Which PLC?

[pattom12 0]

I have an application to control a swimming pool which is essentially a chemical process. Following is a description of what I want to be able to do: 1. Input 2 items via a control panel and read 3 probes-pH, water temp, and orp(oxidation, reduction potential-used for determining chlorine requirement). The input signals will be mV up to 1000mV. 2. The PLC needs to then do an algebraic calculation. 3. Based upon that number an "IF, Then" statement is required. 4. Another simple algebraic addition is then done 5. Another "IF, Then" statement which either sends a signal to a relay or stops sending a signal to a relay. I need a recommendation for a simple PLC which is inexpensive that I can program to do this job. I have zero experience programming PLC's and if necessary can outsource this work although I am an ME with some scientific language programming experience. If someone could point me in the right direction, I would appreciate it. So far the only PLC I have found that will do this is a Panasonic AFPE224300 which can do floating point calculations but the control inputs must be between 21-24 volts and mine are from probes and will be much lower. Thanks in advance. Pat Thomas Tucson, AZ Edited 19 Aug 2006 by pattom12

[Ron_Smith 13]

In my opinion trying to do this with a plc will require a sharp learning curve and a fat wallet. I found this article where a guy designed a control setup using a pc with Linux --- Swimming Pool Control If I was looking for a way to safely and reliably monitor a pool I'd be looking in this direction -- Aquamasters Hope this helps and good luck. Ron

[BobLfoot 302]

Based on your description you are looking for a PLC which can support the following: 1. Three Analog Inputs in the range of 0 - 1000 mv. 2. Input of Data from an Operator Interface (HMI) 3. Output of decision to a Control Relay or Relays. 4. Ability to make If Then type desicions 5. Capable of floating point math and Logarithmic calculations. If this is true, I suggest the FlexLogix, CompactLogix, ControlLogix or MicroLogix family from Rockwell. All can do what you are asking.

[TimWilborne 108]

I think you should start with a simple control system using discrete I/O and then add to it later. It would be a good experience

[larry818 0]

I'd really recommend finding another project. If you make a programming error, you may pump in all the chlorine or acid, or maybe not keep enough in. If anyone gets hurt or sick from this, the DHS will come looking for the reason why... Just buy a commerically available controller from a company with a big product liability insurance policy...

[TimWilborne 108]

Larry I completely agree with you but what is DHS?

[Ken Moore 23]

I'd guess something like: Department of Health and Social services.

[Rodney 21]

Here in the U.K. it is the Department of Health and Safety Rodney

[TimWilborne 108]

Yes that makes sense. Just didn't click

[ssommers 20]

Yeah, what Ron said is probably your best bet. I just did a quick bit of shopping at Allen-Bradley. A Micrologix 1000 with 20 digital I/O, 4 analog inputs & 1 analog output + programming software + cables (and this doesn't include any external monitors) costs about the same as the Aquamasters unit. It will save you time & liability. Good luck, Sue

[larry818 0]

Around here it's Department of Health Services. Nasty tempered folk....

[TimWilborne 108]

Where is "around here". You don't have your flag up or your location

[gravitar 3]

Costs and liability aside, it would be an awful lot of fun to do something like that with a PLC! Maybe it would make more sense to do this in phases. Start first with a system that only MONITORS the process variables. You could then verify the results with traditional methods (chemical reagents, test strips, etc) over time to ensure the reliability of the system. Once you're sure that you are collecting valid data, maybe connect the output deviced but do not fill them with chemicals.. Instead, just monitor the opening and closing of the valves (I assume it would be controlling solenoid valves) to make sure they are open for the appropriate times, not getting stuck open/closed, not chattering, etc. THEN and ONLY THEN, let it controll the process. Set aside some time for you (or someone equally knowledgable) to babysit the operation to make sure it is functioning as attended! (This is much like what we do when starting up a new machine tool..) Is this a public or private (home) pool? If it is a public pool, I'd have to agree with the conventional wisdom of just using a pre-packaged system. Since I see you have a US flag, I don't have to tell you about personal injury lawyers.. If someone were to be harmed in any way at the pool, even if it had absolutely nothing to do with your PLC control system, a shrewd attorney could use it against you. It could be career-ending. Now if this is just your own personal pool, why not? Give it a try, just be cautious. You have much less to worry about as far as law suits and ambulance-chasers, and the local Health Department won't make any inspections. We put up a pool in our backyard this summer and believe me, I've been thinking about doing the same thing! The PLC equipment wouldn't have to be expensive, but I imagine the sensors would be a bit pricey though.

