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jakerjaker

Best PLC/PAC for small remote SCADA system

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Hi all, I'm a newbie to the forum and to PLCs in general so forgive me if this is covered elsewhere and if I get some facts wrong. I am looking at outfitting a few remote sites with SCADA systems for remote control of sluice and crest gates. The gate actuators have fairly simple control schemes (120VAC and 24VDC loops to "raise" and "lower" coils/solenoids) so my I/O is fairly small. These SCADA systems will be accessed through VPN via the internet (any suggestions on security for this type of remote access is welcome too). I was wondering what people would suggest for a fairly small inexpensive system. I have been looking at the Opto22 SNAP PAC controller as a possibility. In theory I like the higher level functional block diagram programming because, for one, I've never programmed ladder logic but it seems fairly limited and focused on discrete relay inputs to me (I'm sure I'll get myself into trouble on this one). For example, I'd like to be able to do slightly more complicated control loops for setting flows/gate setpoints. This would involve constant monitoring of position sensors and River stage/flow sensors. I'm sure this is possible in LL it just seems a little more straightforward to me in FBD. Admittedly I come from a high level programming background. If people have LL vs. FBD experience I'd love to here about it. I can see how the higher level FBD could get you into trouble as simpler is better right? Anyway, here is what I believe I need for I/O and comm for each site. (3) 120VAC digital inputs (sense 120VAC common from actuator in remote control) (6) 120VAC digital outputs isolated (routes 120VAC common from actuator back to raise or lower coil) (5) 4-20mA analog inputs (position sensors from actuators and overcrest inclinometers) (1) 24VDC digital input (sense 24VDC common from overcrest gate) (2) 24VDC digital outputs (routes 24VDC common to raise or lower solenoid) (9) analog voltage inputs w/ completion resistors for 20A CTs (for monitoring current draw on (3) 3-phase motors) (2) analog voltage inputs w/ completion resistors for 200A CTs (for monitoring current draw on (1) dual leg 240VAC motor) (2) ethernet 10/100 Mbps (need to connect to VPN router/DSL modem as well as to an existing Datalogger to retreive other parameters such as Lake and River levels) (1) RS-232 serial (potentially for adding backup dialup modem access) Any suggestions and advice for a small remote system like this would be much appreciated. Thanks!

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I suspect the mixed I/O is going to limit your choices somewhat, particularly the 120 volt inputs mixed with analog inputs. I have used 120 volt terminal block relays as a voltage conversion to get around that limitation on small controllers. You might want to look at Omron CJ2M or CP1H controllers as an option. I have no idea about their availability to you though.

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Do you truly need two ethernet ports, or can you use a switch or hub? In order to keep things small, I'd recommend getting all of one type of digital inputs. For example, you could get all DC inputs, and use relays to convert the AC inputs to DC. Likewise, you can get all relay outputs, which can handle AC or DC. Most major brands should be able to meet your needs. You're better off finding something with good local support. In addition to Omron, you could also check out the Mitsubishi FX3G.

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Good points. I will likely have a router/hub and I hadn't thought of using all DC digital I/O with relays. Of course I still need the analog inputs which seem to be the most costly. I am considering going the AB Micrologix or Compactlogix route after meeting with a good local rep. I don't yet know the cost of an AB system though. I imagine quite a bit more than Opto22 or Omron.

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AB is the cadillac of the PLCs here in the northest, and with that comes the high price. I like working with them, but most of my customers do not like paying for them. The siemens S7-1200 series processors have great software at reasonable cost, with FB programming that is straightforward for someone with your background. LL is standard as well. A lot of AB guys don't like siemens because it is different, but it really comes down to the old Ford vs Chevy argument. They are both doiing the same thing, you just need to get used to the way each performs similar tasks. The TIA portal software is an all inclusive software package that incorporates the addressing schemes of all devices right into the development, and exchange of tags between processors and Operator interface is done by drop and dragging. VPN networking is autodetected as a choice for connection once your client is open, so setting up lenghty communications pathways through multiple softwares is not an issue. The Comfort level HMI also has a web server built in, with trending and data logging built in as well. An example is here http://imtsolar.com/page/Projects/46.html of how this system can perform.
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Have a look at the Omron CP1E - it has built in analogues - you can get Ethernet/serial converters to plug into the units as well - they are well priced but limited in their application - should be OK for SCADA I would think but you need to check the manuals to be sure. USB interface for programming too - no special cables.

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