I don't have experience in this area (yet - still working on the engineering degree), but the first thing that comes to mind is to at least try to patent the device. If you go in there with your concept already patented (or you have a 'patent pending'), your idea is pretty secure. The company will have to deal with you and secure the patent rights from you if they like the product. And if they try to screw you over anyway, hoping that you can't afford a patent lawsuit, then bring your idea to another company that CAN deal with the lawsuit and will respect you.