Allen-Bradley Micrologix model selection

I’m a complete PLC neophyte, and I’m trying to implement some automation for the first time. My system is very basic. I want to control two pneumatic actuators, with two position sensors on each, fired by two solenoid valves respectively, with a PLC. I’d like the position sensors to be my inputs and the valves to be my outputs. (For example: actuator 1 is now in open position, open valve 2...). I’m trying to make the whole system 24VDC.

I’m set on using an Allen-Bradley Micrologix (just from what has been used at my company and the free-ish software), but not sure which one. I don’t have many inputs and outputs, I may add more in the future but not many (an E-stop, etc.). Do you guys have any suggestions/guidance? It’s been impossible getting technical support from AB without already being a customer. And like I said I'm totally new to this.

Hello,
Well just from the "Freeish" software comment you have narrowed your search dramatically. (unless you are referring to using your company's software as being free to you)

Non the less I would say go for the Micrologix1100.
You can download a free version of RSLogix500 that will work with this controller and it has Ethernet & Serial capability for comms.

Also I would suggest using interposing relays between your PLC output and your solenoids.


http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1763-um001_-en-p.pdf


BCS

Welcome to the forum.

The first thing I would do is to write down your sequence of events.
Then look at the document and write down your I/O.
Add 10-20% spares for the future.
Select your controller.

E-STOP is REQUIRED for all automation.

Here's a start.
1. turn power on.
2. press the power on pb. the safety relay energizes and turns on the master control relay (MCR).
3. the mcr relay closes and supplies power to the input and outputs.
4. input 0 (power on) is energized.
5. home the machine using manual controls.
6. When you are at home, turn the system to Automatic mode and press the cycle start pb.

keep on going.

hope this helps,
james

Thank you both, already I'm much farther along than I was.

In regards to the type of output: I should be selecting relay output, correct? Even if I'm adding a relay in between anyway? Or does the relay/FET/MOSFET output matter if I'm adding my own? These may be ridiculous questions, like I said, I'm really new.

And from my research I've gathered that I won't be able to power my solenoids via the PLC, I'll need a separate power supply for those. Is this accurate? And but I should be able to power my position sensors directly from the PLC?

To add on to what James Mcquade said, it is important to know that none of the Micrologix families support safety functions like an Estop internally. An external device like a safety relay would be required. The input to the PLC would be from an Aux contact on the Safety Relay, to let the PLC know that the safety system has shut everything down.

There are PLC's that support failsafe functionality, but unless you specifically select one you should assume that the PLC can't control safety.

jr rider said:

In regards to the type of output: I should be selecting relay output, correct? Even if I'm adding a relay in between anyway? Or does the relay/FET/MOSFET output matter if I'm adding my own? These may be ridiculous questions, like I said, I'm really new.

Click to expand...

If you are using the outputs to control a relay, I would think solid state outputs would typically be fine. I'm not 100% familiar with micrologix, but they are almost always 24V outputs. The advantage solide state has over relay is the useful life. A relay has a limited life span, it might only close 10,000 to 100,000 times before it fails. Thats a big number, but solid state devices typically last a lot longer, as long as they are protected from external electrical problems.

If your PLC is directly converting AC into 24VDC, then yes, you will need an additional power supply. If your PLC is powered by 24V DC, then you can probably use the same power supply to power both. However, if they share a power supply, it is important to separate the circuits. Some people prefer using seperate power supplies for main control and field devices for exactly that reason, to keep everything isolated.

Thanks, I will definitely include an E-stop in my system.

So in regards to the Micrologix 1100 (which is the one I had mostly zeroed in on, and Sparky suggested) it comes with a combination of FET outputs and relay outputs, all 24 VDC. If I was to hook my solenoid valve up, I would go PLC FET output -> external relay -> valve

OR

PLC relay ouput -> valve?

Solenoids Coils are inductive loads, when you turn off a solenoid coil the magnetic field that surrounds it can create excessive amounts of voltage as it collapses.

For this reason it is always a good practice to include a interposing relay between the output of the PLC and inductive loads.

If your solenoids current requirements are small enough "Could" you simply wire it to the relay output of your PLC?

Sure, you could....but when the effects of this back lash voltage finally takes a toll on the contacts, would you rather simply change out a relay, or have to move to another output on your PLC, and rewrite your logic to this new output, and you will then have a dead useless output that you can do nothing with.

