Interview Question

[kcox1980]

I am currently acting as my plants' maintenance supervisor. As such, will be involved in conducting interviews to fill a Tech Level 2 position which requires a basic level of PLC knowledge and troubleshooting ability. While we don't require a formal test, I would like to ask a few technical questions to gauge the candidate's overall technical knowledge. The one PLC related question I came up with was this:

"There is an error in the following logic, can you identify it and how would you fix it?" Then I would show them a printout of the logic(attached below) that I wrote specifically for this question. If they get stumped I will offer the hint "The problem is that the motor will not stop running, even when all operating conditions that should make it stop are met".

My concern is with the difficulty level of the question. Since I came up with the problem the solution is obvious to me. I don't have anyone else in the plant to run this by because the existing crew are mostly old school mechanics with little to no electrical or PLC experience. I'm not so much concerned with the "correct" answer as I am with getting a feel for how they approach a problem. For example, it stumped on of our engineers who claims to know ladder logic programming but instead of admitting it he tried to BS his way through it saying that the logic didn't make any sense at all and there had to be another routine that I wasn't showing him. That is not the answer I am looking for.

So what do you guys think? Is this an appropriate question to ask of someone who would be asked to troubleshoot logic from time to time or is it too much?

 

 

[Mark-]

Maybe I do not understand something. I see a couple of issues.

The first: Rung 0, if the latch Motor_1 is not set the branch around what I assume is the start switch will set the latch.

Perhaps I am all wet.

 

[kcox1980]

Just now, Mark- said:

Maybe I do not understand something. I see a couple of issues.

The first: Rung 0, if the latch Motor_1 is not set the branch around what I assume is the start switch will set the latch.

Perhaps I am all wet.

 

That's one answer I'm looking for, probably the most obvious one. You could also say that there isn't any way to break the seal in circuit. Finally, you shouldn't have a seal in circuit on a latch instruction at all. 

I'm kind of winging it here. Never been in a position to conduct an interview for a technician.

[TimWilborne]

Here's a question for you. Are you looking for an employee that can troubleshoot PLC programs or the machines? 95% of the time there's nothing wrong with the program. The question usually is can he figure out why the motor isn't running.

I think a better test would be a ladder program with numerous permissive to start a motor. Then the question be if the motor will not run what should you look at on the machine to determine why it's not running. Safety gates, e stops, limit switches, Etc

[pcmccartney1]

Seems to me that on an existing machine, it's rarely the code.  Rather, to TW's point it's something physical.  A bad sensor, a worn out relay, a loose wire.  I would think that an ability to read and understand the code, in conjunction with reading and understanding an up to date set of wiring diagrams should be used to troubleshoot, fix and get the machine back into operating order.

[Michael Lloyd]

I agree with the last two posters. I'd rather have a tech with troubleshooting skills than a programmer. If you hired a programmer, and it was me (not looking for a job btw), I'd want to change all of the code you listed to something that would be safe, easy to troubleshoot, and work  EVEN IF WHAT YOU WROTE WOULD WORK (it won't)

I've edited a fair share of working programs to make them work "better" when they worked fine as they were and I will probably do that for as long as I work on PLC's. If you want a tech, test for a tech.

Edited July 6, 2017 by Michael Lloyd

[Kartik Bhise]

Hi..

Actually your motor starts running irrespective of your input condition.

In order to control stoppage of motor you need to control your Motor_1 tag(which is not high),so that when you unlatch the motor it will work properly.

I hope it helps.Do try it and reply the same.

Thank you..

[Mitsm83]

Sounds like you may have a bunch of mechanics and you need a tech. 

A way I have seen used to help pick the people you want is to just ask about three times they had a problem and solved it to return to production. 

1. Electrical Problem

2. Hydraulic or Pneumatic Problem

3. Mechanical Problem 

You can use the answers to get what you are after.

Asking if they are good with computers or if they like using them is another question that can weed out the more mechanical guys.