Looking for Recommendations

Hi all.

I have been asked to pull prices together for a very small system where I will have just a few I/O. I am looking for hardware recommendations. I have only a few experiences with very small PLC systems and those are not too positive. I was hoping some of you have some really good experiences that you will pass on.

This system will require two analog inputs (speed pots), two analog outputs (speed ref to AC drives), about 10 digital I/O (start, stop, limits, etc.), and a small HMI. The operator will only need to enter a move distance (0 to 240 inches with three decimal display) or a rotational move distance (0 to 360 degrees with three decimal display). He will then tell the machine to rotate left or right, or move forward or backward. For safety sake, I will be installing a discrete stop button for both movements as well as an E-stop.

I have used the Siemens S7-200 series PLCs quite a bit and don’t really think I want to use one of them. My main reason is noise. I have had a lot of problems with these units “seeing” noise on the inputs that the PLCs interprets as signals. The built-in adjustable filtering isn’t always enough to overcome this either. Since this system will be going into a heavy industrial environment, I expect electrical noise to be quite abundant.

One of my biggest considerations is, of course, price. As always, I want to keep this as inexpensive as I reasonably can.

Any comments would be appreciated.

Steve

Sounds like a perfect job for a DIRECT LOGIC 05 - Automationdirect.com PLC

Hey Steve,

That must be some major noise. Where is it coming from? Is your I/O wiring in close proximity to motor leads, welding leads, or any other potential source of noise. If so, you may be able to eliminate your noise problems by separating your I/O wiring from those noise sources. I haven't used an s7-200 before, but I wouldn't think that a Siemens plc would be any more susceptible to noise than any other brand. I've definitely been wrong before, but this really sounds more like a wiring problem than a plc problem.

Now on to you request: I have used Mitsubishi FX series plcs in rail welding machines that produce quite a bit of noise and have never had a noise related problem. But, I am always careful to keep the control wiring separate from the welding leads.

Good Luck -
Kevin Hammond

Yeah, I would not be too surprised if a good deal of my noise problems are because of poor wiring practices. I work for an OEM whose history for going through EE's and Electricians is pretty dynamic. Some of the old machines I have seen in the field were wired a bit haphazardly I fear.

Interestingly enough though, The S7-200 series is the only one where I have seen noise problems on the I/O so far. There are about 5 or 6 different brands out there that I know of and none of the others have seen this issue. In fact, it was a Siemens guy that alerted me to the problem in the first place. He said (in his opinion) it was a weakness in the design.

Anyway, I also try to keep my control and power wiring as separate as possible.

Steve

Take a look at the OCS product line from GE Fanuc. It's a PLC with integrated HMI, available as a 'brick' or modular.

http://gefanuc.com/products/controllers/category.asp?cat_id=9

The programming software is free.

How about c7 ?

How will you think about a SIMATIC C7-621 for the project?

Besides I am interested to know if any other plc experienced in the forum had a noise problem with SIEMENS SIMATICs . As I am working on a project with S7 More explanation will be helpful at least for wiring designation.