Specifying a PLC

[jake]

As an individual who has absolutely no experience in PLC's, I need to start somewhere. Apparently I need to select my PLC in part by determining the number of inputs and outputs I need. Should I count one input for each sensing device (photo eyes or prox switches in this case) and one output for each reacting device (motors)? Or is there more to it? I do have some associates with more experience . . . on vacation right now. I'd like some input. Thanks

 

[rsdoran]

You are on the right path but make sure you do a FULL I/O check...ie determine ANY inputs (pushbuttons, sensors, switches, etc) and ANY output devices (motors, air valves, hydraulic valves, any valve, solenoid, relay, contactor etc) that will or may be used. You may need a warning horn for start that would be another output, an alarm output (or many depending on app), the E-Stop ckt you hardwire but you may want to send an input to the PLC to show it GOOD or BAD. There are also analog issues that may need to be addressed...ie temperature, load cells, anything that you may need to take or give a analog signal to. You may need to use an encoder or another high speed device and will need a High Speed Counter. Are you using an AC Inverter (also known as a Variable Frequency Drive (VFD)) or maybe a DC drive, any of these you may use digital/analog signals OR serial/network communications.

The point is to KNOW what you NEED before you purchase anything. Once you KNOW then look at the brands that offer you the best options.

 

[Steve Etter]

Ron says it well.

I would just like to add one other little thought; try not to give in to the idea that two or more devices, wired in series, can be a single input. Instead, insist that each device has its own, separate input point (even if this means rewiring a lot of devices like limit switches and pushbuttons).

Sure, you could go ahead leave them wired serially. It's done too often. The problem is that you have no way of knowing which of the item in the series is preventing your input from going high. This, of course, greatly increases troubleshooting time because you now have to go find and troubleshoot these hardwired components.

Your PLC, when wired well, can be a very effective means of troubleshooting.

Steve

 

[Goody]

Another point is, dont just add up all the I/O you can think of and then order a PLC to match them. The keyword is expandable.

Depending on the size of the project you may be using a brick type or a rack type. Somewhere along the line this system will need more inputs and outputs than you envisaged. If you cant expand or do not have spare I/O it can be embarrasing and costly.

Check that the system is expandable.

 

[Tom Jenkins]

I strongly suggest that as soon as you complete these tasks suggested above, you get in touch with two or three local dostributors of automation products and bring them in for some assistance. It is tough to spec a system all by yourself the first time around - you may not even know the questions to ask. As many here know, I am a big fan of some of the direct online automation suppliers, but that's because after a couple decades of exposure I know what's what. For your first experience for a paying customer or employer the fastest way up the learning curve is to get a good local distributor to help you out in return for a reasonable amount of business.

 

[Allen Nelson]

One more thing....

If you are automating an existing system, you may need to count, not just quantity of inputs and outputs, but TYPE. Sinking/sourcing. 120V/24V. AC/DC. You usually can't mix types on an input module.

Of course, it might be cheaper to replace one 120VAC pushbutton to make everything 24 VDC, rather than buying an entire input module for just one point.

Again, it comes down to KNOWING the machine, as Ron said.