Hello,
I have built custom control panels for about 20 years. Most of my clients still prefer "relay logic" as even a tech in the field with knowledge can repair them .. no need for a pc or special training (and they don't have to pay me to come out into the field).
The other clients I have done work for all had a "factory standard" and so I had to stay with their brand of PLC. For example, my last contract position (I was told there was 6 months work ... I stayed for 10 years finally leaving to take a full time job with the company) used GE. The place I worked for the 5 years previous only used Allen Bradly.
Anyway, I actually have a "home" project for a change! After three years of living onsite in an RV while building (well freezing here in VT when temperatures hit -30) my wife and I moved into the lower unfinished level of our new home. My problem is I need an elevator as my wife is paralyzed from the shoulders down (diving accident when she was 12 ... she is 37 now). We thought we had this taken care of when we started building. We got two quotes ... both for around $15K. We found a grant for $10K so about $5K out of pocket. Now three years later both companies came in with new quotes of about $40K ... way out of our budget!
So, I purchased an electric forklift (30 years old but working great .... $850). I stripped off the mast, pump and motor. The mast is in place, I am building a "car" to fit on top of the forks. I have designed a complete hydraulic power unit. In order to get rid of the "jerk" when starting and stopping, I purchased a flow control valve and control board (can take 0-5, 0-10 or 4-20). I have numerous interlocks to check position, to open, close and lock gates (preventing entry when the car is not at the right location), I have over travel switches (hooked to a contactor and not just the PLC), even a timer to prevent the motor from continuously running if a contactor welds itself on (saw a forklift start on fire because of this once). I even have a back-up hydraulic pump (plus the main pump is running of 24 volt batteries so it will function even if the power goes out).
So, now it is time for controls. We are on a super tight budget (who isn't). I will need to buy a plc with digital I/O and an analog output and software. I will probably even have a PC dedicated to it so if it needs maintenance even 15 years from now, I won't have to worry about finding a PC compatible with 15 year old software. Brands like Allen Bradly are just out of reach for a home project ... I can't even afford the software.
I had been looking at plc's from Automation Direct. They seem reasonably priced ... but then I read the reviews ... some great, some bad! There may be others but I don't know of anyone personally who has tried any of these.
Can anyone suggest a "cheap" brand of PLC that is "good"?
I would even consider converting a PC for use as a PLC (using one of the small industrial ones with a solid state hard drive) and putting the I/O off the USB port but again, I don't know of anyone with personal knowledge of this to see if it is "good" or not.
I would appreciate any advice!
Thanks ...... Mike
PS ... I have been building panels in the unfinished part of the house while we built ... the pad is poured for the new shop ... work starts this spring. I have included a picture of a "typical" relay logic panel I build (built this one while moving). You can also see pictures of the RV I converted (long story but my wife needs care every 4 hours so I converted an RV so she could travel) at http://mcsele.shutterfly.com/2299 .... I designed an build a lift that folds under the RV, a lift on the inside, installed hospital beds, ......