OT: First job interview

[MelissaMiranda]

So, I have my first real job interview this week...and I'm pretty nervous but also VERY excited.

Just graduated as a junior industrial automation engineer. This is my first "real" job interview and man I never thought it would be this exhausting. Can't really imagine how further exhausting it will be to land a full-time job where your brain is constantly "on". Love the field though.

I have to go through a handful of tests: Some personality and IQ tests and lastly I have to solve a case and then present it. The case is actually what I fear the most because I don't really know how to solve it with the little information they've given me in the task. I feel really stupid but I have zero experience with parcel conveyors so I kinda lack the imagination of how one should be designed.

I don't have to design everything from scratch. I have the layout of the whole system. But I need to think about where to place the sensors (for gap detection purposes), mind the required spacing between the parcels (250mm), parcel length (300x300x100mm) and main speed of conveyor of 2.1 m/s.

Here's the exact task:

Your task is to meet the customer requirement of 4000 parcels/hr. in the
dimension 300x300x100mm.

Limitations:
1. Main conveyor feeding line speed can maximum be 2,1 m/s to be within the specifications of the sorter line. Line sorter speed = Main conveyor line speed. The line sorter and main
conveyor feeding line has to run continuously

2. Spacing between parcels must be a minimum of 250 mm upon handover to the line sorter.
3. No conveyor in the system are designed to have a velocity higher than 3,5 m/s or an
acceleration above 4,0 m/s^2.

Task:

As a minimum include your thoughts on:

1. sensor placement (how many and where?)
2. max. capacity for a single BOM feeding line.
3. critical areas that would require attention during development/commissioning

So, there I have it. I'm just puzzled how to figure out the max capacity when I don't have more info of the conveyor dimensions? Maybe I'm missing something? I'm pretty nervous about the interview so my brain is already cloudy .


I changed profession three years ago because I wanted to try something new. Always dreamed of being an "engineer" but never got to do anything about it (because girls just weren't engineers at the time I was a teen in the early 2000s). So I ended up working with HR. The learning curve has been steep but I've managed to get good grades but the "real world" is just so much more complex than what you learn in the university.


I'm not looking for solutions handed to me but maybe some guidance of where to start? If you need an overview drawing of the system, please let me know.


Edit: attached drawing of the layout


Thank you in advance

/Melissa Miranda

 

[rankhornjp]

One of the best pieces of advice I've gotten from this forum was to:
Imagine you had an unlimited number of people and think about what task you could give to those people to get the result(s) you want. (1 person may hold a stopwatch[timer], the next my push the box, etc)
Write it all down. Read and edit until

This should give you a good place to start writing your code.

 

[drbitboy]

most engineering approaches start and end with dimensional analysis.


The case provided has several specifications:

• parcel rate
  • 4000parcel/h
• parcel dimensions
  • 300mm/parcel, dimension 1*
  • 300mm/parcel, dimension 2*
  • 100mm/parcel, dimension 3*
  • 250mm/parcel, minimum gap between parcels
• conveyor rates:
  • 2.1m/s, maximum main conveyor speed
  • 3.2m/s. maximum of any(?) conveyor speed
• conveyor acceleration
  • 4.0m/s**2, maximum of any conveyor acceleration

Can you write a general formula that combines all, or most, of those?

Don't use the numbers, write it down as variables (e.g. pph = parcels/h; L = parcel length along conveyor; g = gap between parcels; s = conveyor speed). That formula will define at least one aspect of the process.

Can you use that formula to determine what is the minimum speed at which the conveyor can run and still deliver the mandated parcel rate?


Can you use that formula to determine what is the maximum parcel rate that can be achieved?



Basically, you want to get to a point where you can evaluate any quantity in the system by multiplying by one; see here.


* Which dimension will be lined up along the direction of conveyor motion? What if the parcel is rotated slighty i.e. could a diagonal dimension be lined up in the direction of conveyor motion? If yes, how long is a "parcel" in that situation?

 

[MelissaMiranda]

Hi all!

Thank you for taking your time and sharing your knowledge on the matter.

drbitboy: I'll try and look further into your tip. Thanks a lot!

 

[parky]

From your description it looks like you only have to worry about the feed to the sorter, well that is how it reads, although I consider it a little unclear. Drbitboy is right, consider the maths it can be worked out.

