Analyzing PLC Requirements

At one time I did a similar type of study with the view of implementing Geographic Information Systems, from what you have outlined you are definitely on the correct track.

Remember that your item 3, I/O details, is a major part of the spec - and can sometimes determine which PLC model (& possibly even which make) you go for.

Make sure to include any/all analogue inputs & their types & ranges, also details of any/all motors to be driven. If you've only got on/off inputs & outputs, it's pretty much plain sailing.

Here's some stuff to think about:

• Is there already a "User Requirement Specification", or a project briefing document of some sort that could be used by, and no disrespects in this, any newly-experienced person that the business might send?
• Is there a "standard survey sheet" that could go along for filling in, so that nothing gets omitted? How valuable would such a thing be to the business' operations?
• Could the Client fill the survey form in remotely, thereby saving a site visit?
• What existing systems does the Client use already, thereby saving on duplicating programming software and spare hardware stockholdings?
• Does the Client have an existing Standard, that limits the choice of PLC equipment and software that may be selected by suppliers?
• Is there a software programming Standard that would guide suppliers in producing software structure, thereby minimising maintenance and debugging time?
• etc.

Hi all,

1.) Process requirements - sometimes a PLC would just not do and you would need a DCS

2.) Legal/insurance requirements - what laws govern that type of process in your country and what does your insurance specify cover.

3.) Zone requirements - Is the area (plant) hazardous, thereby requiring a whole different approach to the control/instrumentation system design.

4.) Types and quantities of IO - do you have alot of Analogue (PID) loops or are the signals mainly discrete

5.) Topography - What is the area of the plant you need to cover and what requirements does the client have (preferences for specific makes)

6.) Supplier support - Do you have suppliers readily available in your area and are the parts easily available

7.) Reliability/Availability - If you put a rubbish system in you will get rubbish out of the system. You will have more downtime, costing you more money in lost revenue.

8.) Interface needs - Does your system have to interface with other systems, MES; Web Browsers; Historians etc etc etc.

9.) Cost - probabely the only thing the bean counters will look at (the clients accountants)

As you can see - this is not a simple process. If you have a prefered vendor, it would be better to sit with them as well as your client to review these requirements and tailor the solution.

Regards,

Craig

Aside from all the great feedback that everyone has provided, also ask your client if they have anyone on board that knows anything about PLC's. This information will help your company decide if you will need a person on call or on site to deal with issues as they come come after the installation. This too could be a very costly oversite if your not up front about it.

Good luck...

typically that is called an add on, most large companies use it as a revenue source.