[TimWilborne 108]

Must be nice, all I got is deep spot in a creek

[larry818 0]

Must be nice....

[Groo 7]

I came into work one morning and found the fire brigade there, our de-ionising water facility had 'a problem' overnight and overflowed with acid, everywhere. Dangerous playing with such things unless your absolutely know the safety steps required.

[entertron 0]

Pat, Interesting application. You will be hard pressed to find a standard PLC that will monitor a 0-1000 mV analog signal directly. You would probably have to use a 0-5 VDC signal and only have 20% resolution, or a signal conditioner...which would be an additional cost. The options presented are rather costly control platforms. In addition to the cost of the hardware, the software is also costly. We have two customers that are using ORP sensors directly to the Smart-PAK PLUS. One is for a portable water filtration plant. The other application involves cyanide destruction. Each application uses the Smart-PAK PLUS in conjuntion with ORP sensors connected directly to the analog input that was configured for 0-1000 mV and +/- 2 VDC on the second application for both ORP and PH. For your water temperature probe, we can also configure RTD and Type K thermocouples in one controller. Other products would be modular based and therefore would require a separate modules for your voltage inputs and temperature inputs, whereby the Smart-PAK PLUS is a board level product, with the circuitry designed into the board and configured according to the application requirements. With regards to your application: 1. Input 2 items via a control panel and read 3 probes-pH, water temp, and orp(oxidation, reduction potential-used for determining chlorine requirement). The input signals will be mV up to 1000mV. ---YES 2. The PLC needs to then do an algebraic calculation using constants, water temp, and two log calculations. ---Will depend on the calculations 3. Based upon that number an "IF, Then" statement is required. --- YES 4. Another simple algebraic addition is then done --- YES 5. Another "IF, Then" statement which either sends a signal to a relay or stops sending a signal to a relay. ---YES Note - we are unable to do floating point, but depending on the screen you use, might be able to address it through the screen. Much will depend on the number of I/O you will require, as to whether the Smart-PAK PLUS is sufficient for your application. Based on the information you have provided, although helpful, isn't complete to provide you exact pricing, but for 1 pc, you will be under $700.00. This includes programming software. If need be, you can email or contact us directly to discuss your application in greater detail. Hope this helps. God Bless, Stephen Luft Edited 8 Aug 2006 by entertron

[TERdON 9]

Converting 0-1V signals to 0-5V signals is basic. All that is needed is an operation amplifier a couple of resistors, and some soldering skills. A few bucks at most if you build it yourself...

[entertron 0]

It may sound basic to do, but in many cases, customers want the product they are purchasing to already have the capability. Makes for a cleaner, less complicated solution. Would you rather get something "out of the box" ready to go, or have something where "batteries are not included"?

[TERdON 9]

I like DIY, thank you. :) But perhaps I'm not the perfect guy to ask (too high geek points), I suppose most other people prefer boxed solutions. Still, what I wanted to show with my post is that the functionality isn't really that expensive to implement, if you're willing to do it yourself. If not, you shouldn't be surprised you have to pay for it!

[Gambit 326]

Why not try a simple Mitsubishi FX1N PLC with a FX2N-2AD card. It has a resolution of 1.25mV when used 0-5V.

[entertron 0]

Even though the analog module will read to 1.25 mV, you are still using a full scale of 0-5 volts with each 1.25 mV = approximately 1 bit. If you have a range of 0-1000 mV rather than 0-5 VDC, your accuracy and resolution is much more precise. (Based on 12 bit analog) For a 0-1000 mV range, each mV = 4.095 bits When using a 0-5 VDC input to read a 0-1000 mV signal, you are only using 20% of the total range. Edited 25 Aug 2006 by entertron