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kevinprior

Speeding up detection response times

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Hi, I am using a PLC 5/40 which uses remote I/O, approx 12 racks worth. The problem I am coming across is I am detecting a tray coming past a thru beam sensor, this sensor then fires a pneumatic solenoid to enable a pump to provide a deposit in the tray. The issue I have is that the position of the deposit is varying and it needs to be in the same place each time, I have added various timers in to "tune" the deposit, but it works then sometimes moves. I am putting this down to a scan time issue as the current scan is about 18mS. I have used the immediate input function and this has made the deposit better but I am still getting variations, aside from using a standalone method has anybody got any ideas? Standalone is not ideal as we trim in the values using a panel view which is located nearby. I have seen the trick of scanning the same rack a number of times in the I/O config chassis table, but was wondering if there are any other ideas out there. Thanks

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Trying to service this one higher speed requirement is going to make the rest of your process starved for processor time. I think putting in a stand alone for this function would be best, perhaps a SLC 5/04, then connect to the rest of the system via DH+. The panel view could still be used. Is the panel view DH+, ethernet, DF1? There may be other options, depending on the panel view protocol used.

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Put your handling code in an STI. That way you can not only service it more frequently if necessary, but more importantly, you can stabilize the scan times to a constant amount, such as say every 10ms or every 20ms regardless of other activity. Beware that obviously you have to do something about the IO within your STI as well. Second idea playing off the other suggestion: Buy a micrologix 1100. It has both built-in Ethernet and RS-232/RS-485 so it can use DF1 or DH-485. 10 digital inputs and 6 digital outputs, and 2 crappy (10 bit) analog inputs for about the price of a single AB I/O card. They program using RS-Logix 500 which is very similar to RS-Logix 5. They have their own LCD and input buttons which you can use if you desire but they will talk to other stuff as well. If that's not a PLC-5/40E, you can either do everything in RS-232 or RS-485, or alternatively you can go with a Digi One IAP for about $600. This has 2 serial ports and one Ethernet port and can bridge protocols any way you want to go. So you could effectively upgrade your PLC-5/40 and Panelview to have "Ethernet" ports and then connect the Micrologix in via Ethernet as well. I have several PLC-5/20's all over my plant with this arrangement. It's a lot more cost effective than AB's $1500+ for a Sidecar or almost $10K for an Ethernet processor (at AB pricing for PLC-5, you may as well upgrade to a different PLC platform).

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Your 18ms scan time does not seem high to me unless your trays are very small and/or are moving very fast. Before spending a lot of money try this simple test. If conditions permit try this experiment. Run a long wire to local I/O for your tray sensor and run a long from from local I/O for your trigger signal. See if this improves things. Here is why. The RIO scanner operates Asynchronus to the PLC scan. Racks are serviced in an order {believe rack number but not sure} so one time you may be jsut about to read that rack when the sensor makes another time you may have just passed it. I am pretty sure your RIO update time is much greater than 18 ms using 12 racks and this is the issue. IIM and IOM cannot help this since you are still at the mercy of the RIO Scanner. IF the long wire works and you have a spare port on the 5/40 you might try creating a second RIO network to that rack only. THis should mitigate the RIO update time issue. To look at it fully you'll need to work thru the following: 1. How long is the tray {inches or mm} 2. How long is the tray front to tray front interval {not the tray} 3. What is the rate of speed trays are travelling. {Inches per second or mm per second} 4. Where in the tray is target deposit point. 5. What is the RIO Update Time. 6. What is the scan time. Convert Items 1 to 4 into terms of item 5 and item 6. Each event 1-4 should have at leaast 2 and preferrably 5 to 10 RIO Updates and a greater number of Scans per occurrence.

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I did not know the RIO scanner operated on a different scan to the scan time dispayed I thought it was a combined number. Your idea is very feasable, as it happens this RIO rack I am using was an addition a number of years back and is in fact a few feet away from the main Local I/O and there is a few points free now so I can try this. I will give this a try.

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I have actually put a relay in parallel to the input and used the relay contact to drive the output. This way the PLC still thinks it is doing it's job but the hardwiring which is repeatable is doing the job. If you need a time delay, use a timer instead of a relay. Make sure you document this is the program.

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