 

[TurpoUrpo]

When I look the questions presented to you, it seems for me, that they are testing on your ability to think "how would this work" and there really is no one right answer. They want to see how you think, what problems and possibilities you spot. I'm sure, that they are not expecting ready and full solution from fresh grad.

So I would say you should think:

What is the process and what is needed for the parcel to be processed, how would you detect input parcels, how would you avoid collisions etc.

Calculate the theoretical maximum speed.

And think about commissioning, this I would imagine is hardest for you. Think about what kind of problems could be there in operating such a system and then try to find how would you tackle them when designing the system and commissioning it.

 

[parky]

The explanation is a bit vague to say the least, that looks like a check weigher of some description and something else after that before the sorter these usually run faster than the line to get separation, without dimensions of these units then it would be difficult to judge if the pack separation is enough to allow only one parcel on that unit. maybe it's just a bar code reading station who knows to be honest if one of my customers gave me a drawing & description like that I would want to know what each piece of equipment was and some description of the operation etc.

 

[MelissaMiranda]

Hi all! Thanks for all the replies to my enquiry. Thank you for taking your time to aid.

Just one detail: I forgot to tell you that the spacing between parcels must be a minimum of 250 mm upon handover to the line sorter.

So, that must mean that a single BOM max cap = element speed (m/s) divided by parcel dimension (m) * 3600 (s/hr). Correct?

Anything else I can calculate with the numbers provided?

 

[drbitboy]

So, that must mean that a single BOM max cap = element speed (m/s) divided by parcel dimension (m) * 3600 (s/hr). Correct?

the units look correct, yes.



Define "parcel dimension."




but yes, you have the gist of it and are well on your way; as for whatever else can be calculated I'll leave that to you.



Well done! This is a public forum so let's hope your interviewer is not following, although I think everyone has tried to be circumspect in the level of detail provided, so you can certainly claim the bulk of the credit, and for the rest saying "I don't know" is often an important first step in understanding.



If the interviewer is watching, I have this to say: in my experience I would not expect one recent grad in ten (or more) to have the fundamentals MelissaMiranda demonstrates with that formula, espeecially as it is in a domain about which she was more or less completely unaware a few days ago; that makes her pretty special.

 

[MelissaMiranda]

the units look correct, yes.

Define "parcel dimension."

but yes, you have the gist of it and are well on your way; as for whatever else can be calculated I'll leave that to you.

Hi again drbitboy. Thanks for the quick reply.

Parcel dimensions in meters is 0.3m

It's been so long since I've used math this way. I know it's a pretty basic calculation for real engineers but I'm quite rusty.

And thank you for the kind words. I'm still learning.

 

[drbitboy]

Parcel dimensions in meters is 0.3m

yes, that is how many meters of parcel there are per parcel.


but if you are trying to determine parcels per hour via you formula:



(conveyor m/s) / (???m/parcel) * 3600 s/h



the ???m/parcel must be (conveyor m/parcel), which is different than (parcel m/parcel).

 

[MelissaMiranda]

yes, that is how many meters of parcel there are per parcel.


but if you are trying to determine parcels per hour via you formula:



(conveyor m/s) / (???m/parcel) * 3600 s/h



the ???m/parcel must be (conveyor m/parcel), which is different than (parcel m/parcel).

Ah, okay. So I need to know the conveyor length? When you say "conveyor", do you mean main feed line or BOM line? I'm a little confused now, because with the info I got, I can't determine that...can I?

 

[drbitboy]

Ah, okay. So I need to know the conveyor length? When you say "conveyor", do you mean main feed line or BOM line? I'm a little confused now, because with the info I got, I can't determine that...can I?

I mean the distance along the conveyor from the front edge of one parcel to the front edge of the next (i.e. the edge that triggers the proxes).

 

[MelissaMiranda]

I mean the distance along the conveyor from the front edge of one parcel to the front edge of the next (i.e. the edge that triggers the proxes).

Sorry, English is not my native language, so I hope I'm not testing your patience too much

Let's see if I got this right: 0.3m + the minimum required gap distance of 0.25m? So 0.55m?

 

[L D[AR2P#0.0]]

Whilst not adding anything to this party, I find it particularly confusing that the boxes shown in the diagram (they look 100x100x300 to me) are not representative of the boxes in the specification (300x300x100) leading to the question of what orientation are the boxes placed on the conveyor that feeds the line sorter