they said they made the change to the point but the location4 attribute is the same before and after, they're just BSing you.

There are a lot of considerations that need to get balanced when figuring out this stuff.  What's working against you is the default settings that come out-of-the-box with PI... as well as a generation taking the path of least resistance.

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