In state driven EMs I am able to set my sequence (execution order) for all the states, but am I somehow able to set my sequence order for each individual state?
For example, lets say I have a pump with a downstream and upstream valve. When I start the pump, I would like to open the valves first, then start the pump. On the other hand, when I stop the pump I would like to stop the pump first, then close the valves. So between the "Start" and "Stop" states in the EM the execution order is effectively reversed. While there are plenty of workarounds, being able to set the execution order per state (rather than for all the states in the EM) could save a lot of configuration headaches.
This not only applies to pumps, sometimes I'd like to open a valve before the rest, but on closing I'd like it to be first to close. I can't imagine Emerson would not have this functionality in their EMs.
Matt,
You only have two choices:
- List the driving of the pump/valve SP multiple times and ignore the ones to get the operation required for each state
- Move to using a Command Driven EM which can do whatever you want as you are in control of the logic and the order it executes for each Command State.
We very rarely use these EMs because of this limitation and other technical reasons.
Your request to be able to set the order per state is probably not alone asking for this enhancement request for these EM types. You can submit this or any other enhancement request to the User Driven Enhancement Program at www.userideas-emerson.com.
Regards,
Matt
In reply to Matt Stoner:
Thanks Matt, I submitted the enhancement request, hopefully we'll see this feature implemented in the not too distant future.
In reply to Matt Postak:
Hi Matt,
I would highly recommend the Command Driven EM for sequences. The State-Driven works well for valve configurations and anything that doesn't require a sequence.
The State-Driven EMs react to new commands more quickly (about 1 or 2 scans) than Command-Driven EMs and give a some unique functionality in runtime, but the Command-Driven can handle the scenario you described easily.
I am not sure about listing the valve / motor setpoints multiple times technique, but there are was to handle in the State-Driven EM using logic inserted into the EM in the "Function Block" view. (I think that is the correct term.) Making use of this area, you can write to intermediate setpoints and use function blocks such as Timers, AND, Condition, etc.. However, unless simple, consistentent and straightforward, use the Command-Driven EM.
Michael Moody
In reply to Michael Moody:
Yes, Command Driven EMs would be appropriate, but what is nice about state driven EMs is how fast they are to configure and/or re-configure. It is also beneficial that you can see all the EM set points for each device on one screen. This can especially be handy when configuring line routes and such. Having to drill down into multiple sequences to check set-points isn't ideal. As I said in the first post there are plenty of workarounds, but having the ability to set a sequence number per state would really help streamline configuration.