I have seen it done both ways, but I prefer just to use a relay between the PLC and inductive load Every time no matter what type of output it is.

You can even take precautions a step further and place a RC Snubber , MOV or Diode (depending on your application) across the inductive load to help suppress the effects of this back lash voltage.

But you can look into that once you get your PLC project off the ground.

EDIT:
Actually the coil on the relay you will install is the same kind of inductive load as the Solenoid Coil.
However the load on this relay coil is so small, it not much of a factor. (Depending on the actual relay you use of course)
If you are using 24vdc you could also put a diode across the coil of the relay if you wanted to really get picky about it.
I usually do but I have seen may applications that do not and they work just fine.

BCS

jr rider said:

PLC FET output -> external relay -> valve

OR

PLC relay ouput -> valve?

Click to expand...

The thing with relays is that they are mechanical components and will eventually fail. Do you want to replace (a) a single socketed relay or (b) an entire PLC just because one relay output failed? This is why interposing relays (your first option) are recommended.

That said, we usually drive valve solenoids directly from the FET outputs, with no relays. Of course, you need to keep the current consumption of the solenoid in mind. Most of our valve solenoids are in the vicinity of 0.25A which is well below the typical FET limit of 0.5A. Many here would advise against this method but I can tell you we've experienced very few failures with hundreds of valves being controlled in this manner. Get in the habit of putting diodes across the solenoid coils and the failure rate should go to almost zero. Comparing that to relays... well the number of relay output modules we've replaced would fill a good sized trashcan.

Adding to what Bering C Sparky said.

Relay outputs have a cycle life, so you need to look at the surge / run current of the output and on / off frequency.

I have wired output relays to interposing relays and directly to the device, it all depends on the application.

When we use dc outputs, we purchase terminals with diode protection.

With solid state outputs, you must know what group of people will be working on the units.

In regards to triac outputs, the outputs are on all the time !! Doesn't matter if the output is on or off, they are
LIVE. The only thing that turns off the triac output is the attached load. So when you measure the voltage on an output and there is 0 volts, the load is forcing the triac off, but when you remove that load, the triac will turn on because there is no load.

When writing the program,
1. write down the sequence first.
2. assign the I/o
3. go through the sequence again and add the I/o to the documentation.
4. write the code in order and keep the logic simple to understand. Just because you understand it, doesn't mean maintenance can. So document your program and keep maintenance in the loop. Let them know what you are doing, and welcome their suggestions. They will accept the machine you are programming better and will respect you more. If they cannot fix the problem, guess who gets the calls at all hours of the night. Hint, Hint.
5. ask lots of what if questions in regards to the machine operation. if you can ask the question and do not have an answer, plan for the problem - it will be the first thing to go wrong when you show the machine to your customer or boss.
6. write an instruction manual and document everything as best you can. Include the sequence of events as well with the I/o.

I am adding the programming information for your next step.
Others will add to it, do things differently.
Eventually, you will need to develop your own style of programming.

regards,
james

You guys are all very helpful. Thanks.

Because of the configuration of the micrologix (4 FET outputs and 2 relay outputs) I may run out of FET outputs and only be left with relay ouputs, eventually. When I do should I go:

PLC relay output -> external relay -> valve

in order to protect the PLC, as most are saying?

Unless your valve solenoids pull lots of current, I don't really see much benefit to the relay-relay setup. If there was some reason to think the external relay would fail before the PLC relay then it would make sense. But depending on the relay you use, it may be just as detrimental to the PLC relay as connecting the valve direct.

Most of the relay failures I've seen are in high cycle applications--like once every few seconds for 16 hours a day. If your switching is infrequent then you probably don't have anything to worry about. I'd suggest using a diode across the coil as others have suggested.

All right last thing, I think, before I send in my order.

Does anyone know what the 'Fast' means in AB's fast 24 VDC FET input?

I think I will order the 1763-L16BBB, which has two 'fast' and two regular FET inputs. I'm just curious as to what that means before I actually plac emy order.

The "fast" input is one that can read very quick switching signals and be used with the high speed counter functions in RSLogix. Commonly used for reading encoders and linear transducers (thousands of cycles per second). Without additional configuration, however, the fast inputs will just function as regular inputs and you won't notice any difference in the programming.