Auxiliary control systems for cars

I have designed a few control systems, but always for some factory environment. Now I'm facing a new challenge. I need to build a control system for some mobile equipment. The task is in short to modify a small lorry truck and build some machinery on top of it. It's a farm machine that must be allowed for a lot of beating, dirt, water flushing and all kinds of abuse. It's going to be a rent out machine. Nothing wrong said about farmers, but I expect it will not always be treated nice.
Any of you with experience on building such auxiliary control systems for vehicles? I have questions regarding just about everything: Enclosures, power supply, buttons and handles, weather resistance, vibrations and not the least: which controller should I choose? I have most experience with Siemens S7-200 and S7-300/400. I also have some experience with several Allen Bradley types. But for me these PLC systems appear to be far too delicate, likely to fall apart by hitting the first pot hole in the road.

What sort of voltage (and variations) are you dealing with?

xzen,

as far as the enclosure and controls, i would look into NEMA 4, 4X type devices.

regards,
james

Pay very close attention to wiring type and routing. Cut no corner and spare no expense. "Farmers" spend a lot of time and money repairing wiring breaks. The fields are very abusive. What is the application? Will the operator be controlling in the cab or outside or both? Also remember that no matter how tight you seal something it will be exposed to the elements. Install lighting and illuminating button/switches because it could be used late in the night.

Before you start to look at specifics you need to have a think, it is way too easy to totally over specify a machine. For instance what are the consequences of the machine failing, is there any danger, are there big financial losses, how far away from you/support will the machine be. Then think about simplifying the system, the less there is on the machine, the less there is to go wrong.

The machines I am involved with aren't on a farm but are dragged around by farm tractor and then sit vibrating at the side of the road for a few hours. We chose to use a cheap system that allowed them to have cheap spare parts. So the PLC is a S7-200, for the HMI I use Beijer (PLCs and HMIs have so far lasted about 10 years but with occasional failures). Most machines have an on board 400/3/50 generator and I use standard Siemens power supplies. For other machines I use DC/DC converters to give me a stable 24V from the 12V battery system.
http://www.whatpowersupply.com/products.asp?cat=DC to DC Convertor

I don't generally use anything special for the enclosures but they are mounted undercover inside the machine with only the lid facing the weather. I use standard Moeller pushbuttons/indicators because they can easily pick up spares and no matter what you fit they will break them anyway.

Don't have any soldered wire connections because they will break, do use spring type terminals and do oversize cables.

Bryan

IFM has stuff just for this:
http://www.ifm.com/ifmuk/web/pselect2!4_10_10_30.html

Works in the most varying temperatures, wetness and voltage variations.

Programmed in Codesys.

I thank you all for good suggestions. Especially you BryanG seem to have some experience in this area. Minimizing complexity and maximizing robustness seems to be a good approach. Also what Mtech mention about lighting and illuminating button/switches is important.