Ideas on a good PLC to use...

[flyingsparks]

I need a PLC for control of a mill. I was thinking of using a Siemens Logo, I think (havn't fully worked it all out yet) but 4 I/O should be enough and LOGO's come in this for a reasonable price. My problem is this...

I havn't done much PLC work since I left college, (8 years ago!) and even then it wasn't studied that much so a may be a bit rusty!
Are LOGO's capable of having timers programmed?
Is the PC software ladder logic and is it cheap? (I know you can program them on the screen, but figured it would be easier using PC software!)

Anyone have any other suggestions as a good alternative brand and any help appreciated!?

Thanks
Chris

 

[bernardgreen]

What sort of mill and how complex will the control funtions be.

Four I/O seems impossibly too small. START and STOP will take two leaving only two for the mill motors etc.

The one time I used Siemens PLC's was a few years ago. Timers were possible and easy to impliment but we found a few odd things like some registers ( data values ) were used for internal functions without it being clear in the documentation. We found it when two of our variables were changing even though the machine was at standstill.

Last PLC I used was from B and R Automation ( UK office in Peterborough ) which was extremely flexible and once the programming language was understood easy to program ( I seldom use PLCs so if I could cope any one could ) Also B&R have ( had ) a very helpful applications department who wrote most of the code for the project. There would a cost to buy the programming suite of software tools.

A client is using a very small " PLC " like system called a "Visual Touch Controller " CT1721 from Comfile which is a mini PC with colour touch screen, 80K of flash memory, up to 82 I/O points and 3 serial ports. He likes it as it can be programmed using Visual Basic on a PC and then the file downloaded to the VTC.

If it really is only four I/O then I would look at a PIC processor with a bit of support hardware.

 

[flyingsparks]

Thanks BG. It is a fairly small and simple set up really, hence only the need of 4 I/O. I have done a bit of design on the LOGO software, I'l upload a screen shot of it below.

Another question, if I have a 240V signal as an input, would that be classed as analogue or digital? This is probabily a silly question, but cant quite work it out! Ie if a switch turns on and sends 240v back and into the I1 of the PLC, to make the out put on.?

The reason I want to use a PLC instead of contactors etc, is because I am keen to learn more anout PLC and programming, and figured this would be a good project to use one on. The job is for my brother, and cost (to a degree) is not so important as I will be learning along the way. Thanks Chris

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• flyingsparks
• 15 Jan 2012
• 1

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[bernardgreen]

I make it 6 I/O , 4 in and 2 out.

Ie if a switch turns on and sends 240v back and into the I1 of the PLC, to make the out put on.?

As the switch is ON or OFF it is a digital signal as regards logic but like all inputs it has an analogue value in volts which is either above the ON threshold or below the OFF threshold.

Most PLC's have inputs that react to 24 volt ( or lower ) DC sources. If the PLC cannot take 230 volts directly on it's inputs ( few can ) then a 230 AC volt to 24 volt DC ( or maybe 5 volt ) interface is require. I use Opto 22 modules which were designed for the function and are very reliable. The one you would need takes a 230 volt ON OFF signal and produces an opto isolated switched output that can drive a PLC input ( may require a pull up resistor depending on the way the PLC input operates )

http://www.opto22.com/site/pr_details.aspx?cid=4&item=IAC24A (24 volt PLC )

http://www.opto22.com/site/pr_details.aspx?cid=4&item=IAC5A (5 volt PLC )

 

[bhm1712]

Ladder code is easier than function block and other methods in my opinion, the software you have posted a pic of looks fairly straight forward but not my cup of tea. Allan Bradley RSlogix5000 and Mitsubishi GX developer (now GX Works) are my 2 favourite pieces of software for PLC's. However these cost in excess of £1000 to buy for the pc.

