truck identification to plc

Hi i have an application that when a truck goes near my system it must be identified by an ID so my plc knows who is there (just want to know ID of truck maybe 1 to 25 truck.
My plc can either talk with modbus, receive ascii or ethernet/ip
Is there system that already exist for this. i know there are some rfid system but i would prefer a system without cards or keys, maybe just a tag under the hood or on windshield and have 25-30 ft range
an already build kit would be perfect

RFID is going to be what you want, and they can be very unobtrusive, a window decal or hidden tag is about as easy to use and reliable as I can think of. I have not worked with RFID systems, only viewed demonstrations and did a lot of online research a few years ago.

http://www.vizinexrfid.com/sentry-ast-long-range/

RFID is an excellent option. However RFID readers aren't cheap, especially long range readers, but the RFID tags themselves are dirt cheap. RFID is a dependable mature technology.

Another option is a vision system. If the trucks can have number decals on them then a vision system can recognize those numbers. It won't really read the numbers, but it will recognize a pattern and match it to a stored pattern. You'll have to teach it the pattern with the truck. Disadvantage is mud and dirt will prevent pattern recognition. Advantage is humans can also read it as it goes by.

if i use the rfid like the sentry mid range, how do i interface it with plc, it just seems to be a big chip, do they make kits ready to use

Without knowing the physical setup of the place, it's hard to comment. When I work on a system 10 years ago they were ASCII only so I had to deploy a Prosoft ASCII to Ethernet/IP gateway. These days, you can probably get Etherenet/IP ready kits.

One of the major issue I had work with and you will have to resolve to is when to read the tag. Sounds like you have to deploy some kind of proximity trigger so the tags are read all the time.

Bar code is another option and they work well on trucks. Afterall, bar code started when railroad company needed a way to ID railcars. However, RFID system is now more integrated with PLC so I would try RFID first. Distance will be an issue, there's no RFID that works outside ~50cm that I know of so you will have to mount the head closer.

Pepperl & Fuchs is what I worked with, give them a call and they can help you draw up a system.