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Old February 21st, 2010, 07:06 PM   #1
NOContact
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PLC selection advice needed for a simple system

The application is a control system for four fuel dispensers.
There are eight inputs:
Four contact closures (or 122V AC)
Four 122V AC sources
Six outputs:
Contact closures (or 120V AC)
The site is now controlled by:
Four time delay relays
Eight SPST relays
The logic is:
Fuel control system closes contact for a dispenser (contact closure or 122 VAC input to PLC)
Timer starts
And Start Pump & pipe leak detector (same contact closure or 120 VAC output from PLC)
When time for leak detection ends (timer), and dispenser's meter starts (122VAC input to PLC), then dispenser-valve opens (120 VAC output from PLC)
There are two fuel pumps, & four dispensers, two dispensers for each fuel.
The goal is to provide more reliable service, fewer points of contact failure, easier service, and lower cost. The cost is now $155 for relay parts and 8 hours for installation.
Are some models or brands better?
Thanks in advance for your advice.
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Old February 21st, 2010, 08:40 PM   #2
Christoff84
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It generally comes down to using what you know/what is available and supported in your area, and how much you want to spend. For something that seems relatively simple, you might try looking at Automation Direct or some other PLC that provides free/lost cost programming software.
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Old February 21st, 2010, 08:59 PM   #3
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You might want to look at something like an Allen Bradley pico. Basically a programable relay. The software is free. They can also be programmed through the small hmi on them. Not the most fun to do, but usable on a budget.
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Old February 22nd, 2010, 01:18 AM   #4
ghettofreeryder
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Take a look at beckhoff automation PLCs. The programming software is free and it is IEC-61131 compliant. Very affordable as well
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Old February 24th, 2010, 09:01 PM   #5
NOContact
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Thanks

Thank you for your comments.

What do you think of the Click series?

"CLICK PLC CPU, 8 AC in / 6 relay out, requires a 24 VDC power supply; 8K steps program memory, 6.4K words data memory, Relay Ladder Logic programming; built-in RS232C programming port and additional RS232C Modbus/ASCII communications port (configurable up to 115.2K baud). Discrete Inputs: 8 AC inputs, 100-120 VAC, 2 isolated commons. Discrete Outputs: 6 relay outputs, 6-240 VAC / 6-27 VDC Form A (SPST) relays, 1.0A/point, 2 isolated commons. Removable I/O terminal block."
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Old February 25th, 2010, 07:10 AM   #6
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Quote:
Originally Posted by NOContact View Post
What do you think of the Click series?
It looks like it fulfills your requirements that were listed in your first post. Because the I/O of the CPU is exactly what you need with no spares, the weak link is going to be the relay outputs. This is a common problem for all most all PLC brands.If you have a relay output go bad, you will not have to buy another CPU unit just buy an output module. If you leave room for the output module in your cabinet this will be easy. Relay outputs generally fail at at around 100,000 cycles. Pay attention to the surge suppression guidelines listed in the manual to get the best life from the relay contacts.

I did not see any current requirements in your original post, so I do not know if the relay outputs will be robust enough for your application. Another thing you did not mention is if your "relay closure" requirements need to be isolated from each other. In the CLICK model you are looking at I believe the first four relays have one "common" terminal and the next two relays have another "common" terminal.

The software is free and intuitive.
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Last edited by milldrone; February 25th, 2010 at 07:39 AM.
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Old February 25th, 2010, 08:38 AM   #7
L D[AR2,P#0.0]
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What are the implications of plc failure causing fuel to be pumped all the time ?
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Old February 25th, 2010, 09:53 PM   #8
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Quote:
Originally Posted by L D[AR2,P#0.0] View Post
What are the implications of plc failure causing fuel to be pumped all the time ?
The result would be loss of usage tracking for filling school buses and owner's trucks. No financial impact as such. Oh, a phone call to me and rush to the site too.

Why do you ask? Is this a common mode of failure?

The same thing could happen with a welded contact though.

The system has a hand valve on the hose so there will not be fuel loss.

The PLC contacts are driving relays and solenoid valves. I have not checked the currents but should be able to find them in the manufacturer's data.
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Old February 26th, 2010, 12:59 AM   #9
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Quote:
Originally Posted by NOContact View Post
The result would be loss of usage tracking for filling school buses and owner's trucks. No financial impact as such. Oh, a phone call to me and rush to the site too.

Why do you ask? Is this a common mode of failure?

The same thing could happen with a welded contact though.

The system has a hand valve on the hose so there will not be fuel loss.

The PLC contacts are driving relays and solenoid valves. I have not checked the currents but should be able to find them in the manufacturer's data.
One should design with the idea of how to make this thing work as we want.

One should put double the time into design criteria
"what do we do if it screws up"
(power loss failed sensor etc etc). Couple best examples aircraft, train control, submarines, fuses, overload protection, emergency gravity brakes on elevators etc etc etc.

Dan Bentler
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Old March 7th, 2010, 08:58 AM   #10
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So, add code to detect and respond to unusual activity or lack thereof?
Timers on usage so that if left unattended, the delivery cannot be longer than it would take to fill an empty tank?
Also, it would be possible to easily reduce the wait time between filling the first and second tank by keeping the pump on and starting the dispenser without the normal time delay required by starting the pump. Naturally would have to include a time limit if no fueling is happening. . .
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Old March 7th, 2010, 01:55 PM   #11
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I do not wish to detract from the thread since this is a plc website but I think a commercially available dedicated product would be a safer alternative.
See this link for a suggestion.:
http://www.gasboy.com/page/fleet_com...as_pump_gasboy
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Old March 17th, 2010, 08:53 AM   #12
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Trouble with the link

The link "http://www.gasboy.com/page/fleet_com...as_pump_gasboy " takes IE to a generic home page. Which device did you have in mind? The system now has a competitive controller (different manufacturer's), but it just controls a dispenser with one contact closure.
The installation needs to have a contact closure or 120VAC source on for five seconds before the dispenser can alow fuel to flow. This powers on a pump and permits a leak check on the piping before fuel can be dispensed. If the dispenser comes on at the same time as the pump, and someone opens the nozzle, the leak detector will restrict the flow from the pump. This results in very angry folks who have to wait a L O O O O O N N N N N G G time to fill the tanks. (There are also other controls that must operate in a particular order. The fueling system just does not provide the sequencing or contacts necessary.)
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