In this case i would use something like a mitsy FX1N. hows the control circuit going to be wired? you can get various plc's with 230 volt control circuits in sink and source ( source = 230 volts on the input for on, sink = loss of 230 volts on the input for on)

Are you using the outputs to drive contactor/relay coils? or do the outputs connect direct to the mills PCB inputs??

 

[bhm1712]

having just noticed something in your first post: Just to point out that a PLC should not be used to drive motors or other equipment! it sounds like that what you want it to do?

The PLC is a controller only and will not handle the loads from motors etc. It should be used to control contactors and relays which turn the equipment on or off.

 

[bernardgreen]

you can get various plc's with 230 volt control circuits in sink and source ( source = 230 volts on the input for on, sink = loss of 230 volts on the input for on)

This is true but those that have the 230 volt interface circuitry built as non replacable parts on the main board can become write-offs when spikes or surges on the 230 volt system damages the board's I/O components. Using separate I/O modules for the 230 volt interfaces often reduces repairs after mains problems to a £10 plug in component being changed.

 

[bhm1712]

you can get various plc's with 230 volt control circuits in sink and source ( source = 230 volts on the input for on, sink = loss of 230 volts on the input for on)

This is true but those that have the 230 volt interface circuitry built as non replacable parts on the main board can become write-offs when spikes or surges on the 230 volt system damages the board's I/O components. Using separate I/O modules for the 230 volt interfaces often reduces repairs after mains problems to a £10 plug in component being changed.

On the low cost units this may be the case. This is why i ask about the control circuit - it sounds as though he wants to supply power to the motors using the plc, but since this is not really possible, then it makes perfect sense to design a seperate 24 volt control circuit and buy a cheaper PLC

 

[ricicle]

Bear in mind that the PLC cannot be used for the safety control of the mill - this must be done with hard wiring (probably with a safety relay depending on the assessed risk level)

 

[bhm1712]

on the new Allan Bradley GuardLogix, the safety relay is part of the PLC, but the SIL rating still needs to be taken into account, as most ( despite what the rockwell salesman will say) do not conform to the higher SIL ratings.

Is the mill going to be used in a commercial setting or purely for home DIY projects?

 

[flyingsparks]

Thanks for all your points. Yes the PLC would be switching contactors to control the motors, and the E-Stop circuit is going to be seperate from the PLC with another contactor. I will look into those other types mentioned - Thank you all!!

 

[flyingsparks]

on the new Allan Bradley GuardLogix, the safety relay is part of the PLC, but the SIL rating still needs to be taken into account, as most ( despite what the rockwell salesman will say) do not conform to the higher SIL ratings.

Is the mill going to be used in a commercial setting or purely for home DIY projects?

The mill is on a farm, so yes you could say it is commercial, but its my brothers farm and no one works there!

 

[ricicle]

...its my brothers farm and no one works there!

Not even your brother ?

 

[ericmark]

For simple programming the mitsubishi was my preferred. But as it became more complex then Alan Bradley I found better. Much depends what you want to do SCADA was easy with Alan Bradley when compared with mitsubishi but HMI was easy with mitsubishi.

Some systems give away software seem to remember IFM ASii was free others charge a fortune and with some there as a few options. mitsubishi started with Medoc and you can down load that for free I think. Then it progressed to another name escapes me and then GX Developer with is expensive.

RSLinx and RSLogix for Alan Bradley is also expensive.

Some times suppliers will on QT give away software to get custom. But start point look at LC Automation.

 

[bhm1712]

Medoc is very outdated and not the easiest to use. Mitsubishi GX developer is not really much more complex than RSlogix (AB) its just different, but i do enjoy writing code in Logix compared to GX developer.

The problem with AB is the cost of the stuff. Compared to other manufacturers its really expensive, but you do get what you paid for.

LC automation are not much help as of late, even to paying customers!! still trying to get an answer from 6 months ago about a CC-Link fault i am having issues with at work!

For this mill, i think going down the route of buying PC software is extreme, especially if its being treated as a training exercise. Might be best to get the basics by using the handheld/displays on some lower end products.