Thread Starter

ravi chandran

Dear sirs,

I am in automation business. How is costing done for plc design, plcb programming and trouble shooting works. Please advise.

thanks in advance,


R A Peterson

What a question.

Here's the answer.

If you can get away with it, the best way to handle troubleshooting and install is on a time and material basis. You charge the customer a flat rate per hour, plus expenses, and whatever materials might be needed. The reason for this is the near impossibility of estimating accurately (within 50%) the amount of time these activities will take.

As for design and programming, you will need to look at how long it took you to do similar projects. There really is no other way. Anything else is just a wild guess. Once you have enough projects under your belt (maybe 5000
hours worth or so) you can begin to make some correlations between type of project, I/O count, type of PLC, etc.

One thing you cannot do is multiply the number of I/O times some fixed number and come up with a realistic estimate of programming hours, although this is generally the most common way it is done. Its why so many projects come in so far overbudget on engineering hours. This is not as big a deal as you might think though. The amount of money spent on engineering is typical a small percentage of the overall budget so it generally is not that important to the project as a whole. OTOH - if you take on a chunk of the engineering
of a project at a fixed cost, it could be a big deal to you personally when you find that the 500 hours you thought it would take ends up being 1000

Some things I have learned from long experience.

The number of hours of engineering is generally proportional to the number of pages in the spec. In fact, at a former place of employment, we had a spreadsheet we used for estimating hours. It was based on previous projects going back over 5 years. One of the consistant things we found was that jobs from A&E firms (like Bechtel, S&L, etc.) consistantly took 2-3 times as much engineering time as similar jobs that did not have an A&E firm in the middle.

Another thing we found was that, again very consistantly, the more engineers you put on the job, the more total hours it took. We found that a project that had 2 engineers on it, would typically require about 25% more hours then if the same project had been done by a single engineer. For 3 engineers the number was about 50%. We never had a project with more then 3 engineers on it, so I have no numbers for anything above 3 engineers. Intuitively, this
is a fairly obvious observation, since instead of one engineer going to meetings, it was now two. Now two engineers have to read the 300 page spec,
etc. Any project with less then 500 hours will do far better (at least time wise) with a single engineer doing the entire project (hardware and software design) IF you have people capable of doing the whole thing. There are good reasons for having multiple engineers on a project other then efficiency though, so its not always feasable or practical to have a single guy do the
whole thing.

One thing we discovered was that the type of PLC used made a big difference. We had solid experience with a couple brands and those projects that used those PLCs went much faster than PLCs we had less experience with (big

I wish I had kept a copy of the sheet we used. You would be amazed at the things we had on it that made a lot of difference in how many hours projects took to engineer. I put the sheet together based on researching about 5 years worth of projects. Before we started using the sheet our estimates were regularly off by as much as 200%. After the sheets, it was rare to be off by more then 25% (except for new people - but we added a factor for that later).

Only one item on the sheet was kind of subjective. That was the reusability factor. after estimating the number of total hours as if it was a total new project, there was an estimate made as to how much stuff could be taken from
previous projects. the rule was that only stuff that could be snacthed with NO changed could be considered under this category (for instance an entire sheet of drawings with only title block changes), If this number was 25%, then that was how much could be deducted from the estimated
design/programming hours. It could not be deleted from the hours estimated to come from reading the spec, etc. This elimnated the old slaes theory of "its an exact duplicate except".

Over the years it was used, it was remarkably accurate.

Bob Peterson