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kcor

micrologix 1400 and temperature control

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I am looking at controlling the temperature of some aluminum molds that we use on our equipment that currently have fuji temperature controllers and j type thermocouples. This setput works great, but our Director is now wanting us to look at using the plc to do this since he saw this at a trade show being done. If i understand correctly if you set the card up as Scaled for PID then with a J type thermocouple which has a temperature range of -346 degrees F up to 2192 degrees F then -346 would equal 0 and 2192 would equal 16383. In our current machines we have a fuji controller going to a solid state relay that has a 4-28vdc input and the fuji is set up as PID Control. If i take the value in from the thermocouple and use that in a PID to control the temperature then with the micrologixs analog output cards you have 0-10v or 4-20ma, what would i have to replace the solid state relay with? Is there a solid state relay that will work on a 4-20ma signal or 0-10vdc? I may be way out in left field on this. has anyone done this and if so could you share some insight and let me know if i am way out in left field or at least half way thinking right. if you have a sample code you would share that would be very helpful. thanks

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Is there any reason to do the upgrade other than the trade show? Temperature controllers work well, especially if they are already tuned and working properly. Switching to a new controller (any new controller) will require you to retune the PID, especially if the new controller uses a different algorithm, which is likely. That said, there are a lot of advantages to using a PLC, just make sure they outweigh the pain of the upgrade process. Are you using a thermocouple input card to wire the thermocouple directly to the PLC, or are you using a standard analog input card and a thermocouple transmitter to change the millivolt signal to 4-20mA? That will make a large difference in how you set things up. Generally speaking, thermocouple cards are a better way to go, but a lot depends on what you're trying to accomplish. For the output a 4-20mA or 0-10V signal is analog, with many different states. They would be used to control the speed of a VFD or the position of a control valve. You're trying to control a heating element which is ON/OFF, which is why you have a solid-state relay. Forget about using an analog output, what you want to do is turn the control variable output of the PID instruction into a pulse width modulation (PWM) signal, which is what the Fuji temperature controller does. With PWM, you control the amount of ON-time in a given period (cycle). So, for example, say you have a cycle of 1 seconds, or 1000ms. When the control variable is at 50%, PWM will turn on the heating element for 500ms, then turn it off for 500ms. At 25%, it will be on for 250ms and off for 750ms. And so forth. Some PLCs have PWM instructions or build the functionality into the PID instruction, but you don't have that luxury with the Micrologix 1400. However, it's fairly simple logic to write. First, set up a self-resetting timer with a millisecond (0.001) time base. Then, set the preset of the timer to the cycle time that you want (in milliseconds). Shorter times usually give you tighter control, but this will be application dependent. You could probably find out what the Fuji is using as a starting point. Next, set up the PID instruction to limit the control variable to that same value. So if you're using a 1 second cycle time, put 1000 in as the control variable max. Finally, add a rung that compares the accumulated value of the timer to the control variable. When the timer is less than the control variable, turn on the heating element. That essentially it. You may want to put in some limits on the PWM to eliminate chatter on the heater at the extreme ends of the control variable. For example, keep it on whenever the control variable is 95% or above. But that should give you enough to get started.

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Thanks. The main reason is he is convinced that it can be done cheaper with a plc since the machine builder told him they could do it with Siemens cheaper. He is brain washed into now thinking Siemens is the way and is cheaper than Fuji controllers or better than Allen Bradley since this machine builder told him that Siemens and temp control with the plc is the way everyone in the world does it. Bottom line is he thinks he knows just enough about controls to be able to decide what we should use which caused a lot of headaches. I was looking at using a solid state realy that has a 4-20ma input. Continental is the one we use with the Fuji and they make one that uses a 4-20ma input. The reason i was looking at doing this is only because a few years ago i did a autoclave machine that used a 4-20ma steam valve to control temperature and with a PID in a micrologix 1200 that project works very well.

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Ah. Enough said. Here is some food for thought. Sounds like the decision is made and that you're moving into PLCs. However, upgrading older machinery can be difficult and you always run into gotchas. And don't forget that you have to take a machine down for a length of time, which halts production. Upgrading can definitely be worthwhile, but only if you are doing it for the right reasons: Parts are obsolete or hard to get, which could lead to a long down time. Machine is difficult to maintain, and upgrading improves this with better diagnostics or replacing worn-out parts. You need to add or improve the functionality (different products, faster speeds, etc.). I've used PID on both AB and Siemens. AB is my strong preference, but that's only familiarity. I've heard from a number of reliable sources that Siemens is great if you're used to it. But if you're background is in ladder logic and/or the AB world, it is a bear to learn. If you have AB experience and not Siemens, then stick with AB. A machine builder might be able to do it cheaper with Siemens, but that's because they can spread the cost of the learning curve over multiple machines. I've never used an analog controlled solid-state relay, but they would certainly make control easier. It would save you development and testing time, though you'll have to weigh that with the added hardware cost. I've done a number of systems with on/off heat sources, and have used the logic I described for those. Including, incidentally, a number of autoclaves, though the ones I worked on had on/off steam valves instead of analog control valves.

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I know this is a relatively old post, but I'm a little behind on my reading. JRoss is on the money with this. At a previous employer, we had a bunch of machines with similar heater circuits. All used PID control in SLC 5/04 PLCs. Some used analog SSRs,to control the heaters, others used digital. Both were from Crydom. I don't think we ever did any testing or tracking to see which was better in the long run, either by performance or cost (long term part replacement). The analog SSRs were more expensive and required more expensive PLC outputs. One one line we were looking at upgrading, we would have had to use a more powerful PLC to handle that many analog outputs (several dozen heater circuits). The digital SSRs are cheaper and use much cheaper PLC outputs, but are a little bit trickier to program. If I had to guess, I would say that the digital units are more likely to fail (more on/off cycles) but don't recall very many of either type failing, certainly not enough to make a huge difference. As for Siemens vs AB, AB is the 900 lb gorilla in North America, Siemens is the 900 lb gorilla everywhere else. We have a bunch of stuff from both and far prefer the AB. I got started on both at about the same time and for me there is no way I will volunteer to work with Siemens hardware when given an option. I've seen way too many weird glitches, bugs, pains and non-intuitive workarounds with it. The AB stuff just works. I've never really done a side-by-side price comparison, apples to apples, between the two. Even if Siemens stuff comes out a bit cheaper, I would rather pay a little more for AB and have something that's much easier to support down the road.

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