How long should it take?

I know that this is like asking how long is a piece of string but I only need a general idea.

A machine purchased last year (from a supplier who has since gone out of business) gave trouble yesterday and somehow two pneumatic cylinders that normally do not operate together decided to collide. This is not the first time that we have had this type of failure as it previously occurred with a different pair of cylinders. I added some interlocking code to prevent that fault from occuring again but this latest failure makes me wonder would I be better off starting from scratch and rewriting the program (or get it rewritten by someone else). Also the original program had no documentation apart from identifying the I/O.

It basically consists of 5 pnematic cylinders, 1 hydraulic lifting platform, and 1 vacuum pump, a few photocells to determine when various objects are present and of course the usual stuff such as stop/start, emergency stop/guarding etc.
There are also 5 conveyor belts but these are simply sequentially started according to demand.

The PLC is an Allen-Bradley SLC 5/03

I need to be able to give my boss a rough idea of how long it will take to rewrite the program as opposed to just dealing with this latest problem.

I understand that a more experienced programmer would be quicker than I would be, but even knowing how long it would take an experienced programmer would be useful as my boss would then have an idea of how much it would cost.

If I rewrite it then I will ask to be allowed to work on it without any interruptions for the duration of the project (instead of being called to other equipment).

Thanks in advance for any responses.

Kevin

Here's my piece of string.

5 days of design (including design for test/simulation), 3 days of coding, 5 days of testing (off the machine), 3 days of testing on the machine (including forced fault conditions, power down and recovery from randomly timed power offs).

I would ask for a week if i was you!
Yes you might write the code in a day but testing it and properly commisioning it should be done with great care.
The added bonus being that if you get it done inside a week you might get considered for other more important projects but your boss will have more faith in you and you will get some browning points!

All the best mate

We use a formula of

0.5 hour per digital I/O point
1 hour per analog I/O point
2 hours per PID loop
4 hours per HMI screen

Of course this is assuming we have a process narrative and a P&ID diagram. Without that, you first need to study the machine and determine what it should do and create either a flowchart or process narrative for yourself.

Give yourself 1 week to commission and debug once the code has been developed and downloaded.
Ian

Depending on the complexity of your process is what will determine the time frame. As for cost I use $100.00 per I/O point as a rule of thumb. If your were to break that up into I/O per hour try three I/O per hour, I know it seems like a far stretch, but when doing arithmatic functions, and analogs it will all even out in the wash.

IF you have an accurate list of IO items...

AND IF you have a good idea of how the system ought to run (i.e., state diagram, timing diagram, flow chart, etc...)


THEN a week seems like it ought to be adequate.


If you don't have an accurate list of IO or if you really aren't sure how the system is supposed to run, it'll obviously take longer.

A week sounds pretty good but, I like to keep something in reserve, I have found that even if you are promised " no interuptions blah, blah.." that there will be interuptions and somehow and the end of your week everyone will have forgotten about them and will be saying "you said a week" add a couple of days. It may get you out of the @#$%. And . if nothing goes wrong and yoyu finish within the week you get promoted to GOD status for being under budget...
One mor ething if you go ahead with it give yourself a few days "playing with the PLC" to get familar with it and any possible "traps for inexperienced players"

I'd agree with Simons piece of string and say around 2weeks.

If the 2 cylinders should never operate together then apart from software you may want to look into hardwiring (interlocking) these together as well.

How many lines of code does the existing program use ? (Or how much memory of the 5/03 is used)

I loved visiting Ireland and want to return. Can you hire me to jet over and do it for you? please?

Thanks for all the replies (I had naively thought a couple of days would do it). At this stage the powers that be have decided that another "patch" will do for now.


In answer to your question 504Bloke there about 80 lines of code with 464 memory used (3632 left) in the current version.

I will also look into the possibility of hard wiring the interlocking, however most of the solenoid valves are of the spring return variety which will cause difficulties.

Kevin

Kevin(spark) said:

In answer to your question 504Bloke there about 80 lines of code with 464 memory used (3632 left) in the current version.

Kevin

Click to expand...

80 lines of code is not a lot.

If you know the machine and have drawings of IO ext then it will probably be quicker than a few weeks!

The thing i find is if you tell em 2 days then on the 3rd day you'll have all the white coats hanging around asking stupid questions and giving you hassle!

I have never been to Ireland either and am closer than jstolaruk

Would love to be able to bring both of you over, however they appear to want quick and cheap, not expensive and right.


I'll probably end up working on it in my spare time, instead of lurking here

Kevin(spark) said:

Would love to be able to bring both of you over, however they appear to want quick and cheap, not expensive and right.


I'll probably end up working on it in my spare time, instead of lurking here

Click to expand...

Can you post the code, i'm sure there are plenty here who would help!

Kevin,

There are ways pneumatically to interlock the cylinders.
You could use pilot operated check valves to make sure that the cylinders don't extend at the same time. Call a reputable distributor for help with the specific patrs you need.

504Bloke

I'm not sure if I should post the code (I don't know who the guy who wrote it is and he might be a lurker here).
When I get around to writing the new code I will certainly post it.

PLC Rookie,
That's a great idea to interlock pneumatically, I will look into it.

Kevin