PLC Test Panel Advice

Guest,

I've built several of these test fixtures and they are worth their weight in gold for debugging PLC programs as opposed to trying to do it on the target hardware. My favorite vendor for these things is http://web4.automationdirect.com/adc/Home/Home

Four things I would suggest you consider before purchasing hardware: First, make sure you have at least 20% spare space on the front panel and internal panel (these projects always seem to expand in scope midstream). Second, get a power supply that is rated at twice what you need, for the above reason as well as running cooler.Third, consider adding connectors on the side of the box so that you can test switching the actual target load rather than a lamp. Also good for testing with actual sensors rather than a switch on the front panel.

And lastly use real labels that are easily removed/changed for the lights/switches/pushbuttons i.e. make it look professional. Your boss will feel better about spending the money if he/she doesn't see loads of masking tape with Sharpie writing on it.

Good Luck.

The picture is for you to assess the size of the display vs the size of the box.

Don't forget to provide a 4-20 mA circuit to test your analog inputs of the ADC of the PLC. You could reroute the 4-20mA output to the input but you might have some ground problems when you want to display the value. If you need a cheap 4-20mA transmitter at the turn of a 10 turn pot, I can provide it (resolution guaranteed 14 bit).

For the bezels, the manufacturer provides a rectangular cutting tool for up to $150. Buy it, it saves you a lot of problems.

Stevem gave you good advice, take into account.

Hi, Guest, - please register (so we'll know you're the Real Michael!).

Do you have PC programming ability available? If so, it would be worth considering using a PC with some kind of general purpose I/O (internal board or external module via USB - several possibilities come (fuzzily) to mind (it's late!)).

This would give you maximum flexibility for changing the test rig configuration, and minimum expenditure on display devices (for both digital & analog outputs). You can display loads of information on a PC monitor, & if you can program (or have a tame programmer), you can set up automatic test sequences to simulate various scenarios, and to detect & display error conditions.

You may consider it easier to use a panel with pushbuttons and/or toggle switches for the PLC inputs - it wouldn't be too hard to design a hybrid system (PC for status display & hard-wired switches for inputs).

In fact is a better idea, to use a smart device as a testing device, than al those switches. One or two displays should do the rest. Price-wise you come even lower that with switches. You can get a PLC for 10 bucks on ebay.

You just deserve a GA

I have previously designed ATE for production burn-in of power metering and building controller products my company manufactured. I found that for burn-in testing I could effectively test most of the inputs and outputs just by providing loop-back from inputs to outputs, and using test software to drive the output and monitor for the correct change in the inputs. If it's critical to test that your outputs are capable of driving to some specific voltage, or checking that the inputs respond to low levels, it's possible to add some intermediate buffering circuit to provide those conditions inside the test rig. Most of our ATE consisted of nothing more than such buffering circuits with some calibration voltages and contactors to switch high voltages during meter input testing. Solid state relays (which you can make out of a pair of back-to-back MOSFETS) can be quite useful in place of contactors and have a lot less on resistance if you design them properly.

Anyway I'd recommend automating the test process as much as you can - either use the unit itself, another unit of the same kind, or another controller / PC to generate and monitor your test signals. Automation will save you a lot of time and money in the long run if there's any quantity of devices to test. We even had out test program print out a calibration certificate for each unit, with teh pass/fail results for the discrete and analog I/O included.

Dear "Guest" Michael,

Since you say that you work for an industrials controls company I assume that you will need a solution that is flexible.

Have you considered the use of PLC simulation software. I am sure of you google "PLC simulation software" you will get a number of hits. Contact the PLC supplier and they will be able to asssit if possible.

This approach would provide maximium flexibility without incurrring the costs associated with physically building a panel that may or may not require modification for the next project.

KInd Regards

Mr. W.A Snow

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