plc food conveyer

Im doing a assignment at college, where i have to decide which plc's and what components to use to make a mince pie factory such as cameras to check size and thermo couples to check temp, but I need to explain why these plc's are compatable with the components just wondered if anybody had any ideas on this or know of any decent website
PLEASE HELP IM DESPERATE.

SLC500 (Allen Bradley) have thermocouple analog input units for sale.
Pie size, trainable form-detector from Omron for digital input.
Use internet for search.

"I need to explain why these plc's are compatable with the components"
History is destiny.

Industrial controls pre-date the PLC. When PLCs entered the process control world they had to make the inputs and outputs compatible with the sensors that were already being used, so they developed modules compatible with the thermocouples and 4-20 mA signals and 120 VAC and 24 VDC switch singals and relays that were already out there.

Then came serial communications, again compatible with the teletype and computer signals that were in common use.

Then came things like cameras, and they made those compatible with the serial data links that were in use.

So most any brand of PLC has equipment compatible with the devices you want to use because there asn't any profit in re-inventing the wheel.